Innovative Characterization Methods and Further Insights on OER Based Reversal Tolerance for PEMFC Anodes

Abstract

Anode durability is a major lever to achieve durability targets for commercial applications of low temperature proton exchange membrane fuel cells (PEMFCs). To achieve reversal tolerant anode catalyst layers (ACLs), the introduction of oxygen evolution reaction (OER) co-catalysts to the ACL has been established as a material based mitigation strategy.[1,2] The presence of OER co-catalyst improves the PEMFC tolerance to gross fuel starvation events and protects the membrane electrode assembly (MEA) from severe damage. [1]We developed a cyclic reversal (CR) accelerated stress test (AST) alternating normal (Air/H2) and reversal (Air/N2) operation (cf. Fig. 1a) on a differential PEMFC. The AST is mimicking recurring short-term reversal events in field operation due to freeze start-ups. The results indicate MEA failure during normal operation and a simultaneous OER catalyst recovery effect for recurring short-term reversal events. This degradation behavior is in contrast to findings for commonly applied non-cyclic reversal (NCR) ASTs where OER catalyst deactivation/degradation is assumed as root cause for MEA failure (cf. Fig. 1b).[2,3]Incorporating further characterization methods such as cyclic voltammetry, electrochemical impedance spectroscopy and hydrogen pump, a hydrogen oxidation reaction (HOR) overpotential distribution analysis of the ACL is derived. The results for CR AST show MEA failure due to an increase in HOR mass transport overpotential, strongly indicating a structural change within the ACL causing MEA failure during normal operation, while still maintaining significant OER activity.[4]Figure Caption:Fig. 1: (a) Schematic CR AST (cyclic reversal AST) cell voltage profile. (b) MEA cell voltage during reversal operation for CR AST and prolonged reversal operation (non-cyclic reversal; NCR).