PEMFC AST Protocol for Air-Air Starts: Screening of Cathode Tolerances

Abstract

The long-term durability of the cathode catalyst layer continues to be one of the main obstacles for the widespread commercialization of proton exchange membrane fuel cells (PEMFCs) for transportation applications. A major challenge is air/air-start events that occur after long shut-down periods where an H2/air gas front passes through the anode and causes polarization of the cathode electrode to ≈1.4-4.5 V. To screen catalyst materials for their air/air-start tolerance, several accelerated stress test (AST) protocols have been developed. The two most commonly used AST protocols have been recommended by the U.S. Department of Energy (DoE): (1) catalyst support protocol consisting of triangle sweep cycles: 500 mV/s between 1.0 V and 1.5 V (H2/N2, 80ׄ°C, 100% relative humidity (RH)) designed to trigger corrosion of the carbon support; (2) unmitigated start-up/shut-down durability protocol involving an H2/air front, whereby each cycle consists of a start-up event, intermittent fuel cell operation, an unmitigated shut-down event, and an idle period under relevant air/air-start conditions (35°C, 100% RH). While the second protocol is a closer representation of air/air-start events that occur in a PEMFC system, it requires specialized fuel cell testing equipment to allow for an H2/air front in the anode electrode. Conversely, the triangle sweep protocol can be used for catalyst benchmarking on any fuel cell test bench and even in model systems like rotating disc electrode (RDE), or gas diffusion electrode (GDE) setups.In this work, we compared H2/air front-based start-up/shut-down (SUSD) aging to the voltage-controlled carbon corrosion AST for four catalyst coated membranes (CCM). Using in-situ electrochemical and ex-situ characterization techniques (e.g., μXRD, SEM, and TEM), the degradation is compared with respect to the rate of loss in the electrochemically active surface area (ECSA). Particularly the correlation between voltage losses and ECSA, Pt particle size growth, catalyst layer thinning, and homogeneity of aging were investigated. Our results with the catalyst support AST confirms that this method exclusively triggers the carbon corrosion aging mechanism without a significant increase in the Pt particle size and is therefore a suitable screening protocol for support stability. However, the more application relevant SUSD protocol resulted in a combination of comparatively mild carbon corrosion and severe Pt dissolution/redeposition that caused significant Pt size growth and Pt band formation. Furthermore, SUSD resulted in inhomogeneous aging where the Pt size growth is more pronounced towards the anode inlet of the CCM. Due to the difference in the dominating aging mechanisms, the voltage loss response at nominally the same loss of ECSA varies greatly for the two protocols. Therefore, typical voltage-controlled AST (1-1.5V) is found to be unsuitable for predicting the air/air start tolerance of catalyst material. Instead, we propose to use an alternative voltage-controlled protocol that combines the two main degradation mechanisms of air/air starts: triangle sweep cycles at 500 mV/s between 0.7 V and 1.4 V (H2/N2, 45ׄ°C, 100% relative humidity).