Tuning the oxygen electrocatalytic performance of metal-doped graphitic carbon nitride for the development of zinc-air battery

Abstract

Efficient and durable non-precious cathode catalysts are needed at this hour for the development of fuel cells and metal-air batteries. The instability of one of the well-studied non-precious catalysts, Fe–N–C, in acidic electrolytes and its inferior bifunctional electrocatalytic activity in alkaline electrolytes, shifts the attention towards other electrocatalysts based on Ni and Co. Herein, we demonstrate the synthesis of nitrogen and transition metal (M=Co, Ni) co-doped mesoporous carbon (Co/Ni–N–mC) catalysts for bifunctional oxygen electrocatalysis. The synthetic approach involves the thermal annealing-induced transformation of the graphitic carbon nitride (g–C₃N₄) to nitrogen-doped graphitic mesoporous carbon (N–mC). The Co–N–mC catalyst has superior bifunctional oxygen electrocatalytic activity. It promotes the 4-electron pathway for the reduction of oxygen to water and is highly durable in alkaline electrolyte. The bifunctional activity is evaluated in terms of the potential gap (ΔE). The small ΔE for Co–N–mC makes it suitable for metal-air batteries. The rechargeable zinc-air battery is fabricated with Co–N–mC and it delivers a specific capacity of 718 mAh g⁻¹Zn and a power density of 122.2 mW/cm² with long-time charge-discharge cycling stability for 100 h. The synergistic effect between metal nanoparticles and nitrogen-doped carbon matrix, as well as the post-synthetic surface engineering-induced morphological changes, account for the enhanced activity.