Close
About
FAQ
Home
Collections
Login
USC Login
Register
0
Selected
Invert selection
Deselect all
Deselect all
Click here to refresh results
Click here to refresh results
USC
/
Digital Library
/
University of Southern California Dissertations and Theses
/
Studies of transport phenomena in hydrotalcite membranes, and their use in direct methanol fuel cells
(USC Thesis Other)
Studies of transport phenomena in hydrotalcite membranes, and their use in direct methanol fuel cells
PDF
Download
Share
Open document
Flip pages
Contact Us
Contact Us
Copy asset link
Request this asset
Transcript (if available)
Content
STUDIES OF TRANSPORT PHENOMENA IN
HYDROTALCITE MEMBRANES, AND THEIR USE IN DIRECT
METHANOL FUEL CELLS.
by
Tae Wook Kim
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMICAL ENGINEERING)
December 2008
Copyright 2008 Tae Wook Kim
ii
Dedication
To my heroes my father, mother, wife, and my lovely children
iii
Acknowledgements
“This is what LORD says, he who made the earth, the LORD who formed it and
established it – the LORD is his name call to me and I will answer you and tell you great
and unsearchable things you do not know.” – Jeremiah 33:2, 3-
I thank God for everything. God has led me in his best way.
I would like to express my profound gratitude, admiration and respect to my advisors,
Professor Muhammad Sahimi and Professor Theodore T. Tsotsis for their guidance,
encouragement and direction throughout this research work. This dissertation is a result
of their help, support and patience. Also, I would also like to thank Professor Robert Bau
for serving on my dissertation committee and for his helpful advice during the period that
the work for this dissertation was being done.
Thanks are due to the administrative staff of the Mork Family Department of Chemical
Engineering and Materials Science, Ms. Karen Woo, Ms. Heather Alexander, and Mr.
Brendan Char for all their help and support throughout my graduate studies, and to all the
fellow graduate students in our research group for their help and fruitful discussions.
iv
Finally, I would like to thank the members of my family for their support and
unconditional love, especially my wife and two children (Sung Jin and Ji Won) for their
patience, encouragement and love. This dissertation would not have been possible
without the support and love of my family.
v
Table of Contents
Dedication ii
Acknowledgements iii
List of Tables viii
List of Figures x
Abstract xiii
Chapter 1: Introduction 1
1.1 Motivation & background 1
1.2 CO
2
selective membranes 2
1.2.1 Zeolite membranes 3
1.2.2 Carbon molecular-sieve membranes 5
1.2.3 Silica membranes 5
1.3 Gas transport through Membranes 9
1.4 Conductive membrane for DMFC 13
1.5 Hydrotalcite 17
1.6 Scope of this present work 23
Chapter 2: Preparation of HT membrane using an electrophoretic technique 26
2.1 Introduction 26
2.2 Experimental process 29
2.2.1 Preparation of the colloidal suspension 29
2.2.2 Preparation of films 29
2.2.3 Characterization 32
2.3 Results and discussion 33
2.3.1 Characterization of the HT powders and films 33
2.3.2 Permeation results 41
2.4 Conclusion 45
Chapter 3: HT membrane preparation by vacuum suction method 47
3.1 Introduction 47
3.2 Experimental 49
3.2.1 Materials 49
3.2.2 Membrane preparation 50
3.2.3 Membrane characterization 51
3.3 Results and discussion 53
3.3.1 Characterization results 53
vi
3.3.2 Permeation results 55
3.4 Conclusion 66
Chapter 4: Micromembrane 68
4.1 Outline of micromembrane 68
4.2 Micromembrane preparation 70
4.2.1 Preparation of the colloidal suspensions 70
4.2.2 Silicon-based micromembranes 72
4.2.3 Micromembranes prepared on stainless steel supports 74
4.3 Results and discussion 75
4.3.1 Silicon-based micromembranes 75
4.3.2 Stailess steel-based micromembranes 79
4.3.3 Permeance results 80
4.4 Conclusion 83
Chapter 5: The preparation and characterization of Hydrotalcite (HT) –
sulfonated polyetheretherketone (SPEEK) cation-exchange
membranes for DMFC (Direct Methanol Fuel Cell) 84
5.1 Introduction 84
5.2 Experimental 87
5.2.1 Chemical and materials 87
5.2.2 Preparation of polymers 87
5.2.3 Preparation of membranes 90
5.2.4 Characterization 91
5.2.5 Water gain 92
5.2.6 Methanol permeability 93
5.2.7 Proton conductivity 94
5.3 Results and discussion 96
5.3.1 Hydrotalcite 96
5.3.2 Sulfonation degree and ion exchange capacity 96
5.3.3 FTIR analysis 99
5.3.4 Thermogravimetric study 100
5.3.5 Water gain and IEC 102
5.3.6 Methanol permeability 103
5.3.7 Proton conductivity 104
5.4 Conclusion 110
Chapter 6: Hybrid Hydrotalcite (HT)-sulfonated polyetherether ketone (SPEEK)
cation-exchange membranes prepared by in situ sulfonation 112
6.1 Introduction 112
6.2 Experimental 115
6.2.1 Chemical and materials 115
6.2.2 Preparation of sulfonated polymers 116
6.2.3 Characterization 118
vii
6.3 Results and discussion 119
6.3.1 Characterization of membranes 119
6.3.2 Thermal stability 123
6.3.3 Water gain and proton conductivity 125
6.3.4 Methanol permeability and proton conductivity 126
6.4 Conclusion 129
Glossary 131
Bibliography 134
Appendix 147
Apendix References 150
viii
List of Tables
Table 1.1: Structural characteristics and properties of different zeolite 4
Table 1.2: Differenct type of zeolite membranes and their performance 4
Table 1.3: Summary of results from different carbon membranes 6
Table 1.4: Summary of results from different silica membranes 7
Table 1.5: Transport mechanism for various membranes 10
Table 1.6: Units used in membrane science 13
Table 1.7: The properties of hybrid membranes 16
Table 1.8: Composition, crystallographic parameters, and symmetry for some 20
natural anionic clays
Table 1.9: Values of c’ for some inorganic anions 22
Table 2.1: Surface area and pore size of the Mg70D powder treated under various 36
Conditions
Table 2.2: The CO
2
and N
2
permeances and ideal selectivity of supports used in 42
the preparation of EPD membranes
Table 2.3: The CO2 and N2 permeances and the ideal selectivity of EPD 43
membranes (Colloidal suspension: 0.76wt% Mg 70DS, pH 7,
ΔP=30psi)
Table 2.4: The conditions of various coating methods used for preparation 44
of silicon-based HT micromembrane
Table 3.1: Pore volumes and diameters for the supported HT membrane 53
as well as the support disk
Table 3.2: The permeation characteristics of HT membranes prepared by 58
the vacuum-suction method (temperature, 25 °C)
Table 3.3: The temperature effect on the transport properties of the 59
HT membranes (HT-3, ΔP=20 psi (137,895.1 Pa))
ix
Table 3.4: The effect of pressure-drop on the transport properties of the HT 60
membranes (temperature, 25°C)
Table 3.5: Permeation properties before and after the silicone-coating 62
(Sil-HT-1) Permeance unit: ×109 (mol/m
2
s Pa),temperature, 25°C)
Table 3.6: The permeabilities and ideal selectivities of the silicone coating 63
(temperature, 25°C)
Table 3.7: Comparison between the calculated and experimental values for 64
the HT membrane films (temperature, 25 °C, Sil-HT-1)
Table 3.8: The permeation characteristics of a silicone-coated HT membrane 65
(Sil-HT-2), and of a silicone film
Table 3.9: Mixed-gas experimental data for a silicone-coated HT membrane, 66
temperature, 25 °C, ΔP =30 psi (206,842.7 Pa)
Table 4.1: The results of EDX analysis on HT - γ-alumina layer 78
Table 4.2: Kinetic diameters of molecules 81
Table 4.3: The permeation characterization of a HT micro-membranes 82
(2
nd
coat , Temp. R.T)
Table 5.1: The SD measured by the titration method and by
1
H NMR 90
Table 6.1: The initial and final concentrations of Al and Mg 121
Table 6.2: The concentration of sulfur, the SD and the proton conductivity 122
of different polymers
x
List of Figures
Figure 1.1: Transport mechanisms for meso- and microporous 10
Figure 1.2: Schematic representation of the hydrotalcite-type anionic 21
clay structure
Figure 1.3: The thermal evolution of Mg-Al-CO
3
as a function of temperature 22
Figure 1.4: Main industrial applications of anionic clays 23
Figure 2.1: Particle size distributions of the Mg70D and the Mg70Ds powders 30
Figure 2.2: EPD membrane preparation apparatus 31
Figure 2.3: TEM images of EPD HT coatings (a) 0.5h of deposition time 34
(b) 1h of deposition time
Figure 2.4: The characterization of Mg70D and HTU powders 38
(a) PXRD spectra (b) zeta potential
Figure 2.5: FTIR spectra (a) HTU (b) Mg70D 39
Figure 2.6: The morphology of the support tube ((a) top surface, 40
(b) cross-section) and the EPD film prepared (deposition voltage
1 V, 2 coatings, with 1 h of deposition/coating) on the support tube
((c) top surface, (d) cross-section)
Figure 2.7: The morphology of (a) a Mg70DS EPD film and (b) a Mg70D 41
EPD film, both prepared using α-alumina support tubes
(magnification: ×500)
Figure 3.1: The schematic of permeance test apparatus 54
Figure 3.2: SEM picture of the surface (top view) of a vacuum-suction 56
membrane (a) Magnification ×10K (b) Magnification ×40K
Figure 3.3: The cross-section of (a) the HT membrane and (b) the silicone- 57
coated HT membrane (Magnification × 5K)
xi
Figure 4.1: The particle size distributions (PaSD) of the Col 1 and Col 2, 71
Col 1 (av. particle size 4.4 μm); Col 2 1 (av. particle size 0.17 μm)
Figure 4.2: The FTIR spectra (a) Col 1 (b) Co1 2 72
Figure 4.3: The design of (a) 4 and (b) 20 microholes with 800 μm 73
Figure 4.4: The fabrication process for silicon based micromembrane 73
Figure 4.5: The design of 20 microholes 74
Figure 4.6: SEM picture of silicon microchannel (a) top view 75
(b) cross-sectional view
Figure 4.7: SEM pictures of a membrane deposited on a silicon microchannel 76
by coating with an HT colloidal solution. (a) top surface, (b) cross-
sectional view(Magnification, top view: × 20K, cross: × 10K)
Figure 4.8: The morphology of (a) top view and (b) cross for 1st γ –alumina 77
layer. (Magnification, top view: × 20K, cross: × 10K)
Figure 4.9: HT and alumina layers on the left, and the top HT layer on the right 77
Figure 4.10: The result of EDX analysis for Figure 4.9(b) 78
Figure 4.11: SEM picture of microhole membrane (a) Magnification ×100 79
(b) Magnification ×20K (top view)
Figure 4.12: SEM picture of microhole membrane (cross section view) 80
(Magnification ×50)
Figure 5.1: The sulfonation reaction of the PEEK 88
Figure 5.2: The PaSD of hydrotalcite particles 91
Figure 5.3: A schematic of the conductivity cell 95
Figure 5.4: (a) The TGA spectrum, and (b) the X-ray powder diffraction 97
pattern of the hydrotalcite
Figure 5.5: The SD and IEC of the SPEEK polymer as a function of sulfonation 98
reaction time
Figure 5.6: The FTIR Spectra of the PEEK and SPEEK 99
xii
Figure 5.7: The TGA graphs for the PEEK and SPEEK 101
Figure 5.8: TGA graphs for SPEEK and HT-SPEEK 102
Figure 5.9: Water gain and the IEC for the SPEEK polymers of varying 103
sulfonation degree
Figure 5.10: Methanol permeability of (a) the SPEEK, and (b) the HT-SPEEK 104
Figure 5.11: Conductivity of (a) the SPEEK, and (b) the HT-SPEEK 106
Figure 5.12: Conductivity and water gain of the HT-SPEEK membranes 108
as a function of the HT content
Figure 5.13: Log[C/P] of a number of the membranes 109
Figure 5.14: Proton conductivity versus methanol resistance plot 110
Figure 6.1: The sulfonation reaction of both PEEK and HT in excess H
2
SO
4
116
Figure 6.2: FTIR Spectra of SPEEK and in-situ HT-SPEEK 120
Figure 6.3: Solid
27
Al NMR spectra of R120-HT materials 123
Figure 6.4: Solid
27
Al NMR spectra of R72_10HT and R120_10HT materials 124
Figure 6.5: Thermograms for both the SPEEK and the in situ sulfonated 125
120 HT-SPEEK materials
Figure 6.6: The water gain and proton conductivity of in-situ sulfonated 127
HT-SPEEK polymers
Figure 6.7: The MeOH permeability and proton conductivity of in-situ 128
sulfonated HT-SPEEK polymers
Figure 6.8: Proton conductivity versus methanol resistance plot 129
xiii
Abstract
Currently, the humanity is encountering two major crises: energy deficiency
and global warming. In order to resolve these crises, we should consider maximizing
energy efficiency and minimizing its usage. Furthermore, we should develop alternative
energy sources (e.g. wind, solar, biomass), instead of hydrocarbon products. Moreover,
we need to commercialize well-known techniques such as fuel cells, which are
environment-friendly and high efficiency systems for various applications, such as power
generation and transportation. In addition, we need to continue research on CO
2
capture
and separation processes.
This study presents the synthesis and characterization of hydrotalcite (HT)
membranes with several techniques. In addition, this study explores the possibility of
using HT materials as inorganic fillers for conductive membranes in direct methanol fuel
cells (DMFC). Due to their properties, hydrotalcites also known as layered double
hydroixde compounds, are a potentially good candidate as CO
2
-selective membranes and
inorganic filler of conductive membrane.
Chapter 1 presents a general Introdcution to the various topics discussed in this
Thesis. Chapter 2 describes the use of electrophoretic deposition as a new method for the
preparation of HT thin films. The films are deposited on macroporous alumina substrates
and on alumina substrates, which were previously coated by conventional dip-coating
techniques using slurries of HT colloidal particles. Their permeation properties are
investigated by single and mixed-gas permeation tests. The films are shown to be
xiv
permselective towards CO
2
, consistent with the prior studies of these materials, which
showed them to be effective CO
2
adsorbents.
In Chapter 3 several methods are used for synthesis of effective CO
2
-selective HT
membranes. Single gases and mixtures of gases are tested and their permeation is studied.
Unfortunately, the dip-coating method results in mesoporous membranes with Knudsen
flow. But the vacuum-suction method shows that the He/CO
2
separation factor for these
membranes is significantly higher than the corresponding Knudsen values, despite the
fact that these membranes are not CO
2
-permselective. In order to decrease voids and
pinholes, a silicone layer is coated by vacuum suction on the HT membranes. The
silicone coating appears to improve the separation characteristics of these membranes.
Chapter 4 describes preparation of a miniature-type micromembrane using silicon
wafers and stainless steel (SS) foils as templates. Silicon-based micromembranes show
the potential for application for microreactor systems, but their pressure resistance is not
high enough to carry out the permeation test. HT micromembranes, prepared by coated
HT colloid solution with 0.1~0.2 μm diameter on SS substartes, are characterized by
several analytical techniques and by single-gas permeation experiments. Most of the HT
micro-membranes exhibit Knudsen transport behavior with He and N
2
-transport being
favored when compared to CO
2
. Some of the HT micromembranes turned out to be CO
2
-
selective, however.
Chapters 5 and 6 demonstrate how both hybrid and in-situ hydrotalcite-SPEEK
(sulfonated polyetheretherketone) membranes are synthesized and investigated for the
possibility of making a conductive membrane in direct methanol fuel cell. Our study’s
xv
goal is to develop a new, cost-effective membrane with superior methanol barrier
properties, and reasonable proton conductivity in order to replace commercial Nafion®
membranes. We prepare HT-SPEEK membranes by incorporating HT particles into
SPEEK and by in-situ sulfonation polymerization from PEEK and HT. The hybrid HT-
SPEEK membranes exhibit good resistance for methanol permeability and reasonable
proton conductivity. Their properties depend strongly on the sulfonation degree of the
polymer matrix, and on the fraction of the HT present in the hybrid membranes.
Therefore, HT-SPEEK membranes are potentially viable candidates for replacing
Nafion
®
membranes. Moreover, the in-situ membrane’s properties depend on the reaction
time, and the fraction of hydrotalcite initially added to the PEEK materials prior to
sulfonation. The MeOH permeability for the in-situ membranes is 3 ~ 5 times smaller
than the one for the commercial Nafion
®
115 film.
1
Chapter 1 : Introduction
1.1 Motivation & background
Currently, the world is encountering two major crises: energy deficiency and
global warming. An energy crisis arises from a disruption of energy supplies (oil and
natural gas), accompanied by rapidly increasing energy prices that threaten economic and
national security. The threat to economic security is represented by the possibility of
declining world economic growth, increasing inflation and rising unemployment. Global
warming is also another potential disaster for our planet. The climate changes due to
global warming are a serious challenge for the global community. In order to address
these threats, we should try to maximize energy efficiency and minimize its usage.
Furthermore, we should develop alternative sources of energy (e.g., wind, solar, and
biomass), instead of relying on hydrocarbon resources. Moreover, we need to
commercialize well-known products, such as fuel cells, which are environment-friendly
and represent high-efficiency systems for various applications, such as power generation
and transportation. In addition, we need to continue research on CO
2
capture and
separation processes.
2
1.2 CO
2
-selective membranes
We must focus on reducing the atmospheric concentrations of several greenhouse
gases (e.g., CO
2
, CH
4
, nitrous oxide, and chlorofluorocarbons), which have increased by
about 25 percent since the industrial revolution in the mid 19th century. In particular,
CO
2
emissions have increased dramatically, due to the burning of fossil fuels, such as
coal, petroleum, and natural gas for the production of electricity, and gasoline or diesel
for transportation [Shekhawat et al., 2003]. As a result, many studies in recent years have
tried to reduce the emissions of CO
2
. In order to accomplish this goal, it is very important
to be able to separate and capture CO
2
from its mixtures with other gases [White et al.,
2003]. Although there are several methods available, membrane separation processes are
a relatively simple and energy effective technique [Baker, 2004].
Membrane materials can be divided into two groups: inorganic and polymeric.
Currently, the industry is dominated by polymeric membranes that have been used in a
variety of applications, ranging from food and beverage processing, to desalination of
seawater, gas separations, and medical devices. Recently, research has been focused on
the development and application of inorganic membranes, due to their high demand in
new applications, such as fuel cells, membrane reactors, and other high-temperature
separations. Caro et al. [2000] listed various advantages and disadvantages of inorganic
membranes, in comparison with polymeric membranes.
Inorganic membranes are very
stable at high temperatures, and can be resistant to harsh conditions. In addition to purely
inorganic membranes, hybrid, mixed-matrix membranes have also been developed. They
3
consist of both inorganic and polymer materials [Vu et al., 2003; Bouma et al., 1997], and
combine merits of both type of materials. In what follows, we describe recent research
trends in the field of CO
2
-selective inorganic membranes.
1.2.1 Zeolite Membranes
Zeolites are alumino-silicate-type crystalline materials with uniform pore
structures and cavities that are interconnected by pore openings through which molecules
can pass. Pore channel diameter typically range from 0.3 to 1.0 nm [Kanellopoulos,
2000]. Such properties make membranes made of zeolite materials a good candidate for
gas separation. The electrical charge or polarity of the zeolites also makes it possible for
them to attract or adsorb molecules. The ability to selectively adsorb molecules based on
size and polarity is a key for the use of zeolite membranes in various important separation
applications. Almost 100 different structural types of zeolite are known, each with its
own distinct pore size, shape, and interconnectivity (Table 1.1) [Yan et al., 1995] A
variety of methods for the fabrication of zeolite membranes have also been recently
reported. These include dip-coating, spin-coating, sputtering, and laser ablation.
However, zeolite membranes have usually been prepared by in-situ hydrothermal
synthesis in various porous supports. Various types of zeolite membranes and their
performance are summarized in Table 1.2.
4
Table 1.1 Structural characteristics and properties of various zeolite types
Zeolite type Structural Formula [Si]/[Al]
Window
Dimension/Ǻ
Silicalite 1
Na
n
[Si
96-n
Al
n
O
192
]16H
2
O
∞
5.2×5.7
ZSM-5 10-1000
Zeolite A Na
12
[Si
12
Al
12
O
48
]27H
2
O 1(1.2-3.7) 4.1
Zeolite X Na
96-x
[Si
96+x
Al
96-
x
O
348
]240H
2
O
1-1.5 7.4
Zeolite Y ≥2.5 7.4
Clathrasils e.g., Sodalite and Dodecasil 1-∞ 2.2-2.8
Table 1.2 Different types of zeolite membranes and their performance
Memb.
Type
Permeation
Temp.( ℃)
CO
2
Permeance
(mol/m2 s Pa)
CO
2
/N
2
CO
2
/CH
4
Ref.
PS SF PS SF
ZSM-5
30 5×10
-8
9.3 Kusakabe
et al., 1996 100 6×10
-8
8.6
SAPO-
34
27 2.4×10
-8
6 19 30 Poshusta et
al., 1998 200 1×10
-8
2 2 3.4
27 1.5×10
-7
5 16 16 36 Poshusta et
al., 2000 200 2×10
-8
2 3 4 5
Y-type
30 1.2×10
-7
5 18 2 21 Kusakabe
et al., 1997 130 3×10
-7
11 9 6 8
PS: Permselectivity (Single gas), SF: Separation Factor (Mixture gas)
5
1.2.2 Carbon Molecular-Sieve Membranes
Carbon membranes are typically made by the pyrolysis of thermosetting
polymers, such as polyimide, polyvinylidene chloride (PVDC), polyfurfuryl alcohol
(PFFA), cellulose, and polyacrylonitrile (PAN) [Koresh and Soffer, 1980] During the last
few years, the properties of porous carbon molecular-sieve (CMS) membranes and their
gas separation capacities have been studied and have been shown to depend on two main
factors: the choice of the polymer precursor, and the pyrolysis conditions (temperature,
heating rate, heating atmosphere, etc.) [Koresh and Sofer, 1983]
Pyrolysis of polymeric
compounds leads to carbon materials with a very narrow micropore size distribution,
typically below molecular dimensions (<1 nm), which makes it possible to separate gases
with very similar molecular sizes. Hence, the predominant transport mechanism of most
carbon membranes is molecular sieving. The various types of CMS membranes available
are shown in Table 1.3.
1.2.3 Silica Membranes
Silica is considered a reasonable starting material for the preparation of CO
2
-
selective membranes, due to its relatively good stability. Unlike alumina, which tends to
undergo a phase transition at relatively low temperatures, or carbon which can exhibit
substantial changes in pore size in oxidizing environments, silica has good thermal,
chemical, and structural stability [Kusakabe et al., 1998; Yamamoto et al., 1997; Centeno
and Fuertes, 2000].
6
Table 1.3 Summary of results for different carbon membranes
Precursor
Carbonizatio
n Temp ( ℃)
Perm.
Temp
( ℃)
CO
2
Permeance
(mol/m
2
sec Pa)
Selectivity
Ref. CO
2
/
N
2
CO
2
/
CH
4
BPDA-
pp’ODA
600 30 6×10
-8
31 80 Hayashi
et al.,
1995
700 30 3×10
-8
55 60
700 100 9×10
-8
15 16
BPDA-
pp’ODA
(oxidized)
300-700 65 4×10
-7
8 20
Hayashi
et al.,
1997
Polyetherimi
de
800
25
150
1×10
-10
4×10
-10
15
9
25
20
Fuertes
et al.,
1998
Matrimid®
in NMP
475
25
2.7×10
-9
15 33
Fuertes
et
al.,1999
650 2.4×10
-9
4 8
650
a
3.1×10
-9
22
(S.F)
Polyimide
270
50 2.6×10
-8
36 27
Kusuki
et al.,
1997
120 4.0×10
-8
10 10
700
50 4.0×10
-8
17 30
120 1.0×10
-8
14 21
a
Feed: (20% CO2 + 80% N2) binary gas mixture
7
The fabrication procedures, while not as advanced as those for carbon and alumina
membranes, are capable of producing defect-free silica membranes with molecular-
sieving pores.
Table 1.4 Summary of results for different silica membranes
Precursor
Permeatio
n Temp
( ℃)
CO
2
Permeance
(mol/m
2
sec Pa)
Selectivity
Ref. CO
2
/
N
2
CO
2
/
CH
4
TEOS(400)
25 2.3×10
-7
23 325
De Vos and
Verweij,
1998
100 2.9×10
-7
15 190
200 2.3×10
-7
8 75
TEOS(600)
100 9.4×10
-9
200 6.1×10
-9
300 4.6×10
-9
TEOS,
Methylated
(400)
100 5.1×10
-7
1.8 1.9
De Vos et
al., 1999 300 3.0×10
-7
1.3 1.1
TEOS+ C8TES
(200)
b
(600)
b
(600)
c
100
a
7.0×10
-8
3.5 2.5
Kusakabe et
al., 1999 100
1.0×10
-8
3.0 2.0
2.2×10
-8
7.0 7.0
8.0×10
-9
8.0 4.5
a
:No calcinations,
b
:Gel formation temperature 80°C,
c
:Gel formation temperature 60 °C
8
Molecular-sieving silica membranes can be divided into two different types
[Kanellopoulos, 2000]:
those prepared by chemical vapor deposition (CVD), and those
prepared by sol-gel methods.
The silica sols utilized for the preparation of silica membranes by the sol-gel
approach are obtained from the hydrolysis condensation reaction of alkoxysilanes, such
as tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), or chlorosilane. CVD-
prepared membranes are produced by reacting a gaseous silica precursor, such as tetra-
ethyl-ortho-silane (TEOS), with an oxidizing agent inside the pores of a macro- or
mesoporous support [De Vos and Vereij, 1998]. The various types of silica membranes
available are shown in Table 1.4. One challenge that silica membranes face is their
hydrothermal stability, as they are known to be unstable in the presence of high-
temperature steam.
