Sub-1 nm Cu2O Nanosheets for the Electrochemical CO2 Reduction and Valence State–Activity Relationship

The copper-based (Cu-based) electrocatalytic materials effectively carry out the electrocatalytic carbon dioxide reduction reaction (CO₂RR) toward C₂₊ products, yet the superiority and stability of the oxidation state of Cu are still worth studying. Herein, we designed and prepared three Cu-based electrocatalysts with different oxidation states to study the valence state-activity relationship. Among these Cu-based electrocatalysts, the Cu₂O nanosheets with thickness of only 0.9 nm show an extremely high C₂₊ Faraday efficiency (FE_C2+) of ∼81%, and the FE_C2+ has an increase of 37% compared with the traditional CuO_x phase. The ultrathin two-dimensional (2D) nanosheet structure with abundant oxygen vacancies can stabilize the oxidation state of Cu to improve the selectivity for C₂₊ products in CO₂RR. In situ Raman spectroscopy and density functional theory calculations demonstrate that the rich Cu⁺ in the ultrathin 2D Cu₂O nanosheets is the most suitable oxidation state for *CO adsorption and coverage on the catalyst surface, which promotes the C-C coupling reaction in CO₂RR. This work provides an excellent catalyst for CO₂RR toward C₂₊ products.