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Understanding the factors affecting the performance of iron and nickel electrodes for alkaline nickel-iron batteries

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Understanding the factors affecting the performance of iron and nickel electrodes for alkaline nickel-iron batteries

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Content I

UNDERSTANDING THE FACTORS AFFECTING THE PERFORMANCE OF IRON
AND NICKEL ELECTRODES FOR ALKALINE NICKEL-IRON BATTERIES

by

Chenguang Yang

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)

May 2016

Copy Right 2016 Chenguang Yang
II

Dedications
To my families, especially to my parents and my wife, who have supported and
encouraged me in many ways

III
Acknowledgements
I would like to extend my immeasurable gratitude to my advisor Prof. Sri. Narayan for
his untiring encouragement, support and guidance during my graduate studies. I have
been extremely fortunate to have such a knowledgeable and selfless mentor.

I would like to thank Prof. Steven Nutt and Prof. Katherine Shing for valuable
discussions on my research work and for being on my qualifying examination and
dissertation committees.

I would like to thank Prof. Surya Prakash for his guidance and support on my research
and for being on my qualifying examination committees. I would also like to thank Prof.
Malancha Gupta for being on my qualifying examination committees.

I would like to thank Dr. Aswin Manohar for his encouragement and support during my
graduate studies. I would like to thank all the members of the Narayan group. I am
grateful to work with a group of talented and hardworking scientists, and I am thankful
for their academic and personal support.

IV
TABLE OF CONTENT
DEDICATIONS .......................................................................................................... II
ACKNOWLEDGEMENTS ..................................................................................... III
LIST OF TABLES ................................................................................................... VII
LIST OF FIGURES ................................................................................................ VIII
LIST OF ABBREVIATIONS ................................................................................ XVI
ABSTRACT .......................................................................................................... XVII
CHAPTER 1. INTRODUCTION ............................................................................... 1
1.1 BACKGROUND ON ALKALINE NICKEL-IRON BATTERIES FOR LARGE SCALE ENERGY STORAGE ..... 1
1.2 OVERVIEW OF ALKALINE NICKEL-IRON BATTERIES ....................................................................... 2
1.3 CHALLENGES IN DEVELOPING HIGH-PERFORMANCE ALKALINE NICKEL-IRON BATTERIES ............ 4
1.3.1 Low Charging Efficiency of the Iron Electrode ...................................................................... 5
1.3.2 Poor Discharge Rate Capability of the Iron Electrode ........................................................... 6
1.3.3 Understanding the Formation Process of the Iron Electrode ................................................. 7
1.3.4 Understanding the Factors Affecting the Active Material Utilization of the Iron Electrode .. 9
1.3.5 Understanding the Factors Affecting the Cycle Life of the Iron Electrode ........................... 10
1.3.6 Developing High-Performance Nickel Hydroxide Electrode for the Nickel-Iron Battery .... 12
CHAPTER 2. EXPERIMENTAL APPROACHES ............................................... 13
CHAPTER 3. PRESSED-PLATE IRON ELECTRODES .................................... 21
3.1 PRESSED-PLATE FABRICATION METHOD ...................................................................................... 21
3.2 IMPROVEMENTS IN CHARGING EFFICIENCY ................................................................................... 22
3.2.1 Effect of Carbonyl Iron as Active Material and Bismuth Sulfide as Additive on Improving
Charging Efficiency ....................................................................................................................... 23
3.2.2 Effects of Bismuth Oxide Additives on Improving Charging Efficiency ................................ 29
3.2.3 Studies on Electro-Reduction of Bismuth Oxide to Bismuth ................................................. 31
3.2.4 Corrosion Studies of Iron Particles ...................................................................................... 35
3.2.5 Studies of Kinetic Parameters for Hydrogen Evolution Reaction on Iron Electrodes .......... 38
3.3 STUDIES ON IMPROVING DISCHARGE RATE CAPABILITIES ............................................................ 43
3.3.1 Improving Discharge Rate Capability of Iron Electrodes with Sulfide Additives ................. 43
3.3.2 Studies of De-Passivation Characteristics of Sulfide Additives ............................................ 46
3.3.3 In-situ Formation of Iron Sulfide Compounds in the Presents of Soluble Sulfide Ions ......... 49
V
3.4 STUDIES ON FORMATION OF IRON ELECTRODES............................................................................ 50
3.4.1 Changes in Electrode Potential and Discharge Capacity during Formation ....................... 51
3.4.2 Effects of Active Surface Area on Formation ........................................................................ 54
3.4.3 Understanding of the Mechanism of Iron Electrode Formation ........................................... 58
3.4.4 Phenomenological Model for Formation of Carbonyl Iron Electrodes ................................ 63
3.4.5 Model Validation .................................................................................................................. 68
3.5 CYCLE LIFE STUDIES ON PRESSED-PLATE IRON ELECTRODE ........................................................ 73
3.5.1 Cycle Life Testing of Iron Electrode with Different Additives .............................................. 74
3.5.2 Comparison of Sulfide Incorporation in Iron Electrodes with Different Additives. ............. 76
3.5.3 Change of Discharge Rate Capability with Cycling ............................................................. 78
3.5.4 Changes in Overpotentials during Constant Current Charging ........................................... 82
3.5.5 Charge and Discharge Studies on Magnetite Electrodes ..................................................... 85
3.5.6 Effect of Overcharge and Charge Rate on the Recovery of Capacity ................................... 89
3.5.7 Effect of Discharge Rate on the Recovery of Capacity ......................................................... 92
3.5.8 Effect of Depth of Discharge (DoD) on Capacity Fade ........................................................ 94
3.5.9 Comparison of the Cycling Behavior of Iron Electrode with Different Additives ................. 96
CHAPTER 4. SINTERED IRON ELECTRODES ................................................ 99
4.1 EFFECT OF POROSITY AND PORE SIZE OF SINTERED IRON ELECTRODE ON ACTIVE MATERIAL
UTILIZATION ..................................................................................................................................... 100
4.1.1 Model for Calculating Active Material Utilization from Porosity of Iron Electrode .......... 100
4.1.2 Understanding the Differences between Calculated and Experimental Material Utilization103
4.1.3 The Effect of Pore Size on Utilization of Iron Active Material ........................................... 108
4.2 FORMATION OF SINTERED IRON ELECTRODE .............................................................................. 111
4.2.1 Effect of Sulfide Additives on the Formation of Sintered Iron Electrodes .......................... 111
4.2.2 Changes in Active Material Morphology during Formation of Sintered Iron Electrodes .. 113
4.2.3 Effect of Conducting Carbon Additives on the Rate of Formation of Sintered Iron Electrodes
..................................................................................................................................................... 120
4.3 CHARGING EFFICIENCY OF SINTERED IRON ELECTRODES ........................................................... 121
4.3.1 Achieving High Charging Efficiency on Sintered Carbonyl Iron Electrodes ..................... 121
4.3.2 Understandings on the Reasons for the High Charging Efficiency on Sintered Carbonyl Iron
Electrodes .................................................................................................................................... 122
4.3.3 Charging Efficiency of Sintered Iron Electrodes at Various Charging Rates .................... 126
4.4 DISCHARGE RATE CAPABILITY OF SINTERED IRON ELECTRODE ................................................. 127
4.4.1 Discharge Rate Capability of Sintered Iron Electrode with Soluble Sulfide Additives ....... 127
4.4.2 Discharge Rate Capability of Sintered Iron Electrode with Iron Sulfide Additive ............. 128
VI
4.5 CYCLE LIFE TESTING OF SINTERED IRON ELECTRODES ............................................................... 130
4.5.1 Cycle life testing of Sintered Iron Electrode with Soluble Sulfide Additives....................... 130
4.5.2 Cycle life testing of Sintered Iron Electrodes with Iron Sulfide Additives .......................... 134
4.5.3 Understanding the Cycle Life Performance of the Sintered Iron Electrode ....................... 135
CHAPTER 5. NICKEL HYDROXIDE ELECTRODES ..................................... 140
5.1 INTRODUCTION ON NICKEL ELECTRODES.................................................................................... 140
5.2 BACKGROUND ON THE STATE-OF-ART NICKEL BATTERY ELECTRODES ..................................... 141
5.3 PERFORMANCE OF COMMERCIAL SINTERED NICKEL ELECTRODES ............................................. 142
5.3.1 Charge and Discharge Cycling of Commercial Sintered Nickel Electrodes at Various Rates142
5.3.2 Charge and Discharge Cycling of Commercial Sintered Nickel Electrodes at Reduced Capacity
Utilization .................................................................................................................................... 145
5.3.3 Specific Capacity of Commercial Sintered Nickel Electrode .............................................. 147
5.4 NICKEL-FOAM ELECTRODE ......................................................................................................... 148
5.4.1 Advantage of Foam Based Electrode .................................................................................. 148
5.4.2 Charge and Discharge Performances of Nickel-Foam Based Nickel Electrodes ............... 150
5.4.3 Effect of Conducting Carbon Additives on Nickel-foam Electrodes ................................... 153
5.4.4 Effect of Amount of Binder on Nickel-foam Electrodes ...................................................... 157
5.4.5 Cycle Life Testing of Nickel-Foam Electrode ..................................................................... 159
CHAPTER 6. NICKEL-IRON CELL STUDIES ................................................. 161
6.1 FACTORS AFFECTING THE PERFORMANCE OF NICKEL-IRON CELL .............................................. 161
6.1.1 The Effect of Limiting Electrode ......................................................................................... 161
6.1.2 The Effect of the Formation Procedures ............................................................................. 163
6.1.3 Cell Sizing and Specific Capacity Estimation Based on Electrode and Component Properties
..................................................................................................................................................... 166
6.2 PERFORMANCE OF NICKEL-IRON CELL ....................................................................................... 172
CHAPTER 7. CONCLUSIONS ............................................................................. 176
CHAPTER 8. SUMMARY ..................................................................................... 184
APPENDIX............................................................................................................... 186
REFERENCES ........................................................................................................ 188

VII
List of Tables
Table 1 Kinetic parameters for hydrogen evolution reactions on various iron electrodes.
Charging efficiencies calculated based on Eq.9. Exchange current was normalized for
discharge capacity at twenty-hour rate ................................................................... 40
Table 2 Effect of electrode design on the electrode properties governing formation ...... 63
Table 3 Experimental data on carbonyl iron electrodes charged at 200 mA and discharged at
20 mA (* - Calculated, ** - Experimentally measured) .......................................... 69
Table 4 Active area and roughening coefficient of the iron electrode under different
conditions (ipass = 1.53 mA*cm
-2
, Q1 for the different electrodes was taken from data
presented in Table 3) ............................................................................................... 73
Table 5 Model to calculate specific capacity of nickel-iron cell based on electrode and
component properties ............................................................................................ 171

VIII
List of Figures
Figure 2.1 Pressed-plate electrodes constructed from carbonyl iron powder .................. 13
Figure 2.2 Sintered iron electrodes made from carbonyl iron powder ............................ 14
Figure 2.3 Pasted nickel electrode on nickel-foam substrate with nickel hydroxide ...... 15
Figure 2.4 Test cell configuration showing working electrode, reference electrode, counter
electrodes and electrolyte ........................................................................................ 17
Figure 3.1 Typical charge and discharge voltage profiles for carbonyl iron electrodes with
and without bismuth sulfide additive ....................................................................... 24
Figure 3.2 Charging efficiency at C/2 rate for three types of electrode compositions .... 24
Figure 3.3 Relative rates of hydrogen evolution of various electrodes when charged at C/2
rate ........................................................................................................................... 25
Figure 3.4 Charging efficiency as a function of cycling at C/2 rate of charge and C/20 rate
of discharge. The band refers to the charging efficiency of state-of-art commercial
electrodes from nickel-iron batteries (Sichuan Changhong Battery Co., Ltd., Sichuan)
................................................................................................................................. 25
Figure 3.5 Hydrogen overpotential of various iron electrode materials during charging at
C*/10 rate, where C* is the theoretical capacity based on the mass of the electrode
material .................................................................................................................... 26
Figure 3.6 Morphology of carbonyl iron powder in the electrode .................................. 27
Figure 3.7 X-ray Diffractogram for a charged iron electrode prepared from carbonyl iron
electrode and bismuth sulfide. Powder Diffraction Files: Fe (00-006-0696), Fe(OH)
2

(00-013-0089), Fe
3
O
4
(00-071-6766), Bi (00-044-1246), Fe
3
S
4
(01-089-1998),FeS
(01-076-0964) .......................................................................................................... 28
Figure 3.8 Charging efficiency of a commercial iron electrode and carbonyl iron electrodes
with bismuth oxide as an electrode additive ............................................................ 30
IX
Figure 3.9 Variation of charging efficiency of a carbonyl iron electrode containing bismuth
oxide and iron sulfide with repeated cycling. Note that cycles 9 to 11 were dedicated to
rate-capability measurements and hence charging efficiency data was not collected31
Figure 3.10 Galvanostatic electro-reduction of bismuth oxide electrode in 30% potassium
hydroxide electrolyte ............................................................................................... 32
Figure 3.11 XRD pattern for the bismuth oxide electrode after reduction ...................... 33
Figure 3.12 XRD pattern for the carbonyl iron electrode containing different amounts of
bismuth oxide (a) 5 w/w% and (b) 10 w/w% .......................................................... 34
Figure 3.13 SEM micrographs of iron particles with different additives in 30% potassium
hydroxide electrolyte. (a,b) carbonyl iron, (c,d) carbonyl iron + iron sulfide, (e, f)
carbonyl iron + bismuth oxide ................................................................................. 37
Figure 3.14 Cathodic Tafel polarization plots for fully-charged iron electrodes of various
compositions. Parameters of the Tafel Equation are also shown ............................ 39
Figure 3.15 Discharge capacities of iron electrodes as function of the normalized discharge
rate. Normalized discharge rate expressed as 1/n times the nominal capacity in
Ampere-hours, where n is the number of hours of discharge .................................. 44
Figure 3.16 Discharge capacities of blank carbonyl iron electrodes and electrodes modified
with bismuth oxide as function of the normalized discharge rate ........................... 45
Figure 3.17 Discharge capacities of commercial iron electrode and carbonyl iron electrodes
with various additives as function of the normalized discharge rate ....................... 46
Figure 3.18 Anodic polarization curve for a fully charged iron electrode with different
compositions in 30 w/v% potassium hydroxide at a scan rate of 0.17 mV s
-1
........ 47
Figur 3.19 XRD pattern for the carbonyl iron electrode modified with bismuth oxide and
with sodium sulfide added to the electrolyte (3.0g/L) after extended cycling ........ 49
Figure 3.20 Change of electrode potential during charging at 200 mA for 2 hours during
electrode formation .................................................................................................. 51
Figure 3.21 Change of electrode potential and discharge capacity of carbonyl iron electrode
during discharge at 20 mA to a cut- uring electrode
formation ................................................................................................................. 52
X
Figure 3.22 Growth of discharge capacity of carbonyl iron electrode during formation
cycling ..................................................................................................................... 53
Figure 3.23 Change in overpotential for hydrogen evolution on carbonyl iron electrode at
the end of charging during formation cycling ......................................................... 54
Figure 3.24 Effect of addition of 0.125% Triton® X-100 to the electrolyte on the discharge
capacity during formation of carbonyl iron electrode ............................................. 55
Figure 3.25 Scanning electron micrographs of carbonyl iron electrode (a) before formation
cycling in the “as- prepared” state, (b) at the end of formation cycling in the discharge
state .......................................................................................................................... 56
Figure 3.26 Scanning electron micrographs of carbonyl iron electrode with 10 w/w%
potassium carbonate as pore-former, before formation ........................................... 57
Figure 3.27 Effect of pore-former, 10 w/w% potassium carbonate, on the growth of capacity
during formation ...................................................................................................... 58
Figure 3.28 Anodic polarization curve for completely charged iron electrode in 30%
potassium hydroxide ................................................................................................ 59
Figure 3.29 Anodic polarization curve for completely charged iron electrode after 10 cycles
and 20 cycles of formation ...................................................................................... 61
Figure 3.30 Effect of sodium sulfide on the anodic polarization behavior of carbonyl iron
electrode in 30% potassium hydroxide .................................................................... 62
Figure 3.31 Effect of sodium sulfide additive in the electrolyte on the formation of the iron
electrode .................................................................................................................. 62
Figure 3.32 Electrochemical processes during formation of the iron electrode .............. 64
Figure 3.33 Comparison of measured and predicted discharge capacity of carbonyl iron
electrode with (a) Triton® X100 in the electrolyte (b) pore former additive; dashed
lines show predicted data ......................................................................................... 70
Figure 3.34 Variation in the discharge capacity of a carbonyl iron electrode with bismuth
sulfide additive as a function of cycling .................................................................. 74
Figure 3.35 Recovery of discharge capacity of an iron electrode with the addition of sodium
sulfide to the electrolyte .......................................................................................... 75
XI
Figure 3.36 Discharge capacity of a carbonyl iron electrode with iron (II) sulfide and
bismuth oxide additives during cycle life testing .................................................... 76
Figure 3.37 Discharge capacities at different discharge rates of a carbonyl iron electrode
prepared with bismuth sulfide (Bi
2
S
3
) additive at different stages of cycling and a
sulfide-free iron electrode ........................................................................................ 79
Figure 3.38 Anodic polarization curves (Broken arrow indicates direction of potential scan)
of iron electrodes measured in the fully charged state at different stages of cycling80
Figure 3.39 Discharge current at an electrode potential of -0.85V of iron electrodes
measured in the fully charged state at different stages of cycling ........................... 81
Figure 3.40 Potential – charge curves measured during charging of carbonyl iron electrode
at the cycle numbers indicated (a) prepared with bismuth sulfide, and (b) with iron (II)
sulfide and bismuth oxide additives ........................................................................ 82
Figure 3.41 Overpotential at 50% State-of-Charge during charging of carbonyl iron
electrodes with different additives as a function of cycling .................................... 83
Figure 3.42 XRD of a carbonyl iron electrode prepared with bismuth sulfide additive
measured after capacity fade ................................................................................... 85
Figure 3.43 Potential-charge curves measured during discharge of an electrode prepared
with magnetite powder in different electrolytes ...................................................... 87
Figure 3.44 Effect of sulfide addition on the charging characteristics of an iron electrode
that has faded in capacity. Potential – Charge curves measured on a bismuth sulfide
modified iron electrode during charging before and after the addition of sodium sulfide
to the electrolyte ...................................................................................................... 88
Figure 3.45 Effect of overcharge on the discharge capacity of an iron electrode with
bismuth sulfide additive that is fading in capacity .................................................. 90
Figure 3.46 Effect of various charging rates on the discharge capacity of an iron electrode
prepared with bismuth sulfide additive ................................................................... 91
Figure3. 47 Discharge capacity of a carbonyl iron electrode prepared with bismuth sulfide
additive at different charging rates. The discharge rate was kept constant at C/5 ... 92
XII
Figure 3.48 Effect of different discharge rates on the cycling behavior of a carbonyl iron
electrode with bismuth sulfide additive ................................................................... 92
Figure 3.49 Effect of cycling at high discharge rates on an iron electrode prepared with
bismuth sulfide additive. Potential – Charge curves measured during charging at
different stages during cycling at high discharge rates ............................................ 93
Figure 3.50 Discharge capacity of a carbonyl iron electrode prepared with bismuth sulfide
additive as a function of cycling at different DoDs. Experiments at low DoD were
performed by charging the iron electrode at the C/2 rate and discharging at the C/5 rate
to a cut off potential of –0.75V (vs MMO). The C rates for these experiments were
based on 83% of the maximum rated capacity of the electrode .............................. 94
Figure 3.51 Potential – Charge curves measured during charging of a carbonyl iron
electrode prepared with bismuth sulfide additive when cycling at 83% DoD. (Cycle
numbers are shown adjacent to the curves) ............................................................. 95
Figure 3.52 Schematic of the cycling behavior of a carbonyl iron electrode with (a) bismuth
sulfide additive and (b) iron (II) sulfide + bismuth oxide additives ........................ 97
Figure 4.1 Experimental discharge capacities of sintered iron electrode for electrodes of
varying initial porosities when discharged at C/20 rate. Predicted results are from
calculations using Eq. 28, and with the modification by Equation 30 for k
per
=0.2102
Figure 4.2 (a) SEM images of the cross-section of sintered iron electrodes after discharge (b)
magnified picture of the surface portion in picture (a), (c) magnified picture of the
inner portion in picture (a) ..................................................................................... 105
Figure 4.3 Schematic of cross-section of sintered iron electrode during discharge (a) early
stage of discharge (b) final stage of discharge....................................................... 107
Figure 4.4 SEM characterization of sintered iron electrode with pore former of different
particle sizes (a) 90-105μm, (b) 20-25μm at as-prepared states ............................ 109
Figure 4.5 Discharge capacities of sintered iron electrode with different pore sizes after
formation ............................................................................................................... 110
Figure 4.6 Schematic of cross-section of sintered iron electrode with different pore sizes
before and after discharge...................................................................................... 110
XIII
Figure 4.7 Formation of sintered iron electrode with 0.5% w/v of sodium sulfide added after
10 cycles of passivation (electrode 1), and added at the beginning of formation
(electrode 2) ........................................................................................................... 112
Figure 4.8 The potential-current curves during potentiodynamic polarization of electrodes
with 3.0g/L sodium sulfide additives and without sodium sulfide additives......... 112
Figure 4.9 (a) Sintered iron electrode in the “as-prepared” state (b) Sintered iron electrode
in the charged state following 40 cycles of C/2 charge and C/20 discharge ......... 114
Figure 4.10 (a) SEM images of sintered iron electrode after discharge without sulfide. (b)
Surface in Figure 4.10 a at higher magnification .................................................. 115
Figure 4.11 (a) Discharge capacity of sintered iron electrode during formation (b) Discharge
capacity of high-porosity sintered iron electrode during formation ...................... 117
Figure 4.12 SEM pictures of sintered iron electrode with high porosity at (a) discharged
state during formation (b) charged state during formation .................................... 118
Figure 4.13 Discharge capacities during formation of sintered iron electrode containing 5
wt. % acetylene black ............................................................................................ 120
Figure 4.14 Charging efficiencies of different types of iron electrodes under C/2 charge and
C/20 discharge rates at 100% depth of discharge .................................................. 122
Figure 4.15 Charging curves of sintered and pressed-plated iron electrodes ................ 123
Figure 4.16 Tafel plot for hydrogen evolution reaction on sintered iron electrode ....... 125
Figure 4.17 Discharging curves of sintered and pressed-plated iron electrodes ........... 126
Figure 4.18 Charging efficiencies of sintered iron electrodes at different charging rates127
Figure 4.19 Comparison of discharge rate capabilities of commercial iron electrode,
pressed-plate iron electrode, and a sintered iron electrode .................................... 128
Figure 4.20 Discharge rate capability of commercial iron electrode and sintered iron
electrode with iron sulfide ..................................................................................... 129
Figure 4.21 Discharge capacities of sintered iron electrode with different cycling rates130
XIV
Figure 4.22 Discharge curves of sintered iron electrode correspond to different points
marked in Figure 4.21 ............................................................................................ 131
Figure 4.23 Concentration of sulfide ions in the electrolyte during cycling of sintered iron
electrode at open-circuit period between charge and discharge. ........................... 132
Figure 4.24 Cycle life experiment of sintered iron electrode at 1C rate charge and discharge
and 100% depth of discharge ................................................................................ 135
Figure 4.25 SEM photograph of sintered iron electrode made with carbonyl iron particles
............................................................................................................................... 136
Figure 4.26 SEM characterization of: (a) sintered iron electrode at as-prepared state (b)
sintered iron electrode after initial discharge (c) partially charged sintered iron
electrode (d) magnified picture of c ...................................................................... 138
Figure 5.1 Commercial sintered nickel electrode from Highstar Corporation. The rated
capacity of the electrode is 2.7Ah. The active area of the electrode without the tab is
140mm*70mm*0.63mm ....................................................................................... 143
Figure 5.2 Discharge capacities of commercial sintered nickel electrodes at different cycling
rates ....................................................................................................................... 144
Figure 5.3 Faradaic efficiencies of commercial sintered nickel electrodes at different cycling
rates when charged to its rated capacity ................................................................ 144
Figure 5.4 Faradaic efficiencies of commercial sintered nickel electrodes at different cycling
rates when charged to 80% of its rated capacity.................................................... 146
Figure 5.5 Cycle life testing of commercial sintered nickel electrodes at C/10 cycling rate
and charged to its rated capacity ............................................................................ 147
Figure 5.6 Nickel-foam used as electrode substrate for nickel hydroxide electrode ..... 149
Figure 5.7 Discharge capacity of foam based nickel electrode during C/5 formation cycling
when charged to its theoretical capacity right after fabrication ............................. 151
Figure 5.8 Faradaic efficiencies of foam based nickel electrodes at different cycling rates
............................................................................................................................... 152
Figure 5.9 Discharge capacities of foam-based nickel electrode with conducting carbon
additives during C/5 cycling when charged to its therotical capacity ................... 154
XV
Figure 5.10 Faradaic efficiencies of foam based nickel electrodes modified with conducting
carbon additives at different cycling rates ............................................................. 155
Figure 5.11 Charging curves of foam based nickel electrode with and without conducting
carbon additives at C/2 charging rate during faradaic efficiency measurements .. 156
Figure 5.12 Discharge capacities of foam based nickel electrodes modified with conducting
carbon additives at different cycling rates ............................................................. 157
Figure 5.13 Discharge capacities of foam based nickel electrode with different amounts of
binder during C/5 cycling when charged to its theoretical capacity right after
fabrication .............................................................................................................. 159
Figure 5.14 Charge and discharge capacity of foam-based nickel electrode with 5 w/w%
conducting carbon additive and 10 w/w% binder during cycle life testing at C-rate
charge and discharge to 100% depth-of-discharge ................................................ 160
Figure 6.1 Discharge capacities of iron electrode when cycled at C/10 charge and discharge
rate with 50% overcharge during formation cycling ............................................. 166
Figure 6.2 Charge and discharge capacity of 5Ah nickel-iron cell at various charge and
discharge rates ....................................................................................................... 172
Figure 6.3 Faradaic efficiencies of 5Ah nickel-iron cell at various charge and discharge
rates ....................................................................................................................... 173
Figure 6.4 First 10 cycles of in-cell formation cycling of 10Ah nickel-iron battery at C/5
charge rate and C/10 discharge rate with 30% overcharge .................................... 174
Figure 6.5 Formation cycling of 10Ah nickel-iron battery at C/2 charge rate and C/10
discharge rate with 30% overcharge following the charge and discharge cycling
presented in Figure 6.4 .......................................................................................... 175