Among other inorganic membranes, with good potential for gas separation at
elevated temperatures and in oxidative atmospheres, where polymeric membranes are not
functional, are those made of hydrotalcite (HT) materials. They are potentially very
attractive materials for the preparation of gas-separating membranes, due to their high
CO
2
adsorption capacity at elevated temperatures, thermal stability, and their ability to be
regenerated, and their low cost.
We are not aware of any published studies reporting the preparation of HT
membranes. Many researchers have been interested in adsorption and diffusion properties
of HT. Only a few researchers, however, have made thin films of these materials. One
such study was by Pinnavaia and coworkers, who prepared thin HT films using colloidal
9
suspensions [Gardner et al., 2001]. Considering their potential advantages, HT CO
2
-
selective membranes could be a new good candidate for the separation of CO
2
from its
mixtures.
1.3 Gas Transport Through Membranes
Inorganic membranes can be classified as either porous or dense. Porous
inorganic membranes can be further classified, based on their average diameter d
p
as
macroporous (d
p
>50 nm), mesoporous (2 nm< d
p
<50 nm), and microporous (d
p
<2 nm).
In addition, they can be classified as symmetric (a homogeneous pore structure prevails
throughout the membrane), or asymmetric (they typically have a thin top permselective
layer sitting on top of the macroporous support). The dense inorganic membranes consist
of a thin layer of metal, such as palladium and its alloys (metallic membrane), or solid
electrolytes, such as zirconia. The transport mechanism through the membrane varies
according to the membrane type, as shown in Table 1.5.
Fick’s first law is typically applied for the macroscopic description of the
transport processes through porous membranes:
c c D J ∇ − = ) ( (1.1)
where J is the flux (mol/m
2
s), D(c) is the concentration-dependent diffusivity (m
2
/s), and
c is the concentration (mol/m
3
). The permeability of a membrane with respect to gases is
a function of its properties (physical and chemical structure), the nature of the permeate
10
species (size, shape, and polarity), and the interaction between the membrane and
permeate species [Stern, 1994; Burggraaf and Keizer, 1991].
Table 1.5 Transport mechanisms for various membranes
Type Pore size Transport mechanism
Porous
membrane
Macroporous > 50 nm
Poiseuille flow
(No separation)
Mesoporous 2 nm ~ 50 nm Knudsen diffusion
Microporous < 2 nm
Surface diffusion,
Capillary Condensation
Molecular sieving
Dense membrane - Solution- Diffusion
Figure 1.1. Transport mechanisms for meso and microporous membranes
In dense membranes, the permeability P
e
1
of a given penetrant 1 [(defined by E.S. (1.9)
and (1.10)] is taken to be the product of the solubility coefficient or the sorptivity (a
thermodynamic parameter), S
1
, and the diffusion coefficient (a kinetic parameter), D
1
:
11
1 1 1 e
P S D =
(1.2)
The permeability indicates the rate at which a penetrant traverses a membrane.
The solubility/sorptivity coefficient provides a measure of the amount of gas that is
adsorbed by the membrane when equilibrated with a gas at a given pressure and
temperature. The diffusion coefficient is a measure of how fast a penetrant is transported
through the membrane in the absence of obstructive sorption.
Membranes utilized in separations need to possess both high selectivity and high
permeation. The selectivity of the membrane to specific gas or liquid molecules relates to
the ability of the molecules to transport through the membrane. The permselectivity or
ideal separation factor (based on pure gas permeation), α, is simply the ratio of two gas
permeabilities [Baker, 2004].
=
e
e
P
P
2
1
2 , 1
α
(1.3)
From E.S. (1.2) and (1.3)
=
D
D
S
S
2
1
2
1
2 , 1 α
(1.4)
The actual separation factor (for mixed gas separation) is defined:
x
x
y
y
1
2
2
1
2 , 1
=
α
(1.5)
where x
1
and x
2
are mole fractions of species 1 and 2, respectively, in the feed, and y
1
and
y
2
are their mole fractions in the permeate.
12
Temperature-dependence of the permeability of a gaseous penetrant in a
membrane is typically described by the Arrhenius expression:
=
RT
E -
exp P P
p
0 e e
(1.6)
where P
e
0
is a pre-exponential factor, and E
p
is the apparent activation energy for
permeation. Similar expressions are used to describe the temperature dependence of
diffusivity and solubility/sorptivity. Thus, the permeability may be expressed as follows:
+ −
=
RT
E H
D S
D s
) (
exp P
0 0 e
(1.7)
Therefore, the apparent activation energy for permeation is the sum of the apparent heat
of sorption (solubility), H
s
, and the apparent activation energy of diffusion, E
D
.
The gas flux (J) through the membrane is defined by the following expression:
At
V
J
Δ
=
(1.8)
where ∆V is the volume of the permeated gas, A is the membrane area, and t is the time.
The permeance (P
r
) through the membrane is defined by
p
J
P
r
Δ
= (1.9)
L P
r
⋅ =
e
P
(1.10)
where ∆p is the partial pressure difference between the upstream and downstream sides
of the membrane, and L is the membrane thickness. Permeance can be thought of as
reciprocal resistance against mass transport through a porous medium [Uhlhorn et al.,
13
1989]. Since a multilayer system can be considered as a number of resistances in series,
the overall permeance P
r
of a membrane is related to the permeance of different layers of
the membrane and the support permeability:
Λ + + + =
) 2 (
1
) 1 (
1
) support (
1 1
layer P layer P P P
r r r r
(1.11)
Table 1.6 presents units and unit conversion related to membranes.
Table 1.6 Units used in membrane science
Type SI Others
Permeance mol / m
2
·s·Pa
GPU
a
(Gas Permeation Unit)
Permeability mol·m/ m
2
·s·Pa Barrer
b
a
GPU : 10
-6
cm
3
(STP)/cm
2
·s·cmHg
b
Barrer : 10
-10
cm
3
(STP) ·cm/cm
2
·s·cmHg =3.347
× 10
-16
mol·m /m
2
·s· Pa
1.4 Conductive membranes for DMFC
Fuel cells have application in transportation, and in stationary and portable power
generation. Among them, direct methanol fuel cells (DMFC) are attracting considerable
attention, due to the fact that they can be operated using liquid methanol as a fuel, both
for transportation as well as for portable power generation [Hoogers, 2003]. In addition,
methanol is a naturally occurring, biodegradable, liquid fuel at room temperature. It is
14
one of the easiest fuels to reform because of the lower reforming temperature [Olah,
2006] required. The DMFC relies upon the oxidation of methanol on a catalyst layer to
form carbon dioxide. Positive ions (H
+
) are transported across the proton-exchange
membrane to the cathode, where they react with oxygen to produce water. Electrons are
transported via an external circuit from the anode to the cathode providing power to
external devices. The half reactions are:
Anode: CH
3
OH + H
2
O → CO
2
+ 6H
+
+ 6e
-
Cathode: (3/2)O
2
+ 6H
+
+ 6e
-
→ 3H
2
O
Net reaction: CH
3
OH + (3/2)O
2
→ CO
2
+ 2H
2
O
E
0
cell
= 1.213V (theoretical value)
However, DMFC faces today two major technical problems: one is slow methanol-anode
oxidation kinetics, and the other is methanol crossover (diffusion) through the polymer
electrolyte membrane. This causes a decrease in the fuel cell voltage, loss in its
efficiency, and a poisoned cathode catalyst. [Hoogers, 2003]. Commercially available
membranes like Nafion® are very expensive; it has been found, furthermore, that
sometimes over 40% of the methanol is lost through crossover during fuel cell operation
[Tricoli, 1998]. In addition, Nafion® loses proton conductivity above 80°C due to the
difficulty in maintaining fully hydrated membranes. Thus, many researchers are currently
trying to improve the efficiency of DMFC using nonfluorinated polymer materials and
hybrid membrane as conductive membrane [Jannasch, 2003].
A number of nonfluorinate-
sulfonated polymers has been developed, such as polybensimidazole (PBI), sulfonated
polyetherketones (sPEK), sulfonated polyetheretherketones (SPEEK), polyetersulfone
15
(PES), and sulfonated polyphenyl quionxaline (SPPQ) [Roziere and Jones, 2003]. These
polymers are thermally stable due to polyaromatic or polyheterocyclic repeat units in
them, and are modified by the sulfonic function group. Among them, different types of
polyetherketones have been a focus for many researchers [Li et al., 2005; Li and Wang,
2003; Regina et al., 2006; Xing et al, 2004]. The reason for this interest is that the
membranes based on polyetherketones exhibit good chemical and mechanical stability,
high proton conductivity (0.01 to 0.05 [Scm
-1
]), and both a reduced methanol
permeability and a lower cost with respect to those of the Nafion® membranes. The
properties are affected by the degree of sulfonation of the polymer. Furthermore, various
hybrid membranes have been investigated in the last few years in order to modify the
membrane’s properties. Some hybrid membranes are shown in Table 1.7. Generally,
several inorganic fillers have been reported for hybrid membranes based on both
fluorinated and non-fluorniated proton- conducting polymers, such as layered zirconium
phosphate
[Carbone et al., 2004; Silva et al., 2005], SiO
2
[Dimitrova et al., 2002; Bauer
and Willert-Porada, 2005; Wu and Ma, 2004; Kim et al., 2005], HT [Lee et al., 2005],
layered silicate [Chang et al., 2003], TiO
2
[Bauer and Willert-Porada, 2005],
phosphotungstic acid (PWA) [Wu and Ma, 2004], and heteropolyacids [Dimitrova et al.,
2002]. Arico et al. showed that the conductivity of hybrid membranes tracks the acidity
of the fillers and follows the series: SiO
2
-PWA > SiO
2
> ZrO
2
> n-Al
2
O
3
(neutral) > b-
Al
2
O
3
(basic) [Arico et al., 2003]. That means that the membranes with the best
performance are the most acidic. Some fillers (e.g., ZrO
2
and HT) have been reported to
decrease the methanol permeability [Lee et al., 2005; Silva et al., 2005].
16
Table 1.7 The properties of hybrid membranes
Hybrid membrane
Test Cond.
(Filler %,
Temp)
Conductivit
y (S/cm)
MeOH
Permeability
(× 10
7
cm
2
/sec)
Ref.
Polymer Inorganic filler
Nafion
®115
Zirconia(IV)
phosphotungstic acid
(PWA)
10wt%, 110 ℃ 0.1 -
Carbone
et al.,
2004
20wt%, 110 ℃ 0.09 -
Nafion
®117
Molybdophosphoric
acid
3~9wt%, 30 ℃ 0.06 ~ 0.2 50 ~ 240
Dimitrova
et al.,
2002 Aerosil A380 (SiO
2
) 4wt%, 30 ℃ 0.09 200
Nafion
®115
Hydrotalcite
(Mg/Al: 2,4,6)
1~3wt%, R.T. - 1.51 ~ 3.54
Lee et al.,
2005
Nafion
®117
SiO
2
, TiO
2
SiO
2
13~26wt%,
130 ℃,
0.084 ~
0.088
-
Bauer and
Willert-
Porada,
2005
SPEEK Silica (SiO
2
)/PWA 2~8wt%,R.T.
0.013 ~
0.018
2.9 ~ 4.4
Wu and
Ma, 2004
SPEEK ZrO
2
5~12.5wt%,
50 ~ 90 ℃
0.01 ~ 0.4 ~ 25
Silva et
al., 2005
SPEEK
Layered Silicates
(Laponite MMT)
5~10wt%,R.T
0.00023 ~
0.0032
-
Chang et
al., 2003
Polyanil
ine
Sn(IV)phosphate 6~16wt%,R.T.
0.008 ~
0.000095
-
Khan and
Inamuddi
n, 2006
PVA/
PAA
Silica(SiO
2
)
Several type,
R.T.
0.001~
0.01
2.1 ~ 25
Kim et al.,
2005
17
Finally, SiO
2
-added hybrid membranes increase both the conductivity and methanol
permeability [Kim et al., 2005].
1.5 Hydrotalcites
Layered double hydroxide (LDH) clays, also called hydrotalcite-like compounds,
have a variety of uses in various scientific and technological processes as ion exchange
materials, catalysts, antacids, catalytic supports, and modified electrodes [Cai et al.,
1994]. The first chemical formula for hydrotalcite, [Mg
6
Al
2
(OH)
16
][(CO
3
)
.
4H
2
O], was
reported by Manasse in 1915, who was also the first to recognize that carbonate ions were
essential for its structure [Manasse, 1995]. Generally, LDH are also known as anionic
clays. Anionic clays are natural or synthetic lamellar mixed hydroxides with interlayer
spaces containing exchangeable anions. There are a variety of them, depending on the
composition and polytype form of the minerals (Table1.7). In particular, in general terms,
HT compounds or LDH are used very widely.
Generally, the chemical structure of anionic hydrotalcite-type clay is [M
1−x
(II)
M
x
(III)
(OH)
2
]
x+
[A
mi
x/m
]·nH
2
O, where M
(II)
is a divalent metal cation (Mg, Mn, Fe, Co, Ni,
Cu, Zn,Ga) and M
(III)
is a trivalent metal cation (Al, Cr, Mn, Fe, Co, Ni, and La). A
m−
represents an interlayer anion, such as m-valence inorganic (CO
3
2−
, OH
−
, NO
3
−
, SO
4
2-
,
ClO
4
−
), heteropolyacid (PMo
12
O
40
3−
,PW
12
O
40
3-
), or even organic acid anions. The
following sequence of the affinity for the interlayer anions can be derived for both mono-
18
and divalent anions (the divalent anions are more strongly held in the interlayer than the
monolayer anions, and a carbonate is held most strongly): [Miyata, 1983]
I NO Br Cl F OH SO CO
− − − − − − − −
〉 〉 〉 〉 〉 〉〉 〉〉
3
2
4
2
3
The range of x=M
(III)
/(M
(II)
+M
(III)
) is typically, 0.2≤ x≤ 0.33, but significantly
higher values have also been reported, 0.1≤ x ≤ 0.5 [Yamanoto, 1995]. Anionic clays
have structures similar to that of brucite Mg(OH)
2
, and crystallize in a layer-type lattice.
Each Mg
2+
ion is octahedrally surrounded by six OH
-
ions and the different octahedra
combine to form infinite sheets. The sheets are stacked one on the top of the other, and
are held together by weak interactions through hydrogen bonding. If some Mg
2+
ions are
replaced isomorphously by cations with higher charge but of the same radius, the brucite-
type sheets become positively charged. The electrical neutrality is then maintained by
anions distributed in disordered interlayer domains also containing water molecules (Fig.
1.2) [Kim et al., 2005].
HT crystallizes with a rhombohedral 3R stacking sequence, the parameters of the
unit cell being a and c=3c’ (where c’ is the thickness of one layer consisting of a brucite-
like sheet and one interlayer, which is about7.6 Å.), while the polytype form, manasseite,
crystallizes with a hexagonal 2H stacking sequence, with the parameter of the unit cell
being a and c=2c’ (Table 1.7).
Therefore, the thickness of the corresponding interlayer
region for HT is the difference between c’ and the thickness of the brucite-layer (4.8 Å.)
[Vaccari, 1998]. Values of c’ for some inorganic anions are presented in Table 1.8
[Miyata, 1983].
19
The structure of hydrotalcite can be changed by thermal treatment. During
thermal decomposition, the water and hydroxyl groups of the interlayer region are
eliminated first. In particular, Yang et al. [2002] have studied the thermal evolution of the
Mg-Al-CO
3
structure. According to their study, as temperature increases, the basal
spacing and corresponding interlayer distance decrease. The detailed evolution sequence
is shown in Figure 1.3.
There are several methods for the preparation of HT. Among them, co-
precipitation is the most commonly used method. Co-precipitation is the chosen method
for the M
(II)
and M
(III)
hydroxides, utilizing dilute NaOH and NaHCO
3
or NaCO
3
solutions. Small particle sizes are generally obtained by this method. However, to prepare
large particle samples, hydrothermal treatment at 180-200°C is needed under pressure.
In
an attempt to get large and well-crystallized HT particles, the urea method has been
developed. Urea has a number of properties that makes its use very attractive as a
precipitation agent from a homogenous solution. The hydrolysis of urea in the presence
of a mixture of M
(II)
and M
(III)
salts leads to the formation of HT with good crystallinty,
and a narrow distribution of particle sizes [Costantino et al., 1998]. The sol-gel method is
another good method to prepare HT with larger particles, which also have surface area
and pore volume larger than the HT prepared by the co-precipitation method [Aramendia
et al., 2002].
Synthetic anionic clays, after their thermal decomposition, have also found
many industrial applications, and show good potential for a variety of other applications
in the future (Fig. 1.4).
20
A number of prior studies focused on the use of HT for CO
2
adsorption, and
studied diffusion in these materials. Rodrigues et al.
studied the CO
2
diffusivity for
commercial HT at high temperature.
Table 1.8 Composition, crystallographic parameters, and symmetry for some natural
anionic clays [Drits et al., 1987]
Mineral Chemical Composition
Unit Cell
Parameter Symmetry
a (nm) C (nm)
Hydrotalcite Mg
6
Al
2
(OH)
16
CO
3
· 4H
2
O 0.3054 2.281 3R
Manasseite Mg
6
Al
2
(OH)
16
CO
3
· 4H
2
O 0.310 1.56 2H
Pyroaurite Mg
6
Fe
2
(OH)
16
CO
3
· 4H
2
O 0.3109 2.341 3R
Sjogrenite Mg
6
Al
2
(OH)
16
CO
3
· 4H
2
O 0.3113 1.561 2H
Stichtite Mg
6
Cr
2
(OH)
16
CO
3
· 4H
2
O 0.310 2.34 3R
Barbertonite Mg
6
Cr
2
(OH)
16
CO
3
· 4H
2
O 0.310 1.56 2H
Takovite Ni
6
Al
2
(OH)
16
CO
3
· 4H
2
O 0.3025 2.259 3R
Reevesite Ni
6
Fe
2
(OH)
16
CO
3
· 4H
2
O 0.3081 2.305 3R
Meixnerite Mg
6
Al
2
(OH)
16
(OH)
2
· 4H
2
O 0.3046 2.292 3R
Coalingite Mg
10
Al
2
(OH)
24
CO
3
· 2H
2
O 0.312 3.75 3R
21
The measured reciprocal of the time constant for micropore diffusion (
l D c
2
/ ) [sec
-1
]
ranges from 6.0~11.3×10
-3
, while the activation energies ranged from 4.0~6.2 kJ/mol
[Soares et al., 1989]. Ding et al. studied high temperature CO
2
adsorption on a
hydrotalcite adsorbent during steam reforming of methane. In the presence of water vapor
at 480 °C, they reported the adsorption capacity of hydrotalcite to be 0.58 mol/kg
[Ding
et al., 2000].
Figure1.2 Schematic representation of the hydrotalcite-type anionic clay structure
[Cavani et al., 1991].
22
Table 1.9 Values of c’ for some inorganic anions
[Yang et al., 2002]
Anion OH
-
(CO
3
)
2-
F
-
Cl
-
Br
-
I
-
(NO
3
)
-
(SO
4
)
2-
(ClO
4
)
-
c’(Å) 7.55 7.65 7.66 7.86 7.95 8.16 8.79 8.58 9.20
C
E
Mg
0.71
Al
0.29
(OH)
2
(CO
3
2-
)
0.15
0.46H
2
O
Interlayer Water
-0.44H
2
O from OH
-
Group:
Al-OH
-
: 190
o
C-280
o
C
Mg
0.71
Al
0.29
O
0.44
(OH)
1.12
(CO
3
2-
)
0.15
-CO
2
405
o
C-580
o
C
CO
3
2-
Solid Solution
MgO + Al
2
O
3
Amorphous Phase
Brucite-like Layers
A
B
D
Mg
0.71
Al
0.29
(OH)
2
(CO
3
2-
)
0.15
-0.46H
2
O
70
o
C-190
o
C
OH
-
Group in
Mg-OH
-
-0.56H
2
O
280
o
C - 405
o
C
Figure 1.3 The thermal evolution of Mg-Al-CO
3
LDH as a function of temperature [Yang
et al., 2002].
23
Figure 1.4 Main industrial applications of anionic clays [De Roy et al., 1992].
1.6 Scope of the present work
This thesis studies the synthesis and characterization of HT membranes with
several techniques. In addition, we investigate the possibility of HT materials as
inorganic fillers for conductive membrane in direct methanol fuel cells.
As summarized in the preceding section, HT, which has high CO
2
adsorption
capacity at high temperatures, can be regenerated easily, and is inexpensive, is an
appropriate candidate for the preparation of CO
2
-selective membranes. The first purpose
of this study is the synthesis of HT membranes, and the understanding of their transport
mechanism. HT have been reported having slit-type micropores with sizes below 5 ~ 6 Å
Catalysis
- hydrogenation
- Polymerization
- CH4 reforming or
partial oxidation
- DeNox, N2O decomp
Catalysis Supports
- Ziegler-Natta
- DeSOx
- DeNOx
- Complexes
Industry
- flame retardant
- molecular sieve
- ion exchanger
Medicine
- antiacid
- antipeptin
- stabilizer
Adsorbents
- halogen scavenger
- PVC stabilizer
- waste water
Hydrotalcite-type anionic clays
24
[Huston et al., 2004], and, in principle, one should be able to prepare microporous
membranes from these materials. Both regular-size HT membranes, as well as
micromembranes are prepared by several methods. For the preparation of the latter
membranes, the technique of photolithography is utilized. Conventional coating methods
and electrophoretic deposition are used for the synthesis of regular HT membranes. To
understand the transport characteristics of the membranes, both single-gas and mixed-gas
permeation experiments are carried out under various conditions.
As the second goal of this study, HT-SPEEK hybrid membranes are prepared, and
their characterization and performance are investigated. HT is not only a slightly basic
material, but is also an LDH-type material with a positive layer and a negative interlayer
[Miyata, 1983; Yang et al., 2002]. This material is thermally stable with high CO
2
adsorption capacity at high temperatures. First, HT particles can make methanol diffusion
decrease when particles are well dispersed in the hybrid system. Second, the retention of
water is around 11.9 wt% at the ratio of Mg/Al of 3 from their chemical structure. The
water content plays a key role in transporting the protons [Arico, 2003]. We investigate
the effect of HT inorganic filler in HT-SPEEK membranes for their methanol
permeability, proton conductivity, and the possibility of being a replacement for
Nafion®.
The outline of this dissertation is as follows: the present chapter already
elucidated the introduction and motivation for the work. In Chapter 2, the carbon dioxide
selective membranes are prepared by an electrodepositon method and are investigated. In
25
Chapter 3, the synthesis and characterization of HT membranes which were prepared by
various conventional methods are discussed. In Chapter 4, HT micromembranes are
studied. The micromembranes exhibit good potential for application in microfuel cells. In
Chapters 5 and 6 hybrid and in-situ HT-SPEEK membranes are synthesized and
investigated for the possibility of being conductive membrane, in DMFC’s.
26
Chapter 2 : Preparation of HT membrane using
an electrophoretic technique
2.1 Introduction
Hydrotalcites (HT) are layered double-hydroxide (LDH) clays. Generally, LDH
are natural or synthetic lamellar mixed hydroxides with their interlayer spaces containing
a variety of exchangeable anions; many different synthetic and natural LDH are known to
exist with different chemical composition or with the same composition and different
mineral polytype forms
[Dritis et al., 1987]. The chemical structure of an anionic HT-
type clay, for example, is
[M
1−x
(II)
M
x
(III)
(OH)
2
]
x+
[A
m-
x/m
] ·nH
2
O,
where M
(II)
is a divalent metal cation (Mg, Mn, Fe, Co, Ni, Cu, Zn, Ga) and M
(III)
a
trivalent metal cation (Al, Cr, Mn, Fe, Co, Ni, and La). Here, A
m−
may represent an m-
valence inorganic (CO
3
2−
, OH
−
, NO
3
−
, SO
4
2-
, ClO
4
−
), a heteropolyacid (PMo
12
O
40
3−
,
PW
12
O
40
3-
), or even an organic acid interlayer anion [Miyata, 1983]. HT have a number
of existing and potential scientific and technological uses, for example, as ion-exchange
materials, catalysts, antacids, catalytic supports, chemical sensors, optical devices, and as
clay-modified electrodes [Cai et al., 1994; Roto and Villemure, 2006].
Most recently, the synthesis of thin HT films is attracting interest. Such thin films
can potentially find applications as detection layers for chemical sensors, separation
layers for inorganic membranes, and electrodes for fuel cells. A number of different
27
approaches to prepare such thin HT films and coatings have been investigated. For
example, dip-coating and hydrothermal aging using an HT solution have been recently
utilized by other researcher [Othman et al., 2006] to prepare HT coatings on zeolite
pellets. Pinnavaia and coworkers [Gardner et al., 2001] prepared thin HT films on glass
substrates using alkoxide-intercalated Mg-Al LDH colloidal suspensions. A number of
studies [Szekeres et al., 2005; Hornok et al., 2005; Costan and Imae, 2004] report on the
preparation using a “layer-by-layer” deposition technique, based on the formation of
Langmuir-Blodgett films, of hybrid films consisting of LDH and various polymeric
macromolecules.
Dékány and coworkers [Hornok et al., 2005; Costan and Imae, 2004],
for example, prepared multilayer (12–18 layers) films using (Mg–Al-NO
3
) LDH
particles, which were alternately deposited with either polyacrylic acid (PAA) or
polysodium 4-styrenesulfonate films on modified gold surfaces using the “layer-by-
layer” deposition method. The LDH suspension concentration strongly influenced the
number of layers one could deposit and their characteristics. A different approach to
prepare hybrid LDH/polymeric thin films was utilized by other researcher [Lee et al.,
2006]. After coating a Mg-Al-CO
3
LDH film on a silicon wafer, they formed a polymeric
film on the top by a direct ion-exchange reaction with a PAA binary-solvent solution.
