Systematic exploration of N,C coordination effects on the ORR performance of Mn-Nx doped graphene catalysts based on DFT calculations

A single-atom TM-N-x (TM = Fe, Co, Mn, etc.) embedded graphene matrix is known for its excellent activity and durability in oxygen reduction reaction (ORR) catalysis. Among them, Mn-N-4 sites have been theoretically proved to undergo a complete 4-electron pathway with low ORR overpotentials and low activation barriers in O-2 dissociation. However, in reality there still remain significant activity gaps between such Mn-N-4 based catalysts (such as MnPc and MnP) and Fe-N-4 or Pt-group metal catalysts. The inferior ORR performance of MnPc and MnP could be attributed to the strong binding ability of Mn that causes great difficulties in removing the ORR products from the surface sites. On this basis, 17 types of Mn-N-x models containing various three-, four- and five-coordination groups were established. Systematic density functional theory (DFT) calculations were performed to investigate the N,C coordination effects on their corresponding ORR activities. Scaling relations were found among the binding strengths of key ORR intermediates, which could be modulated by the N doping level among different coordination groups. A volcano plot for ORR overpotentials (eta(SHE)) as a function of *OH adsorption free energy (Delta G(*OH)) was further established. The 3D five-coordination sites exhibit much higher ORR activity due to the great decrease in strong binding abilities compared with 2D three- or four-coordination sites. Particularly, (Cyan)Mn-N-4/D is positioned near the apex of the volcano plot with an eta(SHE) of 0.33 V even lower than that of Pt(111) (0.34 V). Furthermore, the electron withdrawing/donating mechanisms among Mn, N, C, and O were investigated and related to the binding abilities of different coordination groups. Electronic structure calculations indicate that the binding abilities of Mn-N-x well correlate with the sigma-type anti-bonding components between Mn-3d and O-2p states near the Fermi energy level.