Investigation of Degradation Heterogeneities in PEMFC Stack Aged under Reformate Coupling In Situ Diagnosis, Post-Mortem Ex Situ Analyses and Multi-Physic Simulations

Abstract

Proton Exchange Membrane Fuel cells (PEMFCs) durability of stacks operated under reformate is investigated with a special focus on the heterogeneity of aging. During an aging test at constant load, the local performances were investigated in situ using a segmented circuit board and a specific CO poisoning diagnostic tool based on the transition from pure hydrogen to reformate containing carbon monoxide. The heterogeneities analyses are supported with multi-physic simulations which highlight the heterogeneous CO coverage along the anode, as well as the competition between both electrodes leading to non-monotonous current density profiles. At the end of life, electrochemical and transmission electron microscopy analyses were performed on three characteristic zones (air inlet/H2 outlet, middle and air outlet/H2 inlet) of the Membrane Electrode Assembly (MEA). These experimental investigations put in evidence that the cathode outlet aged more severely than the cathode inlet, while more CO tolerance was lost at the anode outlet. The degradation by the electrochemical Ostwald ripening mechanism of the Pt3Co nanoparticles at the cathode outlet is suspected to pollute the ionomer, leading to the observed accelerating performance losses. Finally, optimized MEAs have been designed to mitigate the suspected degradation mechanisms, and tested at stack level demonstrating a clear durability improvement.