Sub-1 nm Cu 2 O Nanosheets for the Electrochemical CO 2 Reduction and Valence State–Activity Relationship

The copper-based (Cu-based) electrocatalytic materials effectively carry out the electrocatalytic carbon dioxide reduction reaction (CO 2 RR) toward C 2+ 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 Cu 2 O nanosheets with thickness of only 0.9 nm show an extremely high C 2+ Faraday efficiency (FE C 2+ ) of ∼81%, and the FE C 2+ 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 C 2+ products in CO 2 RR. In situ Raman spectroscopy and density functional theory calculations demonstrate that the rich Cu + in the ultrathin 2D Cu 2 O nanosheets is the most suitable oxidation state for *CO adsorption and coverage on the catalyst surface, which promotes the C–C coupling reaction in CO 2 RR. This work provides an excellent catalyst for CO 2 RR toward C 2+ products.