See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/236614123
ChemInform Abstract: Alkaline Polymer Electrolyte Membranes for Fuel Cell
Applications
Article in Chemical Society Reviews · May 2013
DOI: 10.1039/c3cs60053j · Source: PubMed
CITATIONS
READS
253
860
4 authors, including:
Yan-Jie Wang
Jinli Qiao
Dongguan University of Technology
Donghua University
45 PUBLICATIONS 1,763 CITATIONS
176 PUBLICATIONS 3,874 CITATIONS
SEE PROFILE
SEE PROFILE
Jiujun Zhang
National Research Council Canada
280 PUBLICATIONS 21,835 CITATIONS
SEE PROFILE
Some of the authors of this publication are also working on these related projects:
Low cost production and high value-added application of bacterial nano-cellulose View project
Advanced electrode for unitized regenerative fuel cells & Metal-Air batteries View project
All content following this page was uploaded by Yan-Jie Wang on 29 May 2014.
The user has requested enhancement of the downloaded file.
Chem Soc Rev
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
REVIEW ARTICLE
Cite this: Chem. Soc. Rev., 2013,
42, 5768
View Journal | View Issue
Alkaline polymer electrolyte membranes for fuel
cell applications
Yan-Jie Wang,ab Jinli Qiao,*a Ryan Bakerb and Jiujun Zhang*bc
In this review, we examine the most recent progress and research trends in the area of alkaline polymer
electrolyte membrane (PEM) development in terms of material selection, synthesis, characterization, and
theoretical approach, as well as their fabrication into alkaline PEM-based membrane electrode
Received 6th February 2013
DOI: 10.1039/c3cs60053j
assemblies (MEAs) and the corresponding performance/durability in alkaline polymer electrolyte
membrane fuel cells (PEMFCs). Respective advantages and challenges are also reviewed. To overcome
challenges hindering alkaline PEM technology advancement and commercialization, several research
www.rsc.org/csr
directions are then proposed.
College of Environment Science and Engineering, Donghua University,
Shanghai 201620, PR China. E-mail: jiujun.zhang@nrc.gc.ca;
Fax: +86-21-67792159; Tel: +86-21-67792379
b
NRC Energy, Mining & Environment Portfolio, National Research Council Canada,
Vancouver, BC V6T 1W5, Canada. E-mail: qiaojl@dhu.edu.cn;
Fax: +1604 221 3001; Tel: +1604 221 3087
c
Research Institute of Donghua University, Shanghai 201620, PR China
such as fuel cells, batteries, supercapacitors, and water electrolysis have been recognized as some of the most feasible and
efficient technologies for portable, stationary and transportation applications.1 Among these technologies, fuel cells can, in
principle, supply energy with efficiencies in excess of 80% in
contrast to the limiting efficiency of the Carnot cycle. Fuel cell
technology is expected to be one of the more promising
environmentally-friendly power sources for transportation and
stationary applications.2 As a key component, polymer electrolyte membrane fuel cells (PEMFCs) including direct methanol
fuel cells (DMFCs) (depending on the different fuels used)
utilize a proton (H+) or hydroxide (OH )-conducting polymer
Dr Yan-Jie Wang obtained his MS
and PhD in materials science
from North University of China
in 2002 and Zhejiang University
in 2005, respectively. He then
carried out postdoctoral research
at Sungkyunkwan University,
Korea, and Pennsylvania State
University on advanced functional
materials. In 2009, he was
co-hired by the University of
British Columbia and the National
Research Council of Canada
Yan-Jie Wang
working on fuel cell catalyst
research. Since October 2012, he has been a visiting associate
professor at Donghua University working on fuel cell membranes.
He is particularly interested in physicochemical synthesis methods
and electrochemical technology in energy conversion and storage.
Dr Jinli Qiao is a professor, PhD
supervisor and Disciplines Leader
at Donghua University. She
received her PhD in Electrochemistry
from
Yamaguchi
University, Japan, in 2004 before
joining the National Institute of
Advanced Industrial Science and
Technology (AIST), Japan, as
a research scientist. From
2004–2008, she carried out
7 fuel cell projects including two
NEDO projects of Japan on the
Jinli Qiao
development of novel protonconducting membranes, new binders for MEA fabrication, and
non-platinum catalysts. Since March 2008, she has carried out a
total of 7 projects funded by Chinese Government including the
National Natural Science Foundation of China.
1. Introduction to alkaline polymer
electrolyte membrane fuel cells
With the growing demand for alternative energy worldwide,
electrochemical energy storage and conversion technologies
a
5768
Chem. Soc. Rev., 2013, 42, 5768--5787
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
membrane as a solid electrolyte. In this way, corrosion of the
cell is overcome. In fact, due to their high energy and power
densities, high efficiency and also low/zero emissions, PEMFCs
have been developed for almost five decades with reducing
environmental pollution and improving energy efficiency as
strong driving forces. However, for real fuel cell technology
commercialization, several challenges have been identified,
including their high cost and insufficient durability, caused
by two key fuel cell components: electrocatalysts and the
polymer electrolyte membrane (PEM).
In PEMFCs, electrocatalysts are used to promote the cathode
oxygen reduction reaction (ORR) and the anode fuel (hydrogen
or methanol) oxidation reaction, while the PEM is used to
separate the anode from the cathode as well as to conduct
reactants such as proton (H+ cations) for acidic fuel cells or
hydroxide ion (OH anions) for alkaline fuel cells. Therefore,
PEM high ionic conductivity is a key factor for PEMFCs since it
allows higher performances to be achieved. Currently used
PEMs are fluoropolymer based materials such as Nafion,
the perfluorosulfonic acid (PFSA) membranes, developed by
DuPont, or A901/A201, the anion-exchange membranes,
developed by Tokuyama. These Teflon-like molecular backbones give these materials excellent long-term stability in both
oxidative and reductive environments. Unfortunately, their
high cost and some unsolved technological problems such as
high methanol crossover rates, difficulty in synthesis and
processing, and instability in an operating fuel cell environment, still delay PEMFC commercial production. To overcome
these challenges, tremendous effort has been put into developing
cost-effective and durable electrocatalysts and PEMs in the course
of PEMFC research, development and commercialization over the
past several decades.2,3
Regarding PEMFC technology, there are two types depending
on the PEM used: one is an acidic PEMFC in which the PEM
conducts protons, while the other is an alkaline PEMFC in which
Ryan Baker is a technical officer
and assistant project manager
with the National Research
Council of Canada’s Energy
Mining
and
Environment
portfolio. He received his BSc in
chemistry from the University of
Victoria in 2000. During his BSc
he gained two years’ work
experience with Canfor R&D,
Cominco Engineering Services
Ltd.,
Stuart
Energy
(now
Hydrogenics)
and
Nova
Research
Ryan Baker
and Technology Centre. After
graduation, he worked for Stuart Energy in California on their
prototype electrolyser and vehicle refueling station. In 2002 he was
hired by NRC, and in 2008 received his M.A.Sc. in chemical
engineering from the University of British Columbia.
This journal is
c
The Royal Society of Chemistry 2013
Chem Soc Rev
Fig. 1
Schematic illustration of the H2/O2 alkaline PEMFC’s working mechanism.
the PEM conducts hydroxide ions. Alkaline PEMFCs, and in
particular alkaline PEMs, are the focus this review.
In the mid-1960s, alkaline fuel cells with aqueous KOH
liquid electrolyte showed much success as the devices powering
the famous Gemini and Apollo spacecraft, even taking man to
the Moon.4,5 However, for commercialization, CO2 in the air
formed acidic carbonates in the alkaline electrolyte, poisoning
it, and electrode flooding/drying, were identified as limiting
factors. Therefore, it was necessary to replace this liquid
alkaline electrolyte using solid-state alkaline PEMs which conduct OH , and at the same time play a role in separating the
anode and cathode.6–8
To illustrate the working principle, Fig. 1 shows a schematic
of an alkaline PEMFC (H2 as fuel and O2 as the oxidant), where
the catalyzed cathode oxygen reduction reaction (O2 + 4e +
2H2O - 4OH ) can produce OH ions which then pass
through the alkaline PEM to the anode where they react with
hydrogen to produce H2O (2H2 + 4OH - 4H2O + 4e ). The
electrons released from H2 will pass through the external circuit
to be consumed in the reduction of oxygen. This electric current
between the anode and cathode is used to produce power.
Dr Jiujun Zhang is a Principal
Research Officer and Fuel Cell
Catalysis
Core
Competency
Leader at the National Research
Council of Canada Institute for
Fuel Cell Innovation (NRC-IFCI,
now the Energy, Mining & Environment
Portfolio
(NRCEME)).
Dr Zhang received his BS and
MSc in electrochemistry from
Peking University in 1982 and
1985, respectively, and his PhD
in electrochemistry from Wuhan
Jiujun Zhang
University in 1988. He then
carried out three terms of postdoctoral research at the California
Institute of Technology, York University, and the University of
British Columbia. He is also the Adjunct Professor at the
University of British Columbia, the University of Waterloo,
Peking University, and Donghua University.
Chem. Soc. Rev., 2013, 42, 5768--5787
5769
View Article Online
Chem Soc Rev
Review Article
Table 1 Typical fuels and their corresponding electrode reactions for alkaline PEM fuel cells using an O2 cathode. Reproduced from ref. 9 with copyright permission
(The Royal Society of Chemistry)
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Fuel
E0a (V/SHE) Cathode reaction (oxidant)
Anode reaction (fuel oxidation)
H2 + 2OH - 2H2O + 2e
CH3OH + 6OH - CO2 + 5H2O + 6e
CH3CH2OH + 2OH - CH3CHO + 2H2O + 2e ,
CH3CH2OH + 4OH - CH3COOH + 3H2O + 4e ,
CH3CH2OH + 12OH - 2CO2 + 9H2O + 12e
Iso-propanol
CH3CHOHCH3 + 2OH - CO3COCH3 + 2H2O + 2e ,
CH3COCH3 + 16OH - 3CO2 + 11H2O + 16e
Ethylene glycol
(CH3OH)2 + 14OH - 2CO32 + 10H2O + 10e or
(CH3OH)2 + 10OH - (CO2)22 + 8H2O + 6e
Sodium borohydride NaBH4 + 8OH - NaBO2 + 6H2O + 8e
Hydrogen
Methanol
Ethanol
The overall alkaline PEMFC reaction is: O2 + 2H2 - 2H2O. The
theoretical open circuit voltage (OCV) of such a fuel cell is
1.229 V in ambient conditions. If the current passes through
the cell, the cell voltage would be reduced depending on the cell
operation conditions. In DMFCs, on the other hand, methanol
is oxidized as the fuel: CH3OH + 6OH - CO2 + 5H2O + 6e ,
and the whole cell reaction will be: CH3OH + 32O2 - CO2 + 2H2O
with a theoretical OCV of 1.213 V. To date, there are also many
different fuel choices for applications/power ranges of alkaline
PEMFC systems. Power demand plays a major role in operating
strategy and fuel choice. Table 1 summarizes some typical fuels
and their corresponding electrode reactions for alkaline PEMFCs
using an O2 cathode.9 Although each individual fuel has some
specific requirements, hydrogen and alcohols (e.g. methanol and
ethanol) have been considered the major fuels. Unfortunately,
when CO2-generating fuels are used, the carbonate issue, discussed later, needs to be solved for the improvement of alkaline
fuel cell efficiency. Even using hydrogen as fuel, the tolerance of
ambient CO2 and self-purging of carbonate are also needed.
In Fig. 1, it can be seen that the major mass transfers within
the PEM are OH ions and water. Regarding OH transport
within the PEM, both water uptake and transport play an
important role in affecting PEM conductivity and in turn fuel
cell performance. For example, during H2/O2 alkaline PEMFC
operation (Fig. 1), OH transports through the PEM from the
cathode to the anode, then reacts with H2, forming water,
which is different from that in acidic PEMFCs where the water
is formed at the cathode through H+ transport from anode to
cathode. This OH transportation through an alkaline PEM can
be significantly affected by the degree of water uptake, water
self-diffusion/permeation as well as electro-osmotic drag.
Therefore, to improve both performance and durability of
alkaline PEMFCs, the water related properties of alkaline PEMs
should be considered.