The focus of this study is on the preparation of HT thin films as separation layers
for inorganic membranes and sensors for separation and detection applications.
Microporous inorganic thin films are attracting attention in recent years for applications
under conditions where conventional polymeric films face technical challenges; such
conditions include high temperatures and oxidative gas atmospheres, which tend to
28
degrade the polymeric materials, and the presence of high pressure CO
2
and hydrocarbon
vapors, which tend to plasticize these films. Nanoporous zeolite, carbon molecular sieve,
and silica membrane films have been widely studied, so far, for gas phase applications
[Kusakabe et al., 1996; Poshusta et al., 1998; Hayashi et al., 1995; Sedigh et al., 2000]
HT materials have also received attention in recent years as adsorbents for CO
2
gas
separation at elevated temperatures, and in oxidative atmospheres [Soares et al., 1989;
Huston et al., 2004; Kim et al., 2004]. The (Mg-Al-CO
3
)-HT has been shown, for
example, to be an effective and reversible CO
2
adsorbent for a variety of both reactive
and conventional separations [Harale et al., 2007; Yong and Rodrigues, 2002]. The same
HT materials, in the form of thin films deposited on macroporous supports, also show
good potential as selective membrane layers for CO
2
and other species that show strong
affinity for the HT structure, and can accommodate themselves in the interlayer region.
Other than the efforts by our group [Kim et al., 2006], we are not aware, however,
of any other group that has managed to prepare HT thin films that are indeed CO
2
-
permselective. In that study (See Chap. 3.3.2), we used a vacuum-suction approach for
the preparation of HT asymmetric membranes. Such membranes, after they are “surface-
caulked” by a silicone coating to repair film cracks and defects, exhibit CO
2
/N
2
separation factors as high as 35. In the present study, a different technique for preparing
compact and well-intergrown HT thin films for inorganic membrane and sensor
applications is described, namely, electrophoretic deposition (EPD) on macroporous
supports using HT colloidal solutions. Electrophoresis is the process whereby suspended
particles travel through a fluid in response to an applied electric field [Gombocz, 1995].
29
The advantages of EPD include the uniformity of the deposition process, even for
complex and large forms, and control of the thickness of the deposit through adjusting the
various deposition parameters [Morikawa et al., 2004]. The goal of this study is preparing
thin HT films using EPD under various conditions, and validating their potential utility in
membrane and sensor applications.
2.2 Experimental process
2.2.1 Preparation of the colloidal suspension
The colloidal suspension used for the formation of the thin HT films is prepared
using a commercial HT powder (Sasol Mg70D, provided to us by the Sasol Corporation
with a Mg/Al ratio equal to 3.0, as determined by ICP-MS analysis), which contains 4.6%
lactic acid in order to improve its dispersion characteristics. Prior to its use in the EPD
process, the HT powder was ground by ball-milling (at the NETZSCH company). Figure
2.1 shows the particle size distribution of the original powder and of the ground powder
(hereinafter referred to as Mg70DS). A colloidal suspension of the Mg70DS powder in
distilled water was used for the EPD.
2.2.2 Preparation of the films
The EPD unit used for film preparation has been previously described [Kanamura
and Hanagami, 2004]. (Fig. 2.2) The thin films described here were deposited on two
different types of supports.
30
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
0.01 0.10 1.00 10.00 100.00
Micron meter
% in C h a n n e l
Mg70D
Mg70DS
Figure 2.1 Particle size distributions of the Mg70D and the Mg70Ds powders [Mg 70D
(av. particle size 12.8μm); Mg70DS (av. particle size 0.17 μm)].
One type of support is a tubular α-alumina substrate, with an I.D of 7 mm, O.D. of
11mm, an average pore radius in the range of 0.85~1.0 μm (calculated based on
permeation measurements with an inert gas), and ~30 % porosity (using the Archimedes
method). The second type of supports are the same tubular alumina substrates after thin
HT films have been deposited on them by dip-coating using the original Mg70D powder
(the colloidal dispersion utilized contained 5 wt% of HT). The α-alumina substrates were
dipped in the suspension for a period of 10 s, and then drawn out of the suspension at a
withdrawal speed of 0.5 cm/s. This procedure is repeated for each additional coating (the
substrates reported here were coated twice). The tubes were then dried overnight in
ambient air at 25
o
C for 24 h, and in an oven at 150
o
C for 12h.
31
Fig. 2.2 EPD membrane preparation apparatus.
For EPD deposition on these tubular substrates, a stainless steel rod was inserted inside
the support tube and used as the cathode. The stainless steel vessel itself was then used as
the anode (in general, the choice in electrodes may vary, depending on the zeta potential).
A DC power supply was used to generate the potential difference between the two
electrodes. The electrical potential selected for deposition of LDH films varied from 1 to
20 V. The solution pH was adjusted by using 0.09 M H
2
SO
4
and 0.09 M NaOH solutions.
The films, after coating, were dried overnight in ambient air, and at 150
o
C for 12 h. The
mass M (Kg) of the film deposited during EPD is described by the following equation:
∫
=
t
o
p
Edt aAC M μ (2.1)
Power supply
(-) Cathode
α-alumina
tube
(+) anode
Stainless steel
vessel
Stainless steel
rod
32
where t is the deposition time (s), a a coefficient related to the friction characteristics,
near the electrode, of the particles that are being deposited, A the electrode surface area
(m
2
), C
p
the particle concentration in the suspension (kg/m
3
), μ the electrophoretic
mobility (m
2
/Vs), and E the electric field (V/m). The electrophoretic mobility (μ) is given
by the following equation [Satta, 2000; Chen et al., 1999].
η π
ε
μ
e
d
L
ZV
4
=
(2.2)
where Z is the zeta potential, V the applied voltage, ε
d
the dielectric constant of the
medium in which the particles are suspended, η the viscosity of the suspension, and L
e
the
electrode separation distance. During the preparation of the thin films we investigated the
effect of applied voltage, support type, and number of coatings, while maintaining
constant the suspension concentration, the zeta potential, and the distance between
electrodes.
2.2.3 Characterization
The FTIR spectra of the HT materials were recorded using a Genesis II (Mattson,
FTIR) instrument; the experimental operating conditions were a scan-range from 4000
cm
-1
to 400 cm
-1
, scan numbers 50, and a scan resolution of 2 cm
-1
. XRD analysis was
carried out using a Rigaku X-ray diffractometer, with the CuK
α
line used for the X-ray
source with a monochromator positioned in front of the detector. Scans were performed
over a 2θ range from 5
o
to 75
o
with scan rate 2°/min and step rise 0.05. The surface area,
BJH (Barret-Joyner-Halenda) pore volume, Horvath-Kawazoe (HK) pore volume, and
33
pore size distribution (PSD) of the HT were calculated from the N
2
adsorption isotherm at
77K using a Micrometrics ASAP 2010 instrument. The morphology and thickness of the
HT membrane were investigated by scanning electron microscopy (SEM). The SEM
photographs were obtained on a Cambridge 360 Scanning Electron Microscope.
Transport properties of the HT EPD membranes were measured using a permeation
apparatus using a bubble-flow meter to measure the gas flows. Gas Chromatography was
used for mixed-gas experiments. (See Figure 3.2)
2.3 Results and discussion
2.3.1 Characterization of the HT powders and films
Since HT are crystalline materials, the challenge, similar to the one faces in the
preparation of zeolite films, is to prepare a polycrystalline thin film, with crystallites that
are well-compacted and intergrown, so that the intercrystalline region is minimized, and
transport is primarily through the intracrystalline region. The mechanism by which
crystallite intergrowth and closure of the intercrystalline space typically takes place is
Ostwald ripening, involving the growth of larger crystals from those of smaller size,
which have a higher solubility than the larger ones. Ostwald ripening is a well-known
phenomenon in the field of hydrotalcite and anionic clay research. A number of recent
studies [Chen et al., 1999; Xu et al., 2006; Hickey et al., 2000; Benito et al., 2006], for
example, have presented clear experimental evidence that Ostwald ripening is the key
34
mechanism leading to crystallite formation and growth both during the preparation as
well as the hydrothermal aging of these materials. There is a clear evidence, furthermore,
in our studies that Ostwald ripening also takes place during the formation of thin films
using the electrophoretic deposition method. For example, TEM pictures taken during our
study indicate clearly that LDH crystallite growth occurs during film formation by EPD
(see, for example, Figure 2.3). The EPD method, therefore, shows good promise for the
preparation of compact and well-intergrown thin films.
(a) (b)
Figure 2.3 TEM images of EPD HT coatings (a) 0.5h of deposition time (b) 1h of
deposition time.
200 nm 200 nm
35
The additional difficulty one may face in the preparation of defect-free, thin HT films is
that sintering of these materials presents challenges, since they begin to undergo
structural changes at temperatures higher than ~300
o
C, where their layered structure
begins to collapse [Kim et al., 2004], though these changes can be reversed by exposing
the material to an appropriate gas atmosphere, something which is known as the memory
effect. At lower temperatures these materials may still undergo some reversible structural
change; however, the layered LDH structure is maintained intact throughout the whole
material volume. Table 2.1, for example, presents the structural characteristics of the
original Mg70D powder evaluated by N
2
adsorption. The microporosity in Table 2.1 is
evaluated using the HK equation with a slit-type pore model, while the mesoporosity is
evaluated using the BJH model. Prior to nitrogen adsorption, the powder was evacuated
overnight at an elevated temperature, as indicated in the second column in Table 2.1.
Below ~280
o
C, which is the range for the high temperature application we envision for
the LDH films, for use in low temperature shift catalytic membrane reactors, the
structural characteristics remain fairly unchanged. However, above this temperature the
surface area begins to increase, but still remaining fairly low, and the average mesopore
size, but also the micropore size (probably reflecting the basal spacing, see below)
decrease. Treating the powder at much higher temperatures (see row 3, in Table 2.1 –
similar observations have been made by other groups [Yong et al., 2001]) results in very
high surface areas and very significant reductions in the average mesopore size, while the
average micropore size remains fairly invariant, consistent with the collapse of the
36
layered LDH structure throughout the whole volume of these materials [Iglesias et al.,
2005].
Other inorganic materials also used as membrane films (e.g., SiO
2
, Pd and its
alloys, etc.) undergo dynamic changes in response to their environment, and the
challenge is for the designer to be aware of such changes, and to use them to their
advantage.
Table 2.1 Surface area and pore size of the Mg70D powder treated under various
conditions
Pretreatment
( C
0
)
Surface
area (m
2
/g)
BJH
average pore
size (mesopores)
HK median
pore size
(micropores)
Mg70D
(uncalcined)
150 1.5 18.0 nm 10.7 Å
300 23.9 12.3 nm 8 Å
Mg70D
(calcined @ 550
o
C
for 4 h in Ar)
150 219.9 3.1 nm 8.4 Å
The purpose, therefore, of our studies, as previously noted, has been to study the
dynamic behavior of these materials in a variety of environments, and to understand the
implications in terms of preparing well-intergrown and compact thin films, which are
substantially free of pinholes and defects. The commercial HT was characterized by a
37
variety of techniques (PXRD, FTIR, and TGA analysis) and was shown to be very similar
to the HT synthesized in our laboratories [Yang et al., 2005], other than the aspects
relating to the presence of the lactic acid. For example, Figure 2.4a shows the PXRD
spectra for the in-house synthesized HT (hereinafter referred to as HTU, with a Mg/Al
ratio of 2.9) and the Mg70D. The interlayer basal spacing (as determined by the 003
Miller index) of the HTU is 7.73 Å (2θ=11.45), while that of the Mg70D is 7.76 Å
(2θ=11.4), showing only a minor influence of the lactic acid being present. These basal
spacings are in the range of values (7.6 ~ 7.8 Å) typically reported in the literature for HT
with the same Mg/Al ratio [Cavani et al., 1991; Titulaer, 1993]. In the FTIR spectra, see
Figure 2.5, a band at 1593.2 cm
-1
in the Mg70D spectrum indicates the presence of the
lactic acid. After calcination of the material for 12 h at 150
o
C, this band completely
disappears. The presence of lactic acid has more impact on the zeta potential of the
resulting colloidal solutions, which is an intrinsic property of a colloidal system of
importance during EPD, defined as the potential between the Stern and the diffuse layers
in the double-layer colloidal model. The zeta potentials for the HTU and the Mg70D HT,
measured by a Zeta Meter 3.0 (Zeta-Meter Inc.), are shown in Figure 2.4b. Though the
qualitative trends for both HT are similar, and close to those reported by other researcher
[Wilson et al., 1999], the Mg70D zeta potential is larger than the HTU potential; this is
because lactic acid in Mg70D makes the zeta potential shift, because it increases the
positive charge of the colloids. The isoelectric point (IEP) of the Mg70D colloidal
solution (12.5) is also larger than that of the HTU solution (10.7). The zeta potential
38
curves remain fairly flat in a range of pH values from 6 to 9. In this study, we have used
LDH suspensions with a pH ~7.
(a)
(b)
-20
-10
0
10
20
30
40
50
60
70
3 4 5 6 7 8 9 10 11 12 13 14
pH
Zeta potential
(mV)
Mg70D
HTU
IEP = 10.7
IEP = 12.5
Figure 2.4 The characterization of Mg70D and HTU powders (a) PXRD spectra (b) zeta
potential.
39
500.0 1000.0 1500.0 2000.0 2500.0 3000.0 3500.0 4000.0
Wavenumbers (cm-1)
Transmittance
Mg-O
CO
3
2-
C
3
bonding
H
2
O Bending
CO
3
-H
2
O
OH
-
3505.0
3502.6
3058.2
3020.0
1636.7
1659.0
1372.3
1371.3
657.6
663.8
1593.2
a
b
Fig 2.5 FTIR spectra (a) HTU (b) Mg70D.
The morphology of a film prepared by EPD using the Mg70DS powder
suspension on a HT dip-coated alumina substrate is shown in Figure 2.6 (SEM
photographs of the top-surface and the cross-section of the substrate are also shown in the
same Figure). The EPD technique results in smooth layers. In addition to the parameters,
relating to the EPD process, included in Equations (2.1) and (2.2) above, the size of the
particles in the suspension used for deposition also has a significant effect on the quality
of the final films prepared. Figure 2.7 shows two different films (using, in this case, pure
alumina substrates) prepared by EPD utilizing a suspension of the original Mg70D
powder, as well the Mg70DS powder.
40
(a) (b)
(c) (d)
Figure 2.6 The morphology of the support tube ((a) top surface, (b) cross-section) and
the EPD film prepared (deposition voltage 1 V, 2 coatings, with 1 h of
deposition/coating) on the support tube ((c) top surface, (d) cross-section). For the top
surfaces the magnification is 5K, while for the cross-sections the magnification is 3K.
The Mg70DS powder produces smooth films on the alumina substrates. The same is not
true, however, when using the original Mg70D powder. The films produced appear
rough, not compact, and without fully-intergrown crystallites.
41
(a) (b)
Figure 2.7 The morphology of (a) a Mg70DS EPD film and (b) a Mg70D EPD film, both
prepared using α-alumina support tubes (magnification: ×500).
2.3.2 Permeation results
Table 2.2 shows the permeation features of some of the support tubes utilized. As
previously noted, the alumina tubes are macroporous and transport through them is by
convective, non-separatory flow. The other three supports are prepared by dip-coating of
the original tubes using the Mg70D powder. The differences, in Table 2.2, between the
support tubes designated as Sa and Sb relate to where the HT layer is placed; for tubes
designated as Sa, the film is placed on the inside, while for Sb-type tubes it is placed on
the outside. The dip-coating of the HT layer on the alumina supports decreases their
permeance by an order of magnitude, while the transport mechanism transitions into the
Knudsen regime, indicative of the mesoporous structure of the HT layers (the Knudsen
CO
2
/N
2
separation factor is 0.80).
42
Table 2.2 The CO
2
and N
2
permeances and ideal selectivity of supports used in the
preparation of EPD membranes
Support
Δp
(psi)
Permeance (mol/m
2
sPa)
CO
2
/N
2
CO
2
N
2
α-alumina
(E1)
30
7.20×10
-6
6.87×10
-6
1.04
Sa(E2) 30
1.37×10
-7
1.68×10
-7
0.81
Sb(E3) 30
1.93×10
-7
2.29×10
-7
0.84
Sb(E4) 30
2.18×10
-7
2.57×10
-7
0.85
Table 2.3 presents the single gas transport data (the pressure difference during the
transport measurements was set at ∆P=206.8 kPa) for several membranes prepared with
various supports, some of whose properties are shown in Table 2.2. The voltage of
deposition, the number of coatings, and the time of deposition per coating are also
indicated in the Table. The EPD films have, typically, a permeance which is lower that
that of the original supports (an order of magnitude lower for the films prepared on
alumina substrates, and typically two times lower than the permeance of the HT dip-
coated substrates). Their thickness increases with the voltage and time of deposition
consistent with Equation (2.1). They show, furthermore, modest selectivity towards CO
2
,
their ideal separation factors, defined as the ratio of the single gas permeances, being
typically 40 – 70 % higher than the corresponding Knudsen values. (see Table 2.3). Table
2.4 shows the transport characteristics of some of the same membranes in Table 2.3,
43
using a CO
2
:N
2
mixture of gases (mostly an equimolar mixture, other than membrane E1
for which a 0.3:0.7 mixture was utilized) while maintaining the overall ∆P=206.8 kPa (30
psi) as with the single gas experiments. For most of the membranes we tested, the
permeance of the gases in the mixed gas experiments at 298 K was close to that of the
single gas experiments.
Table 2.3 The CO
2
and N
2
permeances and the ideal selectivity of EPD membranes.
(Colloidal suspension: 0.76wt% Mg 70DS, pH 7, Δp=30psi)
Name
Support
type
EPD conditions Single gas permeation
Voltage
Coating
times
EPD
Time
Temp
(K)
CO
2
Permeance
(mol/m
2
sPa)
CO
2
/N
2
E1
α-
alumina
1V 1 1.5h 298 4.91 ×10
-7
1.21
E2 Sa 1V 2 1h
298 7.66×10
-8
1.12
373 9.47×10
-8
1.17
473 9.41×10
-8
1.32
E3
Sb
1V 2 1h 298 1.43×10
-7
1.14
E4 20V 2 1h 298 2.07×10
-7
1.14
E5 20V 3 1h 298 2.07×10
-7
1.33
As the temperature increases, the CO
2
/N
2
separation factor also increases, which is
mostly due to an increase in the permeance of CO
2
. Similar increases in the separation
44
factor of the fastest moving component are typically observed with other microporous
membranes [Elyassi et al., 2007; Coronas et al., 1997]. This is, typically, indicative of
two transport paths, namely a mesoporous region through which the slowest moving
component transports through, and a microporous region accessible only to the fast
moving species.
Table 2.4 The CO
2
and N
2
permeances and mixture selectivity of EPD membranes
Memb.
Δp
(psi)
Feed
Gas
N
2
:CO
2
Temp.
(K)
Permeance ×10
7
(mol/m
2
sPa)
S.F.(CO
2
/N
2
)
CO
2
N
2
Mixed Single
E1 30 0.7:0.3
298 2.70 1.94 1.39 1.24
473 3.16 1.55 2.03 -
E2
30 0.5:0.5
298
0.758 0.646 1.17 1.12
373
1.01 0.792 1.27 1.16
473
1.11 0.636 1.75 1.17
E3 30 0.5:0.5 298 1.33 1.23 1.10 1.14
E5 30 0.5:0.5 298 2.34 1.53 1.53 1.33
One cannot a priori exclude that surface diffusion may also be, at least partially,
responsible for the CO
2
preferential transport. However, the observed temperature
dependence of CO
2
permeance (see Table 2.4), would tend to indicate otherwise, as
45
surface diffusion typically decreases as temperature increases; on the other hand, the
experimentally measured CO
2
permeance increases with temperature.
2.4 Conclusions
In summary, we have presented here results of our ongoing studies, the goal of
which is the preparation of thin HT films for membrane and sensor applications. The
EPD technique was utilized as a new method for the preparation of these membrane
films. The films were deposited on macroporous alumina substrates and on alumina
substrates, which were previously coated by conventional dip-coating techniques using
slurries of HT colloidal particles. The permeation properties of the resulting HT
membrane films were investigated by single and mixed-gas permeation tests. The EPD
films show selectivity towards CO
2
, consistent with the prior studies of these materials,
which show them to be effective CO
2
adsorbents.
As noted in the introduction, LDH show also high affinity towards a variety of
other species, all of which can accommodate themselves as anions in the interlayer
region. The LDH films show, therefore, promise for applications as sensor and
membranes for all such species. Before these materials find commercial applications,
however, as sensors and membranes, a means must be found to minimize the mesoporous
intercrystalline region. Surface-caulking by a silicone coating was shown previously very
effective [Kim et al., 2006] to repair film cracks and defects of HT thin films (see Chapter
3). Though useful for liquid phase and low temperature applications, the technique is not
appropriate, however, for films to be used at high temperatures. For the latter application,
46
other techniques must, therefore, be found to prepare such materials. Of all techniques we
know, EPD shows the best promise in that regard, based on process flexibility and the
ability of the technique to prepare well-intergrown thin films, as the preliminary data
presented here indicate.
47
Chapter 3 : HT membrane preparation by the
vacuum suction method
3.1 Introduction
Atmospheric concentrations of CO
2
have increased significantly in modern times,
attributed mostly to increased emissions from the burning of fossil fuels [Shekhawat et
al., 2003]. There is concern in recent years about reducing the emissions of CO
2
. To
accomplish this goal, it is important to be able to separate and capture the CO
2
from its
mixtures with other gases [White et al., 2003]. Although there are several methods
available, membrane separation processes are a relatively simple and energy-efficient
technique
[Baker, 2004]. Polymeric membranes are the current industry standard finding
a variety of applications, including CO
2
separation. Inorganic membranes are making
recent inroads due to their increased demand in new applications, such as fuel cells,
membrane reactors, and high-temperature separations. One advantage of the inorganic
membranes is that they are stable at high temperatures, and resistant to the harsh
conditions encountered in industrial environments. In particular, zeolite membranes,
carbon molecular-sieve (CMS) membranes, and silica membranes have been widely
studied as CO
2
-selective membranes at higher temperatures [Kusakabe et al., 1996;
Poshuta et al., 1998; Hayashi et al., 1995; de Vos and Verweij, 1998].
CO
2
separation and
capture from power-plants, however, involves exposing the membranes to high-
temperature oxidative environments, and to gases with high steam concentrations. Some
48
of the conventional inorganic membranes are not capable of functioning in such
environments (e.g., CMS membranes in oxidative environments, and zeolite and silica
membranes in the presence of high-temperature steam). Membranes made of hydrotalcite
(HT) materials show promise in this regard, as they are stable in high-temperature steam,
and in oxidative environments.
Hydrotalcites are also known as anionic layered double hydroxides (LDH), and
are being studied today for a variety of scientific and technological uses, such as ion-
exchange materials, adsorbents, catalysts and catalytic supports, modified electrodes, and
antacids
[Cai et al., 1994]. The LDH clays are natural or synthetic lamellar mixed
hydroxides with interlayer spaces containing exchangeable anions as noted in the
previous chapters.
Though hydrotalcites have been widely studied for a variety of applications, we
know of only a few published studies about their use for preparing thin films and
membranes. A number of different approaches to prepare such thin HT films and coatings
have been investigated. Pinnavaia and coworkers [Gardner et al., 2001] were the first to
prepare thin HT films on glass substrates using alkoxide-intercalated Mg-Al LDH
colloidal suspensions. Several other studies [Szekeres et al., 2005; Hornok et al., 2005;
Costan and Imae, 2004; Lee et al., 2006] were discussed in chapter 2. In the chapter 2,
we used an electrophoretic deposition (EPD) method for the preparation of hydrotalcite
thin films and membranes. The permeation properties of these films were investigated by
single and mixed-gas permeation tests, and the films were shown to be slightly
permselective towards CO
2
[Kim
et al., 2008a].
49
In this chapter, we report on the preparation of HT membrane films using a
vacuum-suction method. The goal of this study is to further validate the use of these
materials in the preparation of CO
2
-selective membranes and films, and to understand
their transport characteristics through single and mixed-gas permeation tests under
various conditions. The advantage of the vacuum-suction over the EPD method is that it
is more convenient to utilize for the preparation of larger area films and membranes.
3.2 Experimental
3.2.1 Materials
The hydrotalcite powder (Mg70D, with a Mg/Al ratio of 3.0, as measured by ICP-
MS analysis), and the silicone coating material (RTV 615A, B) were supplied by the
Sasol and GE Silicone Companies, respectively. Alumina powder, from which the
membrane support is made, and heptane (which is used in the preparation of silicone
films) were purchased from Accumet Materials and from Aldrich, respectively. In
addition, a hydrotalcite (herein after referred to as HTU) was also prepared in our
laboratories, through the co-precipitation method, in order to compare its characteristics
with those of the commercial HT [Yang et al., 2005]. The Mg/Al ratio for the HTU was
2.89, as measured by ICP-MS analysis.
50
3.2.2 Membrane preparation
Porous α-Al
2
O
3
discs, to be used as support for membrane preparation, were
prepared by pressing 5 g of alumina powder with 1000 kg
f
/cm
2
for 10 min, and then
calcining at 1000 ºC for 3 h. The thickness of the disks was ~ 2 mm, and their porosity
~0.34, as measured by the Archimedes method [Suwanmethanond et al., 2000]. For the
preparation of the HT membrane, we utilized the Mg70D powder after its particle size
was reduced by ball-milling at the NETZSCH Corporation. The particle size distribution
(PaSD) for the Mg 70D powder before and after ball-milling (the ground powder is
referred to, in this study, as Mg70DS) is shown in Figure 2.1 [Kim et al., 2008a].
