FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Membranes: Design and Characterization

Membranes: Design and Characterization SM MacKinnon, TJ Fuller, FD Coms, MR Schoeneweiss, CS Gittleman, Y.–H Lai, R Jiang, and AM Brenner, General Motors R&D Fuel Cell Laboratory, Honeoye Falls, NY, USA & 2009 Elsevier B.V. All rights reserved.

Introduction Background Advanced perfluorosulfonic acid (PFSA)-based protonexchange membranes (PEMs) are the industry standard for automotive fuel cell vehicles. The currently available PFSA membranes provide nearly all the functional requirements necessary for commercialization, including (1) high performance to meet power density requirements; (2) mechanical durability to stand up to the rigors of relative humidity (RH) cycling; and (3) chemical stability to maintain both power and function. The complexity of membrane optimization leads to inherent trade-offs in selecting a membrane because no single existing technology provides best-in-class technology for all requirements. The present state of commercial PFSA membranes includes at least 10 industrial fluoropolymer suppliers which should provide a potentially competitive market, yet PEM costs are considerably greater than the high-volume targets of US$10–20 m2 published by the Department of Energy. The motivations to develop alternative membrane technologies include (1) achieving high-volume costs that are oUS$10 m2 at 15 million m2 per year, suitable for approximately 1 million vehicles per year; (2) the elimination of passivation coatings to inhibit metal plate corrosion in the presence of fluoride released upon operational degradation of PFSA membranes; and (3) the need for materials with reliable fuel cell performance and durability at low RH (e.g., 25–50%) between 60 and 80 1C, with intermittent excursions up to 95 1C. Furthermore, PEMs that perform at low RH would reduce the parasitic power loads and capital costs of humidifiers and compressors, enabling simplified system designs. Alternative PEM research and development continues to advance in academic, industrial, and government laboratories with an ever-increasing number of contributions to the field. Presently, research has appeared to be largely focused on developing cost-effective membranes with competitive performance compared to commercial PFSA membranes. While this emphasis on cost and performance is appropriate, more detailed characterizations of both mechanical and chemical durability are required to identify competitive PEM alternatives that meet all performance, durability, and cost targets for automotive applications. During automotive fuel cell operation, a PEM must provide a number of functions over a broad range of RH and temperature. These functions include (1) a low-

resistance Hþ transport pathway; (2) electrical isolation of anode from cathode; and (3) fuel and oxidant gas separation. Proton-exchange membranes must be able to provide all three functions concurrently over the entire operating range of a fuel cell system and the life of a fuel cell vehicle. Our current understanding of fuel cell membrane failure mechanisms centers on the observation that all membranes eventually fail mechanically, resulting in pinhole formation and the inability to function as a fuel and oxidant separator. During fuel cell operation, a membrane may simultaneously experience physical deterioration as a result of hydration–dehydration cycles (i.e., swelling and shrinking), as well as chemical degradation, hydrolysis, or other membrane-specific chemical reactions. The accumulation of both physical and chemical degradation will hasten the time to pinhole formation and a mechanical breach. Automotive Proton-Exchange Membrane Targets The effort to attain durable membranes for automotive applications has resulted in a series of PEM targets that are provided to interested research groups. Table 1 outlines the PEM automotive targets as determined based on a series of in situ and ex situ characterization tools that are utilized to evaluate membrane technologies. The tools were originally developed to correlate PFSA membrane technology to stack life and performance, yet also provide a method to develop design guidelines for alternative membranes. Through a methodical approach, an understanding develops of the relationship between membrane morphology and chemistry to that between in situ performance and automotive accelerated durability measures. Herein, the objective is to (1) describe both ex situ and in situ characterization tools used to evaluate PEMs, and (2) provide a guideline for the development of alternative PEMs to overcome current performance and durability limitations.

Proton-Exchange Membrane Development Screening Experiments Ex Situ Membrane Physical Properties Hydrophilic volume ion-exchange capacity

The measure of ion-exchange capacity (IEC) allows for the determination of the concentration of available

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742 Table 1

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

Automotive PEM targets

Membrane performance

Conditions

Target

Hþ resistance Fuel cell voltage at 1.0 A cm2

25–100% RHin 50–100% RHout

25 mO cm2 40.65 V

Membrane in situ durability

Conditions

Target

Accelerated mechanical durability Accelerated chemical durability Accelerated chemical–mechanical durability

0–150% RH cycle, 80 1C OCV at 95 1C 0–150% RH cycle, 80 1C, 0.1 A cm2

420 000 cyclesa o–100 mV h1 420 000 cyclesa

Leak rate limit of 10 mL min1. PEM, proton-exchange membrane; RH, relative humidity.

a

conductive sites in a membrane on a weight basis and may be readily tailored. Generally, increases in membrane IECs lead to better-performing membranes and also to greater membrane swelling in water. The accepted method to measure the available IEC involves a titration to neutral pH 7, using a standardized sodium hydroxide (NaOH) solution (B0.010 0 mol L1), of a solution containing an ion-exchanged membrane immersed in 2 mol L1 sodium chloride (NaCl). The standard IEC is calculated by eqn [1]: IEC meq Hþ g1   l NaOH  0:0100 M NaOH  1000 ¼ g ionomer

½1

An approach to correct the gravimetric IEC to a global volumetric IEC is done by accounting for the density of the dry membrane. The resulting IEC expressed in meq cm3 captures the molar concentration of protons in the volume of the membrane. The global volumetric IEC may be further corrected to reflect an approximation of the volume of water in the membrane from the boiling water volume swell data expressed in cm3 H2 O , which we refer to as the hydrophilic volume IEC. Therefore, by expressing the IEC in units of meq Hþcm3 H2 O allows for the best approximation of the actual molar concentration of Hþ in the membrane under fully humidified conditions. The hydrophilic volume IEC is calculated by eqn [2]:

standard die of known length and width is used to die cut samples and to ensure a consistent membrane shape to the measurement. Each sample is weighed at room temperature and the thickness is measured and recorded prior to the die-cut pieces of membrane being immersed in liquid water at both 25 and 100 1C for 1 h, respectively. Post-swelling measurements are taken by measuring the length, width, and thickness changes, followed by recording the wet weight after wiping any excess water from the surface of the membrane samples. The percent water uptake is measured using eqn [3]: % water uptake ¼

  Wwet  Wdry  100% Wdry

½3

where Wwet and Wdry are the wet and dry weights, respectively. The percent water uptakes can also be normalized for absolutely dry membranes by substituting Wdry for the vacuum-dried weight of the membranes. The volume swell is measured using eqn [4]: %volume swell ¼

