The efficient electroreduction of CO2 has received significant attention, as it is one of the crucial means to develop a closed-loop carbon cycle. Here, we describe the mechanistic workings of an electrochemically-deposited CuSx catalyst that can reduce CO2 to formate with an excellent Faradaic efficiency (FEHCOO-) of 75% and current density (jHCOO-) of -9.0 mA/cm2 at -0.9 V vs. the reversible hydrogen electrode. At this potential, the formation of other CO2 reduction products such as hydrocarbons and CO were significantly suppressed (total FE < 4%). The formate intermediate (HCOO*) was identified by operando Raman spectroscopy. A combination of electrochemical and materials characterization techniques revealed that the high selectivity towards 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* towards formate is favored.
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This work is supported by an academic research fund (R-143-000-683-112) from the Ministry of Education, Singapore. Y.D., Y.H. and D.R. acknowledged Ph.D. research scholarships from the Ministry of Education, Singapore. P.H. acknowledges the financial support from the National Nanotechnology Center (NANOTEC), Thailand Research Fund (TRF), and computing resources from National e-Science Infrastructure Consortium (Thailand) and NANOTEC. Z.W.S. acknowledges the support of the Institute of Materials Research and Engineering, A*STAR (IMRE/17-1D0314) and the Singapore National Research Foundation (NRF-NRFF2017-04).