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Studies on iron-chloride redox flow battery for large scale energy storage

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Content i

Studies on Iron-Chloride Redox Flow Battery
for large Scale energy storage
by
Kyu Min Kim

Submitted in partial fulfillment of the requirements
For the degree of Master of Science

Thesis Advisor: Dr. Sri. Narayan

Department of Chemistry
University of Southern California
December 2015
ii

Kyu Min Kim

iii

Acknowledgments

I am indebted to my advisors, Professor Dr. Sri.Narayan for their endless support and
understating my difficult situation. As the first student of him, I have learned not only doing
research in the right way but also thinking positively. He was always with me whenever I have
been struggling.

I am also grateful to Professor Dr. G.K. Surya Prakash for being generous with his time
whenever I need to discuss with him.

I wish to thank all members of the Sri.Narayan ’s group. Especially, I would like to thank Dr.
Aswin K. Manohar for many inspiring discussion and supporting my research. Dr. Bo Yang,
Phong, Lena, Derek, Chenguang, and Dan, they were more than enough helpful lab mates. I
really appreciate it.

Finally, I would like to special thank my family. My mom, daddy, sister, bro-in-law, brother, and
lovely two nieces Brynn and Reina. Without them, I could not endure my whole college years.

I thank the US Army RDECOM CERDEC CP&I and the Loker Hydrocarbon Research Institute
for funding this research.
iv

Table of Contents

Studies on Iron-Chloride Redox Flow Battery for large Scale energy storage

Chapter 1: General Introduction 1
1.1 Grid energy storage system 1
1.2 Important parameters 3
1.2.1. Efficiency and Lifetime 3
1.2.2. Power and Discharge time 4
1.2.3. Cost 5
1.3 Motivation 5
1.4 References 7

Chapter 2: Redox Flow Battery Systems 8
2.1 Iron-Chloride redox flow batteries 8
2.1.1. Principle of the new Iron-Chloride redox flow battery 9
2.1.2. Test system for the Iron-Chloride redox flow battery 10
2.1.3. Electrochemical process in the Iron-Chloride redox flow battery 11
2.1.4. Electrolyte Composition 12
v

2.1.5. Membrane 13
2.1.6. Electrode 14
2.1.7. Construction of the Iron-Chloride redox cell 14
2.2 Advantages of Iron-Chloride redox flow battery 15
2.3 Critical technical issue of Iron-Chloride redox flow battery 17
2.4 References 18

Chapter 3: Experimental Methodology 19
3.1 Half-cell studies 19
3.1.1. Iron plating experiment to study the effect of pH 22
3.1.1.1. Materials and Reagents 22
3.1.1.2. Electrochemical experiments 22
3.1.2. Iron plating experiment by additives 24
3.2 Full cell studies in a flow cell 24
3.2.1. Material and reagents 26
3.2.2. Electrochemical experiments 26
3.3 Measures of cell performance 27
vi

3.3.1. Energy density and Power density 27
3.3.2. Efficiency 28
3.3.2.1. Energy efficiency 28
3.3.2.2. Voltage efficiency 29
3.3.2.3. Coulomb efficiency 29
3.4 References 30

Chapter 4: Iron Plating Electrokinetics by pH control 31
4.1 Background 31
4.2 The features of supporting ligands 32
4.2.1. Structures for iron-ligand complexes 32
4.2.2. Solubility and pKa of supporting ligands 33
4.3 Half Cell results with different ligands 34
4.4 Iron Chloride redox flow battery results 40
4.5 Conclusions 42
4.6 References 43

vii

Chapter 5: Iron plating electrokinetics by additives 44
5.1 Background 44
5.2 Additives effects on Iron plating 44
5.2.1. Plating with Indium Chloride 44
5.2.2. Plating with filtered Bismuth Chloride 45
5.2.3. Plating with Choline Chloride 46
5.3 Half Cell results with different additives 47
5.3.1. Plating with Indium Chloride 47
5.3.2. Plating with filtered Bismuth Chloride 48
5.3.3. Plating with Choline Chloride 49
5.4 Iron Chloride redox flow battery results 50
5.5 Conclusions 53
5.6 References 54

Chapter 6: Conclusions 55

viii

List of Figures

Figure 1.1: The levelized cost of energy (LCOE) of various types of energy sources 2
Figure 1.2: Comparison of the energy storage and power capability of various technologies 3
Figure 1.3: Power and energy densities of various energy-storage systems 5
Figure 2.1: Schematic of the iron-chloride redox flow battery 9
Figure 2.2: The main structure of Iron-Chloride Redox Flow Battery 10
Figure 2.3: Tokuyama anion exchange membrane 14
Figure 2.4: The internal perspective structure of the cell 15
Figure 2.5: Photograph of the assembled cell used in the work 15
Figure 3.1: Half-cell experiment setup 20
Figure 3.2: Working electrode (Glassy carbon) 21
Figure 3.3: Reference electrode (Ag/AgCl) and Counter electrode (Platinum) 21
Figure 3.4: Dissolving solution under argon gas 23
Figure 3.5: Potentiostat and Rotating electrode speed control 24
Figure 3.6: The detailed view of the components of a flow battery 25
ix

Figure 3.7 : Experimental set-up of Iron chloride redox flow battery 26
Figure 4.1: Suggested structures for iron-ligand complexes for 32
a) ascorbic acid, b) citric acid, and c) iminodiacetic acid
Figure 4.2: Charging/discharging curve of half cell 34
Figure 4.3: Charging efficiency with ascorbic acid additive according at various pHs 35
Figure 4.4: schematic of hydrogen evolution in various pHs and iron deposition. 36
Figure 4.5: E Vs. I curve in charging with various pHs (ascorbic acid additive) 37
Figure 4.6: E Vs. I curve in discharging with various pHs (ascorbic acid additive) 37
Figure 4.7: The effect of ferric ion concentration in electrolyte for charging efficiency 38
Figure 4.8: Charging efficiency with citric acid additive according to various pHs 38
Figure 4.9: Charging efficiency with iminodiacetic acid additive according to various pHs 39
Figure 4.10: Discharge capacity of full cell per cycle number with pH adjustment 40
Figure 4.11: Charging efficiency of full cell with three different condition 41
of negative electrolyte
Figure 5.1: Volcano curve for hydrogen evolution 45
Figure 5.2: A possible mechanism of hydrogen evolution reaction with choline chloride 46
Figure 5.3: Charging efficiency with indium chloride additive 47
Figure 5.4: Charging efficiency with filtered bismuth chloride 48
x

Figure 5.5: Charging efficiency with choline chloride additive 49
Figure 5.6: Charging/discharging cycle of iron chloride redox flow battery 51
Figure 5.7: Discharge capacity of full cell per cycle number with additives 51
Figure 5.8: Charging efficiency of full cell with four different conditions 52
1) No additives, No pH adjustment, I
charging
=20mA/cm
2

2) 0.25M Ascorbic acid, pH = 2, I
charging
=20mA/cm
2

3) 0.25M Ascorbic acid, pH = 2, I
charging
=40mA/cm
2

4) 15mM Indium chloride,
0.25M Ascorbic acid, pH = 2, I
charging
=40mA/cm
2

xi

List of Tables

Table 1.1: Comparison of the materials used in various redox flow battery systems 6
Table 2.1: The properties of Tokuyama membrane 13
Table 2.2: Summarize several types of redox flow batteries 16
Table 4.1: Solubility and pKa of the selected supporting ligands 33

xii

Abstract

Iron Chloride redox flow battery has been introduced as large scale energy storage
battery which uses cost effective, abundant, and non-toxic materials. This study focused on
finding optimal composition of electrolyte and changing experimental conditions for achieving
high faradaic efficiency for Iron chloride redox flow battery.
Two methods for solving parasitic reaction, hydrogen evolution, at the negative electrode
were investigated: pH control and increasing overpotential by surface inhibition of hydrogen
evolution. The charging efficiency from half-cell experiments showed an increase from 26.5% to
93.95% with 0.25M ascorbic acid and pH controlled with ammonium hydroxide at a value of 2.
Also, by increasing overpotential of electrode surface by Indium chloride additive, the charging
efficiency in the half-cell experiment could reach 96%.
The full redox flow battery cell was cycled with different compositions of the negative
electrolyte. The full cell showed stable cycleability. The charging efficiency of the full cell was
improved significantly from 40% to 91.5% when the electrolyte was at pH 2 in the presence of
ascorbic acid and indium chloride additives.

