Modulated Sn oxidation states over Cu2O-derived substrate for selective electrochemical CO2 reduction.

Pursuing a 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 cuprous oxide (Cu2O)-derived substrate. A Sn electrodeposition of 120 s led to a cathode with a formate faradaic efficiency of around 81%, which is over 37% higher than 684 s and Sn foil at -1.1 V vs. RHE. This result highlights the significant role of interfaces between deposited Sn and cuprous-derived substrate in determining the selectivity of CO2R. High-resolution X-ray photoelectron spectra revealed that the residue cuprous species at the Cu/Sn interfaces could stabilize Sn species at 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 Sn(2+) but higher than that of Sn(4+). Our work highlights the significant role of the substrate on the selectivity of the deposited catalyst and provides a new avenue to advance selective electrodes for CO2 electrochemical reduction.