Sm(OH)₃-Modified CuO_x-Promoted Electroreduction of CO₂-to-C₂₊ Products through a Proton-Regulated Mechanism

The electrocatalytic reduction of CO₂ to multicarbon (C₂₊) products can be boosted through regulating the proton supply. This promotes *CO hydrogenation and subsequent C-C coupling through enabling the activation and dissociation of H₂O. However, this process has often been overlooked, as it may also cause the unfavorable hydrogen evolution reaction. Herein, we present a proton-regulation strategy by introducing Sm(OH)₃ on CuO_x as an active site for water dissociation to supply protons, thereby enabling efficient conversion of CO₂ into C₂₊ compounds. The constructed 2.4%-Sm(OH)₃/CuO_x exhibits a faradaic efficiency as high as 84.3% for C₂₊ production, with an absolute partial current density of up to 627.5 mA cm^-2 at -0.77 V versus the reversible hydrogen electrode. Combined experimental results and density functional theory calculations show that the incorporation of appropriate amounts of Sm(OH)₃ enhances the interaction between the interfacial H₂O and the catalyst, thereby promoting H₂O dissociation to generate protons. This process facilitates the hydrogenation of *CO intermediates to *CHO and subsequent C-C coupling via *CHO-CHO, thus enhancing the formation of C₂₊ products. This work provides insight into the tuning of interfacial H₂O dissociation for CO₂ conversion.