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 WOx 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 WOx|solution interface via rapid H-diffusion in the bulk of the WOx. Consequently, the rate of HOR is principally controlled by the rate of H-spillover at the Pt|WOx boundary. Our results establish MEPC membrane electrodes as a platform for spatially separating the critical bond activation and charge transfer steps of electrocatalysis.