Craig S. Gittleman

Viscoelastic Stress Analysis of Constrained Proton Exchange Membranes Under Humidity Cycling

Many premature failures in proton exchange membrane (PEM)fuel cells are attributed to crossover of the reactant gas from microcracks in the membranes. The formation of these microcracks is believed to result from chemical and/or mechanical degradation of the constrained membrane during fuel cell operation. By characterizing the through-membrane leakage, we report failures resulting from crack formation in several PEMs mounted in 50 cm 2 fuel cell fixtures and mechanically stressed as the environment was cycled between wet and dry conditions in the absence of chemical potential. The humidity cycling tests also show that the failure from crossover leaks is delayed if membranes are subjected to smaller humidity swings. To understand the mechanical response of PEMs constrained by bipolar plates and subjected to changing humidity levels, we use Nafion® NR-111 as a model membrane and conduct numerical stress analyses to simulate the humidity cycling test. We also report the measurement of material properties required for the stress analysis-water content, coefficient of hygral expansion, and creep compliance. From the creep test results, we have found that the principle of time-temperature-humidity superposition can be applied to Nafion® NR-111 to construct a creep compliance master curve by shifting individual compliance curves with respect to temperature and water content. The stress prediction obtained using the commercial finite element program ABAQVS® agrees well with the stress measurement of Nafion® NR-111 from both tensile and relaxation tests for strains up to 8%. The stress analysis used to model the humidity cycling test shows that the membrane can develop significant residual tensile stress after humidity cycling. The result shows that the larger the humidity swing and/or the faster the hydration/dehydration rate, the higher the residual tensile stress. This result is confirmed experimentally as PEM failure is significantly delayed by decreasing the magnitude of the relative humidity cycle. Based on the current study, we also discuss potential improvements for material characterization, material state diagnostics, and a stress model for PEMs.

Chapter 2 – Membrane Durability: Physical and Chemical Degradation

Publisher Summary Proton exchange membranes (PEMs) are the most promising membranes for automotive applications because of their relatively high proton conductivity at low temperatures. This chapter addresses the three primary root causes of membrane failure in automotive fuel cell systems. PEM fuel cells for high power density operation utilize perfluorosulfonic acid (PFSA) membranes. Ultimately, fuel cells fail because of pinholes that develop and propagate within the polymer membranes and can also fail if electronic current passes through the membranes causing the system to short. Both chemical and mechanical degradation of PFSA membranes are extensively studied and effective mitigation strategies are developed that can significantly extend PFSA-PEM lifetimes. The approaches described in this chapter can be applied to such alternative PEMs, but new fundamental models for both chemical and mechanical degradation are likely required. Even for the case of PFSA membranes, further work is required to develop the predictive models required to enable accurate estimation of membrane life in a real system. This chapter includes the first in-depth fundamental study of membrane shorting of PEMs in subsequent thermal decay of the PEM. Results and analysis suggest that the PEM type is not the limiting factor in preventing membrane shorting, and that mitigation is best achieved by a combination of design and operating strategies.

The Effect of Mechanical Fatigue on the Lifetimes of Membrane Electrode Assemblies

Long-term durability of the membrane electrode assembly (MEA) in proton exchange membrane (PEM) fuel cells is one of the major technological barriers to the commercialization of fuel cell vehicles. The cracks in the electrode layers of the MEA, referred to as mud-cracks, are potential contributors to the failure in the PEM. To investigate how these mud-cracks affect the mechanical durability of the MEA, pressure-loaded blister tests are performed at 90°C to determine the biaxial fatigue strength of Gore-Primea® series 57 MEA. In these volume-controlled tests, leaking rate is determined as a function of fatigue cycles. The failure is defined to occur when the leaking rate exceeds a specified threshold. Postmortem characterization using bubble point testing and field emission scanning electron microscopy (FESEM) was conducted to provide visual documentation of leaking failure sites. The analysis of the experimental leaking data indicates that the MEA has much shorter lifetimes at the same nominal stress levels than membrane samples without the electrode layers. FESEM photomicrographs of leaking locations identified via the bubble point testing show cracks in the membrane that are concentrated within the mud-cracks of the electrode layer. These two pieces of information indicate that the mud-cracks within the electrode layers contribute to the leaking failures of the MEA assembly. For the fuel cell industry, this study suggests there is an opportunity to reduce the likelihood of membrane pinhole failures by reducing the size and occurrence of the mud-cracks formed during the MEA processing or by increasing the fatigue resistance (including the notch sensitivity) of the membrane material within the MEA.

