The role of nitrogen and sulfur dual coordination of cobalt in Co-N4−xSx/C single atom catalysts in the oxygen reduction reaction

Abstract

Single-atom catalysts (SACs) have been considered as a potential candidate for fuel cell application due to the fact that they exhibit good oxygen reduction reaction (ORR) activity. In this study, the ORR catalytic activities of Co-N₄/C, Co-N₃S/C, Co-N₂S₂/C, Co-NS₃/C, and Co-S₄/C catalysts are studied using density functional theory (DFT) calculations based on the BEEF-vdW functional. The reduction of *OH into H₂O is found to be the potential determining step of the ORR on Co-N₄/C catalysts. This implies that the activity of the Co-N₄/C system could be improved by weakening the binding energy of the *OH intermediate. The DFT results revealed that the adsorption energy of the *OH intermediate bound on Co-N₃S/C, Co-N₂S₂/C, Co-NS₃/C, and Co-S₄/C is weaker than that on the Co-N₄/C catalyst. The results show that the overpotentials of Co-N₄/C, Co-N₃S/C, Co-N₂S₂/C, Co-N₃S/C, and Co-S₄/C catalysts are 0.57, 0.37, 0.41, 0.40, and 0.47 V, respectively. Thus, the Co-N₃S/C catalyst revealed a lower overpotential pathway. A slightly smaller number of charges are transferred from the Co atom in Co-N₃S/C to ORR intermediates as compared to Co-N₄/C and the d-band center of the Co atom changes from −0.71 eV (Co-N₄/C) to −0.91 eV (Co-N₃S/C). This can explain the weaker adsorption energy of *OH on the Co-N₃S/C catalyst. Therefore, Co-N₃S/C is a promising non-precious single-atom catalyst for efficient ORR activity in acidic solutions in fuel cells.