XVI
List of Abbreviations
LCOE: Levelized Cost of Energy
MMO: Mercury-Mercury Oxide Electrode
NHE: Normal Hydrogen Electrode
C-Rate: Discharge current in Amperes equal to the numerical value of the capacity “C”
in Ah. The discharge rate is expressed as 1/n times the nominal capacity (C), where n is
the number of hours of discharge
SOC: State-of-Charge
DoD: Depth-of-Discharge

XVII
Abstract
Inexpensive, robust and efficient large-scale electrical energy storage systems are
vital to the utilization of electricity generated from solar and wind resources. In this
regard, the low-cost, robustness, and eco-friendliness of aqueous iron-based rechargeable
batteries are particularly attractive and compelling. Commercial nickel-iron batteries are
mainly limited by the low charging efficiency and poor discharge rate capability of the
iron electrode, and the low specific capacity at the cell level. Therefore, our study has
focused on demonstrating critical electrical performance and cycling properties to enable
the widespread use of nickel-iron batteries in stationary and distributed energy storage
applications.
In electrodes fabricated by a low-cost pressed-plate method, we have demonstrated
new chemical formulations of the rechargeable iron battery electrode to achieve a
ten-fold reduction in the hydrogen evolution rate, an unprecedented charging efficiency
of 96%, a high specific capacity of 0.3 Ah/g, and a capability of being discharged at the
3C rate. We show that modifying high-purity carbonyl iron by in situ electro-deposition
of bismuth leads to substantial inhibition of the kinetics of the hydrogen evolution
reaction. The in situ formation of conductive iron sulfides mitigates the passivation by
iron hydroxide thereby allowing high discharge rates and high specific capacity to be
simultaneously achieved. These major performance improvements are crucial to
XVIII
advancing the prospect of a sustainable large-scale energy storage solution based on
aqueous iron-based rechargeable batteries.
The formation process of the iron electrode could adversely impact the cost of
nickel-iron battery; we have focused our efforts on understanding the effect of electrode
design on formation. We have investigated the role of wetting agent, pore-former
additive, and sulfide additive on the formation of carbonyl iron electrodes. The wetting
agent increased the rate of formation while the pore-former additive increased the final
capacity. Sodium sulfide added to the electrolyte worked as a de-passivation agent and
increased the final discharge capacity. We have proposed a phenomenological model for
the formation process that predicts the rate of formation and final discharge capacity
given the design parameters for the electrode. The understanding gained here will be
useful in reducing the time lost in formation and in maximizing the utilization of the iron
electrode.
We have demonstrated iron electrodes containing iron (II) sulfide and bismuth oxide
additives that do not exhibit any noticeable capacity loss even after 1200 cycles at 100%
depth of discharge in each cycle. In iron electrodes prepared with bismuth sulfide
additive, capacity loss occurred during cycling, accompanied by a decrease in discharge
rate capability and rapid passivation. The recovery of capacity by adding sulfide ions to
the electrolyte confirmed that the electrode that suffered capacity fade did not have an
adequate supply of sulfide ions. We also found that the loss of cycleability was
XIX
accompanied by the steady accumulation of magnetite and loss of iron sulfides at the
iron electrode. The use of sparingly soluble iron (II) sulfide as an electrode additive
ensured a sustained and steady supply of sulfide preventing the accumulation of
magnetite during cycling. Thus, we have gained understanding of the critical role of
sulfide additives in achieving long cycle life in rechargeable alkaline iron electrodes.
We have demonstrated for the first time an advanced sintered iron electrode capable
of over 3500 charge and discharge cycles at the 1-hour rate and 100% depth of discharge,
with an average coulombic efficiency of over 97%. The high coulombic efficiency of 97%
can be achieved even without bismuth additives. Such an efficient and robust sintered
iron electrode is also capable of continuous discharge at quite high rates (even as high as
3C) without any noticeable degradation. We have shown that the porosity, pore size and
thickness of the electrode can be chosen systematically to optimize specific capacity and
robustness. By tuning the electrode design, the initial “formation” cycling process can be
reduced to less than 10 cycles. The sintered electrode structure combined with the
rational electrode design bestows extraordinary cycle life, rate capability, and robustness
to this alkaline iron electrode.
We have demonstrated a pasted nickel hydroxide electrode with nickel-foam
substrate and cobalt encapsulated nickel hydroxide active material. The faradaic
efficiency and rate capability were significantly improved compared to the commercial
sintered nickel electrode that we have tested. By using the nickel-foam substrate, we
XX
were able to achieve almost double the specific capacity at the electrode level compared
to the commercial sintered nickel electrode. The material utilization, faradaic efficiency
and rate capability of the pasted nickel electrode could be further improved by the
incorporation of conducting carbon additives into the electrode. The cycle life test of the
pasted nickel hydroxide electrode showed stable discharge capacities with high
efficiencies over 1400 deep-discharge cycles. This cycle life performance of the nickel
electrode is promising for developing a nickel-iron battery with the high-performance
iron electrode we have demonstrated.
We have discussed the effect of the limiting electrode and the formation process on
the performance of nickel-iron batteries. We have also developed a model to estimate the
specific capacity of the nickel iron cell based on the cell designs and electrode and cell
component properties. We have demonstrated the cycling performance of the nickel-iron
cell using the iron and nickel electrodes we have developed, and the effect of formation
procedures on the charge and discharge properties of the cell.
1
Chapter 1. Introduction
1.1 Background on Alkaline Nickel-Iron Batteries for Large Scale Energy Storage
Electrical energy storage systems will enable the seamless integration of the
electricity generated from wind turbines and solar photovoltaics into the electricity grid.
Rechargeable batteries are particularly good candidates for large-scale energy storage
because of their high round-trip efficiency, inherent scalability, and flexibility to being
located almost anywhere.
1-8
Vanadium–redox, lead-acid and lithium-ion rechargeable
batteries are under consideration for such application as these battery technologies are
readily available. However, these systems are currently limited by their materials cost,
lifetime, safety concerns, and the ability to meet the cost targets for the delivered
energy.
9, 10
The state-of-art commercially-available lithium-ion, lead-acid, nickel-metal hydride
and vanadium redox flow batteries may be rated by the levelized cost of energy
delivered (LCOE). LCOE is calculated as the ratio of the cost (capital and operating
costs) to the total amount of energy delivered by the battery over its useful lifetime. The
LCOE for commercially-available batteries is, at least, five to ten times higher than the
DoE target of $0.10 to $0.20/kWh.
9,10
Also, since energy storage will be required at the
scale of thousands of gigawatt-hours, we are faced with the challenge of providing a
sustainable solution, as the global material reserves are quite limited for the materials
2
used in state-of-art batteries.
8
Therefore, the development of inexpensive, efficient,
robust and sustainable battery systems for grid-scale energy storage is a topic of intense
research.
6,7,9,10
Rechargeable nickel-iron batteries have unique advantages that make
them particularly attractive for meeting the emerging demands of grid-scale electrical
energy storage systems.
8, 11-14
Under an effort funded by ARPA-E, we have been
focusing on developing such batteries.
8, 15-20

1.2 Overview of Alkaline Nickel-Iron Batteries
Invented by Thomas Edison in the USA and Waldemar Jungner in Sweden in the
early 1900s, nickel-iron batteries have been used in various stationary and mobile
applications for over 70 years in the USA and Europe until the 1980s when the
iron-based batteries were largely supplanted by sealed lead-acid batteries.
15
The
nickel-iron battery is based on the use of nickel oxyhydroxide (NiOOH) at the positive
electrode and iron at the negative electrode, with concentrated potassium hydroxide
solution as the electrolyte. The cell reaction when limited to the first discharge step is
given by Eq. 1 with the reaction proceeding to right-hand side during discharge and the
reverse reaction occurring during charge.
2NiOOH + 2Fe + 2H
2
O  2Ni (OH)
2
+ Fe (OH)
2
E
o
cell
= 1.37 V (1)
Iron, one of the primary raw materials for nickel-iron batteries, is globally abundant,
inexpensive, eco-friendly and recycled readily. Thus, iron is particularly attractive as a
3
raw material for batteries required on a very large scale.
8
The utilization of such low-cost
materials could bring down the capital cost of energy storage systems, therefore
decreasing LCOE. In addition, the nickel-iron battery is historically well known for its
extraordinary robustness to repeated cycles of charge and discharge. Over 3000
deep-discharge cycles have been reported on nickel-iron batteries.
21-24
Such robustness is
uncommon among rechargeable batteries; other types of batteries in use today degrade in
about 1000 cycles.
15
For example, lead-acid batteries typically offer 500-800 cycles and
lithium-ion batteries usually last no more than 1000 cycles especially under
deep-discharge conditions.
25
Such robustness of the nickel-iron battery could further
bring down LCOE because the capital cost of the energy storage system could be spread
over a longer lifetime of the battery.
The promise of iron-based batteries for large-scale energy storage has spurred
renewed interest in their development.
18, 11-14
During the 1970s and 1980s,
Westinghouse Corporation, Eagle-Picher Industries, and the Swedish National
Development Corporation have worked on developing iron-air and nickel-iron batteries
for electric vehicle applications.
21-23,26-29
Micka et al. have studied the effect of starting
materials, additives and impurities on iron electrodes.
30, 31
Between 1980 and 2000 and
then again more recently after 2012, Shukla et al. have reported extensive studies on the
mechanism and performance of iron electrode for nickel-iron and iron-air batteries.
12,
4
32-39, 44
Periasamy et al. have reported studies on improving the iron electrode for
nickel-iron batteries.
40-43

While the nickel-iron battery needed improvements to overcome the limitations
from the iron electrode, the advancements in alkaline nickel electrode has spurred the
development and widespread utilization of nickel-cadmium and nickel-metal hydride
batteries. Milner and Thomas have presented a detailed survey of the nickel hydroxide
electrode.
45
Barnard and Randell have studied the reaction mechanisms and various
factors affecting the nickel hydroxide electrode.
46-53
Conway et al have studied the nickel
hydroxide electrode and the oxygen evolution reaction extensively.
54-60
Zimmerman has
reported performance improvements and detailed understanding on the sintered nickel
electrode for nickel-cadmium and nickel-metal hydride batteries.
61-66

1.3 Challenges in Developing High-Performance Alkaline Nickel-Iron Batteries
Despite the use of low-cost materials and long cycle life of the alkaline nickel-iron
battery, low efficiency, poor rate-capability of the iron electrode, and heavy and
bulky cell structures limit the ability of commercial nickel-iron batteries to be
competitive in large scale energy storage applications. In this thesis, I will focus on
understanding the factors that affect the charge and discharge performance of alkaline
iron and nickel electrodes, approaches to improving these electrode properties, and
exploring the design parameters at the electrode and cell level, in an effort to help
5
improve the overall performance of the nickel-iron battery to be able to reach the LCOE
targets of large scale energy storage applications.
1.3.1 Low Charging Efficiency of the Iron Electrode
The electrochemistry of the iron electrode in alkaline batteries is based on the redox
process involving the inter-conversion of iron (II) hydroxide and elemental iron:
Fe(OH)
2
+2e
-
 Fe +2OH
-
E
o
= -0.877 V (2)
The forward reaction occurs during charging of the electrode and the reverse reaction
occurs during discharge. The principal limitation of the iron electrode is its low
charging-efficiency that is in the range of 55-70%.
13-16
This limitation arises from the
hydrogen evolution reaction that occurs during charging according to the following
reaction.
2H
2
O +2e
-
→ H
2
+2OH
-
E
o
= -0.828 V (3)
The hydrogen evolution reaction occurs during charging of the iron electrode because
the electrode potential for this reaction is positive to that of the iron electrode charging
reaction (Equation 2). Consequently, the batteries will have to be overcharged by
60-100% to achieve full capacity. The hydrogen evolution that occurs during charging is
undesirable because it lowers the round-trip energy efficiency and results in loss of water
from the electrolyte. Thus, suppressing hydrogen evolution at the iron electrode has
far-reaching benefits of raising the overall energy efficiency, lowering the cost, and
6
increasing the ease of implementation of iron-based batteries in large-scale energy
storage systems. However, suppressing hydrogen evolution and achieving an iron
electrode with a charging-efficiency close to 100%, without interfering with the other
performance features of the electrode, has been a formidable challenge for many years.
In this thesis, I will present the various methods we have used to achieve a charging
efficiency on the iron electrode as high as 96%. I will also present understanding on the
mechanisms underlying the achievement of high charging efficiency through the use of
additives and alternate fabrication techniques.
1.3.2 Poor Discharge Rate Capability of the Iron Electrode
Batteries for large-scale energy storage must also be capable of delivering their
entire capacity quite rapidly, for example, in just one hour. Some of the grid-scale
ancillary services such as frequency regulation require that the battery to be capable of
even higher rates of discharge for short periods.
10
Present-day commercial iron
electrodes cannot be discharged in less than five hours if an electrode utilization of 0.2
Ah/g or greater is desired. During discharge of the iron electrode, the iron active material
is converted to iron (II) hydroxide that is electrically insulating.
30,34,67,68
Consequently,
at high rates of discharge, the formation of an insulating layer of iron hydroxide results
in a sharp reduction in the cell voltage.
24, 32
This inability to discharge at high rates due
to the formation of an insulating layer of iron hydroxide is termed “passivation”. Various
researchers have noted that sulfides mitigate electrode passivation and help sustain high
7
discharge rates at iron electrodes. Micka and Zabransky reported an appreciable increase
in the discharge capacity for sulfide-modified electrodes.
30
Shukla et al described the
beneficial role of sulfide additives in increasing the electronic conductivity of the
electrodes.
32, 34, 67
Caldas et al. reported that the sulfide ions were incorporated in the
passive layer resulting in an increase in ionic conductivity of the passive film.
69
Berger
and Haschka found that sulfides increased the solubility of the discharge products of the
iron electrode and prevented rapid passivation.
70
While sulfide appears to be a useful
additive to improve the discharge performance of the iron electrode, the relationship
between electrode passivation, discharge rate capability, and the mechanism of
de-passivation by sulfides has not been adequately understood.
In this thesis, I will present the effect of using sulfide additives with carbonyl iron
electrodes to improve its discharge rate capability and reach a discharge capacity of 0.2
Ah/g at a discharge rate as high as 3C. The relationship between discharge rate capability
and passivation, the mechanism of de-passivation, and the effect of sulfide in improving
discharge rate capability over long periods of charge and discharge cycling are also
discussed.
1.3.3 Understanding the Formation Process of the Iron Electrode
A newly-prepared iron electrode in a nickel-iron battery undergoes “formation”
during which the electrode is charged and discharged repeatedly for 20 to 50 cycles after
which a stable discharge capacity is reached.
24, 35, 69-72
The formation of iron electrodes
8
is a critical step in manufacturing before the cells are ready for use. Such a formation
step is also encountered with the more the commonly-known battery types including
lithium-ion, lead-acid, and nickel-metal hydride
73-76
, although the chemical processes
specific to the formation process vary with the battery type.
The electrode formation step in battery manufacturing has at least two significant
implications: (1) The stable discharge capacity attained at the end of formation
determines the extent of utilization of the active material in the electrodes, (2) The
increased manufacturing time and capital cost of equipment for formation will impact
the final cost of the battery. Therefore, understanding the various factors that influence
the formation process could help us reduce the cost of iron-based batteries. While there
have been extensive studies of the poor charging efficiency and discharge rate capability
37, 71,79-81
of iron electrodes, the mechanism of formation, and the factors that affect the
rate of formation and the capacity at the end of formation are not completely understood.
Vijayamohanan et al. have investigated the formation of iron electrodes prepared by
the chemical reduction of ferrous oxalate.
35
The authors attributed the increase in
capacity during formation to changes in the morphology and conductivity of the iron
electrode. In addition, they have also reported the beneficial effect of the addition of iron
(II) sulfide to the electrode active material.
The negative electrode of a lead-acid battery also undergoes formation to produce a
lead matrix and fine lead crystals that ensure high electrode conductivity, utilization and
9
rechargeablility.
77, 78
The lead electrode typically forms in a single cycle, while multiple
cycles of charge and discharge are required for the formation of the carbonyl-iron-based
electrode. Also, prior to formation, the negative electrode of the lead-acid battery
consists of a mixture of materials largely in the oxidized state (lead oxide and lead
sulfate) that is transformed into metallic lead in a single step during formation. However,
the carbonyl iron electrode starts with materials in the reduced state (metallic iron) that
achieve progressively larger active area in each cycle during formation. For these
reasons, it is not viable to directly apply the models used for describing the formation of
the lead-acid battery electrode to the iron electrode.
In this thesis, I will present the influence of electrode design on the rate of formation
and the final utilization of high-performance rechargeable electrodes based on carbonyl
iron for both “pressed-plate” and “sintered” type electrodes. I will also offer a simple
phenomenological model that provides insight into the factors affecting the rate of
formation and the stable capacity achieved at the end of formation.
1.3.4 Understanding the Factors Affecting the Active Material Utilization of the Iron
Electrode
Due to the large difference in molar volume between the charged and discharged
phases (iron and iron hydroxide), adequate porosity is essential to realizing high specific
discharge capacity in the iron electrode. With the molar volume of iron and iron
hydroxide being 7.09 cm
3
/mole and 26.47 cm
3
/mole, respectively, there is about 300%
10
increase in volume when iron is converted to iron hydroxide. Therefore, electrode
porosity is needed to accommodate this significant volume change. If all the iron active
material in an electrode were completely converted to iron hydroxide, a porosity of 73%
is required and the theoretical discharge capacity of iron of 0.962 Ah/g could be
achieved.
28
However, such a high level of utilization is not realistic as there would not be
any electrically conducting phase to connect up the particles of iron (II) hydroxide. Thus,
a mixture of iron and iron (II) hydroxide is needed to have a functional electrode. While
sintering conditions have been shown to affect the porosity and discharge capacity of
iron electrodes, the effect of pore size on the utilization is still unclear.
27, 28
In this thesis, I will present the relationship between porosity and pore size on the
utilization of the iron active material in a systematic manner to provide a rational design
approach to maximize specific discharge capacity.
1.3.5 Understanding the Factors Affecting the Cycle Life of the Iron Electrode
To achieve the targeted LCOE for large-scale energy storage, the batteries must
have a long cycle life.
6, 9, 82
For example, at a battery cost of $100/kWh, to realize an
LCOE of $0.025/kWh, about 5000 deep cycles of charge and discharge without
significant loss in capacity will be required.
9
Lead-acid batteries cannot be expected to
support more than 500 cycles when subjected to cycling to 100% depth-of-discharge and
operated at slightly elevated temperatures of 50
o
C.
83-85
Similarly, lithium-ion
batteries can offer about 1000-2000 deep- discharge cycles with a strict control of
11
temperature during cycling. Consequently, the LCOE for lead-acid batteries is about
$0.38/kWh, at a capital cost of $150/kWh and a round-trip efficiency of 80%. The
alkaline iron electrode has been reported to be very robust to cycling, overcharge,
overdischarge and elevated temperature operation.
86
But, systematic experimental data
on the cycling stability of iron-based battery systems in the scientific literature is scarce.
Eagle-Picher Industries, Inc. has reported cycling studies on nickel-iron cells and found
no capacity fade for more than 1000 cycles of operation in some cell configurations.
29

The nickel-iron cells tested in most studies were designed to be limited by the positive
electrode and possible causes for the loss of performance of the iron electrode with
cycling are not available.
While anecdotal reports of the robustness of the iron electrode are consistent with
the insoluble nature of iron (II) hydroxide, changes in the electrode properties over
thousands of cycles of charge and discharge are not unreasonable to expect. Such
changes could result from the depth of discharge, the rate of charge and discharge, and
the concentration of sulfide additives.
Therefore, in this thesis, I will focus on understanding the changes in the iron
electrode during extended life testing and the role of sulfides in preserving the properties
of the iron electrode during extended cycling at 100% depth of discharge.
12
1.3.6 Developing High-Performance Nickel Hydroxide Electrode for the Nickel-Iron
Battery
While the use of the nickel electrode in rechargeable batteries has been known for
almost a 100 years, substantial improvement in materials and fabrication techniques have
occurred over the years. The state-of-art nickel electrodes exhibit excellent cycle life,
rate capability and efficiency characteristics. In this thesis I will describe our efforts to
make improvements to the state-of-art nickel electrode by altering the fabrication
methods and the formulation of the active material mix used in the preparation of the
electrodes. These improvments assist the development of nickel electrodes to attain
comparable charge and discharge performances and specific capacities to the
high-performance iron electrode that we have developed, ensuring good overall
performance of the nickel-iron battery.

13
Chapter 2. Experimental Approaches
Pressed-plate Iron Electrode Preparation
Pressed-plate iron electrodes consisted of carbonyl iron powder (80 wt %, BASF
SM grade), polyethylene powder (10 wt. %, MIPELON, Mitsui Chem USA) and
potassium carbonate (10 wt. %). The polyethylene powder and potassium carbonate
served as the binder and pore former, respectively. In some of the carbonyl iron
electrodes studied, bismuth sulfide, bismuth oxide or iron (II) sulfide were added to the
extent of 5-10 wt % of the powder mixture. The iron electrodes were fabricated by
hot-pressing the mixture on to a nickel grid at 140
o
C. The electrodes were about 5 cm x
5 cm and 0.5-1 mm thick and with a typical mass of 3-5 g. (Figure 2.1)

Figure 2.1 Pressed-plate electrodes constructed from carbonyl iron powder
5 cm
(a)
(b)
14
Sintered Iron Electrode Preparation
Sintered iron electrodes samples were prepared by sintering carbonyl iron powder
(BASF SM grade) onto a degreased nickel mesh. Ammonium bicarbonate (10-100% of
the mass of iron powder, ReagentPlus®, ≥99.0%) was added to the electrode mix to
increase porosity. The electrodes were sintered in a quartz tube furnace under argon
atmosphere at 850° C for 15 minutes. The electrodes typically consisted of about 2 grams
of carbonyl iron powder (Figure 2.2) and the discharge capacity was in the range of 0.2
to1 Ah depending on the formulation and the conditions of preparation. In some of the
electrodes studied, 5 wt. % (of the iron active material) of iron sulfide is mixed with the
iron powder before sintering.

Figure 2.2 Sintered iron electrodes made from carbonyl iron powder
15
Nickel Electrode Preparation
Nickel electrodes were prepared by a pasting method. Ethyl cellulose binder
(ETHOCEL, Dow Chemical Company, Standard 45, 5-15 wt. %) is first dissolved in
isopropanol and the solution is sonicated until homogeneous. Nickel hydroxide powder
(BASF, AP87 grade) is then mixed with the binder solution to make an active material
paste. The paste is then spread on to a piece of nickel-foam with a size of 5 cm x 5 cm x
1.5 mm with a spatula. The pasted electrode was dried in an oven at 85 ℃ overnight.
(Figure 2.3) In some of the electrodes been tested, 5 wt. % of carbon nano-tubes (Cnano,
Flo tubes 9000) was added to the paste.

Figure 2.3 Pasted nickel electrode on nickel-foam substrate with nickel hydroxide
16

Porosity Measurements of Iron Electrodes
Porosity measurements were made by first immersing the iron electrode in
isopropanol. After immersion for 5 minutes, the electrode was removed from the
isopropanol bath. The excess isopropanol on the electrode was quickly removed using
a paper towel, and the electrode was weighed. The differences in the electrode mass
before and after immersing in isopropanol bath indicated the amount of isopropanol
retained in the pores of the electrode. The pore volume was calculated from the mass
uptake and the density of isopropanol. The porosity was calculated from the ratio of the
volume of pores to the geometric volume of the electrode.
Test Cell Construction
The electrodes were tested in a three - electrode cell. (Figure 2.4) All potentials were
measured against a mercury/mercuric oxide (MMO) reference electrode (E
MMO
o
= +0.098
V vs. the normal hydrogen electrode). For iron electrode tests, a nickel oxide battery
electrode of the sintered type was used as the counter electrode, and a solution of
potassium hydroxide (30 w/v %), similar that used in iron-based rechargeable batteries,
was used as the electrolyte. For the nickel electrode tests, blank nickel mesh was used as
the counter electrode, and a solution of potassium hydroxide (30 w/v %) with lithium
hydroxide (1 w/v %) was used as the electrolyte.
17

Figure 2.4 Test cell configuration showing working electrode, reference electrode,
counter electrodes and electrolyte
Formation of Electrodes
The charge-discharge experiments were performed using a 16-channel battery
cycling system (MACCOR- 4200). For iron electrode formation, the electrodes were
charged at 200 mA for 2 hours and then discharged at 20 mA to a potential of -0.75 V vs.
MMO. This protocol for charge/discharge cycling was used for the first 10-20 cycles,
and the charging time was increased to 4 hours in the following formations cycles with
all the other parameters unchanged. For nickel electrode formation, the electrodes were
charged at 120 mA for 5 hours and then discharged at 120 mA to a potential of 0 V vs
MMO. Nickel electrode formation usually takes less than 10 cycles.

