Rational Design of Sulfur-Doped Copper Catalysts for the Selective Electroreduction of Carbon Dioxide to Formate

The selective electroreduction of CO₂ to formate (or formic acid) is of great interest in the field of renewable-energy utilization. In this work, we designed a sulfur-doped Cu₂ O-derived Cu catalyst and showed that the presence of sulfur can tune the selectivity of Cu significantly from the production of various CO₂ reduction products to almost exclusively formate. Sulfur is doped into the Cu catalysts by dipping the Cu substrates into ammonium polysulfide solutions. Catalyst films with the highest sulfur content of 2.7 at % showed the largest formate current density (jHCOO- ) of -13.9 mA cm^-2 at -0.9 V versus the reversible hydrogen electrode (RHE), which is approximately 46 times larger than that previously reported for Cu(110) surfaces. At -0.8 V versus RHE, the faradaic efficiency of formate was maintained at approximately 75 % for 12 h of continuous electrolysis. Through the analysis of the evolution of the jHCOO- and jH2 values with the sulfur content, we show that sulfur doping increases formate production and suppresses the hydrogen evolution reaction. Ag-S and Cu-Se catalysts did not exhibit any significant enhancement towards the reduction of CO₂ to formate. This demonstrates clearly that sulfur and copper acted synergistically to promote the selective formation of formate. A hypothesis about the role of sulfur is proposed and discussed.