Revealing the oxygen reduction reaction activity origin of single atoms supported on g-C3N4 monolayers: a first-principles study

Herein, the potential of single transition metal atoms (TM, from Ti to Au) supported on g-C3N4 (TM/g-C3N4) for the oxygen reduction reaction (ORR) was investigated by first-principles calculations. It was demonstrated that the TM atoms can remain stable in the cavity of g-C3N4 and interact with the substrate via charge transfer from the TM atoms to g-C3N4. Among all the TM/g-C3N4 samples, Pd/g-C3N4 stands out with a low overpotential of 0.46 V, showing good performance for ORR; thus, it has great potential to replace the noble Pt catalyst. The ORR activity of TM/g-C3N4 is a function of Delta E-*OH (an energy descriptor). Furthermore, the d-band center and ICOHP (electronic structure descriptors) can quantitatively describe the variation trend of Delta E-*OH in addition to Bader charge analysis (a charge transfer descriptor). Considering the number of d orbital electrons and the electronegativity of TM, phi (an intrinsic descriptor) can be applied to predict and reveal the origin of the ORR activity. A bridge from intrinsic characteristics to electronic structures, to charge transfer, to electronic structures and then to adsorption energy has been established, which is conducive to better reveal the ORR activity origin and provide guidance for designing effective ORR electrocatalysts.