Prior to membrane formation, the surface of the support disks was polished with
600 and 2400 grit-sand paper, and cleaned several times with deionized water in an
ultrasonic bath. Before coating, the cleaned alumina disc support was dried in air at 473K
for aboot 6 h. For membrane preparation, colloidal suspensions of 0.76 wt% Mg70DS HT
in deionized water were prepared by dispersing the powder with the aid of an ultrasonic
bath. The colloidal solution was then coated drop-wise, with the aid of a micro-pipette,
on the top of the support, while a mild vacuum of ~10 Torr (1,333.2 Pa) was applied at
the bottom of the disk using a mechanical pump, with the goal of enhancing the adhesion
of the HT films on the underlying support. The membranes thus prepared were dried in
air at 150
o
C for 12 h. Some of the membranes were also coated with a 3.5 wt.% silicone
solution (GE Silicones, RTV 615A, B) in heptane [Jansen et al., 2006]. The goal of this
treatment was to plug the intercrystallite voids and the pinhole defects.
51
3.2.3 Membrane characterization
The HT powders and the resulting membrane films were characterized by a
variety of techniques. The FTIR spectra of the HT were recorded using a Genesis II
(Mattson, FTIR) instrument; the experimental operating conditions were a scan-range
from 4000 cm
-1
to 500 cm
-1
, scan numbers 50, and a scan resolution of 2 cm
-1
. XRD
analysis was carried out using a Rigaku X-ray diffractometer, with the Cu-K
α
line used
for the X-ray source, with a monochromator positioned in front of the detector. Scans
were performed over a 2θ range from 5
o
to 75
o
with scan rate of 2°/min and a step rise
0.05. The surface area, the BJH (Barret-Joyner-Halenda), and the Horvath-Kawazoe (H-
K) pore volumes, and the pore size distribution (PSD) of the HT were calculated from N
2
adsorption at 77 K using a Micrometrics ASAP 2010 instrument. The samples for
adsorption were pretreated by heating in vacuum overnight. The surface morphology of
the HT membrane films was investigated by scanning electron microscopy (SEM), using
a Philips/FEI XL-30 Field Emission Scanning Electron Microscope.
Transport properties of the HT membranes were measured using a Wicke-
Kallenbach type permeation apparatus using a bubble flow-meter to measure the flow,
and a mass spectrometer to measure the gas concentration. For the silicone-coated HT
membranes, their permeation characteristics were measured, using the constant volume
(or diffusion time-lag) method [Fielding, 1980]
with the apparatus shown in Fig. 3.1.
To
measure the gas permeance, the permeate side pressure was kept at ~1×10
−2
Torr (1.33
Pa), while the feed side was maintained at a fixed pressure of either 30 or 40 psi
(206,842.7 or 275,790.3 Pa). Gas permeance (
r
P [mol/m
2
s Pa]) is calculated from Eqn
52
(3.1). Permeability (
e
P ) is calculated by multiplying
r
P by the film thickness, if known
(
e
P =
r
P × L).
p A T
dt dp V
P
r
Δ × × × ×
× ×
=
400 , 22 325 , 101
/ 273
0
(3.1)
where
0
V [cm
3
] is the volume of the downstream side of permeation cell,
dt
dp
[Pa/s] the
rate of change of pressure in that side with time, A [m
2
] the membrane area, △p [Pa] the
pressure-drop across the membrane, and T [K] the temperature. The ideal selectivity
(α
1/2
) or permselectivity of the HT membrane for a pair of gas components 1 and 2 is
expressed as the ratio of the two pure gas permeances.
2 ,
1 ,
2 / 1
r
r
P
P
= α (3.2)
The separation factor (S.F.) for the mixed-gas permeation experiments is defined as:
p
J
p
J
F S
Δ
Δ
=
2
1
2
1
. .
(3.3)
where J
1
and J
2
are the fluxes (mol/m
2
s) of the individual gases 1 and 2.
53
3.3 Results and discussion
3.3.1 Characterization results
FTIR spectra of the Mg70D HT and the HTU, reported in chapter 2, are very
similar to each other, and in line with what has been previously published for the same
materials [Roelofs et al., 2002]. The only difference between the two spectra is a single
band at 1593.2 cm
-1
, which results from the lactate acid (CH
3
-HCOH-COOH) added to
the Mg70D HT powder to improve its dispersion characteristics. PXRD spectra for the
HTU indicate a basal spacing of 7.72 Å (2θ=11.45, λ=0.1542 nm) according to Bragg’s
equation [Kim et al., 2008a]. The PXRD patterns for the Mg70D HT indicate a basal
spacing of 7.76 Å (2θ=11.4, λ=0.1542 nm).
Table 3.1 Pore volumes and diameters for the supported HT membrane as well as the
support disk
HK pore volume
(cm
3
/g)
[ > 1.7 nm]
BJH average pore
volume (cm
3
/g)
[1.7 – 300 nm]
BET surface area
(m
2
/g)
HT membrane 0.001258 0.01271 4.47
Support 0.000560 0.00791 1.91
54
Figure 3.1 The schematic of permeance test apparatus.
54
55
These results are consistent with those reported in the literature, indicating a typical basal
spacing ~7.8 Å
[Titulaer, 1983]. The XRD spectra of the prepared HT films are very
similar to those of the powders (other than the bands characteristic of the support),
indicating that the films retain their crystallinity during deposition.
Table 3.1 shows the BJH and HK pore volumes and the surface area of one the
HT membranes measured by BET analysis, as well as the corresponding values for the
alumina support disk. Despite the fact that the HT layer is a small fraction of the total
weight in the composite structure, the surface area of the supported membrane is more
than twice that of the alumina disk.
Figure 3.2 shows the morphology of the top surface of a membrane prepared by
the vacuum-suction method. The surface is smooth, as a result of using for coating the
suspension of the finely-gound Mg70DS HT powder. Figure 3.3a shows the cross-section
of one of these films. The film thickness is ~ 6-7 μm, and the layer appears to consist of
well-intergrown HT crystallites. Figure 3.3b shows the cross-section of one of the
silicone-coated HT membranes; clearly shown on this figure are the HT layer, and the
silicone layers sitting on its top.
3.3.2 Permeation results
Table 3.2 shows the permeance and ideal permselectivity for single gases, such as
He, N
2
, Ar and CO
2
, measured at two different transmembrane pressure drops of 30 or 40
psi (206,842.7 or 275,790.3 Pa), at room temperature, for two different supported HT
membranes prepared by the vacuum-suction technique.
56
(a)
(b)
Figure 3.2 SEM picture of the surface (top view) of a vacuum-suction membrane (a)
Magnification ×10K (b) Magnification ×40K.
57
(a)
(b)
Figure 3.3 The cross-section of (a) the HT membrane and (b) the silicone-coated HT
membrane (Magnification × 5K).
Silicone layer
HT layer
HT layer
58
The ideal permselecitivities of the HT membranes are higher than the ideal
Knudsen values, with the He permeance being of the order of ~10
-8
mol/m
2
s Pa. By
comparison, the permeance of the underlying alumina support is 2 orders of magnitude
higher than that of the HT membranes, with ideal permselectivities lower than the
Knudsen values. This is indicative of the presence of substantial microporosity in the HT
films.
Table 3.2 The permeation characteristics of HT membranes prepared by the vacuum-
suction method (temperature, 25 °C)
Memb
Gas
(MW))
Permeance × 10
8
(mol/m
2
s Pa)
Permselectivity
He/gas
p Δ
Ideal
Knudsen
value
Experimental Result
30 psi 40 psi 30 psi 40 psi
su#1
He (4) 5.292 4.93 1.0 1.0 1.0
N
2
(28) 1.18 1.24 2.65 4.48 3.98
Ar (40) 0.873 0.883 3.16 6.06 5.58
CO
2
(44) 0.611 0.604 3.32 8.66 8.16
su#2
He (4) 2.02 1.90 1.0 1.0 1.0
N
2
(28) 0.869 0.881 2.65 2.32 2.16
CO
2
(44) 0.261 0.278 3.32 7.74 6.83
α-alumina
Support
He (4) 647.9 686.2 1.0 1.0 1.0
N
2
(28) 326.5 354.8 2.65 1.98 1.93
Ar (40) 279.9 292.1 3.16 2.32 2.35
CO
2
(44) 308.4 337.5 3.32 2.10 2.03
59
A simple model was used to analyze the permeation characteristcs of these composite HT
membranes (see the Appendix). The model assumes that transport in the support layer is
a combination of convective flow and Knudsen transport, whereas in the HT layer it is
due to the combined surface and Knudsen diffusion. Using this model, indicates that for
the supported HT membranes more than 99% of the resistance to transport resides in the
HT layer, indicative that the transport characteristics measured reflect the properties of
the HT film itself. The effect of temperature on the permeation characteristics of the HT
membranes is shown in Table 3.3.
Table 3.3 The temperature effect on the transport properties of the HT membranes (HT-
3, p Δ =20 psi (137,895.1 Pa))
Temp.
(K)
Permeance × 10
8
(mol/m
2
s Pa)
Permselectivity
He N
2
Ar CO
2
N
2
/CO
2
He/CO
2
He/N
2
He/Ar
298 2.04 0.543 0.42 0.336 2.04 6.07 3.76 4.86
373 1.51 0.455 0.315 0.341 1.51 4.43 3.32 4.79
423 1.1 0.319 0.241 0.232 1.1 4.74 3.45 4.56
473 0.745 0.204 0.13 0.138 0.745 5.40 3.65 5.73
503 0.701 0.192 0.134 0.111 0.701 6.32 3.65 5.23
The permeance of all the gases decreases as the temperature increases (from 298 to 503
K), with the exception of CO
2
for which the permeance first increases as the temperature
slightly increases from 298 to 373, and then decreases. Notice, that the ideal separation
60
factors first decrease with temperature and then increase. These results are qualitatively
consistent with the changes in the structural characteristics and the average pore size
measurements made with HT powders, as shown in Table 2.1.
Table 3.4 shows the effect of pressure-drop on the single gas permeances, and the
ideal separation factor for three different membrane samples. The He:N
2
ideal separation
factor remains fairly invariant with respect to the pressure-drop (particularly for
membranes HT-4 and HT-5); the He:CO
2
and to a lesser extent the N
2
:CO
2
separation
factors, on the other hand, decrease as the pressure-drop increases.
Table 3.4 The effect of pressure-drop on the transport properties of the HT membranes
(temperature, 25°C)
Membr.
p Δ
(psi)
Permeance × 10
8
(mol/m
2
s Pa)
Permselectivity
He N
2
CO
2
He/CO
2
He/N
2
N
2
/CO
2
HT-2
20 2.19 0.881 0.240 9.13 2.49 3.67
30 2.02 0.869 0.261 7.74 2.32 3.33
40 1.90 0.881 0.278 6.83 2.16 3.17
HT-4
20 3.35 0.691 0.249 13.45 4.85 2.78
30 3.15 0.659 0.331 9.52 4.78 1.99
40 3.27 0.688 0.394 8.30 4.75 1.75
HT-5
20 2.94 0.619 0.303 9.70 4.75 2.04
30 2.85 0.600 0.319 8.93 4.75 1.88
40 2.97 0.619 0.358 8.30 4.80 1.73
61
It is interesting to note, that the CO
2
permeance increases with pressure-drop, reflecting
the fact that CO
2
has a significant affinity for the HT surface; in fact, the HT materials are
known to be excellent high-temparature CO
2
adsorbents [Kim et al., 2004]. As noted in
Tables 3.2 and 3.4, these HT films are microporous with ideal permselectivities that
exceed the Knudsen values. On the other hand, these membrane films show no
preferential transport towards CO
2
, as one would have expected, if transport through
these films were dominated by surface diffusion alone, since CO
2
is known to adsorb
preferentially in these materials.
The obsereved behavior is, in our opinion, the outcome of the complex three-
dimensional porous structure of these materials, which consist of completely interwoven
mesoporous and microporous regions [Chen et al., 2008]. In principle, if one were able to
completely “plug” the mesoporous regions, so that transport through these HT films was
only through the interlayer HT space in the individual crystallites (assuming that they are
well-intergrown to from a percolating region of such interlayer spacings throughout the
film), one would observe significantly higher permeation rates for CO
2
, when compared
to such inert gases such as He, Ar, and N
2
.
To test this hypothesis, we utilized a silicone material (RTV 615A, B supplied by
GE Silicone Companies) which is composed of vinyl-polydimethylsiloxane (VPDMS)
and modified silica. This compound has good thermal resistance, with the supplier’s data
indicating the operating temperature to be as high as 204
o
C. Silicone materials are
known to be CO
2
-permselective, due to their high CO
2
solubility, when compared to that
of fixed gases such as N
2
, He, H
2
[Kim et al., 2004; Chen et al., 2008].
62
Table 3.5 Permeation properties before and after the silicone-coating (Sil-HT-1)
Permeance units: ×10
9
(mol/m
2
s Pa), temperature 25
o
C
p Δ
(psi)
Gas
HT membrane
before coating
HT membrane after
coating
Silicone-coated
Alumina membrane
Perme-
ance
CO
2
/
N
2
Permeance
CO
2
/N
2
Perme-ance
CO
2
/N
2
30
N
2
26.5
0.68
0.0159±0.001
4
31.1±
2.5
0.204
9.6
CO
2
18.1 0.494±0.002 1.97
40
N
2
24.9
0.70
0.0165±0.001
3
27.3±
1.8
0.204
10.0
CO
2
17.5 0.450±0.008 2.04
The supported HT membranes were coated utilizing this silicone material using
the procedure described in the Experimental section. To study the effect of coating, we
first measured the properties of the membrane without the coating, and then, after the
membrane was coated, measured its transport properties again. As expected, after the
silicone coating, the HT membrane permeances significantly decreased, as shown in
Table 3.5, which also shows the permeances and separation factors of an alumina disk
also coated with the silicone layer. We calculated the permeability and ideal separation
factors for the silicone layer alone, based on the experimental data with the silicone-
coated alumina membranes, using the resistance-in-series model presented in the
Appendix, and the thickness measured from the SEM pictures. The values are shown in
Table 3.6, where they are compared with the corresponding values for PDMS from the
63
literature (we have been unable, unfortunately, to locate permeation measurements in the
literature with the VPDMS material). Using the calculated permeabilities for the silicone
layer, its measured thickness (SEM) on the supported HT membrane, and the
experimentally-measured permeances of the silicone-coated HT membrane, we
calculated the permeances and ideal selectivities of the HT membrane itself using the
“resistance-in-series” model (see the Appendix).
Table 3.6 The permeabilities and ideal selectivities of the silicone coating (temperature,
25
o
C)
p Δ
(psi)
Permeability (Barrer)
[Permeance ×10
9
(mol/m
2
s Pa)]
Ideal S.F. for CO
2
/N
2
Refs.
Experimental Reference (PDMS)
Experiment Ref.
CO
2
N
2
CO
2
N
2
15
2645
[25.3]
251.9
[2.41]
10.5
Jha et
al.,
2006
a
30
19.44
[1.97]
2.01
[0.204]
1300
[4.35]
299
[1.00]
9.6 4.35
Orme et
al.,
2003
b
40
20.13
[2.04]
2.01
[0.204]
- - 10.0 -
58 -
[1758-
8040]
Jiang et
al., 2008
Thickness of the silicone film
a
: 100μm,
b
: 35 μm
64
The calculated values (based on the experimentally measured permeances of the silicone-
coated HT membranes after accounting for the effect of silicone coating itself) and the
experimental values (measured with the HT membranes prior to being coated with the
silicone layer) are compared in Table 3.7.
Table 3.7 Comparison between the calculated and experimental values for the HT
membrane films (temperature, 25
o
C, Sil-HT-1)
p Δ
(psi)
Permeance ×10
9
(mol/m
2
s Pa)
Permselectivity
CO
2
/N
2
Calculated (after coating
with silicone)
Experimental (before
coating with silicone)
Calcula
ted
Experime
ntal
CO
2
N
2
CO
2
N
2
30 0.659 0.0172 18.1 26.5 38.23 0.68
40 0.577 0.0180 17.5 24.9 32.15 0.70
The experimental values are significantly higher than the calculated values, which
indicates that the silicone layer penetrates deep into the underlying HT film’s structure
(since, when accounting for the effect of the silicone layer, we used the thickness of the
silicone layer on the top of the HT membrane, calculated from the SEM images). Table
3.8 shows the permeation characteristics of a different silicone-coated HT membrane for
CO
2
as well as other molecules like He, N
2
and H
2
. In the same Table we also present the
values of the silicone layer itself, measured in composite structures which are utilizing
the alumina disks as supports again, the silicone-coated HT membrane film exhibits
enhanced CO
2
permeation.
65
Table 3.8 The permeation characteristics of a silicone-coated HT membrane (Sil-HT-2),
and of a silicone film
Memb
r.
p Δ
(psi)
Permeance X 10
9
([mol/m
2
·sec·Pa]) Permselectivity
CO
2
N
2
He H
2
CO
2
/N
2
CO
2
/H
2
CO
2
/H
e
Sil-
HT-2
30
0.546±
0.003
0.0159±
0.0014
0.0440±
0.0005
0.108±
0.001
34.4±
3.1
5.0±
0.1
12.4±
0.1
40
0.489±
0.009
0.0165±
0.0013
0.0504±
0.0016
0.102±
0.001
29.7±
1.8
4.8±
0.1
9.70±
0.2
Silico
ne
30 1.97 0.204 0.291 0.504 9.6 3.9 6.8
40 2.04 0.204 0.296 0.516 10.0 3.9 6.9
Furthermore, the measured ideal separation factors of the silicone-coated HT membrane
significantly exceed those of the silicone layer itself, likely pointing out that the
separation characteristics of the silicone-coated HT membrane reflect the intrinsic
properties of the HT material itself. They likely, furthermore, reflect the strong affinity of
the CO
2
molecules for the intercrystalline space of the hydrotalcite material itself.
Finally, Table 3.9 shows the results of mixed-gas permeation tests with two
silicone-coated HT membranes. These experiments were carried out using the permeation
apparatus of Fig. 3.1 connected with a mass-spectrometer to measure the composition of
the permeated gas mixture. The He/CO
2
mixed-gas separation factors, as defined by E.S.
(3.3), are somewhat lower, but still fairly close to the measured single-gas values.
66
Table 3.9 Mixed-gas experimental data for a silicone-coated HT membrane, temperature,
25
o
C, 30 = Δp psi (206,842.7 Pa)
Sample
Mole fraction Permeance (mol/m
2
s Pa) S.F.
Single-gas
CO
2
/He
CO
2
He CO
2
He CO
2
/He
Sil-HT-2 0.5 0.5
5.65±0.035
×10
-10
4.83±0.37
×10
-11
11.69±
0.9
12.4±0.1
Sil-HT-3 0.5 0.5
4.94±0.03
×10
-10
5.15±0.3
×10
-11
9.60±
0.7
11.2±0.2
3.4 Conclusions
We have presented the results of our ongoing research, the goal of which is the
preparation and study of CO
2
-selective hydrotalcite films and membranes. HT films are
prepared by a vacuum-suction method using alumina macroporous disks as supports. The
membrane films were tested for their transport characteristics using single and mixed-gas
permeation tests, as well as by a variety of other characterization techniques, including
SEM, FTIR, XRD and BET. Microporous membrane films were prepared, which show
significantly higher permeation rates for gases with smaller kinetic diameters, such as He,
as compared with gases with larger kinetic diameters, such as Ar. On the other hand,
these membranes show no preferential transport towards CO
2
, even though the HT films
are expected to be CO
2
-selective. Coating the HT membranes with a silicone layer
changes their separation characteristics, making them significantly more permeable
towards CO
2
. We attribute this to the plugging, by the silicone layer, of the mesoporous
67
regions of these HT films, which then forces the molecules to transport mostly through
the microporous regions, where one expects surface transport by the CO
2
molecules to
dominate, due to their strong affinity towards the HT surface.
68
Chapter 4: Micromembranes
4.1 Introduction
The term MEMS (Micro Electro Mechanical System) is used to describe
miniaturized mechanical machinery, such as motors, turbines, pumps, and actuators used
in micro-systems. [Madou, 2002] MEMS fabrication techniques are also today finding
wide use in the design of microchemical systems (MCS), particularly membrane
microreactors (MMR), which are devices that combine reaction and separation into a
single unit. The reason why MCS are finding increasing use in chemical systems is
because they are more efficient in chemical production and cause less pollution. In
particular, functional “Lab or Factories-on-a-Chip” show good potential for application in
a number of industries [Fletcher et al., 2002]. In addition, these devices have a good
potential as analytical tools for chemical, biochemical and biomedical devices and as
diagnostic tools for rapid disease screening [Leung and Yeung, 2004]. MMR have been
fabricated, for example, using silicon (Si) wafers as micro-fuel processors to function as
an integral component of miniature, portable methanol fuel cells [Jense, 1999; Bravo et
al., 2004; Goerke et al., 2004]. The fabrication of micromembranes has been studied by
several investigators who investigated the preparation of zeolite and Pd membranes on Si
wafers [Wan et al., 2001; Wilhite et al., 2004; Pattekar and Kothare, 2004]. Santamaria
and coworkers, [Mateo et al., 2003] for example, prepared silicalite-1 micromembranes
on laser-perforated stainless steel (SS) sheets and tested them for propane and N
2
gas
69
separation. Leung and Yeung [2004] prepared MFI zeolite membranes on Si
microchannels, and tested them for gas separation.
The synthesis of thin HT films and membranes is also attracting recent interest
due to their unique separation properties as noted in previous chapters. The HT
membranes also show promise for application in membrane reactors for the production of
hydrogen through the steam reforming or the water gas shift reactions. The advantage of
the HT over the Pd membranes is that they are permselective to CO
2
, and the H
2
product
is, therefore, delivered at high pressures, thus requiring no re-pressurization for further
use. The ability to prepare high quality HT micromembranes will make possible their use
in MMR, and as components of miniaturized fuel cells [Fletcher et al., 2002]. Though the
HT have been widely studied for various applications, we know of only few published
studies on the preparation of HT thin films and membranes (and, so far, of no study that
details the preparation of micromembranes). Pinnavaia and coworkers [Gardner et al.,
2001] were the first to prepare thin HT films on glass substrates using alkoxide-
intercalated Mg-Al LDH colloidal suspensions. Othman et al.
[2006]
studied coating HT
films on zeolite adsorbents, in order to prepare a new class of adsorbents. A number of
studies [Szekeres et al., 2005; Hornok et al., 2005; Costan and Imae, 2004] reported on
the preparation of LDH hybrid films with various polymers, using a “layer-by-layer”
deposition technique, through the formation of Langmuir-Blodgett films. A different
approach to prepare hybrid LDH/polymeric thin films on Si wafers was utilized by Lee et
al. [2006] After coating a Mg-Al-CO
3
LDH film on a Si wafer, they formed a polymeric
film on the top by a direct ion-exchange reaction with a PAA binary-solvent solution. In
70
our prior study (chapter 2), we used an electrophoretic deposition method for the
preparation of HT membranes. These films were shown to be slightly permselective
towards CO
2
. In a subsequent study (chapter 3), a vacuum-suction deposition method was
applied to prepare the HT films on macroporous alumina substrates. Permeation tests
indicated the films to be substantially microporous, permeating gases like He, with
smaller kinetic diameters, faster than larger molecules, such as N
2
or Ar (2-3 times faster
than what is expected from Knudsen diffusion alone). Coating the HT films with a
silicone layer, to plug potential defects, resulted in membranes with substantial selectivity
of CO
2
vs. N
2
. The focus in this chapter is on preparing thin HT micromembranes and on
understanding their transport characteristics. (100) p-type Si wafers and perforated SS
discs were used as supports in this chapter.
4.2 Micromembrane preparation
4.2.1 Preparation of colloidal suspensions
The collidal suspension (Col 1, Mg/Al=3.1 by ICP) was prepared by the
following method after synthesis of HT by the co-precipitation method. [Yang et al.,
2005] ACS reagent grade anhydrous Na
2
CO
3
, Mg(NO
3
)
2
·6H
2
O and Al(NO
3
)
3
·9H
2
O from
Aldrich were used. An aqueous solution (45 ml) of Mg(NO
3
)
2
6H
2
O and Al(NO
3
)
3
9H
2
O
was prepared with a Mg/Al molar concentration of 3:1. This solution was added all at
once to a second solution (70 ml) containing NaOH (0.35 mol) and Na
2
CO
3
(0.09 mol) at
333 K. After 2 h of reaction under vigorous stirring at a temperature of 60
o
C, the
71
reaction mixture was washed with distilled water by centrifuging it for 10 min at 2500
rpm in sealed containers until the pH of the solution became ~ 8. It was then allowed to
stand for one day after 4h of sonication. The resulting mixture was separated in three
layers i.e. a dark bottom layer, a translucent middle layer, and a transparent top layer. The
translucent layer was used in this process. Also, another colloidal suspension (Col 2,
20wt% in D.I, Mg/Ai =3.0 by ICP) was prepared by heating the ball-mill treated 7.6wt%
suspension of commercial HT powder (Mg 70D purchased from Sasol) with an average
particle diameter of 0.17 μm at 60 °C for 6h. Figure 4.1 shows the particle size
distribution for both Col 1 and Col 2 respectiviely. The average diameter of Col 1 particle
is around 0.32 ㎛ measured by Brookhaven BI-200SM dynamic light scattering
instrument at R.T.
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
0.01 0.10 1.00 10.00 100.00
Micro meter( )
% in Channel
Mg70DS (Col 2)
HTU (Col 1)
Figure 4.1 The particle size distributions (PaSD) of the Col 1 and Col 2, Col 1 (av.
particle size 0.322 μm); Col 2 1 (av. particle size 0.17 μm)
72
Figure 4.2 shows the FTIR spectra of the HT powders used to prepare the Col 1 and Col
2 colloidal solutions. The FTIR spectra contain peaks which are typical of the HT, as
reported in the technical literature. In the Col 2 spectrum, a band at 1593.2 cm
-1
indicates
also the presence of the lactic acid used as a dispersant material.
