Mechanisms insight into oxygen reduction reaction on sulfur-doped Fe–N2 graphene electrocatalysts

The density functional theory (DFT) calculations were performed to investigate the stability of the S-doped Fe–N2G electrocatalysts, as well as ORR mechanism and activity. The most stable configuration is the Fe–N2S1G because of forming a strong bond structure of Fe–S. In addition, the structures of Fe–N2S3G and Fe–N2S4G also exhibit the higher stability compared to the undoped Fe–N2G. According to the distinct charge difference on the surface, the O-contained intermediates would like to adsorb on the active sites of Fe–N2 complex active sites. The binding strength of OH on these different catalysts follows the increasing order of Fe–N2S4G < Fe–N2S3G < FeN2G < Fe–N2S1G < Fe–N2S5G < Fe–N2S2G < Fe–N2S6G < Fe–N2S7G, implying the opposite order of the catalytic activity. The calculations of the free energy diagrams show that all elementary reaction steps on Fe–N2S4G, Fe–N2S3G, FeN2G and Fe–N2S1G are downhill. Besides, the rate determining step (RDS) for these catalysts (excluded Fe–N2S4G) is the fourth reduction step (OH*+H++e−→H2O+*). The study of the reaction mechanisms predicted that the direct 4-electron reduction process is the favorable ORR pathway, and the alternative reaction pathways containing the formation of OH* + OH* co-adsorbate also process without the formation of H2O2 for these catalysts. Particularly, Fe–N2S4G also exhibits the outstanding performance for H2O2 reduction. In general, since the higher stability and working potential for ORR, Fe–N2S4G is predicted to be the prior candidate site for ORR among S-doped FeN2G catalysts.