Instability of Pt/C electrocatalysts in proton exchange membrane fuel cells: A mechanistic investigation

Equilibrium concentrations of dissolved platinum species from a Pt/C electrocatalyst sample in 0.5 M H ₂SO ₄ at 80°C were found to increase with applied potential from 0.9 to 1.1 V vs reversible hydrogen electrode. In addition, platinum surface area loss for a short-stack of proton exchange membrane fuel cells (PEMFCs) operated at open-circuit voltage (-0.95 V) was shown to be higher than another operated under load (∼0.75 V). Both findings suggest that the formation of soluble platinum species (such as Pt ²⁺) plays an important role in platinum surface loss in PEMFC electrodes. As accelerated platinum surface area loss in the cathode (from 63 to 23 m ²g _Pt in ∼100 h) was observed upon potential cycling, a cycled membrane electrode assembly (MEA) cathode was examined in detail by incidence angle X-ray diffraction and transmission electron microscopy (TEM) to reveal processes responsible for observed platinum loss. In this study, TEM data and analyses of Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes: (i) Ostwald ripening on carbon at the nanometer scale, which is responsible for platinum particle coarsening from ∼3 to ∼6 nm on carbon, and (ii) migration of soluble platinum species in the ionomer phase at the micrometer scale, chemical reduction of these species by crossover H ₂ molecules, and precipitation of platinum particles in the cathode ionomer phase, which reduces the weight of platinum on carbon. It was estimated that each process contributed to ∼50% of the overall platinum area loss of the potential cycled electrode.