1

Chapter 1: General Introduction
1.1 Grid Energy Storage System
The awareness on the energy policy from all over the world has been raised with the
concern of the exhaustion of the energy crisis owing to the rising of the international oil price
and fossil energy. The interest in renewable energy is also because of the climate change issues
associated with the increased levels of carbon dioxide. Therefore, new renewable energy supplies
like wind power and solar power generation which is the core of the low carbon green growth are
rapidly expanded all over the world. However, this type of energy generation requires large-scale
energy storage systems to deal with the intermittency of generation. The large grid energy
storage system is required for commercialization by using renewable energy including the solar
and wind power generation. Since wind and solar power are controlled by natural systems, it
must be used in conjunction with an energy storage device in order to meet the human demands.
Grid energy storage system can also produce revenues by storing energy when it is
inexpensive and delivering it when the cost of utility derived power is expensive. When the
demand for energy is low energy can be stored. Then the accumulated energy can be used when
the demand for energy is large and the cost of energy is high. Therefore, economic benefit can
be derived from matching the supply with the demand, currently not possible without energy
storage.
There are various methods for solving the challenge of large-scale energy storage (Figure
1.2). These include, pumped-hydro storage, high-speed flywheels, various types of rechargeable
batteries, compressed-air storage etc. Pumped-hydro based energy storage is relatively
inexpensive, but has disadvantages of being highly dependent on the geography near the energy
2

storage site. Conventional recharageable batteries are also widely studied as energy storage
system. However, the specific energy storage (Wh/kg) in such conventional batteries is still
inadequate for meeting the targets for large grid energy storage system.

Figure.1.1: The levelized cost of energy (LCOE) of various types of energy sources [1]
Renewable energy source needs high levelized cost of energy (LCOE), which shown in Figure
1.1. The levelized cost of energy (LCOE) represents the present value of the total cost of
building and operating a power generating plant over an assumed financial life and duty cycle [1]
Further, in these conventional batteries, the power and energy are coupled. Unlike typical
batteries, a flow battery allows the battery’s power (the rate of electricity flow) to be decoupled
from the battery’s capacity (the total amount of energy held based on the size of reservoir). As a
result, people are free to adjust the battery’s specifications to their specific needs.
3

Figure 1.2: Comparison of the energy storage and power capability of various technologies [2]

1.2 Important parameters
In this section, we will discuss some of the important parameters for choosing a battery
for grid-scale applications. We cannot say that there is a single battery is most suitable. The
battery must be chosen based on how well the requirements are satisfied. Such requirements are
based on the power output needed, how long it must last, and how much efficiency is required.
1.2.1. Efficiency and Lifetime
The efficiency and lifetime can be affected by the operating conditions of the battery.
Batteries vary considerably in their characteristics. For examples there are batteries that
can work in high temperature environment, while others rapidly degrade under such
conditions. Also, we cannot say that only high efficiency and long lifetime of battery is
4

the best. For example, the supercapacitor has high efficiency and long lifetime but it
cannot be used for grid energy storage system because it has a low specific energy
characteristic. Also, the efficiency can be affected by the current density and charge and
discharge characteristics. Therefore, the conditions under which the battery will be used,
largely determine the candidate battery.
1.2.2. Power and Discharge time
The power capability and discharge rate characteristics of various energy storage systems
are shown in Figure 1.2. The power requirement for a battery can vary from a few watts
(W) to a thousand Megawatts (MW). Also, batteries may be required to deliver power
only for a few seconds sometimes. For example, the UPS (Uninterruptible Power Supply)
application is highly suited for supercapacitors, flywheels, and superconductive coils.
These three types of storage are able to deliver high power in few seconds but only for a
short while for a few minutes. Also these batteries can be cycled consciously with high
quality power. For storing large amounts of energy, pumped hydro batteries and
compressed-air storage system are suitable for delivering megawatts of power. However,
as mentioned above, those kinds of system require special geological formations.
Therefore, discharging time and power is an important requirement for selecting the
battery system.
5

Figure 1.3: Power and energy densities of various energy-storage systems [3]
1.2.3. Cost
Another important consideration to select the battery is the price. For example, a lithium
battery is the most widely used in the electric vehicle battery system but it is expensive
for large-scale energy storage.
1.3 Motivation
A redox flow battery is a type of rechargeable battery where rechargeability is given by
two dissolved chemical components in electrolytes. The liquid energy sources can create
electricity and has ability to be recharged in the same system. The main advantage of flow
batteries is that they can be recharged by replacing the electrolyte liquid in outside reservoirs.
Consequently, the redox flow battery has been attractive for grid-scale scale energy storage
system because the electrolyte tanks can be scale up for energy storage capacity while the battery
stack can be sized for meeting the power requirements. [8]. Several research groups are currently
studying a variety of redox flow battery systems. All-vanadium redox flow battery (all-VRFB)
6

[4-5], bromine-polysulfide [6], iron-chromium [7], and zinc-bromine redox flow battery [8] have
been actively studied. Of these systems, the all-vanadium redox flow battery has reached an
advanced stage of development and is now a commercially available system. However,
vanadium which is main material in the all –vanadium redox flow battery is expensive and toxic
material. Also, the energy density is limited by the solubility of vanadium.
In this thesis, we introduce Iron Chloride redox flow battery system as large scale energy
storage battery. This study is motivated by the early research of Hurska and Savinell. [9] They
report round trip current efficiencies of 90% and energy efficiencies of 50% for an all-iron flow
battery operated at 60 °C. However, the most significant losses occurred at the negative electrode
due to slow plating/stripping kinetics with hydrogen evolution and the associated pH change in
the electrolyte. The objective of this study is to solve the technical problems with this this
technology and improve its performance. This battery system is very attractive because it uses
iron and chloride that are cost effective, abundant, and non-toxic materials. Therefore, this type
of battery can be competitive compared to other redox flow battery systems discussed above.
Material Cost, $/Kg Reserves, Million tons Toxicity
Iron 0.2 100,000 as iron ore None
Lead 2.2 95 High
Vanadium 27 38 High
Chromium 10 1.8 High
Chloride 0.3 100,000 as NaCl None
Bromine 0.60 15,000 as NaBr High

Table 1.1: Comparison of the materials used in various redox flow battery systems

7

1.4 References
1. http://www.safremaenergy.com/strategy/commercial/lcoe/#!prettyPhoto
2. H. Ibrahim, A. Ilinca and J. Perron, Renewable & Sustainable Energy Reviews, 12, 1221
(2008).
3. http://knowledge.electrochem.org/encycl/art-b03-flow-batt.htm
4. M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A.G. Fane, M.A. Green, “New all-
vanadium redox flow fell”, Journal of The Electrochemical Society 133 (1986) 1057-
1058.
5. M. Skyllas-Kazacos, M. Rychick, R.G. Robins, Patent: US 4,786,567 All-vanadium
redox battery, (1988), USA.
6. R.J. Remick, PGP. Ang, “Electrically rechargeable anionically active reduction-oxidation
electrical storage-supply system”, USA Patent 4,485,154 (1984).
7. L.H. Thaller, “Redox flow cell energy storage systems”, Department of Energy,
Washington, DC, DOE/NASA/1002-79/3; National Aeronautics and Space
Administration: Washington, DC, NASA TM-79143 (1979).
8. K.J. Cathro, K. Cedzynska, D.C. Constable, “Preparation and performance of plastic-
bonded-carbon bromine electrodes”, Journal of Power Sources 19 (1987) 337-356.
9. L. W. Hruska and R. F. Savinell, Journal of the Electrochemical Society, 128, 18 (1981).

