Sulfur-incorporated copper catalysts for enhanced ethylene production in membrane electrode assembly CO₂ electrolysis

Copper sulfides typically favor formate production in CO₂ electrolysis, yet their behavior under membrane electrode assembly (MEA) conditions remains largely uncharted. In this study, Cu_xS/Cu electrodes with tunable morphologies, including spheres, films, polygons, and cubes, were synthesized, and the surface sulfur content was systematically tuned through electrodeposition followed by controlled sulfidation. Performance evaluation in MEA systems reveals that sulfidation markedly enhances C₂H₄ selectivity and partial current density (PCD). For spherical Cu electrodes, sulfidation to a S/Cu surface ratio of 0.33 resulted in a 1.76-fold increase in C₂H₄ PCD, from 58.2 to 102.5 mA/cm² at 3.2 V. In situ Raman spectroscopy suggests that sulfur coordination stabilizes bridge-bonded CO and promotes the formation of the CH₂CHO intermediate, which is consistent with the enhanced C₂H₄ activity. Density functional theory calculations reveal that Cu-rich sulfide surfaces, including Cu₂S and Cu₃₁S₁₆, stabilize *OCCO intermediates, suggesting a thermodynamically favorable C-C coupling reaction pathway through synergistic Cu-S coordination. These findings identify surface sulfur incorporation as a structural motif that actively promotes CO dimerization and enhances C₂H₄ selectivity under MEA conditions.