Membrane and active layer degradation upon PEMFC steady-state operation - I. Platinum dissolution and redistribution within the MEA

We studied proton exchange membrane fuel cell membrane electrode assemblies (MEAs) degradation after fuel-cell operation. Anode and cathode pronounced degradation was monitored by chemical [energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS)], physical [scanning electron microscopy (SEM), transmission electron microscopy], and electrochemical (ultramicroelectrode with cavity) techniques. Aged MEAs underwent severe redistribution of most elements (Pt, C, F), coupled to a dramatic change of Pt particles shape, mean particle size and density over the carbon substrate. Among the various scenarios for Pt redistribution, Pt dissolution into Ptz+ species and transport in the ionomer or the proton exchange membrane play important roles. The Ptz+ dissolution/transport is likely favored by activators/ligands (F- or SOx-containing species) originating from the alteration of the polymers contained in the MEA. From SEM observations, the source of Ptz+ species is the cathode, while EDS and XPS show some SOx- and F-containing species origin from the anode. Local chemical analyses (SEM-EDS and XPS) revealed the excess Pt monitored in aged MEAs is associated with F excess. For instrumental limitation concerns, we could not detect the S element, but SOx-containing species could also act as counter ions during Ptz+ transport within the MEA. Pt corrosion/redistribution is associated with the decrease of Pt-active area as revealed by COad-stripping voltammograms.