The physical and chemical state of proton exchange membrane fuel cell ͑PEMFC͒ electrocatalysts ͑Pt/C͒ were investigated after 529 h of operation under fuel cell relevant conditions ͑333 K, 0.12 W cm −2 ͒ and 123 h of rest time under inert atmosphere ͑N 2 ͒. Upon aging, pronounced corrosion of the cathode electrocatalyst ͑carbon-supported platinum nanoparticles: Pt/C͒ was evidenced by field-emission gun scanning electron microscopy ͑FEG-SEM͒, high-resolution transmission electron microscopy ͑HRTEM͒, and electrochemical techniques. Carbon corrosion was witnessed by the decrease of the cathode thickness ͑−60%͒ and by the presence of nonsupported Pt particles inside the electrode. At the cathode, the corrosion of Pt nanoparticles produces Pt ͑*͒ z+ ions which diffuse in the ionomer phase or in solution ͓͑*͒ stands for ionic species present in the ionomer phase or in solution͔. These ions are highly mobile inside the membrane electrode assembly ͑MEA͒ and may cross over from the cathode to the anode through the PEM. The driving force for that is the electro-osmotic drag ͑if Pt ͑*͒ z+ ions combine with anions and carry a negative charge͒ and the chemical diffusion ͑concentration gradient of oxidized platinum species͒. Consequently, Pt ͑*͒ z+ ions were detected by ultraviolet ͑UV͒ spectroscopy in the PEM. Due to the high mobility of Pt ͑*͒ z+ ions, FEG-SEM and HRTEM analysis of the cross sections of MEAs revealed a pronounced change of Pt distribution after operation. Size distributions of both anode and cathode electrocatalysts evidenced an increase of the mean particle size, tailing toward large particle sizes and particle agglomeration. Nonspherical Pt nanoparticles were detected inside the PEM, the size and shape distribution of which strongly depend on the distance from the cathode. We believe Pt ͑*͒ z+ ions are reduced chemically inside the membrane by H 2 crossing over the PEM and both chemically and electrochemically at the anode/cathode.

We studied proton exchange membrane fuel cell membrane electrode assemblies ͑MEAs͒ degradation after fuel-cell operation. Anode and cathode pronounced degradation was monitored by chemical ͓energy dispersive spectrometry ͑EDS͒, X-ray photoelectron spectroscopy ͑XPS͔͒, physical ͓scanning electron microscopy ͑SEM͒, transmission electron microscopy͔, and electrochemical ͑ultramicroelectrode with cavity͒ techniques. Aged MEAs underwent severe redistribution of most elements ͑Pt, C, F͒, 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 Pt z+ species and transport in the ionomer or the proton exchange membrane play important roles. The Pt z+ dissolution/transport is likely favored by activators/ligands ͑For SO x -containing species͒ originating from the alteration of the polymers contained in the MEA. From SEM observations, the source of Pt z+ species is the cathode, while EDS and XPS show some SO x -and F-containing species origin from the anode. Local chemical analyses ͑SEM-EDS and XPS͒ 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 SO x -containing species could also act as counter ions during Pt z+ transport within the MEA. Pt corrosion/ redistribution is associated with the decrease of Pt-active area as revealed by CO ad -stripping voltammograms.

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