2. Advantages and challenges of alkaline
PEMFCs
Compared to acidic PEMFCs, alkaline PEMFCs have some
advantages.10 The major advantages can be summarized as
follows: (1) fuel cell reaction kinetics are fast under alkaline
conditions. These fast kinetics could reduce or remove the need
5770
Chem. Soc. Rev., 2013, 42, 5768--5787
E0c (V/SHE) E (V/SHE)
0.83
0.81
0.77
1/2O2 + H2O + 2e - OH
3/2O2 + 3H2O + 6e - 6OH
3O2 + 6H2O + 12e - 12OH
0.40
0.40
0.40
1.23
1.21
1.17
0.67
9/2O2 + 9H2O + 18e - 18OH
0.40
1.07
0.72
1/2O2 + H2O + 2e - 2OH
0.40
1.12
1.24
1/2O2 + H2O + 2e - 2OH
O2: 0.40
1.64
H2O2: 0.88 2.12
for precious metal catalysts such as Pt-based catalysts.11 For
example, inexpensive, non-noble metal catalysts can be used,
such as nickel for anodic fuel oxidation, and silver, iron
phthalocyanines and even transition metal chelates with
simple nitrogen-containing ligands etc. for cathodic oxygen
reduction reaction;12–15 (2) corrosion problems can be minimized under alkaline conditions; and (3) the use of small
organic fuels as fuels is favored due to their fast kinetics of
electrooxidation in alkaline medium. Such fuels as methanol
and ethanol have the advantages of easier storage and transportation and also have higher volumetric energy densities
when compared to hydrogen. Furthermore, the C–C bonds
present in higher alcohols such as ethanol and propanol can
be easily broken in an alkaline environment when compared to
that in an acidic environment.16,17 As a result of these advantages, alkaline PEMFCs have in recent years been considered
the next generation of fuel cell technology, and great effort has
been put into their research and development.9
However, there are still some challenges with alkaline
PEMFCs. These are all related to the PEMs’ relatively low ionic
conductivities, insufficient stabilities, fuel crossover, carbonation, as well as challenges with water management. These
challenges can be summarized as follows.
(1) Low anionic conductivity when compared to that of
acidic PEMs
This low conductivity can lead to a large voltage drop across the
PEM when a current passes through the cell, resulting in poor
fuel cell performance. A typical alkaline PEM is composed of a
polymer backbone with OH exchange groups, as shown in
Fig. 2. Since the diffusion coefficient of OH ions is four times
less than that of H+ ions, a fourfold increase in OH ion
concentration is needed to achieve similar results to those
obtained with an acidic PEM, suggesting that the polymer(s)
used alkaline PEMs need a higher ion exchange capacity.7
Unfortunately, high ion exchange capacities can lead to excessive polymer swelling upon hydration and concomitant loss of
mechanical properties.
(2)
Insufficient stability
Instability of the alkaline PEM in fuel cell operation at high pH
(pH 4 14) and elevated temperature (460 1C) is the major
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
Chem Soc Rev
quaternary ammonium groups are prone to degradation when
exposed to hydroxyl attack, especially in a strongly basic
environment and high temperatures above 60 1C.26–30 This is
primarily attributed to E2 Hofmann elimination and SN2 substitution reactions. The former could result in a cleavage of the
quaternary ammonium as shown in Reaction (2), while the
latter is a nucleophilic substitution as shown in Reactions (3)
or/and (4), in which the hydroxide ions could strongly attack
a-hydrogen on the ammonium to form two alcohol groups and
an amine.19
Fig. 2 Illustration of hydrophilic/hydrophobic domain separation in an alkaline
PEM: the hydrophilic domains that contain water molecules (red) and OH
(green) are intercalated in the hydrophobic network in alkaline PEM. The
hydrophobic domain is represented by a dark color.
cause of PEMFC degradation. Under operating alkaline PEMFC
conditions, the major cause of degrading stability is caused by
chemical processes. The degradation mainly occurs at two locations
in the PEM, one on the polymer backbone, and the other on the
functional ionic groups, both of which are directly related to its
ionic conductivity, mechanical properties and physicochemical
stability. Generally, alkaline PEMs are constructed from copolymers
whose quaternized co-monomers feature an anion as the charge
carrier. Since quaternary ammonium hydroxides are strong
bases of which the basicity is comparable to that of KOH, the
stability of the polymer backbone (no degradation of polymer
main chain) should be an important issue, or an alkaline PEM
will be useless because of the loss in mechanical strength. The
main mechanisms of degradation are supposed to be Hofmann
elimination when b-hydrogens are present (E2, b-hydrogen
elimination) and the direct nucleophilic attack (SN2, nucleophilic
substitution at a-carbons) by OH ions at the cationic sites as well
as radical attack. In a high pH environment, hydroxyl attack, a
dehydrofluorination reaction, can produce some internal CQC
double bonds, leading to polymer chain scission of and a
reduction in the membrane’s mechanical properties:18
(1)
The stability of functional ionic groups, i.e. the ion-exchange
groups, is equally important as that of the polymer backbone.
Among different ion-exchange groups, ammonium groups were
reported to be chemically and thermally more stable than
quaternary phosphonium or tertiary sulfonium groups,19,20
although phosphonium groups seemed to have potential to
be more stable towards the attack of hydroxide ions than
conventional quaternary ammonium groups.21 Some recent
publications22–25 have shown that guanidinium group-based
alkaline PEMs exhibited improved properties such as thermal
and chemical stability compared to their ammonium-bearing
equivalents. In general, as commonly used anion-exchange groups,
This journal is
c
The Royal Society of Chemistry 2013
(2)
(3)
(4)
It follows, to overcome PEM degradation, that the solvation
of OH anions is being considered as an important implication
in the design of advanced alkaline PEMs, because a membrane
with poor solvation easily exhibited a faster degradation of the
cationic groups.29
(3)
Fuel crossover from anode to cathode
Regarding fuel crossover, in alkaline PEM fuel cells, fuel (such
as hydrogen) crossover from the anode to cathode should be
considered, due to its negative effects in terms of fuel efficiency
and cathode voltage depression. When fuel crossover becomes
serious, a mixed potential can be caused by fuel oxidation on
the cathode such that fuel cell performance decreases significantly. Certainly, this fuel crossover also causes a reduction in
fuel efficiency due to cathode fuel oxidation which is a parasitic
side reaction. Actually, fuel crossover through an alkaline PEM
is less severe than is the case for that in an acidic PEM due to
the different water transfer directions mentioned above. For
transporting OH ions from the cathode to the anode in an
operating alkaline PEMFC with an alcohol feed (methanol,
ethanol, propanol, or ethylene glycol, see Table 1), it was
reported that alcohol crossover was insignificant because of
its low permeability and unfavorable dragging by water and ion
flow.31–33 Even though fuel crossover is reduced in an alkaline
PEM, it cannot be completely avoided.
(4) Carbonation due to CO2 in the air oxidant and/or the
product of small fuel oxidation at the anode
This carbonation, forming carbonate (CO32 ) and bicarbonate
(HCO3 ) within the catalyst layer and PEM, could reduce the
conductivity, leading to a large performance drop.34,35
Chem. Soc. Rev., 2013, 42, 5768--5787
5771
View Article Online
Chem Soc Rev
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
(5)
Difficulty in water management
Compared to that in acidic PEMFCs, water management issues
could still cause performance drop, although the electrode
flooding/drying issues would be improved when using alkaline
PEMs.15,16,36
To overcome these challenges, new and innovative PEM
synthesis appears to be the most important approach in alkaline
PEMFCs. In this review, we attempt to summarize recent significant progress in the synthesis of novel alkaline PEMs together
with the help of the results obtained in the authors’ laboratory
and reported in the literature. Less attention is given to acidic or
H+ cation conducting PEMs since these subjects have been welltreated in recent reviews.37,38 Searching for better membrane
materials, such as developing effective membrane systems with
lower cost, facile preparation and simple structure as well as
improving the chemical and structural stability of current
membrane materials are the motivation for these developments,
especially for low temperature alkaline fuel cell operation.
3. Synthesis, characterization and alkaline
PEM fuel cell validation
In alkaline PEM synthesis various polymers, whose main polymer chain structures range from poly(olefin)s, poly(styrene)s,
poly(phenylene)s, poly(ether sulfone)s, poly(phenylene oxide)s,
poly(ether imide)s, and poly(arylene ether)s to organic–
inorganic hybrid composites, have been used to produce
membranes specifically for alkaline PEMFC applications. In
general, typical alkaline PEMs can be catalogued into two types:
homogeneous and heterogeneous membranes. Details of PEM
structure, synthesis, characterization, and performance will be
discussed in the sub-sections below.
3.1
Homogeneous membranes
Homogeneous alkaline PEMs are composed of an anionexchanging material with a single phase structure, in which
the cationic charges are covalently bound to the polymer backbone. Generally, some cationic sites such as quaternary ammoniums are grafted and fixed on the skeleton of the polymer
chain through chemical reactions and/or physical treatment.
With a mobile counteranion such as OH associated to each
cationic group such as –NH4+, the whole PEM maintains
electroneutrality. Both the performance and stability of alkaline
PEMs are mainly determined by the polymer backbone and its
fixed cationic groups. Therefore, optimized synthesis methods
with innovative materials are necessary to obtain high performance, stable alkaline PEMs for fuel cell applications.
3.1.1 Direct polymerization. To attach cationic groups onto
the skeleton of the polymer chain, an easy way is to directly
polymerize a functionalized monomer containing cationic
groups or use the co-polymerization of a functionalized monomer and other monomers. The direct polymerization method
can be summarized in the following steps: first, the active
group in the monomers can be initiated via a possible physical/
chemical process (e.g. a radical group can be initiated by a
5772
Chem. Soc. Rev., 2013, 42, 5768--5787
Review Article
photo or thermal initiator); second, a corresponding polymerization process happens to form a functional co-polymer. After
quaternization and alkaline (OH ) exchange, the PEM will
become conductive. However, without a cross-linked structure,
the PEM will not be mechanically strong enough for fuel cell
applications. For example, polymerized chloromethylstyrene,
after quaternization, will have poor stability and mechanical
properties when it is used in alkaline PEM fuel cells.39 A typical
chemical, divinyl benzene was used as a cross-linking agent
during polymerization or during quaternization.40,41 However,
the resulting membrane showed both low water content and
high electrical resistance, which are not favorable for fuel cell
applications. For example, the water content in such a PEM was
below 0.28 g (H2O) g 1 (Cl-form) dry membrane, and its
ion exchange capacity was in a range of 1.03–1.89 meq. g 1
(Cl-form) dry membrane.40 When measured with 1000 Hz AC
after equilibration with a 0.5 N sodium chloride solution, the
anion exchange membranes showed an electrical resistance of
1.7–7.0 O cm 2.
Another direct polymerization route to prepare alkaline
PEMs is to condense a functionalized monomer. However,
the PEM obtained often exhibits insufficient mechanical
stability.11,42 Therefore, a PEM with a single conducting phase
prepared by monomer direct polymerization may not be suitable in obtaining high performance alkaline PEMs for fuel cell
applications. Even by using cross-linking to create some different microphase structures for improvement, the resulting
PEMs still do not seem satisfactory in terms of both mechanical
stability and strength.
3.1.2 Grafting method. To prepare homogeneous membranes, the grafting method has been shown to be one of the
most effective approaches in synthesizing commercially available alkaline PEMs. The process of grafting can be summarized
as follows: first, a monomer can be grafted into a polymer
membrane using physical/chemical techniques; second, the
grafted monomer can be functionalized according to the
expected requirements. If the monomer carries a functional group,
the second step can be ignored. In the grafting reaction, some
cationic functional groups such as ammonium, phosphonium,
sulfonium, pyridinium, guanidinium and imidazolium can be
created on the preformed membrane (see Fig. 3). It has been
Fig. 3
Six cationic groups used as anion-exchange sites.
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
Chem Soc Rev
realized that the grafting process can be controlled to yield
target architectures.43 According to the reported literature,
many popular physical techniques such as irradiation, UV
and plasma methods are often utilized to graft the functional
monomers onto polymer films via copolymerization.31 The
employed polymer films mainly include non-fluorinated films
(e.g. polyethylene (PE)), partially fluorinated films (e.g. PVDF
and P(E-co-TFE)), fully fluorinated films (e.g. poly(fluorinated
ethylene propylene) (PFEP), as well as poly(tetrafluoroethyleneco-fluoroethylenepropylene) (P(TFE-FEP)). In terms of functional ionomers, chloromethyl styrene (CMS), which has a
better stability,11 has been used to prepare alkaline PEMs by
radiation-grafting of ethylene-co-tetrafluoroethylene (ETFE)
to introduce quaternary ammonium groups, as shown in
Reactions (5–7):
(5)
(6)
(7)
where three steps are used, one to radiation-graft ETFE in the
presence of CMS to form a ETFE-g-CMS co-polymer film, the
second to do a quaternization to produce quaternary ammonium
groups through a functionalized reaction of methylene chloride,
and the last to carry out an alkaline exchange process.44 In this
process, the film produced exhibited a good ion-exchange
capability (IEC) and also a high OH conductivity, even at
ambient temperature, and could be used as a PEM in alkaline
PEMFCs. In particular, the ETFE-g-CMS copolymer film formed
with OH as the counterion showed an IEC of 0.92 meq. g 1
(dry membrane) and a power density of 110 mW cm 2 at a 60 1C
H2/O2 in an alkaline fuel cell.16,44 Other polymer based alkaline
PEMs were also synthesized, and showed a low IEC, probably
due to physical degradation of the backbone and thus might
not be sufficient for use in either alkaline PEMFCs or even other
electrochemical devices.45,46 In addition, both the mechanical
and chemical properties of these aforementioned alkaline
PEMs need to be further improved in terms of their fuel cell
applications.
Other monomers were also studied for the grafting process,
including aminosiloxane groups, hydrogenated olefins with an
This journal is
c
The Royal Society of Chemistry 2013
aliphatic ammonium, glycidyl methacrylate, trifluorostyrene, as
well as vinylimidazole. For example, among some trifluorostyrene
(i.e. a monomer) radiation-grafted polymers, the polyethylenebased functional membrane exhibited the lowest electrical resistance (i.e. 1.4 O cm 2) and an IEC of 0.86 10 3 eq. g 1.
