A theoretical investigation on the OER and ORR activity of graphene-based TM-N and TM-NX (X = B, C, O, P) single atom catalysts by density functional theory calculations.

Single-atom catalysts (SACs) have shown promising activity in electrocatalysis, such as CO reduction (CORR), the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Transition-metal-embedded N-doped graphene (M-N-C) with TM-N active sites (where TM represents a transition metal) is a representative SAC family that has attracted the most attention in both experimental and theoretical studies. However, TM-N type M-N-C has received less attention than TM-N, although some experimental studies have reported its excellent activity in OER and CORR. To fully explore the electrocatalytic activity of TM-N type M-N-C, in this work we systematically investigate the OER and ORR activity of TM-N (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) and TM-NX (X = B, C, O, P) using density functional theory (DFT) calculation. We examine the formation energies, OER/ORR free energy diagrams, overpotentials, charge density, d-band center and electronic structure of each candidate. Our computational screening shows that CuN is a promising bifunctional electrocatalyst for both OER and ORR with low overpotentials of 0.31 V (OER) and 0.44 V (ORR), while CrN and CuNB are predicted to be promising OER catalysts, with overpotentials of 0.26 V and 0.50 V, respectively. A volcano plot derived from the scaling relationships suggests that substituting one nitrogen atom with a hetero atom significantly affects the potential-limiting step in OER/ORR, leading to worse activity in most cases. Density of states and d-band center analyses indicate that the change in OER/ORR activity is strongly correlated with the binding strength of *OH, which is dominated by the location of the d-band center. Our simulation results introduce a comprehensive insight into the activity of the TM-N site in TM-N-C, which could benefit the further development of graphene-based SACs for fuel cells and renewable energy applications.