Figure 4.2 FTIR spectra (a) Col 1 (b) Co1 2.
4.2.2 Silicon-based micromembranes
Microchannels were etched on the Si wafers using a standard photolithographic
technique (details of the procedure can be found in Wan et al. [2001]), with the design
pattern utilized shown in Figure 4.3. The design consists of two different channel types,
with a width of 500 ㎛ and 1000 ㎛ correspondingly. A colloidal solution (Col 1),
prepared using an HT synthesized in our laboratory, was used for the HT film preparation
on the Si wafer microchannels using a micropipette.
OH
-
CO 3-H 2O
H 2O Bending
C 3 bonding
CO 3
2-
Wavenumber (cm
-1
)
Transmittance
(a)
(b)
Mg-O
73
Figure 4.3 The design of silicon microchannel.
Figure 4.4 The fabrication process for the silicon based micromembranes.
. The films were dried in air at 110
o
C for 12 h. The films were deposited either
directly on the Si base of the microchannel, or on top of a γ-Al
2
O
3
intermediate film
deposited on the Si channel, prior to the deposition of the HT film, using a boehmite-sol,
74
and drying the resulting film at 600
o
C for 2 h. After the HT film deposition, the Si base
of the channel was etched using a 44 wt% KOH solution at 359 K. Figure 4.4 shows
schematically the fabrication process of the HT micromembranes on silicone wafer.
4.2.3 Micromembranes prepared on stainless steel supports
We have also prepared micromembranes on stainless steel supports. These
membranes are potentially more useful than silicon-wafer based membranes due to their
better pressure resistance.
(a) (b)
Figure 4.5 The design of (a) 4 and (b) 20 microholes with 800 μm.
Holes or channels with characteristic dimensions within the range of 50 ~100 ㎛
are typically fabricated by drilling or laser ablation. Unfortunately laser drilling is very
expensive. In order to prepare micromembranes using the SS substrates, prior to
deposition a square pattern of circular holes (typically 4 holes, but substrates with as
many as 20 holes were also used (Fig. 4.5)), each with a diameter of ~800 μm, were
drilled through the substrate. The micromembranes were prepared by coating the holes
12 mm
9 mm
75
with a colloidal solution (Col 2) prepared using a commercial HT powder, with the aid of
a micropipette, and by drying the resulting films at 150
o
C for 24 h.
4.3 Results and discussion
4.3.1 Silicon-based micromembranes
SEM pictures of the microchannel etched on silicon wafers and used for the
preparation of micromembranes are presented in Fig. 4.6.
(a) (b)
Figure 4.6 SEM picture of silicon microchannel (a) top view (b) cross-sectional view.
(Magnification × 100).
SEM observations indicate that the films prepared by coating the HT solution directly on
the Si channels are well-adhering, with submicron-size crystallites (Fig. 4.7). No major
defects/cracks appear on the films, though some intercrystalline voids do exist, as can be
seen in Fig. 4.7. Using an intermediate γ-alumina layer (Figs. 4.8 (a,b)), prior to the HT
film deposition avoids the formation of visible voids in the HT top-film, the surface of
76
which appears more uniform (Fig. 4.9 (a)) and with crystallites which are smaller than
those of the HT film formed without the aid of the γ-Al
2
O
3
intermediate layer. Moreover,
the HT and γ-Al
2
O
3
layers appear well-adhering with each other, the total composite film
thickness being ~10 μm (Fig. 4.9 (b)).
(a) (b)
Figure 4.7 SEM pictures of a membrane deposited on a silicon microchannel by coating
with an HT colloidal solution. (a) top surface, (b) cross-sectional view. (Magnification,
top view: × 20K, cross: × 10K).
EDX (Energy Dispersive X-ray spectroscopy, 15KV, Philips/FEI XL-30) analysis was
carried out at 5 spots (Fig. 4.9 (b)) along the film’s cross-section. Table 4.1 shows the
results of the EDX analysis which indicate that the concentration of Mg, Al, O and Si
vary according to the position in the membrane layer. Mg only exists in the HT layer.
Thus, the presence of Mg indicates also that the HT is present, on the top of the γ-
alumina layer. Note, that the concentration of Al increases as we move towards the γ-
alumina layer. Section 3 appears to be the border between the two layers, as indicated
77
from the sharp changes in the Mg and Al concentrations. Therefore, the above results
indicate the formation of the HT layer (manifested by the presence of Mg) on top of the
γ-Al
2
O
3
layer, and a ‘diffuse” interface where the two films blend.
(a) (b)
Fig. 4.8 The morphology of (a) top view and (b) cross for 1
st
γ –alumina layer
(Magnification, top view: × 20K, cross: × 10K).
(a) (b)
Figure 4.9 The morphology of (a) top view and (b) cross for HT coating layer with
intermediate layer. (Magnification, top view: × 20K, cross: × 5K).
γ-Alumina
HT-γ-Alumina
78
Figure 4.10 The section of EDX analysis on Figure 4.9 (b).
Table 4.1 The results of EDX analysis on HT - γ-alumina layer
Atoms
Atomic concentration (%) in HT- γ-alumina layer
Section1 Section 2 Section 3 Section 4 Section 5
O 48.73±0.94 44.79±0.80 35.37±0.75 34.21±0.68 30.81±0.71
Al 35.73±0.56 43.93±0.54 62.32±0.69 64.25±0.34 60.10±0.68
Mg 14.68±0.38 10.81±0.29 1.80±0.15 0.95±0.11 0.80±0.12
Si 0.85±0.13 0.47±0.11 0.51±0.11 0.59±0.10 8.29±0.27
1
2
3
4
5
EDX analysis
79
4.3.2 Stainless steel-based micromembranes
Figure 4.11 shows the morphology of the HT membrane prepared on the SS disk.
The HT surface is smoother than that of the films prepared in the Si microchannels. This
is, likely, because in the preparation of these membranes we used Col 2, which was
prepared with an HT powder with a somewhat smaller average particle size than Col 1,
but which also contains a small quantity of lactic acid as an additive to improve its
dispersion characteristics. The films are dense, and even under a high magnification
(Figure 4.11(b)), individual crystallites are hardly visible. Figure 4.12 indicates that the
thickness of the microhole membrane is around 300 μm.
(a) (b)
Figure 4.11 SEM picture of microhole membrane (a) Magnification ×100 (b)
Magnification ×20K (top view).
80
Figure 4.12 SEM picture of microhole membrane (cross section view)
(Magnification ×50).
4.3.3 Permeance results
The permeation characteristics of the micromembranes were tested by single gas
(He, Ar, CO
2
, and N
2
) permeation experiments. Due to experimental limitations, we were
unable to measure the permeation characteristics of the Si-based HT micromembrane
(~10μm thick), as they developed ruptures even for relatively small transmembrane
pressure gradients. The molecular diameters of molecules used for studying membrane
permation in this chapter are presented in Table 4.2. We were able to generate reliable
data, however, with several of the SS micromembranes, as indicated in Table 4.3, which
includes permeances measured at room temperature, and the calculated single-gas
selectivities (by comparison the Knudsen ideal gas pair selectivities are α(He/Ar)=3.16,
α(He/CO
2
)=3.32, α(N
2
/CO
2
)= 0.8 – missing entries in Table 4.3 typically indicate that
81
the membranes developed cracks during the experiments, and we were, therefore, unable
to complete the permeation tests).
Table 4.2 Kinetic diameters of molecules [Baker, 1974]
Molecules Diameter(nm)
H
2
0.289
He 0.26
CO
2
0.33
N
2
0.364
Ar 0.34
The permeances of these membranes are relatively low (~10
-8
-10
-9
mol/m
2
sPa) due
to their micropore nature, and their relatively large thicknesses (~300 μm). These
membranes have a complex three-dimensional porous structure, which consists of
completely interwoven mesoporous and microporous regions [Kim et al., 2008b; Chen et
al., 2008]. CO
2
has a unique affinity for the LDH interlayer space, which is why these
materials are known to be effective, reversible CO
2
adsorbents
[Dadwhal et al., 2008].
As can be seen in Table 4.3, several of the SS micromembranes show separation
factors that are higher than the corresponding Knudsen values, indicative of the presence
of a substantial microporous region.
82
Table 4.3 The permeation characteristics of a HT micro-membranes
(2
nd
coat, Temp.: R.T)
Mem
b.
p Δ
(psi)
Permeance×10
9
(mol/m
2
s Pa)
Permselectivity
He N
2
Ar CO
2
He
/Ar
He
/CO
2
He
/N
2
CO
2
/N
2
M#1 30 16.5 - 3.66 2.21 4.51 7.49 - -
M#2 20 21.0 5.66 6.79 4.18 3.11 5.03 3.71 0.74
M#3 20 26.4 7.47 - 8.05 - 3.28 3.53 1.08
M#4 30 7.55 1.17 0.41 2.04 18.6 3.7 6.5 1.7
M#5 30 0.272 0.386 0.704
Not all the membranes are CO
2
-permselective, however. Preferential transport of CO
2
with respect to a noble gas with a smaller size (like He) – see membrane #5 – is
determined by the complex interplay between molecular and surface transport in the
membrane’s microporous and mesoporous regions. In general, the more “compact” the
membrane is, as manifested by a lower He permeance, the more permselective it is
towards CO
2
. The fact that these membranes contain both mesoporous and microporous
regions, was also verified experimentally by BET investigations, which indicated clearly
the presence of both regions.
83
4.4 Conclusions
In summary, we have investigated the fabrication of HT micromembranes using
Si wafers and perforated SS discs as templates. Electron microscopy indicates that the HT
films are compact and dense, and well-adhering on the substrates. For the Si-based
membranes, the presence of an intermediate γ-Al
2
O
3
layer seemed to improve the
characteristics of the deposited HT films. The permeation properties of the HT
micromembranes were investigated through single-gas permeation tests. Several of the
micromembranes exhibited ideal separation factors that exceeded the corresponding
Knudsen values. Some of these membranes were also permselective towards CO
2
. These
micromembranes show good promise for application in sensor devices and in MMR. In
this study, we have shown it feasible to prepare such membranes through traditional
microfabrication and membrane thin film deposition methods. Much remains to be
learned, however, about the optimal preparation of these materials, as their permeation
properties depend strongly on the geometry and morphology of their complex three-
dimensional structure and, in the case of CO
2
, on its strong affinity towards the LDH
surface.
84
Chapter 5 : The preparation and characteri-
zation of Hydrotalcite (HT)- sulfonated
polyetheretherketone (SPEEK) cation-exchange
membranes for DMFC (Direct Methanol Fuel
Cell)
5.1 Introduction
Fuel cells find application in the transportation industry and in stationary and
portable power generation. Among them, direct methanol fuel cells (DMFC) are
attracting considerable attention due to the fact that they can be operated using liquid
methanol as a fuel. However, DMFC face today two major technical hurdles, the first
being their slow methanol oxidation kinetics at the anode, and the other unwanted
methanol transport (also known as crossover) through the polymer electrolyte membrane.
The latter phenomenon decreases the fuel cell voltage, resulting in loss in fuel cell
efficiency, and poisons the catalyst at the cathode [Hoogers, 2003].
The commonly utilized, commercially available Nafion
®
membranes are
expensive, and suffer from significant MeOH crossover, with as much as 40% of the
methanol reported lost due to crossover during fuel cell operation [Tricoli, 1998]. In
addition, Nafion
®
loses its proton conductivity at temperatures above 80
o
C, due to its
inability to stay fully hydrated at such elevated temperatures. Because of the challenges
conventional Nafion
®
membranes face, significant research in recent years has been
devoted to the development of more effective membranes, including those made using
85
non-fluorinated polymers, or from hybrid materials consisting of a polymer base
intermixed with one or more fillers [Jannasch, 2003].
Examples of non-fluorinated
polymers used to prepare conductive membranes include poly-benzimidazole (PBI),
sulfonated poly-etherketones (SPEK), sulfonated poly-etheretherketones (SPEEK), poly-
ethersulfone (PES), and sulfonated poly-phenyl-quionxaline (SPPQ) [Roziere and Jones,
2003]. The SPEK and SPEEK membranes, in particular, have been the focus of many
investigations [Li et al, 2005; Li et al., 2003; Regina et al., 2006; Xing et al., 2004], and
have been shown to have good chemical and mechanical stability, and high proton
conductivity (0.01 to 0.05 Scm
-1
), together with reduced methanol permeability, and a
lower cost than Nafion
®
membranes. In addition to the SPEK and SPEEK, all the other
aforementioned polymers are also thermally stable, due to their polyaromatic or poly-
heterocyclic nature. Furthermore, their sulfonic functional groups generated through
sulfonation, endow them with ionic conductivity that depends strongly on the degree of
sulfonation.
Membranes made of hybrid materials (polymer + filler) have also attracted recent
attention for fuel cell applications, and some of the key published studies are listed in
Table 1.7. In the majority of these studies, inorganic fillers, such as layered zirconium
phosphate
[Carbone et al., 2004; Silva et al., 2005], SiO
2
[Dimitrova et al., 2002; Bauer
and Willert-Porada, 2005; Wu and Ma, 2004; Kim et al., 2005], HT [Lee et al., 2005],
layered silicate [Chang et al., 2003], TiO
2
[Bauer and Willert-Porada, 2005],
phosphotungstic acid (PWA) [Wu and Ma, 2004], and heteropolyacids [Dimitrova et al.,
2002] are dispersed in either fluorinated or non-fluorinated proton conducting polymers.
86
For example, Arico et al. [2003] studied a number of such fillers, showing that the
conductivity of hybrid membranes relates to the acidity of the fillers, with the membranes
containing the most acidic fillers (in the order SiO
2
-PWA > SiO
2
> ZrO
2
> n-Al
2
O
3
--
neutral > b-Al
2
O
3
–basic) showing the best performance. For some fillers (e.g., ZrO
2
,
HT), methanol permeability was reported to decrease as the filler concentration increased
[Lee et al., 2005; Silva et al., 2005]. This is not generally true, however, with SiO
2
addition, for example, shown to increase both the conductivity and the methanol
permeability of hybrid membranes [Kim et al., 2005].
In this chapter, the HT-SPEEK hybrid membranes are prepared and characterized,
and their performance is evaluated. The HT is a slightly basic material, also known as the
layered double hydroxide (LDH), made from positively-charged layers containing two
kinds of metallic cations, and exchangeable interlayer charge-balancing anions [Miyata,
1982; Yang et al., 2002].
The
HTs are known to retain substantial amounts of water in
their interlayer structure (~11.9 wt% for the MgAl-CO
3
LDH used in this study, with a
Mg/Al ratio equal to 3). Water plays a key role in the proton transport, significantly
increasing its mobility
[Arico et al., 2003]. Well-dispersed HT filler particles, on the other
hand, are expected to hinder methanol diffusion in the hybrid membranes. The goal of
this study has, therefore, been to investigate the effect of the HT inorganic fillers on the
properties of the HT-SPEEK membranes, in terms of their methanol permeability and
proton conductivity, and to study the suitability of the materials as a potential
replacement for Nafion
®
membranes.
87
5.2 Experimental
5.2.1 Chemical and materials
Victrex PEEK (poly-etheretherketone) grade 450F particles and the HT (Pural Mg
70, with Mg/Al the ratio ~3.0 by ICP analysis) were kindly provided to us by the Victrex
and Sasol companies, respectively. Sulfuric acid, dimethylacetamide (DMAc) and
methanol (HPLC grade, 99.99% pure) were purchased from VWR. Nafion
®
115 and
Nafion
®
112 were purchased from DuPont. The properties of these two commercial
membranes were also studied, in order to compare them with the hybrid membranes.
5.2.2 Preparation of the polymers
The PEEK and the sulfuric acid were mixed together at room temperature (1g of
PEEK per 45 ml of acid), following the sulfonation procedure described in the literature
[Li et al., 2003; Huang et al., 2001], and also shown schematically in Fig. 5.1. The
sulfonation degree of the polymers (defined by Equation (5.1) below) was varied by
changing the reaction time from 24 to 144 h. When the desired sulfonation degree (SD)
was achieved, the polymer solution was immersed into a large excess of ice-cold water
under mechanical stirring. The polymer precipitate formed was filtered and washed
repeatedly with deionized water, until the pH of the filtrate solution became equal to 5.
88
Fig. 5.1 Diagram of the sulfonation reaction of PEEK.
The procedure involves immersing a sample (0.25 g) of the sulfonated PEEK into 100 ml
of 0.01 M NaOH aqueous solution for 2 days, in order to neutralize the SPEEK polymer,
and to fully convert it into its sodium salt form, i.e., the SPEEK–Na. Dilute sulfuric acid
(0.005M) is then employed to back-titrate the NaOH aqueous solution containing the
polymer sample, which is partially neutralized by the SPEEK, using a pH-meter to
determine the neutral point (pH=7). By measuring the amount of H
2
SO
4
consumed during
the titration, the number of moles of SO
3
Na units contained in the neutralized sample
(corresponding to the SO
3
H units in the original sample) is determined. The SD is then
described by the following equation
unit PEEK the of number molar unit Na SO - PEEK the of number molar
unit Na SO - PEEK the of number molar
3
3
+
= SD (5.1)
H
2
SO
4
+
R.T.
Poly Ether Ether Ketone
Sulfonated Poly Ether Ether Ketone
Reaction time (24h – 144h)
89
while the IEC is given by
) / ( 1000 ) / ( g meq W N IEC
sample Na SPEEK
× =
−
(5.2)
where W
sample
is the weight of the SPEEK sample and
Na SPEEK
N
−
is the molar number of
the Na SO - PEEK
3
units .
According to Huang et al. [Huang et al., 2001], the SD and IEC relate to each other as
follows:
102(IEC) - 000 1
(IEC) 288
= SD (5.3)
When SD =100%, the calculated limiting IEC value of the SPEEK is 2.56 meq/g.
The SD can be also quantitatively determined by evaluating the peak area
represented by all protons in the aromatic phenyl ring in the
1
H NMR spectra. Jin et al.
[Jin et al., 1985] have reported that the sulfonic group occurred only on the phenyl ring in
between two ether groups of the PEEK repeat unit. Therefore the following equation is
suggested [Nolte et al., 1993; Wu et al., 2006; Zaidi et al., 2000; Robertson et al., 2003].
D C B B A A
E
H
H
A
A
n
n
, ,
'
, ,
'
,
2 12 Σ
=
−
; , 1 0 ≤ ≤ n (%) 100 × = n SD (5.4)
where
E
H
A is the peak area of the H
E
(i.e., the hydrogen located next to the sulfonic acid
groups) signal and
D C B B A A
H
A
, ,
'
, ,
'
,
is the integration of the peak area corresponding to all
aromatic hydrogen. The
E
H
A increase, as the SD increases. Table 5.1 compares the SD
calculated by both methods. The SD values from the
1
H NMR study are similar to the
values from the titration method.
90
Table 5.1 The SD measured by the titration method and by
1
H NMR.
Samples
Sulfonation Degree (%)
Titration method
1
H NMR study
SD 45 45 45.3
SD 53 53 52.7
SD 60 60 58.4
SD 81 81 75.3
5.2.3 Preparation of the membranes
Dried SPEEK powders were dissolved (9.1 wt %) in DMAc, and the resulting
solution was filtered using fine-glass filters (Whatman syringe-filters) with 1.0 μm pore
size [Zaidi et al., 2000]. In order to prepare the composite membranes, a suspension of
submicron-sized HT particles (mean diameter of ~0.189 μm) in DMAc was prepared by
ball-milling by the Netzsch grinding company. The particle size distribution (PaSD) of
the suspension is shown in Fig. 5.2 The HT suspension was added to the prepared
SPEEK solution, and the resulting mixture was stirred, first mechanically for 2 h, and
then in an ultrasonic bath for an additional 1 h. The membranes were cast onto a Petri
dish, using the HT-polymer solution, and then dried at 60 °C in vacuum oven for 24h.
The thickness of the resulting membranes was in the range of 100 ~ 150 μm. The content
of the HT in the composite membranes was 2 ~ 10 wt% of the total mass (HT +
polymer).
91
Fig. 5.2 PSD of Hydrotalcite particles.
5.2.4 Characterization
The FTIR spectra of the polymers were recorded using a Genesis II (Matteson)
FTIR instrument. The experimental operating conditions were a scan-range from 4000
cm
-1
to 500 cm
-1
, and a scan-resolution of 2cm
-1
. The powder sample was composed of
20 mg of the polymer intermixed with 1000 mg of IR spectroscopic-grade KBr. A
thermogravimetric analyzer (TA Instruments model 2050) was used for evaluating the
thermal stability of the membranes. Membrane samples for thermogravimetric analysis
(TGA) were preheated at 150
o
C for 30 min under a nitrogen atmosphere in order to
remove moisture and other minor organic residues that may be present. Membrane
samples were then heated from 90 to 700
o
C under a nitrogen atmosphere, with a heating
92
rate of 10
o
C/min. The hydrotalcite filler was also subjected to TGA, from room
temperature to 700
o
C with the same operating conditions. XRD analysis of the HT was
also carried out using a Rigaku X-ray diffractometer, with the Cu-K
α
radiation used as the
X-ray source. Scans were performed over a 2θ range from 5
o
to 75
o
, with a scan rate
2°/min and a step rise 0.05. The, BJH (Barret-Joyner-Halenda) surface area and pore
volume, the HK (Horvath-Kawazoe) micropore diameter, and the pore size distribution
(PSD) of the HT were calculated from the N
2
adsorption isotherm at 77 K using a
Micrometrics ASAP 2010 instrument. The isotherms were measured using samples that
had been pretreated by heating them at 150 C
0
in a vacuum oven overnight. The
1
H NMR
spectra were recorded using a Varian Mercury 400 instrument in order to determine, as
noted above, the SD of SPEEK samples by a method other than direct titration. A 2 wt %
polymer solution in dimethyl sulfoxide-d6 was prepared for each analysis. The data
acquisition was conducted with 128 scans, a range from -1 to 10 ppm, a pulse angle of 45
degrees, acquisition time of 2 s, and a relaxation delay of 1 s at room temperature.
5.2.5 Water gain
The water gain by the membranes is determined by measuring the change in the
weight before and after the membranes are hydrated. In order to measure the gain, the
sample was immersed in distilled water at room temperature for 1 day in order to be fully
hydrated, and then the excess water on the surface of the membrane was removed using
filter paper. Immediately after that, the weight of the wetted membrane,
wet
W , was
93
measured. The weight of the membrane,
dry
W , was also determined after completely
drying the membrane in the oven at 100 °C for 1 day [Silva et al., 2005]. The relative
water gain is defined by the following equation (5.5).
100 (%) ×
−
= ⋅
dry
dry wet
W
W W
gain Water (5.5)
5.2.6 Methanol permeability
Methanol permeability was determined at room temperature using a simple
diffusion cell, which consists of two chambers, each with a volume of 150 ml. To
measure the permeability, the membrane is clamped in between the two chambers, which
are kept under constant stirring during the experiment. To begin the experiments, one of
the chambers (chamber A) is filled with an aqueous solution of methanol (C
A
=5M), while
the other chamber (chamber B) is filled with distilled water. Prior to testing, the
membrane is completely hydrated in distilled water for at least a period of 24h. The
methanol concentration in chamber B, resulting from the flux across the membrane due to
the concentration difference between the two chambers, is measured by gas
chromatography (using a Gow-mac 580 GC with a TCD detector and a Hayesep T
100/120 column). During the experiment, the methanol concentration in chamber B
remained negligible when compared to the concentration in chamber A, the latter
remaining essentially unchanged. Based on these observations, the change in
concentration C
B
in chamber B as a function of time is expressed by Equation (5.6), and
94
the methanol permeability P
m
by Equation (5.7) [Tricoli, 1998; Pivovar et al., 1999;
Chang et al., 2003].
(5.6)
(5.7)
where A is the membrane area (cm
2
), and L the membrane thickness (cm).
Experimentally, the permeability is calculated from the slope of the plot of the methanol
concentration C
B
in the chamber B versus time.
5.2.7 Proton conductivity
The proton conductivity of the membrane is calculated from the AC impedance
spectroscopic data, which are obtained over a frequency range of 0.1 ~ 8 MHz, using a
Zahner IM 6 instrument. We used the two-electrode method, in order to measure the
proton conductivity in the longitudinal direction at room temperature
[Liu et al., 2006].
The diagram of the electrochemical cell utilized is shown in Fig. 5.3 [Zawodzinske et al.,
1991; Cahan and Wainright, 1993; Elabd et al., 2004].
Prior to testing, the membrane
sample (20 mm × 10 mm) was immersed in 0.1M sulfuric acid for 2 days in order to be
fully hydrated. Each sample was then placed in a cell block open to the air through a
pinhole, in order to be equilibrated with the ambient pressure. Each end of the membrane
sample strip was tightened in a frame in between two stainless steel electrodes. The
A
B
B
C
L V
DK A
dt
dC
×
×
×
=
) (
) (
A
B B
C A
L V
dt
dC
Pm
×
×
× =
95
Zahner IM 6 instrument measures the resistance of the membrane, from which the
conductivity σ (S cm
-1
) is calculated using Equation (5.8) [Liu et al., 2006].
Wt R
L
m
= σ (5-8)
where L (cm) is the distance between the electrodes, R
m
(Ω) the membrane resistance, W
(cm) the width of the membrane, t (cm) the thickness. The resistance R was derived using
both the Nyquist and Bode diagrams. The resistance in the Bode diagram is calculated for
the phase angle close to zero at high frequency, and in the Nyquist plot from the low
intersect of the high-frequency semi-circle with the real axis in the complex impedance
plane. Both techniques yield indistinguishable results.
