Grain refining enables mixed Cu+/Cu0 states for CO2 electroreduction to C2+ products at high current density

The oxidation status of Cu-based materials have been proved to be essential to the catalytical performances of electrochemical CO2 reduction. The coexistence of Cu+ and Cu0 species is generally considered as the origin of superior catalytic performance, yet the Cu+ moieties are subject to reduction under negative potentials especially at high current density. In this work, we report a grain refining approach to tune the oxidation states of Cu-based catalysts by modulating the electron transfer during electrochemical CO2 reduction reaction (CO2RR) process when the in-situ electroreduction of Cu+ species occurs. Cu2O nanospheres with abundant grain boundaries exhibited lower electron conductivity compared with Cu2O nanospheres with less grain boundaries, which can hinder the complete reduction of Cu2O and maintain Cu+ species under high current densities. As a result, the multi-grain Cu2O showed a maximum FE of ∼79% for C2+ products at a high current density of 800 mA cm−2, notably surpassing the later. Experimental and theoretical analyses indicated that mixed Cu+/Cu0 states of multi-grain Cu2O during reaction, favoring the C-C coupling process towards C2+ products. This work demonstrates the feasibility to tune the real valence state of catalytic sites under operational conditions by nanostructure engineering.