Manipulating Cu Nanoparticle Surface Oxidation States Tunes Catalytic Selectivity toward CH₄ or C₂₊ Products in CO₂ Electroreduction

Herein, a facile seed-assisted strategy for preparing Cu nanoparticles (NPs) with polyvinyl pyrrolidone (PVP) capping is presented. Compared to the Cu NPs with deficient PVP protection, the Cu NPs capped with a sufficient amount of PVP remain almost completely as Cu⁰ species. In contrast, the Cu NPs that are considered PVP deficient form an oxide structure in which the inner layer is face-centered cubic Cu and the outer layer is, at least in part, made up of Cu₂O species. Furthermore, to eliminate CO₂ molecule diffusion and simultaneously obtain significant current density (200 mA cm⁻²) for industrial applications, a flow cell configuration is used for carbon dioxide electro reduction reaction (CO₂RR) testing in 0.5 m potassium hydroxide solution. The Cu NPs with zero valence deliver Faradaic efficiencies (FEs) for the CO₂ reduction to CH₄ of over 70%, with a current density exceeding 200 mA cm⁻², outstripping the performances of the majority of the reported CO₂ electrocatalysts. Interestingly, the distribution of products catalyzed by the Cu NPs with +1 valence includes multicarbon products (C₂₊) such as C₂H₄, C₂H₅OH, CH₃COOH, and C₃H₇OH with combined FEs of >80%, with current densities of up to 300 mA cm⁻². The above results unambiguously establish that surface oxidation of Cu species plays a crucial role in the CO₂RR.