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.