Chapter 8
Materials for Proton
Exchange Membrane Fuel
Cells (PEMFCs)
1. Introduction
Proton exchange membrane (PEM) fuel cell
technology is a promising alternative for a secure
and clean energy source in portable, stationary,
and automotive applications.
However, it has to compete with cost, reliability,
and energy efficiency with established energy
source such as batteries and internal combustion
engines.
Many of the major challenges in PEM fuel cell
commercialization are closely related to three
critical materials considerations: cost, durability,
and performance.
The challenge is to find a combination of
materials that will give an acceptable result for
the three criteria combined.
State-of-the-art PEM fuel cells, using thinner membranes (< 40 μm)
and Pt/C electrodes (< 1 mg Pt/cm2) for cost reduction, are less
expensive (but still higher than DOE cost targets) but only have a
demonstrated lifetime of less than 15,000 h operating on reformate.
The idea of using an ion-conductive polymeric membrane as a gaselectron barrier in a fuel cell was first conceived by William T. Grubb, Jr.
(General Electric Company) in 1955.
In his classic patent, Grubb described the use of Amberplex C-1, a
cation exchange polymer membrane from Rohm and Haas, to build a
prototype H2-air PEM fuel cell (known in those days as a solid-polymer
electrolyte fuel cell).
Today, the most widely used membrane electrolyte is
DuPont’s Nafion due to its good chemical and
mechanical stability in the challenging PEM fuel cell
environment.
A perfluorinated polymer with pendant sulfonated side
chains, Nafion was initially developed in 1968 by Walther
G. Grot of DuPont for the chlor-alkali cell project of the
National Aeronautics and Space Administration (NASA)
space program.
Several manufacturers provide other perfluorinated
polymers, composite polymers, and hydrocarbon
polymers as membrane electrolytes.
A membrane electrode assembly (MEA) usually refers to a five-layer
structure that includes an anode gas diffusion layer (GDL), an anode
electrode layer, a membrane electrolyte, a cathode electrode layer,
and a cathode GDL.
Most recently, several MEA manufacturers started to include a set of
membrane subgaskets as a part of the MEA package.
This is often referred to as a seven-layer MEA. In addition to acting
as a gas and electron barrier, a membrane electrolyte transports
protons (H+) from the anode, where H2 is oxidized to produce H+
ions and electrons, to the cathode, where H+ ions and electrons
recombine with O2 to produce H2O.
For economic reasons, air is usually used as the cathode feed rather
than pure O2.
Electrons are carried from the anode to the cathode through the
external electric circuit.
The anode and cathode electrode layers are typically made of Pt or
Pt alloys dispersed on a carbon support for maximum catalyst
utilization.
Ionomers and polytetrafluoroethylene (PTFE) resins can be added
to the electrode layers. The former extends the proton transport path
beyond the electrode-membrane interfaces；latter facilitates liquid
water removal from the electrode layers. Both can also help bind
together various electrode components.
GDLs are made of porous media such as carbon paper or carbon
cloth to facilitate the transport of gaseous reactants to the electrode
layers, as well as the transport of electrons and water away from the
electrode layers.
An MEA is sandwiched between two bipolar plates to form a single
fuel cell. The word bipolar refers to a plate’s bipolar nature in a series
of single cells (known as a stack) in which a plate (or a set of half
plates) is anodic on one side and cathodic on the other side.
Bipolar (half) plates often have gas channels on the side facing an
MEA and channels for temperature control on the other side and,
together with the GDLs, they provide structural support for MEAs in
addition to serving as transport media for reactants/products,
electricity, and heat.
2. Electrode Materials
The hydrogen oxidation reaction (HOR) occurs at the
anode electrode of an H2/O2 PEM fuel cell (reaction 1):
H2  2 H+ + 2 e-
E0= 0 V
This is a thermodynamically reversible process that often
serves as a standard reference electrode, known as the
reversible hydrogen electrode (RHE), for all other
electrochemical process.