Regarding the effect of radiation conditions, accelerated-electron
radiation was also tested. For example, some novel polysulfonebased alkaline PEMs from New-Selemion and Asahi Glass
showed higher coulombic and energy efficiencies compared
to Nafions membranes in an all vanadium redox flow battery.47
In addition, plasma technology was also used to prepare
cross-linked alkaline PEMs. In a plasma polymerization, a
uniformly thin 4-VP-based membrane with a high degree of
cross-linking was obtained and showed an ionic conductivity
of B0.54 10 3 S cm 1 and an electric resistance as low as
1.9 O cm 2. The low ionic conductivity could result from a high
degree of cross-linking.48 Instead of monomers, grafted SOCl2
groups were also explored using UV-induction, in which the
presence of sulfur dioxide and chlorine could enable a nucleophilic substitution of a diamine to functionalize polyethylene
into an alkaline PEM.49 This provides a new route for graft
polymerization of alkaline PEMs.
3.1.3 Chemical modification. In the preparation of homogeneous membranes, chemical modification involves creating
cationic moieties on pre-prepared polymer–polymer blends
which are then dissolved/cast to form a film. After alkaline
exchange, this film becomes an alkaline PEM. Two papers have
given detailed classifications of alkaline PEMs prepared by
chemical modification according to this structure and preparation procedure.11,31 For example, a styrene-based alkaline
PEM was synthesized using the two-step process shown in
Reactions (8) and (9).
(8)
(9)
The first step is chemical modification to produce an ammonium salt type styrene-based polymer through direct chemical
polymerization (a process called the halogen (either chlorine or
bromine)-methylation process); the second step is to introduce
a mobile anion charge by an alkaline exchange process.
If the PEM produced by chemical modification is intended
to be used in an alkaline PEMFC, backbone polymer selection
seems to be important in obtaining both enhanced performance and stability. Some polymer materials such as polyether
sulfone (PSU) and its modified derivatives, which showed
relatively high chemical and physical stabilities, have been
Chem. Soc. Rev., 2013, 42, 5768--5787
5773
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Chem Soc Rev
used as polymer backbones to produce alkaline PEMs.50 After
the chloromethylation step for quaternization, the quaternary
ammonia polysulfone obtained exhibited an ionic conductivity
above 10 2 S cm 1 at room temperature and also good mechanical strength in a fuel cell.51 The power density achieved in an
alkaline fuel cell was 110 mW cm 2 at 0.55 V. Although the
quaternization can be carried out using different amines for
improving OH conductivity,52,53 PSU-based membranes tend
to lose their mechanical properties due to swelling even when
immersed in water. To improve their dimensional stability,
approaches such as cross-linking and block copolymers could
be utilized. Unfortunately, these approaches could also have a
negative impact on ionic conductivity.11
As mentioned above, alkaline stability is generally of concern due to known degradation mechanisms under alkaline
conditions: b-hydrogen elimination and direct nucleophilic
substitution at an a-carbon. Generally, the elimination reaction
pathway can be avoided by using quaternary ammonium
groups that do not have b-hydrogens, but the substitution
pathway cannot be avoided so easily. Therefore, some new
approaches should be attempted in order to reduce the ammonium group’s susceptibility to the substitution reaction. It has
been found, for example, that an alkaline PEM based on a
poly(phenylene) backbone showed perfect chemical stability in
4.0 M NaOH solution at 60 1C for 30 days, and did not exhibit
any changes in appearance or flexibility during the testing, nor
was there any measurable change in the IEC values.27 By
increasing the length of the alkylene spacer between the
aromatic polymer backbone and the nitrogen atom of the
ammonium group, chemical degradation in an alkaline
environment was slowed substantially.39 The degradation or
loss of the quaternary ammonium also reduced as the length of
the aliphatic chain of diamine increased.28 For membranes
created with quaternary ammonium groups using diamine
4,40-diazabicyclo[2.2.2]octane (DABCO), the quaternary ammonium groups were stabilized by through space sharing of an
electron pair from the non-quaternized nitrogen on the same
DABCO molecule.26
Another polymer, chitosan (CS) was found to have excellent
film-forming properties and high mechanical strength.54,55
Recently, chitosan was modified to form derivatives in which
the quaternary ammonium groups were anchored to the
chitosan matrix to form alkaline PEMs, and then employed in
PEMFCs. Typically, a novel cross-linked quaternized chitosan
PEM could exhibit a high conductivity (B10 2 S cm 1).54 When
the PEM was assembled between two commercial electrodes
with a catalyst loading of 1 mg cm 2 20 wt% Pt/C to form an
MEA and tested in an alkaline PEMFC, a current density of
65 mA cm 2 was obtained at a cell voltage of 0.2 V. Meantime,
to improve the mechanical strength of poly(vinyl alcohol) (PVA),
chitosan was also used to cross-link PVA with PVA-chitosanblend-based PEMs.56
As a typical fluorine-based polymer, Nafions is one popular
material with phase-separation morphology, whose chemical,
mechanical and electrochemical properties seem very promising for optimal and long-term fuel cell performance whether it
5774
Chem. Soc. Rev., 2013, 42, 5768--5787
Review Article
is used in acidic or alkaline PEMFCs. According to the
literature,57,58 two kinds of modified Nafions such as
carboxylic Nafions and sulfonic Nafions have already been
used in alkaline PEM applications. Although both these
alkaline PEMs exhibited an electrical resistance and could
directly impregnate the electrodes, they presented an insufficient chemical stability in alkaline media. Recent research has
moved to utilize Nafions as hybrid polymer electrolytes membranes for alkaline PEM fuel cells. To overcome the degradation
of fluorinated polymers under alkaline conditions, some fluorinated co- and ter-polymers were conceived as alkaline PEMs.
The chlorotrifluoroethylene (CTEF)-based co-polymer was processed in the quaternization with a substitution of chlorine to
iodine and then installed using OH charges in an ion exchange
procedure.59 It was either used as a binder for alkaline PEMFCs
or processed into membranes. The measured IEC and ionic
conductivity were 3.65 10 3 meq. g 1 and 1 10 2 S cm 1,
respectively.
In fact, many polymers have been explored as materials for
alkaline PEM synthesis in which cationic moieties are created
on the backbones using chemical modification. Although the
chloromethylation of membranes based on polystyrene,
poly(ether imide) or polysulfone, has been widely carried out
using chloromethylmethyl ether, its toxicity (e.g. carcinogenicity)
has restricted its development for alkaline PEMFC applications.
With respect to this, safer and more facile synthesis routes need
to be developed. The synthesis of poly(propylene oxide) (PPO)based PEMs seems to meet both safety and facile synthesis
requirements, but quaternized PPO shows a relatively low IEC
(B10 3 meq. g 1).60,61 The recently reported guanidinium
functionalized-PPO (GPPO) by benzyl bromination reached a
hydroxide conductivity of up to 0.071 S cm 1 at room temperature. At present, the alkaline stability of this membrane (stable
in 1 M KOH solution at 25 1C) should be improved, and
the power density in a H2/O2 fuel cell test at 50 1C is only
16 mW cm 2.62 Other membranes based on epichlorohydrin
and PVA have shown good ionic conductivities, but their
dimensional stability is poor after high water uptake, which
in turn is unfavorable for MEA stability in alkaline PEMFCs.63,64
Therefore, efforts to develop PEMs for alkaline PEMFC applications should not only focus on achieving high conductivity and
IEC, but also on maintaining sufficient physical-chemical
stability at high water uptake levels, especially in high concentration KOH solutions and at elevated temperatures.
3.2
Heterogeneous membranes
Generally, a heterogeneous membrane is composed of a hydroxide
salt and an inert matrix material. The former can be embedded
inside the latter. Sometimes the inert matrix material can potentially be composed of plasticizers and/or inorganics.
3.2.1 Synthesis of polymer–salt complex membrane. Since
the polyethylene oxide (PEO)–salt complex was disclosed 30 years
ago,65,66 similar polymer–salt complexed membranes have
been used in alkaline PEMFC applications. In these complexes,
the salt like hydroxide alkaline (KOH) has the electrochemical
and conductive properties while the polymer, which can provide
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
the mechanical properties, contains electronegative heteroatoms such as oxygen, nitrogen, or sulfur, suggesting their
strong interaction with the salt cations favorable for alkaline
ionic conduction. This ionic conduction is associated with the
segmented motion of the polymer chain and the binding
energy between cations and anions.
Although the PEO–KOH complex was considered for alkaline
PEMFCs, PEO tends to crystallize, especially at high salt concentrations, resulting in a dramatic decrease of conductivity.
Compared to PEO, PVA has a good chemical stability and high
hydrophilicity due to its hydroxyl group, which associates
strongly with water. Using a simple solution casting method,
an alkaline PVA–KOH complex film was obtained and showed a
good conductivity of 4.7 10 2 S cm 1 at room temperature
when the quantities of KOH and water were increased in the
PVA.67 After chemical cross-linking modifications, the alkaline
PVA–KOH membranes were found to be very stable even in a
10 M KOH solution up to 80 1C without losing any membrane
integrity or ionic conductivity.68 However, at higher KOH concentrations, the PVA–KOH complex films seemed to be not
strong enough and thus may not be favorable for fuel
cell applications. To enhance the mechanical properties of
polymer–salt films, polymer blend-structures, such as PVA/
poly(epichlorohydrin) (PECH)69 and PEO/PVA,70 and crosslinking structures such as PVA/poly(acrylic acid) (PAA)71 and
PVA/poly(vinylpyrrolidone) (PVP)72 were considered in the preparation of polymer alkaline PEMs. In particular, PVA/PVP
cross-linking structures showed perfect alkaline stability even
upon exposure to a 10 M KOH solution at up to 120 1C.
Scanning electron micrographs revealed a highly ordered
microvoid structure uniformly dispersed on the membrane
surface with a pore size of ca. 200 nm after heat-curing, which
imparted the membrane with good liquid electrolyte (KOH)
retention ability. Unfortunately, in the fuel cell their resistance
increased with the presence of higher KOH concentrations,
probably leading to a decrease in fuel cell performance.11
Additionally, chitosan and polybenzimidazole (PBI) were
also used in the development of alkaline PEMs. Although
chitosan has a high degree of hydrophilicity and is an abundant, low-cost, weakly alkaline polymer electrolyte, its semicrystallinity intrinsically restricts the polymer’s flexibility, and
also has low conductivity. To enhance ionic conduction, Wan
et al.73 used a casting manufacturing method to obtain alkaline
chitosan-based three layer composite membranes, in which a
porous intermediate layer was made from KOH and chitosan
while the two cross-linked layers were made from chitosan and
glutaraldehyde in order to hold the KOH inside the membrane
(see Fig. 4). The membrane showed a conductivity near
10 2 S cm 1. In H2/O2 fuel cell testing with Pt as the electrode
catalysts, fuel cell performance showed an open-circuit potential
around 1.0 V and a current density of about 35 mA cm 2 at a
voltage of about 0.2 V when an MEA was fabricated by directly
pressing two gas-diffusion electrodes onto the two opposite
surfaces of this composite membrane.
With high thermal and chemical stability, PBI was also
used to form a PBI–KOH complex via immersing a PBI film in
This journal is
c
The Royal Society of Chemistry 2013
Chem Soc Rev
Fig. 4 SEM micrographs: (a) three-layer structure of chitosan-based composite
membrane, and (b) porous structure of inner layer. Reproduced from ref. 73 with
copyright permission (Elsevier B.V.).
a KOH solution.74 The film had a conductivity as high as
B0.1 S cm 1. After the PBI–KOH film and commercial electrodes
from E-Teck were assembled into an MEA for testing and put
into a single H2/O2 fuel cell, the performance was measured with
a current of 0.62 A cm 2 at 0.6 V. Under pressurized H2 and O2,
a significant improvement in performance was obtained compared that with non pressurized fuels.
3.2.2 Inorganic filled polymer–salt complex. It is known
that some inorganic materials, such as oxides (e.g. Al2O3, TiO2,
SiO2, and ZrO2), alkoxysilane, and hydroxyapatite, can be added
into a polymer matrix to enhance the mechanical, thermal and
chemical properties.31 In this way, the hybrid membrane
formed is composed of inorganic filler, polymer matrix and
hydroxide alkaline salt. To distribute inorganic materials into a
polymer matrix, approaches such as intercalation, blending,
in situ polymerization, sol–gel technology and molecular
self-assembly have been utilized in the preparation of hybrid
alkaline PEMs. With the addition of inorganic fillers, most
PEO-based and PVA-based hybrid membranes exhibited a clear
improvement in IECs while their main drawbacks lie with less
efficient OH conductivity than those of homogeneous membranes.
Using sol–gel technology, Wu et al.75 prepared poly(2,6-dimethyl-1,4phenylene oxide) (PPO)-based organic–inorganic hybrid alkaline
membranes with high silica content and high degrees of crosslinking. This series of hybrid membranes seemed visibly homogenous due to a favorable interaction between the organic PPO
and inorganic silica phases via both covalent and weak bonds
(such as van der Waals, hydrogen and ionic interactions76).
