Novel test protocols and characterization techniques for OER based reversal tolerant PEFC anodes for automotive applications

Abstract

In a world facing climate change, the application of low temperature polymer electrolyte fuel cells (PEFCs) for automotive and stationary applications gained major attention recently. In particular, commercialization advances are being made for their usage in medium and heavy-duty vehicles. Durability is a key aspect for commercial success of PEFCs. To resist reversal events originating from gross fuel (i.e. H2) starvation in affected cells, the introduction of oxygen evolution reaction (OER) co-catalysts to the PEFC anode has been established as material-based mitigation strategy. This work focuses on the development of test protocols and characterization techniques on single cell level to investigate iridium-based reversal tolerant PEFCs regarding performance and degradation. The first part of this thesis aims at techniques which are providing insights into reversal tolerance of OER based PEFCs. An accelerated stress test (AST) was developed investigating short-term recurring reversal operation to meet the expectations of automotive field application. An OER recovery effect, indicated by unaffected OER activity, was observed for short-term reversal events while normal operation caused PEFC failure. In addition, a significant dependence between hydrogen oxidation reaction (HOR) catalyst and reversal tolerance was found. Using further characterization methods such as hydrogen pump polarization curves, PEFC failure for short-term reversal events could be ascribed to hydrogen oxidation mass transfer increase originating from severe carbon corrosion and structural collapse within the anode catalyst layer. The electrochemical results were validated analyzing scanning electron microscope images (SEM). The second part of the thesis focuses on PEFC degradation by transient anode conditions originating from start-up/shut-down (SUSD) events. SUSD ASTs were developed to provoke substantial anode degradation while minimizing cathode degradation due to the so-called reverse-current effect. Advanced characterization methods to investigate the significant degradation of the HOR and OER catalyst were developed, uncovering a structural change of the anode catalyst layer and a substantial decline in reversal tolerance when PEFCs were exposed to SUSD events. In addition, characterization methods are presented to investigate the crossover of IrO2 based OER catalyst to the cathode catalyst layer, promoted by transient anode conditions. Iridium crossover was found to significantly impact the determination of electrochemical surface area (ECSA) for platinum-based catalysts by state-of-the-art characterization methods. The introduction of a voltage clipping step during SUSD events was showing to have a minor impact on anode degradation and iridium crossover. Using energy dispersive X-ray spectroscopy (EDX) and SEM imaging, the electrochemical degradation characteristics were substantiated.