Chapter 8Chapter 8

Materials for Proton Materials for Proton Exchange Membrane Fuel Exchange Membrane Fuel

Cells (PEMFCs)Cells (PEMFCs)

1. IntroductionProton 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 gas-

electron 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 four-

electron oxygen reduction reaction (ORR) is the dominant

electrochemical process (reaction 2):

O2 + 4 H+ + 4 e- 2 H2O 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 MaterialsAs 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- Heyrovsky reaction (5)

and

MHads + H2O → H3O+ + e- + M Volmer reaction (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 rate-

limiting 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 MaterialsA 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 ：

The final two steps are the same for both mechanisms:

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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 open-circuit

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 CatalystsCathode 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 MaterialsThe 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 MaterialsMembranes 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 PBI-

based 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 MaterialsThe 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 MaterialsBipolar 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, Materials for the Hydrogen Economy, Jones, R. H. and Thomas, G. J., ed., CRC Jones, R. H. and Thomas, G. J., ed., CRC Press, Boca Raton, 2008.Press, Boca Raton, 2008.