Chem. Soc. Rev., 2013, 42, 5768--5787
5775
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Chem Soc Rev
Hydroxide anion conductivities up to 0.011 S cm 1 were
obtained at room temperature and up to 0.035 S cm 1 at 90 1C.
When a 140 mm thick membrane with 5.4% silica was tested in a
single fuel cell in which the anode and cathode electrodes were
both commercial (E-Tek) type with Pt/C (20% mass) catalyst,
a power density of 32 mW cm 2 was recorded.
3.3 Interpenetrating polymer network (IPN)-based
membranes
An IPN structure (Fig. 5(a)) is composed of two polymers with a
network, in which two polymers are synthesized and/or crosslinked without any covalent bonds between them, while a semi
IPN structure (Fig. 5(b)) is formed via only one polymer’s crosslinked structure and one or more linear and branched polymers.
Both of these structures are promising in recent research trends
due to their excellent combination of electrochemical and mechanical properties. In simple terms, the hydrophobic polymer in the
structure can provide good thermal, chemical and mechanical
properties, while the conductive polymer can transport anions.
Because PVA’s highly chemically reactive hydroxyl functions
favor chemical cross-linking, it was used to prepare a series of
Fig. 5 Two polymer networks: (a) interpenetrated polymer network (IPN)
structure, and (b) semi-interpenetrated polymer network (semi-IPN) structure.
5776
Chem. Soc. Rev., 2013, 42, 5768--5787
Review Article
IPN/semi-IPN-based membranes, in which other polymers
(such as a water soluble quaternized copolymer), could offer
anions as charge carriers to conduct hydroxide (OH ). After
chemical cross-linking modifications using glutaraldehyde
as the cross-linking agent, these water soluble quaternized
copolymers could be ‘‘trapped’’ in the cross-linked PVA matrix,
thus providing the membrane with good mechanical strength
and flexibility, and strong dimensional stability as well as alkaline
stability.77–79 However, after ion exchange these membrane
gave conductivities around 1–7 10 3 S cm 1, which were still
unsatisfactory for alkaline fuel cell applications, because the
conducting groups of the polyelectrolyte in the system were
bound only to a restricted area and a slow extraction took place
over the time observed. Most recently, by a combined thermal
and chemical cross-linking method and using poly(diallyldimethylammonium chloride) (PDDA) as an anion charge
carrier, a PVA/PDDA–OH alkaline anion-exchange membranes
has been successfully prepared and applied in an alkaline PEM
fuel cell. The resulting membrane exhibited a high OH conductivity of 0.025 S cm 1 at 25 1C, which enabled a high power
density of 32.7 mW cm 2 to be achieved at room temperature in
a real H2/O2 fuel cell. The membranes exhibited a high alkaline
stability in 8 M KOH at 80 1C for 360 h due to effectively
suppressed b-hydrogen elimination.80 To enhance ionic conduction, a semi-IPN-based alkaline membrane was also prepared by
heat-treating the blend base membranes of chloroacetylated
poly(2,6-dimethyl-1,4-phenylene oxide) (CPPO) and bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO).81
The FT-IR spectra (see Fig. 6) of these two kinds of membranes
showed behavior of C–Br (B1025 cm 1) and C–Cl (B1082 cm 1),
suggesting that below 80 1C, only chloroacetyl groups contribute
to the formation of the cross-linked structure and above 90 1C,
both chloroacetyl and bromomethyl groups will tend to form
cross-links. This structure was also formed via a Friedel–Crafts
reaction without adding any cross-linking reagent or catalyst.82
Fig. 6 IR spectra of (a) unheated membrane and (b) heated membrane.
Reproduced from ref. 81 with copyright permission (Elsevier B.V.).
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
The alkaline membrane’s hydroxide conductivity was found to
be 3.2 10 2 S cm 1 at 25 1C. Compared to this membrane,
another alkaline PEM with semi-IPN structure was synthesized
from poly(methyl methacrylate-co-vinylbenzyl chloride) (PMV)
using divinylbenzene (DVB) as the crosslinker. DVB can be
polymerized as a crosslinked rigid network due to its phenyl
ring structure and the two vinyl groups on one ring.83 Therefore, poly(divinylbenzene) (PDVB) can hold the QPMV providing
more mechanical support. The hydroxide conductivity of
the resultant alkaline QPMV–PDVB membrane could reach
10 2 S cm 1 above 50 1C. This membrane gave a power density
of 80 mW cm 2 at 70 1C and can operate continuously for 420 h
in a fuel cell without significant performance issues such as the
decline of voltage, current and power density.
A semi-IPN structure was also synthesized from quaternized
chitosan (QCS) and polystyrene (PS), in which the PS was
synthesized by polymerization of styrene monomers in an
emulsion of the QCS in acetic acid aqueous solution under a
nitrogen atmosphere at elevated temperature.84 The semi-IPN
structure was formed by post-cross-linking of the QCS.
The membrane showed a hydroxyl ionic conductivity of 2.8 10 2 S cm 1, with a strong chemical and mechanical stability in
KOH solution at 80 1C probably because PS is more hydrophobic than QCS.
3.4
Chem Soc Rev
were formed in BPPO polymer in order to avoid the utility of
toxic (often carcinogenic) reagents in chloromethylation reaction for the preparation of alkaline PEMs. The hydroxide
conductivity of obtained Im-APEMs increased with IEC due to
the higher concentration of charge carriers and the volume
fraction of water. Compared to QA-APEM with a hydroxide
conductivity of only 18 10 3 S cm 1, the Im-APEMs showed
a higher conductivity up to 32 10 3 S cm 1 at room temperature. The improved conductivities of Im-AAEMs are probably induced from the pre-functionalizing strategy that
regulates the formation of ionic clusters during the solution
casting process. The atomic force microscopy (AFM) tapping
phase images in Fig. 7 clearly prove that there are two regions:
a hydrophilic area (dark) from the imidazolium ionic clusters,
and a hydrophilic area (bright) from the aromatic polymer
backbones. As the concentration of ionic groups increases,
the hydrophilic regions become more interconnected, yielding
superior ion conducting channels. After an MEA was fabricated
using Pt/C as the electrode catalyst, its H2/O2 fuel cell test
exhibited a peak power density of 30 mW cm 2 at a current
density of 76 mA cm 2. It was suggested in this study that a
chemically compatible imidazolium-based alkaline ionomer
IL-based ionomer membranes
Ionic liquids (ILs) are defined as organic salts that melt at or
below room temperature. IL-based polymer electrolytes have
emerged as an interesting class of materials in some wideranging electrochemical energy applications such as batteries
and fuel cells. More recently, imidazolium-type IL functionalized polymers have been popular and attractive in the preparation of alkaline PEMs due to their negligible vapor
pressure, high conductivity, good thermal stability and excellent ion-exchange capability. Compared to typical alkaline
PEMs prepared from quaternary ammonium (QA) cationic
groups (QA-APEMs), the imidazolium-type IL functionalized
alkaline PEMs (Im-APEMs) could possess different properties
such as high hydroxide conductivity, good chemical stability,
and desired selective solubility. They may dissolve in nonalcohol low-boiling-point water-soluble solvents (e.g. acetone,
ethyl acetate, and tetrahydrofuran), which favors solution casting of the ionic polymers in the preparation processes and the
use of alkaline PEMs as ionomers to enhance the ionic contact
in the catalyst layers of the fuel cell electrodes.85,86
Generally, the QA groups are introduced via postfunctionalization, which introduces the ionic functionality
after the membranes are cast. However, this method cannot
restrict ion aggregation or phase separation. It seems that
imidazolium chemistry may be more amenable to alkaline
PEM fabrication through a pre-functionalized strategy, favoring
the formation of ion clusters for efficient alkali-anion transport.
In this regard, a typical series of imidazolium-type alkaline PEMs
were synthesized via the functionalization of bromomethylated
poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) using 1-methylimdazole.87 In the preparation process, bromomethyl groups
This journal is
c
The Royal Society of Chemistry 2013
Fig. 7 AFM tapping phase images for two selected bromomethylated
poly(phenylene oxide) (BPPO)-based imidazolium alkaline PEMs (Im-APEMs):
the feed molar ratio of 1-methylimidazole/–CH2Br in membranes (A) and (B)
are 60.4/100 and 86.3/100, respectively. The dark areas correspond to the ‘‘soft’’
hydrophilic imidazolium ionic clusters and the bright areas correspond to the
hard structures of the hydrophobic aromatic polymer backbones. Reproduced
from ref. 87 with copyright permission (Elsevier B.V.).
Chem. Soc. Rev., 2013, 42, 5768--5787
5777
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Chem Soc Rev
should be developed as an ionic polymer binder in electrode
catalyst layers.
Although most of the recent synthesized alkaline Im-PEMs
have exhibited a high hydroxide ion conductivity (B10 2 S cm 1)
at room temperature and also showed good alkaline stability,
probably due to the resonance effect of the conjugated imidazole rings, which weaken the interaction of imidazolium
groups and hydroxide ions, the stability of these membranes
still needs to be improved with a focus on the degradation of
imidazolium cations in alkaline environments. Compared to
the aliphatic main-chain polymers, the aromatic main-chain
polymers are expected to be combined with imidazolium functional groups for the development of high-performing IL-based
alkaline PEMs with both high mechanical strength and thermal
stability.85,87
4. Theoretical study on alkaline PEMs and
their PEMFC applications
Theoretical studies such as modeling and simulation have been
used extensively to investigate the application of acidic PEMs in
PEMFC applications in the past two decades. However, not
enough research literature exists on alkaline PEM theoretical
studies. In fact, theoretical studies can be utilized to not only
provide some fundamental insight but also down-select PEMs
under fuel cell operating conditions.
4.1
Ionic conductivity/transport of alkaline membranes
As discussed above, alkaline PEMFCs operate in an alkaline
environment and transport hydroxide ions across the alkaline
membrane. Therefore, it is very important to understand and
predict the ionic transport and conduction of alkaline membranes as a function of relative humidity and membrane
properties under operating conditions.
Although fundamental understanding and modeling to
describe hydroxide transport through an alkaline membrane
are not abundant in the literature, an analytical dusty fluid
model has been explored.88,89 Based on acidic PEM literature as
a reference, three transport mechanisms such as the Grotthuss
mechanism, diffusion and migration, and convection are discussed for the OH transport process within alkaline PEMs, as
shown in Fig. 8. In the Grotthuss mechanism hypothesis, OH
exhibits a different Grotthuss behavior from that of protons
(H+). Membrane structure was thought to have a close relationship with the transport process. At 25 1C, the transport coefficient for the hydroxyl ion was reported to be 5.3 10 9 m2 s 1
while that for the proton is 9.3 10 9 m2 s 1 in liquid water.
When these transport coefficients were compared to the differences in conductivity among Nafions 115 membrane, the
alkaline fully fluorinated poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), and the alkaline partially fluorinated
poly(ethylene-co-tetrafluoroethylene) (ETFE) membranes (0.078,
0.018, and 0.012 S cm 1, respectively), these substantial discrepancies were found to be larger than those for the bulk transport
coefficients. Several possible reasons were discussed such as
5778
Chem. Soc. Rev., 2013, 42, 5768--5787
Review Article
Fig. 8 Three dominant hydroxide transport mechanisms in alkaline membranes.
Reproduced from ref. 88 with copyright permission (The Electrochemical Society).
insufficient OH dissociation and solvation, the morphology of
the structure, the impact of the structure on inhibiting transport mechanisms, the interactions between the OH in
solution and the quaternary ammonium side chains, and/or
the formation of carbonate/bicarbonate due to the presence of
air during the measurement. This research work relative to the
dusty fluid model can provide valuable information about how
to improve membrane performance.
4.2
Membrane degradation
As discussed above, although more progress has been made on
issues related to performance, the commercial viability of
alkaline PEMFCs is still being limited by the low durability
and fast degradation of alkaline PEMs. There are relatively few
theoretical studies on the issue of alkaline PEM degradation
in fuel cells probably because of the costs, timescales and
difficulties associated with experimentation. For example,
membrane degradation tests require thousands of hours of
operation and there are difficult challenges in visualizing the
degradation and measuring the very low concentrations of
species that can provide direct evidence of failure. Despite
these challenges, a simple degradation mechanism for
ammonium-type alkaline membranes has already been studied
using density functional theory (DFT) methods.
To figure out and predict what degradation will happen to
the alkaline membrane, Chempath et al.29 and Kiss et al.89 used
DFT calculations and took a tetraalkylammonium-based functional group as a model to optimize the geometries of all
species. Two different mechanisms were found for tetramethylammonium in alkaline media: one where the methyl group
suffers an SN2 attack by a hydroxyl ion and thus directly forms
methanol ([N(CH3)4]+ + OH - N(CH3)3 + CH3OH), and the
other in which an ylide (trimethylammonium methylide) and a
water molecule are formed by the abstraction of a proton from
a methyl group ([N(CH3)4]+ + OH - CH2QN(CH3)3 + H2O),
consequently, the ylide can react with water to form methanol
(CH2QN(CH3)3 + H2O - N(CH3)3 + CH3OH). Interestingly, the
formation of methanol was found to be thermodynamically
favorable ( 28.7 kcal mol 1) while this degradation reaction
was practically irreversible.90 The small barrier for the ylide
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
formation reaction was calculated to be 8.4 kcal mol 1, suggesting an equilibrium between starting materials and ylide. The
formation of ylide played a key role in the decomposition of
tetramethylammonium hydroxide.
