Acid Etching Strategy: Optimizing Bifunctional Activities of Metal/Nitrogen-doped Carbon Catalysts for Efficient Rechargeable Zn-Air Batteries

Transition metal-embedded heteroatom carbon composites are regarded as an important branch of bifunctional catalysts for rechargeable Zn-air batteries. The inevitable transition metal particles on the carbon skeleton may affect the availability of the metal-heteroatom-carbon catalytic site. Herein, we propose an acid treatment strategy to remove the bare transition metal particles, thus regulating the electrochemical surface area. The OER activities are highly related to the electrochemical surface area for the catalysts with different acid treatment times. In addition, there exists an optimal acid treatment time to achieve the highest ORR and OER activities with the ΔE value of 0.75 V. Given the superior bifunctional activities after acid treatment, we further assemble the rechargeable Zn-air batteries with the optimal catalyst, which achieves a peak power density of 364 mW cm^-2 and long cycling life of 500 h at 10 mA cm^-2 . This work affords an efficient strategy to enhance the ORR/OER activities and may guide the design of transition metal/heteroatom carbon composites.