Fig. 5.3 Schematic design of conductivity cell [Pivovar et al., 1999].
1. Cell block
2. Thumbscrew
3. Open area to allow equilibrium
4. Membrane
5. Stainless steel electrode
96
5.3 Results and discussion
5.3.1 Hydrotalcite
The HT filler was characterized by various analytical methods. Figure 5.4(a), for
example, shows the weight-loss curve as a HT sample is heated in a N
2
atmosphere from
room temperature to 700 °C. In the figure, the left y-axis represents the weight loss
wt(%), while the right axis is the derivative thermogravimetric, dwt(%)/dT, curve (DTG)
with respect to temperature. The weight loss and the DTG curves are typical of what were
previously reported for the HT [Yang et al., 2002], reflecting first the loss of the
interlayer water around 205°C, then the simultaneous loss of CO
3
2-
(in the form of CO
2
)
and OH
-
(in the form of water) from the structure, and finally the complete
decomposition (accompanied by loss of CO
2
) to form mixed oxides or a spinel structure.
Figure 5.4(b) shows the HT PXRD patterns, which are indicative of the LDH
structure of the material. Based on the Bragg’s equation, the HT basal spacing is 7.8 Å
(2θ=11.35, λ=0.1542 nm), again typical of what was previously reported in the literature
[Yang et al., 2002]. The BET surface area and the HK micropore mean diameter of the
material are 9.5 Å and 15.8 m
2
/g, respectively.
5.3.2 Sulfonation degree and ion exchange capacity
The SD plays a key role in determining the methanol permeability and proton
conductivity. As the SD increases, so also do the charge density and the proton
conductivity.
97
(a)
0 100 200 300 400 500 600 700
50
60
70
80
90
100
0.2
0.0
-0.2
-0.4
-0.6
-0.8
DTG [dwt(%)/dT]
391
0
C
TG
Weight(%)
Temp(
o
C)
179
0
C
DTG
(b)
10 20 30 40 50 60 70
0
2000
4000
6000
8000
10000
12000
14000
Intensity
2 Θ
11.35 d-spacing 7.8
0
A
Fig. 5.4 (a) TGA spectrum and (b) X-ray powder diffraction pattern of hydrotalcite.
98
The IEC and the SD of a number of SPEEK samples, which were prepared by varying the
time of sulfonation, are shown in Fig. 5.5. As expected, the SD and IEC increase as the
sulfonation reaction time increases. For the remainder of the paper, the various SPEEK
polymer samples are identified according to their SD, e.g., SD45, SD53, or SD60,
referring to three of the samples in Fig. 5.5. Generally, the PEEK and SPEEK (as long as
the SD is less than 40%) are insoluble, at room temperature, in convectional solvents
such as DMAc and NMP. Sulfonation, however, makes the polymer progressively more
soluble, to the point where for a SD higher than 70%, it becomes soluble in methanol,
and also shows poor chemical stability in hot water [Wu et al., 2006] (for membrane
preparation we used the SPEEK polymer films with a SD of no more than 60%).
Fig. 5.5 SD and IEC of SPEEK polymers.
2.18
1.73
1.54
1.36
0.98
0.32
81%
60%
53%
45%
32%
10%
0
0.5
1
1.5
2
2.5
3
24h 48h 72h 96h 120h 144h
Reaction time(h)
IEC
0%
20%
40%
60%
80%
100%
SD
IEC
S.D(%)
99
5.3.3 FTIR analysis
FTIR was used to track the progress in converting the PEEK into the SPEEK
during sulfonation. As Fig. 5.6 shows, there are significant differences between the FTIR
spectra of the SPEEK and of the unreacted PEEK. The spectrum of the SPEEK in Fig.
5.6 is typical of what was previously reported by other investigators [Zaidi et al., 2003;
Zhang and Zhou, 2005] for that material. When studying the SPEEK spectrum, one
observes several peaks that are attributed to the presence of the sulfonic acid groups, and
which are not found in the PEEK spectrum.
Fig. 5.6 Spectra of PEEK and SPEEK.
1489.4
PEEK
706.4
1075.3
1252.4
1471.5
1491
3410
SD 60
706.8
1076.8
1252.5
1471.6
1491.6
3409
SD 45
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Absorbance
1000 1500 2000 2500 3000 3500
Wavenumbers (cm-1)
SD 60
SD 45
PEEK
100
They include the absorption band (3409-3410 cm
-1
) for the hydroxyl vibration in the
sulfonic acid group, the band (1252.4-1252.5 cm
-1
) corresponding to the asymmetric
O=S=O stretch, the band at (1075.3-1076.8 cm
-1
) corresponding to the O=S=O
symmetric stretch, and the band at (706.4-706.8 cm
-1
) corresponding to the S-O stretch.
In the SPEEK spectrum, furthermore, the aromatic C-C band (1489.4 cm
-1
) is split into
two bands as the PEEK is progressively transformed into the SPEEK, with the size of the
newly formed band (1471.5-1471.6 cm
-1
) increasing with the degree of sulfonation. In
summary, the FTIR analysis provides a very sensitive means for following the progress
of the sulfonation reaction, and insight into the accompanying structural changes.
5.3.4 Thermogravimetric study
The weight-loss curves of the PEEK and SPEEK membranes, when heated in
flowing nitrogen, are shown in Fig. 5.7. This graph indicates that sulfonation has a
detrimental effect on the thermal stability, with the SPEEK polymer being less thermally
stable than the original PEEK polymer. Furthermore, as can be seen in Fig. 5.7, the
higher the SD is, the less thermally stable the polymer is. The addition of the HT filler, on
the other hand, improves the thermal stability of the polymer. Figure 5.8, for example,
shows the weight-loss curves of the membrane films made with the SD60 SPEEK
material using two different concentrations of the HT filler (4 and 7%). Shown in the
same figure is the weight-loss curve for the commercial Nafion
®
115, as well the SD60
SPEEK without any filler. Note that the thermal stability of the SD60 SPEEK membranes
improves significantly with the addition of the HT filler.
101
100 200 300 400 500 600 700
40
50
60
70
80
90
100
100 200 300 400 500 600 700
40
50
60
70
80
90
100
SD 45
SD 53
Weight(%)
Temp. (Degree)
PEEK
SD 60
Fig. 5.7 TGA curves for both the PEEK and SPEEK.
The weight-loss curve of the 7 wt% HT hybrid membrane is very close to the Nafion
®
115
curve up to 350
o
C. However, whereas the Nafion
®
115 material begins to decompose
above 400
o
C, the hybrid HT-SPEEK material remains fairly stable even at much higher
temperatures.
102
100 200 300 400 500 600 700
0
20
40
60
80
100
0
20
40
60
80
100
100 200 300 400 500 600 700
SD 60
7HT(%)
Weight (%)
Temp. (
0
C)
4HT(%)
Nafion ®115
Fig. 5.8 TGA curves for both the SPEEK and HT-SPEEK materials.
5.3.5 Wate gain and IEC
Figure 5.9 shows the water gain of a number of the SPEEK membrane films with
varying SD and IEC. The water gain (as well as the IEC) depends strongly on the SD.
Moreover, both are known to have a profound effect on the membrane conductivity
[Thomassin et al., 2006]. In general, the higher the water gain of a given polymer film is,
the higher one expects its proton conductivity to be, as the mobility of the ions in the
membrane phase increases with increasing water content. Proton conductivity directly
103
relates with IEC, of course, as a higher IEC implies a membrane with a higher charge
density.
0%
20%
40%
60%
80%
100%
0.984 1.358 1.540 1.730 2.180
IEC (meq/g)
Water gain(%)
SD45
SD53
SD60
SD81
SD32
Fig. 5.9 Water gain and IEC for SPEEK polymer.
5.3.6 Methanol permeability
Figure 5.10 (a) shows the methanol permeability of a number of the SPEEK
membrane films (each with a different SD and the corresponding IEC), and several sets
of data from the literature [Li et al., 2003; Regina et al., 2006]. The permeabilities of our
own SPEEK films range from 4.4×10
-7
– 6.24×10
-7
cm
2
/s. By comparison, the
permeability for the commercial Nafion
®
115 film is ~2.4×10
-6
cm
2
/s. As the SD (and the
IEC) increases, so also does the methanol permeability. The somewhat higher
permeabilities for our SPEEK films, when compared to the literature data, are attributed
104
to the higher methanol MeOH concentration used during the measurements, in order to
eliminate the concentration effect often encountered during such experiments.
Figure 5.10(b) shows the effect that the addition of the HT filler has on the
permeability of the SPEEK membrane films. The HT-SPEEK hybrid membranes have
significantly lower methanol permeability than the original SPEEK, and this appears to
be true for films of varying SD. For comparison, the SD45-10HT membrane (which
contains 10% by weight HT) has 16 times lower MeOH permeability than the
commercial Nafion
®
115. The effect on the permeability of the HT filler is consistent with
the idea (and expectation) that the inorganic filler will block the methanol from passing
through the polymer film.
5.3.7 Proton conductivity
Figure 5.11(a) shows the conductivity of a number of the SPEEK membranes
prepared by us, as well as some data from the literature [Li et al., 2003; Regina et al.,
2006; Xing et al., 2004]. The measured conductivities range from 0.0046 to 0.055 S/cm.
As the IEC increases, the proton conductivity also increases, as expected. Figure 5.11(b)
shows proton conductivity data of the HT-SPEEK hybrid membranes. Two different
types of behavior are indicated by the figure. For membranes with a large SD, the proton
conductivity decreases proportionally to the fraction of the HT in the hybrid membranes.
On the other hand, for membranes with a lower SD the proton conductivity first increases
and then decreases slowly with the HT content.
105
(a)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
1E-8
1E-7
1E-6
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
1E-8
1E-7
1E-6
Exp
Lei Li et al
A. Regina et al
MeOH permeability [cm
2
/s]
IEC(meq/g)
(b)
0 2 4 6 8 10
1E-7
1E-6
1E-7
1E-6
Nafion 112
SD 45HT
Permeability (cm
2
/sec)
Hydrotalcite wt (%)
Nafion 115
4.6 times
18 times
SD 60HT
SD 53HT
Fig. 5.10 Methanol permeability of (a) SPEEK and (b) HT-SPEEK.
106
(a)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
1E-4
1E-3
0.01
0.1
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
1E-4
1E-3
0.01
0.1
Exp
Lei Li et. al
A. Regina et. al
P. Xing et. al
Conductivity (S/cm)
IEC (meq/g)
(b)
0 2 4 6 8 10
1E-3
0.01
0.1
0 2 4 6 8 10
1E-3
0.01
0.1
Conductivity (S/cm)
Hydrotalcite wt (%)
3 times
SD 60 HT
SD 45 HT
6.48× 10
-2
SD 81 HT
SD 53 HT
Nafion® 115
Nafion® 115
Fig. 5.11 Conductivity of (a) SPEEK and (b) HT-SPEEK.
107
The same behavior is also mirrored by the water gain of these materials, as can be seen in
Fig. 5.12, where for materials with a higher degree of sulfonation water gain and proton
conductivity are both a decreasing function of the HT content, while materials with a
lower SD show non-monotonic behavior. What causes this behavior is still not entirely
clear, as the interaction between the sulfonated polymer matrix and the inorganic filler is
rather complex.
Proton transport in the SPEEK membranes is known to occur by two
mechanisms [Kreuer et al., 1982; Kreuer, 1996; Kreuer, 2001], the hopping mechanism
(also known as the Grøtthuss mechanism), and the vehicle mechanism. In the hopping
mechanism, the proton is passed down a chain of water molecules, transferred from one
to another through the formation and breaking of hydrogen bonds (proton-hopping). In
the vehicle mechanism, the proton combines with the solvent molecules, producing H
3
O
+
(hydronium ions) or CH
3
OH
2
+
complexes, which are then transported through the
membrane. The same mechanisms are likely to be responsible for proton transport in the
hybrid membranes, and the filler particles are potentially active in both. Protons are likely
to be transported along the surface and/or the edges of the particles, which have proton
acceptor (acid) sites [Winter et al., 2006]. The HT filler particles also contain substantial
amounts of interlayer water (11.9 wt% for the HT used in this study). Protons can
conceivably pass through the interlayer water molecules through the hopping mechanism.
The presence of the HT filler can also help improve the connectivity of the hydrophilic
domain, which may explain the initial increase in the proton conductivity for membranes
108
with low SD, for which the connectivity of the hydrophilic region may be limiting the
proton conductivity.
0 2 4 6 8 10
1E-3
0.01
0.1
0 2 4 6 8 10
30
40
50
60
70
80
90
100
1E-3
0.01
0.1
SD 81 Cond.
SD 45 Cond.
Conductivity (S/cm)
Hydrotalcite wt (%)
Water gain (%)
SD 45 W.G.
SD 60 W.G.
SD 81 W.G.
SD 60 Cond.
Fig. 5.12 Relationship between conductivity and water gain for HT-SPEEK membranes.
A common measure of performance for the DMFC membranes is the (C/P) index
[log(C/P)], which is defined as the ratio of the proton conductivity of the membrane
divided by its methanol permeability. The higher the (C/P) index of a membrane is, the
most likely is for it to find application in a DMFC. Figure 5.13 shows the log(C/P) values
for two commercial Nafion
®
materials, as well as for several SPEEK, and hybrid HT-
SPEEK membranes that we have prepared. Several of the membranes that we have
prepared have log(C/P) values which are higher than those of Nafion
®
112 and
109
Nafion
®
115, which are ~ 4.5. Figure 5.14 shows the data for several of the membranes
we have prepared and the Nafion
®
112 and Nafion
®
115 membranes, as well as the prior
literature data with Nafion
®
117 [Li et al., 2005] in the conductivity vs. inverse methanol
permeability plot. The membrane performance target is the upper right hand part of the
figure. A number of the hybrid HT-SPEEK membranes show quite desirable
characteristics, and good potential for DMFC application.
3.6
3.8
4
4.2
4.4
4.6
4.8
5
Nafion 112
Nafion 115
SD45
SD53
SD60
SD45_2HT
SD45_4HT
SD45_7HT
SD45_10HT
SD53_4HT
SD53_7HT
SD53_10HT
SD60_4HT
SD60_7HT
SD60_10HT
L o g [C /P]
Fig. 5.13 Log[C/P] of a number of the membranes.
110
100000
1E-3
0.01
0.1
(Methanol resistance)
Nafion
117(Ref.)
Nafion
112
10
5
SD 45
SD 60
10
6
10
7
10
8
SPEEK
SPEEKHT
Nafion
Conductivity (S/cm)
[Methanol permeability (cm
2
/S)]
-1
10
9
SD 53
Target
Nafion
115
Log[C/P]=4.5
Fig. 5.14 Proton conductivity versus methanol resistance plot.
5.4 Conclusions
Various SPEEK and HT-SPEEK membranes were prepared and tested for their
methanol permeability and proton conductivity. The membrane properties depend on the
sulfonation degree, and the fraction of the HT that is incorporated in the hybrid
membranes. In general, the presence of the HT filler in the hybrid HT-SPEEK
membranes helps decrease their methanol permeability, which is significantly smaller
than that of Nafion
®
membranes. For some of the HT-SPEEK membranes (those with a
low SD, which are most likely to find application) addition of the HT filler also increases
the proton conductivity. Typically, a certain filler fraction exists that maximizes proton
111
conductivity which at its maximum is up to three times higher than the values of the pure
SPEEK membrane. It is still unclear what causes this beneficial effect, but it is likely to
result from the complex interplay between the two active mechanisms of proton transport
(hopping and vehicle) and the unique characteristics of the inorganic filler particles. The
(C/P) performance index of some of the membranes is comparable to that of the
commercially available Nafion
®
membranes and, therefore, the membranes are promising
candidates for DMFC applications.
112
Chapter 6 : Hybrid hydrotalcite – suflonated
polyetheretherketone (SPEEK) cation exchange
membranes prepared by in situ sulfonation.
6.1 Introduction
Direct methanol fuel cells (DMFC) are attracting considerable attention as noted
in chapter 5. However, DMFC face two major technical hurdles. The first challenge is the
slow anodic oxidation kinetics, and the other the inadvertent methanol transport through
the polymer electrolyte membrane (a phenomenon known as crossover). Methanol
crossover decreases the fuel cell voltage, resulting in fuel cell efficiency loss, and in the
poisoning of the catalyst at the cathode [Hoogers, 2003].
The commonly utilized Nafion
®
membranes are expensive, and in addition suffer
from significant MeOH crossover, with sometimes over 40% of the methanol reported
lost during fuel cell operation [Tricoli, 1998]. Significant research in recent years has,
therefore, been devoted to developing new membranes to replace the commercial
Nafion®. The most common approach, involves modifying Nafion® through
copolymerization with other polymers. For example, Pintauro and coworkers [Wycisk et
al., 2006] fabricated proton conducting membranes from blends of Nafion® and
polybenzimidazole (PBI) by solution casting. They reported that selectivity, which is
defined as the ratio of proton conductivity to methanol permeability and is thought to be a
favorable predictor of better DMFC performance, was up to four times greater for their
113
membranes than that of Nafion 117. Kang and coworkers [Kim et al., 2007] prepared
and tested Nafion®/poly(p-phenylene vinylene) (PPV) composite proton-conductive
DMFC membranes. They reported that the power density of a MEA using the
Nafion/PPV composite membrane was 34mW/cm
2
, which is a 30% improvement over
the pure Nafion®.
Another approach involves getting completely away from Nafion® by using non-
fluorinated polymers instead [Jannasch, 2003]. Examples of non-fluorinated polymers
used to prepare proton-conductive membranes include PBI, sulfonated poly-etherketones
(SPEK), sulfonated poly-etheretherketones (SPEEK), sulfonated poly(arylene ether ether
nitrile) (SPAEEN), poly-ethersulfone (PES), and sulfonated poly-phenyl-quionxaline
(SPPQ) [Roziere and Jones, 2003]. Guiver and coworkers [Kim et al., 2008c] reported,
for example, that several SPAEEN copolymer membranes showed better proton
conductivity than Nafion® and MEA made out of these membranes showed significantly
improved DMFC performance compared to MEA using sulfonated polysulfone (BPSH)
or Nafion® membranes. SPEK and SPEEK membranes have also been the focus of many
investigations [Li et al, 2005; Li et al., 2003; Regina et al., 2006; Xing et al., 2004], and
have been shown to have good chemical and mechanical stability, high proton
conductivity (0.01 to 0.05 Scm
-1
), together with reduced methanol permeability, and a
lower cost than Nafion
®
. In addition to SPEK and SPEEK, most of the aforementioned
polymers are thermally stable, due to their polyaromatic or poly-heterocyclic nature.
A third approach involves preparing hybrid membranes, which are either made by
dispersing various inorganic filler materials in a proton-conducting polymer base (see
114
Table1.7) or by dispersing the inorganic fillers into a monomer base, followed by in-situ
polymerization [Zhang and Simon, 2005; Haggenmueller et al., 2006; Zhang et al.,
2004]. The addition of fillers is intended to improve the performance characteristics of
the pure polymeric membranes. Arico et al. [2003], for example, studied a number of
such fillers, showing that the conductivity of hybrid membranes relates to the acidity of
the fillers, with the membranes containing the most acidic fillers (in the order SiO
2
-PWA
> SiO
2
> ZrO
2
> n-Al
2
O
3
--neutral > b-Al
2
O
3
–basic) showing the best performance.
Generally, methanol permeability was reported to decrease as the filler concentration
increased [Lee et al, 2005; Silva et al., 2005]. Guiver et al. [Su et al., 2007] prepared
nanocomposite proton-exchange membranes from SPPEK) and various amounts of
sulfonated silica nanoparticles (silica-SO
3
H). The hybrid membrane had improved
thermal stability and decreased methanol crossover, which Guiver et al. attributed to the
strong –SO
3
H/–SO
3
H interaction between the SPPEK chains and silica-SO
3
H particles.
Inorganic clay materials have been utilized by a number of groups in the
preparation of hybrid membranes [Lee et al, 2005; Chang et al., 2003]. In the resulting
membrane the polymer macromolecules are inserted into the clay interlayer space,
providing for a more “intimate” interaction between the polymer base material and the
inorganic filler. Often, this results in the complete delamination of the clay, with the
individual sheets being dispersed in the polymer matrix. In chapter 5, we have previously
prepared composite membranes by incorporating hydrotalcite particles into SPEEK.
These HT-SPEEK hybrid membranes exhibit good properties (relatively high proton-
115
conductivity and reduced MeOH crossover), and their properties depending strongly on
the sulfonation degree of the polymer matrix, and on the fraction of HT present.
In this chapter, HT-SPEEK polymer membranes are prepared by a different
technique (referred to as the in situ sulfonation method), which involves dispersing the
HT filler into the PEEK polymer and sulfonating the resulting mixture using H
2
SO
4
. The
goal of this study has been to investigate the effect of the HT inorganic filers on the
properties of the HT-SPEEK membranes prepared by in-situ sulfonation in order to
compare it with the hybrid HT-SPEEK membranes prepared by the more conventional
technique, in terms of their methanol permeability and proton conductivity. In addition to
developing a material as a potential replacement for Nafion
®
the aim of this study is to
provide additional fundamental understanding into the properties of clay-polymer
nanocomposites.
6.2. Experimental
6.2.1 Chemicals and materials
Victrex PEEK (poly-etheretherketone) grade 450F particles and hydrotalcite
(Pural Mg 70, with a Mg/Al the ratio ~3.0 by ICP analysis) were kindly provided to us by
the Victrex and Sasol companies respectively. Sulfuric acid, dimethylacetamide (DMAc)
and methanol (HPLC grade, 99.99% pure) were purchased from VWR. Nafion
®
115 and
112 were purchased from DuPont and their properties were also studied, in order to
compare them with our own membranes.
116
6.2.2 Preparation of sulfonated polymers
The PEEK, the hydrotalcite (Mg70), and concentarted sulfuric acid (18M) were
mixed together at room temperature (1g of combined (PEEK+HT) solid per 45 ml of
sulfuric acid). The reaction that we believe takes place during the sulfonation of the
(PEEK+HT) mixture is shown in Figure 6.1 [Li et al., 2003]. The sulfonic group of
SPEEK is located on the phenyl ring in between the two ether groups of the PEEK
repeating unit. Experimental evidence indicates (see below), that the polymer completely
delaminates the clay structure, while the concentrated sulfuric acid selectively dissolves
away the Mg from the HT sheet.
Fig. 6.1 The sulfonation reaction of both PEEK and HT in excess H
2
SO
4
.
The aluminum and what is left of the Mg in the HT sheets reacts with the sulfonic
functional groups in the SPEEK [Arai, 2004]. The HT-SPEEK material, prepared by the
117
in situ sulfonation technique, can be therefore viewed as a co-polymer composed of
randomly intermixed SPEEK and SPEEK-Al polymer segments. This is in contrast to the
HT-SPEEK material prepared by the more conventional approach (during which the HT
filler particles are dispersed into an already sulfonated polymer) in which the HT filler
maintains its structure intact.
In this chapter, the fraction of HT added to the PEEK material (prior to
sulfonation) was varied from 0 to 10 wt%. The HT-PEEK materials were sulfonated for
various times, between 72 h and 120 h. To terminate the reaction, the clay-polymer
solution would be immersed into a large excess of ice-cold water under mechanical
stirring. A precipitate would form which was filtered and washed repeatedly with
deionized water, until the pH of the filtrate solution became 5. The polymer was then
dried under vacuum at 45 °C for 24 h. Dried polymer powders were subsequently
dissolved in DMAc, and the resulting solution (9.1 wt %) was filtered using fine-glass
filters (Whatman syringe-filters with 1.0 μm average pore size) [Zaidi et al., 2000].
Membranes were cast onto a Petri dish, using the filtered polymer solution, and then
dried at 60 °C in a vacuum oven for 24 h. The thickness of the resulting membranes was
in the range of 100 ~ 150 μm. The sulfonation degree (SD) [Huang et al., 2001]
representing the ratio of the number of sulfonated PEEK repeat units to the total initial
number of PEEK repeat units is quantitatively determined by evaluating the peak area
represented by all protons in the aromatic phenyl ring in the
1
H NMR spectra. Jin et al.
[1985] have reported that the sulfonic group occurred only on the phenyl ring in between
118
two ether groups of the PEEK repeat unit [Arai, 2004; Nolte et al., 1993] We have also
reported previously (chapter 5), that the SD determined from the NMR spectra matches
well with the SD determined by direct titration.
6.2.3 Characterization
Several analytical techniques and characterization methods are used in this study
as noted in Chapter 5. Also, XPS analysis (using a VJ X-ray Photoelectron
Spectrophotometer) was carried out in order to analyze the elemental (C, O, and S)
concentrations using polymer samples with dimensions of 1cm × 1cm. The conditions of
analysis were beam intensity of 15 KV, scan number of 60 scans, and a scan range of 0 ~
1000 eV. The Al and Mg concentrations for the materials prepared with a reaction time of
120 h were determined by ICP-MS (inductively coupled plasma mass spectrometry).
These materials have a SD approaching 70, and as noted also with the HT-SPEEK
materials prepared by the more conventional technique, as the SD approaches 70 they
begin to solubilize in methanol and in hot water. Indeed, we generated samples
appropriate for ICP-MS samples by managing to dissolve ~150mg/liter of the in situ
sulfonated HT-SPEEK in 60 ºC water. We were unable to analyze by ICP-MS the
samples prepared with 72 h of reaction-time because they did not dissolve in hot water.
Solid
27
Al- NMR (using a Bruker Avance 500 MHz Spectrometer) was utilized in order
to investigate the presence of the aluminum and its chemical environment in the HT-
SPEEK materials prepared by in-situ sulfonation. The data acquisition was conducted
119
with 8192 scans, 1 MHz spectrum width, acquisition time of 2 ms, magic angle spinning
of 11.5 MHz, and a relaxation delay of 2 s at room temperature.
