▎ 摘　　要

Developing highly efficient, durable, low-cost electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is very important for electrochemical devices. Herein, a series of homonuclear and heteronuclear transition metals and nitrogen codoped graphene (M,N-6-Gra, FeMN6-Gra, M = Ru, Rh, Pd, Ag) serving as bifunctional electrocatalysts for ORR and OER are investigated by means of the density functional method. For ORR, FeRhN6-Gra has the best performance. The formation of *OH is the rate-determining step with a low energy barrier of 0.40 eV (compared with 0.80 eV for pure Pt) and a small overpotential of 0.26 V. For OER, Pd2N6-Gra has the best catalytic activity with a small overpotential of 0.30 V, followed by 0.43 V for FeRhN6-Gra. Therefore, FeRhN6-Gra shows excellent bifunctional catalytic activity toward both ORR and OER. Further experimental study of this compound should be rewarding. Interestingly, the ratio between electronegativity and the atomic radius of transition metals is found to be a good descriptor for evaluating the catalytic activity.