Furthermore the transition state structures and the corresponding activation barriers of four modified cations in the SN2
mechanism were assessed within a small error to have very
similar geometries and activation barriers in the range of
12–14 kcal mol 1. This means that the degradation effect
should be insignificant after the addition of phenyl and benzyl
groups to the tetramethylammonium. If a dielectric medium
was considered for alkaline membranes, Chempath et al.29,91
found the effect of water and solvation on reactivity. By exploring a polarizable continuum model (PCM), they compared the
calculated energies versus the different dielectric constants, and
concluded that OH ions and cations were not solvated and
became much more reactive in the absence of water, suggesting
a state of poor stability for cations. All of these modeling results
have important implications for the design of alkaline PEMs,
especially for the cationic groups.
4.3
Alkaline membrane water transport and carbonate ions
In H2/O2 alkaline PEMFCs, water is one of the reactants in the
reduction reaction (2O2 + 4H2O + 8e - 8OH ). If an alkaline
PEM suffers from low water retention at the membrane–cathode
interface, there will not be enough water at the cathode for the
reduction reaction, leading to a significant decrease in fuel cell
performance. To study how the membrane could retain water
without affecting its adherence or bonding to fuel cell electrodes,
a modified commercial Morgan ADP membrane was prepared by
Follain et al.92 using plasma technology. They used the Park
model to model and analyze the sorption isotherms of untreated
and plasma-treated membranes. It was found that the interactions between water molecules and the membrane are not
qualitatively modified by plasma treatment. The plasma-treated
membranes could be less water permeable in the context of
membranes for alkaline fuel cells without qualitatively modifying the water transport properties. This could partially explain
the enhancement of fuel cell performance recently depicted with
a more pronounced effect for a highly cross-linked film by
plasma modification than the untreated film.
Besides, one problem in the development of alkaline
PEMFCs is performance decay caused by the accumulation of
carbonate ions in the alkaline membrane. Siroma et al.93
developed a mathematical model to determine the relationship
between the current density and the residual carbonate ions in
an alkaline PEM. In the mathematical model, three assumptions were proposed: one regarding the composition of counteranions, another regarding the boundary condition of the
concentration of carbonate ions, and the third regarding
the balance of diffusion and migration of carbonate ions. The
mathematical analysis could explain the effects of thickness
and conductivity of the membrane on the amount of carbonate
ions in the alkaline PEM during fuel cell operation. Their
model is helpful in developing high performance membranes
for alkaline PEMFCs.
This journal is
c
The Royal Society of Chemistry 2013
Chem Soc Rev
5. Validation of alkaline PEMs using PEMFC
performance
5.1
Requirements for alkaline PEMs in fuel cells
Improving alkaline PEMs has been identified as the single
greatest challenge in alkaline PEMFC technology, because both
their architectures and properties predominantly determine
the performance in terms of both power density and durability.
For PEMFC applications, ideal alkaline PEMs are expected to
meet the following criteria: (1) low cost; (2) high ionic conductivity (410 2 S cm 1); (3) high chemical and thermal
stability during manufacturing and fuel cell operating conditions; (4) serve as barriers to electrons (electronic insulator with
high electrical resistance); (5) low gas and/or fuel permeability
to reduce crossover; (6) be as thin as possible (50–80 mm) to
reduce ionic resistance and reduce the cost; (7) high mechanical
strength and low degree of swelling when immersed in water;
and (8) excellent capability of being used in solution form (called
the ionomer solution) in order to facilitate the preparation of
polymer impregnated electrodes and the MEA. Unfortunately, to
date, no alkaline PEM can completely fulfil all of the requirements above. Therefore, exploring new membranes using innovative synthesis methods seems to be the most important
approach in making technological breakthroughs.
To obtain a high performance alkaline PEMFC both the
alkaline PEM properties, which are strongly dependent on the
polymer matrix, and the nature/concentration of the cationic
charges, must be optimized in terms of their interaction and
synergistic effects. In general, the polymer matrix (or backbone
and structure) determines the mechanical/thermal stabilities
which are strongly affected by water content, membrane thickness, dimensional stability as well as tensile strength, while the
nature/concentration of the cationic charges play important
roles in anion exchange capacity, ion transfer number and
conductivity.
In an operating fuel cell environment, the cationic groups
bearing a hydroxyl counteranion must be stable in an alkaline
environment so that integral stability can be obtained. On the
other hand, the flexibilities of both polymer chains and the
alkaline PEM’s backbone play important roles in both its
chemical and mechanical stabilities. For example, a flexible
chain generally corresponds to a low glass transition temperature (Tg). If Tg is below the operating temperature, some
structural changes of polymer(s) can be avoided for the
membrane in the chemical reactions, and it can be thermally
stable at a higher temperature above 100 1C. In this regard, new
strategies to develop membranes composed of block copolymers, polymer blends, hybrid materials or ionic liquid based
ionomers should be explored to produce alkaline PEMFCs with
both high power density and durability.
5.2
MEA assembly and fuel cell performance
The performance and durability of an alkaline PEMFC are
strongly dependent on the PEM used and the corresponding
MEA, as well as the fuel cell operating conditions, which
correlate with each other in terms of synergetic effects for
Chem. Soc. Rev., 2013, 42, 5768--5787
5779
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Chem Soc Rev
Fig. 9 A typical schematic process for MEA fabrication. The right half of the
figure is reproduced from ref. 94 with copyright permission (Elsevier B.V.).
optimal performance. For example, even if one has a great
alkaline PEM, if the MEA fabrication is not optimized, the fuel
cell performance and/or durability may be poor. On the other
hand, even a great MEA may not give good performance if the
fuel cell operation conditions are not optimized.
Regarding MEA assembly, Fig. 9 shows a typical schematic
process for MEA fabrication.94 Normally, the MEA consists of
an alkaline membrane, catalyst layers, and gas diffusion layers
(GDL). These components are fabricated individually and then
pressed together under elevated temperatures and pressures.
The catalyst layers are formed by coating a catalyst slurry on
both sides of the membrane. The catalyst slurry can be prepared using an electrocatalyst, a binder such as Nafions, and a
solvent such as isopropyl alcohol (IPA). Typical key steps are
shown in Fig. 9. The fabrication method used to incorporate a
membrane into the MEA’s electrode layers is a complicated
process for an alkaline PEM fuel cell and several factors such as
the membrane’s morphology, mechanical properties, scalability, materials cost as well as processing methods should be
considered. Compared to acidic PEM fuel cells, alkaline PEM
fuel cells have had a lag in development with respect to the
methods and materials used for MEA fabrication because of
such challenges as low hydroxide conductivity, poor durability
of anion conducting polymer binders, the absence of solvents
with suitable and safe low boiling points, water-soluble organic
solvents for catalyst ink preparation, as well as the absence of
fast and efficient membrane/electrode attachment fabrication
techniques similar to those for hot-pressing and decal methods
for acidic PEM fuel cell MEAs. However, in the basic environment of alkaline PEM fuel cells, a major benefit is that the
cathode oxygen reduction over-potential can be significantly
decreased, resulting in a high fuel cell efficiency and an
increase in catalyst durability. Furthermore, facile cathode
kinetics allow the use of non-precious metals in place of noble
or precious metals normally used as catalysts, thus significantly
reducing the alkaline PEMFC’s cost.
A soluble ionomer with high ionic conductivity and
chemical stability is required to build up an efficient threephase boundary95 (see Fig. 10) in the MEA catalyst layer and
5780
Chem. Soc. Rev., 2013, 42, 5768--5787
Review Article
Fig. 10 A three-phase medium consisting of a carbon/platinum phase (electronic
conductor), the ionomer (ionic conductor) and gas pores (oxygen or hydrogen
supply). Reproduced from ref. 95 with copyright permission (Elsevier B.V.).
improve the catalyst particle utilization as well as reduce the
internal resistance. For example, Gu et al.21 developed a high
conductivity quaternary phosphonium-based ionomer as the
catalyst layer electrode binder in an alkaline PEM fuel cell. This
could be because quaternary phosphonium containing polymers can show an excellent solubility in low-boiling-point
water-soluble solvents like methanol and ethanol, which are
easy and safe to handle and are easily removed during MEA
preparation. An MEA with an active area of 5 cm2 was fabricated
using an alkaline PEM (70 mm thick FAA commercial membrane
from Fuma-Tech GmbH, conductivity: 1.7 10 2 S cm 2 in
water at 20 1C), and hot-pressed at 60 1C, where Pt-black was
used as both the anode and cathode electrocatalysts. A power
density of 138 mW cm 2 was achieved at 70 1C at a loading of
0.2 mg (Pt) cm 2 and 0.05 mg (ionomer) cm 2 for both the
anode and cathode with a pressurized H2 and O2 feed. When
using 0.5 mg (Pt) cm 2 and 0.125 mg (ionomer) cm 2, the
power density was increased to 178 mW cm 2 at 70 1C. This
H2/O2 alkaline PEM fuel cell exhibited a substantially improved
peak power density and significant reduction in internal resistance. This phosphonium-based ionomer showed excellent
solubility, high ionic conductivity and physicochemical stability,
and also helped to build a more efficient three-phase boundary
in the catalyst layer compared to quaternary ammonium hydroxide
containing polymers as the ionomer binder.
Despite the recognition of an alkaline PEM fuel cell’s
potential advantages prototypes of these fuel cells still use
Pt-based materials as catalysts (especially at the anode), while
the long-awaited non-precious alkaline PEM fuel cell has not
yet appeared. To investigate the effect of non-precious metal
catalysts (NPMCs) on fuel cell performance, different MEAs
were fabricated using Tokuyama membrane (# A201, 28 mm,
4.2 10 2 S cm 1 at 23 1C with 90% RH) and various cathode
catalysts such as Co- and Fe-phthalocyanines,13 Co– and/or
Fe–EDA chelates14 and Co based commercial catalysts from
GPMaterials.96 5 wt% ionomer (AS-4 ionomer, Tokuyama
Corporation, Japan) GPM) or 30 wt% polyvinylbenzyl chloride
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
(PVBC) were used as an ionomer binder. Using these NPMCs as
cathode catalysts, the fuel cells gave a maximum power density of
120 mW cm 2 (0.6 mg cm 2, 45 1C), 196 mW cm 2 (4 mg cm 2,
50 1C) and 260 mW cm 2 (3 mg cm 2, 60 1C), respectively, with
an anode Pt loading of 0.4–0.5 mg cm 2.
To broaden the technical applications of NPMCs in alkaline
PEM fuel cells and find standard alkaline MEA prototypes,
Lu et al.97 tried Ag as an anode catalyst and Cr-decorated Ni
as a cathode catalyst, and a self-synthesized quaternary ammonium polysulfone ionomer was used as the membrane and
electrode binder, respectively. Under their testing conditions,
the fuel cell gave a maximum power density of 50 mW cm 2 at
60 1C with a Ni loading of 5 mg cm 2 and Ag loading of 1 mg cm 2.
Although the reported power densities were far below those
obtained using Pt/C electrodes (600–700 mW cm 2 at 60 1C),
it demonstrated the feasibility of using NPMCs as catalysts.
With the use of NPMCs, the MEAs should be more promising in
commercial applications of alkaline PEM fuel cells.
Regarding fuel cell testing, Fig. 11 gives a simple schematic
diagram of the measurement setup.98 Normally, there are four
major components in the fuel cell test system: (1) a single fuel
cell or a fuel cell stack consisting of many single cells connected
in parallel, (2) a fuel cell test station consisting of a reactant
supply unit for supplying both fuel (H2 or liquid alcohols such
as methanol) and oxidant (air or O2) separately into the fuel
cell, (3) a reactant pressurizing unit to control the reactants’
pressures, (4) a humidification unit for humidifying the reactant
gases, (5) a thermal management unit for controlling the fuel cell
temperature, (6) a load bank for controlling fuel cell current
density or voltage to obtain the voltage–current polarization and
power density curves, as well as the lifetime voltage–time curves,
and (7) a computer to control the testing and data acquisition/
analysis. As shown in Fig. 12, the fuel cell test station can provide
control over the reactant gas humidification temperature and
cell operating pressure, as well as anode and cathode mass flow
rates. Fig. 12 shows a typical current–voltage curve recorded
using an alkaline PEM fuel cell.98
Fig. 11 Schematic of test system. Reproduced from ref. 98 with copyright
permission (Elsevier B.V.).
This journal is
c
The Royal Society of Chemistry 2013
Chem Soc Rev
Fig. 12 Fuel cell performance of MEAs with Co- and Fe-phthalocyanine modified MWCNTs along with E-TEK and Tanaka Pt/C catalyst based cathodes using
Tokuyama’s A201 series anion exchange membrane. Reproduced from ref. 13
with copyright permission (Elsevier B.V.).