At the cathode electrode, the thermodynamically irreversible fourelectron oxygen reduction reaction (ORR) is the dominant
electrochemical process (reaction 2):
+
-
O2 + 4 H + 4 e  2 H 2 O
E0 = 1.229 V
When connect through an external circuit, the net result of these two
half-cell reactions is the production of H2O and electricity for H2 and
O2. Heat is also generated in the process.
In the absence of a proper catalyst, however, neither of these two
half reactions takes place at meaningful rates under PEM fuel cell
operating conditions (50 to 80 °C, 1 to 5 atm).
Despite decades of effort in search of cheaper alternatives, platinum
is still the catalyst of choice for both the HOR and ORR.
Anode Catalyst Materials
As mentioned above, an anode serves as
the HOR site in a H2/O2 fuel cell. As such, it
must fulfill the following basic functional
requirements:
1. transport H2 to the catalyst sites
2. catalyze the HOR process
3. carry proton away from the reaction sites to the
membrane electrolyte
4. remove electrons from the anode
5. transfer heat in or out of the reaction zone
The HOR process is believe to proceed through the following steps
(Reactions 4 to 6: M = metal catalyst):
H2 + 2 M → 2 MHads
Tafel reaction
(4)
or
+
H2+ H2O + M → MHads + H3O + e
and
+
-
MHads + H2O → H3O + e + M
- Heyrovsky reaction
Volmer reaction
(5)
(6)
The rate-determining step varies depending on the specific catalysts
and the reaction conditions.
For a PEM fuel cell with a Pt anode, the HOR process involves only
the Tafel and Volmer reactions, with the Tafel reaction being the
rate-determining step. The rate of the overall HOR process can be
express in the Bulter-Volmer form (equation 12.1):
j = j0 [ e (1-β1) F ηs /RT – e β1 F ηs /RT ]
(12.1)
The exchange currant density (j0) depends on the natural of the
catalyst morphology, the catalyst-electrolyte interface, the properties
of the reaction media (pH, electrolyte, temperature, concentration,
etc.), and the levels of contaminants such as CO, Cl-, and sulfur
species.
For the HOR, the measurement of j0 is further complicated by the
lower H2 gas diffusivity in a strong electrolyte solution known as the
“salting out” effect.
As a result, the reported value ranges from 10-5 to 10-2 A/cm2 for
different Pt electrodes in various acidic media. However, the ratelimiting process of a H2/O2 fuel cell is the ORR on the cathode
because j0 for the ORR is 10-6 ~10-11 A/cm2.
Anode materials research has been centered mostly on Pt-loading
reduction, CO-tolerant catalysts for DMFCs and systems operating
with CO-contaminated fuels (such as reformate), and low-cost Pt
alternatives.
Cathode Catalyst Materials
A cathode serves as the site for the ORR in a
H2/O2 fuel cell. It should fulfill the following
basic functional requirements:
1. transport O2 to the catalyst sites
2. catalyze the ORR process
3. carry proton from the membrane electrolyte to the
catalyst sites
4. remove electrons to the reaction sites
5. remove product water
6. transfer heat to or from the reaction zone
The exact ORR mechanism is still a topic of much debate.
Two representative mechanisms, namely, a dissociative
mechanism (reactions 11, 15 and 16) and an associative
mechanism (reactions 12 to 16), are illustrated here：
1
O2  M  M  Oads
2
(11)
or
O2  M  M  O2
(12)
M  O2  H   e   M  O2 H
(13)
M  O2 H  H   e   H 2O  M  Oads
(14)
The final two steps are the same for both mechanisms:
M  Oads  H   e   M  OH
(15)
M  OH  H   e   H 2O  M
(16)
Recent studies pointed to the formation of a peroxy
intermediate on the Pt surface, suggesting that the more
complex associative mechanism is at work on a Pt
electrode.
However, the DFT calculations by Nørskov at al.
suggested that the associative mechanism was only the
dominant pathway at ORR overpotentials greater than
0.8 V.
At realistic ORR overpotentials (< 0.8 V), the two
pathways run parallel to each other.