18
Charging Efficiency Measurements
Charging efficiency was measured on electrodes after completion of formation. The
discharge capacity at the end of formation was used to determine the charge and
discharge rates. The discharge capacity divided by the charge capacity is the charging
efficiency, and this value has been reported here as a percentage value. For iron
electrodes, the charging efficiency is measured by charging the electrode at C/2 rate for 2
hours, and discharging the electrode at C/20 rate to the cut-off potential of -0.75 V vs
MMO. The C-Rate is an expression of discharge current in Amperes where the discharge
rate is expressed as 1/n times the nominal capacity (C) in Ampere-hours, where n is the
number of hours of discharge
For nickel electrodes, the charging efficiency is measured by charging the electrode
at C/5 rate for 5 hours, and discharging the electrode at C/5 rate to the cut-off potential
of 0 V vs MMO.
Rate Capability Measurements
Charge and discharge rate capability measurements were also performed on “formed”
electrodes. For the iron electrodes, to measure discharge rate capabilities, the electrode is
charged at C/2 rate for 2 hours, and discharged at rates ranging from C/20 to 3C to -0.75
V vs MMO with correction for the voltage drop across the series equivalent resistance
(or IR drop). Similarly, to measure the charge rate capability, the electrodes were
19
charged at various rates from C/5 to 4C to the capacity value attained at the end of
formation, and discharged at C/5 rate to -0.75 V vs MMO. To measure the rate capability
of nickel electrodes, the electrodes were charged at various rates from C/10 to C to the
capacity value attained at the end of formation, and discharged at various rates from
C/10 to C to 0 V vs MMO.
Polarization Studies
Anodic polarization experiments were performed on fully-charged iron electrodes
using a PAR-VMC-4 potentiostat/galvanostat. The polarization curves were measured by
conducting a potentiodynamic scan from the open-circuit potential to -0.75 V (vs MMO)
at a scan rate of 0.167 mV/s.
Cycling Studies
After measuring the charging efficiency and discharge rate capability, the electrodes
were subjected to extended cycling tests. The depth-of-discharge during cycling was 100%
unless specified otherwise. For pressed-plate iron electrodes, the electrodes were charged
to its rated capacity at C/2 rate and discharged at C/5 rate. A cut-off potential of –0.75 V
(vs. MMO) was used to terminate the discharge. For the sintered iron electrodes, the
cycling experiments were carried out at C/2 or 1C charge and discharge rates with
charging to the formed capacity and discharging to the cut-off potential of -0.75 V vs
MMO (with correction for IR drop). For the nickel electrodes, the electrodes were cycled
20
at 1C charge and discharge rates with charging to the formed capacity and discharging to
the cut-off potential of 0 V vs MMO.
SEM Characterization
A JSM-7001 scanning electron microscope (SEM) was used for physical and
morphological characterization. “As-prepared” iron electrodes samples were used
directly for the SEM characterization. “Cycled” iron electrode samples were washed first
with de-ionized water to remove any entrained potassium hydroxide; the samples were
then immersed in isopropanol to remove the excess moisture and to prevent oxidation,
following which the samples were then dried in a vacuum chamber for 4-5 hours at room
temperature, before introducing into the microscope.
XRD Characterization
The phase composition of materials in the electrodes following cycling was
investigated by X-Ray Diffraction (XRD) measurements using a Rigaku Ultima IV
diffractometer (Cu K  source). The electrode samples were washed first with de-ionized
water to remove any entrained potassium hydroxide; the samples were then immersed in
isopropanol to remove the excess moisture and to prevent oxidation, following which the
samples were dried in a vacuum chamber for 4-5 hours at room temperature. The sample
was then ground with a mortar and pestle to break down any large particles before
introducing into the X-Ray powder diffractometer.
21
Chapter 3. Pressed-Plate Iron Electrodes
3.1 Pressed-Plate Fabrication Method
The manufacturing cost is a very important consideration for meeting the
challenging cost goals for large-scale energy storage as envisioned by the U. S.
Department of Energy -ARPA-E. Consequently, we have focused on a low-cost
approach to preparing electrodes. Traditional iron based electrode employs a pocket
plate method which uses pockets made of perforated steel plates as the current collector
filled with reduced iron powder. The resulting structure is heavy and bulky, and much
more active material is used to maintain a good cycle life. Among the various electrode
manufacturing techniques, including pocket plate, pressed-plate, sintered plate and fiber
plate, the pressed-plate method is relatively simple, easily scaled and more cost effective.
Therefore, we have prepared pressed-plate type electrodes by combining the iron active
material with a polyethylene binder material followed by the application of heat and
pressure (Figure 1). Such electrodes are inexpensive to fabricate, electrode additives
could be easily incorporated, and the size of the electrode produced could be readily
scaled with fairly good uniformity of electrodes. Therefore, we have chosen
pressed-plate as our primary electrode fabrication technique in our studies.
22
3.2 Improvements in Charging Efficiency
When the iron electrode is cathodically polarized during charging, the primary
electrochemical reaction that happens on the iron electrode is the reduction of iron
hydroxide to iron. However, because of the more positive potential of the hydrogen
evolution reaction, hydrogen gas will evolve on the iron electrode during charging and
part of the charging current will be diverted to this parasitic hydrogen evolution reaction,
resulting in a low charging efficiency for the iron electrode. The charging efficiency of
the commercial iron electrode is usually in the range of 55-70%.
The low charging efficiency of iron electrodes mainly affects iron-based alkaline
batteries in two respects: Firstly, the evolution of hydrogen causes loss of water in the
electrolyte, therefore, most commercial nickel iron batteries require frequent
maintenance (adding distilled water to the cell). Secondly, the low charging efficiency of
iron electrode requires excess charge to fully charge the battery, thus adversely
impacting the levelized cost of energy when using alkaline iron batteries are used in
large scale energy storage applications.
Therefore, in our effort to develop alkaline iron batteries for large-scale energy
storage, high charging efficiency on the iron electrode is one of the key improvements
that we were trying to achieve.
23
3.2.1 Effect of Carbonyl Iron as Active Material and Bismuth Sulfide as Additive on
Improving Charging Efficiency
Prior to measuring the charging efficiency, the pressed-plate iron electrodes were
charged and discharged about 30-40 times during which the discharge capacity increased
to a stable value. The process of attaining a stable discharge capacity, termed
“formation”, has been recognized previously for iron electrodes.
34, 35
We have found that
at the end of formation, the electrodes show a lower hydrogen evolution rate compared
to the beginning of formation, a result that has not been reported for commercial
electrodes. The formation process involves the repeated conversion of iron to iron (II)
hydroxide followed by re-deposition as iron. This process could be expected to purify
the carbonyl iron electrode further by the removal of any soluble impurities. All charging
efficiency measurements were thus performed on such “formed” electrodes. The voltage
profiles during charge and discharge (Figure 3.1) show that the charge input is almost
completely recovered during discharge.
Specifically, the charging efficiency of the carbonyl iron electrode was found to be
90 ± 1%. The electrodes formulated with carbonyl iron and bismuth sulfide showed an
even higher charge efficiency of 96 ± 1% (Figure 3.2).

24

Figure 3.1 Typical charge and discharge voltage profiles for carbonyl iron electrodes
with and without bismuth sulfide additive

Figure 3.2 Charging efficiency at C/2 rate for three types of electrode compositions
This high value of charge efficiency for the carbonyl iron electrode with bismuth
sulfide represents a ten-fold decrease in the amount of hydrogen evolved during charging
25
(Figure 3.3).
24, 32, 47
Repeated cycling of these electrodes did not show any decline of
this high value of charging efficiency (Figure 3.4).

Figure 3.3 Relative rates of hydrogen evolution of various electrodes when charged at
C/2 rate

Figure 3.4 Charging efficiency as a function of cycling at C/2 rate of charge and C/20
rate of discharge. The band refers to the charging efficiency of state-of-art commercial
electrodes from nickel-iron batteries (Sichuan Changhong Battery Co., Ltd., Sichuan)
40
60
80
100
0 10 20 30
Charging Efficiency %
Number of Cycles
Carbonyl Iron+ bismuth sulfide
Carbonyl Iron
commercial
electrodes
55-70%
26

Figure 3.5 Hydrogen overpotential of various iron electrode materials during charging at
C*/10 rate, where C* is the theoretical capacity based on the mass of the electrode
material
The increase in charging efficiency found with the high-purity carbonyl iron
electrode is attributed to the high overpotential for hydrogen evolution on carbonyl iron.
Of the various iron electrode materials that were tested, the ones made from carbonyl
iron have the highest overpotential for hydrogen evolution reaction (Figure 3.5).
Carbonyl iron powder consists of spherical iron particles (3-5 micron diameter)
produced by the decomposition of iron pentacarbonyl. (Figure 3.6) This material was
post-treated in hydrogen at 300
º
C to remove any residual oxygen and carbon. Upon such
heat treatment the unique “onion” structure of carbonyl iron is erased and the
microstructure becomes homogeneous. Carbonyl iron powder is one of the purest forms
-0.35
-0.33
-0.31
-0.29
-0.27
-0.25
Carbonyl Iron Carbonyl Iron +
Bismuth Sulfide
Commercial
Electrode
(magnetite)
Reduced Iron
Oxide
Overpotential (V)
27
of iron available commercially, it does not contain the common impurities such as
manganese, sulfur and phosphorus that are present in the reduced oxides. In general,
these impurities decrease the hydrogen overpotential and facilitate hydrogen evolution
by increasing the ease of formation of adsorbed hydrogen species on the surface of
iron.
93
While the importance of purity of the iron materials has been emphasized in
previous research, such high values of charging efficiency of >90% have not been
reported in the literature.
94

Figure 3.6 Morphology of carbonyl iron powder in the electrode
A further decrease in the rate of hydrogen evolution has been achieved by the
addition of bismuth sulfide to the carbonyl iron material. Bismuth sulfide is an
electrically conducting solid, insoluble in the potassium hydroxide electrolyte. During
charging, the bismuth sulfide is transformed into elemental bismuth (Eq. 4).
28
Bi
2
S
3
+ 6e
-
 2Bi + 3S
2-
E
0

= -0.818V (4)
The electrode potential for the reduction of bismuth sulfide to bismuth is more
positive than that of the iron electrode reaction (Eq.2) and thus the charging process
conducted at -1 V (vs NHE) facilitates the formation of elemental bismuth.
95
The
presence of elemental bismuth in the charged electrodes was confirmed by X-ray powder
diffraction (XRD) studies (Figure 3.7).

Figure 3.7 X-ray Diffractogram for a charged iron electrode prepared from carbonyl iron
electrode and bismuth sulfide. Powder Diffraction Files: Fe (00-006-0696), Fe(OH)
2

(00-013-0089), Fe
3
O
4
(00-071-6766), Bi (00-044-1246), Fe
3
S
4
(01-089-1998),FeS
(01-076-0964)
It is the presence of elemental bismuth that increases the overpotential for hydrogen
evolution on carbonyl iron (Figure 3.5). The high hydrogen overpotential on bismuth is
29
due to the unfavorable energetics for the electro-sorption of surface-bonded hydrogen
intermediates.
96

3.2.2 Effects of Bismuth Oxide Additives on Improving Charging Efficiency
Since our goal is to attain a charging efficiency close to 100%, we aimed for further
improvement of the carbonyl iron electrode by using additives that selectively inhibited
hydrogen evolution. Elements such as lead, cadmium, mercury and bismuth are known
to exhibit the highest overpotentials for the hydrogen evolution reaction. Consequently,
the addition of these elements has been considered to reduce hydrogen evolution rates.
93,
96
Among the inorganic additives, bismuth is relatively non-toxic and eco-friendly.
Besides the use of bismuth sulfide as an electrode additive to increase charging
efficiency, with 5% of bismuth oxide as an additive, the charging efficiency of the
carbonyl iron electrode was about 90% (Figure 3.8).
30

Figure 3.8 Charging efficiency of a commercial iron electrode and carbonyl iron
electrodes with bismuth oxide as an electrode additive
When the concentration of bismuth oxide in the iron electrode was raised to 10%,
the charging efficiency improved to 92%. This value of charging efficiency was
considerably higher than that of the commercial electrode at 70%. It was clear that the
presence of bismuth oxide further improved the efficiency of the carbonyl iron electrode
by suppressing the hydrogen evolution reaction. In addition, this high value of charging
efficiency of the bismuth-oxide-modified iron electrode was found to be stable over at
least 20 cycles of repeated charge and discharge (Figure 3.9) suggesting the practical
viability of using bismuth oxide for improving charging efficiency of the iron electrode.
31

Figure 3.9 Variation of charging efficiency of a carbonyl iron electrode containing
bismuth oxide and iron sulfide with repeated cycling. Note that cycles 9 to 11 were
dedicated to rate-capability measurements and hence charging efficiency data was not
collected
3.2.3 Studies on Electro-Reduction of Bismuth Oxide to Bismuth
To investigate the changes that the bismuth oxide additive underwent during
charging of the iron electrode, we studied the electro-reduction of bismuth oxide without
any iron active material. In these electrodes, bismuth oxide was combined with a
polyethylene binder and hot pressed onto a nickel grid. In another formulation, 12 w/w %
of acetylene black was mixed with the bismuth oxide and the binder before hot pressing.
These electrodes were polarized cathodically at a constant current of 500 mA (0.33A-g
-1
)
in the battery electrolyte. The potential-charge curves showed a well-defined plateau
corresponding to the reduction of the bismuth oxide to elemental bismuth (Figure 3.10).
After the reduction of bismuth oxide was complete, hydrogen evolution was the only
32
reaction that took place, as indicated by the inflection in the potential-charge curve
(Figure 3.10). The total charge input in the plateau region corresponded to the reduction
of bismuth oxide to elemental bismuth as shown in Eq. 5.
Bi
2
O
3
+3H
2
O+ 6e
-
 2Bi + 6OH
-
E
0

= 0.460V (5)

Figure 3.10 Galvanostatic electro-reduction of bismuth oxide electrode in 30%
potassium hydroxide electrolyte
When acetylene black was present in this electrode, the plateau potential was
significantly closer to the electrode potential predicted from the Nernst equation
(corrected for 5.36 M potassium hydroxide). The difference between the plateau
potentials for the electro-reduction of bismuth oxide, with and without acetylene black,
was about 500 mV. This higher overpotential for the reduction of bismuth oxide in the
absence of acetylene black was due to the poor electronic conductivity of bismuth oxide.
The ohmic resistance of the bismuth oxide electrode measured at 10 kHz (2 mV
33
peak-to-peak AC signal) was 0.66 Ohm. With the addition of acetylene black, an
electrically conductive carbon, the high-frequency resistance of the electrode reduced to
0.27 Ohm and the plateau potential approached the reversible potential for the bismuth
oxide/bismuth couple (Eq. 5). XRD investigation of these electrodes confirmed the
complete reduction of bismuth oxide to elemental bismuth (Figure 3.11).

Figure 3.11 XRD pattern for the bismuth oxide electrode after reduction
When bismuth oxide was present as an additive in the iron electrode, the
high-frequency resistance of this electrode was similar to that of the electrode with just
bismuth oxide and acetylene black because the iron particles provided a conductive
matrix. Since the standard reduction potential for the bismuth oxide/bismuth couple (Eq.
5) is more positive to that for the reduction of iron (II) hydroxide to iron (Eq. 2), the
bismuth oxide was expected to undergo electro-reduction to elemental bismuth (Eq. 5)
when an iron electrode was charged. The XRD measurements on iron electrodes
34
modified with bismuth oxide that had been subjected to charging, confirmed the
presence of elemental bismuth (Figures 3.12 a, b).

Figure 3.12 XRD pattern for the carbonyl iron electrode containing different amounts of
bismuth oxide (a) 5 w/w% and (b) 10 w/w%
a)
b)
35
3.2.4 Corrosion Studies of Iron Particles
In addition to performing the charging efficiency measurements on the iron
electrode, we also examined the surface morphology of carbonyl iron powder exposed to
a 30 w/v % solution of potassium hydroxide in the presence of various additives. The
scanning electron micrographs of the iron particles obtained after 30 days of exposure to
the electrolyte are shown in Figures 3.13 a - f.
When carbonyl iron was exposed to potassium hydroxide electrolyte, the smooth
particles of iron became rough and covered with iron hydroxide (Figure 3.13 a, b).
Further, we noticed the generation of a considerable number of hydrogen bubbles that
corresponded to the corrosion reaction (Eq. 6).
Fe +2H
2
O Fe (OH)
2
+ H
2
(6)
Similar corrosion was also noticed when carbonyl iron was mixed with iron sulfide.
Iron sulfide is known to prevent the passivation of iron. As a result, it was not surprising
that the corrosion of iron to iron (II) hydroxide was accelerated by the presence of iron
sulfide (Figure 3.13 c, d).
In the presence of bismuth oxide however, the surface of the iron particles appeared
to be smooth and did not seem to have suffered any corrosion by the alkaline medium
(Figure 3.13 e, f). Also, no hydrogen bubbles were observed during the exposure period.
36
Therefore, it was clear that the corrosion of iron was substantially inhibited by the
deposition of bismuth according to Eq. 7.
Bi
2
O
3
+3H
2
O + 3Fe  2Bi +3 Fe (OH)
2
(7)
Also, once the bismuth was deposited, the formation of iron hydroxide ceased to
occur. A similar experiment was also conducted with bismuth sulfide, and we did not
observe any hydrogen evolution and the morphology of the carbonyl iron particles was
similar to the case with bismuth oxide. Thus, we were able to confirm directly, the role
of the elemental bismuth in preventing hydrogen evolution.

37

Figure 3.13 SEM micrographs of iron particles with different additives in 30% potassium
hydroxide electrolyte. (a,b) carbonyl iron, (c,d) carbonyl iron + iron sulfide, (e, f)
carbonyl iron + bismuth oxide
38
3.2.5 Studies of Kinetic Parameters for Hydrogen Evolution Reaction on Iron Electrodes
To measure the effect of bismuth on the rate of hydrogen evolution, we determined
the kinetic parameters (exchange current and Tafel Slope) for the hydrogen evolution
reaction on the carbonyl iron electrodes with the bismuth additives. Steady-state
polarization data (Figure 3.14 a and b) was obtained on the iron electrode in the
fully-charged state where the only reaction occurring during cathodic polarization was
hydrogen evolution. The kinetic parameters obtained by fitting the steady-state
polarization data to the Tafel equation (Eq. 8) are summarized in Table 1.
Log
10
(I
H2
/I
o
) = (E-E
H
r
)/b (8)
where I
o
and b are the exchange current and the Tafel slope, respectively. E
H
r
is the
reversible electrode potential for the hydrogen evolution reaction in the battery
electrolyte. I
H2
is the current associated with hydrogen evolution and the E is the
electrode potential during cathodic polarization. The apparent exchange current was
determined from the intercept of the Tafel line at zero overpotential. For comparing
various electrodes, the exchange current was normalized to the discharge capacity of the
electrodes, as the discharge capacity is proportional to the electrochemically-active area
of the electrode.

39

Figure 3.14 Cathodic Tafel polarization plots for fully-charged iron electrodes of various
compositions. Parameters of the Tafel Equation are also shown
a)
b)
40
Table 1 Kinetic parameters for hydrogen evolution reactions on various iron electrodes.
Charging efficiencies calculated based on Eq.9. Exchange current was normalized for
discharge capacity at twenty-hour rate

The normalized exchange current decreased by a factor of six in the presence of 5%
bismuth oxide and decreased even further when the concentration of bismuth oxide in
the electrode was 10%. However, the normalized exchange current for the electrodes
with the bismuth oxide additive was about 60% higher than that of the electrode with
bismuth sulfide. Such differences in normalized exchange current could arise from the
differences in the morphology of bismuth formed by electrodeposition from bismuth
oxide and bismuth sulfide particles that will affect the electrochemically-active area over
which the bismuth is distributed. For example, the differences in the initial particle size
of the additive could give rise to a different final distribution and morphology of bismuth.
The slightly lower values of charging efficiency observed with bismuth oxide compared
to bismuth sulfide were consistent with the higher normalized exchange current for
hydrogen evolution observed on the bismuth oxide electrodes.
41
The observation of bismuth as a separate phase even at a low fraction of 5% is
consistent with the insolubility of bismuth in iron predicted by the Hume-Rothery
rules.
97
This insolubility combined with the low surface energies of bismuth makes the
re-distribution of bismuth into the iron matrix highly unfavorable.
98
Consequently, the
bismuth can be expected to remain on the surface of iron as nano-crystals or “ad-atoms”
suppressing hydrogen evolution during charging. The bismuth present on the iron
electrode does not undergo oxidation during the discharge process because the necessary
electrode potential for electro-oxidation is not reached. In the event of over-discharge
of the iron electrode, the bismuth will be oxidized to insoluble bismuth oxide. This
bismuth oxide will be readily reduced to the elemental bismuth during the subsequent
charge cycle.
99
These characteristics of the bismuth deposits are consistent with the
stable charging efficiency values observed in repeated cycles of charge and discharge
(Figure 3.4, Figure 3.9).
We also found that the addition of iron sulfide to the bismuth oxide modified iron
electrode did not change the kinetic parameters for hydrogen evolution significantly.
Consistent with this finding, the charging efficiency of the iron electrode modified with
bismuth oxide and iron sulfide was not different from the iron electrode with just the
bismuth oxide additive (Figure 3.8).
The Tafel slope of the unmodified carbonyl iron electrode was higher than that of
the electrodes with bismuth oxide additive. The higher value of Tafel slope could be due
42
to the resistance of the poorly-conducting oxide layer present on the iron electrode. Such
high values of Tafel slopes for hydrogen evolution on film-covered electrodes have been
reported for stainless steel and zirconium in alkaline media.
100, 101
For a conductive
surface resulting from the deposition of bismuth, the Tafel slopes were substantially
lower than on plain carbonyl iron. The similar values of Tafel slope for electrodes
containing bismuth oxide and bismuth sulfide confirmed that a bismuth-covered surface
was exposed to the solution on both these electrodes.
Using the values of exchange current and Tafel slope for hydrogen evolution on the
various electrodes (Table 1), we calculated the charge efficiency and compared these
with the values obtained by direct measurement of discharge capacity (Figure 3.2 and
Figure 3.8).
The charging efficiency was calculated using Eq. 9.
Charging Efficiency (%) = {(Q
charging
− Q
H2
) /Q
charging
} x 100 (9)
Where Q
charging
was the total input charge and Q
H2
was the charge used up in hydrogen
evolution.
Q
H2
was calculated from the cumulative value of the product of the time during
charging and the hydrogen evolution current, I
H2
, calculated from the Tafel relationship
(Eq. 8). The magnitude of I
H2
varies during charging since the electrode potential
gradually becomes more negative during charging. The values of charging efficiency
43
predicted from the kinetic parameters followed the same trend as the experimental values
(Table 1). Therefore, it was clear that the kinetics of hydrogen evolution was being
modified to different extents by the various additives.
3.3 Studies on Improving Discharge Rate Capabilities
3.3.1 Improving Discharge Rate Capability of Iron Electrodes with Sulfide Additives
To meet the demands of large-scale energy storage, the batteries must be capable of
being completely charged and discharged in one to two hours. The performance at
different discharge rates is described by the term “rate-capability”. The higher the
rate-capability the smaller the battery required for a particular amount of stored energy.
For many of the redox-flow type batteries, charging and discharging at high rates results
in significant loss of efficiency.
102
With the new carbonyl iron electrode containing
bismuth sulfide, high discharge rate capability is achieved along with the improved
charge efficiency. At a two-hour rate of discharge, with the addition of bismuth sulfide
we observe a twenty-fold increase in capacity compared to the commercial electrode and
a fifty-fold increase compared to the plain carbonyl iron electrode (Figure 3.15). We also
note that the specific mass loading of the commercial electrodes is approximately 8 times
higher than that of the carbonyl electrodes. This higher loading could also contribute to
the lower rate capability of the commercial electrodes.
44
The specific discharge capacity of the electrode with bismuth sulfide even at a
one-hour discharge rate corresponds to about 60% of the maximum discharge capacity of
the electrode. The commercial electrode yields almost no capacity at these high
discharge rates.

Figure 3.15 Discharge capacities of iron electrodes as function of the normalized
discharge rate. Normalized discharge rate expressed as 1/n times the nominal capacity in
Ampere-hours, where n is the number of hours of discharge
In addition, we studied the passivation behavior of bismuth oxide containing iron
electrodes with two types of “de-passivating” additives: (1) sodium sulfide at a
concentration of 3.0 g/L in the electrolyte, and (2) 5 w/w % of iron sulfide added to the
iron active material during electrode fabrication.
45
Electrodes with carbonyl iron or with just bismuth oxide exhibited very poor rate
capability. Specifically, the bismuth-oxide-modified carbonyl iron electrode did not give
any appreciable capacity at rates higher than C/5 (Figure 3.16).

Figure 3.16 Discharge capacities of blank carbonyl iron electrodes and electrodes
modified with bismuth oxide as function of the normalized discharge rate
However, when sodium sulfide was added to the electrolyte, the same electrode
delivered 8 times greater capacity at the C/1 rate compared to the experiment without
sodium sulfide (Figure 3.17). Further, the addition of iron sulfide increased the delivered
capacity by 18 times of that without any additive. The electrode containing bismuth
oxide and iron sulfide could be discharged at 3C rate with an electrode utilization value
of almost 0.2 Ah/g (Figure 3.17). The 3C rate observed here is the highest discharge rate
reported with iron electrodes, and makes the electrode highly suitable for supporting a
wide range of power demands of grid-scale energy storage systems. We also note that
46
the ability to discharge at such high rates did not compromise the charging
characteristics in that the high charging efficiency of 92% was maintained.

