Directed heterogeneous hydride transfer enables selective CO2 reduction using a Janus palladium membrane electrode.

Electrochemical CO2 reduction is typically constrained by proton-coupled electron transfer, which intrinsically intertwines CO2 conversion with the competing H2 evolution reaction. Here we introduce a Janus palladium membrane electrode that circumvents proton-coupled electron transfer by enabling selective CO2 reduction through heterogeneous hydride transfer. The membrane spatially separates hydrogen generation from CO2 reduction and independently polarizes the CO2-facing interface, converting permeated hydrogen atoms into Pd-H species with tunable hydricity. This design sustains directional hydride flux from water to CO2, enabling highly selective formate production under mild cathodic polarization. We achieve excellent Faradaic efficiencies and turnover frequencies in aqueous electrolytes, while isotope labelling experiments confirm hydride transfer as the dominant reaction pathway. Continuous hydrogen delivery stabilizes a hydrogen-rich surface of the electrode, suppressing CO formation and enabling stable operation. This electrode's success for CO2 reduction in both aqueous and fully aprotic electrolytes offers a general route to access heterogeneous hydride reactivity beyond proton-mediated electrocatalysis.