8

Chapter 2: Redox Flow Battery Systems
In the previous chapter, we provided a general introduction to batteries for the grid-scale
energy storage system and some important considerations. After this, we described the
motivation for pursuing the Iron-Chloride redox flow battery system, based on the advantages of
this system as a new technology of large scale energy storage applications. In the following, we
describe the technical aspects of the iron-chloride battery in greater detail.

2.1. Iron-Chloride Redox Flow Batteries
Hruska and Savinell proposed an all-iron flow battery in 1981[1] as a possibility for
redox flow battery applications. The all-iron chemistry utilizes a single element iron in three
oxidation states (Fe
0
, Fe
2+
, and Fe
3+
) for both the negative electrolyte and positive electrolyte.
The performance of an all-iron flow battery as a function of electrolyte composition, cell
membrane, and operating temperature has been studied. They report round trip current
efficiencies of 90% and energy efficiencies of 50% for an all-iron flow battery operated at 60 °C.
However, when the battery was charged and discharged, the largest voltage loss occurred at the
iron plating electrode (or negative electrode) and also a large voltage drop due to the electrolyte.
Also, hydrogen evolution resulted in lower Coulombic efficiency and corrosion of the plated iron.
Although this first version of the all-iron flow battery system had several technical issues
given the overall advantages of the system, this technology is still meaningful as large scale
energy storage. So, we have designed a new configuration of iron chloride battery that
overcomes some of the disadvantages.
9

2.1.1. Principle of the New Iron-Chloride Redox Flow Battery

Figure 2.1: Schematic of the iron-chloride redox flow battery
A schematic of a new iron chloride redox flow battery is provided in Figure 2.1. There
are 3 parts in the cell; positive electrode, negative electrode, and an anion exchange
membrane. At the positive electrode, ferric/ferrous redox reaction occurs. The standard
reduction potential for this reaction is 0.77 volt. At the negative electrode, ferrous/iron
redox reaction occurs. The standard reduction potential for this negative electrode
reaction is -0.44 volt. These values of standard reduction potential allow us to predict a
cell voltage of 1.21 V.
During discharge from 100% of state of charge, ferric chloride pumped from the
electrolyte reservoir to the positive electrode is converted to ferrous chloride. At the same
time, elemental iron is converted to ferrous chloride at the negative electrode. In order to
avoid mixing the positive and negative electrolytes, an anion-conducting membrane that
can selectively transport only chloride anion, is used.

10

2.1.2. Test System for the Iron-Chloride Redox Flow Battery
The design of the test system for the iron-Chloride redox flow battery is shown in Figure
2.2. The positive electrolyte and negative electrolyte are contained in separate reservoirs.
. As current collector, it must have a high electrical conductivity and reliable cycling in
acidic conditions, titanium current collector was adopted. An extended electrode
structure made of reticulated vitreous carbon was used because of its desirable properties
of stability, conductivity and porosity. Ag/AgCl reference electrodes are used on both
the positive and negative sides of the cell. Two pumps were used to flow the electrolytes
to both sides of the cell. It was possible to change the flow rate of the electrolytes by
altering the pump speed.

Figure 2.2: The main structure of Iron-Chloride Redox Flow Battery

11

2.1.3. Electrochemical Processes in the Iron Chloride Redox Flow Battery
The overall cell reaction in the iron-chloride redox flow battery is the conversion of iron
and iron (III) chloride to iron (II) chloride. The reactions at the positive and negative
electrode are shown below.
( )

(Eq. 2.1)
( )

(Eq. 2.2)

(Eq. 2.3)
During the discharge, electrons are transferred from the negative electrode to the positive
electrode (Fig. 2.1); the ferric ion at the positive electrode accepts an electron from the
generated from the oxidation of iron to the ferrous ion at the negative electrode. So the
net reaction is the conversion of iron and ferric ion to the ferrous ion. The flow of
electrons is reversed when the battery is charging; during charge iron is deposited at the
negative electrode and ferrous ion is oxidized to the ferric ion at the positive electrode.
However, besides the above reactions in the iron chloride redox battery several other
parasitic reactions that occur in the cell. These parasitic reactions are listed below.
1) Hydrogen evolution occurs during charging :
H
+
+ e
-
 ½ H
2

= 0.0 V (Eq. 2.4)
Reversible potential for hydrogen evolution is more positive to that of iron
deposition
2) Oxidation of iron (II) to iron (III) by oxygen from air
2Fe
2+
+ ½ O
2
+ 2H+ + H
2
0  2Fe
3+
(Eq. 2.5)
12

3) Precipitation of Iron (III) and iron (II) hydroxides by hydrolysis :
Fe
2+
+ 2H
2
0  Fe(OH)
2
(s) + 2H
+
(Eq. 2.6)

Fe
3+
+3 H
2
0  Fe(OH)
3
(s) + 3H
+
(Eq. 2.7)

The electrolyte composition and the operating conditions are chosen to suppress these
parasitic reactions.

2.1.4. Electrolyte Composition

While the electroactive component of the electrolyte in the iron-chloride redox flow
battery is just the iron (II) chloride, iron (III) chloride and iron (0), there are several
additives used to achieve optimal performance:
1. Ammonium Chloride is used for supporting electrolyte in order to improve
conductivity.
2. Ammonium hydroxide and hydrochloric acid are used to adjust the pH of the
electrolyte.
3. Additives are used to maintain pH and prevent the precipitation of iron (II) at
pH 2-3. Such additives include citric acid and ascorbic acid.
4. Additives used to suppress hydrogen evolution include indium chloride and
choline chloride.

13

2.1.5. Membrane
An anion- exchange membrane is needed to separate the two electrode compartments of
the cell. Previous work has used porous membranes [1], however with such an
arrangement will permit cross diffusion of ions leading to loss of energy efficiency. To
prevent cation permeation from one side of the cell to another, an anion exchange
membrane was used. Such a membrane was chosen to allow for the transport of chloride
ions during the charge and discharge processes. Specifically, a membrane type A201
obtained from Tokuyama Corporation was used (Figure 2.4). The exact chemical
composition of the Tokuyama membrane is proprietary. However, the membrane had a
thickness of 28 micrometers and an ionic conductivity of 27mS /cm in 1 M chloride at 20
o
C.

1) Two probe method for in plane conductivity measurement at 23 ℃, 90%RH under N2 atmosphere, at OH- form
Table 2.1: The properties of Tokuyama membrane [2]
14

Figure 2.3: Tokuyama anion exchange membrane [2]
2.1.6. Electrode
The electrodes used in the iron-chloride redox flow battery consisted of a macro-porous
carbon material called as RVC (Reticulated Vitreous Carbon). RVC is chemically stable
in acidic electrolytes and operates across a wide range of electrode potentials with
minimal hydrogen evolution. Being macroporous, the RVC electrode allows for
transport of liquids through the electrode structure. In addition, reticulated vitreous
carbon can be cut into sheets of required size. RVC has adequate electrical conductivity
and is suitable for use at the positive and negative electrodes of the cell. RVC is available
at a reasonable price from BASi. Inc. With such properties, RVC is suitable for a wide
range of applications both in research laboratories and in industry. [3]
2.1.7. Construction of the Iron-Chloride Redox cell
In Figure 2.5, we present a photograph showing the internal structure of the cell used in
this study. Also, in the Figure 2.6, the assembled cell is shown.
15

Figure 2.4: The internal perspective structure of the cell

Figure 2.5: Photograph of the assembled cell used in the work
The specific explanations of each part in the cell will describe in the next chapter.

