Imaging Reactivity of the Pt–Ionomer Interface in Fuel-Cell Catalyst Layers

The interface between perfluorosulfonic acid polymer electrolytes and Pt nanoparticles in a model hydrogen fuel-cell catalyst layer was analyzed using high-resolution scanning transmission X-ray microscopy. After electrochemical cycling, thin films of damaged ionomer, enriched in C═C bonds, are observed near the Pt surface. This local degradation of the Pt–ionomer interface triggered by electrochemical oxidation contrasts with that from peroxide radical exposure. Peroxide damage is catalyzed by Pt and leads instead to thin films containing carboxylic acids. Direct mapping of the degradation at these nanoscale interfaces inside hydrogen fuel-cell catalyst layers exposes a previously unknown electrochemical reactivity of perfluorinated ionomers.