Lifetime Prolongation of PtCo/C Cathode Catalysts for PEM Fuel Cells By Understanding Triggers for Pt Surface Area Loss and Co Dissolution in Voltage Cycling-Based Accelerated Stress Tests
Authors: Meier Vivian , Schilling Markus Achim , Lazaridis Timon , Gasteiger Hubert Andreas 
Published: December 2024

Proton exchange membrane fuel cell (PEMFC) applications in the heavy-duty mobility sector have attracted increasing attention due to their flexible scalability in terms of energy and power density. The challenging high fuel efficiency targets in the heavy-duty sector can be met by introducing platinum-cobalt alloy (PtxCo/C) catalysts which exhibit a 1.5-3x higher kinetic activity for the oxygen reduction reaction (ORR) as compared to carbon supported platinum (Pt/C) catalysts.1 Despite the better beginning-of-life performance of PtxCo/C, the dissolution of Co2+ ions into the ionomer phase of the membrane electrode assembly (MEA) during aging results in significant performance losses.2 The release of Co2+ ions from the catalyst influences the fuel cell performance in two major ways: i) a decrease in specific ORR activity, most prominent at low-current densities, and thus in fuel cell efficiency, and ii) mass transport-related performance losses due to Co2+ cation contamination of the ionomer phase, being especially relevant at high-current densities.2 Besides increasing the lifetime by using advanced catalysts (e.g., core-shell particles), longer durability can be achieved by optimizing the fuel cell operation conditions to minimize Pt surface area loss and Co dissolution.3

In this study, we investigate the triggers for the performance decay of PtxCo/C ORR catalysts by separately evaluating Pt and Co loss by varying the conditions of voltage cycling accelerated stress tests (ASTs). All voltage cycling ASTs are performed using 5 cm2 active area single-cells with membrane electrode assemblies (MEAs) based on PtxCo/C cathode catalysts at a loading of 0.3 mgPt cmMEA
-2, conducted in H2/N2 at 80 °C, 95% RH, and ambient pressure. Whereas the lower potential limit (LPL) is kept constant for all tests, different upper potential limits (UPLs), UPL hold times, and wave profiles are investigated. The MEA performance at different aging steps (i.e., number of voltage cycles) is evaluated as a function of Pt ECSA (electrochemically active surface area) loss and Co dissolution. Whereas previous voltage cycling studies for Pt/C catalysts revealed that the H2/air performance losses are solely determined by the cathode roughness factor (rf, in cm2
Pt/cm2
cathode), independent of the aging protocol,4 we show that this relation does not hold for PtxCo/C catalysts. Instead, the H2/air performance needs to additionally account for the extent of Co dissolution. Both loss terms are partly independent and can be triggered to varying extents depending on the aging protocol. Based on our findings, we propose guidelines for possible operation strategies to minimize PtxCo/C degradation under automotive conditions.

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Acknowledgement

This research has received funding from Bosch.