6.3. Results and discussion
The HT-SPEEK polymers prepared by in situ sulfonation are categorized by the
sulfonation reaction time and the amount (wt%) of the HT added to the polymer prior to
the sulfonation reaction. For example, a material containing 10 wt% HT being subjected
to 72 h of in situ sulfonation is referred to as R72-10HT. Pure PEEK sulfonated for 72 h
is referred to as SP72.
6.3.1 FTIR, XPS and NMR analysis (characterization of membranes)
FTIR has been used to study the structure of both the SPEEK and the in situ
sulfonated HT-SPEEK materials and to compare them with the structure of the
unsulfonated polymer. Fig. 6.2 shows the structure of the SP120 and the R120-10HT
materials. As Fig. 6.2 shows, there are significant differences between the FTIR spectra
of both polymers. The spectrum of the SP120 is typical of what was previously reported
for that material by other investigators and in chapter 5. Its spectrum contains several
peaks attributed to the presence of the sulfonic acid groups that are not found in the
original PEEK spectrum. They include, for example, the absorption band (3410.6 cm
-1
)
for the hydroxyl vibration in the sulfonic acid group, the band (1252.4 cm
-1
)
corresponding to the asymmetric O=S=O stretch, the band at (1011.4 cm
-1
) corresponding
to the O=S stretch, and the band at (706.4 cm
-1
) corresponding to the S-O stretch. In the
120
R120-10HT spectrum, the O=S=O stretch absorption peak (~1252 cm
-1
) is smaller than
the corresponding peak in the SP120 spectrum, as one may have expected as a result of
the reaction occurring as shown in Fig. 6.1. In addition, the intensity of sulfonic acid
group absorption peak in the R120-10HT spectrum is diminished from its original size in
the SPEEK polymer. In the spectrum of both the SP120 and the R120-10HT materials,
the aromatic C-C band (1489.4 cm
-1
) is split into two bands, as the PEEK polymer is
progressively transformed into the SPEEK or the in-situ HT-SPEEK materials, with the
size of the newly formed band (1471.0-1471.6 cm
-1
) increasing with the degree of
sulfonation, as one may have expected.
Fig. 6.2 FTIR spectra of SPEEK and in-situ sulfonated HT-SPEEK.
The concentrations of Al and Mg (as determined by ICP-MS), both prior to and
after in-situ sulfonation, are shown in Table 6.1. The Al:Mg molar ratio in the in situ
sulfonated HT-SPEEK polymer is ~6 ~ 7, very different from the initial Al/Mg ratio of
0.33 in the starting HT material, indicating the preferential dissolution of the Mg
component of HT. The final concentration of Al in the in-situ sulfonated HT-SPEEK
SP120
R120_10HT
R72_10HT
Absorbance
121
polymers increases in proportion with the Al concentration present in the mixture prior to
sulfonation. Table 6.2 shows the SD of the SPEEK and the in situ sulfonated HT-SPEEK
materials as calculated from
1
H NMR spectra. The sulfur concentration (wt%), as
determined by XPS, and indirectly from the from the SD, together with the proton
conductivity are also shown in Table 6.2.
Table 6.1 The initial and final concentrations of Al and Mg
Polymers
Initial concentration wt % from
chemical formula
Final concentration wt.% from
ICP-MS
Al Mg Al Mg
R120_4HT
0.357 0.97 0.0671 0.0077
R120_7HT
0.626 1.69 0.1267 0.0161
R120_10HT
0.894 2.42 0.1667 0.0221
The SD and the sulfur concentration in the membranes increase with the time of
sulfonation as expected, and so does the proton conductivity. The HT containing
materials have a significantly higher (a factor of ~2) proton conductivity than the pure SP
materials. The results in Table 6.2 indicate the presence of a maximum, with proton
conductivity first increasing and then decreasing with increasing HT content.
Solid state
27
Al NMR was used to study the in-situ sulfonated HT-SPEEK
polymers in order to investigate the presence of the aluminum functional group. Figure
6.3 shows the NMR spectra of three different hybrid materials all subjected to 120 h of
122
sulfonation but containing initially varying amounts of hydrotalcite (for all three
materials we used a similar sample amount, ~ 75 mg).
Table 6.2 The concentration of sulfur, the SD and the proton conductivity of different
polymers
Polymers
Reaction
time (h)
S.D
from NMR
analysis (%)
Proton
Conductivity
[S/cm]
The concentration of
Sulfur (wt %)
Element analysis
From XPS
From
S.D
SP72
72
43.0 0.0046 3.36 4.27
R72_4HT 41.2 0.0119 4.04 4.11
R72_7HT 44.2 0.0134 4.13 4.37
R72_10HT 43.9 0.0105 4.12 4.35
SP120
120
58.4 0.0243 4.38 5.58
R120_4HT 63.8 0.0403 4.56 6.02
R120_7HT 68.3 0.0454 5.48 6.38
R120_10HT 67.6 0.0412 5.87 6.32
Figure 6.3 clearly indicates the presence in all three materials of the octahedral
structure of aluminum hydroxide, ~ 0 PPM. [Klein et al., 2000; Han et al., 2003].
Aluminum hydroxide is known to form an almost perfect octahedral complex of
Al(H
2
O)
6
3+
surrounded by water molecules [Ma et al., 2003]. As Figure 6.3 indicates, as
the initial HT fraction increases so does the size of the Al octahedral complex peak, as
expected. Figure 6.4 shows the NMR spectra of two materials containing the same initial
123
HT fraction (~ 10% HT) which were subjected to different times of sulfonation (72 h and
120 h).
150 100 50 0 -50 -100 -150
-2000
0
2000
4000
6000
8000
10000
12000
14000
150 100 50 0 -50 -100 -150
-2000
0
2000
4000
6000
8000
10000
12000
14000
100 0 -100
-2000
0
2000
4000
6000
8000
10000
12000
14000
R120_4HT
R120_7HT
PPM
R120_10HT
Intensity
Octahedral
Structure
Figure 6.3 Solid
27
Al NMR spectra of R120-HT materials.
As Figure 6.4 shows, the larger the reaction time, the larger is the amount of
aluminum octahedral complex that gets directly incorporated into the polymer.
6.3.2 Thermal stability
The weight-loss curve of a SPEEK material, when heated in flowing nitrogen, is
compared with that of an in-situ HT-SPEEK polymer both prepared with the same
sulfonation reaction time of 120 h, in Fig. 6.5. Adding the hydrotalcite appears to
124
generally improve the thermal stability of the HT-SPEEK polymer, but an optimum HT
fraction exists beyond which the thermal stability begins to deteriorate (sulfonating the
PEEK materials degrades its thermal stability [Xing et al., 2004], with polymers with a
higher SD being typically less stable). The weight-loss curve of the R120-4HT material is
very close to that of the Nafion
®
115 up to 350
o
C. However, while Nafion
®
115 begins to
decompose above 400
o
C, the R120-4HT material remains stable.
150 100 50 0 -50 -100 -150
-2000
0
2000
4000
6000
8000
10000
12000
14000
R72_10HT
Intensity
PPM
R120_10HT
Figure 6.4 Solid
27
Al NMR spectra of R72_10HT and R120_10HT materials.
125
6.3.3 Water gain and proton conductivity
Figure 6.6 shows the water gain and proton conductivity of a number of in-situ
sulfonated HT-SPEEK films with a varying HT content.
100 200 300 400 500 600 700
0
20
40
60
80
100
0
20
40
60
80
100
100 200 300 400 500 600 700
10HT(%)
SP120
7HT(%)
Weight (%)
Temp. (
0
C)
4HT(%)
Nafion ®115
Figure 6.5 Thermograms for both the SPEEK and the in situ sulfonated 120 HT-SPEEK
materials.
Both the water gain and proton conductivity depend strongly on the initial HT fraction
added prior to sulfonation. For lower HT contents both the conductivity and the water
gain increase as the fraction of HT added increases. At larger HT concentrations,
however, the effect of HT added saturates and the conductivity begins to decrease as the
HT content increases further. The hybrid HT-SPEEK materials show substantially
126
improved conductivities when compared to the pure SPEEK materials (by comparison
the conductivity for the commercial Nafion
®
115 films was measured as 0.065 S/cm in
this study). As previously noted, NMR analysis indicates that the in-situ sulfonated HT-
SPEEK polymer contains a partially acidic aluminum hydroxide group from, which may
explain the higher conductivity of the HT-SPEEK materials over the pure SPEEK
materials. In general, for the HT-SPEEK materials the water gain behavior mirrors the
proton conductivity behavior, as expected, since a mobility of ions in the membrane
phase increases with increasing water content [Thomassin et al., 2006].
6.3.4 Methanol permeability and proton conductivity
Figure 6.7 shows the methanol permeability, and for side-by-side comparison, the
corresponding proton conductivity of a number of in-situ HT-SPEEK membrane films
with varying initial HT content. The MeOH permeability increases ~30% from the value
in the original SPEEK polymer as the fraction of HT increases. This in contrast to the
behavior observed with the HT-SPEEK by the more conventional technique for which
methanol permeability decreased as the HT fraction contained in the material increased.
As previously noted, for the HT-SPEEK materials prepared by the more conventional
technique, the filler appears to retain its structure intact and thus provide an effective
blockage to methanol transport. For the in situ sulfonated HT-SPEEK materials on the
other hand, Al (and to a much lesser extent Mg) appear to embed themselves into the
polymer structure and, therefore, not to provide effective hindrance for MeOH transport.
127
0 2 4 6 8 10
30
35
40
45
50
1E-3
0.01
0.1
R72
Water Gain
R72
Conductivity [S/cm]
Water Gain (%)
The Fraction of HT (wt%)
R120
Conductivity
Figure 6.6 The water gain and proton conductivity of in-situ sulfonated HT-SPEEK
polymers.
Since, increasing HT content resulted for the most part in higher proton conductivity for
the polymer film, it is not surprising that methanol conductivity increases as well, since it
can be transported in the form of CH
3
OH
+
ions through membrane by the vehicle
mechanism [Kreuer et al., 1982; Kreuer, 1996]. Despite the failure of HT to block
methanol transport, it should be noted that the MeOH permeability of in-situ sulfonated
HT-SPEEK polymers ranges from 4.7×10
-7
to 7.9×10
-7
[cm
2
/s], which is still 3 – 5 times
smaller than the permeability we measure with the commercial Nafion
®
115 film,
~2.46×10
-6
cm
2
/s.
128
0 2 4 6 8 10
4E-7
6E-7
8E-7
1E-6
1.2E-6
1.4E-6
1.6E-6
1.8E-6
2E-6
1E-3
0.01
0.1
MeOH Perm
R72
R72
R120
The Fraction of HT (wt%)
Conductivity [S/cm]
MeOH Permeability [cm
2
/s]
R120
Conductivity
Figure 6.7 The MeOH permeability and proton conductivity of in-situ sulfonated HT-
SPEEK polymers.
Figure 6.8 shows data in the form of a proton conductivity vs. inverse methanol
permeability (or methanol resistance) plot for several of the membranes we have prepared
and the Nafion
®
membranes, as well as prior literature data for Nafion
®
117 [Li et al.,
2005]. The desired membrane performance target is the upper right hand part of this
figure. A common measure of performance for conductive membranes is the (C/P) index
(Log[C/P]), which is defined as the ratio of the proton conductivity of the membrane
divided by its methanol permeability. The (C/P) index line with a value of 4.5 is shown in
Figure 6.8. From Figure 6.8, we note that commercial Nafion® films have ~ 4.5 for
Log[C/P]. Note that the in-situ sulfonated HT-SPEEK materials show improved the (C/P)
129
index over the original SPEEK materials as well as the HT-SPEEK materials prepared by
the more conventional technique. We have observed that the in situ R120 membranes
have the (C/P) values, around 4.7 or 4.8 which are higher than those of Nafion
®
112 and
Nafion
®
115 materials. But these membranes are excluded in the figure because these in
situ R120 membranes were dissolved in hot water (at 60
0
C) environments.
100000
1E-3
0.01
0.1
(Methanol resistance)
Nafion
117(Ref.)
Nafion
112
10
5
SP60
SP120
10
6
10
7
10
8
SPEEK
SPEEKHT
Nafion
In situ HT-SPEEK
Conductivity (S/cm)
[Methanol permeability (cm
2
/S)]
-1
10
9
Target
Nafion
115
Log[C/P]=4.5
in situ R72
Fig. 6.8 Proton conductivity vs. methanol resistance plot.
6.4. Conclusion
In this study, various in-situ sulfonated HT-SPEEK membranes have been
prepared and tested for their methanol permeability and proton conductivity. The
130
membrane properties depend on the reaction time, and the fraction of hydrotalcite
initially added to the PEEK materials prior to sulfonation. The MeOH permeability
increases with the fraction of HT initially added to the polymer prior to the sulfonation.
The same can be said for the proton conductivity for HT fractions below a certain optimal
level. Despite the fact that the HT material did not block methanol permeability, its value
for the HT-SPEEK hybrid materials is 3 ~ 5 times smaller than the one for the
commercial Nafion
®
115 film.
131
Glossary
a a coefficient related to the friction characteristics in EPD process
A membrane area (m
2
)
i
A peak area of the i
peak
in the sulfonated polymer
O S
B
,
viscous-flow membrane parameter (m
2
)
C gas concentration in the pore (mol/m
3
)
C
A
methanol concentration in chamber A (M)
C
B
methanol concentration in chamber B (M)
C
p
particle concentration in the suspension (kg/m
3
),
d
p
nominal membrane pore diameter (m)
D diffusion coefficient (m
2
/s)
0
,K S
D Knudsen diffusion coefficient (m
2
/s)
eff
K S
D
,
Effective Knudsen diffusion coefficient (m
2
/s)
E Electric field (V/m)
E
p
apparent activation energy for permeation (kJ/mol)
IEC ion exchange capacity (meq/g)
J gas flux (mol/m
2
s)
L nominal membrane thickness (m)
L
e
electrode separation distance (m)
132
M deposition amount in EPD process (kg)
i
M molecular weight of component i (g/mol)
N molar number
P pressure (Pa, psi)
e
P membrane permeability ( 1barrer = 1×10
-10
cmHg s cm
cm STP cm
⋅ ⋅
⋅
2
3
) (
)
m
P methanol permeability (cm
2
/s)
r
P gas permeance through the membrane (mol/m
2
s Pa)
1
p pressure on the permeate side (Pa, psi)
12
p interlayer pressure (Pa, psi)
2
p pressure on feed side (Pa, psi)
R universal gas constant (8.314 J/mol K)
R
m
membrane resistance of polymer (Ω)
SD sulfonation degree (%)
S. F. separation factor for mixed gases
t thickness of polymer membrane (cm)
V applied voltage (V)
0
V volume of downstream side of permeation cell (cm
3
)
W width of polymer membrane (cm)
W
dry
weight of dry sample polymer (kg)
W
wet
weight of wetted sample polymer (kg)
Z zeta potential (mV)
133
Greek letters
α ideal separation factor or permselectivity
ε membrane porosity
ε
d
dielectric constant of the medium
η viscosity (Pa-s)
μ electrophoretic mobility (m
2
/Vs)
σ conductivity of polymer (S/cm)
τ tortusity
Subscripts
K knudsen flow
S support
SL selective layer (HT or silicone layer in this study)
tot total
134
Bibliography
Arai, T. "Proton conductive membrane and production method thereof." U. S. Patent
Application Publication: (2004) US2004-0081823A1.
Aramendia, M. A.; Borua, V. et al. "Comparative study of Mg/M(III) (M = Al, Ga, In)
layered double hydroxides obtained by coprecipitation and the sol-gel method."
Journal of Solid State Chemistry (2002) 168(1): 156-161.
Arico, A. S.; Baglio, V. et al. "Influence of the acid-base characteristics of inorganic
fillers on the high temperature performance of composite membranes in direct
methanol fuel cells." Solid State Ionics (2003) 161(3-4): 251-265.
Baker, D. W. "Zeolite Molecular Sieves." John Wiley & Sons Inc. (1974)
Baker, R. W. "Membrane Technology and applications." 2nd Ed.: (2004) chap. 2.
Bakker, W. J. W.; Broeke, L. J. P. et al. “Temperature dependence of one-componet
permeation through a silicate-1 membrane." AICHE Journal (1997) 43: 2203-
2214.
Bauer, F.; Willert-Porada, A. "Characterisation of zirconium and titanium phosphates and
direct methanol fuel cell (DMFC) performance of functionally graded Nafion(R)
composite membranes prepared out of them." Journal of Power Sources (2005)
145(2): 101-107.
Benito, P.; LabajoS, F. M. et al. "Influence of microwave radiation on the textural
properties of layered double hydroxides." Microporous and Mesoporous Materials
(2006) 94(1-3): 148-158.
Bouma, R. H. ; Checchetti, B., A. et al. "Permeation through a heterogeneous membrane:
The effect of the dispersed phase." Journal of Membrane Science (1997) 128(2):
141-149.
Bravo, J.; Karim, A. et al. "Wall coating of a CuO/ZnO/Al
2
O
3
methanol steam reforming
catalyst for micro-channel reformers." Chemical Engineering Journal (2004)
101(1-3): 113-121.
Burggraaf, A. J.; Keizer, K. "Synthesis of Inorganic Membranes,” in Inorganic
Membranes: Synthesis, Characteristics, and Applications." R.R. Bhave ed. Van
Nostrand Reinhold: New York. : (1991) 10-63.
135
Cahan, B. D.; Wainright, J. S. "Ac-Impedance Investigations of Proton Conduction in
Nafion(Tm)." Journal of the Electrochemical Society (1993) 140(12): L185-L186.
Cai, H.; Hillier, A. C. et al. "Nanoscale Imaging of Molecular Adsorption." Science
(1994) 266(5190): 1551-1555.
Carbone, A. ; Casciola, M. et al. "Composite nafion membranes based on PWA-zirconia
for PEFCs operating at medium temperature." Journal of New Materials for
Electrochemical Systems (2004) 7(1): 1-5.
Caro, J.; Noack, M. et al. "Zeolite membranes - state of their development and
perspective." Microporous and Mesoporous Materials (2000) 38(1): 3-24.
Cavani, F.; Trifiro, F. et al. "Hydrotalcite-type anionic clays: preparation, properties and
applications." Catalysis Today (1991) 11: 173-301.
Centeno, T. A. ; Fuertes, A. B. "Carbon molecular sieve gas separation membranes based
on poly(vinylidene chloride-co-vinyl chloride)." (2000) Carbon 38(7): 1067-1073.
Chang, H. Y.; Lin, C. W. "Proton conducting membranes based on PEG/SiO
2
nanocomposites for direct methanol fuel cells." Journal of Membrane Science
(2003) 218(1-2): 295-306.
Chang, J. H.; Park, J. H. et al. "Proton-conducting composite membranes derived from
sulfonated hydrocarbon and inorganic materials." Journal of Power Sources
(2003) 124(1): 18-25.
Chen, C. Y.; Chen, S. Y. et al. "Electrophoretic deposition forming of porous alumina
membranes." Acta Materialia (1999) 47(9): 2717-2726.
Chen, F.; Mourhatch, R.; Tsotsis, T. T.; Sahimi, M. “Pore network model of transport and
separation of binary gas mixtures in nanoporous membranes.” Journal of
Membrane Science. (2008) 315: 48–57
Coronas, J.; Falconer, J. L. et al. "Characterization and permeation properties of ZSM-5
tubular membranes." Aiche Journal (1997) 43(7): 1797-1812.
Costa, A. S.; Imae, T. "Morphological investigation of hybrid Langmuir-Blodgett films
of arachidic acid with a hydrotalcite/dendrimer nanocomposite." Langmuir (2004)
20(20): 8865-8869.
136
Costantino, U.; Marmottini, F. et al. "New synthetic routes to hydrotalcite-like
compounds - Characterisation and properties of the obtained materials." European
Journal of Inorganic Chemistry (1998) 10: 1439-1446.
Dadwhal, M.; Kim, T. W.; Sahimi, M. et al. "Study of CO
2
diffusion and adsorption on
calcined layred double hydroxides: the effect of particle size." Industrial &
Engineering Chemistry Research (2008) 47: 6150-6157.
Datta, S. "Application of design of experiment on electrophoretic deposition of glass-
ceramic coating materials from an aqueous bath." Bulletin of Materials Science
(2000) 23(2): 125-129.
de Roy, A.; Forano, C. et al. "Anionic Clays: Trends in Pillaring Chemistry, Synthesis of
Microporous Materials, ed. M. L. Occeli and E. R. Robson, Van Nostrand
Reinhold." New York (1992) (Chap. 7): 108-169.
de Vos, R. M. ; Verweij, H. "Improved performance of silica membranes for gas
separation." Journal of Membrane Science (1998) 143(1-2): 37-51.
de Vos, R. M.; Maier, W. F. et al. "Hydrophobic silica membranes for gas separation."
Journal of Membrane Science (1999) 158(1-2): 277-288.
Dimitrova, P.; Friedrich, K. A. et al. "Modified Nafion((R))-based membranes for use in
direct methanol fuel cells." Solid State Ionics (2002) 150(1-2): 115-122.
Ding, Y.; Alpay, E. "Equilibria and kinetics of CO
2
adsorption on hydrotalcite
adsorbent." Chemical Engineering Science (2000) 55(17): 3461-3474.
Drits, V. A.; Sokolova, T. N. et al. "New Members of the Hydrotalcite-Manasseite
Group." Clays and Clay Minerals (1987) 35(6): 401-417.
Elabd, Y. A.; Walker, C. W. et al. "Triblock copolymer ionomer membranes Part II.
Structure characterization and its effects on transport properties and direct
methanol fuel cell performance." Journal of Membrane Science (2004) 231(1-2):
181-188.
Elyassi, B. ; Sahimi, M.; et al. "Silicon carbide membranes for gas separation
applications." Journal of Membrane Science (2007) 288(1-2): 290-297.
Fielding, R.; "Determination of Small (Less-Than 30 Seconds) Diffusional Time Lags in
Permeation Experiments." Polymer (1980) 21(2): 140-142.
Fletcher, P. D. I.; Haswell, S. J. et al. "Micro reactors: principles and applications in
organic synthesis." Tetrahedron (2002) 58(24): 4735-4757.
137
Fuertes, A. B.; Centeno, T. A. "Carbon molecular sieve membranes from
polyetherimide." Microporous and Mesoporous Materials (1998) 26(1-3): 23-26.
Fuertes, A. B.;D. M. Nevskaia, et al. Carbon composite membranes from Matrimid (R)
and Kapton (R) polyimides for gas separation." Microporous and Mesoporous
Materials (1999) 33(1-3): 115-125.
Gardner, E.; Huntoon, K. M. et al. "Direct synthesis of alkoxide-intercalated derivatives
of hydrotalcite-like layered double hydroxides: Precursors for the formation of
colloidal layered double hydroxide suspensions and transparent thin films."
Advanced Materials (2001) 13(16): 1263-1266.
Goerke, O.; Pfeifer, P. et al. "Water gas shift reaction and selective oxidation of CO in
microreactors." Applied Catalysis a-General (2004) 263(1): 11-18.
Gombocz, E.; Cortez, E. "Separation, Real-Time Migration Monitoring and Selective
Zone Retrieval Using a Computer-Controlled System for Automated-Analysis."
Applied and Theoretical Electrophoresis (1995) 4(4): 197-209.
Haggenmueller, R.; Du, F. M. et al. "Interfacial in situ polymerization of single wall
carbon nanotube/nylon 6,6 nanocomposites." Polymer (2006) 47(7): 2381-2388.
Han, S. H.; Hou, W. G. et al. "Coordination structure of aluminum in magnesium
aluminum hydroxide studied by Al-27 NMR." Chinese Chemical Letters (2003)
14(6): 605-608.
Harale, A.; Hwang, H. T. et al. "Experimental studies of a hybrid adsorbent-membrane
reactor (HAMR) system for hydrogen production." Chemical Engineering Science
(2007) 62(15): 4126-4137.
Hayashi, J.; Yamamoto, M. et al. "Simultaneous Improvement of Permeance and
Permselectivity of 3,3',4,4'-Biphenyltetracarboxylic Dianhydride-4,4'-
Oxydianiline Polyimide Membrane by Carbonization." Industrial & Engineering
Chemistry Research (1995) 34(12): 4364-4370.
Hayashi, J.; Yamamoto, M. et al. "Effect of oxidation on gas permeation of carbon
molecular sieving membranes based on BPDA-pp'ODA polyimide." Industrial &
Engineering Chemistry Research (1997) 36(6): 2134-2140.
Hickey, L.; Kloprogge, J. T. et al. "The effects of various hydrothermal treatments on
magnesium-aluminium hydrotalcites." Journal of Materials Science (2000)
35(17): 4347-4355.
138
Hoogers, G. "Fuel Cell Technology Handbook." CRC PRESS: London, (2003) Chap. 7.
Hornok, V.; Erdohelyi, A. et al. "Preparation of ultrathin membranes by layer-by-layer
deposition of layered double hydroxide (LDH) and polystyrene sulfonate (PSS)."
Colloid and Polymer Science (2005) 283(10): 1050-1055.
Huang, R. Y. M.; Shao, P. H. et al. "Sulfonation of poly(ether ether ketone)(PEEK):
Kinetic study and characterization." Journal of Applied Polymer Science (2001)
82(11): 2651-2660.
Huang, A. S.; Lin, Y. S. et al. "Synthesis and properties of A-type zeolite membranes by
secondary growth method with vacuum seeding." Journal of Membrane Science
(2004) 245(1-2): 41-51.
Hutson, N. D.; Speakman, S. A. et al. "Structural effects on the high temperature
adsorption of CO
2
on a synthetic hydrotalcite." Chemistry of Materials (2004)
16(21): 4135-4143.