  Volwet  Voldry  100% Voldry

½4

The wet and dry volumes, Volwet and Voldry, are also reported as a swelling factor according to eqn [5]: Swelling factor ¼

  Volwet Voldry

½5

IEC meq Hþ cm3 H2 O ¼

ðIEC meq Hþ g1 Þ  ðg ionomerÞ ðVolwet  Voldry Þ cm3 H2 O

½2

where Volwet and Voldry are the wet and dry volumes, respectively. Water uptake and volume swell

Both gravimetric and volumetric water uptake are critical parameters impacting PEM performance and durability. The dimensional change in each direction upon swelling are critical parameters impacting the cumulative membrane stress experienced during RH fluctuations. A

Direct current proton conductivity

Historically, there is excellent agreement between membrane conductivity as measured by alternating current (AC) impedance and by direct current (DC) methods. Membrane DC conductivity is determined using a fourpoint probe conductivity cell from BekkTechTM where a membrane sample (with dimensions of 4.5 cm  0.9 cm and measured thickness) is placed onto a Teflon fixture in contact with four platinum electrodes. The fixture sits within two end plates of 50 cm2 active area fuel cell hardware to facilitate the use of humidified nitrogen and hydrogen at a standard flow rate of 2 L min1 from a fuel

RHinlet (%) and temperature (°C)

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization 110 100 90 80 70 60 50 40 30 20 10 0

(a)

membranes; i.e., the conductivity of the SPTES-50 surpasses Nafion 112 at high RH (480%) but drops off more sharply when the RH is decreased. Membrane humidity stability factor, Fx

Temperature Relative humidity 60

0

120

180

Specific H+ conductivity (S cm−1)

Time (min) 1

0.1

0.01 20.00 (b)

743

SPTES-50, 1.85 meq g−1 N112, 0.9 meq g−1 PFSA, 1.25 meq g−1 SParmax, 1.81 meq g−1 Target >0.1 S cm−1

The mechanical durability of non-PFSA-based PEMs is directly linked to the relationship of the measured percent elongation at break in the x- or y-dimension with the percent dimensional change in the same direction after exposure to liquid water. Membranes that undergo greater dimensional change after exposure to boiling water as compared to the dry elongation at break in the same dimension may fail mechanically if membrane stresses experienced upon humidification and drying exceed the tensile strength of the membrane. Thus, the humidity stability factor, Fx, was developed as a metric of the likelihood of a membrane to withstand repeated humidity cycling. Fx can be calculated according to eq [6]: Fx ¼

40.00

60.00

80.00

100.00

RHinlet (%)

Figure 1 (a) Protocol for the evaluation of ex situ specific Hþ conductivity. (b) specific Hþ conductivity for N112, a high-IEC PFSA and sulfonated polyarylene-thioethersulfone (SPTES-50, 1.8 meq g1). IEC, ion-exchange capacity; PFSA, perfluorosulfonic acid; SPTES, sulfonated polythioethersulfone.

cell test stand. Current is measured from applied voltages supplied by a personal computer. Figure 1(a) outlines the testing protocol whereby candidate membranes are equilibrated with an initial soak at 80 1C and 100% RH for 1 h, followed by drying to 20% RH at 80 1C. The RH is increased stepwise in 10% increments and maintained for 30 min, followed by five consecutive measurements from 0 to 0.6 V at a sweep rate of 10 mV s1 prior to the next increase in RH. There is a strong relationship between proton conductivity and membrane IEC for PFSA membranes as higher IECs display improved conductivity at low RH. For example, Figure 1(b) illustrates that a low equivalent-weight (o800) PFSA (IECB1.25 meq g1) has a specific proton conductivity of 0.06–0.07 S cm1 at 50% RH and 80 1C, compared to 0.04–0.05 S cm1 for Nafions 112. Also shown in Figure 1(b) are data for a sulfonated poly-thioethersulfone (SPTES-50, 1.85 meq g1), as described by Dang and coworkers, and sulfonated polyphenylene (SParmax, IEC 1.81 meq g1), both of which were synthesized in our lab. The profile of conductivity as a function of RH is characteristic of most hydrocarbon

% Elongation at break ð25 1C; 50% RHÞ % Length change ð100 1CH2 OÞ

½6

The percent elongation at break was determined under environmental conditions of 25 1C and 50% RH according to ASTM standard D882-02 ‘Tensile Properties of Thin Plastic Sheeting’. The percent change in length is determined from the characterization of volume swell after immersion in boiling water for 1 h. Because of the directional dependence of the humidity stability factor, both the x- and the y-planar dimensions should be considered as the nature of membrane anisotropy may contribute to varying values of percent elongation at break and percent length change. Fuel Cell Performance Evaluation In situ fuel cell membrane performance is evaluated relative to benchmarks in the same unit cell configuration using catalyst-coated diffusion media (CCDMs). For preliminary screening evaluations, a standard membrane–electrode assembly (MEA) is constructed and evaluated in identical unit cell hardware. Membrane– electrode assembly are prepared using PEMs with a thickness of 15–50 mm and are then evaluated in fuel cells with 50 cm2 active catalyst area of 0.4 mg Pt cm2 coated on diffusion media with a sintered microporous layer on both the anode and the cathode. By using a standardized automated protocol, a break-in period is followed by a series of polarization curves and measurements of cell voltage versus RH experiments. Further MEA optimizations (i.e., processing parameters, gas diffusion layer (GDL), and electrode materials) are investigated once a potential membrane candidate has been identified relative to incumbent PFSA membranes and technology targets.

