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

Abstract

Low-cost copper (Cu)-based electrocatalysts have been widely established with the special capability of generating C 2+ products from the CO 2 reduction reaction (CO 2 RR). However, efficient formate production has been rarely achieved due to the instant reduction of most reported Cu-based catalysts upon CO 2 RR, and the derived metallic Cu compromises the C 1 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 CO 2 RR, 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 Cu 2 S, favorably promoting the reaction kinetics. Accordingly, the RbCu 7 S 4 as a representative catalyst achieves a Faradaic efficiency of 90.4 ± 1.3% for formate at only −0.7 V versus reversible hydrogen electrode (V RHE ), 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 CO 2 RR.