6. Current status and approaches in alkaline
PEMs
As discussed previously, four major PEM issues, which determine both the performance and durability of alkaline PEMFCs,
are as follows: (1) transport/conductivity; (2) stability; (3) CO2
poisoning; and (4) insufficient optimization of fuel cell performance. The following subsections will give discussions about
the current status of and approaches to these four aspects.
6.1
Transport/conductivity of alkaline PEMs
In general, PEM conductivity is the first key property for ionconducting polymers for their actual applications. According to
the literature,99,100 alkaline PEM conductivity, which is lower
than that of their acidic PEM counterparts, is mainly due to the
slow conduction of hydroxide ions in PEMs compared to that of
protons, leading to a larger ohmic loss when a current passes
through a PEM fuel cell. Therefore, developing an alkaline PEM
with high OH conductivity is probably the first priority for
alkaline PEMFC technology.
According to the performance target set by the US Department of Energy (DOE), the conductivity of an alkaline PEM used
in PEMFC applications should be above 0.1 S cm 1 to support a
large current with minimal resistive losses.11 At the current
state of technology, most of the performance levels are in the
range of 10 3–10 2 S cm 1, which is still far from the DOE
target. To further improve conductivity, the following
approaches have been taken during the past several decades.
(1) Synthesis of alkaline PEMs. The design, synthesis, and
optimization of novel molecular structures in terms of backbone chemistries, form of phase separation, and a cross-linking
frame have been such as to obtain high OH conductivity. For
example, a structural design similar to Nafions phase-separation
has been considered, in which there are two domains: hydrophilic and hydrophilic (see Fig. 2), where the hydrophilic
domains can serve as the path for OH transportation.
Chem. Soc. Rev., 2013, 42, 5768--5787
5781
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Chem Soc Rev
(2) PEM characterization technical development. To assist
and accelerate the development of alkaline PEMs, several
characterization parameters have been developed such as the
determination of hydration number per base site, water uptake
and dimensional changes, water self-diffusion/permeation
coefficient, and the conductivity relative to different RH, as
well as polymer structures. Some standard experimental conditions and protocols were also built to deeply understand the
relationship between transport/conductive properties and
molecular structure.
(3) MEA and fuel cell design for PEM validation and optimization. The PEMs developed have been integrated into electrodes to form membrane electrode assemblies (MEAs), and then
tested in an optimized fuel cell. Based on the alkaline PEM fuel
cell’s performance, further modifications to PEM synthesis and
MEA fabrication in terms of conductivity are carried out for
optimization.
6.2
Stability of alkaline PEMs
A second important challenge is the insufficient stability of
alkaline PEMs when they are used in PEMFCs. The DOE target
for the stability of alkaline PEMs is only 1–3% loss of performance after 5000 h by 2017.101,102 Recently, PEMs in alkaline
PEMFCs have shown some improvement in durability from
hundreds to thousands of hours in a temperature range of
60–80 1C,103–105 which is still far below current perfluorosulfonic
acidic PEMs and far from the required target performance.105
Currently, regular alkaline PEMs have 10% loss in performance
after 1000 h. The best performance level to date is only 1–3% loss
after 2000 h for the new Tokuyama membrane.101,105
Stability of alkaline PEMs includes both chemical and
mechanical stabilities, which can be affected by several factors
such as relative humidity (RH) cycling, high temperature operation, as well as chemical attack beyond hydroxide (oxidative or
radical attack).
Regarding chemical stability, the decomposition of alkaline
PEMs can probably occur at two locations: the cationic groups
and the backbone. Normally, the cationic group includes a
functional cation and a tether. The tether is a chemical linkage
between the backbone and cation although the cation is
incorporated within the backbone for some functional alkaline
PEMs.106 Therefore, improving both stability of cations and
tethers has been an ongoing research activity and is also
immediately needed for the development of advanced alkaline
PEMs. Numerous investigations on the backbone have been
focused on hydrocarbon-based structures, most of which have
demonstrated stability under target operating conditions (wet,
high temperature, and high pH). The research approaches to
improving PEM chemical stability are mainly to cross-link more
common polymers to improve some of the properties of the
materials (e.g. thermal, mechanical or physicochemical properties).107 Another factor causing PEM instability is chemical
attack by hydroxide and radical species formed during fuel cell
operation. These radicals include HOO /HO2 , [HOOCO2] ,
[OOCO2]2 , and O2 . To decrease radical attack, approaches
have been carried out such as the use of radical scavengers to
5782
Chem. Soc. Rev., 2013, 42, 5768--5787
Review Article
deactivate these species, the decrease of metal contaminants to
reduce the production of radical species, and the use of radical
inhibitors or peroxide-decomposition catalysts deposited
within membrane to decrease the severity of radical attacks,
or an incorporation of sacrificial material to prevent direct
attack on the membrane.108 Although almost no attention
has been placed on this issue to date, related work has been
considered important in researching the influence of radical
species in the degradation of alkaline PEMs.
Regarding the mechanical stability of alkaline PEMs, the
major requirement is to have enough strength and stability
when the PEMs are used in the membrane electrode assembly
(MEA) fabrication process and the fuel cell stack compression.
Normally, the mechanical properties of alkaline PEMs are
characterized in both the dry and wet states by stretching the
membranes to determine their tensile properties.109 Although
many hydrocarbon based alkaline PEMs exhibited both high
ion exchange capacities (IEC) and water uptake, low mechanical durability was observed when using accelerated testing
protocols during RH cycling, which was probably caused by
membrane dimensional changes induced by both higher stiffness
(Young’s modulus) and water uptake. Actually, the mechanical
stability is strongly dependent on the testing devices for specific
applications. For example, fuel cell dynamic operation for automobile applications contains frequent start-up, shut down and
load changes, where a high PEM mechanical stability in addition
to high chemical stability is definitely required.
To improve the mechanical stability, several approaches
have been taken, such as cross-linking structures,79,80,110 and
chemical modifications.111 For example, physical or chemical
grafting can be widely expected to give rise to stable structures
such as block structures in the membranes with high mechanical strength. At the same time, these structures can exhibit
good flexibility and dimensional stability in the presence of a
basic medium. Also, the grafting method can decrease the cost
when suitable materials are chosen. Certainly, the introduction
of such structures in developmental membranes should be
based on the balance between mechanical stability and
conducting/transport properties so that the overall fuel cell
performance can be improved.
6.3
Carbon dioxide poisoning
Even in using an alkaline PEM to replace liquid alkaline
electrolyte in alkaline fuel cells, the alkaline PEM still suffers
from a hydroxide/carbonate/bicarbonate equilibrium, which is
not an issue for acidic PEMs. CO2 from air or as a by-product of
hydrocarbon fuel oxidation (i.e. methanol) will lead to the
formation of carbonates inside the membrane, which could
seriously degrade the performance of PEMs such as a significant reduction in OH conductivity, leading to a degradation in
fuel cell performance. For instance, Yan and Hickner34 and
Yanagi and Fukuta105 clearly reported a decrease of OH
conductivity in alkaline PEMs due to the influence of CO2 in
ambient air. In the case of a H2/air fuel cell, although the CO2
concentration in air is relatively low (B400 ppm), when air is
fed into the cathode of an alkaline PEMFC fuel cell, CO2 quickly
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
Chem Soc Rev
equilibrates in the alkaline membrane forming carbonates.
However, it is very difficult to precisely control or test the ratio
of different anions within an alkaline PEM at any given time. To
reduce the negative effect of CO2 on alkaline PEMFC operation,
some approaches have been taken, such as the use of liquid
hydrogen to condense the CO2 out of air, eliminating the
presence of CO2 using pure fuels in the anode feed stream,
and the preparation of a high performance membrane with
strong resistance against CO2. However, new strategies to solve
the CO2 poisoning issue are needed for commercialization.
6.4
Alkaline PEMFC performance optimization
As mentioned previously, alkaline PEMFCs have received
increased attention due to their major advantage over acidic
PEMFCs in using non-noble metal materials as the catalysts. In
recent years, some advances in alkaline PEMFCs have been
achieved in terms of both power density and durability. For
example, before 2006, the typical power densities reported for
alkaline PEMFCs were in the range of 10 to 100 mW cm 2, and
the durability obtained was very poor in most cases.
Recently, commercial (or semi-commercial) alkaline PEMs
and ionomer solutions such as those from the Tokuyama
Corporation have appeared on the market, signaling an
advancement in alkaline PEM development. For example,
a hydrogen fed alkaline PEMFC could reach a performance as
good as that of an acidic PEMFC.101 At 80 1C, the highest power
density (B500 mW cm 2) has been achieved with a current
density as high as 2 A cm2, although the durability was still
insufficient and the issue of CO2 poisoning still existed.
It should be mentioned that more recently, using the mutual
radiation grafting technique, two good candidates were reported
Table 2
for H2/O2 fuel cells using TMA functionalized (LDPE-co-VBC)3
and poly(ETFE-g-VBC) membranes112 by introducing nonfluorinated or partially fluorinated fluorine groups. With a
catalyst loading of 0.4 mg(Pt) cm 2, the cell displayed the
highest power densities of 823 mW cm 2 at 60 1C, 718 mW cm 2
at 50 1C and 648 mW cm 2 at 20 1C at the voltage of 0.5 V,
respectively. The advancement can be attributed to both
the increased fundamental understanding and the mitigation
strategies developed for a number of critical issues in alkaline
PEMFCs. Tables 2 and 3 summarize the reported best performances and optimum durabilities of alkaline PEM fuel cells
from both the Tokuyama Corporation (including Penn State
contributions)113 and CellEra Inc.114 The data show that H2
alkaline PEM fuel cell performance has taken great strides
forward and is approaching the performance and durability
of acidic PEM fuel cells.
Regarding the membrane electrode assembly (MEA) for an
alkaline PEMFC, a MEA consists of anode and cathode gas
diffusion layers, catalyst layers, and a PEM, as shown in Fig. 9.
In the catalyst layer, there are at least two essential components, one is the catalyst particle (normally, it is a carbon
supported catalyst particle), and the other is the ionomer.
Due to the porous structure of the catalyst layer, the threephase boundary (catalyst particle/ionomer/gas channel or /liquid
in the case of direct fuel cells) can be formed, which serves as
the fuel cell reaction zone for either the fuel oxidation or oxygen
reduction reactions. Besides the catalyst, ionomer, a kind
of solid conductive polymer electrolyte (SPE) (in alkaline
PEMFCs, this SPE is conducting hydroxide ions) plays a critical
role in fuel cell performance. This ionomer is composed of either
some perfluorinated- or hydrocarbon-based polymer material.
Several best performances for H2 alkaline PEMFCs
Company
Membrane
Ionomer
(i.e. binder)
Pt amount
(mg cm 2)
Best performance
(peak power
density vs. current)
(mW cm 2, A cm 2)
Tokuyama
corporation
A901
AS-4
0.5
300, 0.8
A201
AS-4
0.5
250, 0.6
A801
AS-4
0.5
95, 0.4
A801
A3V2
0.5
22, 0.1
A901
AS-X
0.5
450, 1
A201
AS-4
0.5
340, 0.8
—
—
—
500, 1.3
CellEra Inc.
Table 3
Operation conditions
Anode: 95% RH, H2 (1.0 L min 1); cathode: 95% RH, clean air
(2.0 L min 1); catalyst: Pt/C catalyst; cell temperature: 50 1C.
Anode: 95% RH, H2 (0.5 L min 1); cathode: 95% RH, clean air
(1.0 L min 1); catalyst: Pt/C catalyst; cell temperature: 50 1C.
Anode: 95% RH, H2;cathode: 95% RH, air; catalyst: Pt/C catalyst;
cell temperature: 50 1C.
Anode: 95% RH, H2; cathode: 95% RH, air; catalyst: Pt/C catalyst;
cell temperature: 50 1C.
Anode: 95% RH, H2 (1.0 L min 1); cathode: 95% RH,
O2 (1.0 L min 1); catalyst: Pt/C catalyst; cell temperature: 50 1C.
Anode: 95% RH, H2 (1.0 L min 1); cathode: 95% RH, clean air
(CO2 o 0.1ppm, 2.0 L min 1); catalyst: Pt/C catalyst;
cell temperature: 50 1C.
H2/O2, 80 1C
Good durability data for H2 alkaline PEMFCs
Researcher
Durability
Tokuyama corporation
Penn State
CellEra
This journal is
c
100 mA cm
100 mA cm
400 mA cm
The Royal Society of Chemistry 2013
Operation conditions
2
2
2
constant current after 700 h
constant current after 5000 h
constant current after 700 h
H2/air, 50 1C
H2/air, 50 1C
H2/air, 60 1C
Chem. Soc. Rev., 2013, 42, 5768--5787
5783
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Chem Soc Rev
As shown in Fig. 11, the PEM is a necessary component of the
MEA, which conducts hydroxide ions and also serves as a
separator for separating the anode from the cathode. It has
been recognized that this MEA is the fuel cell’s heart; all
challenges including fuel cell performance and durability are
related to this MEA. For example, the formation of carbonates,
leading to low conductivity, occurs in both ionomer and PEM;
catalyst degradation could happen in the catalyst layer, anode
flooding and cathode drying could also happen within this
MEA, resulting in both low performance and durability. Therefore, to improve both the fuel cell performance and durability,
several approaches have been carried out: (1) using a high
conductivity and durability PEM as well as ionomer; (2) new
and cost-effective catalyst development to make high activity
and durability catalysts for both anode and cathode reactions;
(3) advanced MEA design and fabrication through optimizing
the catalyst layer structure and ink fabrication with respect to
the fuel cell performance and durability; (4) improving fuel cell
performance using optimized operating conditions such as
high temperature operation to increase the MEA’s water management ability; (5) optimizing gas diffusion layers (GDLs) to
improve reactant mass transport; (6) using engineering
approaches and mitigation strategies as well as specific operating conditions such as high temperature to reduce the negative
effect of CO2 poisoning; and (7) optimizing the fuel cell system
design and operation to increase the system efficiency.