2.2 Advantages of Iron Chloride Redox Flow Battery
The advantages of the iron chloride redox flow batteries are mentioned briefly in
previous chapter. In this section, several differences and advantages between the iron chloride
redox flow battery and the other types of redox flow batteries is discussed in detail.
16

Reaction E
cell
0
Electrolyte
Anode/Cathode
characteristics
All
Vanadium
(-) :

(+) :

1.26V H
2
SO
4
/H
2
SO
4

1) Energy density is limited by
the solubility of vanadium.
2) Expensive
materials($13.29/Kg)
3) Strong sulfuric acid
electrolytes are highly
corrosive
Iron-
Chromium
(-) :

(+) :

1.18V HCl / HCl
1) chromium redox couple has
significantly slower kinetics
and requires electrocatalysts
2) low output voltage and
efficiency
Bromine
-Polysulfide
(-) :

(+) :

1.5V NaS
2
/ NaBr
1)highly soluble materials
2)easily mixing of the
electrolytes, which can lead to
precipitation of sulfur species
and the formation of H
2
S and
Br
2
.

Table 2.2: Summarize several types of redox flow batteries [4]
Compared with several types of redox flow batteries which describe in table 2.1, the iron
chloride redox flow battery has several advantages.
 Iron Chloride redox flow battery uses non-toxic materials. There are no aggressive chemical
reagents inside of the both electrolytes. Therefore, the new technology of Iron Chloride redox
flow battery is eco-friendly.
 Since Iron is the fourth abundant material in the earth, the cost of Iron is low. Also, there are
abundant quantities of Chloride from sea salt (NaCl). Compared with other systems, their
abundance is superior.

17

2.3 Critical technical issue of Iron Chloride Redox Flow Battery
While the iron chloride redox flow battery presents many advantages, the major issue
limiting performance is the low charging efficiency. The low charging efficiency results from
the evolution of hydrogen at the negative electrode. Since the standard reduction potential of
hydrogen evolution is positive to that of the charging reaction of ferrous to metallic iron,
hydrogen is evolved readily during charging. Typical charging efficiencies can be as low as 50%
depending on the pH. The low charging efficiency lowers energy efficiency and utilization of the
active materials. Further, the low charging efficiency causes the pH of electrolyte to rise, leading
to precipitation of ferrous hydroxide in the negative electrolyte impairing rechargeability. The
charge imbalance of the negative and positive electrode also causes a capacity drift at the
positive electrode leading to overcharge and oxygen evolution. Unless the charging efficiency is
nearly 100% the iron chloride battery is not a viable energy storage system. Consequently, the
focus of this work is to raise the charging efficiency of the negative electrode. We explore
specific strategies to improve the performance of the iron chloride battery.

18

2.4 References
1. L. W. Hruska and R. F. Savinell, Journal of the Electrochemical Society, 128, 18 (1981).
2. http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/amfc_050811_fukuta.pdf
3. J.M. Friedrich,C. Ponce-de-Le_on, G.W. Reade, F.C. Walsh, Journal of eleoanalytical
Chemistry, 561 (2004) 203–217
4. Adam Z. Weber, Matthew M. Mench, Jeremy P. Meyers, Philip N. Ross, Jeffrey T.
Gostick , Qinghua Liu , J Appl Electrochem (2011) 411,137–1164

19

Chapter 3: Experimental Methodology
The previous chapters introduced some general ideas on redox flow batteries and also the
specific advantages and technical challenges of the iron chloride redox flow battery. As pointed
out earlier, one technical issue is the low charging efficiency due to the relative ease of hydrogen
evolution on the negative electrode during charging of the battery. In this chapter, we describe
the experimental methodology in half-cell and full-cell configurations used for studying the
performance and addressing the technical issues of the iron chloride battery.

3.1 Half-cell Studies.
The goal of the half-cell studies is to measure the faradaic efficiency during the
electrodeposition of iron. This measurement is carried out by depositing iron at various current
densities and then determining the charge associated with the anodic stripping of the deposited
iron. The ratio of the amount of charge needed for dissolution vs. deposition is reported as the
faradaic efficiency. These measurements were made: (1) as a function of electrolyte pH, (2) in
the presence of additives the complex the ferrous ion, and (3) with inhibitors of hydrogen
evolution. In Figure 3.1, we show the half-cell set up used for making these measurements.
20

Figure 3.1: Half-cell experiment setup
The electrodeposition of iron is carried out on a glassy carbon rotating disk electrode.
(RDE). The Rotating Disk Electrode (RDE) is a conductive disk (e.g. glassy carbon,
noble metal) embedded in an inert non-conductive polymer and connected to an electric
motor that allows for controlled rotation of the electrode at various speeds (Figure
3.5). By using high enough rotation rate a high steady rate of convective mass transport
can be achieved to the surface of this electrode [1]. In this study, the RDE was the
working electrode. A Teflon wrapping in the boundary between the working electrode
and the rotating shaft protected from corrosion by the electrolyte solution. A silver-silver
chloride electrode was used as the reference electrode (Figure 3.3) and a platinum
electrode was used as the counter electrode (Figure 3.3).
21

Figure 3.2: Working electrode (Glassy carbon)

Figure 3.3: Reference electrode (Ag/AgCl) and Counter electrode (Platinum)
22

In a typical experiment, the cell is filled with the electrolyte solution containing iron (II)
chloride containing the additives of interest. An argon flow is maintained over the cell to
prevent the oxidation of iron (II) to iron (III). The glassy carbon RDE (Pine Instruments, Grove
City, PA, area = 0.196 cm
2
) was polished with 1μm and then 0.05μm alumina on polishing pads
(Buehler, Lake Bluff, IL), then sonicated in D.I. water for five minutes. The RDE is then rotated
and subjected to a constant current deposition followed by anodic stripping. The deposition is
controlled by charge, where the anodic stripping was controlled by a voltage cut off. To ensure
complete stripping of the surface the electrode was held at 1.0 V vs. the reference for a few
seconds prior to start of a new experiment.
3.1.1. Iron plating experiment to study the effect of pH
3.1.1.1. Materials and Reagents
All chemicals were of reagent grade. Ferric chloride hydrate was obtained from J.T.
Baker (Center Valley, PA). Ammonium chloride, ascorbic acid, citric acid, succinic acid,
and iminodiacetic acid were obtained from Sigma- Aldrich (St. Louis, MO).
Hydrochloric acid and Ammonium hydroxide were obtained from Fisher Scientific
(Waltham, MA).
3.1.1.2. Electrochemical experiments
Blank electrolytes were prepared with 3M FeCl
2
∙4 H
2
O and 2M NH
4
Cl unless otherwise
noted. The pH was adjusted by using concentrated solutions of either HCl or aqueous
ammonia. The additional volume from adjusting the pH was accounted for, and
concentrations were adjusted appropriately. Chelating agents such as, Ascorbic acid,
Citric acid, Succinic acid, and Iminodiacetic acid was added to the blank electrolyte at a
23

concentration of 0.25 M. Dissolving the ferrous chloride solutions must be carried out
under argon gas flowing before the solution is transferred to the cell (Figure 3.4). In this
manner, iron (III) formation is avoided. Iron (III) reduces the faradaic efficiency for iron
deposition.

