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-N(2)G electrocatalysts, as well as ORR mechanism and activity. The most stable configuration is the Fe-N(2)S1G because of forming a strong bond structure of Fe-S. In addition, the structures of Fe-N(2)S(3)G and Fe-N(2)S(4)G also exhibit the higher stability compared to the undoped Fe-N(2)G. According to the distinct charge difference on the surface, the O-contained intermediates would like to adsorb on the active sites of Fe-N-2 complex active sites. The binding strength of OH on these different catalysts follows the increasing order of Fe-N(2)S(4)G < Fe-N(2)S(3)G < FeN(2)G < Fe-N(2)S1G < Fe-N(2)S5G < Fe-N-2. S2G < Fe-N(2)S6G < Fe-N(2)S7G, implying the opposite order of the catalytic activity. The calculations of the free energy diagrams show that all elementary reaction steps on Fe-N(2)S4G, Fe-N(2)S3G, FeN(2)G and Fe-N(2)S1G are downhill. Besides, the rate determining step (RDS) for these catalysts (excluded Fe-N(2)S4G) 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-N(2)S4G also exhibits the outstanding performance for H2O2 reduction. In general, since the higher stability and working potential for ORR, Fe-N(2)S4G is predicted to be the prior candidate site for ORR among S-doped FeN(2)G catalysts. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.