Regardless of the actual ORR mechanism, one thing is
clear: the ORR is the rate-limiting process in a H2/O2 fuel
cell.
The slow ORR kinetics is responsible for the steep
slope in the activation polarization region (Figure
12.2).
A small but noticeable H2 crossover current at the
open circuit (OC) is largely to blame for the opencircuit voltage (OCV) loss even for a state-of-the-art
membrane electrolyte.
Much of the cathode research has been directed at
finding a cathode catalyst to improve the slow ORR
kinetics and to find a cheap replacement for Pt.
Pt and Pt alloys are the most active catalysts for
the ORR.
Stability of Pt Cathode Catalysts
Cathode lifetime durability presents a special
challenge in PEM fuel cells.
The major problems associated with the catalysts are
Pt agglomeration, sintering, dissolution, and
redistribution.
The cathode environment is highly oxidative and
corrosive due to high voltage (e.g., 0.6 to 1.0 V), low
pH, elevated temperature, and the presence of water
and oxygen.
Although Pt has low solubility at normal cell operating
voltages, its solubility increases significantly with cell
voltage and reaches the highest dissolution rate at
round 1.1 V.
Furthermore, freshly formed PtOx and Pt(OH)x are less
stable in acidic media than material that have been aged.
This makes a PEM fuel cell very susceptible to Pt
dissolution when its voltage cycles are between 0.75 and
1.2 V.
Some dissolved Pt ions (e.g., Pt2+) will migrate into the
ionomers or membrane. Where they are reduce to Pt by
H2 that has diffused from the anode side.
These Pt particles in membrane/ionomers are unlikely to
participate in the ORR process because of the lack of
electrical continuity.
Some dissolved Pt can redeposit into other Pt particles,
which results in the growth of the Pt particles.
It is believed that the exact band location is determined
by the H2 crossover rate.
Electrode Support Materials
The primary functions of a good catalyst support are
to (1) maximize catalyst utilization, (2) transport
electrons, and (3) transfer heat.
Other desirable attributes include high chemical and
electrochemical stability, good mechanical integrity,
and, last but not least, low cost.
Carbon materials, such as Vulcan-X72 by Cabot
Corp., are widely used as PEM fuel cell catalyst
supports because of their high surface area, low cost,
excellent electric/thermal conductivity, and adequate
stability and mechanical properties.
The use of carbon-supported Pt in place of Pt black is
directly responsible for the reduction of Pt loading in
PEM fuel cells.
However, this also substantially increase involve gaseous reactants
(H2/O2), protons, and electrons, the active catalyst sites must have
access to these species at the same time.
Such reaction sites are often referred to as catalyst-electrolyte-reactant
triple-phase boundaries (TPBs).
In order to increase catalyst utilization and reduce the activation
overpotential loss, it is critical to maximize the TPB regions within an
electrode.
Proton-conducting materials (such as Nafion ionomers) are usually
added to the electrode layer to improve the proton transport beyond
the electrode-membrane interface, thus increasing the Pt utilization.
The ionomers and PTFE resin also serves as a binder to keep the Pt/C
particles together and to create hydrophobic/hydrophilic domains for
facile gas and water transport.
Others have attempted to use more stable carbon materials such as
graphitized carbon and carbon nanotubes (CNTs).
3. Membrane Electrolyte Materials
Membranes are a critical and challenging component in PEM fuel
cell applications.
In order to meet the cost and durability target for residential and
automotive applications, a membrane electrolyte must meet several
functional requirements:
1. high proton conductivity over a wide RH range
2. low electrical conductivity
3. low gas permeability, particularly for H2 and O2 (to minimize H2/O2
crossover)
4. good mechanical properties under cycles of humidity and temperature
5. stable chemical properties
6. quick start-up capability even at subzero temperatures
7. low cost
The life time of a membrane is related to its original
thickness, its mechanical integrity, and the chemical
stability of the constituent ionomers.
