Understanding of Anion Effect on Iron Redox Behavior and Oxygen Reduction Activity of Fe-N-C Electrocatalysts

Abstract

Fe-N-C catalysts are the most promising platinum group metal (PGM) free electrocatalysts for oxygen reduction reaction (ORR). Significant progress has been made in understanding the active site structure and activity and improving the ORR performance. It is generally accepted that the Fe-N-C sites are associated with FeNx centers embedded in a defective nitrogen-doped graphene. As reported in literature, both pyrrolic N and pyridinic N can support Fe single site; however, it is difficult to distinguish between the two. Furthermore, experimental characterization, such as in-situ X-ray absorption spectroscopy (XAS) and Mössbaruer, has elucidated the redox behavior of Fe2+ to Fe3+ during the reaction. Interestingly, in different acidic environments, the ORR activity exhibits differently. For instance, in comparison between H2SO4 and HClO4 solutions at the same pH value, the Fe redox happens at lower potential in H2SO4; however, the right shift of half-wave potential (E1/2) of ORR in H2SO4, compared with HClO4, indicates higher ORR activity. This indicates that the electrolyte systems have a significant impact on ORR activity. To further understand the active site structure and their ORR activity, density functional theory (DFT) calculations utilizing explicit solvation effect are carried out. XANES simulation using FDMNES suggests that as prepared catalysts could consist of Fe single atom sites supported by both pyrrolic and pyridinic N atoms; however, the pyrrolic species are not as stable as pyridinic species in solution. As presented by DFT simulation, the anions in solutions can interact with FeN4 site differently. SO4 2- binds strongly with the FeN4 center, while HSO4- and ClO4- interact rather weakly. Furthermore, the redox potential and limiting potential are evaluated via mechanistic study. By focusing on more stable Fe center supported by pyridinic N, the results suggest that the transformation of Fe2+ to Fe3+ happens at 0.52 V in H2SO4, which is lower than that in HClO4, 0.69V. More interestingly, the limiting potential (activity) of ORR in H2SO4 is higher than that in HClO4 (0.94 vs 0.68 V). DFT simulation indicated that the anions in electrolyte can impact the Fe redox behavior and ORR performance, which agrees well with the experimental investigation. This work also provides significant guidance to further improve ORR activity of Fe-N-C catalysts.