On the Role of Sulfur for the Selective Electrochemical Reduction of CO 2 to Formate on CuS x Catalysts

The efficient electroreduction of CO 2 has 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 CuS x catalyst that can reduce CO 2 to formate with a Faradaic efficiency (FE HCOO – ) of 75% and geometric current density ( j HCOO – ) of −9.0 mA/cm 2 at −0.9 V versus the reversible hydrogen electrode. At this potential, the formation of other CO 2 reduction 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.