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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

Table 2

Fuel cell conditions for polarization curves

RHout (%)

Stoich (A/C)

Anode RHin (%)

Cathode RHin (%)

Inlet pressure (kPa gauge) Anode

Cathode

150 110 85 80 63

2/2 2/2 3/3 2/2 3/3

100 100 50 35 32

50 50 50 35 32

170 50 75 50 50

170 50 75 50 55

Relative humidity sweep sensitivity

The sensitivity of new PEMs to operation at low RH is evaluated through the analysis of RH sweep curves generated using an automated protocol. The protocol is graphically shown in Figure 3(a), and involves a stepwise 10% decrease in inlet RH from 100% to 10% RH in 15min intervals. A 2  2 matrix of current density and temperature is used to evaluate the performance dependence on RH. Specifically, humidity sweeps at 0.4 and 1.2 A cm2 are performed at both 80 and 95 1C. The average cell potential over the last 5 min of each interval

1.6

160 A 140 120

1.4 1.2

RHout (%)

B 100

1 C D E

80 60

0.8 0.6

40

0.4

20

0.2

0

0 50

0

100

150

Time (min)

(a) 1.2

110% RHout NRE 211, 0.9 110% RHout SParmax, 0.9 63% RHout NRE 211, 0.9 110% RHout SPTES 1.85 110% RHout SPamax 1.81

1.0 Cell voltage (V)

A series of standard polarization curves have been developed to help evaluate fuel cell performance relative to conditions that would be experienced during stack operation. An understanding of the dynamics of 50-cm2 active area testing platforms helps to identify potentially useful membranes, especially when the relative performance is compared to a baseline. The series of cathode outlet RHs (RHout) has been chosen to evaluate candidate membranes over a wide window of operating conditions. Table 2 outlines the fuel/oxidant stoichiometry, inlet pressures, and inlet RH set points in polarization curves that result in the desired outlet RH. The operating protocol (without break-in) for the experiment is outlined in Figure 2(a). The current density is increased in a stepwise manner to 0.02, 0.04, 0.08, 0.10, 0.20, 0.40, 0.60, 0.80, 1.0, 1.2, and 1.5 A cm2 at 15-min intervals and the average cell performance over the last 5 min of each soak is plotted versus current density. Figure 2(b) outlines the polarization curves for NRE-211, sulfonated polyphenylene (SParmax, 1.81 meq g1), and sulfonated polyarylene-thioethersulfone (SPTES-50, 1.85 meq g1) at both 110% RHout and 63% RHout. The sulfonated hydrocarbon-based membranes of polyphenylene, such as SParmax (1.81 meq g1), typically have performances that are 50 mV lower at 1.0 A cm2 under wet conditions (110% RHout) and lack the ability to operate at low RH (63% RHout) at 1.0 A cm2. NRE-211 membranes have acceptable performance under wet conditions but fall short by 100 mV under 63% RHout conditions. Our goal is to maintain the performance achieved under wet conditions over all operating conditions listed in Table 2.

Current density (A cm−2)

Fuel cell polarization curves

0.8

Target 0.65 at 1.0 cm

−2

0.6 0.4 0.2 0.0 0

(b)

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Current density (A cm−2)

Figure 2 (a) Protocol for the evaluation of in situ fuel cell performance: (A) 150%, (B) 110%, (C) 85%, (D) 80%, and (E) 63%. (b) Polarization curves for DuPontTM Nafions NRE-211, 25-mm sulfonated polyphenylene (SParmax, 1.81 meq g1), and 25-mm sulfonated polythioethersulfone (SPTES-50, 1.85 meq g1).

is plotted as a function of inlet RH for each condition. Figure 3(b) illustrates the cell voltage versus RH for cast DuPontTM Nafion 1000 and sulfonated polyphenylene (SParmax, 1.81 meq g1) at a constant 1.2 A cm2 for both 80 and 95 1C cell temperature. Typical PFSA membranes display a 30-mV loss under extremely dry conditions of 20% inlet RH at 1.2 A cm2 and 95 1C. Comparatively, the SParmax (1.81 meq g1) performance is more than 200 mV lower over the same RH sweep at 80 1C, 1.2 A cm2 and precipitously drops when below 80% RH at 95 1C.

745

1.4

100

1.2 1

80

0.8 60 0.6 40

0.4

20

0.2 50

0 (a)

100

160 140

Cycle

120 100 80 60 40 20 0

0 150

0

RHinlet (%) and temperature (°C)

120

Current density (A cm−2)

RHinlet (%)

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

(a)

0

2

4

8

10

40

0.7

HC 1

0.5 0.4 0.3

80 C Nafion 1000 80 C SParmax, 1.81 meq g−1 95 C Nafion 1000 95 C SParmax, 1.81 meq g−1

0.2 0.1

Leak rate (mL min−1)

35 0.6 Cell voltage (V)

6

Time (min)

Time (min)

HC 2

30

NRE-211

25

Nafion N111 IP (extruded)

20

Target of 20 000 cycles

15 Failure measured at 10 mL min−1 leak rate

10 5 0

0

0 0

(b)

20

40

60

80

100

120

RHinlet (%)

Figure 3 (a) Protocol for the evaluation of in situ relative humidity (RH) sensitivity at 1.2 A cm2. (b) RH sensitivity curves for cast 25 mm DuPont DE 2020 Nafion 1000 and 25-mm sulfonated polyphenylene (SParmax, 1.81 meq g1) at 1.2 A cm2, 80 and 95 1C.

Durability Accelerated mechanical durability

Earlier, we stressed the importance of mechanical durability in the development of non-PFSA PEMs. Accelerated mechanical durability is an in situ experiment in which a standard MEA with anode and cathode platinum loadings of 0.4 mg cm2 is built into a 50 cm2 active area with 2 mm-wide straight-channel flow fields separated by 2 mm lands. Membrane–electrode assembly are compressed between two pieces of commercially available carbon-fiber gas diffusion media and a 50 cm2 unit cell is constructed. Figure 4(a) illustrates the accelerated protocol where deep hydration–dehydration cycles are induced by toggling between 2.0 SLPM (standard litres per minute) flow of dry air for 2 min and 2.0 SLPM flow of 150% RH air for 2 min, while holding the cell temperature at 80 1C and ambient pressure. The time to failure is defined as the number of cycles until a 10 mL min1 crossover leak is measured under a 3 psi pressure differential from one side of the membrane. Figure 4(b) shows the very poor accelerated mechanical durability for typical sulfonated aromatic hydrocarbon membranes (HC 1 and

(b)

5000

10 000 15 000 # of cycles

20 000

25 000

Figure 4 (a) Accelerated mechanical durability protocol. (b) Accelerated mechanical durability for hydrocarbon membranes (HC 1 and HC 2), cast DuPontTM Nafions (NRE211) and an extruded version of Nafions 111 (Ion Power).