Figure 3.17 Discharge capacities of commercial iron electrode and carbonyl iron
electrodes with various additives as function of the normalized discharge rate
3.3.2 Studies of De-Passivation Characteristics of Sulfide Additives
To understand the enhanced discharge rate capability achieved with the sulfide
containing electrodes, we investigated the passivation characteristics of various iron
electrodes by potentiodynamic anodic polarization. Consistent with the results of
discharge rate capability (Figure 3.15-3.17), the presence of sulfide additives was found
to modify considerably the passivation characteristics of the iron electrode. When a
carbonyl iron electrode without additives was polarized anodically, the current increased
in the potential range of 1.00 V to 0.90 V (Figure 3.18). Polarization of the electrode
47
positive to 0.85 V resulted in a decrease of current. We term this value of electrode
potential where the current begins to decrease with increasing anodic polarization as the
passivation potential (E
pass
) and the corresponding peak current as the passivation current
(I
pass
). Since the onset of passivation limits the discharge process, the passivation
current is a measure of the maximum discharge rate achievable with the iron electrode.
This type of passivation behavior was also exhibited by the iron electrodes containing
just bismuth oxide. The value of passivation current observed in the anodic
polarization curves corresponded approximately to the maximum discharge rate
observed with the carbonyl iron and bismuth oxide electrodes (Figure 3.16).

Figure 3.18 Anodic polarization curve for a fully charged iron electrode with different
compositions in 30 w/v% potassium hydroxide at a scan rate of 0.17 mV s
-1

48
When sulfide is present in the electrolyte, or when iron sulfide is present in the
electrode, the polarization measurements did not show any current limitation from
passivation. With both these types of sulfide additives, the current continued to increase
even when the electrode potential was 0.75 V. Thus, with the potentiodynamic
polarization studies we were able to confirm directly that sulfide additives mitigated iron
electrode passivation.
15, 17
These results on the “de-passivation” of the iron electrode
with the sulfide additives are consistent with the high discharge rates of 3C observed
(Figure 3.17). We also note that the anodic polarization behavior of the iron electrode in
the presence of sodium sulfide and iron sulfide was similar to the behavior of a carbonyl
iron electrode modified with bismuth sulfide (Figure 3.18), although significantly higher
currents were sustainable with the electrodes containing bismuth oxide and iron sulfide.
49
3.3.3 In-situ Formation of Iron Sulfide Compounds in the Presents of Soluble Sulfide
Ions

Figur 3.19 XRD pattern for the carbonyl iron electrode modified with bismuth oxide and
with sodium sulfide added to the electrolyte (3.0g/L) after extended cycling
For the iron electrode formulated with bismuth sulfide, and the electrodes with
sodium sulfide added to the electrolyte, we attribute the excellent discharge rate
capability to the in situ formation of iron sulfides. In the XRD measurements on cycled
electrodes that incorporated bismuth sulfide, and in the electrode cycled in electrolyte
containing sodium sulfide, we were able to detect iron sulfide phases corresponding to
FeS and Fe
3
S
4
(Figure 3.7 and Figure 3.19). We may infer that sulfide ions (from soluble
sodium sulfide or from reduction of bismuth sulfide (Eq. 4)) reacted with the iron (II)
hydroxide to form iron (II) sulfide (Eq.10).
S
2-
+ Fe(OH)
2
= FeS + 2

OH
-
(10)
50
The iron (II) sulfide can react with sulfide ions to form various mixed-valence iron
sulfides that are electronically conductive like iron (II) sulfide. The in situ incorporation
of such electronically conductive iron sulfides will counter the passivation caused by the
discharge product, iron (II) hydroxide, an electronic insulator.
34
Thus, the iron sulfide compounds maintain the electronic conductivity at the
interface allowing the discharge reaction to be sustained at high rates. This is supported
by previous work on the beneficial effect of sulfide additives.
30, 34, 35, 47, 68

3.4 Studies on Formation of Iron Electrodes
After an iron electrode is prepared, it usually goes through 30 to 50 cycles of
repeated charge and discharge, during which the discharge capacity of the electrode
increases to a stable value. This process is referred to as “formation”. For large scale
alkaline iron batteries, the formation step could significantly impact the cost of the
batteries because formation not only requires additional cycling equipment, it also
increases the time needed to produce alkaline iron batteries. In addition, the stable
discharge capacity reached at the end of formation determines the extent of utilization of
the iron active materials, which also affects the final cost of the alkaline iron battery.
Therefore, understanding on the formation process is essential in the development of
high-performance and low-cost nickel-iron batteries.
51
3.4.1 Changes in Electrode Potential and Discharge Capacity during Formation
The potential - time curves during charging of the carbonyl iron electrode
undergoing formation (Figure 3.20) showed no evidence of a voltage plateau
corresponding to the reduction of iron (II) hydroxide to iron for the initial 20 cycles
indicating that most of the input charge was diverted to the hydrogen evolution reaction
(Eq. 3).

Figure 3.20 Change of electrode potential during charging at 200 mA for 2 hours during
electrode formation
After 30 cycles of formation, a voltage plateau corresponding to the reduction of
iron (II) hydroxide appeared between 1.1 V and 1.2 V. With further cycling, this
voltage plateau became dominant and distinct from the second voltage plateau that
corresponded to the hydrogen evolution reaction at 1.22 V. The charge corresponding
to the iron electrode reaction increased with a simultaneous decrease in the charge
52
diverted to hydrogen evolution. This decrease in hydrogen evolution rate was also
reflected in a corresponding increase of discharge capacity (Figure 3.21).

Figure 3.21 Change of electrode potential and discharge capacity of carbonyl iron
electrode during discharge at 20 mA to a cut-
electrode formation
The discharge capacity was very low in the first 30 cycles and the capacity rapidly
grew in the next 30 cycles reaching a stable value of about 0.14 Ah/g after 60 cycles
(Figure 3.22).
53

Figure 3.22 Growth of discharge capacity of carbonyl iron electrode during formation
cycling
We found that as formation proceeded, the plateau potential during charging
increased (Figure 3.20). This increase in electrode potential indicated that the
overpotential for the reduction of iron (II) hydroxide to iron decreased progressively
with each cycle during formation. The decrease in overpotential for the iron electrode
reaction was consistent with the increase in the electrochemically active area of the iron
electrode that accompanied formation.
We also observed that the electrode potential at the end of the charging process
decreased as formation proceeded (Figure 3.20). At the end of the charging process the
only reaction that occurred was the hydrogen evolution reaction. Consequently, a more
negative value of electrode potential suggested an increase in the overpotential for the
hydrogen evolution reaction at the end of formation (Figure 3.23).
54

Figure 3.23 Change in overpotential for hydrogen evolution on carbonyl iron electrode at
the end of charging during formation cycling
3.4.2 Effects of Active Surface Area on Formation
The electrochemically active area grew very slowly in the first 30 cycles (Figure
3.22). This slow growth was attributed to the poor wettability of the electrode; the use of
polyethylene as a binder rendered the electrode surface hydrophobic and it took several
cycles of charge and discharge for the electrolyte to penetrate the pores of the electrode.
The capacity of the electrode increased much faster after the pores were wetted by the
electrolyte. The high degree of hydrophobicity observed initially was mitigated by the
addition of a wetting agent such as Triton® X-100 to the electrolyte (Figure 3.24).
55

Figure 3.24 Effect of addition of 0.125% Triton® X-100 to the electrolyte on the
discharge capacity during formation of carbonyl iron electrode
Once the pores became accessible to the electrolyte, the charge and discharge
process produced a progressively rougher surface resulting in an increase in
electrochemically active surface area and discharge capacity. The scanning electron
micrographs of the iron electrode surface at the beginning and at the end of formation
confirmed this increase in surface area (Figure 3.25 a, b). An iron electrode before
formation was characterized by particles of carbonyl iron of uniform size (3-5 microns)
surrounded by the polyethylene binder (Figure 3.25 a). At the end of formation, we
found the surface covered with acicular crystals of iron (II) hydroxide and a very rough
surface with features of the size of 0.1 micron (Figure 3.25 b).

56
a)

b)

Figure 3.25 Scanning electron micrographs of carbonyl iron electrode (a) before
formation cycling in the “as- prepared” state, (b) at the end of formation cycling in the
discharge state
In addition to the increase of the electrode surface area, the discharge step filled the
pores with increasing amounts of iron (II) hydroxide. The electrode reached its
maximum capacity when the iron (II) hydroxide produced during discharge limited
57
further access of the electrolyte to the pores. When no additional iron (II) hydroxide
could be formed a stable capacity was reached and this signified the end of formation.
The benefit of having a larger pore area was confirmed by the high discharge
capacity exhibited by electrodes with a high initial porosity. Electrodes fabricated with
carbonyl iron and 10% w/w of potassium carbonate resulted in a highly porous electrode
with a volume porosity of 50%; the potassium carbonate dissolved into the electrolyte
and worked as a “pore-former” (Figure 3.26).

Figure 3.26 Scanning electron micrographs of carbonyl iron electrode with 10 w/w%
potassium carbonate as pore-former, before formation
These highly porous electrodes exhibited a significant increase in discharge capacity
right from the start of the formation process. The electrodes with the pore former were
easily wetted even at the start of formation. At the end of formation, the electrode with a
pore-former had 20% higher capacity than the electrodes without the pore-former
(Figure 3.27).
58

Figure 3.27 Effect of pore-former, 10 w/w% potassium carbonate, on the growth of
capacity during formation
3.4.3 Understanding of the Mechanism of Iron Electrode Formation
While the results provided above identified the role of the various factors that
determined the end of formation discharge capacity, the need for 30-40 cycles for
completing formation required explanation. To understand the need for a large number
of cycles to achieve complete formation, we focused on the phenomenon of passivation
of the iron electrode that limited the discharge capacity in every cycle. When an iron
electrode is discharged, it is anodically polarized and iron is converted to iron (II)
hydroxide (Eq. 2). The anodic polarization of the iron electrode indicated an
active-passive behavior (Figure 3.28) similar to iron in sulfuric acid
103
. As the iron
electrode was polarized anodically, iron (II) hydroxide was produced and the discharge
current increased over the potential range of 1.00 to 0.90V and at about 0.85V
59
further increase in electrode potential resulted in a decrease in current. This type of
behavior indicated that the electrochemical conversion of iron to iron (II) hydroxide was
inhibited beyond 0.85V as the electrode surface became “passive”. The consequence of
such passivation was that the discharge process was arrested and no more capacity could
be realized. The potential at which this phenomenon occurs is the passivation potential
and the corresponding current (I
pass
) normalized for the electrode area is the critical
passivation current density (i
pass
).
104, 105

Figure 3.28 Anodic polarization curve for completely charged iron electrode in 30%
potassium hydroxide
The passivation potential (Figure 3.28) was found to be in good agreement with the
potential corresponding to the inflexion in the discharge curves (Figure 3.21) measured
during formation. The electrode potential at which the inflexion occurred did not vary by
more than 10 mV throughout formation cycling. We were thus able to confirm that the
60
passivation process restricted the discharge capacity realized in every cycle during
formation.
For a discharge current I
dis
, the effective discharge current density is given by,
i
dis
= I
dis
/ A(t), where A(t) is the time-dependent active area of the electrode. As the iron
was converted to iron (II) hydroxide during discharge, the decreasing area available for
reaction resulted in an increase in the effective discharge current density of the iron
electrode. When this discharge current density became equal to the critical passivation
current density (i
pass
) of the iron electrode, no more capacity was obtained from the
electrode (Eq. 11) and the end of discharge was determined by the condition
i
pass
= i
dis
= I
dis
/ A (t) (11)
We infer from Eq. 11 that the longer the duration for which i
dis
stayed smaller than
i
pass
, the higher was the discharge capacity of a given electrode. Since every cycle
produced fresh iron electrodeposits, progressive roughening of the electrode surface
occurred in every cycle. This roughening process increased the surface area available for
reaction and allowed a higher current density to be sustained on discharge prior to
passivation. This increase in area was readily seen in the increased current required for
passivation in the 20
th
cycle compared to the 10
th
cycle of formation (Figure 3.29).
Thus, in spite of the capacity limit resulting from the passivation process, the discharge
capacity grew in every cycle due to the progressive roughening of the electrode.
61

Figure 3.29 Anodic polarization curve for completely charged iron electrode after 10
cycles and 20 cycles of formation
We therefore concluded that to realize higher discharge capacity in every cycle
either the active area of the electrode or the critical passivation current density (i
pass
) of
the electrode must be increased. The active area of the iron electrode can be varied by
the addition of a pore-former to the electrode active material or by the addition of a
wetting agent to the electrolyte. The critical passivation current density can be
increased with sulfide additives that prevent the formation of an insulating passive layer
during discharge.
30, 34, 47, 68
In fact, we found that iron electrodes anodically polarized
in potassium hydroxide solutions containing 3.0 g/l of sodium sulfide did not exhibit the
usual passivation characteristic, and thus the effective passivation current density may be
considered to be very high (Figure 3.30).
62

Figure 3.30 Effect of sodium sulfide on the anodic polarization behavior of carbonyl iron
electrode in 30% potassium hydroxide
The de-passivating effect of the sulfide additive also resulted in 10% higher
discharge capacity at the end of formation compared to the carbonyl iron electrode
without any additive (Figure 3.31). The rate of formation of the electrode cycled in
sulfide-containing electrolyte is also slightly higher than that of the electrode cycled in
sulfide-free electrolyte.

Figure 3.31 Effect of sodium sulfide additive in the electrolyte on the formation of the
iron electrode
63
3.4.4 Phenomenological Model for Formation of Carbonyl Iron Electrodes
Based on the above results, we have identified that two electrode properties have a
direct impact on the rate of formation and the discharge capacity achieved at the end of
formation:
(1) Electrochemically active surface area available for the charge/discharge
reaction and
(2) Critical current density for passivation during discharge.
These two electrode properties are influenced by the electrode and electrolyte
composition as summarized in Table 2. To provide a quantitative analysis of the results
above, we provide a phenomenological model that relates the electrode properties in
Table 2 to the rate of formation and the final discharge capacity.
Table 2 Effect of electrode design on the electrode properties governing formation
Electrode/Electrolyte
Composition
Electrode Properties Determining
Formation Characteristics
Electrochemical
Active Area
Critical
Passivation
Current Density
Wetting Agent √ -
Pore-Forming Additive √ -
Sulfide in Electrolyte - √
64
We propose a phenomenological model based on the observed effect of various
electrochemical factors on the formation process. Our understanding of the basic
electrochemical processes occurring during formation is shown schematically in Figure
3.32. During discharge, the surface of the iron particles is converted to iron (II)
hydroxide. When the surface is completely covered with iron (II) hydroxide, passivation
of the electrode occurs and no more discharge capacity can be obtained from the
electrode. The iron produced by the reduction of iron (II) hydroxide during the
subsequent charging step is deposited on areas different from where the iron was
dissolved. This type of deposition results in a progressive roughening of the electrode
and more parts of the iron electrode are converted by repeated cycling to a rechargeable
form.

Figure 3.32 Electrochemical processes during formation of the iron electrode
Let A
o
be the electrochemically active area of a freshly-prepared iron electrode.
When the electrode is discharged at a constant current, I
dis ,
iron is converted to iron (II)
hydroxide and the electrochemical active area of iron continuously decreases. As the
65
area of the iron electrode decreases during discharge, the effective discharge current
density, i
dis
, increases. When i
dis
reaches the value of the critical passive current density,
i
pass
, the electrode potential falls below the set cut-off value and the discharge terminates.
If t
p
is the time at which i
dis
becomes equal to i
pass
, then the capacity of the electrode in
the first discharge, Q
1
, is given by,
Q
1
= I
dis
t
p
(12)
The volume of iron (II) hydroxide that is produced during the first discharge is given by
V
Fe(OH)2
=  I
dis
t
p

(13)
Where β = M
Fe(OH)2
/ (2F ρ
Fe(OH)2
)
where M
Fe(OH)2
, 
Fe(OH)2
, and F are the molecular weight, density of iron (II) hydroxide,
and the Faraday constant, respectively.
Since the carbonyl iron particles are spherical with a radius r, the volume change of
V
Fe(OH)2
caused by the discharge process produces a change in surface area of
A
Fe(OH)2 ,
is given by,
A
Fe(OH)2
= V
Fe(OH)2
(3/ r) (14)
At the end of the first discharge step characterized by time t
p
, the change in area is
A
Fe(OH)2
= (3/ r)  I
dis
t
p
(15)
66
This change in area resulting from the formation of iron (II) hydroxide reduces the
area of the iron surface that is available for discharge. Consequently, as the discharge
proceeds, the effective discharge current density increases and the critical passivation
current density is reached at time t
p
. The area of the electrode when passivation sets in is
obtained by combining equations (12) to (15):
A
pass
= A
o
– A
Fe(OH)2
= A
o
- (3/ r)  I
dis
t
p
(16)
Where A
pass
is the area available at passivation.
Consequently, the passivation current density is
i
pass
= I
dis
/A
pass
(17)

From the equations (12) – (17), we relate the discharge capacity in the first cycle, Q
1,

to the passivation current density and the geometry of the iron particles by,
Q
1
= (A
o
- I
dis
/i
pass
)( r/3 ) (18)
For the discharge capacity to increase in the subsequent cycles, the area of the
electrode surface available for discharge must increase. The charging process results in
the electrodeposition of iron in parts different from where the iron was dissolved during
discharge (Figure 3.31). As a result of this roughening, the active area available for
discharge in the second cycle increases. The area available for discharge at the end of the
first re-charge is therefore given by,
67
A
1
=A
o
+ kA
o
(19)
where k is the “roughening” coefficient. The value of k will depend on the morphology
of the electrodeposited iron.
In the second cycle, the area A
1
is now available for discharge. Using Eq. 18, the
discharge capacity of the electrode at the end of the second cycle is given by,
Q
2
= (A
1
- I
dis
/i
pass
)

(r/3 )

(20)
The active area of the electrode at the end of the second charging step is,
A
2
= A
1
+ k A
0
= ( A
o
+ kA
o
) + kA
o
= A
o
(1+2k) (21)
Subsequent steps of discharge and charge will result in further increase in area as in Eq.
21. Thus, the generalized expression for the area of the electrode after (N-1) charging
steps is given by
A
N-1
= A
o
{1 + (N-1) k }

(22)
And the capacity at the end of N cycles is given by,
Q
N
= (A
N-1
- I
dis
/i
pass
)

(r/3 )

(23)
Combining Eqs. (22) and (23),
68
Q
N
= {A
o
[1+(N-1)k]

– I
dis
/i
pass
} ( r/3 ) (24)
Differentiating Q
N
with respect to N, the rate of formation is given by,
dQ
N
/dN = A
o
k (r/3 ) = k[Q
1
+ {( I
dis
/i
pass
) ( r/3 )}] (25)
The simple expressions for Q
N
and dQ
N
/dN shown in Eqs. (24) and (25) allow us to
understand and predict the capacity increase during formation in terms of the electrode
properties. After a certain number of cycles during formation, the iron (II) hydroxide
produced during discharge will limit electrolyte access to the pores and prevent further
discharge. At this stage, the electrode reaches the end of formation with a stable
discharge capacity Q
EF.
Thus, Q
EF
is equal to Q
N
when the filling of the pores limits the
discharge. By knowing the values of r, Q
1
and the roughening coefficient in Eq. 25, the
rate of formation of the iron electrode can be predicted.
3.4.5 Model Validation
To validate the model, we compare the rate of formation and final capacities
predicted by the model to experimental measurements. The experimental data for
carbonyl iron electrodes prepared with and without pore-former (with wetting agent) are
presented in Table 3.
The electrode with the wetting agent started with a higher initial surface area, A
o

compared to the electrode “with pore former”. The higher initial area led to a higher
69
discharge capacity at the start of formation. In the case of the electrode with pore-former,
the pore-former dissolved in the electrolyte making more active area available as the
formation process proceeded. The initial electrochemical active area of the electrode, A
o

was estimated from porosity measurements and SEM studies. The porosity of the
electrode was measured by immersing a freshly prepared iron electrode in isopropyl
alcohol for about 10-15 minutes. The difference in the weight of the electrode before and
after soaking can be used to estimate the total pore volume of the electrode (V
pores
). From
the SEM images, the radius of a pore (r
pore
) was found to be about 15 microns. The initial
electrochemical active area of the electrode, A
o
was calculated using the expression, A
o
=
2V
pores
/ r
pore
.
Table 3 Experimental data on carbonyl iron electrodes charged at 200 mA and
discharged at 20 mA (* - Calculated, ** - Experimentally measured)
Electrode Property
Carbonyl iron electrode
without pore former
(with wetting agent)
Carbonyl iron
electrode with pore
former
Porosity**, % 37.5 50.7
Area of the electrode*, cm
2
7.73x 10
2
5.20x 10
2

Total pore volume**, cm
3
0.29 0.39
Capacity at the start of
formation**, Ah g
-1

0.04 0.027
Capacity at end of formation**,
Ah g
-1

0.12 0.17
Number of cycles for formation** 17 42
Rate of formation**, Ah g
-1
cycle
-1
0.005 0.0035
70
The rate of formation is governed by the difference between the initial and final
capacity and also by the number of cycles of formation. So, even though the pore former
electrode started with a lower capacity and ended with a higher capacity, the large
number of cycles taken for formation resulted in a lower rate of formation.
We have determined the passivation current density, i
pass
for iron in alkaline medium
as 1.53 mA cm
-2
from experiments on high-purity iron disk electrodes.

Figure 3.33 Comparison of measured and predicted discharge capacity of carbonyl iron
electrode with (a) Triton® X100 in the electrolyte (b) pore former additive; dashed lines
show predicted data
71
The simulated evolution of capacity for an iron electrode with particle radius of 2
micron and ‘k’ value of 0.1 based on Eq. 25 is shown in Figures 3.33 a and b. Two cases
are considered: the iron electrode cycled in the electrolyte containing a wetting agent,
and an iron electrode containing a pore-former. The evolution of the capacity predicted
by Eq. 25 seems to be in good agreement with the measured formation data. The
electrode cycled in the sulfide-containing electrolyte, and the blank carbonyl iron
electrode was incompletely wetted in the initial cycles of formation. However, these
electrodes progressively became wetted by the electrolyte in subsequent cycles. This
extremely slow wetting of the electrodes precluded analysis of the rate of formation by
the proposed model.
The initial pore volume calculated from the porosity measurements was used to
predict the end of formation capacity (Q
EF
) of the electrode. Assuming 50% filling of the
pore volume as the limit for end of formation, the corresponding volume of iron (II)
hydroxide was calculated. The charge corresponding to this volume represented the
end of formation capacity of the electrode. We found a close agreement between the
measured and the calculated values of Q
EF
(Figure 3.33). The 50% limit of filling of pore
volume is also consistent with the pore-loading in commercial sintered nickel-cadmium
battery electrodes for realizing good utilization of active materials
106
. The end of
formation capacity Q
EF
was calculated using the following expression
Q
EF,calculated
= (0.5 V
pores
) / β
72
Using the experimentally observed initial capacity (Q
1
) and the rate of formation
(dQ
N
/dN), we predicted the value of k, the roughening coefficient, using Eq. 25. The
value of k was found to be in the range of 0.10 to 0.12 which is also in agreement with
the k value of 0.10 chosen to the simulate the measured formation curves (Figure 3.33).
The value of k is found to be similar for the electrode with wetting agent and the pore
former, as would be expected from the model (Table 4). The values of k that range from
0.10 to 0.12 suggest that the roughening of the electrodes is a slow process and this is
consistent with the low solubility of the iron (II) species in the electrolyte.
107
The
parameter k determines the increase in roughness of the iron electrode resulting from the
electro-reduction of iron (II) hydroxide. Specifically, iron forms by the electro-reduction
of the ferrite ion in solution and thus we could expect the parameter k to be governed by
the spatio-temporal distribution of the concentration of the ferrite ion during deposition.
However, the solubility of the ferrite ion is extremely low and hence the spatial
re-distribution of soluble iron is limited. Thus, a small and constant value of k observed
in this study is justified.
The active area of a fresh iron electrode, A
o
, predicted using Eq. 13 was in good
agreement with the areas estimated from porosity measurements and SEM studies (Table
4).
73
Table 4 Active area and roughening coefficient of the iron electrode under different
conditions (ipass = 1.53 mA*cm
-2
, Q1 for the different electrodes was taken from data
presented in Table 3)
Electrode
Roughening
coefficient,
k
A
o
, Predicted
using Q
1
(Eq.
13) cm
2

Initial Area from SEM
and porosity
measurements, A
o
, cm
2

Electrode with binder
and Triton®-X-100
(Wetting Agent)
0.107 730 773
Electrode with binder
and Pore Former
0.122 452 520
3.5 Cycle Life Studies on Pressed-Plate Iron Electrode
We have been working on building alkaline iron electrodes with low-cost materials
and processes, and improving its performances during charge and discharge cycling to
decrease the levelized cost of energy for large-scale energy storage applications. Another
factor that strongly affects the levelized cost of energy is the cycle life of alkaline iron
batteries. While alkaline iron batteries are known for its longevity, systemic studies on
the changes on the iron electrode during long-term cycling in the scientific literature are
lacking. Therefore, we have focused on understanding the changes in the iron electrode
during long-term cycle life testing, and proposing methods that could help maintain the
high performance of iron electrodes over long periods of charge and discharge cycling.
74
3.5.1 Cycle Life Testing of Iron Electrode with Different Additives
Two types of carbonyl iron electrodes with the following additives were studied:
(1) 5% bismuth sulfide and (2) 5% iron (II) sulfide and 10% bismuth oxide.
The discharge capacity of the iron electrode containing bismuth sulfide was 0.3
Ah/g after formation, and the charging efficiency was 96%. During repeated cycling, the
discharge capacity of this electrode remained stable for about 50 cycles and then began
to gradually decrease. This gradual fade in the discharge capacity resulted in about 50%
loss in capacity after about 150 cycles (Figure 3.34).