On the Use of Pressure-Loaded Blister Tests to Characterize the Strength and Durability of Proton Exchange Membranes

The use of pressurized blister specimens to characterize the biaxial strength and durability of proton exchange membranes (PEMs) is proposed, simulating the biaxial stress states that are induced within constrained membranes of operating PEM fuel cells. PEM fuel cell stacks consist of layered structures containing the catalyzed PEMs that are surrounded by gas diffusion media and clamped between bipolar plates. The surfaces of the bipolar plates are typically grooved with flow channels to facilitate distribution of the reactant gases and water by-product. The channels are often on the order of a few millimeters across, leaving the sandwiched layers tightly constrained by the remaining lands of the bipolar plates, preventing in-plane strains. The hydrophilic PEMs expand and contract significantly as the internal humidity, and to a lesser extent, temperature varies during fuel cell operation. These dimensional changes induce a significant biaxial stress state within the confined membranes that are believed to contribute to pinhole formation and membrane failure. Pressurized blister tests offer a number of advantages for evaluating the biaxial strength to bursting or to detectable leaking. Results are presented for samples of three commercial membranes that were tested at 80°C and subjected to a pressure that was ramped to burst. The bursting pressures exhibit significant time dependence that is consistent with failure of viscoelastic materials. Rupture stresses, estimated with the classic Hencky’s solution for pressurized membranes in conjunction with a quasielastic estimation, are shown to be quite consistent for a range of blister diameters tested. The technique shows considerable promise not only for measuring biaxial burst strength but also for measuring constitutive properties, creep to rupture, and cyclic fatigue damage. Because the tests are easily amenable to leak detection, pressurized blister tests offer the potential for characterizing localized damage events that would not be detectable in more commonly used uniaxial strength tests. As such, this specimen configuration is expected to become a useful tool in characterizing mechanical integrity of proton exchange membranes.

Viscoelastic Stress Model and Mechanical Characterization of Perfluorosulfonic Acid (PFSA) Polymer Electrolyte Membranes

Many of the premature failures in the PEM fuel cells are attributed to crossover of the reactant gas from pinholes or through-the-thickness flaws in the membranes. The formation of these pinholes is not fully understood, although mechanical stress is often considered one of the major factors in their initiation and/or propagation. This paper reports evidence of pinhole failure from mechanical stress by cycling between wet and dry conditions in a normally built single 50cm2 fuel cell. In an effort to understand the source of the mechanical stress, to quantify the magnitude, and to correlate its role in membrane failure, a membrane stress model based on linear viscoelastic theory was developed. The effects of temperature, water content, and time are accounted for in the membrane stress model. To satisfy the inputs for the membrane model and to characterize the mechanical behavior of the polymer electrolyte membrane, a series of experiments was performed. Using commercially available Nafion® NR111 membrane as a model material, swelling of 15% and shrinkage of 4% were found from a hydration and de-hydration cycle. Data on elastic moduli versus relative humidity showed discontinuity at the vapor and liquid water transition. We also found that creep compliance master curves can be obtained by double-shifting the compliance curves according to the time-temperature-moisture superposition principle, which significantly simplifies the modeling effort. Combining data on hygro-expansion, elastic moduli, and creep compliance data through the membrane stress model, it was found that the de-hydration process induces significant stress in the membrane. Due to fluctuations in fuel cell operating conditions, the membrane and the associated components are subject to mechanical fatigue which may mechanically degrade the membrane of PEM fuel cells and eventually lead to pinhole formation.Copyright

Through-Plane Proton Transport Resistance of Membrane and Ohmic Resistance Distribution in Fuel Cells

Ohmic losses in fuel cells contain both ionic (mainly protonic) and electronic contributions. In this work, we developed a method to distinguish the resistance contributions from individual components. Proton transport resistance of proton exchange membranes (PEMs) in the through-plane direction, the major proton transport direction in in situ fuel cells, was measured using electrochemical impedance spectroscopy combined with compression-controlled fuel cell hardware. Membrane resistance was obtained by subtracting the nonmembrane contributions from the total resistance. Proton conductivities of PEMs with different equivalent weights, calculated from through-plane resistance measurements at various relative humidity conditions, were compared with good agreement to the in-plane measurements. Fuel cell electronic resistance and membrane-electrode interfacial resistance were also evaluated using both ex situ and in situ methods. The membrane-electrode interface resistance was nearly constant over a range of relative humidity conditions. The effects of test procedure and cell build strategy were investigated and had a significant impact on the membrane resistance measurements.

