Degradation heterogeneities induced by repetitive start/stop events in proton exchange membrane fuel cell: Inlet vs. outlet and channel vs. land
Résumé
This paper investigates aging heterogeneities that set up during \PEMFC\ operation in start-up and shut down conditions. The spatially-resolved analyses were based on in situ measurements of local current densities, electrochemical surface area of Pt at the cathode in a segmented cell and overall \CO2\ generation. In complement, ex situ physicochemical analyses were performed after the \PEMFC\ testing, using scanning and transmission electron microscopy as well as focused ion beam scanning electron microscopy, to probe the micro and nano-scale of the cathode catalyst layer. In the present cell configuration (counter-flow mode, impact of the shut-down events negligible versus that of start-up), two kinds of aging heterogeneities are witnessed. Firstly, the performances loss at the air outlet/H2 inlet is less important than at the air inlet/H2 outlet; fuel starvation events are prevalent in this latter region; the resulting local loss of performances are linked to the distribution of the internal currents along the cell, and to larger physicochemical changes of the cathode catalyst layer in the air outlet/H2 inlet region. The faradic part of the internal currents (generated during the fuel starvation events) was identified to not only come from the electrooxidation of the carbon support of the cathode catalytic layer (CL): internal currents may also originate from Pt dissolution, carbon corrosion in the gas diffusion layer (GDL) and/or water oxidation. Secondly, the degradations at the air inlet/H2 outlet also depend on the position during the aging, either under a channel or a land. Fuel starvation events are more pronounced in land regions, due to slower removal of oxygen trapped under a land (the source of the fuel starvation) in the anode compartment during a start-up. Finally, it is wise pointing out that the particular degradation mechanism witnessed in this study would not have been observed if the \MEA\ had been aged following a standardized stress test (potential cycling under N2-atmosphere); therefore, one may question whether the \DOE\ standardized stress-test procedure is relevant to mimic real fuel cell operation.