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

Abstract

We have systematically investigated a metal-free bifunctional electrocatalyst of heteroatom-doped carbon nitride (X-Y-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 sp(3)-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.