The role of S in the Co-N-S-C catalysis system towards the ORR for proton exchange membrane fuel cells

Density functional theory (DFT) was used to calculate the thermodynamic free energy changes of the possible basic reaction steps of the oxygen reduction reaction (ORR) on both sides of the active center of the S-doped CoN4-G catalyst. It is found that the adsorption energies on CoN4 active site of these catalysts increase in the range: O < OH < Co-ad OH < OOH < O2 < H2O2 < H2O on these catalysts and the trend of adsorption energy change for all O-contained intermediates is almost consistent. The large thermodynamic driving forces for the OOH reduction to O* (or to 2OH*) and the small thermodynamic driving forces for the H2O2 generation indicate that the four electron pathway is more is more advantageous to proceed than the 2e− pathway. Moreover, the last reduction step (OH* + H+ + e− → H2O + *) with the largest ΔG value for all catalysts is the rate-determining step (RDS) in thermodynamic. In the most cases of S-doped catalysts, it is helpful to increase the activity of CoN4 active center. Moreover, the active center on the dented side has a higher ORR activity than its corresponding raised side for the case that S is adjacent to the CoN4 active center. Dented side of CoN4S1-G exhibits the largest on-set potentials of 0.65 V which is larger than the on-set potential of un-doped S CoN4-G catalyst (0.40 V). The d-band center of Co is much farther from the Fermi level or a larger HOMO-LUMO gap imply that the catalyst is not beneficial to the adsorption of the O-contained species, thus showing a larger on-set potential and more outstanding ORR activity.