Platinum electrocatalysis: Novel insights into the dissolution mechanism and oxygen reduction reaction

Formation of hydrogen peroxide and oxygenated radical species are the leading cause of chemical degradation observed in polymer electrolyte membranes (PEM) in polymer electrolyte fuel cells. Recent experimental studies have shown that Pt nano-deposits in the PEM, which originate from Pt dissolution in the catalyst layer, play an important role in radical-initiated membrane degradation. Surface reactions at Pt particles facilitate the formation of reactive oxygen species. The net effect of Pt surface processes on membrane degradation depends on the local equilibrium conditions around the Pt nano-deposits, specifically, their equivalent local electrode potential. In this thesis, we first present a multi-step theoretical approach, validated by a collaborative experimental study, to understand the impact of environmental conditions around the Pt nanodeposits on membrane chemical degradation. In the first step, we developed a physical analytical model for the potential distribution at Pt nanodeposits in the PEM. Given the local potential, we identify the surface adsorption state of Pt. Thereafter, density functional theory (DFT) was used to investigate the influence of the Pt adsorption state on the mechanism of oxygen reduction reaction (ORR), particularly the formation of hydrogen peroxide and hydroxyl radical as the two important reactive oxygen species for membrane degradation. In a separate work, we employed DFT to study the atomistic mechanism for interfacial place-exchange between surface Pt atom and chemisorbed oxygen at oxidized Pt (111)-water interfaces. Understanding the criteria for Pt oxide growth is a crucial step to comprehend the mechanisms of Pt dissolution during electrochemical processes.