Figure 3.34 Variation in the discharge capacity of a carbonyl iron electrode with bismuth
sulfide additive as a function of cycling
However, this loss in electrode capacity was not permanent and could be recovered
completely when cycling was continued after addition of sodium sulfide to the
electrolyte (Figure 3.34). The sodium sulfide added was enough to reach a concentration
75
of 2 mM in the electrolyte. Although the electrode capacity increased and stabilized after
the addition of sodium sulfide, the electrode lost 60% of its capacity gradually again in
300 more cycles. This repeated loss could be recovered completely by further addition of
sodium sulfide (2 mM) to the electrolyte (Figure 3.34). The recovery to 100% of the
capacity after the addition of sodium sulfide occurred in just 5 cycles (Figure 3.35).
While the iron electrode containing bismuth sulfide exhibited a capacity fade, the
iron electrode prepared with iron (II) sulfide and bismuth oxide additives did not show
any decrease in capacity even for 1200 cycles at 100% depth of discharge (Figure 3.36).
Also, these iron electrodes maintained a charging efficiency of 92% through the cycling
tests. Such robustness to repeated cycling at 100% depth-of-discharge, combined with
high charging efficiency is particularly attractive for deployment of energy storage at the
grid-scale that will require 15 years or more of stable and efficient operation.

Figure 3.35 Recovery of discharge capacity of an iron electrode with the addition of
sodium sulfide to the electrolyte
76

Figure 3.36 Discharge capacity of a carbonyl iron electrode with iron (II) sulfide and
bismuth oxide additives during cycle life testing
These results also suggested that sulfide addition in the soluble form was repeatedly
needed to sustain cycle life, whereas the insoluble sulfide in the form of iron (II) sulfide
was quite effective in preserving the cycling properties.
3.5.2 Comparison of Sulfide Incorporation in Iron Electrodes with Different Additives.
In the iron electrode prepared with bismuth sulfide additive, the electrochemical
reduction of bismuth sulfide takes place during the first few charging cycles during the
electrode formation process.
15, 108
This reduction of bismuth sulfide results in the
deposition of elemental bismuth on the iron electrode and release of sulfide ions into the
electrolyte (Eq. 4, page 28). The sulfide ions generated as per Eq. 4, combines with the
iron (II) hydroxide formed during discharge (Eq. 2, page 5) to form iron (II) sulfide on
77
the iron electrode (Eq. 10, page 48). The in situ production of iron sulfide by this process
has been confirmed in our earlier studies.
15

The amount of conductive iron sulfides formed in situ (Eq. 10) would depend on the
amount of iron (II) hydroxide available on the iron electrode during formation, the
availability of soluble sulfide by Eq. 4, and the permeation of the sulfide ions into the
electrode structure. During the first few cycles of electrode formation, the capacity of the
iron electrode is very low and the amount of iron (II) hydroxide produced is very small.
17, 85
Further, the sulfide ions produced during the electro-reduction of bismuth sulfide
(Eq. 4) during the first few cycles of formation will be oxidized at the nickel oxide
counter electrode to sulfate and the sulfide would be irreversibly depleted. Under such
conditions, it is reasonable to expect that the continued in situ incorporation of iron
sulfide would cease for the bismuth sulfide modified iron electrode after the initial few
cycles of electrode formation. Consequently, the amount of iron sulfide formed in situ
was likely to be very small relative to any deliberate additions of iron (II) sulfide at the
level of 5-10 wt. % of the electrode during fabrication.
In the case of the carbonyl iron electrode prepared with iron (II) sulfide additive, the
high concentration and the sparingly soluble nature of the iron (II) sulfide additive
provided a reservoir for sustained release of sulfide ions for a long time. Thus, the
depletion of this sulfide reservoir by dissolution into the electrolyte would be a very slow
process. Consequently, even after 1200 cycles, the iron electrode prepared with iron (II)
78
sulfide additive can be expected to provide adequate sulfide ions to meet the need for
ensuring long cycle life and high discharge rate (Figure 3.36). Further studies were
performed on the two types of iron electrodes with bismuth sulfide and iron (II) sulfide
to gain greater insight into the mechanisms underlying capacity fade and the role played
by the sulfide additives on the cycling behavior.
3.5.3 Change of Discharge Rate Capability with Cycling
For the iron electrode prepared with bismuth sulfide, the capacity achieved at any
chosen discharge rate had decreased significantly after 110 cycles (Figure 3.37). For
example, the capacity at the C/2 rate of discharge had decreased from 0.24 Ah/g to 0.08
Ah/g after 110 cycles. This level of discharge performance following capacity loss was
similar to that of an iron electrode prepared without any sulfide additive (indicated as a
“sulfide-free” electrode in Figure 3.37).
15
However, after the addition of sodium sulfide
to the electrolyte, the discharge rate capability of the electrode that had suffered capacity
loss, also recovered to the values observed prior to the extended cycling studies (Figure
3.37).
79

Figure 3.37 Discharge capacities at different discharge rates of a carbonyl iron electrode
prepared with bismuth sulfide (Bi
2
S
3
) additive at different stages of cycling and a
sulfide-free iron electrode
The loss of discharge rate capability was attributed to the rapid passivation of the
iron electrode. The passivation behavior refers to the maximum in the current observed
during an anodic potentiodynamic scan. For a “sulfide-free” electrode, such passivation
readily occurred during anodic polarization (Figure 3.38). Results of such polarization
studies indicated that a sulfide-free iron electrode could not sustain more than 0.05 A/g
or a C/5 rate without undergoing passivation (Figure 3.38).
80

Figure 3.38 Anodic polarization curves (Broken arrow indicates direction of potential
scan) of iron electrodes measured in the fully charged state at different stages of cycling
Prior to extensive cycling, the iron electrodes prepared with bismuth sulfide additive
did not exhibit passivation, and high discharge currents could be sustained during anodic
polarization (Figure 3.38, Figure 3.39). [11] These observations had suggested that
sulfide was available in sufficient quantity to prevent passivation. However, after
repeated cycling of the bismuth sulfide containing iron electrodes, a significant change in
the polarization behavior was observed (Figure 3.38). At an electrode potential of –
0.85V, this electrode could sustain a discharge current of only 0.15 A/g, compared to the
value of 0.4 A/g before the cycling tests (Figure 3.39). Thus, the capacity fade correlated
well with the onset of passivation. Further, upon addition of sodium sulfide to the
electrolyte of the iron electrode that had faded in capacity, the polarization
81
characteristics were restored and higher discharge currents could be observed (Figure
3.39). Therefore, we concluded that: (a) the capacity fade of the electrodes prepared with
bismuth sulfide was correlated to the electrode’s inability to be discharged at high rates,
(b) the sulfide present at the start of cycling studies had been lost, and (c) the addition of
sulfide to the electrolyte can mitigate passivation and restore the discharge rate
capability.

Figure 3.39 Discharge current at an electrode potential of -0.85V of iron electrodes
measured in the fully charged state at different stages of cycling
82
3.5.4 Changes in Overpotentials during Constant Current Charging
The electrode potential during charging was monitored during cycling for the two
types of electrodes, namely: (1) carbonyl iron with bismuth sulfide and (2) carbonyl iron
with bismuth oxide and iron (II) sulfide additives (Figure 3.40).

Figure 3.40 Potential – charge curves measured during charging of carbonyl iron
electrode at the cycle numbers indicated (a) prepared with bismuth sulfide, and (b) with
iron (II) sulfide and bismuth oxide additives
a)
b)
83
The behavior of the two types of electrodes was considerably different. The
overpotential during charging for the iron electrode prepared with bismuth sulfide
additive increased gradually with cycling over 100 cycles. Specifically, the overpotential
for the electrode formulated with bismuth sulfide increased by more than 100 mV in 100
cycles at 50% state-of-charge (Figure 3.41). This change in overpotential was
accompanied by a 40% loss in discharge capacity (Figure 3.34). However, the
overpotential at 50% state-of-charge for the iron electrode containing bismuth oxide and
iron (II) sulfide additives did not increase even by 5 mV after 1200 cycles (Figure 3.41),
remarkably different from the electrode prepared with bismuth sulfide additive.

Figure 3.41 Overpotential at 50% State-of-Charge during charging of carbonyl iron
electrodes with different additives as a function of cycling
An increase of overpotential during charging as seen in Figure 3.41 with the
electrodes prepared with bismuth sulfide suggested that the polarization resistance for
84
charging increased as cycling proceeded. Such a gradual increase of polarization
resistance can occur if iron is discharged to a poorly reversible phase such as magnetite,
Fe
3
O
4
. During discharge of an iron electrode, in addition to iron (II) hydroxide (Eq.2,
page 5), the iron (II, III) oxide, magnetite (Fe
3
O
4
) can also be formed (Eq. 26)
Fe
3
O
4
+ 4H
2
O + 8e
-
 Fe + 8OH
-
E
o
= -0.912V (26)
From the relative values of the standard electrode potentials, the formation of
magnetite is more favorable compared to iron (II) hydroxide. However, the kinetic
reversibility of the reactions in Eq.2 facilitates the formation of iron (II) hydroxide
during electrode discharge.
109, 110
Towards the end of discharge, at electrode potentials
close to the cut-off value of –0.75V vs. MMO, the overpotential is sufficiently high to
lead to the formation of magnetite (Eq. 26). Magnetite is a semiconductor with an
electronic conductivity of 10
2
–10
3
S.cm
-1
. [46] However, the overpotential for the
reduction of magnetite to metallic iron is very high and attempts to reduce magnetite in
alkaline media results in mainly hydrogen evolution.
111
Under similar conditions as
investigated here, the faradaic efficiency of the reduction of magnetite to iron has been
reported to be less than 20% with hydrogen evolution consuming most of the input
charge.
111
A gradual build-up of the poorly-rechargeable magnetite phase on the iron
electrode could result in a decrease in the cycleable active material and an increase of
polarization resistance and overpotential during charging at constant current (Figures
3.40a and Figure 3.41). Thus, as the iron electrode was discharged repeatedly up to –
85
0.75V, the amount of Fe
3
O
4
formed at the iron electrode could gradually increase in each
cycle.

Figure 3.42 XRD of a carbonyl iron electrode prepared with bismuth sulfide additive
measured after capacity fade
The X-ray diffractogram of the iron electrode prepared with bismuth sulfide additive
and subjected to several cycles of charge and discharge confirmed the presence of
magnetite in the electrode (Figure 3.42). We have also found reports in the literature of
the formation of magnetite on iron electrodes when cycled between electrode potentials
similar to our studies.
30, 68

3.5.5 Charge and Discharge Studies on Magnetite Electrodes
We recall here that cycling is accompanied by an increasing tendency for the
electrode to be passivated, resulting in loss of discharge rate capability (Figure 3.37,
Figure 3.38). We also found that the discharge capacity of a faded iron electrode can be
86
recovered completely by the addition of sodium sulfide to the electrolyte (Figure 3.34).
Therefore, we investigated the rechargeability of the magnetite phase in the presence of
sulfide ions. To this end, we performed independent experiments on magnetite electrodes
prepared by pressing a mixture of magnetite with polyethylene binder (20 wt. %) and
acetylene black (10 wt. %) on a nickel grid. The magnetite electrode was charged at a
constant current of 75 mA in potassium hydroxide electrolyte (30 wt. %) for 20 hours.
Subsequently, the electrode was discharged at 15 mA until a cut-off voltage of –0.75V
(vs MMO) was reached. Similar charge and discharge studies on the magnetite electrode
were performed after the addition of 2 mM sodium sulfide to the potassium hydroxide
electrolyte. We found that the discharge capacity increased from 9 mAh to 120 mAh in
the presence of sulfide ions in the electrolyte (Figure 3.43), indicating over a ten-fold
increase in the amount of magnetite converted to metallic iron in the presence of sulfide.
We concluded that in the presence of sulfide ions, the poorly reversible magnetite phase
underwent reduction to form metallic iron during electrode charging.
87

Figure 3.43 Potential-charge curves measured during discharge of an electrode prepared
with magnetite powder in different electrolytes
Thus, when sulfide was not present in adequate amounts, the relatively irreversible
magnetite phase gradually accumulated in each cycle leading to the observed capacity
fade of the carbonyl iron electrode. The X-ray diffractogram of this iron electrode with
bismuth sulfide additive also does not show any iron sulfide peaks (Figure 3.42). This is
in contrast to an iron electrode with similar composition where mixed valence iron
sulfides were observed within a few cycles after electrode formation (Figure 3.7). Thus,
the accumulation of magnetite in the electrode coincided with the loss of iron sulfides
from the bismuth sulfide modified iron electrode.
88

Figure 3.44 Effect of sulfide addition on the charging characteristics of an iron electrode
that has faded in capacity. Potential – Charge curves measured on a bismuth sulfide
modified iron electrode during charging before and after the addition of sodium sulfide
to the electrolyte
The potential-charge curves of the iron electrode prepared with bismuth sulfide
additive that has lost capacity shows two well-defined potential regions (Figure 3.44)
corresponding to the iron electrode reaction (-1.1 V to -1.2 V vs MMO) and hydrogen
evolution (-1.3 V vs. MMO). Upon addition of sodium sulfide to the electrolyte, the
charge corresponding to the iron electrode reaction increases in preference to the
hydrogen evolution reaction. After about 8 cycles, the amount of charge diverted to
hydrogen evolution is minimal and the iron electrode reaction takes up almost all the
input charge. Correspondingly, the overpotential for charging decreased steadily in every
cycle after the addition of sodium sulfide to the electrolyte. This reduction in
89
overpotential was almost 100 mV, and the electrode potential returned to values that
were observed before any capacity loss occurred (Figure 3.44).
The evidence of magnetite on an iron electrode that has suffered capacity fade, the
effect of added sulfide on the charging overpotentials, and the increased rechargeability
of magnetite in the presence of sulfide, pointed to the slow accumulation of the poorly
rechargeable magnetite phase as the underlying cause of capacity fade.
3.5.6 Effect of Overcharge and Charge Rate on the Recovery of Capacity
If the accumulation of poorly rechargeable magnetite was the cause of capacity loss,
we could expect extended overcharge or a slow rate of charge to aid recovery of the lost
discharge capacity. To verify the benefit of overcharge, the electrode that had suffered
capacity loss was periodically overcharged once in every 10 to 15 cycles to the extent of
100% at the C/2 rate.
90

Figure 3.45 Effect of overcharge on the discharge capacity of an iron electrode with
bismuth sulfide additive that is fading in capacity
Following such overcharge we found that the discharge capacity of the electrode
was always higher than with the experiments without any overcharge (Figure 3.45). Also,
when the charge rate was lowered from C/2 to C/10, we observed a further increase in
discharge capacity (Figure 3.46). A decrease in the rate of capacity fade was also
observed for an overcharge of 100% at a reduced charge rate of C/10 (Figure 3.46).
91

Figure 3.46 Effect of various charging rates on the discharge capacity of an iron
electrode prepared with bismuth sulfide additive
These observations were consistent with accumulation of the poor rechargeable
magnetite phase that was slowly converted to iron during prolonged overcharge at a low
rate. Accordingly, higher rates of charging resulted in return of a lowered value of
discharge capacity. For example, when the charging rate of the iron electrode is
increased from C/5 to 2C during cycling, a decrease in discharge capacity is observed
(Figure 3.47). Yet, the high charging rate did not affect the rate of capacity fade. When
charging at rates as high as 2C, the parasitic hydrogen evolution reaction becomes more
favorable. Consequently, the charging efficiency of the iron electrode decreases and a
lower value of capacity is returned during discharge. This decrease in the value of
discharge capacity resulting from the increased charge rate is not a permanent loss and
can be recovered simply by charging at a lowered rate of C/2 (Figure 3.47).
92

Figure3. 47 Discharge capacity of a carbonyl iron electrode prepared with bismuth
sulfide additive at different charging rates. The discharge rate was kept constant at C/5
3.5.7 Effect of Discharge Rate on the Recovery of Capacity
Experiments were performed by fixing the charging rate at C/2 and conducting the
discharge at C/5 or 1C rate (Figure 3.48).

Figure 3.48 Effect of different discharge rates on the cycling behavior of a carbonyl iron
electrode with bismuth sulfide additive
93
When an electrode that had suffered capacity fade was cycled repeatedly at 1C rate
of discharge, a rapid decrease in the discharge capacity was observed (Figure 3.48).
After several cycles at the 1C rate, when the discharge rate was decreased to C/5, a full
recovery of capacity was observed (Figure 3.48). Thus, operating at low discharge rates
reduces the apparent rate of capacity fade. However, despite the recoverability of
discharge capacity at low discharge rates, a slow increase of overpotential of the
electrode during charging was still evident, indicating the formation of the less reversible
magnetite phase during the cycling (Figures 3.48 and Figure 3.49).

Figure 3.49 Effect of cycling at high discharge rates on an iron electrode prepared with
bismuth sulfide additive. Potential – Charge curves measured during charging at
different stages during cycling at high discharge rates
94
3.5.8 Effect of Depth of Discharge (DoD) on Capacity Fade
Despite the robustness of the iron electrode to mechanical stresses induced by
deep-discharge, we found that in the absence of adequate sulfide ions the capacity fade
depended strongly on the depth of discharge (DoD) similar to other rechargeable
batteries.
24, 112
We observed that when cycled at 83% DoD, the discharge capacity of the
iron electrode prepared with bismuth sulfide additive decreased by 26% after about 300
cycles. This rate of capacity fade at 83% DoD is much lower than the fade rate at 100%
DoD when a 25% loss in capacity occurred within 100 cycles (Figure 3.50).

Figure 3.50 Discharge capacity of a carbonyl iron electrode prepared with bismuth
sulfide additive as a function of cycling at different DoDs. Experiments at low DoD were
performed by charging the iron electrode at the C/2 rate and discharging at the C/5 rate
to a cut off potential of –0.75V (vs MMO). The C rates for these experiments were based
on 83% of the maximum rated capacity of the electrode
95
A gradual increase in overpotential during charging over 200 cycles indicated the
accumulation of the magnetite phase on the iron electrode (Figure 3.51). Thus, the
formation of magnetite on the iron electrode occurred even at reduced depths of
discharge. However, the amount of magnetite that was formed during every cycle when
the electrode was cycled under these conditions of reduced DoD was smaller than for the
100% DoD case. Also, a rapid fade in capacity was not observed until the un-utilized
capacity of 17% (at 83% DoD) that served as a reserve was depleted by the formation of
the irreversible magnetite phase.

Figure 3.51 Potential – Charge curves measured during charging of a carbonyl iron
electrode prepared with bismuth sulfide additive when cycling at 83% DoD. (Cycle
numbers are shown adjacent to the curves)
96
3.5.9 Comparison of the Cycling Behavior of Iron Electrode with Different Additives
We also recognize that depending on how the sulfide additives are incorporated in
the electrode, significant differences in performance can result. In the case of the iron
electrode prepared with bismuth sulfide additive, these conductive iron sulfide phases
are formed in situ during the electrode formation process. However, in the case of the
iron electrode prepared with iron (II) sulfide additive, the slow release of sulfide ions
resulting from the sparingly soluble nature of iron (II) sulfide provided the necessary
de-passivating properties during discharge. The iron sulfides are characterized by a very
low solubility constant and therefore are not expected to be rapidly removed from the
iron electrode (Eq. 27).
113

FeS  Fe
2+
+ S
2-
K
eq
= 4 X 10
-19
(27)
In the iron electrode prepared with bismuth sulfide, a limited amount of iron sulfide
formed in situ and consequently, the benefit of sulfide is lost in a few cycles. Thus, in
about 150 cycles, the iron electrode prepared with bismuth sulfide additive lost nearly 50%
of its capacity. Similarly, a gradual loss in capacity in about 100 cycles following
formation has been observed by Shukla et al. with in-situ carbon-grafted iron electrode
prepared with bismuth sulfide additive.
12

In the case of the iron electrode with bismuth sulfide additive, the loss of the in situ
formed iron sulfides and the accumulation of magnetite during repeated cycling
97
manifests in a loss of electrode capacity (Figure 3.52a). The addition of sodium sulfide
to the electrolyte of this electrode leads to capacity recovery in the short term (Figure
3.52a). The reaction of dissolved sulfide with iron (II) hydroxide should result in some in
situ formation of iron sulfide. The iron sulfides so produced could be poorly distributed
throughout the electrode structure to provide a long-term benefit, and consequently,
capacity fade is observed again in about 100 cycles.

Figure 3.52 Schematic of the cycling behavior of a carbonyl iron electrode with (a)
bismuth sulfide additive and (b) iron (II) sulfide + bismuth oxide additives
When iron (II) sulfide was added to the carbonyl iron electrode as an additive during
electrode fabrication, a sustained supply of sulfide within the iron electrode structure was
98
ensured. The consistent presence of sulfide prevented the accumulation of magnetite
phase by facilitating its reduction to metallic iron during charging (Figure 3.52b) and
resulted in an iron electrode with long cycle life. As the leaching of the sulfides from this
iron electrode will deplete the reserves, albeit slowly, we can adjust the amount and
distribution of iron sulfide in the original electrode for achieving the desired cycle life.
99
Chapter 4. Sintered Iron Electrodes
Our efforts with the “pressed-plate” type iron electrode have led to remarkable
advances in its efficiency and rate capability, achieved through electrode re-design and
re-formulation.
15, 17, 18, 20
Such “pressed-plate” electrodes were prepared by hot-pressing
of a mixture of the carbonyl iron particles, sulfide-containing additives, a pore-forming
additive and a polymeric binder. However, greater mechanical robustness is projected for
a “sintered-type” iron electrode when compared to other types of electrodes.
94
Such
robustness results from the particles of the active materials being sintered at a relatively
high temperature to form a rugged porous electrode. Considering the requirements of
long cycle life for large-scale energy storage systems, we have focused on understanding
the design factors affecting the performance of a “sintered-type” iron electrode, and the
designing of a “sintered” iron electrode that is able to achieve the demanding
performance targets of efficiency, long cycle life and rate capability needed for
large-scale energy storage systems.
While the pressed-plate fabrication method is relatively inexpensive for
manufacturing and is easily scaled up, fabricating electrodes by sintering of iron powders
can offer other significant benefits. Some of these benefits include increased mechanical
robustness, higher utilization of active materials and lower Ohmic resistance. W. A.
Bryant of Westinghouse Corporation has demonstrated the effect of sintering conditions
on the porosity and discharge capacity of sintered iron electrodes.
23, 26-28
Periasamy et al
100
described the performance of sintered iron electrodes in nickel-iron alkaline batteries.
42

While these reports focused on the manufacturing of sintered electrodes, they did not
address the fundamental issues of charging efficiency, discharge rate capability,
formation rate, and cycle life that are critical to large-scale energy storage. In this thesis I
have focused on understanding the factors affecting the performance of the sintered iron
electrode and to gain insight into the effect of the various design variables on the
performance characteristics and cycle life. I have implemented the new design learning
into laboratory prototypes and demonstrated the electrical performance required for
integrating such battery electrodes into grid-scale energy storage applications.
4.1 Effect of Porosity and Pore Size of Sintered Iron Electrode on Active Material
Utilization
4.1.1 Model for Calculating Active Material Utilization from Porosity of Iron Electrode
We found that the discharge capacities of the sintered iron electrodes increased as
the electrode porosity was increased (Figure 4.1). We compared these results with the
predictions of a simple model of effect of initial porosity offered by C. S. Tong et al.
114

In this basic model, the maximum specific capacity at any given value of porosity is
attained when the pore volume is filled with the discharge product. Also, all the pore
volume associated with the porosity is assumed to be accessible for the discharge
101
reaction. In this model, the maximum capacity, C
max
for an electrode of initial porosity of
, is given by
C
max
=
ε
1−ε
∙ K [28]
K in equation 28 is a constant based on the material properties of the iron electrode given
by,
K =
α
(V
2
−V
1
)ρ
1
[29]
Where  is the molar specific capacity of the active material, V
2
is molar volume of
discharge product, V
1
is molar volume of charge product and ⍴
1
is density of charge
product. For the iron electrode with iron (II) hydroxide as the discharge product, K=
0.353 Ah/g. The theoretical limit of C
max
is 0.962 Ah/g, and this maximum capacity of
iron would be achieved at an initial porosity of 73% and when all the iron is converted to
iron hydroxide with the original geometric volume of the electrode remaining
unchanged.
102

Figure 4.1 Experimental discharge capacities of sintered iron electrode for electrodes of
varying initial porosities when discharged at C/20 rate. Predicted results are from
calculations using Eq. 28, and with the modification by Equation 30 for k
per
=0.2
We note here that the value of porosity of 73% does not permit rechargeability as
the particles of iron (II) hydroxide are electrically insulating and are no longer
electrically interconnected as all the iron particles would be converted to iron (II)
hydroxide. In practice, if the volume fraction of conducting iron falls below the
percolation threshold, the rechargeability of the iron (II) hydroxide will be compromised.
Thus, for a given value of percolation threshold, the maximum initial porosity 𝜀 in Eq.
28 can be estimated from Eq. 30.
 = {1- (V
1
/V
2
)}/ (1+ k
per
) [30]
Where k
per
is the percolation threshold given by the minimum ratio of the volume
fraction of the conducting materials to the volume of the non-conductive discharge
103
product to maintain electrical connectivity at all times. For a plausible value of k
per
=0.2,
we have porosity value of 61% which is significantly lower than the 73% that does not
consider electrical connectivity.
Experimental results in Figure 4.11 indicate that the discharge capacity increased
with porosity as is to be expected. However, we found the values of discharge capacity
to be significantly lower than that predicted by the model from Eq. 28, and the
divergence between the experimental values and predicted values appeared to increase
with the porosity. In addition, although the predicted maximum capacity with correction
of percolation threshold k
per
was achieved in our experiment at the porosity value of 85%,
the experimental capacities are still relatively low compared to the predicted capacity
with correction of k
per
in the porosity range between 61% and 85%.
4.1.2 Understanding the Differences between Calculated and Experimental Material
Utilization
The reason for the differences between the observed and the predicted values of
discharge capacity became clear upon examining the scanning electron micrographs of
the cross-section of the sintered electrodes in the discharged state (Figure 4.2).

