Simulations of the Oxidation and Degradation of Platinum Electrocatalysts

Improved understanding of the fundamental processes leading to degradation of platinum nanoparticle electrocatalysts is essential to the continued advancement of their catalytic activity and stability. To this end, the oxidation of platinum nanoparticles is simulated using a ReaxFF reactive force field within a grand-canonical Monte Carlo scheme. 2-4 nm cuboctahedral particles serve as model systems, for which electrochemical potential-dependent phase diagrams are constructed from the thermodynamically most stable oxide structures, including solvation and thermochemical contributions. Calculations in this study suggest that surface oxide structures should become thermodynamically stable at voltages around 0.80-0.85 V versus standard hydrogen electrode, which corresponds to typical fuel cell operating conditions. The potential presence of a surface oxide during catalysis is usually not accounted for in theoretical studies of Pt electrocatalysts. Beyond 1.1 V, fragmentation of the catalyst particles into [Pt₆ O₈ ]^4- clusters is observed. Density functional theory calculations confirm that [Pt₆ O₈ ]^4- is indeed stable and hydrophilic. These results suggest that the formation of [Pt₆ O₈ ]^4- may play an important role in platinum catalyst degradation as well as the electromotoric transport of Pt^2+/4+ ions in fuel cells.