7. Summary and research directions in
alkaline PEMs for PEMFC applications
In this review, we have examined the most recent progress and
research trends in the areas of developing alkaline polymer
electrolyte membranes (PEMs) in terms of material selection,
synthesis, characterization, theoretical approaches, the fabrication of these PEM-based membrane electrode assemblies
(MEAs) and their corresponding performance/durability in
PEM fuel cells (PEMFCs).
Using an alkaline PEM instead of a liquid caustic alkali
electrolyte, the corresponding PEMFC has been considered
advanced or next generation fuel cell technology. The use of
alkaline PEM based fuel cells could avoid or reduce the
problems of leakage, carbonation, precipitation of carbonate
salts, and prevent gas electrode flooding. In particular, compared to acidic PEMFCs, these have two major advantages, such
as allowing non-precious metal catalyst use in alkaline PEMFCs
could dramatically reduce the cost per kW of power, and the
fast electrode kinetics for fuel (small organics) oxidation in
alkaline medium allows the use of easy storage and transportation of fuels. Compared to acidic PEMFCs, there are also other
advantages such as increased material stability at high pH,
decreased fuel crossover rates, and potentially improved water
management. However, several challenges still exist, such as
insufficient membrane ionic conductivity, leading to low fuel
cell performance due to high membrane resistance; lower
membrane stability; as well as issues with CO2 poisoning.
5784
Chem. Soc. Rev., 2013, 42, 5768--5787
Review Article
In more recent years, developmental work on anion
conducting PEMs has significantly bloomed, with increasing
attention in alkaline PEMFCs due to the major advantages
mentioned above. Several research directions to overcome
those challenges, which currently hinder technological success
and commercialization, are proposed below.
(1) Developing innovative methodologies for synthesizing
and characterizing materials and their corresponding highperformance alkaline PEMs to overcome both insufficient ionic
conductivity and stability. To enhance mechanical stability,
developing cross-linking strategies has been proved to be an
effective route for membrane synthesis. Introducing different
functional polymer chains can be an effective way to create a
microscopic phase-separated structure in ion conducting polymer membranes similar to Nafions’s polymer structure. With
such structures, ionic conductivity can be realized by a hopping
type mechanism in the aggregation of ionic channels in a wellseparated area. As a result, the ionic conductivity could be
improved. Furthermore, the separate hydrophobic phase could
make the membrane more dimensionally stable so that some
properties such as thermal stability, as well as mechanical and
physicochemical properties can be obtained. To gain a deeper
understanding of the relationship between synthesis strategy
and membrane properties, factors such as the hydration
number (number of water molecules per base site), water
uptake and dimensional changes, water self-diffusion coefficient (using pulsed field gradient NMR), water permeation
coefficient (using a permeation cell), as well as alkaline conductivity should be well characterized.
Normally, some specific conditions such as temperature,
relative humidity and counterion form can have some significant impact on the chemical stability of various alkaline PEM
materials. The stability of materials used for the synthesis of
advanced alkaline PEMs should also be considered in terms of
the improvement of conductivity. Therefore, it is important to
investigate and understand the correlation among the conductivity of alkaline PEMs, relative humidity, counterion form, and
variations in polymer chemistry such as cross-linking site and
additives.
In addition, when employing the cross-linking method, the
polymer cost should also be reduced by using cost-effective
alkaline PEM materials in terms of fuel cell commercialization.
(2) Optimizing the preparation procedure for alkaline PEMbased MEAs and their corresponding design and operation in
terms of improving both performance (power density) and
durability. Building electrode architectures with suitable
three-phase boundaries composed of ionic and electronic conduction and mass transfer is necessary to produce a high
performance MEA. This MEA can be built using non-platinum
group metal catalysts for the electrode design. In the optimization of the catalyst layer, a focus on non-PGM catalysis, ink
formulation and processing represents an identified area of
need to develop advanced alkaline PEMFCs. This would be
significantly different from the traditional acidic PEM systems
with typical carbon supported or unsupported catalysts. The
electrode preparation is a necessary step in achieving high
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
performance of fuel cells. To obtain ideal three-phase boundaries
in the MEA, the ionomer in the catalyst layer would play an
important role in the electrode preparation and the catalyst layer’s
activity. The ionomer should be highly soluble in low-boiling
water-soluble solvents such as ethanol and (n- and 2-)propanol
so that these solvents are easy and safe to handle as well as
remove during electrode preparation. And also the ionomer
should have high hydroxide conductivity and stability in alkaline,
favoring an improvement in fuel cell performance. Unfortunately,
up to now, no ionomer as a commercial binder is available for
alkaline PEM fuel cells. In addition, when air is fed into the
cathode in an alkaline PEMFC, the tolerance or mitigation
strategies of ambient CO2 in air should be considered in terms
of their effect on fuel cell performance. The operating temperature
in the range of 60 1C–80 1C can improve carbonate rejection
because the solubility of CO2 decreases with increasing temperature. At the same time, water management is relative to the
operating temperature and is more challenging due to the direction of ion flow from cathode to anode. Therefore, system
approaches, as well as fuel cell operating conditions, should both
be optimized to enable higher temperature operation.
(3) Further theoretical studies and modeling are needed to
obtain a more fundamental understanding in down-selecting
alkaline PEM materials and optimizing their corresponding
PEMFC performance and durability. Computational modeling
of fuel cells is an important aspect in fuel cell performance
optimization. A fundamental understanding of the electrochemical
and transport processes in alkaline PEMFCs should be necessary
for the improvement of fuel cell performance and durability.
Compared to acidic PEM materials in fuel cell applications, alkaline
PEM materials require less work on modeling and simulation. Also
these theoretical modeling and simulation studies are based on
ex situ conditions. In an operating fuel cell environment, the
conditions are often far from ideal.
It is expected that the modeling work in alkaline PEM fuel
cells should be focused on (i) the membrane properties, such as
ionic conduction and transport, degradation mechanism, water
uptake and transport, and carbonate ion formation, (ii) the
electrocatalytic kinetic reaction mechanism of non-PGM catalysts, (iii) the optimization of MEA including water management and the dynamic design of gas diffusion layers (GDLs),
and (iv) the fuel cell operating conditions. Based on the
modeling and simulation, it is necessary to build a close
relationship between membrane and fuel cell performance to
obtain advanced alkaline PEMFCs. In addition, constructing
models to simulate alkaline PEM performance in an operating
fuel cell is necessary in terms of fundamental understanding
and optimization of fuel cell fabrication and performance.
(4) Development and optimization of high-performing alkaline
PEMFC stacks and systems for portable, automobile and stationary applications. It is known that the stack consists of a series of
individual fuel cells. With modular connections and arrangements, fuel cell stacks can supply power and correspondingly
determine the performance of system. The compact designs of
alkaline fuel cell stacks should be flexible and optimized according to the corresponding type of application. In this regard,
This journal is
c
The Royal Society of Chemistry 2013
Chem Soc Rev
the design, fabrication and assembly of fuel cell components,
such as MEA, bipolar plate (flowfield), current collector and
sealing, are critical for fuel cell performance and durability.
It should be pointed out that for most alkaline PEM fuel
cells, whether it is fed by liquid and/or gas, the estimation of
carbonate formation is considered of critical importance relative to
the system performance as even ambient CO2 levels have resulted
in significant performance loss. The tolerance of ambient CO2
levels and carbonate self-purging needs to be properly assessed
to improve fuel cell stack performance. Some engineering
approaches to deal with CO2 need to be emphasized. For example,
when the operating temperature is increased to B80 1C, the power
density and CO2 tolerance can be improved. At higher temperatures, in addition to more durable materials, water management
should also be optimized to maintain high fuel cell performance
for technological advance. Currently, a 200 mW cm 2 peak power
at 0.5 V cell voltage has been demonstrated at 60 1C, and 2000 h of
operation with less than 10% voltage loss has been achieved at
50 1C. According to the DOE’s alkaline PEMFC milestone,
further advances in performance and durability (i.e. alkaline
PEM technologies in MEA/single cells with non-PGM catalysts
that maintain performance 4350 mW cm 2 for 2000 h at
T 4 80 1C) will be expected to move alkaline PEM fuel cells
closer to commercial viability.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21173039); Specialized Research
Fund for the Doctoral Program of Higher Education, SRFD
(20110075110001) of China and the State Environmental
Protection Engineering Center for Pollution Treatment and
Control in Textile Industry of China. All financial support is
gratefully acknowledged.
References
1 R. Liu, L. Zhang, X. Sun, H. Liu and J. Zhang, Electrochemical Technologies for Energy Storage and Conversion,
Wiley-VCH, Weinheim, 2012.
2 K. D. Kreuer, J. Membr. Sci., 2001, 185, 29–39.
3 M. Mamlouk and K. Scott, J. Power Sources, 2012, 211,
140–146.
4 J. H. Reid, Process of Generating Electricity, US 736016, USA,
1903.
5 P. C. L. Noel, PCL, US 350111, USA, 1904.
6 X. Zhang and J. Chen, J. Power Sources, 2011, 196,
10088–10093.
7 E. Agel, J. Bouet and J. F. Fauvarque, J. Power Sources, 2001,
101, 267–274.
8 T. Xu, Z. Liu and W. Yang, J. Membr. Sci., 2005, 249,
183–191.
9 E. H. Yu, X. Wang, U. Krewer, L. Li and K. Scott, Energy
Environ. Sci., 2012, 5, 5668–5680.
10 B. Y. S. Lin, D. W. Kirk and S. J. Thorpe, J. Power Sources,
2006, 161, 474–483.
Chem. Soc. Rev., 2013, 42, 5768--5787
5785
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Chem Soc Rev
11 G. Merle, M. Wessling and K. Nijmeijer, J. Membr. Sci.,
2011, 377, 1–35.
12 S. D. Poynton, J. P. Kizewski, R. C. T. Slade and J. R. Varcoe,
Solid State Ionics, 2010, 181, 219–222.
13 I. Kruusenberg, L. Matisen, Q. Shah, A. M. Kannan and
K. Tammeveski, Int. J. Hydrogen Energy, 2012, 37, 4406–4412.
14 X. G. Li, B. N. Popov, T. Kawahara and H. Yanagi, J. Power
Sources, 2011, 196, 1717–1722.
15 T. S. Olson, S. Pylypenko and P. Atanassov, J. Phys. Chem.
C, 2010, 114, 5049–5059.
16 J. R. Varcoe and R. C. T. Slade, Fuel Cells, 2005, 5, 187–200.
17 P. K. Shen, C. Xu, H. Meng and R. Zeng, in Advances in
Fuel Cells, ed. X. W. Zhang, Research Signpost, Kerala,
2005, pp. 149–179.
18 B. Ameduri, Chem. Rev., 2009, 109, 6632–6686.
19 A. C. Cope and E. R. Trumbull, Olefins from Amines: The
Hofmann Elimination Reaction and Amine Oxide Pyrolysis
in Organic Reactions, R.E. Krieger Publ. Co., Huntington,
New York, 1975.
20 E. B. Trostyanskaya and S. B. Makarova, Anion exchangers
onium class of compounds, Zhurnal Prikladnoi Khimii,
1966, 39, 1754.
21 S. Gu, R. Cai, T. Luo, Z. Chen, M. Sun, Y. Liu, G. He and
Y. Yan, Angew. Chem., Int. Ed., 2009, 48, 6499–6502.
22 J. Wang, S. Li and S. Zhang, Macromolecules, 2010, 43,
3890–3896.
23 Q. Zhang, S. Li and S. Zhang, Chem. Commun., 2010, 46,
7495–7497.
24 M. Guo, J. Fang, H. Xu, W. Li, X. Lu, C. Lan and K. Li,
J. Membr. Sci., 2010, 362, 97–104.
25 X. Zhu, B. Wang and H. Wang, Polym. Bull., 2010, 65, 502–506.
26 B. Bauer, H. Strathmann and F. Effenberger, Desalination,
1990, 79, 125–144.
27 M. R. Hibbs, C. H. Fujimoto and C. J. Cornelius, Macromolecules, 2009, 42, 8316–8321.
28 E. N. Komkova, D. F. Stamatialis, H. Strathmann and
M. Wessling, J. Membr. Sci., 2004, 244, 25–34.
29 S. Chempath, B. R. Einsla, L. R. Pratt, C. S. Macomber,
J. M. Boncella, J. A. Rau and B. S. Pivovar, J. Phys. Chem. C,
2008, 112, 3179–3182.
30 J. C. Garcia, J. F. Justo, W. V. M. Machado and L. V. C.
Assali, J. Phys. Chem. C, 2010, 114, 11977–11983.
