First-principles study of the oxygen reduction reaction on the boron-doped C9N4 metal-free catalyst

Replacing precious Pt-based catalysts with cheap metal-free catalysts would make the hydrogen economy viable. Herein, based on first-principles calculations, we have systemically investigated the oxygen reduction reaction (ORR) under the acid environment catalyzed by the B-doped C9N4 monolayer, a nodal-line semimetal. It is found that the doped B atom energetically prefers to replace the inner C atom with an exothermicity of 0.89 eV, and the resulting B-doped C9N4 retains the metallic conductivity beneficial for the charge transfer during the electrocatalytic process. The ORR prefers to adopt efficient four-electron pathways by firstly forming two energetically almost degenerate OOH species (OOHα and OOHβ). For further hydrogenation reactions, the formations of OH + OH and H2O + OH with similar energy barriers of about 1.0 eV are the rate-determining steps for OOHα and OOHβ configurations, respectively. Moreover, for both ORR pathways the potential-determining step is the last electron-proton pair transfer reaction with an overpotential of 0.98 V. Electronic structure analysis shows that due to the small electronegativity of the B atom, O2 can be effectively activated by charge transfer and further orbital hybridization, which initiates the ORR. Present study suggests that the metal-free B-doped C9N4 monolayer has great potential application for ORR.