HC 2), which rarely last more than 400 cycles. This test is particularly tough on sulfonated high-temperature thermoplastic polymers due to their high modulus and the lack of membrane elasticity. Even many sulfonated elastomers have difficulty exceeding several thousand RH cycles in this test. Comparatively, DuPont-recast 25 mm Nafion NRE-211 approaches 5000 cycles and yet falls dramatically short of the corresponding extruded version of 25 mm Nafion 111, available through Ion PowerTM, which maintains mechanical integrity up to and beyond 20 000 RH cycles. The results suggest that extruded membranes of novel membrane technologies may be more mechanically robust and able to meet the target of 20 000 RH cycles. However, film casting in the protonic form is the preferred manufacturing process for PFSA membranes compared to the added cost of extrusion and subsequent hydrolysis. Accelerated chemical durability

Accelerated chemical durability tests have been developed with the understanding that hydrogen peroxide along with hydroxyl and hydroperoxyl radicals are present in operating fuel cells and participate in the degradation of PFSA membranes. The widely accepted

746

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization 1.0E-02

1.1 Robust PFSA OCV decay: 50 µVh−1

1.0E-03 Nafion® 112 OCV Decay: 1000 µVh−1

0.9

1.0E-04 1.0E-05

0.8 Nafion® 112: 42% fluoride loss after 200 h 0.7

1.0E-06

0.6

1.0E-07

FRR (gFh−1cm−2)

Open-circuit voltage (V)

1

1.0E-08

0.5 Robust PFSA: < 1% fluoride loss after 200 h

1.0E-09 200

0.4 0

50

100

150

Time (h)

Figure 5 Accelerated chemical durability protocol. Open-circuit voltage (OCV), 95 1C, 50% RH, 50 kPag, 5/5 stoich at 0.2 A cm2 equivalent flow. FRR, fluoride release rate; PFSA, perfluoro sulfonic acid.

Accelerated chemical and mechanical durability

Through the introduction of a chemical stressor into the accelerated mechanical degradation test, we have developed a demanding accelerated test to evaluate membranes that includes both principal stressors that a membrane experiences during fuel cell operation. Membranes are evaluated in this test using 50 cm2-active area-MEAs prepared with CCDMs containing 0.4 mg Pt cm2 on both the anode and the cathode. Membranes are subjected to RH cycling with reactive gases (hydrogen and air) at a constant current density of 0.1 A cm2, as

140

Cycle

0.4

120 100

0.3

80 0.2

60 40

0.1

Current density (A cm−2)

0.5

160 RHinlet (%) and temperature (°C)

method of accelerating chemical degradation in situ involves operating MEAs in fuel cells at open-circuit voltage (OCV), at high temperature and fixed RH to ensure that the membrane does not experience additional mechanical stresses. Similar to the performance evaluations, 50 cm2-active-area MEAs are prepared with CCDMs with 0.4 mg Pt cm2 loadings on both the anode and the cathode. The cell, with a serpentine flow field, is operated at 95 1C and 50% inlet RH and a 5/5 stoichiometric equivalent flow at 0.2 A cm2 and 50 kPa gauge pressure. The OCV is monitored for 200–800 h and the fluoride release rate (FRR) is measured from the analysis of outlet water to determine the percent loss of fluorine versus the total fluorine inventory of the membrane. Figure 5 shows the standard degradation rate of –1000 mV h1 for Nafion 112 resulting in a 42% loss of total fluoride inventory within 200 h. A more chemically robust PFSA membrane exhibits a very low degradation rate of –50 mV h1, resulting in FRR below 107 gF h1 cm2 and a corresponding loss of less than 1% of the total fluoride inventory in the membrane.

20 0 0

2

4 6 Time (min)

8

0 10

Figure 6 Accelerated chemical and mechanical durability protocol.

shown in Figure 6. In these tests, hydrogen air stoichiometry is 20 on both the anode and the cathode to enable nearly uniform RH throughout the cell. The RH cycle is the same as the accelerated mechanical durability test with a 2 min, 0%-RH hydrogen and air flow followed by a 2 min, 150%-RH hydrogen and air flow, at 80 1C and 0 kPag back pressure. The added chemical degradation significantly accelerates the time to 10 mL min1 crossover leak for both the homogeneous 25-mm Nafion membranes and the reinforced GoreTM Primeas MEAs, as shown in Table 3. The observed number of RH cycles to determine failure is reduced by at least a factor of 5 relative to the inert humidity cycling tests without an electrical load. Furthermore, the extruded N111-IP membrane from Ion Power, which passed the accelerated mechanical durability test after 20 000 cycles, develops a crossover leak after only 1800 humidity cycles at

Table 3 Comparison of relative humidity (RH) cycling with inert gases and at 0.1 A cm2

DuPontTM Nafions (NRE-211) Ion PowerTM Nafions (N111-IP) GoreTM Primeas

Accelerated mechanical durability (# of cycles)

Accelerated chemical– mechanical durability (# of cycles)

4500 20 000

800 1800

6000–7000

1300

1

0.8 RH sweep

0.75 Cell voltage (V)

Membrane

747

+

Ex situ specific H conductivity

0.7 0.65 0.1

0.6 0.55 0.5 0.45 0.4 0

20

40

60

80

100

Ex situ specific H+ conductivity (S cm−1)

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

0.01 120

RHinlet (%) 2

0.1 A cm . Clearly, chemical degradation causes mechanical weakening of these PFSA membranes.

Figure 7 Comparison of in situ and ex situ membrane performance as a function of % relative humidity (RH), at 80 1C.

Proton-Exchange Membrane Design Methodology

fuel cell as a result of the electrochemical reduction of oxygen and in-cell water transport. Fuel cell models show that the average membrane RH is greater than either the gas inlet or outlet RHs in cells when the anode and cathode gas streams flow in a counterflow configuration. While a specific proton conductivity of 0.1 S cm1 (0.02 O cm2) at 50% RH remains a valid target for novel PEM development, it should not be considered as the sole measure of membrane performance when screening novel membrane technologies. For example, reduced H2/O2 permeability of non-PFSA membranes may allow a reduction in membrane thickness, thus decreasing the membrane through plane resistance and allowing for lower proton-conducting membranes to perform acceptably in fuel cells Figure 8.