Figure 3.4: Dissolving solution under argon gas
Electrochemical measurements were carried out using a potentiostat (VersaSTAT MC,
Princeton Applied Research) as shown in Figure 3.5. In all the half-cell experiments the
rotation rate was held at 2500 rpm using an electrode speed control instrument (Pine
Instruments).Deposition studies were performed at different current density values in the
range of 10mA/cm
2
to 1A/cm
2
. Anodic stripping experiments were done with the same
current density at 60mA/cm
2
. Electrochemical impedance spectroscopy measurements
were done from 100 kHz to 0.1 Hz with an alternating voltage excitation of ± 2 mV.
24

Figure 3.5: Potentiostat and Rotating electrode speed control
3.1.2. Iron plating experiment by additives
3.1.2.1. Materials and Reagents
All chemicals were of reagent grade. Ferric chloride hydrate ( J.T. Baker, Center Valley,
PA), indium Chloride and choline chloride were obtained from Sigma- Aldrich (St. Louis,
MO). Hydrochloric acid and Ammonium hydroxide were obtained from Fisher Scientific
(Waltham, MA).
3.1.2.2. Electrochemical experiments
All experimental procedures used in the deposition and anodic stripping experiments
were the same as described above, except that the ferrous chloride solution containing
additives in the range of 100 to 1000 ppm was used.

3.2 Full Cell Studies in a Flow Cell
An iron-chloride flow cell for conducting full cell studies was fabricated in house.
Titanium electrodes were used as current collectors. These electrodes were electroplated with
25

gold to increase surface electronic conductivity. The cell housing was fabricated using
polypropylene. Polypropylene is inert to the strongly acidic electrolyte used in the cell. Two
separate orifices on either compartments of the cell were used for placing reference electrodes.
Threaded inlet and outlet ports were provided on each side of the flow cell, for connecting tubing
and circulating the electrolyte. A reticulated vitreous carbon porous electrode was placed on the
top of gold plated titanium electrodes. The pin-like structures on the gold plated titanium
electrode made good contact with the reticulated carbon structure. An anion-exchange
Membrane separated the two sides of the cell. Gaskets made of silicone were used to seal the two
compartments at the edge of the membrane. At least eight bolts were used to fasten the two parts
of the cell and the membrane. After assembly the flow cell was connected using fluoroethylene
polymer tubing through pumps to reservoirs that contained the ferrous chloride solutions.
(Figure 3.6 and Figure 3.7)

Figure 3.6: The detailed view of the components of a flow battery
26

Figure 3.7 : Experimental set-up of Iron chloride redox flow battery
3.2.1. Material and reagents
Materials and reagents used in the half cell tests were also used for the full cell tests. The
anion exchange membrane was procured from Tokuyama Corporation, (Tokuyama,
Japan, A201 (OH) Lot # A-0033). They were procured in thicknesses of 1 mil and 10
mils.
3.2.2. Electrochemical experiments
For the negative electrode side, the experiments were performed at a flow rate of 0.8 L /
min. For positive electrode side, the experiments were performed at maximum flow rate
with 8V of power in the pump which was in the range of 1 to 2 liters/min. The cell
resistance was measured using Electrochemical Impedance Spectroscopy (EIS), from 100
kHz to 0.1 Hz. The high-frequency impedance was used to estimate the cell resistance.
Cycling experiments were conducted to validate their stability and efficiency of the iron
27

chloride redox flow battery. Typical cycling conditions were 500mA of discharge and
500 mA of charge.
3.3 Measures of Cell Performance
In this section, we discuss three important parameters for the evaluation the performance
of the iron-chloride redox flow battery: energy density, power density, and efficiency.
3.3.1. Energy density and Power density
Energy Density is the amount of energy stored per unit volume. The energy density in
determined by the mass of the active materials and their energy content. In the
conventional solid battery system, the active materials are contained in the electrode
structure. However, in case of iron-chloride Redox Flow Battery, the active material is
dissolved in the electrolyte. Consequently, energy density is defined in terms of the cell
voltage and the charge content. The charge content depends on the concentration, and the
volume of the tank. Thus, energy content is given by,

( ) (Eq. 3.1)
Where Q
c
is the charge content in Coulombs, and U
cell
is the cell voltage during operation.
The charge content is given in terms of the concentration C
Fe
, volume of tank V
tank
,
Avogadro number NA and “e” the number of electrons transferred in the reaction.

( ) (Eq. 3.2)
28

Therefore, the energy density it is only necessary divided the

by the volume
of the tank

in order to deduce the formula of the energy density (

). The
equation is following:

( ⁄) (Eq. 3.3)
Power density can be divided the number of hours in which the cell is discharged.

( ⁄) (Eq. 3.4)

3.3.2. Efficiency
The efficiency is an important parameter for determining the cost of electricity stored.
There are three kinds of efficiency measures: energy efficiency, voltage efficiency, and
coulomb efficiency.
3.3.2.1. Energy Efficiency
Energy efficiency (

) is defined as the ratio of the integrals of the energy
recovered during discharging and the energy used for charging. The equation is following:

∫

( )
∫

( )
(Eq. 3.5)

: Power delivered during the discharge

: Power during the charging

29

3.3.2.2. Voltage Efficiency
The voltage efficiency of the cell at any point during the discharge is given by the ratio of
the discharge voltage to the theoretical cell voltage.
Voltage Efficiency =

(Eq. 3.6)

: Voltage during the discharge

: The theoretical cell voltage
3.3.2.3. Coulomb Efficiency
This efficiency shows the ratio of the charge withdrawn during discharge to the amount
of charge used during the charging process.

∫

( )

( )
(Eq.3.7)

: Amount of charge withdrawn during the discharge

: Amount of charge supplied during the charge

30

3.4 References
1. Allen J. Bard, Larry R. Faulkner, Electrochemical Methods: Fundamentals and
Applications, 2000
2. J. Nikolic , E. Expósito , J. Iniesta , J. González-Garcia and V. Montiel , J. Chem.
Educ., 2000, 77 (9), 1191

31

Chapter 4: Iron Plating Electrokinetics by pH control
4.1 Background
The main problem of low efficiency in the Iron Chloride redox flow battery system was
pointed out as hydrogen evolution during deposition of iron. [1] This problem can be solved by
reducing the concentration of protons in the electrolyte, which is equivalent to raising the pH of
the electrolyte.
pH = -log[H
+
]
Raising the pH of the electrolyte will shift the equilibrium potential of hydrogen evolution to
more negative potential according to Nernst equation: If pH of the electrolyte is raised from zero
to one, the equilibrium potential of hydrogen evolution shifts from zero to -0.059 V. As a result
of the reduced concentration of the protons the mass-transport limited current will decrease and
the contribution of the hydrogen evolution current to the net current during charging will also
decrease. As a result, we will have a higher charging efficiency. [2]
However, once we raise the pH of the electrolyte, the precipitation of the ferric ion as
iron(III) hydroxide, Fe(OH)
3
and ferrous ion as iron(II) hydroxide, Fe(OH)
2
will occur. The
following equations can explain this phenomenon.
Fe
2+
+ ¼ O
2
+ 20H
-
+ ½ H
2
0  Fe(0H)
3 (s)

Fe
2+
+ H
2
0  Fe(OH)
2

(s)
+ 2H
+
To prevent precipitation of the ferric ion as Fe(OH)
3
, complexing ligands were investigated. The
addition of complexing ligands to the electrolyte helps to increase the solubility of the ferric ions
and stabilize ferrous/ferric redox kinetics at higher pHs. [3] The complexing ligands must meet
32

several criteria. Iron- ligand complexing agents must have high solubility at the pH value of 2 or
greater because we need at least pH 2 in order to minimize hydrogen evolution in the negative
electrolyte. Also, the ligand itself must be electrochemically inert in the potential window of
operation of the iron chloride redox flow battery.
In this chapter, three different supporting agents were used; Ascorbic acid, Citric acid,
and Iminodiacetic acid.