104
a)

b)

105
c)

Figure 4.2 (a) SEM images of the cross-section of sintered iron electrodes after discharge
(b) magnified picture of the surface portion in picture (a), (c) magnified picture of the
inner portion in picture (a)
We observed that on the surface the crystals of the discharged product were packed
closely and the pores were completely filled (Figure 4.2b). However, in the inner part of
the electrode, the discharged product appeared to be less tightly packed and a
considerable part of the pore volume seemed still available in these inner sections of the
electrode (Figure 4.2c). Since the simple model assumed complete filling of all the pores
throughout the structure of the electrode, it is not a surprise that the measured discharge
capacity was lower than the predictions of the model.
The observed difference in the utilization of the pore volume was understandable
considering that we had a relatively thick electrode structure (thickness of about 2 mm).
106
When the discharged product formed on the outer parts of the electrode, the interior parts
of the electrode became deprived of access to the electrolyte. After the entire pore
volume on the surface was filled with the discharged product, the interior sections did
not have access to the electrolyte needed for the conversion of iron to iron (II) hydroxide.
Thus, the iron particles in the interior of the electrode remained under-utilized (see
schematic in Figure 4.3). Also, electrodes with higher porosities were generally thicker,
and consequently the inner portions of such electrodes would be even more poorly
utilized than the thinner electrodes of lower porosity. These expected trends are
consistent with our observations (Figure 4.1). Even when the electrode rechargeability
was considered by correcting the porosity with the percolation threshold k
per
, the
maximum discharge capacity could not be achieved when electrode porosity reached 61%
because of poor utilization of the inner part in the electrodes. The predicted capacity was
only achieved at porosity of 85%, indicates that the additional 24% porosity was needed
to compensate for the poor electrolyte access to the interior of the electrodes (Figure
4.1).
Therefore, we have learned that the porosity and thickness of the sintered iron
electrodes must be considered together for optimization of discharge capacity at any
particular rate of discharge. Similar limitations to utilization have been observed with
other conversion-type electrodes as for example in the negative electrode of the lead-acid
battery.
115
Despite these limitations, we have been able to achieve a utilization of 0.5
107
Ah/g with the iron electrode. (Figure 4.1) This value of specific discharge capacity is
among the highest reported for battery electrodes of the conversion type.
a)

b)

Figure 4.3 Schematic of cross-section of sintered iron electrode during discharge (a)
early stage of discharge (b) final stage of discharge
108
4.1.3 The Effect of Pore Size on Utilization of Iron Active Material
The size of the pores and their locations in the electrode further modify the effects of
electrode porosity and thickness on utilization. While large pores are desirable for
ensuring good electrolyte access, electrodes with smaller pores can offer more internal
surface area than electrodes with large pores. We controlled the size of the pores in the
electrode by adding pore-former particles of different sizes while keeping the porosity
the same. We have used ammonium bi-carbonate as the pore-former and with two
different ranges of particle size: 20-25μm and 90-105μm.
a)

109
b)

Figure 4.4 SEM characterization of sintered iron electrode with pore former of different
particle sizes (a) 90-105μm, (b) 20-25μm at as-prepared states
The sizes of the pores in the sintered electrode corresponded well with the particle
sizes of the pore-former additives (Figure 4.4). Despite the porosity values of both types
of electrodes being similar (69.6% and 71.6%), we observed a 21% increase in the
utilization of iron active material in the electrode with the smaller particles of the pore
former compared to the one with larger pores (Figure 4.5). As a result, specific discharge
capacities were increased from 0.33 Ah/g to 0.4 Ah/g.
110

Figure 4.5 Discharge capacities of sintered iron electrode with different pore sizes after
formation

Figure 4.6 Schematic of cross-section of sintered iron electrode with different pore sizes
before and after discharge
These observations led us to conclude that with the smaller pores a larger surface of
the active material is available for the electrode reaction during discharge, as depicted in
Figure 4.6.
111
4.2 Formation of Sintered Iron Electrode
4.2.1 Effect of Sulfide Additives on the Formation of Sintered Iron Electrodes
Iron electrodes after been prepared were subjected to repeated charge and discharge
cycles during which the discharge capacities gradually achieved a stable value. This
process is referred to as “formation”. In the presence of sulfide additives (either in the
electrode as iron sulfide or in the electrolyte solution as sodium sulfide) the capacity rise
during formation was rapid. Without sulfide additives, the capacity was low (as in the
first 10 cycles for electrode 1, Figure 4.7). In the absence of sulfide, only very low
discharge currents could be sustained on the electrode. Thus, only a very small amount
of discharge capacity was realized.
As shown in Figure 4.8, when sulfides are present in the electrode or electrolyte,
high values of discharge currents can be sustained as the iron electrode is polarized.
Correspondingly, after the addition of sulfide to the electrolyte for electrode 1 (Figure
4.7), higher discharge capacities were realized. If sulfide were present in the electrode or
in the electrolyte right from the start of the formation cycling, the capacity increase was
rapid from the start, and fewer cycles were needed to achieve the full capacity (electrode
2 in Figure 4.7).
112

Figure 4.7 Formation of sintered iron electrode with 0.5% w/v of sodium sulfide added
after 10 cycles of passivation (electrode 1), and added at the beginning of formation
(electrode 2)

Figure 4.8 The potential-current curves during potentiodynamic polarization of
electrodes with 3.0g/L sodium sulfide additives and without sodium sulfide additives

113
4.2.2 Changes in Active Material Morphology during Formation of Sintered Iron
Electrodes
The change in formation rate with sulfide is also consistent with morphological
changes on the electrode. From Figures 4.9a and 4.9b, it is clear that the electrode
subjected to repeated cycling became covered with finely-divided particles. This
decrease in particle size of the charged phase (Figure 4.9 b) explained how more of the
charged phase was available for discharge. It is this process of sub-division to fine
particles that leads to increase of discharge capacity during formation. These
observations are also consistent with the model for the formation of the iron electrode we
have proposed in previous studies on the pressed-plate iron electrode.
a)

114
b)

Figure 4.9 (a) Sintered iron electrode in the “as-prepared” state (b) Sintered iron
electrode in the charged state following 40 cycles of C/2 charge and C/20 discharge
We also examined the morphology of sintered iron electrode that had passivated in
the absence of sulfide under the conditions of constant current discharge. While the
overall electrode structure was still porous (Figure 4.10 a), the compact morphology
(Figure 4.10 b) suggested no significant formation of the discharged phase. We noticed
small hexagonal plates on the surface that were not present in the charged state (Figure
4.10 b). This type of compact layer of discharge product following passivation is in stark
contrast to the abundance of well-formed crystallites of the discharged product for an
electrode that did not get passivated (Figure 4.2a).

115
a)

b)

Figure 4.10 (a) SEM images of sintered iron electrode after discharge without sulfide.
(b) Surface in Figure 4.10 a at higher magnification
116
During formation of the sintered iron electrode, we also observed that the first
discharge of the electrode usually has a relatively high capacity, and the following
discharge usually has a much lower capacity (Figure 4.11 a,b). The discharge capacity in
the first cycle could be regained and exceeded during formation (Figure 4.11 a). But
usually in electrodes with higher porosity, the electrode did not recover to the high
discharge capacity of the first cycle (Figure 4.11b).
a)

117
b)

Figure 4.11 (a) Discharge capacity of sintered iron electrode during formation (b)
Discharge capacity of high-porosity sintered iron electrode during formation
To understand this phenomenon, we performed scanning electron microscope
characterization of the sintered electrode with high porosity during formation, in the
charged and discharged state. In the discharged state, we observed discharged product
particles with two distinct ranges of sizes, namely 2-5 microns and 10-50 microns
(Figure 4.12a). Upon charge, the electrode did not consist entirely of the charged product;
we observed particles of the discharged product of 10-50 microns in diameter in the
electrode(Figure 4.12b). Thus, the large particles of discharged product of 10-50 microns
in size seen in Figure 4.12 a remained largely unchanged during the charging process.

118
a)

b)

Figure 4.12 SEM pictures of sintered iron electrode with high porosity at (a) discharged
state during formation (b) charged state during formation
119
In a high porosity electrode, because of the large pore volume available for reaction,
extensive conversion of the electrode structure to the discharged product was expected
during the first discharge. Where the sintering necks are narrow, the electrical
conductivity was also expected to be lost due to the formation of non-conducting iron
(II) hydroxide. Such particles of the discharge product that have lost the electrical
connection to the current collector cannot be easily converted to the charged product and
will continue to remain in the discharged state. Consequently, the second discharge
could be expected to yield a lower capacity compared to the first discharge due to a
lower amount of active material and pore volume (Figure 4.11 a,b). Thus, particles of
the discharged product that cannot be returned to the charged state would be be
observable even after charging (Figure 4.12 b). During subsequent cycling of the
electrode, the gradual formation of conductive iron particles in the charged state would
serve to connect up the particles of the discharged product electrically. This process of
re-connection of the electrically isolated particles of the discharged product would lead
to a recovery of discharge capacity during formation (Figure 4.11 a). For electrodes of
higher porosity, where more of the discharged product could become electrically isolated
(Figure 4.12 a and b) fully recovery of the discharge capacity is less likely. Consequently,
the very high capacity of 0.35 Ah/g observed during the first discharge led to lower
discharge capacity in the subsequent formation cycles (Figure 4.11b).
120
4.2.3 Effect of Conducting Carbon Additives on the Rate of Formation of Sintered Iron
Electrodes
From the foregoing analysis of the processes occuring during formation, we
concluded that we could achieve high discharge capacity and also reduce the number
of cycles of formation if we could only keep the particles of discharge product
electrically connected. To this end, we have prepared electrodes with acetylene black as
a conductive additive. Acetylene black has a unique inter-connected structure of
conductive nanoparticles of carbon, which is well-suited for achieving our objective.
With 5 w/w% acetylene black in the electrode, we could achieve stable capacity of 0.25
Ah/g in just 10 cycles of formation (Figure 4.13). The benefits of including conductive
carbon to the iron electrode has also been discussed by Shukla et al, where a grafted
carbon structure was shown to improve the overall electrode utilization.
12

Figure 4.13 Discharge capacities during formation of sintered iron electrode containing 5
wt. % acetylene black
121
4.3 Charging Efficiency of Sintered Iron Electrodes
4.3.1 Achieving High Charging Efficiency on Sintered Carbonyl Iron Electrodes
In our previous studies on the pressed-plate type electrodes, we reported significant
improvements of the charging efficiency of iron electrode by using carbonyl iron as the
active material in conjunction with bismuth-based additives.
15, 18
Compared to 70%
charging efficiency for a commercial iron electrode, these pressed-plate electrodes had a
charging efficiency as high as 96%.
15
The benefits of adding bismuth oxide and bismuth
sulfide as additives therefore have been established for the pressed-plate electrode.
With the sintered iron electrode, the bismuth compounds could not be incorporated
because of the reactivity and volatility of the bismuth compounds at the high
temperatures involved in sintering. However, much to our surprise, the charging
efficiency of sintered iron electrode was as high as 96% even without the bismuth
additives (Figure 4.14). Therefore, the rate of hydrogen evolution during charging of the
sintered iron electrode is low even without the addition of bismuth additives.
122

Figure 4.14 Charging efficiencies of different types of iron electrodes under C/2 charge
and C/20 discharge rates at 100% depth of discharge
4.3.2 Understandings on the Reasons for the High Charging Efficiency on Sintered
Carbonyl Iron Electrodes
To understand the result of high charging efficiency, we compared the electrode
potentials during charging for the pressed-plate electrode (containing bismuth sulfide
and bismuth oxide) and the sintered electrode of the same capacity and geometric area
but without any bismuth additives. The potential of the sintered electrode was 20-60 mV
more positive than that of the pressed-plate electrode during charging (Figure 4.15). The
more positive the electrode potential is during charging, the lower the overpotential
would be for the reactions on the electrode surface, including that for the hydrogen
evolution reaction. Thus, lower overpotential for the hydrogen evolution reaction at the
123
sintered electrode meant less hydrogen production during charging, resulting in an
increase in the charging efficiency.

Figure 4.15 Charging curves of sintered and pressed-plated iron electrodes
To confirm that the reduction in overpotential did explain the increased charging
efficiency on the sintered iron electrode, we conducted detailed polarization studies on a
fully-charged sintered iron electrode. When the electrode is fully charged, the only
reaction that can happen on the iron electrode under cathodic polarization is the
hydrogen evolution reaction. A Tafel plot yielded the exchange current and Tafel slope
for the hydrogen evolution reaction on a sintered iron electrode (Figure 4.16).
With these kinetic parameters for the hydrogen evolution reaction, we were able to
calculate the amount of charge that was directed to the hydrogen evolution reaction at
various potentials during charging. The integrated value of charge diverted for
124
hydrogen evolution reaction was used to calculate the charging efficiency expected at the
electrode. This method yielded a charging efficiency of the sintered iron electrode is
92%, close to the measured charging efficiency of 96%. Therefore, it was clear that the
high charging efficiency of the sintered iron electrode resulted from the lower
overpotential on the sintered electrode.
In addition, the Tafel slope for the hydrogen evolution reaction on sintered iron
electrode is 0.13 V /decade and the normalized exchange current is 1.6*10
-3
A/Ah. We
normalized the exchange current to the capacity as the actual surface area could not be
easily measured, but the capacity is proportional to the active surface area. Comparing
these two parameters with that for the pressed-plate iron electrode modified with
bismuth sulfide that we previously reported, both the Tafel slope and the exchange
current for the sintered iron electrode are slightly lower (Figure 4.16). Therefore, we
may conclude that it is the combined effect of lower Tafel slope, lower exchange current
and lower overpotential on the sintered iron electrode that resulted in the high charging
efficiency.
A plausible reason for the reduced overpotential on the sintered iron electrode could
be the large electrode area provided by the electrically connected particles of the sintered
structure. Thus, for a given charging current, the sintered electrode would be expected to
present a significantly lower current density than the pressed-plate electrode of the same
geometric area. This type of dependence of charging efficiency on the current density
125
may be one of the reasons for the poor charging efficiencies of commercial pocket-plate
electrodes where the electrical resistance is expected to be higher, compared to the
sintered electrode and pressed-plate electrode.

Figure 4.16 Tafel plot for hydrogen evolution reaction on sintered iron electrode
To validate the above hypothesis that the lower overpotential on the sintered iron
electrode arises from its connected structure, we also compared the discharge curves of
the pressed-plate and sintered electrodes (Figure 4.17). It is obvious that when applied
the same current during discharge, the overpotential on the sintered electrode is about 20
mV lower. The lowered overpotential also during discharge is most likely brought by the
connected sintered structure, which is able to lower the current density with the same
discharge current compared to the pressed-plate electrodes.
126

Figure 4.17 Discharging curves of sintered and pressed-plated iron electrodes
4.3.3 Charging Efficiency of Sintered Iron Electrodes at Various Charging Rates
The charging efficiency of the sintered iron electrode stayed above 94% for charging
rates in the range of C/5 to 1C-rate (Figure 4.18). Thus, the charging efficiency could
be maintained over a range of charge rates. While we do not expect to continuously
charge the iron electrode at a rate higher than the 1C-rate for most renewable energy
storage applications, we also found that even at 2C, 3C and 4C the charging efficiency
only reduced to 85%, 81% and 79%, respectively. (Figure 4.18) Compared to the 70%
charging efficiency of the commercial electrodes at C/5, the 96% at C-rate and the 80%
at 4C-rate for our sintered iron electrode establishes a new benchmark for the iron
electrode technology.
127

Figure 4.18 Charging efficiencies of sintered iron electrodes at different charging rates
4.4 Discharge Rate Capability of Sintered Iron Electrode
4.4.1 Discharge Rate Capability of Sintered Iron Electrode with Soluble Sulfide
Additives
In our previous studies, on the pressed-plate iron electrodes we have reported the
ability to discharge at the 3C rate when bismuth sulfide and iron sulfide are added to the
electrode.
15,

18
Such an iron electrode when discharged at 3C-rate showed a utilization
0.2 Ah/g, a remarkable improvement over the commercial iron electrode for no more
than C/5 rate of discharge (Figure 4.19).The sulfide ions prevent passivation of the iron
electrode, allowing such high rates of discharge to be sustained. However, with the
sintered iron electrodes incorporating bismuth sulfide being not feasible (as discussed
here earlier), we studied the effect of sulfide additive of 0.5% w/v of sodium sulfide into
128
the electrolyte. With the sintered electrode, the addition of sulfide to the electrolyte has a
markedly beneficial effect on the discharge rate capability similar to that observed with
the pressed-plate iron electrode (Figure 4.19).

Figure 4.19 Comparison of discharge rate capabilities of commercial iron electrode,
pressed-plate iron electrode, and a sintered iron electrode
4.4.2 Discharge Rate Capability of Sintered Iron Electrode with Iron Sulfide Additive
We found that a sintered electrode could also be prepared with iron (II) sulfide.
The discharge rate capability of such a sintered electrode with iron (II) sulfide was found
comparable to pressed-plate iron electrode containing iron sulfide.
18
A capacity output
greater than 0.2 Ah/g could be realized at the 3C rate (Figure 4.20). Iron sulfide, because
of its extremely low solubility in the electrolyte (FeS ⇌ Fe
2+
+S
2−
K
eq
= 4 × 10
−19
)
provided a reservoir for the slow release of the sulfide ions within the pores of the
electrodes at a concentration of 6.3 E-10 moles/L. Thus, a constant and sufficient supply
129
of sulfide ions needed to prevent passivation was always available at a location where
the discharge process occurred.
20

Figure 4.20 Discharge rate capability of commercial iron electrode and sintered iron
electrode with iron sulfide
130
4.5 Cycle life testing of Sintered Iron Electrodes
4.5.1 Cycle life testing of Sintered Iron Electrode with Soluble Sulfide Additives

Figure 4.21 Discharge capacities of sintered iron electrode with different cycling rates
Following the formation process, the sintered electrode in the sulfide-containing
electrolyte showed a decrease in discharge capacity when cycled at C/2 rate (Figure
4.21). The decrease in capacity was gradual and after 150 cycles, 60% of the capacity
was not recoverable at the C/2 rate. We had also measured the discharge capacity at C/20
rates during the course of the cycling study at junctures marked 1through 4 on Figure
4.21.
131

Figure 4.22 Discharge curves of sintered iron electrode correspond to different points
marked in Figure 4.21
Curves 1 to 4 in Figure 4.22 are the discharge curves at various discharge rates
corresponding to points 1 to 4 during the cycling experiments marked in Figure
4.21. Comparing the discharge curves 1 and 2 (Figure 4.22), both at the end of formation,
we saw the discharge capacity at C/2 (Curve 2) rate is only slightly lower than it was at
C/20 rate (Curve 1). This meant that the electrode had excellent discharge rate capability
and did not passivate easily at C/2 discharge. However after being cycled, the
discharge capacity of the electrode at C/2 rate decreased significantly (Curve 3). The
sloping discharge curve indicated an increasing internal resistance as the discharge
proceeded. But discharging the electrode at C/20 rate even without charging the
electrode after it reached the cut-off potential at curve 3 resulted in almost full recovery
of the capacity (Curve 4), despite its poor capacity at C/2 (Curve 3). This observation
indicated that the capacity decrease during C/2 cycling (Figure 4.21) was not permanent.
132
The sloping discharge curve for curve 3 suggested that the electrode had begun to
passivate at the C/2, reducing the discharge capacity.

Figure 4.23 Concentration of sulfide ions in the electrolyte during cycling of sintered
iron electrode at open-circuit period between charge and discharge.
Since sulfide was identified to be critical to preventing passivation of the iron
electrode, we monitored the concentration of sulfide ions in the electrolyte during
cycling using a sulfide-ion selective electrode during open-circuit between charge and
discharge. The potential readings from the sulfide reference electrode are taken at the
end of the 10 minutes open-circuit period. The concentration of sulfide ions in the
electrolyte was determined from the measured values of potentials on the sulfide
reference electrode according to equation 31,

where E
S
2− is the potential on the sulfide
reference electrode and C
S
2− is the concentration of sulfide ions in the electrolyte.
116

133
E
S
2−= -610.8mV - 28.1 logC
S
2− (31)
The concentration of sulfide at the beginning of formation was 6.4*10E-2 mole/L. After
the capacity decrease at around the 200
th
cycle, the sulfide concentration in the
electrolyte had decreased to a very low value of 10E-17 mole/L. We then restored the
sulfide ion concentration by addition of 6.4*10E-2 mole/L of sodium sulfide to the
electrolyte and we found that the concentration decreased again to 10E-15 mole/L in just
10 cycles.(Figure 4.23) Further, we noted that the concentration of sulfide fluctuated
significantly between charge and discharge. The decrease of sulfide concentration
suggested that sulfide ions were being consumed during cycling. Therefore the high
capacity at C/2 charge and discharge rates could not be obtained when the sulfide
concentration decreased to a very low value. Secondly, the large fluctuation of sulfide
ion concentration in the electrolyte suggested that sulfide ions were incorporated into the
discharge product, and released when iron was formed from iron (II) hydroxide during
charge.
The slow decrease in sulfide ion concentration with cycling was attributed to the
oxidation of sulfide to higher valence sulfur compounds such as sulfite and sulfate at the
nickel hydroxide counter electrode. At the relatively positive electrode potential of the
nickel hydroxide electrode (Eq.32), the oxidation of sulfide would occur readily as per
Equations 33 to 35.
117

134
β-NiOOH + H
2
O + e
-
⇌ β-Ni(OH)
2
+ 2OH
-
E
◦
= 0.49 V (32)
S
2-
⇌ S + 2e
-
E
◦
= -0.508V (33)
2S + 6OH⇌ S
2
O
3
2-
+ 3H
2
O + 4e
-
E
◦
= -0.50V (34)
S
2-
+ 6OH
-
⇌ SO
3
2-
+ 3H
2
O + 6e
-
E
◦
= -0.59 V (35)
We also recognized from these studies that the sulfide ion at a concentration as low
as 10E-15 mole/L was sufficient to prevent de-passivation. This finding was consistent
with our previous studies on electrodes containing iron (II) sulfide. The iron (II) sulfide
is sparingly soluble in the electrolyte, allowing pressed-plate electrodes to be discharged
at rates as high as 3C, and sustain charge and discharge cycling for over 1000 cycles.
18,
20

4.5.2 Cycle life testing of Sintered Iron Electrodes with Iron Sulfide Additives
There are anecdotal claims of commercial nickel-iron batteries that last 3000 cycles
of deep-discharge cycling.
21-24
However, cycling data under controlled conditions of rate
and depth of discharge are not readily available. Also, commercial iron electrodes are
notorious for their low charging efficiencies and poor discharge rate capabilities. In the
present study, we have been able to cycle sintered iron electrodes for over 3500 cycles at
1C-rate charge/discharge at 100% depth of discharge with an average faradaic efficiency
of over 97% and a utilization of ~0.2 Ah/g (Figure 4.24). This extraordinarily long cycle
135
life under deep-discharge conditions, accompanied by a high-charging efficiency and
high discharge rate capability is being reported for the first time.

Figure 4.24 Cycle life experiment of sintered iron electrode at 1C rate charge and
discharge and 100% depth of discharge
4.5.3 Understanding the Cycle Life Performance of the Sintered Iron Electrode
The impressive cycle life performance of sintered iron electrode may be attributed to
the sintered structure. Unlike the pressed-plate electrode and the pocket-plate design, the
active material in the sintered electrode is inter-connected by the solid sintering necks
formed between the iron particles. (Figure 4.25) This type of particle interconnection
leads to a mechanically robust electrode scaffold.
The sintered structure is able to withstand the stresses induced by the volume
changes during charge and discharge. The electrically-conductive scaffold caused by the
136
sintered particles also ensures a more uniform current distribution and a low effective
current density leading to a high charging efficiency.

Figure 4.25 SEM photograph of sintered iron electrode made with carbonyl iron particles
Another inherent factor that endows the iron electrode with long cycle life is the
charge and discharge mechanism. The charge and discharge process occurs by a
dissolution-precipitation process involving two distinct phases namely iron and iron (II)
hydroxide, mediated by a sparingly-soluble intermediate, the ferrite ion, HFeO
2
-
(Eqs.
36 and 37).
21, 22
The solubility of the ferrite ion is about 10
-15
mole/L. During
discharge, iron is oxidized to this sparingly soluble ferrite ion intermediate, HFeO
2
-
at
-0.86 V vs. Hg/HgO. When the concentration of HFeO
2
-

(aq)
increases as discharge
continues, the equilibrium in equation 37 moves towards the right-hand side and the
insoluble iron (II) hydroxide is produced. During charge, the soluble HFeO
2
-

(aq)
is
reduced to elemental iron according to equation 36. As the concentration of the
137
intermediate decreases during the charging process, the equilibrium in equation 37 shifts
towards the left-hand side, favoring further dissolution of the iron (II) hydroxide and
conversion to elemental iron.

