Sulfur-Modified Copper Catalysts for the Electrochemical Reduction of Carbon Dioxide to Formate

The electrocatalytic CO2 reduction reaction (eCO2RR) has been gaining increasing attention owing to its potential to contribute to sustainability in our society, although enhanced catalytic performance is a prerequisite for its implementation. Herein, Cu electrocatalysts modified with sulfur proved to selectively produce formate via aqueous eCO2RR and thus to unexpectedly prevent the mechanistic fingerprint of Cu (i.e., the CO path). Initially, sulfur-modified copper catalysts (Cu–S) were prepared by the in situ reductive reconstruction of nano CuS precursors, revealing a positive correlation between particle size and selectivity toward formate. Subsequent studies over targeted submicron Cu–S particles with varying sulfur content demonstrated their evolution under reaction conditions, attaining a similar surface state comprising metallic Cu and sulfide phases, irrespective of the initial structure of the materials. In accordance, the initial sulfur content showed only a very limited influence on the catalytic performance, which remained at approximately 80% Faradaic efficiency toward formate at −0.8 V vs RHE, outperforming all cost-effective, earth-abundant, and nontoxic electrocatalysts reported to date for the production of formate via the aqueous eCO2RR.