4.2 The features of supporting ligands
In the following section we describe the feature of the supporting ligands.
4.2.1. Structures for iron-ligand complexes
a. b. c.
Figure 4.1: Suggested structures for iron-ligand complexes for a) ascorbic acid, b) citric
acid, and c) iminodiacetic acid

33

4.2.2. Solubility and pKa of supporting ligands

Ascorbic acid
Citric acid

Iminodiacetic acid
Solubility 33 g/100 mL (20 °C) 147.76 g/100 mL (20 °C) 2.9g/100 mL (20°C)
pKa
1
4.10 3.13 1.873

Table 4.1: Solubility and pKa of the selected supporting ligands

For selecting the supporting ligands we must considered their solubility and pKa. We
have selected ascorbic acid, citric acid, and iminodiacetic acid, and used them at a
concentration of 0.25M in 3M of iron chloride. The main effect of the ligands is
maintaining pH of electrolyte so that the pKa should be around 2-3 so that these ligands
can also play the role of a buffer and maintain the pH.

34

4.3 Half Cell Results with different ligands

Figure 4.2: Charging/discharging curve of half cell
The Figure 4.2 explains methodology of measuring charging efficiency. The efficiency
was measured by determining the number of coulombs need to anodically strip the iron from the
electrode and dividing this value by the total number of coulombs used for deposition of the iron.
We used 3M ferrous chloride tetrahydrate and 2M of ammonium chloride as the basic electrolyte
along with the various other additives. The electrode in the half cell was charged at various
values of current density for plating in the range of 10mA/cm
2
to 1A/cm
2
and anodic stripping
was held always at a current density of 60mA/cm
2
.

( )

-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0 0.03 0.06 0.09 0.12 0.15
E Vs Ag/AgCl (V)
Coulomb (Q)
𝑭𝒆
𝟐
𝟐 𝒆
→ 𝑭𝒆
𝑭𝒆 → 𝑭𝒆
𝟐
𝟐 𝒆

35

Figure 4.3: Charging efficiency with ascorbic acid additive according at various pHs
Results in Figure 4.3 suggest that the charging efficiency is affected by pH of the
electrolyte and addition of ascorbic acid as supporting agent. If we compare the results of the
electrolyte with no additive at pH 0.2 with the case of 0.25M in ascorbic acid in the electrolyte at
the same pH, there was significant increasing in charging efficiency simply by adding ascorbic
acid. It can be concluded that ascorbic acid is effectively complexed with ferric or ferrous ion
and does not precipitate. Further, we could see that charging efficiency is increasing as the pH of
electrolyte is increasing. The highest charging efficiency, 93.95 %, can be found in the
electrolyte condition which contains 3M iron chloride tetrahydrate, 2M ammonium chloride, and
0.25M ascorbic acid adjusted to pH 2 (by adding ammonium hydroxide).

20
30
40
50
60
70
80
90
100
0 200 400 600 800 1000
Charging Effieicney (%)
Curremt Density (mA/cm2)
No additive
pH 0,2
pH 1
pH 2
pH 3
36

Figure 4.4: schematic of hydrogen evolution in various pHs and iron deposition.
In Figure 4.3, we could also see the effect of the deposition current density on charging
efficiency. At the low current density, the charging efficiency was found to be lower than at the
higher current density region. This can be explained by the results in Figure 4.4. At small current
density (y-axis), hydrogen evolution is more favorable than iron deposition. Since the charging
efficiency is calculated by total current of iron deposition divided by the sum of current of iron
deposition and hydrogen evolution, the current of hydrogen evolution directly affects charging
efficiency. As long as the pH of the electrolyte is higher, the charging efficiency can be increased
by decreasing the current of hydrogen evolution.

-1 -0.8 -0.6 -0.4 -0.2 0 0.2
potential (V)
pH 0
pH 1
Iron
Current Density (mA/cm
2
)
37

Figure 4.5: E Vs. I curve in charging with various pHs (ascorbic acid additive)

Figure 4.6: E Vs. I curve in discharging with various pHs (ascorbic acid additive)
Figure 4.5 and Figure 4.6 illustrates that the overpotential of the electrode is increasing
with increasing current density and with decreasing pH of the electrolyte.
-2.7
-2.5
-2.3
-2.1
-1.9
-1.7
-1.5
-1.3
-1.1
-0.9
-0.7
0 0.2 0.4 0.6 0.8 1
E vs Ag/AgCl (V)
Current Density (A/cm
2
)
pH 0.2
pH 1.0
pH 2.0
pH 3.0
-0.7
-0.65
-0.6
-0.55
-0.5
-0.45
-0.4
0 0.2 0.4 0.6 0.8 1
E vs Ag/AgCl (V)
Current Denstiy ( A/cm
2
)
pH 0.2
pH 1.0
pH 2.0
pH 3.0
38

Figure 4.7: The effect of ferric ion concentration in electrolyte for charging efficiency
As we discussed before, we need to reduce ferric ion as much as we can in order to avoid
precipitation that affects the charging efficiency. In order to see how much ferric ion affects
charging efficiency, ferric chloride was added to the electrolyte bath, which is maintaining pH2,
from 6.62ppm to 3312ppm.

Figure 4.8: Charging efficiency with citric acid additive according to various pHs
82
84
86
88
90
92
94
0 50 100 150 200 250 300
Charging Efficiency (%)
Current Density ( mA/ cm2)
No FeCl3
6.62ppm
33.12ppm
66.24ppm
3312ppm
15
25
35
45
55
65
75
85
95
0 200 400 600 800 1000
Charging Efficiency (%)
Current Density (mA/ cm2)
No additive
pH 0.2
pH 1
pH 2
pH 3
39

With citric acid as the supporting additive, the charging efficiency cannot be increased
without pH adjustment. Dramatic increase in charging efficiency can be found when the pH was
adjusted to 3. This means that the citric acid itself cannot be act as good buffer agent like
ascorbic acid does. The highest charging efficiency, 93.74 %, can be found in the electrolyte
condition which contains 3M iron chloride tetrahydrate, 2M ammonium chloride, and 0.25M
citric acid with pH adjusted to 3 with ammonium hydroxide.

Figure 4.9: Charging efficiency with iminodiacetic acid additive according to various pHs
With iminodiacetic acid supporting additive, the charging efficiency could increase by
simply by adding the supporting agent itself just as in the case of ascorbic acid did. However, in
this experiment, the effect of increasing pH in the electrolyte was even more significant
compared to ascorbic acid additive experiment (Figure 4.3). The highest charging efficiency,
94.03%, can be found in the electrolyte condition which contains 3M iron chloride tetrahydrate,
2M ammonium chloride, and 0.25M iminodiacetic acid with pH adjusted to 3 with ammonium
hydroxide.
20
30
40
50
60
70
80
90
100
0 200 400 600 800 1000
Charging Effieicney (%)
Current Density (mA/cm2)
No additive
pH 0.2
pH 1
pH 2
pH 3
40

4.4 Iron Chloride Redox Flow Battery Results
The assembled full cell studies were carried out in order to see the performance of the
iron chloride redox flow battery with the various electrolytes. In this experiment, we used
ascorbic acid as supporting additive in negative electrolyte because the half-cell results showed
that the highest charging efficiency was found with ascorbic acid in pH 2 adjustment. Although
the highest charging efficiency with iminodiacetic acid was higher than the highest charging
efficiency with ascorbic acid, the charging efficiency with iminodiacetic acid was found in pH 3
adjustment, which leads up to an unfavorable condition where precipitation could readily occur
ruining the cycleability of the full cell.

Figure 4.10: Discharge capacity of full cell per cycle number with pH adjustment
0
50
100
150
200
250
300
0 1 2 3 4 5 6 7 8 9 10
Discharge Capacity (mAh)
Cycles
41

Discharge capacity was stable for 10 cycles of charge and discharge. This result
demonstrates that the iron deposition electrolyte is suitable for operation in a rechargeable
energy storage system.

