Understanding How Atomic Sulfur Controls the Selectivity of the Electroreduction of CO₂ to Formic Acid on Metallic Cu Surfaces

Electrochemical conversion of CO₂ with renewable energy has recently gained increasing attention for its potential to offset its dramatic exponential increase. Sulfur (S)-modified Cu electrodes were reported to selectively reduce CO₂ into formic acid and fully quench the original CO production on clean Cu surfaces, yet the actual mechanism is still unsolved. To further understand this critical issue, herein, an integrated structure-selectivity investigation on the role of sulfur upon CO₂ electroreduction was carried out. DFT calculations in combination with relevant experimental observations from the literature were utilized to identify the existing forms of sulfur and their impact of the reaction selectivity of CO₂ to HCOOH under realistic conditions. Interestingly, most residual sulfur atoms were found to exist in relatively unstable forms during the reaction, which plays a dominant role in altering the reaction pathway by enhancing CO adsorption. In turn, these strongly adsorbed CO stabilize the unstable sulfur, and a synergetic effect between CO and S is proposed. These findings unravel a groundbreaking comprehensive understanding for the rational development of highly selective CO₂ reduction electrocatalysts.