Fe + 3OH
-
⇌ HFeO
2
-

(aq)
+ 2e
-
+ H
2
O (36)
HFeO
2
-

(aq)
+ H
2
O ⇌ Fe (OH)
2 +
OH
-
K
sp
= 10
-30
(37)
We have observed the co-existence of the separate phases corresponding to the iron
and iron (II) hydroxide formed during charge and discharge (Figures 4.26a and b). The
scanning electron micrographs show the morphology of sintered iron electrode “as
prepared” and after discharge. The increase in molar volume of the product formed
during discharge leads to filling of the pores in the sintered structure. Two distinctly
identifiable phases corresponding to the iron and iron (II) hydroxide are observed
(Figure 4.26 c and d). The charged phase is very rough with subdivided particles. The
distinct crystals of alpha-iron are seen to grow adjacent to the iron (II) hydroxide that has
a layered particle structure (Figure 4.26 d). The co-existence of the two phases and the
development of one phase from the other is a direct verification of the
dissolution-precipitation mechanism of the iron electrode proposed in previous studies.
21,
22
The sparingly soluble nature of iron (II) hydroxide ensures that the growth of iron
crystals from the ferrite ion occurs adjacent to the iron (II) hydroxide phase. Also, the
sparingly low solubility of the ferrite ion results in preferring nucleation over growth,
leading to small crystallites of iron. Such particle morphology is to be contrasted with
138
the zinc electrode where the zinc oxide and the zinc hydroxide are highly soluble in the
alkaline electrolyte leading to dendritic growth at low current densities.
a) b)

c) d)

Figure 4.26 SEM characterization of: (a) sintered iron electrode at as-prepared state (b)
sintered iron electrode after initial discharge (c) partially charged sintered iron electrode
(d) magnified picture of c
Finally, the use of iron sulfide as an electrode additive ensured that the discharge
capacity is stable over 3500 cycles. The presence of iron (II) sulfide in the electrode
avoided electrode passivation and the rate capability of 1C rate was maintained over
139
thousands of cycles. Thus, the combined effect of the sintered structure and the use of
iron sulfide as electrode additive to achieved the robustness and electrical performance
characteristics of the iron electrode needed for large-scale energy storage applications.
140
Chapter 5. Nickel Hydroxide Electrodes
5.1 Introduction on Nickel Electrodes
The nickel hydroxide electrode is based on the redox process shown in Eq. 38.
NiOOH +H
2
O + e- ⇌ Ni(OH)
2
+ OH- E
o
= +0.49 V (38)
The highly positive value of the standard reduction potential for the nickel electrode is
the basis of using it as a positive electrode of the nickel-cadmium, nickel-metal hydride,
nickel-iron and nickel-hydrogen batteries. While the use of the nickel electrode in
rechargeable batteries has been known for almost a 100 years, substantial improvement
in materials and fabrication techniques have occurred over the years and the state-of-art
nickel electrodes exhibit excellent cycle life, rate capability and efficiency characteristics.

45-66

In this chapter we describe our efforts to make improvements to the state-of-art
nickel electrode by altering the fabrication methods and the formulation of the active
material mix used in the preparation of the electrodes. Our result indicates that nickel
electrodes with a utilization >0.2 Ah/g with deep-discharge cycling over 1400 cycles at
100% depth-of-discharge can be realized. In the earlier chapters we describe the
advances made with various types of iron electrodes. These advances include
improvements to their charging efficiency, discharge rate capability, utilization rate,
formation rate, and cycle life. Our goal is to combine these iron electrodes with an
141
advanced nickel electrode of the type described in this Chapter, to realize a nickel-iron
battery that can meet the performance and durability requirements of large-scale and
distributed energy storage applications.
5.2 Background on the State-of-Art Nickel Battery Electrodes
During the normal discharge of the nickel electrode, -nickel oxyhydroxide is
converted to -nickel (II) hydroxide. (Eq. 38) This transformation involves intercalation
of protons into the layered structure of alternating nickel(II) and oxygen atoms. The
reverse process occurs during charging.
45
Since the standard reduction potential of the
nickel electrode is positive to that of the oxygen evolution reaction,( Eq. 39) the charging
of nickel hydroxide electrode is accompanied by oxygen evolution. .
4OH
-
⇌ O
2
+ 2H
2
O+4e
-
E° =0.41V (39)
The evolution of oxygen reduces the charging efficiency.
45,46,54,57
In addition, during
prolonged overcharge β-NiOOH is converted to γ-NiOOH that is accompanied by a
increase in electrode volume and decrease in discharge capacity of the nickel
electrode.
87,88
To improve the performance of nickel electrode, cobalt (II) hydroxide,
Co(OH)
2
is usually incorporated to the extent of 5% of the mass of the positive electrode
materials.
87,89,90
The addition of cobalt raises the overpotential for the oxygen evolution
reaction and thus improves the charging efficiency. In addition, zinc (II) is added to
prevent the formation of the γ-NiOOH phase.
91

142
Three types of nickel electrode designs are used in rechargeable batteries: pocket
plate, sintered plate and fiber plate. In pocket plate nickel electrodes, perforated nickel
plated steel sheets are made to pockets to hold the nickel hydroxide active material.
24

The pocket also serves as the current collector. In the sintered plate nickel electrode, a
porous sintered structure prepared from carbonyl nickel powder is used as the substrate.
The nickel hydroxide is then loaded into the pores of the sintered structure along with
other additives by a chemical or electrochemical impregnation process.
24
In a fiber-plate
nickel electrode, sintered-nickel fiber or nickel-plated fiber mats are used as the substrate
and the impregnation process is used to load the active material.
90

5.3 Performance of Commercial Sintered Nickel Electrodes
5.3.1 Charge and Discharge Cycling of Commercial Sintered Nickel Electrodes at
Various Rates
Among these various types of electrodes, the sintered plate nickel electrodes usually
have the best discharge rate capability and material utilization because the nickel sinter
substrate combines good electronic conductivity and small pore sizes. Such an electrode
has been widely adopted by the battery industry especially for high-rate applications.
24

However, such electrodes are expensive and have a lower mass specific electrode
capacity because of the amount of nickel used in the electrode. For example, sintered
electrodes from Highstar Co. have a rated specific capacity on an electrode basis of 100
143
mAh/g. (Figure 5.1) However, the ready availability of this type of electrode allowed us
to combine the nickel electrode with our high-performance iron electrodes to quickly
develop a high-performance nickel-iron battery and scale to higher capacities.

Figure 5.1 Commercial sintered nickel electrode from Highstar Corporation. The rated
capacity of the electrode is 2.7Ah. The active area of the electrode without the tab is
140mm*70mm*0.63mm
A commercial sintered nickel electrode of the type shown in Figure 5.1 was used in
our experiments with a rated capacity of 2.7 Ah. The electrode was charged and
discharged at various rates from C/10 to C rate. The discharge capacity of the electrode
decreased by 27% when the cycling rate was increased from C/10 to C. (Figure 5.2)
144

Figure 5.2 Discharge capacities of commercial sintered nickel electrodes at different
cycling rates
As a comparison, the high-performance iron electrodes developed in the present
study (Figure 3.16) had only 6% decrease in discharge capacity when the cycling rate
was increased from C/20 to C. In addition, the faradaic efficiency of the nickel electrode
at C/10, C/5 and C rate cycling were 85%, 78%, and 70% respectively. (Figure 5.3)

Figure 5.3 Faradaic efficiencies of commercial sintered nickel electrodes at different
cycling rates when charged to its rated capacity
145
Again, the iron electrode developed in this study had a high faradaic efficiency of
over 95% even when cycled at C rate. Thus, it became clear that even if the iron
electrode had high performance, the efficiency of the commercial nickel electrode could
limit the performance of the nickel-iron cell. Usually additives such as cobalt hydroxide
are incorporated into the nickel electrode to inhibit oxygen evolution that causes the
inefficiency. With the commercial electrodes it is not clear how much of the additives is
used and how well these are distributed across the surface of the electrode.
5.3.2 Charge and Discharge Cycling of Commercial Sintered Nickel Electrodes at
Reduced Capacity Utilization
The rate of oxygen evolution reaction increases when the potential on the nickel
electrode increases. Consequently, a considerable amount of oxygen is evolved towards
the end of charging. Therefore, we have measured the faradaic efficiency of the
commercial sintered nickel electrode by only charging it to 80% of the rated capacity at
different rates. (Figure 5.4)
The efficiency of commercial nickel electrode increased over 10% at both C/5 and
C/10 cycling rates to 91% and 96%, respectively, when only charged to 80% to the rated
capacity. (Figure 5.4) This increase in faradaic efficiency suggested that less amount of
oxygen evolved on the nickel electrode at a lower states of charge. However, a nickel
electrode that is utilized to just 80% of its capacity will entail an increase in cost and
146
decrease in specific capacity of the nickel-iron battery. The nickel electrode could be
fully utilized by overcharging, but because the undesired γ-NiOOH phase is prone to
form during overcharge. Regular and repeated overcharge of the nickel electrode will
result in an increase in the operational cost of the battery, decrease in cycle life, and loss
of water in the electrolyte.

Figure 5.4 Faradaic efficiencies of commercial sintered nickel electrodes at different
cycling rates when charged to 80% of its rated capacity
We have also performed cycle life test of the commercial sintered nickel electrode
by cycling it at C/10 rate of charge and discharge for over 50 cycles. (Figure 5.5) The
stable capacity observed from the electrode during cycle life testing suggested the
robustness of the structure of the sintered nickel electrode.
147

Figure 5.5 Cycle life testing of commercial sintered nickel electrodes at C/10 cycling
rate and charged to its rated capacity
5.3.3 Specific Capacity of Commercial Sintered Nickel Electrode
The weight of a single commercial sintered nickel electrode we have tested is 27.5
grams, using its capacity from C/5 rate cycling when charged to its rated capacity. The
specific capacity of this electrode is calculated to be 78 Ah/Kg. The low specific
capacity of the sintered nickel electrode is mainly due to the use of the nickel sinter
substrate, which contributes significantly to the weight of sintered nickel electrode. The
iron electrode (described in Figure 4.24) has a specific capacity of 170 Ah/Kg as
calculated from the results of cycle life testing at C-rate for over 3500 cycles. The lower
specific capacity of the commercial sintered nickel electrode is expected to reduce the
specific capacity of the nickel-iron battery significantly.
148
When building a nickel-iron battery using the commercial sintered nickel electrode
and the in-house developed high-performance iron electrode, the performance of the
nickel-iron battery will be greatly limited by the commercial nickel electrode. When
using a typical positive-electrode-limited configuration for nickel-iron batteries, the
charging efficiency and discharge rate capability of the battery will be restricted by the
performance of the nickel electrode. Even in a negative-limited configuration, the low
specific capacity of the commercial nickel electrode will decrease the specific capacity
of the nickel-iron battery and also, increase the cost. Therefore, developing a nickel
electrode with good charging efficiency and discharge rate capability, as well as high
material utilization combined with a lightweight design, is crucial for the development of
a high-performance nickel-iron battery. To this end, the subsequent efforts were focused
on developing a light-weight foam type nickel substrate.
5.4 Nickel-Foam Electrode
5.4.1 Advantage of Foam Based Electrode
Nickel-foam is been used in a variety of batteries as the electrode substrate because
of its light weight, good electronic conductivity, and high porosity (Figure 5.6).
Compared to the nickel sinter substrate, nickel-foam uses less nickel, which makes it
lighter and less expensive. But because the pore size of the nickel-foam is usually in the
range of hundreds of microns, the electrochemical impregnation process is not very
149
suitable to use with nickel-foam. So a pasted-type electrode where a paste of nickel
hydroxide active material and binder is loaded into the pores of nickel-foam is more
feasible.

Figure 5.6 SEM graph of nickel-foam used as electrode substrate for nickel hydroxide
electrode
To suppress oxygen evolution reaction and prevent the formation of γ-NiOOH phase
on the nickel electrode, cobalt, cadmium and zinc compounds are usually co-deposited
with nickel hydroxide during electrochemical impregnation process in sintered nickel
electrodes.
24
A more effective way to improve the performance of nickel electrode is to
coat the nickel hydroxide surface with various additives. For example,
cobalt-encapsulated nickel hydroxide powder is used in nickel metal hydride batteries for
its high charging efficiency. However, nickel metal hydride batteries usually use starved
150
electrolyte configuration where there is only enough electrolyte to wet the electrodes and
separators.
92
In the flooded electrolyte configuration, which is typically used in
nickel-iron batteries, the active material powder could detach and float away from the
electrode into the electrolyte easily, resulting in loss of capacity. Therefore, we have
tried to developed a pasted-plate nickel electrode with nickel-foam as the substrate and
cobalt-encapsulated nickel hydroxide powder as the active material to achieve good
charge and discharge performance and improved electrode specific capacity. This
pasted-plate nickel electrode is made by first mix the cobalt-encapsulated nickel
hydroxide powder with binder solution (Ethyl cellulose in isopropanol) to make an
active material paste. The paste is then spread on to a piece of nickel-foam with a spatula
and the pasted electrode was dried in an oven at 85 ℃.
5.4.2 Charge and Discharge Performances of Nickel-Foam Based Nickel Electrodes
After the nickel-foam electrode was fabricated and assembled into the test cell, it
was charged at C/5 rate to the theoretical capacity based on the amount of active material
in the electrode, and then discharged at C/5 rate to 0 V vs MMO. Unlike the iron
electrode, the nickel electrode did not have to be charged and discharged more than three
times for completion of the formation process. (Figure 5.7) The fast formation of the
nickel electrode is attributed to the charge and discharge mechanism of nickel hydroxide.
During charge and discharge, protons de-intercalate and intercalate, with a continuous
change between nickel (II) hydroxide phase and nickel oxyhydroxide phase. Unlike the
151
formation process of the iron electrode, there is no significant change in the volume of
the active material, subdivision of particles and increase of surface area with increase in
capacity.

Figure 5.7 Discharge capacity of foam based nickel electrode during C/5 formation
cycling when charged to its theoretical capacity right after fabrication
The faradaic efficiency of the nickel-foam electrode was 96% and 81% at C/5 and
C/2 cycling rates, respectively, when charged to its capacity after formation. (Figure 5.8)
Compared to the 78% faradaic efficiency of the commercial sintered nickel electrode
when cycled at C/5 rates, an 18% increase in faradaic efficiency was observed with the
nickel-foam electrode at the same rates. The high faradaic efficiency of the nickel-foam
electrode is attributed to the cobalt encapsulation on the particle surface of nickel
hydroxide, along with other additives such as cadmium and zinc in the active materials
that help suppress the oxygen evolution reaction during charging of the nickel electrode.
152
The utilization of the nickel hydroxide material in the nickel-foam electrode is 0.17
Ah/g. Since the theoretical capacity of nickel hydroxide is 0.29 Ah/g, there is still
room for improvement in the utilization of the active materials. The use of the
polymeric binder could be limiting the utilization of active material in nickel-foam
electrode. Some of the active material particles could be completely coated by the binder,
and thus have poor access to the electrolyte or the current collector. Consequently, these
“electrically-isolated” particles would not be able to participate in the electrode reaction
and contribute to the electrode capacity. Therefore, to increase the specific capacity of
nickel electrode and decrease its cost, it is important to increase the utilization of the
nickel hydroxide active material.

Figure 5.8 Faradaic efficiencies of foam based nickel electrodes at different cycling rates
153
5.4.3 Effect of Conducting Carbon Additives on Nickel-foam Electrodes
Utilization of the nickel hydroxide active materials could be increased by improving
the conductivity of the nickel hydroxide particles and electrical connectivity with the
electrode substrate. Therefore, we have added 5 w/w% of conducting carbon additives
such as multi-walled carbon nanotubes to the active material slurry. The carbon nanotube
particles distributed in the electrode mixture could create more electrically conducting
contacts between nickel hydroxide particles and the nickel-foam substrate, and enable
more active materials to actively participate in the electrode reaction and contribute to
the electrode capacity. With 5 w/w% carbon nanotubes, the utilization of nickel
hydroxide active material in nickel-foam electrode was increased to 0.22 Ah/g (Figure
5.9). We found a 30% increase in the utilization rate of the nickel hydroxide material
compared to the electrode without the conducting additive.
Even though there is an increase in the weight of the electrode associated with the
addition of conducting additives, the high level of material utilization caused the specific
capacity of the electrode to rise to 180 Ah/Kg, from 145 Ah/Kg of the electrode without
the conductive additives. Further, compared to a 78 Ah/Kg specific capacity of the
commercial sintered nickel electrode, the foam based nickel electrode presented almost a
130% improvement. The high specific capacity of the foam based nickel electrode could
enable us to develop a nickel-iron battery with significantly higher specific capacity
compared to the batteries that use commercial sintered electrodes.
154
We also measured the faradaic efficiency of the foam-based nickel electrodes after
the conducting carbon additives were incorporated. (Figure 5.10) At C/10 and C/5
cycling rates, the faradaic efficiencies of the modified nickel-foam electrodes were
above 96%, which was similar to the nickel-foam electrode without conducting additives.
At C/2 cycling rate, the electrode with carbon additives had an efficiency of 93%, which
was a 12% higher than the electrode without conducting additives. Even at C-rate
cycling, the nickel-foam electrode with conductive additives had a faradaic efficiency of
over 88%. The high faradaic efficiency of the modified nickel-foam electrode could be
used to build a nickel-iron battery with much-improved cycling performance compared
to using the commercial sintered nickel electrode.

Figure 5.9 Discharge capacities of foam-based nickel electrode with conducting carbon
additives during C/5 cycling when charged to its therotical capacity
155

Figure 5.10 Faradaic efficiencies of foam based nickel electrodes modified with
conducting carbon additives at different cycling rates
Because the same active materials were been used in the nickel-foam electrodes with
and without conducting carbon additives, the improvements in the faradaic efficiency at
higher cycling rates could be attributed to the improved conductivity of the electrode.
When there are no conducting additives in the electrode, the electric current focuses on
the active material particles in direct or closer contact with the substrate, whereas the
particles that are separated further from the substrate or poorly connected experience
much less current density. Thus, oxygen evolution occurs readily at the area of high
current density. At higher charging rates the differences in current density are
emphasized further and more oxygen is evolved.
When carbon additives are used the conductivity of the active materials and the
electrical connection of the particles to the substrate are improved and the current is
156
distributed uniformly throughout the electrode. Such a uniformly low current density
reduces the overpotential at the surface and leads to lower oxygen evolution rates.
.
Figure 5.11 Charging curves of foam based nickel electrode with and without conducting
carbon additives at C/2 charging rate during faradaic efficiency measurements
During faradaic efficiency measurements, the electrodes were charged to their stable
discharge capacity without any overcharge. In Figure 5.11 we compare the voltage-time
curves during charging at C/2 rate for the two types of nickel electrodes ―with and
without conducting additives. When the state of charge reaches over 50%, the potential
of the electrode without the conducting additive increased more rapidly, and the plateau
potential for oxygen evolution reaction was reached around 80% state of charge. In the
case of the electrode with carbon additives, the plateau for oxygen evolution reaction
was reached around 90% state of charge. Thus, at higher states of charge more current
157
was diverted to the oxygen evolution reaction on the electrode without conducting
additives.
The improved conductivity of foam-based nickel electrode along with its improved
faradaic efficiency also resulted in improvements in the capacity delivered at high rates
of charge and discharge. When the cycling rate was increased from C/10 to C rate, the
discharge capacity of the foam-based electrode with conducting additives decreased only
by 16%, (Figure 5.12) compared to the 27% decrease in the commercial sintered nickel
electrode.

Figure 5.12 Discharge capacities of foam based nickel electrodes modified with
conducting carbon additives at different cycling rates
5.4.4 Effect of Amount of Binder on Nickel-foam Electrodes
When the nickel-foam electrode with 5% binder underwent formation cycling,
active material shedding was observed during overcharge because the nickel hydroxide
158
active material was dislodged from the electrode by the formation of oxygen gas bubbles
leading to a decrease in the discharge capacity of the nickel-foam electrode. This finding
suggested that the electrode was not robust to overcharge, and the discharge capacity
could further decrease during prolonged cycling. The loss of the active material from the
electrode also indicated that the amount of binder in the electrode was not sufficient to
retain the active material particles in the electrode. Therefore, we tested electrode with
increased binder content of 10 and 15 w/w% of the active material.
When the amount of binder was increased in the electrode, we did not observe active
material shedding during overcharge, and the discharge capacities of the electrode
remained stable during formation cycling. However, with higher binder content in the
electrode resulted in a low discharge capacity during the first few cycles and a capacity
value that gradually increased with cycling. The number of cycles needed to reach a
stable capacity also increased with the binder content (from 10 to 15w/w %). (Figure
5.13) This gradual increase in the discharge capacity could be attributed to the poor
accessibility of electrolyte to the active material during initial cycling due to the
hydrophobicity of the polymer binder. As the active materials became wetted by the
electrolyte, a stable discharge capacity could be reached.
159

Figure 5.13 Discharge capacities of foam based nickel electrode with different amounts
of binder during C/5 cycling when charged to its theoretical capacity right after
fabrication
5.4.5 Cycle Life Testing of Nickel-Foam Electrode
We have performed cycle life testing of a foam-based nickel electrode with 5 w/w%
conducting carbon additives and 10 w/w% binder. The electrode was cycled at C rate
with 100% depth of discharge. Even under these severe conditions of cycling, the
electrode showed stable discharge capacity for 1400 cycles with an average faradaic
efficiency of over 91%. (Figure 5.14) This stable discharge capacity under high rate
cycling, along with the high faradaic efficiency, makes the modified foam based nickel
electrode very comparable to the high-performance iron electrode that we have
developed, and could bring the same high performance to the nickel-iron battery.
160

Figure 5.14 Charge and discharge capacity of foam-based nickel electrode with 5 w/w%
conducting carbon additive and 10 w/w% binder during cycle life testing at C-rate
charge and discharge to 100% depth-of-discharge

161
Chapter 6. Nickel-Iron Cell Studies
6.1 Factors Affecting the Performance of Nickel-Iron Cell
6.1.1 The Effect of Limiting Electrode
In any type of cell, there will be a difference between the capacity of the positive
and negative electrodes, and thus the electrode with the lower capacity would determine
the overall capacity of the cell. Therefore, the electrode that limits the capacity of the cell
is referred to as the limiting electrode. Commercial nickel-iron batteries usually have
excess capacities on the negative iron electrode, therefore, are limited by the positive
(nickel) electrode. Even in a cell with equal capacities on each electrode by design, the
different charging efficiencies and discharge rate capabilities of the electrode will cause
the cell to be limited by either the positive or the negative electrode during charge and
discharge. Therefore, we have studied the effects of the limiting electrode on the
performance of the nickel-iron battery when using the high-performance iron and nickel
electrodes that have been developed at USC.
In a positive-limited nickel-iron cell, the chance of the negative iron electrode being
subjected to deep-discharge is very low especially if the negative electrode is sufficiently
oversized and does not have any significant capacity fade. The relatively shallow
discharge on the iron electrode compared to the nickel electrode would help reduce the
formation of the magnetite phase on the iron electrode that occurs at high overpotentials
162
during deep-discharge. Formation of the magnetite phase is to be avoided because it
could result in a decrease in discharge capacity. Thus, limiting the cell capacity with the
positive electrode could help preserve the long cycle life characteristics of the electrodes
at the cell level as well.
In addition, due to the excess capacity of the negative electrode, the iron electrode is
will not reach very high states of charge. Since the rate of the hydrogen evolution
reaction increases with the overpotential and state-of-charge on the iron electrode, we
would prefer not to reach very high states of charge. By designing the cell with a
positive-limited configuration, the negative electrode will evolve less hydrogen during
charging. Avoiding hydrogen formation in the cell is highly desirable as the
recombination of hydrogen with oxygen is a hindered process and requires noble metals
like platinum as catalysts. However, if the positive electrode were capacity limiting, the
oxygen evolved on the nickel electrode can be recombined in-situ on the iron electrode
to form iron (II) hydroxide.
Further, nickel-iron cells limited by the positive electrode could have a lower cost
because the less active material on the positive electrode is being used. The cost of the
positive active material is expected to be about 5 to 10 times the cost of the negative
active material. Thus, the lesser the active material used on the positive electrodes would
lower the overall cost of the cell.
163
In a cell with negative-limited configuration, because of the higher faradaic
efficiency of the iron electrode that we have developed at USC, the cell could benefit
from a higher faradaic efficiency. In addition, because of the higher capacity on the
positive nickel electrode, this electrode is unlikely to be subjected to repeated overcharge
and thus the undesirable γ-NiOOH phase is less likely to form on the nickel electrode.
The formation of γ-NiOOH phase results in capacity fade.
Overall, a positive-limited configuration for the nickel-iron cell seems more
advantageous from the viewpoint of preserving the cycle life of the cell and avoiding
hydrogen generation.
6.1.2 The Effect of the Formation Procedures
The formation is an important step for both nickel and iron electrodes to achieve
high material utilization. During individual electrode tests, the capacities of the electrode
increased during the formation process and then reached a stable value. Falk and Salkind
24
described a formation procedure in three cycles for nickel-iron batteries with pocket
plate electrodes as follows:
Cycle 1: Charge 48 hours at one-half the normal 5-hour rate. Discharge at the 5-hour rate
to 0.9V/cell.
Cycle 2: Charge 10 hours at the 5-hour rate and an additional 14 hour at one-half the
5-hour rate. Discharge at the 5-hour rate to 0.9V/cell.
164
Cycle 3: Charge 15 hours at the 5-hour rate. Discharge at the 5-hour rate for 5.75 hours.
However, because the types of electrodes and active materials used in the
high-performance nickel and iron electrodes that we have developed are different from
the traditional pocket plate electrodes, we were trying to identify the best method to
complete the formation of the nickel-iron battery with such electrodes. A procedure to
achieve formation of both the nickel and iron electrodes within the assembled cell can
minimize the cost.
The charge and discharge rates and the amount of overcharge that were used in the
formation process for individual nickel and iron electrodes were different; for example,
the iron electrode was formed at C/2 charge rate and C/20 discharge rate, with usually 50%
overcharge, and such a procedure formed the electrode in 20-30 cycles. However, the
nickel electrode was formed at C/5 charge and discharge rates, with 50% overcharge and
the electrode completed formation in less than 10 cycles. Such different procedures for
individual electrodes present a challenge for formation of both electrodes as part of the
nickel-iron cell.
One direct approach is to form the electrodes individually in half cells with separate
counter electrodes and then assemble the formed electrodes into a nickel-iron cell. Thus,
separate formation procedures can be used to ensure each electrode is completely formed.
However, forming electrodes separately requires more equipment to perform cycling of
individual electrodes, and will increase the capital cost for manufacturing nickel-iron
165
cells. Also, assembly of cell with the formed electrodes that are already wetted with the
electrolyte could be more challenging than assembling with freshly-fabricated dry
electrodes.
The other approach is to assemble the electrodes into cells after electrode fabrication,
add electrolyte, and then start the formation process. For the high performance nickel
and iron electrodes we have developed, we have to understand the implications of a
common formation procedure. The iron electrode requires a high-rate charging process
to produce smaller iron particles that are essential to achieve increase of electrode
surface area and increase in discharge capacity. And the iron electrode cannot sustain
high discharge rates in the beginning of formation due to the poor distribution of sulfide.
Figure 6.1 shows the capacity of an iron electrode being formed at C/10 charge and
discharge rates with 50% overcharge. Even though the discharge rate is low, the low
charging rate is relatively low to achieve the required increase in surface area. Therefore,
the utilization of iron active material was only about 0.15Ah/g and did not increase
substantially. For the nickel electrode, high-rate charging during initial formation
produces a significant amount of oxygen gas on the electrode and could result in loss of
active material from the electrode, and the formation of the γ-NiOOH phase. Therefore,
it is important to consider the effect of cycling procedures on the each electrode while
determining a common procedure for forming the electrodes in an assembled cell.
166