Figure 4.11: Charging efficiency of full cell with three different condition of negative electrolyte
The full redox flow battery cell was cycled with different compositions of the negative
electrolyte. The charging efficiency of the cell was improved significantly from 40% to 76%
when we used pH 2 adjusted with ascorbic acid additive. Also, when we use high current density,
the charging efficiency of the full cell was further improved, which was in agreement with the
results of the negative electrode half-cell studies. A charging efficiency value of 85% was
observed in the full cell at a current density of 40mA/cm
2
.

40
76
85
0
10
20
30
40
50
60
70
80
90
100
1 2 3
Charging Efficiency (%)
No additives
No pH adjustment
I
charging
=20mA/cm
2
0.25M Ascorbic acid
pH = 2
I
charging
=20mA/cm
2

0.25M Ascorbic acid
pH = 2
I
charging
=40mA/cm
2

42

4.5 Conclusions
It has been shown that the charging efficiency of iron plating half-cell studies was
improved with ligand additives such as ascorbic acid, citric acid, and iminodiacetic acid. The
ligand additives can suppress hydrogen evolution effectively allowing the pH of the electrolyte
to be increased without precipitation. Ascorbic acid additive itself could affect charging
efficiency of half-cell experiment. The charging efficiency was increased from 26.5% to 89.7%
at 150mA/cm
2
of current density. The charging efficiency was even increased further with pH
adjustment with ammonium hydroxide and could reach up to 93.95 % in the electrolyte condition
which contains 3M iron chloride tetrahydrate, 2M ammonium chloride, and 0.25M ascorbic acid
with pH adjusted to 2. From half-cell studies, the electrolyte with ascorbic acid at pH 2 was
selected as the negative electrolyte in full cell studies. The assembled full cell shows very stable
cycleability and similar trend of charging efficiency as the half-cell studies.

43

4.6 References
1. L. W. Hruska and R. F. Savinell, Journal of the Electrochemical Society, 128, 18 (1981)
2. F. Hilbert, Y. Miyoshi, G. Eichkorn and W. J. Lorenz, Journal of the Electrochemical Society,
118, 1927 (1971)
3. J. G. Ibanez, C. S. Choi and R. S. Becker, Journal of the Electrochemical Society, 134, 3083
(1987)

44

Chapter 5: Iron Plating Electrokinetics by Additives
5.1 Background
In the previous chapter, we have discussed about how we can control the pH of negative
electrolyte and maintain during cycling. Although ascorbic acid was shown as a good buffer
agent, the hydrogen evolution could not be prevented completely. In this chapter, we will discuss
about a different way to suppress hydrogen evolution. We can create the iron electrode surface
where hydrogen overpotential is high, which means that hydrogen evolution can be inhibited on
this iron electrode surface. By adding several additives which increase overpotential of hydrogen
evolution on the surface, we can suppress hydrogen evolution even more effectively. In this
chapter, we used Indium chloride, choline chloride, and bismuth chloride as additives. The
amount of additives is all different according to their solubility in acid media.

5.2 Additives effects on Iron plating
5.2.1. Plating with Indium Chloride
In order to increase overpotential of hydrogen evolution on the iron electrode, we need to
adsorb a thin layer of the additive with a high overpotential of hydrogen evolution.
Figure 5.1 shows that indium has one of the poorest kinetics for hydrogen evolution
reaction. Since the standard reduction potential of indium is -0.33 V, the indium plating
reaction precedes the iron plating reaction. By plating a thin layer of indium on the
electrode, the parasitic hydrogen evolution reaction can be inhibited. Another reason to
select indium chloride is that it is soluble in acid media. Since we need a little bit of
45

indium deposition during charging, only a small amount of indium chloride was used, 2
mM and 15 mM of indium chloride.

Figure 5.1: Volcano curve for hydrogen evolution [1]
5.2.2. Plating with filtered Bismuth Chloride
Another candidate material with a high overpotential of hydrogen evolution is bismuth.
Since bismuth is eco-friendly material, it seems better candidate rather than indium.
However, bismuth chloride is barely soluble in acid media. Although it seems not soluble,
we only need a little of bismuth soluble electrolyte in order to suppress parasitic
hydrogen evolution because most hydrogen suppression was controlled by pH adjustment.
Therefore, filtered bismuth chloride additive electrolyte was used as a second additive
material.
46

5.2.3. Plating with Choline Chloride
A study by B. Rezaei et al., reports that tetrabutylammonium hydrogen sulfate and
dibutyl ammonium hydrogen sulfate can suppress the hydrogen evolution in batteries. [2,
3] A study by Wei Zhu et al., reports the effect of suppressing hydrogen evolution by
choline chloride which is a simple quaternary amine as a helper catalyst. [4]

Figure 5.2: A possible mechanism of hydrogen evolution reaction with choline chloride [4]
Figure 5.2 suggests a possible mechanism of hydrogen evolution reaction with choline
chloride. By adding small amount of choline cations which is enough to create a thin
layer on the electrode in the electrolyte, the thin layer of choline cations brings up the
surface become positive charged. The positive charged layer decreases the surface
concentration of protons and sterically hinders the adsorption of protons. Therefore, the
overpotential for hydrogen evolution increases. Choline chloride is also eco-friendly
material because it is commonly used as a food additive for livestock feed and also as a
47

dietary supplement for humans. It is low price because it is a waste product of soybean
oil production. [4]

5.3 Half Cell Results with different additives
5.3.1. Plating with Indium Chloride

Figure 5.3: Charging efficiency with indium chloride additive.
Figure 5.3 demonstrates that indium chloride additive electrolyte gave a higher charging
efficiency compared to no additive electrolyte. As we have discussed before, adjusting
pH 2 with ammonium hydroxide and adding ascorbic acid as buffer agent could suppress
hydrogen evolution effectively but the charging efficiency with the condition could not
reach over 95%. However, with 15 mM indium chloride electrolyte, the charging
efficiency could reach 96% which demonstrates the effect of indium chloride additive for
increasing overpotential for hydrogen evolution. The highest charging efficiency was
82
84
86
88
90
92
94
96
98
0 50 100 150 200 250 300
Charging Efficinecy
Current Density (mA/cm2)
No Indium Chloride
2mM InCl3
15mM InCl3
48

found in the electrolyte condition which contains 3M iron chloride tetrahydrate, 2M
ammonium chloride, 0.25M ascorbic acid at a pH value of 2 (adjusted with ammonium
hydroxide), and 15mM indium chloride additive at charging current density of
150mA/cm
2
.
5.3.2. Plating with filtered Bismuth Chloride

Figure 5.4: Charging efficiency with filtered bismuth chloride.
Filtered bismuth chloride electrolyte shows an improvement in charging efficiency at all
the charging current density values compared to the electrolyte without bismuth chloride.
Although bismuth chloride is hardly soluble in acid media, the data demonstrates that
filtered bismuth chloride electrolyte contains enough amount bismuth to increase the
overpotential for hydrogen evolution on the electrode surface. However, the exact
amount of bismuth chloride in the solution could not be measured. The highest charging
efficiency, 96.62%, was found in the filtered electrolyte which contains 3M iron chloride
82
84
86
88
90
92
94
96
98
0 50 100 150 200 250 300
Charging Efficiency (%)
Current Density (mA/ cm2)
No Bismuth
Filtered Bistmuth
49

tetrahydrate, 2M ammonium chloride, 0.25M ascorbic acid with pH 2 control with
ammonium hydroxide, and 2mM bismuth chloride additive at a charging current density
of 100mA/cm
2
.