Fuel Cell Performance Most researchers developing PEMs for fuel cell applications measure ex situ proton conductivity and compare it to that of a Nafion 1100 EW baseline membrane to predict PEM performance. There exists a large variance in how the conductivity experiments are executed, leading to limited correlations between the reported values of new membranes. One common method measures proton conductivity as a function of temperature in liquid water. A more appropriate test protocol to more closely mimic automotive-based targets involves determining the dependence of proton conductivity as a function of RH isothermally, at a temperature typical of fuel cell operation, such as 80 1C. Because subtle differences in RH have dramatic effects on the accuracy of the conductivity measurements, tight experimental control is crucial. The importance of this measurement is not solely the absolute value of conductivity, but rather the dependence of conductivity on RH. There generally exists a threshold RH below which proton conductivity drops precipitously. Therefore, measuring the proton conductivity using both increasing and decreasing RH steps is important when attempting to make a correlation of ex situ proton conductivity to potential in situ fuel cell performance. There are a number of factors that differentiate fuel cell operation at low RH as compared to low RH ex situ proton conductivity. As shown in Figure 7, the decreasing trend of ex situ proton conductivity as a function of % RH at 80 1C does not directly correlate with the fuel cell RH sweep performance results. This discrepancy may be due to the difference between the inlet and the outlet RH in an operating fuel cell. For example, an inlet RH of 35% can result in a cathode outlet RH of 85% due to the humidification by water formed in the

Design methodology for fuel cell performance

Perfluorosulfonic acid membranes exhibit inherent phase separation as a result of the pendant, hydrophilic PFSA groups attached to a hydrophobic, perfluorovinyl polymer backbone. Recent approaches for improving PFSA membrane performance have been limited to the ability to include greater amounts of sulfonic acid functionality (lower EW) and to reduce the membrane thickness. This strategy is evident by the recent availability of higherIEC PFSA membranes with thicknesses in the range of 17–25 mm, compared to the early days of 50 to 200 mmthick membranes. The disparate hydrophilic–hydrophobic domains are presumably maintained in PFSA membranes with increased acid functionality, because the location of the sulfonic acid moieties remains in the periphery of the pendent chains while the spatial separation from the hydrophobic backbone is maintained. The research and development of aromatic hydrocarbon polymer electrolytes has evolved in order to mimic the attributes of PFSA membrane morphology, by focusing on improving fuel cell performance through an increase in the local concentration of acid groups

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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

Specific H+ conductivity (S cm−1)

1 SPTES-50, 1.85 meq g−1 N112, 0.9 meq g−1 PFSA, 1.25 meq g−1 SParmax, 1.81 meq g−1 GM 1, 2.0 meq g−1 Target > 0.1 S cm−1

0.1

0.01 20

40

80

60

100

RHinlet (%)

Figure 8 Comparison of ex situ specific proton conductivity of an example of the GM hydrocarbon membrane. PFSA, perfluorosulfonic acid; SPTES, sulfonated polythioethersulfone.

Table 4

Hydrophilic volume IEC compared to standard and volumetric IEC

PEM

Wt IEC (meq Hþ g1)

Density (g cm3)

Vol. IEC (meq Hþ cm3)

100 1C Vol. swell (%)

VolH2 O IEC (meq Hþ cm3 H2 O)

Nafions NRE-211a Cast Nafions 1000 b SPEEKb Sulfonated polyphenyleneb (SParmax) GM experimental membranesb

0.92 1.0 1.46 1.6–1.7

1.97 1.97 1.3 1.5

1.81 1.97 1.90 2.4–2.5

50 102 1000 120–300

1.83 1.96 0.95 0.8–2.0

1.0–2.0

1.7

1.7–3.4

60–200

2.0–3.4

a

Values taken from NRE-211 product data sheet published on DuPont’s website. Values acquired from membranes cast from commercial materials or prepared in our lab. IEC, ion-exchange capacity; SPEEK, sulfonated polyetheretherketone; PEM, proton-exchange membrane. b

through the separation of hydrophilic and hydrophobic domains. The aggregation of acid groups can be facilitated by grafting pendent acid functionality and the synthesis of block polymers to promote beneficial phase separations. The mere separation of hydrophobic and hydrophilic domains does not result in optimum performance because a preferred, continuous disordered phase separation has been reported to be superior in performance compared to that of well-defined, lamellar morphology. The concentration and proximity of sulfonic acid groups has a profound effect on the performance of PEMs. Previously, we described the measure of global IEC, corrected by considering the density of a membrane, which leads to a dry volume IEC of 1.97 meq Hþ cm3 from a value of 0.92 meq Hþ g1 for Nafion NRE-211. One limitation to this metric is that dry volumetric IEC does not take into account the hydrophilic volume fraction of the membrane where the available proton carriers

are present. Therefore, the IEC in the hydrophilic domain of the membrane is determined with a simple correction for the volumetric water uptake of the membrane, assuming the volume of absorbed water is equal to the volume of the hydrophilic domain. The adjusted metric, referred to as the hydrophilic volume IEC, is dependent on temperature and RH. Here we consider only the 100 1C liquid water volume swell representing the most dilute condition. As summarized in Table 4, the hydrophilic volume IEC for Nafion, SParmax, and GM internal membranes approach or exceed 2.0 meq Hþ cm3 H2 O . Comparatively, sulfonated polyetheretherketone (SPEEK) has a hydrophilic volume IEC less than 1.0 meq Hþ cm3, and has correspondingly poor performance. The reasoning used to calculate the hydrophilic volume IEC is as follows. According to the product data sheet of NRE-211, the gravimetric water uptake is 50% after boiling a 1 cm2 piece of a 25-mm membrane in water for 1 h. The dry membrane volume is equal to 0.002 5 cm3

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

and the mass is equal to 0.004 9 g. Therefore, the estimated volume of the hydrophilic domain is 0.002 46 cm3, based on the 100 1C volume swell. By accounting for the total volume of water and the total equivalents of protons, 0.004 5 meq Hþ in the membrane results in a hydrophilic volume IEC equal to 1.8 meq cm3 H2 O . This value is not changed considerably by a volume correction, because NRE-211 has a swell factor of 2 and a density of 1.97. The significance of this approach becomes more evident when the hydrophilic volume IEC is applied systematically to aromatic hydrocarbon-based membranes. For example, SPEEK with a 1.46 meq g1 IEC has a density of 1.3 g cm3 and a boiling water uptake of greater than 300%. Therefore, the same 1-cm2, 25-mm membrane has a comparable global concentration of 0.004 7 mmol Hþ, but the hydrophilic volume IEC is equal to 0.90 meq cm3, or only 49% that of NRE-211 IEC with 1.83 meq cm3 H2 O . More highly SPEEK does not result in higher proton concentrations as increases up to 2 meq g1 render the membrane soluble in boiling water. By contrast, hydrocarbon membranes based on sulfonated polyphenylene, such as those prepared via random sulfonation of Parmax purchased from Mississippi Polymer (now Solvay Solexis), can have hydrophilic volume IECs approaching 2.0 meq cm3 H2 O , which is more like that of PFSA membranes. Therefore, a simple IEC measurement and a liquid water volume swell experiment at 100 1C can be very useful first screening tools for down-selecting potential membranes before evaluating fuel cell performance. The experiment can also be used to determine the upper limit of proton concentration in new membrane chemistries prior to experiencing excessive swell and dilution of proton conductors. The methodology

may be expanded further to determine the hydrophilic volume IEC as a function of temperature and RH by obtaining water uptake at those conditions.

