Introduction of Carbon Shells on Copper(I) Oxide Nanocrystals Inducing Selective CO₂ Electroreduction to Methane

Large-scale electrochemical CO₂ reduction reaction (CO₂RR) for methane production is crucial for achieving a carbon-neutral economy. However, practical methane production faces challenges from increased C–C coupling and the hydrogen evolution reaction. Here we introduced carbon shells on cuprous oxide nanocubes to preserve the copper oxidation states under reduction potential. As carbon shell thickness increased, methane production significantly improved. Optimizing the shell thickness to 50 nm resulted in a Faradaic efficiency of 50.2 ± 3.9% at a methane partial current density of −201 ± 16 mA cm⁻² in alkaline electrolytes. Operando X-ray absorption confirmed that the carbon shells prevent the reduction of Cu₂O nanocubes under the negative bias and stabilize Cu(I) species. In situ Raman studies observed a remarkable peak assigning Cu hydroxide species. The carbon shells seem to impede the transport of hydroxide ions, leading to a high local pH and stabilizing Cu hydroxide species. Density functional theory calculations confirmed the role of surface-adsorbed *OH molecules to suppress *CO dimerization and the Barder charge of Cu atoms during *OH adsorption which remained under +1. This work presents a simple strategy to control the major products of CO₂RR and reveals the role of hydroxyl groups on the Cu₂O surface.