Modulated Sn Oxidation States over a Cu₂O-Derived Substrate for Selective Electrochemical CO₂ Reduction

Pursuing high catalytic selectivity is challenging but paramount for an efficient and low-cost CO₂ electrochemical reduction (CO₂R). In this work, we demonstrate a significant correlation between the selectivity of CO₂R to formate and the duration of tin (Sn) electrodeposition over a cuprous oxide (Cu₂O)-derived substrate. A Sn electrodeposition time of 120 s led to a cathode with a formate Faradaic efficiency of around 81% at -1.1 V vs reversible hydrogen electrode (RHE), which was more than 37% higher than those of the Sn foil and the sample treated for 684 s. This result highlights the significant role of the interface between deposited Sn and the cuprous-derived substrate in determining the selectivity of CO₂R. High-resolution X-ray photoelectron spectra revealed that the residual cuprous species at the Cu/Sn interfaces could stabilize Sn species in oxidation states of 2+ and 4+, a mixture of which is essential for a selective formate conversion. Such modulation effects likely arise from the moderate electronegativity of the cuprous species that is lower than that of Sn²⁺ but higher than that of Sn⁴⁺. Our work highlights the significant role of the substrate in the selectivity of the deposited catalyst and provides a new avenue to advance selective electrodes for CO₂ electrochemical reduction.