The Effect of Platinum in the Membrane on Fuel Cell Membrane Durability

Development of highly durable fuel cell membranes is essential to achieve the heavy duty fuel cell bus lifetime targets. The bus duty cycle exposes fuel cell membranes to conditions that can eventually lead to membrane degradation, and limit fuel cell lifetime.

               Fuel cell membranes can degrade both chemically and mechanically leading to membrane thinning, as well as pinhole and crack formation over time. Chemical degradation occurs primarily due to radical attack of the membrane.  It has been observed that during bus operation, platinum particles migrate into the membrane due to catalyst degradation, forming a distinguishable band close to the cathode [1].

               The presence of platinum in the membrane (PITM) is known to affect membrane durability. Observations of both negative and positive effects have been reported in the literature [2-5]. The platinum particle size and distribution inside the membrane have been found to be important factors. A low concentration of highly dispersed small particles is believed to result in increased degradation, and on the other hand a Pt band in the membrane with higher Pt concentrations was confirmed to be effective to improve durability [1].

               The effect of PITM on membrane degradation is investigated by deliberately generating a platinum band in the membrane and running an in situ Accelerated Membrane Durability Test (AMDT) on fuel cells containing such membranes. The lifetime of reference baseline cells, without intentionally deposited PITM is compared to the lifetime of cells with PITM under AMDT conditions.

               Our results confirm that the presence of PITM prolongs membrane lifetime, suggesting that PITM is able to decrease membrane chemical degradation, either by decomposing H₂O₂ (to non-radical products) and/or converting crossover gases to water at the Pt band, thus avoiding radical formation. It was also found that higher PITM concentrations result in a longer membrane lifetime. In addition, membranes with PITM show no thinning, while the baseline membranes show severe membrane thinning after being exposed to the same AMDT conditions. Ex situ mechanical tests on AMDT degraded baseline membranes show significantly reduced mechanical strength of the membranes without PITM. Again, the mechanical strength of the membranes with PITM is on the other hand highly preserved.

                These findings support the theory that the platinum band mitigates membrane chemical degradation under the conditions tested. However, it is important to understand the precise structure of the particles located in the Pt band. Thorough analysis of these Pt particles will help improve the general understanding of the lifetime enhancing effects of PITM. Therefore a systematic characterization procedure of the end-of-life AMDT membranes is conducted using transmission electron microscopy (TEM). The Pt particle sizes, distribution, distances between particles and Pt crystallinity are reported, and their role in the mitigation mechanism is discussed.

Acknowledgments

This research was supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada (APC) grant.

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