Facile Synthesis of Cu_xS Electrocatalysts for CO₂ Conversion into Formate and Study of Relations Between Cu and S with the Selectivity

Abstract The conversion of CO 2 into formate (HCOO − ), a techno‐economically feasible product, can be achieved using earth‐abundant Cu x S electrocatalysts, but questions remain regarding how catalyst structure, composition, and reaction environment influence product selectivity. A novel synthesis method based on electrodeposition of Cu foam and its subsequent sulfidation via immersion in sulfur saturated toluene solution resulted in Cu x S foams. Catalytic activity studies found that HCOO − selectivity is dependent on electrochemical activation at higher overpotentials. To understand the effects of activation, determine the active forms of the catalysts, and identify the role of sulfur, the electrodes are carefully characterized as well as gaseous and sulfur dissolved in electrolyte. This included study of the effects of intentional addition of solution sulfur species, identification of the sulfur loss, determination of the electrode composition and relating sulfur speciation to observed product selectivity. It is found that residual sulfur stabilizes Cu + during electrolysis at potentials favoring HCOO − production, in contrast to pristine Cu that undergoes complete reduction and shows poor HCOO − selectivity. Sulfur in both the catalyst and dissolved in electrolyte are of dynamic nature, and surface residues of SO 4 2− species are identified in all activated catalysts which correspond with enhanced HCOO − production.