Sulfur Doping to Cu₃N Electrocatalyst Enhanced CO₂ Reduction to CH₄

Although metal sulfides are promising catalysts for the electrochemical carbon dioxide (CO₂) reduction reaction, repulsion between the lone pair of oxygen electrons of CO₂ and the electronic clouds of surface sulfur atoms is considered to impede the reaction. Nitrogen introduction is one potential solution to this problem; however, the optimal ratio of sulfur to nitrogen has yet to be determined, and the enhanced reaction products that have been reported to date are limited to carbon monoxide (CO) and formic acid. In this work, copper sulfide (Cu₂S) and copper nitride (Cu₃N) composites with varying sulfur-to-nitrogen ratios are synthesized with the objective of enhancing the catalytic activity of the CO₂ reduction reaction to methane (CH₄). 4.20 mol% sulfur-containing Cu₃N exhibits a Faradaic efficiency for CH₄ production that is higher than that of all examined catalysts including bare Cu₃N and Cu₂S. The results of in situ Fourier-transform infrared spectroscopy suggests that increased electron donation from the catalyst to the *CO intermediate by the introduction of sulfur into Cu₃N shifts the selectivity of the reaction pathway from CO to CH₄ production. Taken together, the present findings demonstrate that sulfide-nitride composite structures can function as effective CO₂ reduction electrocatalysts to generate a variety of valuable products.