Figure 6.1 Discharge capacities of iron electrode when cycled at C/10 charge and
discharge rate with 50% overcharge during formation cycling
6.1.3 Cell Sizing and Specific Capacity Estimation Based on Electrode and Component
Properties
We have developed a model to calculate the specific capacity of a nickel-iron cell
based on the specification of the sintered iron electrode and pasted nickel electrode that
we have developed. We have also included other cell components such as the electrolyte,
cell container, connectors, and separators. The model data are shown in Table 5; the
different background colors indicate data on different components of the battery. Data
for the nickel electrode is in pink, the blue color is related to the iron electrode, green
color means the data is for other cell components and yellow color shows the resulting
properties data for the cell.
167
In order to simplify the calculations, there are a few assumptions that were made in
the model. The assumptions are listed as follows:
1. The capacities of the nickel and iron electrodes are equal.
2. The electrodes are stacked with iron electrodes at both ends.
3. The capacities of the electrodes are taken at 100% depth of discharge.
4. The weight of the electrolyte that is needed to wet the electrodes and the separators is
not taken into account.
5. The weight and the volume of the binder and additives to the nickel and iron
electrodes are not taken into account.
6. The weight of the cell hardware (connectors, bolts, o-rings) may be estimated as a
percentage of the total weight of all the electrodes in the cell.
In the model, the red, underlined numbers are the variables, the other numbers are
either constant or are calculated from the variables. The number in bold at the lower
right corner is the resulting specific capacity of the cell.
The specifications of the cell container we used in the calculation are based on a
commercial nickel-cadmium cell container we have purchased. Changing the
specifications of the cell container yields different specific capacities of the cell, which
indicates improvements that could be made to the cell container to build a nickel-iron
cell with higher specific capacity.
168
The height of the nickel electrode could be adjusted between zero and the height of
the cell container. The difference in the height between the electrode and the cell
container will leave space for the electrolyte. The width of the nickel electrode is the
same as the inner width of the cell container, and the thickness of the nickel electrode
was determined by the nickel-foam that was used. The active material utilization of the
nickel electrode is a value that is determined experimentally. Changing the active
material utilization changed the weight of the electrode. In addition, due to the amount of
material loading on the electrode, and the amount of binder and additives used, the
capacity of the nickel electrode could also change with the active material utilization
(Ah/g) been constant. Therefore, the volumetric capacity (Ah/cm
3
) is also taken into
account. The volumetric capacity number is also determined experimentally from the
actual size and capacity of the electrode that has been tested.
The height, width, and capacity of the iron electrode are assumed to be the same as
the nickel electrode. The active material utilization of the iron electrode is a value
determined experimentally. Changing the active material utilization varies the amount of
active material used in the electrode. The electrode porosity is also determined by
experiment. The porosity value, along with the weight of the active material, determined
the thickness of the iron electrode.
The number of electrode pairs is determined from the thickness of the cell container
minus the thickness of an iron electrode, divided by the thickness of one pair of iron and
169
nickel electrodes covered with two layers of separator. The number of electrode pairs,
along with the electrode capacity, determined the capacity of the cell.
The height and width of the separator are the same as the height and width of the
electrode. The thickness and the density of the separator are determined by the type of
separator used. In our case, we have used a polyamide separator from Freudenberg
Corporation.
The height of the electrolyte in the cell is the difference between the height of cell
container and the electrode. The width and thickness are taken from the cell container
along with the height yields the volume of the electrolyte. With the density of the
electrolyte, the weight of the electrolyte is calculated.
The weight of the cell hardware such as connectors, O-rings, and bolts is calculated
as a percentage of the total weight of the electrodes in the cell. We have taken 50% to
calculate the weight of the hardware, and the resulting weight is very close to the weight
of the commercial hardware that we have purchased.
The height, width, thickness and the volume of the battery are given by the outer
dimensions of the container. The weight of the container is estimated from the wall
thickness and density of the polymer material of the container. In our calculation, we
have chosen polypropylene as the container material. The weight of the battery is a sum
of all the components in the battery. With the cell capacity and a nominal voltage of
170
1.2V, the specific capacity of the battery is calculated. The energy density is also
calculated from the capacity and volume of the battery.
171
Table 5 Model to calculate specific capacity of nickel-iron cell based on electrode and
component properties

height
(cm)
width
(cm)
thickness
(cm)
wall
thickness
(cm)
volume
(cm3)
poros
ity
density
(g/cm3)
weight
(g)
utilizat
ion
(Ah/g)
capacity
(Ah/cm3)
capacity
(Ah)
specific
energy
(Wh/kg)
energy
density
(Wh/L)
container
(PP)
15 7 3.3 0.4 346.5 0.946 134.41
nickel foam 14 7 0.15
nickel active
material
0.21 5.00
single nickel
electrode
14 7 0.15 14.7 23.80 0.34 5.00 252.00
nickel mesh 14 7 0.02 1.96 1.54 3.01
iron active
material
14 7 0.05 4.70 55% 16.66 0.3 5.00
single iron
electrode
14 7 0.07 6.66 19.67 5.00 304.93
number of
electrode
pairs
13 13.3
seperator 14 7 0.01 1.24 0.53 0.66
electrolyte 1 7 3.3 29.05 1.3 37.76
connector+ o-
rings+bolts
282.55 50%
battery 15.8 7.8 4.1 505 1040.1 64.97 74.96 154.31
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6.2 Performance of Nickel-Iron Cell
We have built a nickel-iron cell with the high-performance iron and nickel
electrodes that we have developed. In this cell, there are three sintered iron electrodes
and four foam based nickel electrodes with about 1.8 Ah of discharge capacity on each
electrode. The electrodes were formed individually in the half-cell testing configurations
and assembled into the nickel-iron battery. The electrodes were stacked alternately in the
cell with a separator in between. The cell was designed to be limited by the capacity of
the iron electrode to achieve a high faradaic efficiency. Because the capacity of the cell
after assembling was slightly above 5 Ah, we have taken 5Ah as the rated capacity of the
cell for the convenience of calculating the charge and discharge rates.

Figure 6.2 Charge and discharge capacity of 5Ah nickel-iron cell at various charge and
discharge rates
173
The cell was cycled at various charge and discharge rates from C to C/10 rates
(Figure 6.2). At C/10 cycling rate, the cell was able to deliver 4.8 Ah of capacity during
discharge; when the cycling rate rates were increased to C rate, the cell was still able to
deliver a discharge capacity of 4.4Ah, which is only an 8% decrease from the capacity at
C/10 cycling rate. Correspondingly, the faradaic efficiency of this cell only decreased
from 96% at C/10 cycling rates to 91% at C/5 rates and 88% at C rates (Figure 6.3). This
high faradaic efficiency, as well as the good rate capability of the nickel-iron cell, shows
its potential to meet the requirements of a wide range of applications.

Figure 6.3 Faradaic efficiencies of 5Ah nickel-iron cell at various charge and discharge
rates
In another 10Ah nickel-iron cell with nickel electrode limited configuration, we
assembled the cell first and tried to form the electrode afterwards. We chose C/5 charge
174
rate and C/10 discharge rate with 30% overcharge as the formation procedure as a
compromise between the procedures for forming the iron electrode and nickel electrode.
During discharge, the cut-off voltage for the cell was 0V, and the discharge was also
stopped when the potential on the iron electrode reached above -0.85V vs MMO. The
cut-off potential was set on the iron electrode to protect it from forming less reversible
oxide phases at more positive potentials.

Figure 6.4 First 10 cycles of in-cell formation cycling of 10Ah nickel-iron battery at C/5
charge rate and C/10 discharge rate with 30% overcharge
In the first four cycles during formation, the cell capacity increased to 9Ah (Figure
6.4). But upon further cycling, the capacity decreased rapidly to 4 Ah in 6 cycles. After
we examined the cell voltage and the potential of the iron electrode during this capacity
decrease during formation, it was clear that the cut-off potential on the iron electrode
was reached with less and less discharge capacity. This suggested that with the formation
175
procedure we have selected for the cell was not appropriate for the iron electrode. To
help form the iron electrode by increasing its active surface area, we increased the
charging rate to C/2 while kept the discharge rate and the amount of overcharge to the
same level to further form the cell. With the increased charging rate, the discharge
capacity of the cell increased quickly to 9.7 Ah in 16 cycles (Figure 6.5). Therefore,
forming the nickel-iron cell at a higher charging rate and a low discharge rate with 30-50%
of overcharge is suggested for cells with the high-performance iron and nickel
electrodes.

Figure 6.5 Formation cycling of 10Ah nickel-iron battery at C/2 charge rate and C/10
discharge rate with 30% overcharge following the charge and discharge cycling
presented in Figure 6.4

176
Chapter 7. Conclusions
In this thesis, we have shown a high-performance rechargeable iron electrode
formulated with carbonyl iron and bismuth sulfide that is far superior in characteristics
compared to a commercially available iron electrode. We achieved a ten - fold reduction
in hydrogen evolution rate, a high charging-efficiency of 96%, a high discharge capacity
of 0.3 Ah g
-1
, and also a twenty-fold increase in capacity for the two-hour discharge rate.
We have also demonstrated a high-performance iron electrode based on carbonyl iron,
bismuth oxide and iron sulfide additives. These electrodes exhibit a high charging
efficiency of 92% and the electrodes are capable of achieving a specific capacity of
nearly 0.2 Ah/g even at discharge rates as high as 3C. The rate of hydrogen evolution
with bismuth oxide during charging was six fold lower than that of a commercially
available iron electrode. The efficiency and discharge performance were stable with
repeated cycling.
This high charging efficiency of the electrode resulted from the use of high-purity
carbonyl iron and the in situ electrodeposition of elemental bismuth by the
electro-reduction of bismuth sulfide and bismuth oxide. The sulfides added to the
electrolyte and the electrode mitigated electrode passivation to allow the high rates of
discharge to be sustained.
177
In addition to demonstrating an iron electrode of substantially enhanced
performance compared to commercial electrodes, we also present here an improved
understanding of the roles of bismuth and sulfide in enhancing the performance of the
iron electrode. The level of performance demonstrated here is a significant advance for
the design of iron-based batteries for inexpensive and eco-friendly large-scale electrical
energy storage. We have also shown that a viable “pressed-plate” type battery electrode
can be inexpensively fabricated using this new formulation of active materials.
We have identified the role of various factors on the formation of carbonyl iron
electrodes in rechargeable alkaline batteries. Specifically, we have explored the effect of
electrochemically active surface area and the presence of de-passivating agents. The
discharge capacity of freshly-prepared iron electrodes was found to be limited severely
by the low electrochemically active area because of the hydrophobicity of the binder.
The addition of an electrode wetting agent resulted in a significant increase in the
electrochemically accessible active area of the electrode. With a pore-former, the initial
active area and pore volume were substantially increased. Consequently, a high capacity
was reached at the end of formation.
The soluble sulfide additive in the electrolyte worked as a de-passivation agent
during discharge and resulted in high discharge capacities and an increased rate of
formation. The experimental data has been analyzed in terms of a phenomenological
model that successfully predicted the evolution of discharge capacity during formation
178
and the discharge capacity at the end of formation. This model is applicable to electrodes
that are completely wetted by the electrolyte at the beginning of formation. The model
and experimental results can be used to design iron electrodes for achieving a high level
of utilization and a rapid rate of formation.
We have also performed cycle life studies on the pressed-plate iron electrodes. The
carbonyl iron electrode prepared with iron (II) sulfide and bismuth oxide additives did
not show any capacity fade for more than 1200 cycles of charge and discharge at 100%
DoD. However, an iron electrode prepared with bismuth sulfide additive showed about
50% loss in capacity after 150 cycles. This capacity loss was recovered by the addition
of sodium sulfide to the electrolyte, but the slow decay in capacity was found to persist
even after the addition of sulfide ions.
We have confirmed that the capacity fade in the iron electrode was due to the
formation of a poorly reversible magnetite phase on the iron electrode during discharge.
During repeated cycling, the amount of this relatively irreversible magnetite phase in the
electrode increased, resulting in a loss of capacity. Iron sulfides were also no longer
present in the electrode that had suffered capacity fade. The accumulation of magnetite
also results in increase of charging overpotential. We were able to show that re-charging
the magnetite phase to iron is facilitated by the presence of sulfides. Consequently, the
addition of sodium sulfide to the electrolyte of a faded electrode results in capacity
recovery. However, the effect of these sulfide ions is lost in about 200 cycles as the free
179
sulfide in the bulk of the electrolyte is oxidized quite readily at the nickel oxide counter
electrode. The capacity lost could also be recovered by overcharging the electrodes at a
slow rate that allowed the magnetite phase to be converted to iron. However, this process
would have to be repeated periodically to retain electrode performance. This observation
is consistent with literature reports that frequent overcharge improves retention of
electrode capacity. When the sparingly soluble iron (II) sulfide is added to the electrode
in substantial quantities in the range of 5 to 10 weight %, a sustained and slow release of
sulfide is ensured for a long time, and this allows over 1200 cycles to be achieved
without loss of capacity. In the absence of sufficient sulfide, by limiting the depth of
discharge, a capacity reserve of cycleable active material is retained, delaying the
observed onset of capacity fade despite the formation of magnetite in every cycle.
The cycle life studies revealed the critical role of sulfide in retaining capacity output
and discharge rate capability over extended cycling periods. The role of sulfide additives
in the discharge of the iron electrode is two-fold. Firstly, a conductive phase of iron
sulfides prevents the rapid passivation of the iron electrode during discharge. Secondly,
the sulfide ions reduce the overpotential for the conversion of magnetite to metallic iron
during charging. Presence of sulfide ions in the electrode is therefore critical to prevent
the accumulation of the poorly rechargeable magnetite phase on the electrode during
cycling. When the concentration of sulfide-ion generating materials in the electrode
decreases during cycling, the ability to recharge magnetite to metallic iron decreases,
180
resulting in capacity fade. We have been able to prove that such a capacity fade is readily
reversed by the addition of sodium sulfide to the electrolyte. Therefore, the distribution
and concentration of sulfide ions in the electrode is crucial not only as a de-passivation
additive but also for the capacity output of the electrode during long-term cycling.
Sparingly soluble sulfide such as iron (II) sulfide added to the electrode appeared to be
the best expedient in limiting the loss of sulfides during cycling, and ensuring the
longevity of the electrode performance.
We have demonstrated a rechargeable iron battery electrode with over 3500 charge
and discharge cycles at 1C-rate and 100% depth of discharge, with an average columbic
efficiency of over 97%. The exceptional cycle life capabilities of the sintered iron
electrode arose from its sintered structure that provided an interconnected structure that
is robust. Another factor that ensured the longevity of sintered iron electrode is the
underlying two phase dissolution-precipitation charge/ discharge mechanism. With the
charged and discharged phases been two separate phases with low solubility in the
electrolyte, the morphology of the iron electrode was maintained over thousands of
deep-discharge cycles.
The sintered iron electrode was able to achieve the same high charging efficiency (>
96%) observed on pressed-plate iron electrode but without the need for bismuth
additives. The high charging efficiency on the sintered iron electrode resulted from the
lowering of the overpotential for reactions on the electrode resulting from the conducting
181
interconnected sintered structure. Further, the charge efficiency was as high as 80% even
at a charging rate of 4C.
Discharge rate capability of the sintered iron electrode was comparable with that of
the pressed-plate iron electrode that we have previously reported with soluble sulfide and
iron sulfide additives. Although sodium sulfide added to the electrolyte of sintered iron
electrode gave high discharge capacity, a slow decrease in the discharge rate capability
was observed after C/2 charge and discharge rates cycling of the electrode. However, we
have learnt this decrease in discharge rate capability was reversible and was caused by
decrease in sulfide concentration in the electrolyte. However, no decrease in discharge
rate capability was observed in sintered iron electrode containing iron sulfide over 3500
cycles.
We have shown that the sintered electrodes with smaller pores could achieve higher
utilization with the same porosity because of the higher active surface area. Also, we
show that by using thinner electrodes, the utilization of the interior of the electrodes can
be improved. Therefore, we were able to show a path to designing iron electrodes with
high utilization, by combining benefits of porosity, pore size and electrode thickness.
We found that sulfides benefitted the formation of the sintered iron electrode. We
were able to confirm that without sulfides, the iron electrode passivated and only small
particles of discharged phase could be formed. Sulfides helped the iron electrode to
maintain high discharge current and form large discharged particles. We have shown
182
that conducting additives such as acetylene black helped to electrically connect up the
insulating discharged product and ensured that more active material can participate in the
electrode reaction, requiring fewer cycles for formation.

We have demonstrated a pasted nickel hydroxide electrode with nickel-foam substrate
and cobalt encapsulated nickel hydroxide active material that had higher faradaic
efficiency and better rate capability than the commercial sintered nickel electrode that
we have tested. By using the nickel-foam substrate, we were able to achieve almost
double the specific capacity on the electrode level compare to the commercial sintered
nickel electrode. The material utilization of the pasted nickel electrode could be further
improved by the incorporation of conducting carbon additives into the electrode. The
carbon additives could improve the conductivity of the active material mixture, and
increase the conductivity of the active material to the substrate, therefore enable more
active material to participate in the electrode reaction. The faradaic efficiency and rate
capability of the nickel electrode is also improved by the carbon additives because the
improved conductivity of the active material mixture could help lower the current
density on the active material. The higher binder content in the electrode is found to
increase the number of cycles needed for formation of the nickel electrode because of the
hydrophobicity of the binder hinders the accessibility of electrolyte to the active material.
The cycle life test of the pasted nickel hydroxide electrode showed stable discharge
183
capacities with high efficiencies over 1400 cycles. This cycle life performance of the
nickel electrode is promising in developing a nickel-iron battery with the
high-performance iron electrode we have demonstrated.
We have discussed the effect of limiting electrode and the formation process on the
performance of nickel-iron batteries. We have also developed a model to estimate the
specific capacity of the nickel iron cell based on the cell designs and electrode and cell
component properties. We have demonstrated the cycling performances of nickel-iron
cell using the iron and nickel electrodes we have developed, and the effect of formation
procedures on the charge and discharge properties of the cell.

184
Chapter 8. Summary
To summarize, we have developed a high-performance alkaline iron electrode based
on a low-cost pressed-plate fabrication technique. We have studied the charging
efficiency and discharge rate capabilities improvements of the pressed-plate carbonyl
iron electrode with bismuth and sulfide additives. The effect of bismuth additives was
studied by XRD characterization of cycled electrodes, and steady state experiments to
understand the effect of bismuth on hydrogen evolution reaction on iron electrode. The
de-passivation effect of sulfide additives was explained by anodic polarization studies.
The changes in iron electrode during formation process were studied, and various
methods to decrease the number of cycles needed for formation and increase the
electrode capacity at the end of formation were studied. A phenomenological model was
developed to predict the evolution of discharge capacity during formation and the
discharge capacity at the end of formation. Changes in the iron electrode during cycle
life testing were studied, and various methods to improve the cycle life performances of
iron electrode with bismuth sulfide additive were studied. A sintered iron electrode was
also developed for nickel-iron battery. The effect of porosity and pore size on the
material utilization was studied. The charging efficiency of sintered iron electrodes
without bismuth additives was studied and the effect of the sintered structure on
lowering the current density on sintered iron electrode was investigated. The effect of
sulfide additives, sintered structure and the charge/ discharge mechanism of the sintered
185
iron electrode were studied. We have tested a commercial sintered nickel electrode and
observed that its charge and discharge performance were inferior to the iron electrodes
that we have developed. Therefore, we have developed a nickel-foam-based nickel
hydroxide electrode for a high-performance nickel-iron battery. We have studied the
effect of carbon additives and binder content on the performance of nickel-foam-based
nickel electrode. We have discussed the effect of limiting electrode and formation
procedures on nickel-iron battery, and we have developed a model to estimate the
specific capacity of nickel-iron battery based on electrode properties and cell designs.
We have also studied the charge and discharge performance, and the effect of formation
procedures on the nickel-iron battery with the electrodes we have developed.

186
Appendix
Journal publications:
 Aswin K. Manohar, Souradip Malkhandi, Bo Yang, Chenguang Yang, GK Surya
Prakash, and S. R. Narayanan. "A high-performance rechargeable iron electrode for
large-scale battery-based energy storage." Journal of The Electrochemical Society
159, no. 8 (2012): A1209-A1214.
 Aswin K. Manohar, Chenguang Yang, Souradip Malkhandi, Bo Yang, GK Surya
Prakash, and S. R. Narayanan. "Understanding the Factors Affecting the Formation
of Carbonyl Iron Electrodes in Rechargeable Alkaline Iron Batteries." Journal of
The Electrochemical Society 159, no. 12 (2012): A2148-A2155.
 Aswin K. Manohar, Chenguang Yang, Souradip Malkhandi, GK Surya Prakash, and
S. R. Narayanan. "Enhancing the Performance of the Rechargeable Iron Electrode in
Alkaline Batteries with Bismuth Oxide and Iron Sulfide Additives." Journal of The
Electrochemical Society 160, no. 11 (2013): A2078-A2084.
 Aswin K. Manohar, Chenguang Yang, and S. R. Narayanan. "The Role of Sulfide
Additives in Achieving Long Cycle Life Rechargeable Iron Electrodes in Alkaline
Batteries." Journal of The Electrochemical Society 162, no. 9 (2015):
A1864-A1872.
187
 “A High-Performance Sintered Iron Electrode for Rechargeable Alkaline Batteries
to Enable Large-Scale Energy Storage” (Coming)
Conference presentations:
 “Performance of Rechargeable Sintered Iron Electrodes for Large-Scale Energy
Storage”, oral presentation, ECS Conference, May2015, Chicago, Illinois
 “Properties of Nickel-iron Batteries with Advanced Iron Electrodes”, poster
presentation, ECS Conference, May2015, Chicago, Illinois
 “Performance and Properties of Sintered Iron Electrodes for Alkaline Batteries for
Large Scale Energy Storage”, oral presentation, ECS Conference, May2014,
Orlando, Florida
 “Improvements of The Iron Electrode of Iron-Air Battery for Grid Scale Energy
Storage Systems”, poster presentation, ECI Conference on Large-Scale Energy
Storage, June 2013, Newport Beach, California
Patent:
Sri R. Narayan, Aswin K. Manohar, Chenguang Yang, G. K. Surya Prakash, Robert
Aniszfeld, “High Efficiency Nickel-Iron Battery”, US Patent US20150086884 A1,
World Patent WO2015042573A1.

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Abstract (if available)

Abstract Inexpensive, robust and efficient large-scale electrical energy storage systems are vital to the utilization of electricity generated from solar and wind resources. In this regard, the low-cost, robustness, and eco-friendliness of aqueous iron-based rechargeable batteries are particularly attractive and compelling. Commercial nickel-iron batteries are mainly limited by the low charging efficiency and poor discharge rate capability of the iron electrode, and the low specific capacity at the cell level. Therefore, our study has focused on demonstrating critical electrical performance and cycling properties to enable the widespread use of nickel-iron batteries in stationary and distributed energy storage applications. ❧ In electrodes fabricated by a low-cost pressed-plate method, we have demonstrated new chemical formulations of the rechargeable iron battery electrode to achieve a ten-fold reduction in the hydrogen evolution rate, an unprecedented charging efficiency of 96%, a high specific capacity of 0.3 Ah/g, and a capability of being discharged at the 3C rate. We show that modifying high-purity carbonyl iron by in situ electro-deposition of bismuth leads to substantial inhibition of the kinetics of the hydrogen evolution reaction. The in situ formation of conductive iron sulfides mitigates the passivation by iron hydroxide thereby allowing high discharge rates and high specific capacity to be simultaneously achieved. These major performance improvements are crucial to advancing the prospect of a sustainable large-scale energy storage solution based on aqueous iron-based rechargeable batteries. ❧ The formation process of the iron electrode could adversely impact the cost of nickel-iron battery

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Asset Metadata

Creator Yang, Chenguang (author)

Core Title Understanding the factors affecting the performance of iron and nickel electrodes for alkaline nickel-iron batteries

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Publication Date 05/02/2018

Defense Date 03/09/2016

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag battery,energy storage,Iron,large scale,nickel,OAI-PMH Harvest

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Narayan, Sri R. (committee chair), Nutt, Steven R. (committee chair), Shing, Katherine (committee member)

Creator Email chenguay@usc.edu,ycgusc@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-246885

Unique identifier UC11277144

Identifier etd-YangChengu-4396.pdf (filename),usctheses-c40-246885 (legacy record id)

Legacy Identifier etd-YangChengu-4396.pdf

Dmrecord 246885

Document Type Dissertation

Format application/pdf (imt)

Rights Yang, Chenguang

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA