Coarsening of Pt Nanoparticles in Proton Exchange Membrane Fuel Cells upon Potential Cycling

Two different Pt/C samples (Pt/Vulcan and Pt/C-high-surface- area) were subject to potential cycling between 0.6V and 1.0V vs. reversible hydrogen electrode, where significant platinum area loss was found. Both cycled MEA cathode samples were examined and compared in detail by glancing angle X-ray powder Diffraction and transmission electron microscopy (TEM) to reveal processes responsible for observed platinum loss. TEM data and analyses of pristine and cycled Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes as reported recently1: i) Ostwald ripening on carbon at the nanometer- scale; and ii) diffusion of soluble platinum species in the ionomer phase at the micrometer-scale, chemical reduction of these species by cross-over H2 molecules and precipitation of platinum particles in the cathode ionomer phase.