Platinum group metal (PGM)-free catalysts for oxygen reduction reaction are essential for affordable fuel cells.

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Anomalous small-angle X-ray scattering (ASAXS) is a powerful technique for probing catalyst particle sizes on the nanometer scale making it ideal for monitoring particle size distributions (PSDs) of Pt-based cathode electrocatalysts for polymer electrolyte fuel cells. Potential cycling experiments were performed on Pt nano-particles in both aqueous and membrane-electrode assembly (MEA) environments. PSDs were resolved through in-situ ASAXS and used to calculate geometric surface areas (GSA) and mass losses. A comparison of the losses in the GSA and mass of Pt along with changes in the evolutions of the PSDs over 900 potential cycles elucidated differences between degradation mechanisms in the two different environments. The MEA environment induced greater GSA and mass losses while maintaining similar growth in the mean particle size as compared to the aqueous environment. Results show changes in PSDs consistent with dominant mechanisms of Pt dissolution/coarsening in the aqueous environment and dissolution in the MEA environment.