Alkali Metal Ions Stabilizing Copper(I)–Sulfur Bonds for Efficient Formate Production from Electrochemical CO2 Reduction

Low-cost copper (Cu)-based electrocatalysts have been widely established with the special capability of generating C2+ products from the CO2 reduction reaction (CO2RR). However, efficient formate production has been rarely achieved due to the instant reduction of most reported Cu-based catalysts upon CO2RR, and the derived metallic Cu compromises the C1 selectivity. Herein, we demonstrate that the incorporation of alkali metal ions is intrinsically effective for stabilizing the Cu(I)–S bonds by forming ternary copper sulfides (M–Cu(I)–S, M = Na, K, and Rb). The strengthened Cu–S bonds can be well preserved in M–Cu(I)–S during the CO2RR, contributing to the protonation effect and thus highly efficient production of formate. Moreover, the M–Cu(I)–S catalysts also exhibit enhanced electrical conductivity relative to that of Cu2S, favorably promoting the reaction kinetics. Accordingly, the RbCu7S4 as a representative catalyst achieves a Faradaic efficiency of 90.4 ± 1.3% for formate at only −0.7 V versus reversible hydrogen electrode (VRHE), with a high partial current density of 272.1 mA cm–2 and stable operation over 72 h. This study could provide a different series of Cu-based electrocatalysts for efficient formate production on an industrial scale from the CO2RR.