Two-Dimensional Nanoscale-Thick Single-Layer Copper Catalysts for Selective Electrochemical CO ₂ Reduction to Methane

The electrochemical reduction of CO2 (CO2RR) into hydrocarbons has emerged as a promising strategy for sustainable energy conversion and carbon neutrality. Among the various possible products, CH4 is of particular interest because it can be directly used in existing infrastructure, unlike H2 and NH3. However, despite intensive studies, the intrinsic role of Cu catalyst size in CH4 selectivity has remained unclear due to the complex interplay between particle size, morphology, exposed crystal planes, oxidation states, and defect structures. In this work, we aimed to clarify the size effect in the CH4-selective CO2RR by preparing single-layer Cu nanosheets (SLCs). By electrostatically adsorbing Cu2+ ions onto titanate nanosheets followed by electrochemical reduction, SLCs with an atomic thickness were synthesized, and their lateral dimensions were tuned by adjusting Cu2+ loading. Structural characterization confirmed that SLCs maintained single atomic layers of 0.3 nm at Cu2+ loadings of 1.6–7.6 wt %, whereas larger loadings resulted in multilayer Cu (MLCs) at loadings of 7.6–11.7 wt %. CH4 Faradaic efficiency showed a volcano-type dependence on SLC diameter, peaking at 39%. In contrast, H2 selectivity followed an inverse volcano trend. Mechanistic analysis revealed that CH4 selectivity correlates with the fraction of in-plane Cu atoms, whereas H2 selectivity correlates with the edge Cu atoms. These findings establish the intrinsic size–selectivity relationship in two-dimensional Cu catalysts and provide a strategy for designing oxide-supported single-layer catalysts to optimize CH4 production in the CO2RR.