31 G. Couture, A. Alaaeddine, F. Boschet and B. Ameduri,
Prog. Polym. Sci., 2011, 36, 1521–1557.
32 Z. Ogumi, K. Matsuoka, S. Chiba, M. Matsuoka, Y. Iriyama,
T. Abe and M. I. Inaba, Electrochemistry, 2002, 70, 980–983.
33 E. Antolini and E. R. Gonzalez, J. Power Sources, 2010, 195,
3431–3450.
34 J. Yan and M. A. Hickner, Macromolecules, 2010, 43,
2349–2356.
35 J. F. Zhou, M. Unlu, J. A. Vega and P. A. Kohl, J. Power
Sources, 2009, 190, 285–292.
36 H. Bunazawa and Y. Yamazaki, J. Power Sources, 2008, 189,
48–51.
37 H. W. Zhang and P. K. Shen, Chem. Rev., 2012, 112,
2780–2832.
5786
Chem. Soc. Rev., 2013, 42, 5768--5787
Review Article
38 H. W. Zhang and P. K. Shen, Chem. Soc. Rev., 2012, 41,
2382–2394.
39 M. Tomoi, K. Yamaguchi, R. Ando, Y. Kantake, Y. Aosaki
and H. Kubota, J. Appl. Polym. Sci., 1997, 64, 1161–1167.
40 T. Sata, Y. Yamane and K. Matsusaki, J. Polym. Sci., Part A:
Polym. Chem., 1998, 36, 49–58.
41 M. S. Huda, R. Kiyono, M. Tasaka, T. Yamaguchi and
T. Sata, Sep. Purif. Technol., 1998, 14, 95–106.
42 K. Haagen and F. Helfferich, Ion exchange membrane,
Ger. Patent 971,729, Germany, 1961.
43 B. Ameduri and B. Boutevin, Well-architectured Fluoropolymers:
Synthesis, Properties and Applications, Elsevier, Amsterdam,
2004, pp. 347–454.
44 J. R. Varcoe, R. C. T. Slade and E. L. H. Yee, Chem.
Commun., 2006, 1428–1429.
45 T. N. Danks, R. C. T. Slade and J. R. Varcoe, J. Mater. Chem.,
2002, 12, 3371–3373.
46 Y. J. Choi, M. S. Kang, J. Cho and S. H. Moon, J. Membr.
Sci., 2003, 221, 219–231.
47 G.-J. Hwang and H. Ohya, J. Membr. Sci., 1997, 132, 55–61.
48 K. Matsuoka, S. Chiba, Y. Iriyama, T. Abe, M. Matsuoka,
K. Kikuchi and Z. Ogumi, Thin Solid Films, 2008, 516,
3309–3313.
49 R. K. Nagarale, G. S. Gohil and V. K. Shahi, Adv. Colloid
Interface Sci., 2006, 119, 97–130.
50 P. Zschocke and D. Quellmalz, J. Membr. Sci., 1985, 22, 325–332.
51 J. Pan, S. Lu, Y. Li, A. Huang, L. Zhuang and J. Lu,
Adv. Funct. Mater., 2010, 20, 312–319.
52 J. S. Park, S. H. Park, S. D. Yim, Y. G. Yoon, W. Y. Lee and
C. S. Kim, J. Power Sources, 2008, 178, 620–626.
53 G. Wang, Y. Weng, D. Chu, R. Chen and D. Xie, J. Membr.
Sci., 2009, 332, 63–68.
54 Y. Wan, B. Peppley, K. A. M. Creber, V. Tam Bui and
E. Halliop, J. Power Sources, 2008, 185, 183–187.
55 A. Lewandowski, K. Skorupska and J. Malinska, Solid State
Ionics, 2000, 133, 265–271.
56 Y. Xiong, Q. L. Liu, Q. G. Zhang and A. M. Zhu, J. Power
Sources, 2008, 183, 447–453.
57 K. Matsui, E. Tobita, K. Sugimoto, K. Kondo, T. Seita and
A. Akimoto, J. Appl. Polym. Sci., 1986, 32, 4137–4143.
58 K. Matsui, Y. Kikuchi, T. Hiyama, E. Tobita, K. Kondo,
A. Akimoto, T. Seita and H. Watanabe, Fluorocarbon Anion
Exchangers, Toyo Soda Mfg. Co., Ltd., Sagami Chemical
Research Center, Japan, 1986.
59 A. Alaaeddine, B. Améduri, A. Martinent and P. Capron,
Matériau polymère fluorocarboné et procédé de synthèse,
French Patent FR 1000404 (assigned to CEA-CNRS), 2010.
60 W. Borman, Aromatic substituted poly(phenylene oxides),
US 3226361, General Electric Co, USA, 1965.
61 T. W. Xu and W. H. Yan, J. Membr. Sci., 2001, 190, 159–166.
62 X. C. Lin, L. Wu, Y. B. Liu, A. L. Ong, S. D. Poynton, J. R. Varcoe
and T. W. Xu, J. Power Sources, 2012, 217, 373–380.
63 Y. Xiong, J. Fang, Q. H. Zeng and Q. L. Liu, J. Membr. Sci.,
2008, 311, 319–325.
64 Q. G. Zhang, Q. L. Liu, A. M. Zhu, Y. Xiong and L. Ren,
J. Membr. Sci., 2009, 335, 68–75.
This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 02 May 2013. Downloaded by The University of British Columbia Library on 18/06/2013 02:14:18.
Review Article
65 D. E. Fenton, J. M. Parker and P. V. Wright, Polymer, 1973,
14, 589–1589.
66 M. Armand, J. Chabagno and M. Duclot, Polyethers as
solid electrolytes, in Fast Ion Transport in Solids: Electrodes
and Electrolytes, Proceedings of the International Conference on Fast Ion Transport in Solids, Electrodes, and
Electrolytes, 1979, pp. 131–136.
67 C.-C. Yang and S.-J. Lin, Mater. Lett., 2002, 57, 873–881.
68 J. Fu, J. L. Qiao, X. Z. Wang, J. X. Ma and T. Okadad, Synth.
Met., 2010, 160, 193–199.
69 N. Vassal, E. Salmon and J. F. Fauvarque, Electrochim. Acta,
2000, 45, 1527–1532.
70 C.-C. Yang, J. Power Sources, 2002, 109, 22–31.
71 G. M. Wu, S. J. Lin and C. C. Yang, J. Membr. Sci., 2006, 275,
127–133.
72 J. L. Qiao, J. Fu, L. Hong and J. X. Ma, Polymer, 2010, 51,
4850–4859.
73 Y. Wan, B. Peppley, K. A. M. Creber, V. T. Bui and
E. Halliop, J. Power Sources, 2006, 162, 105–113.
74 B. Xing and O. Savadogo, Electrochem. Commun., 2000, 2,
697–702.
75 Y. Wu, C. Wu, J. R. Varcoe, S. D. Poynton, T. Xu and Y. Fu,
J. Power Sources, 2010, 195, 3069–3076.
76 Y. H. Wu, C. M. Wu, T. W. Xu, X. C. Lin and Y. X. Fu,
J. Membr. Sci., 2009, 338, 51–60.
77 M. Kumar, S. Singh and V. K. Shahi, J. Phys. Chem. B, 2010,
114, 198–206.
78 J. L. Qiao, J. Fu, L. L. Liu, L. Xu, Y. Y. Liu and J. W. Sheng,
Int. J. Hydrogen Energy, 2012, 37, 4580–4589.
79 T. C. Zhou, J. Zhang, J. L. Qiao, L. L. Liu, G. P. Jiang,
J. Zhang and Y. Y. Liu, J. Power Sources, 2013, 227, 291–299.
80 J. L. Qiao, J. Zhang and J. J. Zhang, J. Power Sources, 2013,
237, 1–4.
81 L. Wu and T. Xu, J. Membr. Sci., 2008, 322, 286–292.
82 M. P. Tsyurupa and V. A. Davankov, React. Funct. Polym.,
2002, 52, 193–203.
83 Y. T. Luo, J. C. Guo, C. S. Wang and D. Chu, Electrochem.
Commun., 2012, 16, 65–68.
84 J. Wang, R. He and Q. Che, J. Colloid Interface Sci., 2011,
361, 219–225.
85 O. I. Deavin, S. Murphy, A. L. Ong, S. D. Poynton, R. Zeng,
H. Herman and J. R. Varcoe, Energy Environ. Sci., 2012, 5,
8584–8597.
86 X. yan, G. He, S. Gu, X. Wu, L. Du and Y. Wang, Int. J.
Hydrogen Energy, 2012, 37, 5216–5224.
87 J. Ran, L. Wu, J. R. Varcoe, A. L. Ong, S. D. Poynton and
T. Xu, J. Membr. Sci., 2012, 415–416, 242–249.
88 K. N. Grew and W. K. S. Chiu, J. Electrochem. Soc., 2010,
157, B327–B337.
89 A. M. Kiss, T. D. Myles, K. N. Grew, A. A. Peracchio,
G. J. Nelson and W. K. Chiu, ECS Trans., 2011, 1827–1835.
90 B. R. Einsla, S. Chempath, L. R. Pratt, J. M. Boncella, J. Rau,
C. Macomber and B. S. Pivovar, ECS Trans., 2007, 11,
1173–1180.
This journal is
View publication stats
c
The Royal Society of Chemistry 2013
Chem Soc Rev
91 S. Chempath, B. R. Einsla, J. M. Boncella, B. R. Pivovar and
L. R. Pratt, Stability of cationic headgroups in alkaline
anion-exchange membrane fuel cells, in 2007 AIChE
Annual Meeting, 2007.
92 N. Follain, S. Roualdes, S. Marais, J. Frugier, M. Reinholdt
and J. Durand, J. Phys. Chem. C, 2012, 116, 8510–8522.
93 Z. Siroma, S. Watanabe, K. Yasuda, K. Fukuta and
H. Yanagi, J. Electrochem. Soc., 2011, 158, B682–B689.
94 S. Litster and G. McLean, J. Power Sources, 2004, 130, 61–76.
95 P. Berg, A. Novruzi and K. Promislow, Chem. Eng. Sci., 2006,
61, 4316–4331.
96 M. Mamlouk, S. M. Senthil Kumara, P. Gouerecb and
K. Scott, J. Power Sources, 2011, 196, 7594–7600.
97 S. Lu, J. Pan, A. Huang, L. Zhuang and J. Lu, Proc. Natl.
Acad. Sci. U. S. A., 2008, 105, 20611–20614.
98 A. Husa, A. Higier and H. Liu, J. Power Sources, 2008, 183,
240–246.
99 Y. S. Kim, Resonance-Stabilized Anion Exchange Polymer
Electrolytes, presented at the 2010 DOE Hydrogen Program
and Vehicle Technologies Program Annual Merit Review
and Peer Evaluation Meeting, June 7–11, 2010.
100 M. Hibbs, M. Hickner, T. Alam, S. McIntyre, C. Fujimoto
and C. Cornelius, Chem. Mater., 2008, 20, 2566–2573.
101 B. Pivovar, 2011 Alkaline Membrane Fuel Cell Workshop
Final Report, National Renewable Energy Laboratory,
Proceedings from the Alkaline Membrane Fuel Cell
Workshop, Arlington, Virginia, May 8–9, 2011.
102 http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/
fuel_cells.pdf.
103 A. Herring and B. Pivovar, Anion Exchange Membranes for
Fuel Cells, presented at the 2011 AMFC Workshop, May 8,
2011.
104 K. Fukuta, Electrolyte Materials for AMFCs and AMFC Performance, presented at the 2011 AMFC Workshop, May 8, 2011.
105 H. Yanagi and K. Fukuta, ECS Trans., 2008, 16, 257–262.
106 N. J. Robertson, IV, H. A. Kostalik, T. J. Clark, P. F. Mutolo,
H. D. Abruña and G. W. Coates, J. Am. Chem. Soc., 2010,
132, 3400–3404.
107 G. Tillet, B. Ameduri and B. Boutevin, Prog. Polym. Sci.,
2011, 36, 191–217.
108 A. Collier, H. Wang, X. Yuan, J. Zhang and D. P. Wilkinson,
Int. J. Hydrogen Energy, 2006, 31, 1838–1854.
109 Y. P. Patil, W. L. Jarrett and K. A. Mauritz, J. Membr. Sci.,
2010, 356, 7–13.
110 A. Taguet, B. Ameduri and B. Boutevin, J. Polym. Sci., Part
A: Polym. Chem., 2006, 44, 1855–1868.
111 A. Taguet, B. Ameduri and B. Boutevin, Adv. Polym. Sci.,
2005, 184, 27–211.
112 M. Mamlouk, J. A. Horsfall, C. Williams and K. Scott, Int. J.
Hydrogen Energy, 2012, 37, 11912–11920.
113 K. Fukuta, Electrolyte Materials for AMFCs and AMFC Performance, presented at the 2011 AMFC Workshop, May 8, 2011.
114 S. Gottesfeld, Breaking the Fuel Cell Cost Barrier, presented
at the 2011 AMFC Workshop, May 8, 2011.
Chem. Soc. Rev., 2013, 42, 5768--5787
5787