Theoretical design of bifunctional single-atom catalyst over g-C2N2 for oxygen evolution and reduction reactions

The two-dimensional graphitic carbon nitride (g-CxNy) is a promising support for electrochemistry catalysts owing to its stability and controllable chemical composition. Here, by density functional theory (DFT) a serial of transition metal single-atom catalysts (SAC) supported on the g-C2N2 with vacancy (denoted as TM1/C2N2, TM=Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir and Pt) were studied for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The non-noble Ni and Co single atom exhibited relatively strong binding energies over the defective g-C2N2 support. The d-band center of TM performed a good descriptor for adsorption free energies of intermediates. For OER, the overpotential of non-noble Co1/C2N2 was as low as 0.30 V, comparable to Rh1/C2N2. For ORR, both Co1/C2N2 and Rh1/C2N2 displayed excellent performance with overpotentials of around 0.50 V. The volcano plot revealed that the proper binding strength of O* and OH* on Co1/C2N2 rendered it an excellent bifunctional catalyst for both OER and ORR. Considering the superior performance of Co SAC over g-C2N2 than not only g-C3N4 but also defective graphene, we postulate that there could exist an optimized C/N ratio of carbon-rich g-CxNy support for improving the non-noble Co SAC activity on oxygen electrode.