Dynamics of Surface Exchange Reactions Between Au and Pt for HER and HOR

Cyclic voltammetric analysis of the Pt-on-Au system for hydrogen evolution and oxidation reactions (HER/HOR) indicates that dynamic surface exchange reactions occur between Pt and Au. HER/HOR activities depend on the dominant surface species present, which is controllable by the potential applied to the system. Bulk Au is not very active for HER/HOR; however, when Pt is deposited onto the Au surface, the system becomes active. The Pt-on-Au system can subsequently be deactivated by cycling to potentials cathodic of the OH-adsorption and Pt-dissolution potentials (~+1.18 V vs normal hydrogen electrode, NHE, at pH 0). Following deactivation, the system can be reactivated by cycling above this potential, giving an activation potential of ~+1.0 V vs NHE. This deactivation/reactivation can be cycled repeatedly and occurs for various forms of the Pt-on-Au system. This potential-dependent surface exchange reaction is attributed to the lower surface energy of Au relative to Pt causing Au to migrate to the surface. When the system is deactivated, Au is present at the surface. However, Pt migrates back to the surface at higher positive potentials, where PtOx/PtOHx is formed, leading to adsorbate-induced surface segregation. The surface compositions were verified by X-ray photoelectron spectroscopy. Implications for electrocatalyst materials development for polymer electrolyte membrane fuel cells are discussed.