There are many factors that can affect the membrane life
time. These include not only the membrane properties
(such as its chemical structure, composition, morphology,
configuration, and fabrication process), but also several
external factors (such as contamination of membrane,
materials compatibility, and the operating conditions of
the fuel cell).
Perfluorosulfonic Acid Membrane Materials
Perfluorinated membranes are still regarded
as the best in the class for PEM fuel cell
applications.
These materials are commercially available
in various forms from companies such as
DuPont, Asahi Glass, Asahi chemical, 3M,
Gore, and Solvay.
Perfluorosulfonic acid (PFSA) polymers all
consist of a perfluorocarbon backbone that
has side chains terminated with sulfonated
groups.
Polybenzimidazole (PBI) Membrane Materials
Plug Power, working with its European partners PEMEAS (now
BASF Fuel Cell GmbH) and Vaillant, is actively pursuing a PBIbased HT PEM fuel cell system as a CHP system with high
system efficiency and great CO tolerance.
PBI (see chemical structure above) is a hydrocarbon membrane
that has been commercially available for decades. Free PBI has
a very low proton conductivity (~10-7 S/cm) and is not suitable for
PEM fuel cell applications. However, the proton conductivity can
be greatly improved by doping PBI with acids such as
phosphoric, sulfuric, nitric, hydrochloric, and perchloric acids.
4. Gas Diffusion Layer Materials
The GDL functional requirements can be summarized as follows:
(1) allow uniform transport of reactant gases to the electrode；(2)
conduct electrons；(3) remove product water from the electrode；
(4) transfer heat to maintain the cell temperature；and (5)
provide mechanical support for the MEA.
To fulfill these functions, an ideal GDL material should have small
gas transfer resistance, good electron conductivity, and good
thermal conductivity.
The porosity of a GDL structure is the most important parameter
for reactant transfer. Water management is the most challenging
problem in GDL and fuel cell design. The ability to remove water
is one of the key properties in evacuating GDL performance.
The cell performance can be improved by placing a fine layer (such
as a microporous layer (MPL)) between the GDL and the catalyst
layer because it creates a saturation jump across the interface.
Commonly used GDL materials are made of porous carbon fibers,
including carbon cloth and carbon paper.
Carbon cloth is more porous and less tortuous than carbon paper
and has a rougher surface.
Experimental results showed that carbon cloth GDL has better
performance under high-humidity conditions because its low
tortuosity (of the pore structure) and rough textural surface
facilitate droplet detachment.
However, under dry conditions, carbon paper GDL has shown
better performance than carbon cloth GDL because it is capable of
retaining the membrane hydration level with reduce ohmic loss.
5. Bipolar Plate Materials
Bipolar plates play an important role in fuel cell operation.
Generally, the functions of bipolar plates can be summarized
as (1) supply and separate reactant gas without introducing
impurities； (2) conduct electrons； (3) remove the heat and
control the fuel cell operating temperatures； and (4) remove
the product water from the system.
To fulfill these functions, an idea bipolar plate material should
have minimal gas permeability, high electrical conductivity,
high thermal conductivity, and good chemical and
electrochemical stability in the fuel cell environment.
From a manufacturing perspective, the bipolar plate material
should have low density, good mechanical strength, easy
processability, and low cost.
Nowadays, materials commonly used for bipolar plates are
either graphite based or metallic.
Up until the early 1990s, pure graphite was used as the
primary bipolar plate material for its good electron
conductivity, low density, and high corrosion resistance
(e.g.,680 S/cm and 1.78 g/cm3 for POCO AXF-5Q).
However due to its poor strength and ductility, graphite
plates cannot be easily fabricated, and the minimum
thickness is limited to about 5 mm.
This result in a large stack volume and low efficiency in
heat transfer.
The brittleness of the graphite limits the methods of
fabrication, which lead to a high manufacturing cost.
6. Materials Compatibility and
Manufacturing Variables
References
Materials for the Hydrogen Economy,
Jones, R. H. and Thomas, G. J., ed., CRC
Press, Boca Raton, 2008.