Iglesias, A. H.; Ferreira, O. P. et al. "Structural and thermal properties of Co-Cu-Fe
hydrotalcite-like compounds." Journal of Solid State Chemistry (2005) 178(1):
142-152.
Jannasch, P. "Recent developments in high-temperature proton conducting polymer
electrolyte membranes." Current Opinion in Colloid & Interface Science (2003)
8(1): 96-102.
Jansen, J. C.; Buonomenna, M. G. et al. "Asymmetric membranes of modified poly(ether
ether ketone) with an ultra-thin skin for gas and vapour separations." Journal of
Membrane Science (2006) 272(1-2): 188-197.
Jensen, K. F. "Microchemical systems: Status, challenges, and opportunities." Aiche
Journal (1999) 45(10): 2051-2054.
Jha, P.; Mason, L. W. et al. "Characterization of silicone rubber membrane materials at
low temperature and low pressure conditions." Journal of Membrane Science
(2006) 272(1-2): 125-136.
Jin, X. ; Ellis, T. S. et al. "Studies on the Water Induced Plasticization behavior of
Nylon-4-the influencece of crystallinity and radiation crosslinking.”
Makromolekulare Chemie-Macromolecular Chemistry and Physics (1985) 186(1):
191-201.
139
Kanamura, K.; Hamagami, J. "Innovation of novel functional material processing
technique by using electrophoretic deposition process." Solid State Ionics (2004)
172(1-4): 303-308.
Kanellopoulos, N. K. "Recent advances in Gas Separation by microporous ceramic
membranes." Elsevier; New York: (2000) 373-391.
Khan, A. A.; Inamuddin "Preparation, physico-chemical characterization, analytical
applications and electrical conductivity measurement studies of an 'organic-
inorganic' composite cation-exchanger: Polyaniline Sn(IV) phosphate." Reactive
& Functional Polymers (2006) 66(12): 1649-1663.
Kim, D. S.; Park, H. B. et al. "Proton conductivity and methanol transport behavior of
cross-linked PVA/PAA/silica hybrid membranes." Solid State Ionics (2005)
176(1-2): 117-126.
Kim, N.; Kim, Y. et al. "Atomistic simulation of nanoporous layered double hydroxide
materials and their properties. I. Structural modeling." Journal of Chemical
Physics (2005) 122(21): 214713-214724.
Kim, T. W.; Sahimi, M. et al. "Gas Transport Properties of Hydrotalcite Membranes.
(paper #285e)." AIChE Annual Meeting (2006) San Francisco.
Kim, T. W.; Sahimi, M. et al. "Preparation of Hydrotalcite Thin Films Using an
Electrophoretic Technique." Industrial & Engineering Chemistry Research
(2008a) (Varma special issue, DOI: 10.1021/ie071446s, In-Press).
Kim, T. W.; Sahimi, M. et al. "Preparation and Characterization of Hydrotalcite Thin
Films." Industrial & Engineering Chemistry Research (2008b, submit).
Kim, D. S.; Kim, Y. S. et al. “High performance nitrile copolymers for polymer
electrolyte.” Journal of Membrane Science (2008c) 321: 199-208.
Kim, Y.; Yang, W. et al. "A Study by in-situ Techniques of The Thermal Evolution of
The Strutcure of a Mg-Al-CO
3
Layered Double Hydroxide." Industrial &
Engineering Chemistry Research (2004) 43: 4559.
Klein, J.; Ushio, M. et al. "Analysis of aluminum hydroxyphosphate vaccine adjuvants by
Al-27 MAS NMR." Journal of Pharmaceutical Sciences (2000) 89(3): 311-321.
Knudsen, M. Ann. d. Physik (1909) 28: 75.
140
Koresh, J.; Soffer, A. "Study of Molecular-Sieve Carbons .1. Pore Structure, Gradual
Pore Opening and Mechanism of Molecular-Sieving." Journal of the Chemical
Society-Faraday Transactions I (1980) 76: 2457.
Koresh, J. E.; Sofer, A. "Molecular-Sieve Carbon Permselective Membrane .1.
Presentation of a New Device for Gas-Mixture Separation." Separation Science
and Technology (1983) 18(8): 723-734.
Kreuer, K. D.; Rabenau, A. et al. "Vehicle Mechanism, a New Model for the
Interpretation of the Conductivity of Fast Proton Conductors." Angewandte
Chemie-International Edition in English (1982) 21(3): 208-209.
Kreuer, K. D. "Proton conductivity: Materials and applications." Chemistry of Materials
(1996) 8(3): 610-641.
Kreuer, K. D. "On the development of proton conducting polymer membranes for
hydrogen and methanol fuel cells." Journal of Membrane Science (2001) 185(1):
29-39.
Kusakabe, K.; Yoneshige, S. et al. "Morphology and gas permeance of ZSM-5-type
zeolite membrane formed on a porous alpha-alumina support tube." Journal of
Membrane Science (1996) 116(1): 39-46.
Kusakabe, K.; Kuroda, T. et al. "Formation of a Y-type zeolite membrane on a porous
alpha-alumina tube for gas separation." Industrial & Engineering Chemistry
Research (1997) 36(3): 649-655.
Kusakabe, K.; Sakamoto, S. et al. "Pore structure of silica membranes formed by a sol-
gel technique using tetraethoxysilane and alkyltriethoxysilanes." Separation and
Purification Technology (1999) 16(2): 139-146.
Kusakabe, K.; Yamamoto, M. et al. "Gas permeation and micropore structure of carbon
molecular sieving membranes modified by oxidation." Journal of Membrane
Science (1998) 149(1): 59-67.
Ksuki, Y.; Shimazaki, H. et al. "Gas permeation properties and characterization of
asymmetric carbon membranes prepared by pyrolyzing asymmetric polyimide
hollow fiber membrane." Journal of Membrane Science (1997) 134(2): 245-253.
Lai, Z. P.; Bonilla, G. et al. "Microstructural optimization of a zeolite membrane for
organic vapor separation." Science (2003) 300(5618): 456-460.
141
Lee, D. W.; Lee, Y. G. et al. "Improvement in thermal stability of stainless steel
supported silica membranes by the soaking-rolling method." Journal of
Membrane Science (2004) 236(1): 53-63.
Lee, K.; Nam, J. H. et al. "Methanol and proton transport control by using layered double
hydroxide nanoplatelets for direct methanol fuel cell." Electrochemistry
Communications (2005) 7(1): 113-118.
Lee, J. H.; Rhee, S. W. et al. "Ion-exchange reactions and photothermal patterning of
monolayer assembled polyacrylate-layered double hydroxide nanocomposites on
solid substrates." Chemistry of Materials (2006) 18(20): 4740-4746.
Leung, Y. L. A.; Yeung, K. L. "Microfabricated ZSM-5 zeolite micromembranes."
Chemical Engineering Science (2004) 59(22-23): 4809-4817.
Li, L.; Zhang, J. et al. "Sulfonated poly(ether ether ketone) membranes for direct
methanol fuel cell." Journal of Membrane Science (2003) 226(1-2): 159-167.
Li, X. F.; Zhao, C. J. et al. "Direct synthesis of sulfonated poly(ether ether ketone
ketone)s (SPEEKKs) proton exchange membranes for fuel cell application."
Polymer (2005) 46(15): 5820-5827.
Liu, B. J.; Kim, D. S. et al. "Fluorenyl-containing sulfonated poly(aryl ether ether ketone
ketone)s (SPFEEKK) for fuel cell applications." Journal of Membrane Science
(2006) 280(1-2): 54-64.
Ma, D.; Han, X. W. et al. "An investigation of the roles of surface aluminum and acid
sites in the zeolite MCM-22." Chemistry-a European Journal (2002) 8(1): 162-
170.
Madou, A. "Fundamentals of microfabrication." 2nd Ed. CRC PRESS; New York.,
(2002) Chap. 1.
Manasse, E. "Rocce eritree e di aden della collezione issel." Atti Soc. Toscana Sc. Nat.,
Proc. Verb. (1915) 24: 92.
Mateo, E.; Lahoz, R. et al. "Preparation and application of silicate-1 micromembranes on
laser-perforated stainless steel sheets." Journal of Membrane Science. (2008) 316:
28-34.
Miyata, S. "Anion-Exchange Properties of Hydrotalcite-Like Compounds." Clays and
Clay Minerals (1983) 31(4): 305-311.
142
Morikawa, H.; Tsuihiji, N. et al. "Preparation of membrane electrode assembly for fuel
cell by using electrophoretic deposition process." Journal of the Electrochemical
Society (2004) 151(10): A1733-a1737.
Nolte, R.; Ledjeff, K. et al. "Partially Sulfonated Poly(Arylene Ether Sulfone) - a
Versatile Proton Conducting Membrane Material for Modern Energy-Conversion
Technologies." Journal of Membrane Science (1993) 83(2): 211-220.
Olah, G. A. "Beyond Oil and Gas: The Methanol Economy." Wiley: New York., (2006).
Orme, C. J.; Stone, M. L. et al. "Testing of polymer membranes for the selective
permeability of hydrogen." Separation Science and Technology (2003) 38(12-13):
3225-3238.
Othman, M. R.; Rasid, N. M. et al. "Mg-Al hydrotalcite coating on zeolites for improved
carbon dioxide adsorption." Chemical Engineering Science (2006) 61(5): 1555-
1560.
Pattekar, A. V.; Kothare, M. V. "A microreactor for hydrogen production in micro fuel
cell applications." Journal of Microelectromechanical Systems (2004) 13(1): 7-18.
Perez-Ramirez, J.; Ribera, A. et al. "Magnetic properties of Co-Al, Ni-Al, and Mg-Al
hydrotalcites and the oxides formed upon their thermal decomposition." Journal
of Materials Chemistry (2002) 12(8): 2370-2375.
Pivovar, B. S.; Wang, Y. X. et al. "Pervaporation membranes in direct methanol fuel
cells." Journal of Membrane Science (1999) 154(2): 155-162.
Poshusta, J. C.; Tuan, V. A. et al. "Synthesis and permeation properties of SAPO-34
tubular membranes." Industrial & Engineering Chemistry Research (1998)
37(10): 3924-3929.
Poshusta, J. C.; Tuan, V. A. et al. "Separation of light gas mixtures using SAPO-34
membranes." Aiche Journal (2000) 46(4): 779-789.
Regina, A.; Fontananova, E. et al. "Preparation and characterization of sulfonated PEEK-
WC membranes for fuel cell applications - A comparison between polymeric and
composite membranes." Journal of Power Sources (2006) 160(1): 139-147.
Robertson, G. P.; Mikhailenko, S. D. et al. "Casting solvent interactions with sulfonated
poly(ether ether ketone) during proton exchange membrane fabrication." Journal
of Membrane Science (2003) 219(1-2): 113-121.
143
Roelofs, J. C. A. A.; van Bokhoven, J. A. et al. “The thermal decomposition of Mg-Al
Hydrotalcites: effects of interlayer anions and characteristics of the final
structure.” Chem. Eur. J. (2002) 8(24): 5571-5579.
Roto, R.; Villemure, G. "Electrochemical impedance spectroscopy of electrodes modified
with thin films of Mg-Mn-CO
3
layered double hydroxides." Electrochimica Acta
(2006) 51(12): 2539-2546.
Roziere, J.; Jones, D. J. "Non-fluorinated polymer materials for proton exchange
membrane fuel cells." Annual Review of Materials Research (2003) 33: 503-555.
Sedigh, M. G.; Jahangiri, M. et al. "Structural characterization of polyetherimide-based
carbon molecular sieve membranes." Aiche Journal (2000) 46(11): 2245-2255.
Shekhawat, D.; Luebke, D. R. et al. "A Review of Carbon Dioxide Selective
Membranes." DOE/NETL-2003/1200 Report, (2003).
Silva, V. S.; Schirmer, J. et al. "Proton electrolyte membrane properties and direct
methanol fuel cell performance II. Fuel cell performance and membrane
properties effects." Journal of Power Sources (2005) 140(1): 41-49.
Sloot, H. J.; Smolders, C. A. et al. "Surface-Diffusion of Hydrogen-Sulfide and Sulfur-
Dioxide in Alumina Membranes in the Continuum Regime." Journal of
Membrane Science (1992) 74(3): 263-278.
Soares, J. L.; Moreira, R. F. P. M. et al. "Hydrotalcite materials for carbon dioxide
adsorption at high temperatures: Characterization and diffusivity measurements."
Separation Science and Technology (2004) 39(9): 1989-2010.
Stern, S. A. "Polymers for Gas Separations - the Next Decade." Journal of Membrane
Science (1994) 94: 1-65.
Su, Y.-H.; Liu, Y.-L. et al. “Proton exchange membranes modified with sulfonated silica
nanoparticles for direct methanol fuel cells.” Journal of Membrane Science (2007)
296: 21-28.
Suwanmethanond, V.; Goo, E. et al. "Porous silicon carbide sintered substrates for high-
temperature membranes." Industrial & Engineering Chemistry Research (2000)
39(9): 3264-3271.
Szekeres, M.; Szechenyi, A. et al. "Layer-by-layer self-assembly preparation of layered
double hydroxide/polyelectrolyte nanofilms monitored by surface plasmon
resonance spectroscopy." Colloid and Polymer Science (2005) 283(9): 937-945.
144
Thomassin, J. M.; Pagnoulle, C. et al. "Contribution of nanoclays to the barrier properties
of a model proton exchange membrane for fuel cell application." Journal of
Membrane Science (2006) 270(1-2): 50-56.
Titulaer, M. K. "Porous Structure and Particle Size of Silica and Hydrotalcite Catalyst
Precursors." Geologica Ultraiectina, Dutch: (1993) Chap. 9. 207-242.
Tricoli, V. "Proton and methanol transport in poly(perfluorosulfonote) membranes
containing Cs+ and H+ cations." Journal of the Electrochemical Society (1998)
145(11): 3798-3801.
Uchytil, P.; Schramm, O. et al. "Influence of the transport direction on gas permeation in
two-layer ceramic membranes." Journal of Membrane Science (2000) 170(2):
215-224.
Uhlhorn, R. J. R.; Keizer, K. et al. "Gas and Surface-Diffusion in Modified Gamma-
Alumina Systems." Journal of Membrane Science (1989) 46(2-3): 225-241.
Vaccari, A. "Preparation and catalytic properties of cationic and anionic clays." Catalysis
Today (1998) 41(1-3): 53-71.
Vu, D. Q.; Koros, W. J. et al. "Mixed matrix membranes using carbon molecular sieves -
I. Preparation and experimental results." Journal of Membrane Science (2003)
211(2): 311-334.
Wan, Y. S. S.; Chau, J. L. H. et al. "Design and fabrication of zeolite-based microreactors
and membrane microseparators." Microporous and Mesoporous Materials (2001)
42(2-3): 157-175.
White, C. M.; Strazisar, B. R. et al. "Separation and capture of CO2 from large stationary
sources and sequestration in geological formations - Coalbeds and deep saline
aquifers." Journal of the Air & Waste Management Association (2003) 53(6):
645-715.
Wilhite, B. A.; Schmidt, M. A. et al. "Palladium-based micromembranes for hydrogen
separation: Device performance and chemical stability." Industrial & Engineering
Chemistry Research (2004) 43(22): 7083-7091.
Wilson, O. C.; Olorunyolemi, T. et al. "Surface and interfacial properties of polymer-
intercalated layered double hydroxide nanocomposites." Applied Clay Science
(1999) 15(1-2): 265-279.
145
Winter, F.; Xia, X. Y. et al. "On the nature and accessibility of the Bronsted-base sites in
activated hydrotalcite catalysts." Journal of Physical Chemistry B (2006) 110(18):
9211-9218.
Wu, H. L.; Ma, C.-C. M. "SiO
2
/Sulfonated PEEK Dodecatunstophosphoric Acid Hybrid
Materials-Preparation and Properties." Conference on composite Materials (2004)
876-881.
Wu, H. L. ; Ma, C. C. M. et al. "Swelling behavior and solubility parameter of sulfonated
poly(ether ether ketone)." Journal of Polymer Science Part B-Polymer Physics
(2006) 44(21): 3128-3134.
Xing, P. X.; Robertson, G. P. et al. "Synthesis and characterization of sulfonated
poly(ether ether ketone) for proton exchange membranes." Journal of Membrane
Science (2004) 229(1-2): 95-106.
Xu, Z. P.; Stevenson, G. et al. "Dispersion and size control of layered double hydroxide
nanoparticles in aqueous solutions." Journal of Physical Chemistry B (2006)
110(34): 16923-16929.
Yamamoto, M.; Kusakabe, K. et al. "Carbon molecular sieve membrane formed by
oxidative carbonization of a copolyimide film coated on a porous support tube."
Journal of Membrane Science (1997) 133(2): 195-205.
Yamamoto, T.; Kodama, T. et al. "Synthesis of Hydrotalcite with High Layer Charge for
CO
2
Adsorbent." Energy Conversion and Management (1995) 36(6-9): 637-640.
Yan, Y. S.; Tsapatsis, M. et al. "Zeolite Zsm-5 Membranes Grown on Porous Alpha-
Al
2
O
3
." Journal of the Chemical Society-Chemical Communications (1995) (2):
227-228.
Yang, W. S.; Kim, Y. et al. “A study by in situ techniques of the thermal evolution of the
structure of a Mg-Al-CO
3
layered double hydroxide.” Chemical Engineering
Science (2002) 57 2945-2953.
Yang, L.; Shahrivari, Z. et al. “Removal of Trace Levels of Arsenic and Selenium from
Aqueous Solutions by Calcined and Uncalcined Layered Double Hydroxides
(LDH).” Industial & Engineering Chemistry Research (2005) 44: 6804-6815.
Yong, Z.; Mata, V. et al. "Adsorption of carbon dioxide onto hydrotalcite-like
compounds (HTlcs) at high temperatures." Industrial & Engineering Chemistry
Research (2001) 40(1): 204-209.
146
Yong, Z.; Rodrigues, A. E. "Hydrotalcite-like compounds as adsorbents for carbon
dioxide." Energy Conversion and Management (2002) 43(14): 1865-1876.
Zaidi, S. M. J.; Mikhailenko, S. D. et al. "Proton conducting composite membranes from
polyether ether ketone and heteropolyacids for fuel cell applications." Journal of
Membrane Science (2000) 173(1): 17-34.
Zaidi, S. M. J. "Polymer sulfonation- A versatile route to prepare proton-condcution
membrane material for advanced technologies." The Arabian Journal for Science
and Engineering (2003) 28(Number 2B ): 183-194
Zawodzinski, T. A.; Neeman, M. et al. "Determination of Water Diffusion-Coefficients in
Perfluorosulfonate Ionomeric Membranes." Journal of Physical Chemistry (1991)
95(15): 6040-6044.
Zhang, Y. Q.; Lee, J. H. et al. "Polypropylene-clay nanocomposites prepared by in situ
grafting-intercalating in melt." Composites Science and Technology (2004) 64(9):
1383-1389.
Zhang, G. W.; Zhou, Z. T. "Organic/inorganic composite membranes for application in
DMFC." Journal of Membrane Science (2005) 261(1-2): 107-113.
147
Appendix
The reciprocal of the permeance of a multilayer membrane can be thought of as
its resistance for mass transport [Uhlhorn et al., 1989], and can be expressed as the sum
of the transport resistances of the individual membrane layers according to
Λ + + + =
) 2 (
1
) 1 (
1
) support (
1 1
,
layer P layer P P P
r r r tot r
(A1)
For a two-layer membrane at steady-state, the flux through the membrane
tot
J is equal to
the fluxes through the support
S
J , and through the top layer
SL
J .
SL S tot
J J J = = (A2)
The transport of a single gas through the support layer is by combined Knudsen diffusion
and by viscous flow [Sloot et al., 1992].
dx
dp
p B
D
RT
J
S eff
K S S
+ − =
η
0 ,
,
1
(A3)
Integration of Eqn. (A3) for the support disk alone (1-layer membrane) results in the
equation below for the membrane permeance
−
+ =
Δ
p
L RT
B
D
RTL p
J
s
S eff
K S
s
s
η
0 ,
,
1
(A4)
with
2
) (
2 1
p p
p
+
=
−
148
where
2
p is the pressure on the feed-side, and
1
p the pressure on the permeate-side of
the membrane
By plotting the permeance measured with the support disks as a function of
−
p ,
the average pressure, one calculates
eff
K S
D
,
(effective Knudsen diffusion coefficient, m
2
/s)
as the intercept, and
o S
B
,
(viscous-flow membrane parameter, m
2
) as the slope of the plot.
If one is to assume further that the support pore structure consists of straight, parallel,
non-intersecting pores, then
32
2
,
p
o S
d
B
τ
ε
= ,
i
p
K S
eff
K S
M
RT
d
D D
π τ
ε
τ
ε 8
3
0
, ,
= = , (A5)
where ε is the membrane porosity, d
p
[m] the membrane pore diameter, and τ the
membrane tortusity. One can then also calculate
p
d and τ for the support layer.
For a two-layer membrane the flux
S
J through the support layer (equal to the
experimental membrane flux
tot
J , see an Eqn. A2) can be expressed as [Uchytil et al.,
2000]
− + − = ) (
2
) (
1
2
1
2
12
0 ,
1 12 ,
p p
B
p p D
RTL
J
S eff
K S
s
S
η
(A6)
where
12
P is the pressure at the interface between the support and the top layer. From Eqn.
(A6), using the known values of
eff
K S
D
,
, and
o S
B
,
, one can estimate
12
P . One then
calculates the permeance of the support layer (equal to
p
J
s
Δ
, where
1 12
p p p − = Δ ), and
from Eqn. (A1), the permeance of the top layer. As already noted in the paper, for the
149
supported HT membranes the permeance of the support layer is typically two orders of
magnitude higher that the permeance of the top layer, and so the transport characteristcs
of the composite membrane reflect those of the HT film.
150
Appendix References
Sloot, H. J.; Smolders, C. A. et al. "Surface-Diffusion of Hydrogen-Sulfide and Sulfur-
Dioxide in Alumina Membranes in the Continuum Regime." Journal of
Membrane Science (1992) 74(3): 263-278.
Uchytil, P.; Schramm, O. et al. "Influence of the transport direction on gas permeation in
two-layer ceramic membranes." Journal of Membrane Science (2000) 170(2):
215-224.
Uhlhorn, R. J. R.; Keizer, K. et al. "Gas and Surface-Diffusion in Modified Gamma-
Alumina Systems." Journal of Membrane Science (1989) 46(2-3): 225-241.
Abstract (if available)
Abstract
Currently, the humanity is encountering two major crises: energy deficiency and global warming. In order to resolve these crises, we should consider maximizing energy efficiency and minimizing its usage. Furthermore, we should develop alternative energy sources (e.g. wind, solar, biomass), instead of hydrocarbon products. Moreover, we need to commercialize well-known techniques such as fuel cells, which are environment-friendly and high efficiency systems for various applications, such as power generation and transportation. In addition, we need to continue research on CO2 capture and separation processes.
Linked assets
University of Southern California Dissertations and Theses
Conceptually similar
PDF
A study of the application of membrane-based reactive separation to the carbon dioxide methanation
PDF
Methanol synthesis in a membrane reactor
PDF
Dynamics of direct hydrocarbon polymer electrolyte membrane fuel cells
PDF
Design and modification of electrocatalysts for use in fuel cells and CO₂ reduction
PDF
A hybrid adsorbent-membrane reactor (HAMR) system for hydrogen production
PDF
Development of carbon molecular-sieve membranes with tunable properties: modification of the pore size and surface affinity
PDF
On the use of membrane reactors in biomass utilization
PDF
The use of carbon molecule sieve and Pd membranes for conventional and reactive applications
PDF
Design, dynamics, and control of miniature catalytic combustion engines and direct propane PEM fuel cells
PDF
Studies on direct methanol, formic acid and related fuel cells in conjunction with the electrochemical reduction of carbon dioxide
PDF
Fabrication of silicon carbide sintered supports and silicon carbide membranes
PDF
PSSA-PVDF semi-IPN blends for direct methanol fuel cells
PDF
Preparation of polyetherimide nanoparticles by electrospray drying, and their use in the preparation of mixed-matrix carbon molecular-sieve (CMS) membranes
PDF
Process intensification in hydrogen production via membrane-based reactive separations
PDF
Performance prediction, state estimation and production optimization of a landfill
PDF
Hydrogen storage in carbon and silicon carbide nanotubes
PDF
Design of nanomaterials for electrochemical applications in fuel cells and beyond
PDF
Investigation of gas transport and sorption in shales
PDF
The study of CO₂ mass transfer in brine and in brine-saturated Mt. Simon sandstone and the CO₂/brine induced evolution of its transport and mechanical properties
PDF
Molecular-scale studies of mechanical phenomena at the interface between two solid surfaces: from high performance friction to superlubricity and flash heating
Asset Metadata
Creator
Kim, Tae Wook
(author)
Core Title
Studies of transport phenomena in hydrotalcite membranes, and their use in direct methanol fuel cells
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Chemical Engineering
Publication Date
10/11/2008
Defense Date
06/09/2006
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
adsorbent,carbon dioxide,conductive membrane,fuel cell,hydrotalcite,membrane,OAI-PMH Harvest
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Tsotsis, Theodore T. (
committee chair
), Bau, Robert (
committee member
), Sahimi, Muhammad (
committee member
)
Creator Email
kholy7@gmail.com,taewkim@usc.edu
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-m1657
Unique identifier
UC1141567
Identifier
etd-Kim-2407 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-118577 (legacy record id),usctheses-m1657 (legacy record id)
Legacy Identifier
etd-Kim-2407.pdf
Dmrecord
118577
Document Type
Dissertation
Rights
Kim, Tae Wook
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Repository Name
Libraries, University of Southern California
Repository Location
Los Angeles, California
Repository Email
cisadmin@lib.usc.edu
Tags
adsorbent
carbon dioxide
conductive membrane
fuel cell
hydrotalcite
membrane