Mixed Electron-Proton Conductors Enable Spatial Separation of Bond Activation and Charge Transfer in Electrocatalysis

Electrochemical energy conversion requires electrodes that can simultaneously facilitate substrate bond activation and electron-proton charge transfer. Traditional electrodes co-localize both functions to a single solid|liquid interface even though each process is typically favored in a disparate reaction environment. Herein, we establish a strategy for spatially separating bond activation and charge transfer by exploiting mixed electron-proton conduction (MEPC) in an oxide membrane. Specifically, we interpose a MEPC WO membrane between a Pt catalyst and aqueous electrolyte and show that this composite electrode is active for the hydrogen oxidation reaction (HOR). Consistent with H₂ activation occurring at the gas|solid interface, the composite electrode displays HOR current densities over 8-fold larger than the diffusion-limited rate of HOR catalysis at a singular Pt|solution interface. The segregation of bond activation and charge separation steps also confers excellent tolerance to poisons and impurities introduced to the electrolyte. Mechanistic studies establish that H₂ activation at the Pt|gas interface is coupled to the electron-proton charge separation at the WO|solution interface via rapid H-diffusion in the bulk of the WO. Consequently, the rate of HOR is principally controlled by the rate of H-spillover at the Pt|WO boundary. Our results establish MEPC membrane electrodes as a platform for spatially separating the critical bond activation and charge transfer steps of electrocatalysis.