Performance results

Improved fuel cell performance has resulted from empirical studies with membranes prepared by increasing local acid (proton carrier) concentrations and by understanding the structure–property advantages of diverse polymer architectures. Membrane acid contents are increased until an upper limit of IEC is identified by either solubility in boiling water or until in situ fuel cell performance is optimized. Proton-exchange membrane with hydrophilic volume IECs that range between 2.0 and 3.4 meq Hþ cm3 H2 O show competitive performance in our in situ evaluations. The ex situ in-plane proton conductivity of these aromatic hydrocarbon membranes is comparable with that of SPTES, 1.85 meq g1, but falls short of Nafion NRE-211 conductivity at 50% RH. However, as discussed earlier, the ex situ in-plane proton conductivity does not always correlate well to in situ fuel cell performance. With these new membranes, the in situ fuel cell performance at 110% RHout is better than all other aromatic hydrocarbon-based membranes we have prepared and tested and approaches that of NRE-211. The results are displayed in Figure 9. Improved performance has been demonstrated even at low RH, as GM 1 PEM exhibits equivalent performance to that of NRE-211 in a fuel cell operating with 32% inlet RH and an RHout of 63%. Block polymer structures greatly enhance the aggregation of the sulfonic acid functional groups, thus

1.2 110% RHout NRE 211, 0.9 meq g−1 110% RHout SParmax, 1.81 meq g−1 63% RHout NRE 211, 0.9 meq g−1 110% RHout SPTES 1.85 meq g−1 63% RHout SParmax 1.81 meq g−1 110% RHout GM 1, 2.0 meq g−1 63% RHout GM 1, 2.0 meq g−1

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Figure 9 Comparison of in situ fuel cell performance at 110% RHout and 63% RHout.

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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10

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Figure 10 Comparison of block and random GM hydrocarbon membrane in situ fuel cell relative humidity (RH) sensitivity at 1.2 A cm2, 80 1C.

facilitating better performance at lower RH. Figure 10 illustrates the performance sensitivity to inlet RH at 1.2 A cm2 and 80 1C of block and random polymers. The (sulfonated) block membrane demonstrates about a 50mV higher performance at 50% inlet RH and an impressive 0.565 V at 30% inlet RH compared to 0.342 V for the random membrane under the same conditions. Both membranes have the same thickness and IEC. The block structure increases the local acid concentration of sulfonic acid groups within the membranes and helps to reduce the dependence on water to maintain the percolation threshold necessary for proton transport. In randomly sulfonated polyaromatic membranes, with relatively stiff polymer chains, the acid sites have a limited ability to aggregate and bulk free water is forced to act as the conduit for protons over the large distances between charge carriers. The combination of the random distribution of sulfonic acid groups in SPEEK and a smaller hydrophilic domain as compared to PFSA leads to a greater dependence on water to maintain the percolation pathway required to facilitate proton transport via either Grotthus hopping or vehicular transport. For block polymers with the same IEC and chemistry, the improved morphology reduces the dependence on bulk water as the charge carriers are in closer proximity. A tunable structure–property relationship exists for the optimization of fuel cell performance of block copolymers through tailoring the length of both the hydrophilic and the hydrophobic blocks and the degree of sulfonation of hydrophilic block. Proton-Exchange Membrane Durability Mechanical durability

The dynamic drive cycle associated with powering a vehicle can provide a great deal of stress due to frequent wet–dry transitions. The dynamic operation is less prevalent in stationary and portable power fuel cells

except for start-ups and shutdowns. Additional mechanical stresses are imposed on automotive fuel cell membranes due to thermal cycling experienced during cold start or freeze–thaw events. The mechanical durability of candidate PEM technologies is assessed based on the understanding of the intrinsic role of water in membrane failure. In this test, all current- and voltage-driven degradation mechanisms are removed as membranes are subjected to deep hydration– dehydration cycles in an accelerated mechanical durability protocol using only inert gases. The test differentiates between membrane technologies by comparing the number of hydration–dehydration cycles prior to the measure of a 10 mL min1 gas crossover leak. Generally, aromatic hydrocarbon membranes that have both high elastic modulus and low elasticity behave poorly in accelerated fuel cell tests involving deep humidity cycling. The poor mechanical durability further inhibits the characterization of competing current- and voltage-driven degradation mechanisms. This cannot be overstressed: most PEM research does not address the poor mechanical durability of PEMs. There are many claims that the mechanical properties of sulfonated high-performance engineering thermoplastics are suitable for automotive fuel cell applications due to their high modulus and base polymer toughness. These claims are inherently faulty, as a higher modulus actually leads to greater membrane stresses as a result of the hydration and dehydration experienced during dynamic fuel cell operation. Noteworthy is the observation that some membranes that fail in several hundred or thousand humidity cycles in the accelerated durability protocol between 0% and 150% RH can survive 20 000 cycles if the RH is not reduced below 50%. This may be due to the fact that a membrane breach happens only if these membranes are overstressed under extremely dry conditions. Chemical durability

The chemical degradation of PFSA membranes has historically been assessed through the use of Fenton’s reagent: catalytic Fe2þ salts in the presence of hydrogen peroxide. The experiment subjects the membrane to a very large excess of hydroxyl radicals between 60 and 80 1C to assess chemical durability. PFSA membranes have demonstrated days of stability in Fenton’s reagent, whereas most hydrocarbon membranes degrade appreciably within 2 h. The Fenton’s test results do not predict the stability of either PFSA or aromatic hydrocarbon membranes in accelerated chemical durability in situ fuel cell tests. In fact, the opposite trend has been observed as PFSA membranes without free radical stabilizers can actually degrade faster than aromatic sulfonic acid-based hydrocarbon membranes. The degradation of PFSA membranes without stabilizing agents results in

