Theoretical Tuning of the Cu/S Ratio on Two-Dimensional CuSx Materials for the CO2 Electrochemical Reduction Reaction

Copper-based materials are among the state-of-the-art catalysts for the electrochemical reduction reaction of carbon dioxide (CO2RR), while the large surface area and tunable properties of two-dimensional (2D) materials make them interesting catalyst candidates. Thus, their combination can open the opportunity to design alternative catalysts for the conversion of CO2 to new products. In this work, we employ density functional theory calculations within the semiempirical D3 van der Waals corrections and the computational hydrogen electrode model to investigate the Cu/S ratio effects in the CO2RR on the 2D CuSx materials (x = 0.5, 1.0, and 1.5). We found that CuS0.5 shows high onset potential (Uonset) values for CO and HCOOH (−1.67 and −1.35 V vs reversible hydrogen electrode, RHE, respectively), while CuS appears as an interesting candidate for CO or HCOOH formation (Uonset of −0.29 V vs RHE for both products), and CuS1.5 presents small Uonset values for the CO2RR toward CO, HCOOH, CH4, and CH3OH (−0.29 V vs RHE for the first two products and −0.38 V vs RHE for the last two). In terms of potential determining steps (PDS), we obtained the CO2 → *COOH as the PDS for CO and HCOOH formation on both CuS and CuS1.5 and the *CO → *COH for CH4/CH3OH formation on CuS1.5. Interestingly, facile *CO desorption was observed as a possible limiting factor for CH4/CH3OH formation on CuS. Thus, our results also reveal that breaking the *CO and *COOH or *CHO/*COH linear scaling relationships or strengthening the CO adsorption are possible routes for catalyst optimization on this class of materials.