5.3.3. Plating with Choline Chloride

Figure 5.5: Charging efficiency with choline chloride additive.
Figure 5.5 demonstrates that choline chloride additive electrolyte gave higher charging
efficiency compared to the electrolyte with no additive. However, compared to the
previous two experiments with metal additives, the improvement in charging efficiency
with choline chloride additive was not as significant as with indium chloride and the
filtered bismuth chloride electrolyte. Despite the result with choline chloride additive
seems not significantly effective when looking at the figures, the experiment carries
weight with in terms of totally eco-friendly organic material and low price material. The
highest charging efficiency, 93.94%, was found in the electrolyte composition of 3M iron
84
86
88
90
92
94
96
0 50 100 150 200 250 300
Charging Efficiency (%)
Current Density (mA/ cm2)
No Choline chloride
Choline Chloride
50

chloride tetrahydrate, 2M ammonium chloride, 0.25M ascorbic acid with pH 2 control
with ammonium hydroxide, and 0.5M choline chloride additive at a charging current
density of 50mA/cm
2
.

5.4 Iron Chloride Redox Flow Battery Results
Full cell studies were carried out in order to see the difference in the performance of the
iron chloride redox flow battery with and without the hydrogen evolution inhibitor additives. In
this experiment, we used indium chloride to increase the overpotential of hydrogen evolution in
negative electrolyte because the half-cell result demonstrated suppression of hydrogen evolution
even in the pH 2 adjusted solution and has good solubility in acid media. Although the highest
charging efficiency with filtered bismuth chloride electrolyte was higher than the highest
charging efficiency with indium chloride additive, we were not able to measure the exact amount
of bismuth chloride in the solution.

51

Figure 5.6: Charging/discharging cycle of iron chloride redox flow battery

Figure 5.7: Discharge capacity of full cell per cycle number with additives
Figure 5.6 and Figure 5.7 demonstrate stability of the assembled full cell in 5 cycles. It
shows the ability to operate cell as a rechargeable storage system.
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0 5 10 15 20
E Vs Ag/AgCl (V)
Time (Hour)
0
0.5
1
1.5
2
2.5
0 1 2 3 4 5
Discharge Capacity ( Ah)
Cycles
52

Figure 5.8: Charging efficiency of full cell with four different conditions
1) No additives, No pH adjustment, I
charging
=20mA/cm
2

2) 0.25M Ascorbic acid, pH = 2, I
charging
=20mA/cm
2

3) 0.25M Ascorbic acid, pH = 2, I
charging
=40mA/cm
2

4) 15mM Indium chloride,
0.25M Ascorbic acid, pH = 2, I
charging
=40mA/cm
2

The full redox flow battery cell was cycled with four different compositions of the
negative electrolyte. As we have discussed in the previous chapter, pH adjustment improved
significantly charging efficiency. With additional indium chloride additive, the charging
efficiency could reach 91.5 %, which means the iron chloride redox flow battery system can
overcome the technical issues and can be competitive as a large grid energy storage system.

0
10
20
30
40
50
60
70
80
90
100
1 2 3 4
Charging Efficinecy (%)
40
76
85
91.5
53

5.5 Conclusions
It has been shown that the charging efficiency of iron plating half-cell studies was
improved further with the additives which increase overpotential of hydrogen evolution on the
electrode surface. In this chapter, indium chloride, bismuth chloride, and choline chloride
additives were tested. By inhibiting hydrogen evolution reaction at the electrode surface, the
charging efficiency of half-cell experiment increased, compared to the charging efficiency
without additives. The results with indium chloride and bismuth chloride as additives suggest
that these additives can indeed suppress hydrogen evolution effectively even with a little amount
added to the electrolyte. Although the result of choline chloride additive does not seem to be
very effective, it has significance as an eco-friendly organic material at available at a low cost.
Despite the remarkable effect of pH adjustment on suppressing hydrogen evolution, there still
residual evolution of hydrogen. By increasing the ovepotential for hydrogen evolution with
additives, we can have even higher charging efficiency. The assembled full cell shows very
stable cycleability. By achieving a charging efficiency of full cell to 91.5%, the iron chloride
redox flow battery system could be considered as a good candidate for large energy grid storage
system.

54

5.6 References
1. Paola Quaino, Fernanda Juarez, Elizabeth Santos, and Wolfgang Schmickler, Beilstein J.
Nanotechnol. 2014, 5, 846–854
2. B. Rezaei, S. Mallakpour, M. Taki, Application of ionic liquids as an electrolyte
additive on the electrochemical behavior of lead acid battery, Journal of Power Sources 187
(2009) 605
3. B. Rezaei, M. Taki, Effects of tetrabutylammonium hydrogen sulfate as an electrolyte
additive on the electrochemical behavior of lead acid battery, Journal of Solid State
Electrochemistry 12 (2008) 1663
4. Wei Zhu, Brian A. Rosena, Amin Salehi-Khojin, Richard I. Masel, Electrochimica Acta 96
(2013) 18– 22

55

Chapter 6: Conclusions

Iron chloride redox flow battery system is recommended here as a candidate system for
large scale energy storage system. Since iron and chloride are cost-effective, abundant, and non-
toxic materials, the iron chloride redox flow battery system has competiveness compared with
other redox flow battery systems. However, iron chloride redox flow battery system has the
technical issue of parasitic hydrogen evolution at the negative electrode. Two methods for
solving the technical issue at the negative electrode were investigated: pH control and increasing
overpotential by surface inhibition of hydrogen evolution.
In order to achieve a higher value of pH in the range of 2 to 3, pH adjustment was carried
out with ammonium hydroxide in the presence of complexing ligands that help to increase the
solubility of the ferric ions and stabilize ferrous/ferric redox kinetics at higher pHs. The
complexing ligands also maintained the pH of the electrolyte by acting as buffers. Ascorbic acid,
citric acid, and iminodiacetic acid were used as ligand additives. The charging efficiency from
half-cell experiments showed an increase from 26.5% to 93.95% with 0.25M ascorbic acid and
pH controlled at a value of 2.
Although the pH control method has a significant effect on suppressing hydrogen
evolution, the remaining hydrogen evolution restricts us from reaching higher charging
efficiency. Therefore, indium chloride, bismuth chloride, and choline chloride additives that
were increasing the overpotential of hydrogen evolution on the electrode surface was tested. The
charging efficiency in the half-cell experiment could reach 96% with indium chloride additive at
a pH value of 2.
56

The full redox flow battery cell was cycled with different compositions of the negative
electrolyte. The full cell showed stable cycleability. The charging efficiency of the full cell was
improved significantly from 40% to 91.5% when the electrolyte was at pH 2 in the presence of
ascorbic acid and indium chloride additives. With these high values of charging efficiency
suggest one of the major technical issues of the iron chloride redox flow battery system has been
mitigated and the system can become competitive to other large grid energy storage systems.

Abstract (if available)

Abstract Iron Chloride redox flow battery has been introduced as large scale energy storage battery which uses cost effective, abundant, and non-toxic materials. This study focused on finding optimal composition of electrolyte and changing experimental conditions for achieving high faradaic efficiency for Iron chloride redox flow battery. ❧ Two methods for solving parasitic reaction, hydrogen evolution, at the negative electrode were investigated: pH control and increasing overpotential by surface inhibition of hydrogen evolution. The charging efficiency from half-cell experiments showed an increase from 26.5% to 93.95% with 0.25M ascorbic acid and pH controlled with ammonium hydroxide at a value of 2. Also, by increasing overpotential of electrode surface by Indium chloride additive, the charging efficiency in the half-cell experiment could reach 96%. ❧ The full redox flow battery cell was cycled with different compositions of the negative electrolyte. The full cell showed stable cycleability. The charging efficiency of the full cell was improved significantly from 40% to 91.5% when the electrolyte was at pH 2 in the presence of ascorbic acid and indium chloride additives.

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