Electrochemical Formation of Reactive Oxygen Species at Pt (111)—A Density Functional Theory Study

Abstract

Density functional theory was used to investigate the influence of the Pt adsorption state on the formation of hydrogen peroxide (H2O2) and hydroxyl radical (•OH) as two important reactive oxygen species. We generated the free energy diagrams of reduction sequences involving O2 and H2O2 as reactants and H2O2, •OH, or H2O as products. The Pt (111)–water interface was considered in three adsorption states that are encountered in different regions of the electrode potential: an adsorption state with a monolayer of hydrogen (low electrode potential); a clean surface (intermediate potential); and an adsorption state with 1/3 of a monolayer of oxygen (high potential). Results reveal a strong impact of surface water interactions on the pathways of water oxidation and oxygen reduction reactions. In agreement with experimental results reported in the literature, we found that the oxygen reduction pathway is highly sensitive to the hydrogen coverage. Coverage by one monolayer hydrogen renders the surface highly hydrophobic, thereby suppressing its activity for the oxygen reduction. Therefore, for high hydrogen coverage, the formation of H2O2 by a two-electron pathway becomes a preferred path. We discuss results in the context of radical-initiated chemical degradation of polymer electrolyte membranes in polymer electrolyte fuel cells.