Metal-Free Oxygen Evolution and Oxygen Reduction Reaction Bifunctional Electrocatalyst in Alkaline Media: From Mechanisms to Structure–Catalytic Activity Relationship

We have systematically investigated a metal-free bifunctional electrocatalyst of heteroatom-doped carbon nitride (XY-C3N4, where X and Y indicate the dopant and doping site on C3N4, respectively) for oxygen evolution and oxygen reduction reactions (OER and ORR), considering the possible reaction pathways based on the Eley–Rideal (ER) mechanism as well as the doping effects on electrocatalytic activity. In this work, the relative stability of O* and OOH* intermediates was a key factor in determining the ORR pathway; accordingly, ORR follows a two-step reaction pathway governed by O* rather than a four-step reaction pathway governed by OOH*. In addition, we found that P and S codoped C3N4 shows superior OER/ORR activity with synergistic geometric and electronic effects, which coordinatively increase unsaturated sp3-C via structural deformation and improve electrical conductance by modulating the electronic structure with extra electrons from dopants. In particular, PCSC-C3N4 (C3N4 with P and S codoped at the carbon site) shows better bifunctional performance of OER/ORR with competitive overpotentials at 0.42 and 0.27 V, respectively, compared to conventional Pt and RuO2 catalysts. Therefore, our theoretical investigations suggest that PCSC-C3N4 is the most promising bifunctional OER/ORR electrocatalyst with synergistic effects in several electrochemical devices.