Three-Dimensional Phosphorus-Doped Graphitic-C₃N₄ Self-Assembly with NH₂-Functionalized Carbon Composite Materials for Enhanced Oxygen Reduction Reaction

Abstract

Oxygen reduction reaction (ORR) is the major reaction that occurs at the cathodes of fuel cells and metal-air batteries. Development of inexpensive, active, and durable heteroatom doped carbon-based ORR catalysts can lead to significant cost reduction of these electrochemical energy devices, which therefore has recently attracted enormous research attentions. This work reports a three-dimensional porous composite (P-g-C₃N₄@NH₂-CB) for the highly efficient ORR catalyst. P-g-C₃N₄@NH₂-CB was prepared by mixing phosphorus-doped graphitic carbon nitride nanosheets (P-g-C₃N₄ NSs) with NH₂-functionalized carbon black (NH₂-CB) via a novel self-assembly approach. The NH₂-CB was rationally chosen as the spacer that enables the self-assembled with the P-g-C₃N₄ NSs driven by the electrostatic interaction. The intercalation of NH₂-CB induces the transformation of 2-D P-g-C₃N₄ NSs into a 3-D composites material of higher surface area, thereby exposing more ORR active sites. The P-g-C₃N₄@NH₂-CB exhibited a remarkable ORR activity with an electron transfer number of 3.83 and Tafel slope of 89 mV dec^-1 in alkaline electrolyte, which is comparable to the ORR performance on Pt/Vulcan XC-72. It is found that the incorporated P atoms as well as employing NH₂-CB spacer not only reduces the overpotential of ORR, but also enhances the ORR activity of carbon nitride-based materials, owing to the synergistic effect between P and N in tri-s-triazine rings of carbon nitrides and the optimum interaction between the oppositely charged P-g-C₃N₄ and NH₂-CB.