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

membrane thinning and pinhole formation. By contrast, hydrocarbon membranes do not degrade by thinning, but might degrade through the continual addition of hydroxyl radicals to the aromatic moieties resulting in chain scission and the formation of quinone derivatives in the case of sulfonated polyarylethers, as demonstrated in ex situ testing. Moreover, a platinum line develops within PFSA membranes as platinum catalyst dissolves to form ions that migrate into the membrane whereupon hydrogen passing through the membrane from the anode reduces the ions back into platinum metal. In comparison, aromatic hydrocarbon membranes do not develop noticeable platinum metal lines, presumably because hydrogen gas crossover is substantially lower compared to that of PFSA membranes. The observations above suggest the Fenton’s test is not a prognostic test of membrane life in an operating fuel cell and the evaluation does not correlate to the mean time to PFSA failure. Improved membrane resistance to Fenton’s type degradation by end-group-stabilized PFSA membrane has been demonstrated, yet fuel cell operational life as assessed by OCV experiments was unchanged and still poor. Therefore, a more appropriate assessment of fuel cell stability of membranes employs OCV experiments at 95 1C and 50% RH. The experiment provides an acceleration of chemical degradation while minimizing the mechanical stresses on the membrane. Monitoring the FRRs of PFSA membranes in conjunction with the measured decrease in cell voltage has led to a target cell voltage degradation rate of less than 100 mV h1 loss and a targeted fluoride release rate of o108 gF h1 cm2. When evaluating non-PFSA-based membranes that lack

751

fluorine, gas crossover and voltage decay are used as diagnostics. Surprisingly, sulfonated polyphenylene (SParmax 1200) displays a much lower degradation rate of 300 mV h1 compared to that of 1000 mV h1 for Nafion 112. However, as shown in Figure 11, the degradation of a sulfonated polyphenylene is approximately six times greater compared to that of a robust PFSA membrane.

Mechanical–chemical durability

The independent assessment of both chemical and mechanical durability of PEMs, allows for the fundamental development of mitigation strategies, but the effects of chemical degradation on mechanical durability are not addressed. Material scientists have known for many years that surface crazing induced by external chemical stressors can lead to catastrophic failure in load-bearing plastics, which otherwise would not have failed in an inert environment. Similarly, the contributions of chemical degradation agents from electrochemical reactions need to be assessed as a potential accelerant of mechanical degradation. Therefore, an accelerated membrane durability protocol that encompasses both primary membrane stressors was developed by combining the mechanical- and chemicalaccelerated durability protocols as described above. This combined protocol includes extreme wet–dry RH cycling under a constant low current condition of 0.1 A cm2. Perfluorosulfonic acid membranes without chemical stabilizing agents have been found to fail more than five times faster in the combined accelerated chemical– mechanical durability test when compared to the

1.1 Robust PFSA OCV decay: 50 µ Vh−1

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Figure 11 Comparison of accelerated chemical durability of Nafions 112, a robust perfluorosulfonic acid (PFSA) membrane and a 25 mm sulfonated polyphenylene (SParmax, 1.81 meq g1). Open-circuit voltage (OCV), 95 1C, 50% relative humidity (RH), 50 kPag, 5/5 stoich at 0.2 A cm2 equivalent flow.

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

accelerated mechanical durability test alone. The upper target of 20 000 cycles remains the measure of membranes that are considered to be chemically and mechanically robust.

in terms of mechanical properties. Namely, PFSA membranes are rubbers typically having a dry elongation to break greater than 250% and having relatively low dimensional changes upon boiling in water. Thus, the dimensional change induced by boiling in water does not elongate PFSA membranes beyond the dry breaking point. By contrast, hydrocarbon membranes typically exhibit extreme dimensional increases upon swelling in boiling water that extend the membrane beyond the yield and break point when dry. The variation in membrane chemistry through a number of techniques including polymer synthesis strategy, blending additives, and varying the mole fraction of hydrophobic and hydrophilic blocks has led to greatly enhanced mechanical durability. Through the use of the Fx metric, mechanically stable membranes may be achieved facilitating the investigation of chemical degradation mechanisms specific to the chosen membrane chemistry.

Design methodology for durability

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The mechanical durability of PFSA membranes has been improved through incorporation of PFSA ionomers into porous supports such as DSM polyolefin or expanded polytetrafluoroethylene (ePTFE). For example, the GoreSelects family of membranes from W. L. Gore uses an ePTFE support with a PFSA polyelectrolyte and has demonstrated dramatic enhancement in membrane durability under humidity cycling. Supported membranes provide a means of inhibiting crack propagation and reducing the in-plane swelling in favor of volume swell into the thickness dimension. The greatest hindrance of using such supports is that they inherently increase the membrane through-plane resistance to proton transport. To date, most non-PFSA membrane research has not focused on durability issues and, therefore, there has been little work on incorporating non-PFSA ionomers into supports. We believe that the mechanical durability of non-PFSA membranes can be significantly improved through the use of such supports. Previously, we identified the membrane humidity stability factor (Fx), which accounts for the relationship between membrane tensile elongation at break and the dimensional change induced by boiling water. This metric has proven to be a convenient measure to assess various membranes and to predict their relative durability in the accelerated mechanical RH cycling test. The Fx metric has led to the discovery of a fundamental difference between hydrocarbon and PFSA membranes

G

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Figure 12 Accelerated mechanical durability of GM hydrocarbon proton-exchange membrane (PEMs) compared to SParmax and NRE-211; bars ¼ # RH cycles (primary Y); line ¼ ex situ Fx relationship (secondary Y).

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

Table 5

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Design guidelines for the development of automotive PEMs

Design guideline In situ performance Pendent acid groups are more effective than main chain Block copolymers are superior to random copolymers Continuous disorder phase separation better than high degree of orientation High IEC with a volume swell factor less than 2 Maximize local hydrophilic volume IEC (meq Hþ cm3 H2 O ) Mechanical durability Maximize humidity stability factor (Fx )

Potential explanation Facilitates phase separation Improved sulfonic acid aggregation Facilitates phase separation Increased local density of protogenic groups Reduced percolation pathway through membrane Maximize acid concentration without the loss of mechanical properties Increase concentration of protogenic groups Reduce water swelling-induced stress compared to membrane yield and break point

IEC, ion-exchange capacity; PEM, proton-exchange membrane.

which reports a dry elongation to break at ambient conditions of 252% and a length increase of 15% upon boiling in water. The successful preparation of membranes with a wide range of Fx, including membranes with similar swelling and tensile properties to Nafion, has allowed for the correlation of Fx to lifetime in the accelerated mechanical durability test. The relationship between Fx and life under humidity cycling is shown in Figure 12. The results illustrate that by increasing Fx, the mechanical durability of non-PFSA membranes cannot only approach and surpass that of NRE-211, but can actually meet the target of 20 000 RH cycles without membrane failure.