Tear Resistance of Proton Exchange Membranes

Through the thickness flaws or “pinholes” in proton exchange membranes (PEM) allow gas crossover that can lead to fuel cell failure. The formation of these flaws is not fully understood, but one possible mechanism is that small flaws could grow through crack propagation in the fracture mechanics sense. Although relatively brittle features are sometimes observed in failures resulting under simulated fuel cell conditions, the stress strain plots of the membranes themselves exhibit considerable ductility. In an effort to use fracture mechanics principles to characterize PEMs, fracture parameters associated with the essential work of fracture from double edge notch tensile (DENT) specimens; the tear energy obtained from the trouser tear test; and cutting energies associate with knife slitting were measured and compared. Presumably through reducing crack tip blunting, the knife slitting test is able to measure fracture energies as low as 200J/m2 , two orders of magnitude smaller than measured in the other tests. The results are sensitive to rate, temperature, and moisture level. Although the implications of these properties to fuel cell durability are not yet understood, they may have applicability in the more brittle features that are sometimes observed.Copyright

NMR studies of proton transport in fuel cell membranes at sub-freezing conditions

Water uptake activities and transport properties are critical for water management in fuel cell membranes. In this work, three perflourosulfonic acid (PFSA) fuel cell membranes, including Nafion®-117 and two Gore membranes, were evaluated at different relative humidity controlled conditions. These fuel cell membranes were studied using variable temperature 1H spin–lattice relaxation times (T1) and pulsed field gradient (PFG) NMR techniques in the temperature range of 298 to 239 K. Water self-diffusion coefficients and proton transport activation energies in the fuel cell membranes were obtained from the PFG-NMR experiments. The results show that the water self-diffusion coefficients increase with increasing hydration level, and decrease with decreasing temperature. The water molecular motion is significantly slowed at low temperatures; however, the water molecules in these membranes are not frozen, even at 239 K. The water uptake activity and diffusivity in these membranes were compared as a function of temperature and hydration level. At the same temperature and hydration level, the water self-diffusion coefficients of two Gore fuel cell membranes are higher than that of Nafion®-117. This is attributed to the lower EW of the Gore membranes. The presence of an expanded polytetrafluoroethylene (ePTFE) reinforcing layer in the membrane also has an impact on water diffusivity.

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

A series of ex situ and in situ diagnostic tests have been developed to quantitatively screen proton-exchange membranes (PEMs) for automotive fuel cell applications with respect to performance and mechanical and chemical durability. A comparison of the measured lifetimes of perfluorosulfonic acid (PFSA) and sulfonated aromatic hydrocarbon membranes under accelerated test conditions reveals the inherent differences between the two membrane chemistries. Upon subjecting membranes to deep hydration–dehydration cycles, the mechanical durability of PFSA membranes is more robust compared to that of aromatic hydrocarbon membranes, which have higher modulus and lower elasticity. By contrast, under in situ conditions promoting chemical degradation, aromatic hydrocarbon membranes can display improved stability. The next generation of alternative PEMs receiving a lot of attention are low-cost, sulfonated hydrocarbon polymers having controlled molecular architectures. Improved aromatic hydrocarbon PEM performance under conditions of low relative humidity can be facilitated by mimicking the positive attributes of PFSA membranes. New design tools allow for the optimization of nanophase separation of structurally reinforced hydrophobic domains and concentrated hydrophilic domains, thereby improving the performance of aromatic hydrocarbon membranes. A design guideline for polymer scientists is presented outlining the methodology to develop new PEMs for automotive fuel cell applications, including new metrics such as the membrane humidity stability factor and the hydrophilic volume ion-exchange capacity.

The Nonlinear Viscoelastic Properties of PFSA Membranes in Water-immersed and Humid Air Conditions

Proton exchange membranes (PEM) in an automotive fuel cell stack can experience significant temperature and hydration changes as the stack responds to the demanding automotive duty cycle. Since mechanical stresses resulting from the hygrothermal cycles are believed to contribute to the loss of mechanical durability that are sometimes experienced in operating PEM fuel cells, it is important to characterize the mechanical behavior of PEMs over a wide range of hygrothermal conditions. In this study, the linear and nonlinear viscoelastic properties of PEMs equilibrated with both humidified air and liquid water are characterized using a custom-built multistation stress relaxation fixture. Specifically, relaxation data of a commercially available, perfluorosulfonic acid PEM was collected over a temperature range of 30-90°C and strain levels from less than 1% to over 20% or more. A comparison of immersed data to dry conditions and a range of humidity levels is presented in this paper. Significant nonlinearity is observed in the membrane, but becomes less pronounced at longer times. Cyclic tests with various strain levels were carried out on the membranes at 70o C in immersed conditions. The nonlinearity exhibited by the PEM under the larger strain levels was represented quite accurately with a Schapery unaxial hereditary single integral model. For this initial effort, material nonlinear parameters were chosen to simulate the stress output from larger strain levels. Complex loading profiles at various rates were used to validate the model and good agreement was achieved between experimental results and numerical predictions.