On the Role of Sulfur for the Selective Electrochemical Reduction of CO2to Formate on CuSxCatalysts

The efficient electroreduction of CO2has received significant attention as it is one of the crucial means to develop a closed-loop anthropogenic carbon cycle. Here, we describe the mechanistic workings of an electrochemically deposited CuSxcatalyst that can reduce CO2to formate with a Faradaic efficiency (FEHCOO–) of 75% and geometric current density (jHCOO–) of −9.0 mA/cm2at −0.9 V versus the reversible hydrogen electrode. At this potential, the formation of other CO2reduction products such as hydrocarbons and CO was notably suppressed (total FE < 4%). The formate intermediate (HCOO*) was identified by operando Raman spectroscopy with isotopic labeling. A combination of electrochemical and materials characterization techniques revealed that the high selectivity toward formate production can be attributed to the effect of S dopants on the Cu catalyst, rather than surface morphology. Density functional theory calculations showed that the presence of sulfur weakens the HCOO* and *COOH adsorption energies, such that the formation of *COOH toward CO is suppressed, while the formation of HCOO* toward formate is favored.