Conclusions The development of alternative PEMs is currently focused on achieving performance at low RH over the range of temperatures of 60–80 1C, with intermittent excursions up to 95 1C. However, the ultimate goal for automotive applications is the development of membranes that can extend the operating regime to between 100 and 120 1C while maintaining current performance. A number of guidelines are presented herein that provide a series of design tools to develop and evaluate automotive fuel cell membranes for performance and durability. The guidelines in Table 5 summarize what is known in the literature and within the framework of this review and are presented with the intended purpose of future verification and refinement as new PEM structure–property relationships are discovered. The design guidelines may be inherent to PFSA membranes which display phase separation with continuous disordered morphology, and have an increased hydrophilic volume IEC and acceptable membrane humidity stability factor (Fx) for optimum performance and mechanical durability. However, the variety of polymer architectures available

to aromatic hydrocarbon-based PEMs may provide a greater potential to elucidate structure–property relationships, leading to new design guidelines. The strategies to improve the performance and durability of aromatic hydrocarbon membranes generally involve mimicking the attributes of PFSA membranes and evaluating progress with the screening tools identified herein. The demonstration of methods to improve membrane mechanical durability with concurrent consideration of the hydrophilic volume IEC provides an approach to tune and optimize membrane properties for automotive applications. The evaluation of other membrane failure modes associated with chemical degradation is possible only after the improvements in the mechanical durability are achieved. Through the guidance presented, appreciable advances in alternative PEMs with improved performance and mechanical durability can be made which will inherently cost less than PFSAs. With the appropriate design tools and methodologies provided, comparative rankings and metrics will allow a systematic down-selection of competitive membrane technologies for the automotive industry. Furthermore, an improved understanding of the relationship between chemistry, structure, and membrane morphology will allow researchers to tailor alternative PEMs for automotive applications.

Nomenclature Symbols and Units Fx RHinlet RHout Voldry Volwet Wdry Wwet

humidity stability factor inlet relative humidity outlet relative humidity dry volume wet volume dry weight wet weight

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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Design and Characterization

Abbreviations and Acronyms AC CCDM DC DSM ePTFE EW FRR Fx GDL GM HC IEC MEA NRE OCV PEM PFSA RH SLPM SPEEK SPTES

alternating current catalyst-coated diffusion media direct current Company name Remove from list expanded polytetrafluoroethane Equivalent weight fluoride release rate membrane humidity stability factor gas diffusion layer General motors hydrocarbon ion-exchange capacity membrane–electrode assembly Remove from list open-circuit voltage proton-exchange membrane perfluorosulfonic acid relative humidity standard liters per minute sulfonated polyetheretherketone sulfonated polythioethersulfone

See also: Fuel Cells – Proton-Exchange Membrane Fuel Cells: Membrane: Life-Limiting Considerations; Membranes; Membranes: Non-Fluorinated.

Further Reading Department of Energy. Multi Year Research, Development and Demonstration Plan, Planned Activities for 2003–2010 (Draft 6/3/03), vol. 3.4.4, 2003. Beuscher U, Cleghorn SJC, and Johnson WB (2005) Challenges for PEM fuel cell membranes. International Journal of Energy Research 29: 1103--1112. Cleghorn S, Kolde J, and Liu W (2003) Catalyst coated composite membranes. In: Vielstich W, Lamm A, and Gasteiger HA (eds.)

Handbook of Fuel Cells – Fundamentals, Technology and Applications, ch. 44, pp. 538--565. Chichester, UK: John Wiley & Sons. Curtin DE, Lousenberg RD, Henry TJ, Tangeman PC, and Tisack ME (2004) Advanced materials for improved PEMFC performance and life. Journal of Power Sources 131: 41--48. Dillard DA, Budinski M, Lai Y-H, and Gittleman CS (2005) Tear resistance of proton exchange membranes. In: Proceedings of the 3 rd International Conference on Fuel Cell Science, Engineering, and Technology, pp. 153–159. Ypsilanti, MI. Gasteiger HA and Mathias MF (2005) Fundamental research and development challenges in polymer electrolyte fuel cell technology. Proceedings – Electrochemical Society PV 2002-31: 1--24. Ghassemi H, McGrath JE, and Zawodzinski TA, Jr (2006) Multiblock sulfonated–fluorinated poly(arylene ether)s for a proton exchange membrane fuel cell. Polymer 47: 4132--4139. Hickner MA, Ghassemi H, Kim YS, Einsla BR, and McGrath JE (2004) Alternative polymer systems for proton exchange membranes (PEMs). Chemical Reviews 104: 4587--4612. Hickner MA and Pivovar BS (2005) The chemical and structural nature of proton exchange membrane fuel cell properties. Fuel Cells 5: 213--229. Kreuer KD (2003) Hydrocarbon membranes. In: Vielstich W, Lamm A, and Gasteiger HA (eds.) Handbook of Fuel Cells: Fundamentals, Technology, and Applications, vol. 3, ch. 33, pp. 420--435. Chichester, UK: John Wiley & Sons. Kusoglu A, Karlsson AM, Santare MH, Cleghorn S, and Johnson WB (2006) Mechanical response of fuel cell membranes subjected to a hygro-thermal cycle. Journal of Power Sources 161: 987--996. Kusoglu A, Karlsson AM, Santare MH, Cleghorn S, and Johnson WB (2007) Mechanical behavior of fuel cell membranes under humidity cycles and effect of swelling anisotropy on the fatigue stresses. Journal of Power Sources 170: 345--358. Lai Y-H, Gittleman CS, Mittelsteadt CK, and Dillard DA (2005) Viscoelastic stress model and mechanical characterization of perfluorosulfonic acid (PFSA) polymer electrolyte membranes. In: Proceedings of the 3rd International Conference on Fuel Cell Science, Engineering, and Technology, pp. 161–167. Ypsilanti, MI. Mathias MF, Makharia R, Gasteiger HA, et al. (2005) Two fuel cell cars in every garage? The Electrochemical Society Interface 14: 24--35. Paddison SJ (2003) First principles modeling of sulfonic acid-based ionomer membranes. In: Vielstich W, Lamm A, and Gasteiger HA (eds.) Handbook of Fuel Cells: Fundamentals, Technology and Applications, vol. 3, ch. 31, pp. 396--411. Chichester, UK: John Wiley & Sons. Roy A, Hickner MA, Yu X, Li Y, Glass TE, and McGrath JE (2006) Influence of chemical composition and sequence length on the transport properties of proton exchange membranes. Journal of Polymer Science: Part B Polymer Physics 16: 2226--2239.