Molecular dynamics simulations of an electrified water/Pt(1 1 1) interface using point charge dissociative water

Abstract

Water dissociation and structural diffusion of protons at the electrified water/Pt(111) interface are examined using molecular dynamics simulations. A combination of reactive water with a Pt(111) electrode under constant potential conditions is unique and relevant to fuel cell relevant electrochemistry. We use a modified central force model to describe reactive water and electrode charge dynamics to describe the electrode. We perform control simulations using SPC/E water, a contrast that clarifies when the influence of water ions on interfacial water structure and dynamics and electrochemical properties can no longer be neglected. Both mCF and SPC/E water have structured interfacial water layers regardless of electrode potential, but a reactive model is important when considering water structure away from the surface and interfacial dynamics. As opposed to SPC/E water, an applied potential does not induce preferred water orientation for mCF water in the middle of the electrolyte, despite the fact that interfacial OH and H3O ions cannot completely screen the electrode potential. This occurs because fast exchanges among OH/H2O/H3O relax the electric field constraint in the surface normal direction. mCF water accurately describes the timescales of hydrogen bond vibration and structural diffusion of both hydronium and hydroxyl ions. This simple reactive water model distinguishes structural diffusion between H3O and OH ions, where H3O ion transfer is three times faster than OH ion transfer. The model allows us to determine the influence of applied potential and H3O/OH ions on charge transfer effectiveness near the electrode surface, directly relevant to fuel cell electrochemistry.