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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 50cm2 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 ABAQUS® 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.

Various types of hydrocarbon-based ionomer membranes have been proposed for application in fuel cells as an alternative to perfluoroalkylsulfonic acid (PFSA) membranes. The issue of chemical and mechanical degradation of this class of ionomer membranes and the combination thereof, especially under automotive operating conditions, has not been fully resolved at present. Here, we highlight key degradation mechanisms of hydrocarbon-based membranes in the fuel cell and identify necessary developments to mitigate radical induced membrane degradation and mechanical shortcomings of this class of ionomers that may enable them to compete with and, possibly, replace PFSA membranes.

A highly accelerated stress test (HAST) has been developed to generate local stressful conditions that are representative of those in automotive fuel cell stacks. Using a 50-cm 2 cell cycled between 0.05 and 1.2 A/cm 2 with a low inlet RH in the co-flow configuration, the HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the outlets. Conducting HASTs using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a robust membrane containing both a mechanical support and a chemical stabilizing additive, the result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer. One of the key challenges facing the commercialization of automotive fuel cells is the development of membrane electrode assemblies (MEAs) that can meet durability targets. In proton exchange membrane (PEM) fuel cells, the PEM serves to conduct protons from the anode to the cathode of the fuel cell while simultaneously insulating electronic current from passing across the membrane as well as preventing crossover of the reactant gases, H 2 and O 2 . State-of-the-art PEM fuel cells for high power density operation utilize perfluorosulfonic acid (PFSA) membranes that are typically no more than 25 μm thick. To be viable for automotive applications, these membranes must survive 10 years in a vehicle and 8000 hours of operation including transient operation with start-stop and freeze-thaw cycles. Fuel cells cannot operate effectively when gas can permeate the membrane through microscopic pinholes or excessively reduced thickness. Ultimately, fuel cells can fail because such excessive crossover leaks develop and propagate within the polymer membranes. Fuel cells can also fail if electronic current passes through the membranes and causes the system to short. It is thus critical that these membranes are sufficiently robust to thinning, cracking and thermal decomposition over the range of conditions experienced during fuel cell operation. In automotive fuel cell systems, there are three primary root causes of membrane failure: (1) Chemical degradation: polymer decomposition caused by the direct attack on the polymer from radical species generated as byproducts or side reactions of the fuel cell electrochemical reactions; (2) Mechanical degradation: membrane fracture caused by cyclic fatigue stresses imposed on the membrane via hygrothermal fluctuations in a constrained cell; and (3) Thermal degradation: membrane degradation caused by ohmic heating thr...

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