Mechanochemical Doping of Boron & Nitrogen on Graphite as the Metal-Free Bifunctional Electrocatalysts for Zinc-Air Battery

Abstract

The doping of non-metallic heteroatoms on graphene breaks the isoelectronic nature to catalytically active materials towards various electrochemical reactions. The boron and nitrogen-doped (B&N) few-layer graphite is synthesised by stepwise doping of heteroatoms on commercial graphite using Mechanochemistry. The B&N-doped graphite acts as an efficient oxygen reduction electrocatalyst with a potential window of 0.735 V. Further investigations reveal that boron-sites act as a potential active site for oxygen evolution reaction. In contrast, the synergistic effect of the heteroatoms plays a crucial role in the oxygen reduction reaction. The bifunctional activity of the metal-free B&N-doped graphite is tested for zinc-air battery application. The catalysts-coated carbon air electrode shows a power density of 136 mW cm⁻² and a specific capacity of 795 mA h g_Zn⁻¹. Besides, the stability of catalysts is examined using the galvanostatic charge-discharge cycles, which are stable for more than 500 cycles (20 minutes per cycle) or 167 h with the highest zinc utilisation ratio (0.97). The solid-state zinc-air battery is demonstrated to show the material's real-time application. This study opens new avenues to explore more heteroatom(s)-doped carbon catalysts for their bifunctional activity towards metal-air battery applications.