Understanding active sites and mechanism of oxygen reduction reaction on FeN4–doped graphene from DFT study

Abstract

First-principle density functional theory (DFT) calculations are performed to study the active sites in FeN4G electrocatalysts, as well as ORR activity and mechanism. The possible intermediates and transition states existing in the possible reaction paths from Langmuir-Hinschelwood (LH) mechanism are investigated. The results show that the associative pathways of OOH∗ formation is prior to that of O2∗ dissociation. The condition of proton adsorbed on top N sites (T2) is more beneficial to the reduction of O-contained species adsorbed on top Fe site (T1) compared to the conditions of proton adsorbed on top C sites (T3). However, the dissociation of O2∗, OOH∗ and H2O2∗ is more likely to occur on the paired T1-T3 sites, since their lower energy barriers compared to other paired sites. The most favorable four-electron reduction pathway follows the mechanisms: O2∗→ OOH∗→ O∗+H2O→ OH∗+H2O→ 2H2O. The rate determining step for ORR on FeN4G is the reduction of O∗ into OH∗ (barrier, 0.47 eV). The most feasible pathway for ORR is downhill at a high electrode potential (0.76 V vs. SHE at pH = 0) according to the free energy diagram. Compared to the ideal catalyst, the adsorption energy of OOH∗ on FeN4G is much lower in free energy, while those of OH∗ and O∗ are slightly higher. Additionally, the elementary reaction rate for OOH∗→O∗+H2O is much larger than that of OOH∗→H2O2 based on the parameter of activation barrier. Therefore, the formation of H2O2 (l) is unfavorable on FeN4G catalysts.