Hybrid Nanostructures of Bimetallic NiCo Nitride/N-Doped Reduced Graphene Oxide as Efficient Bifunctional Electrocatalysts for Rechargeable Zn-Air Batteries

A nonprecious electrocatalyst with high efficiency in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is extremely crucial for the development of high-performing metal-air batteries. In this work, a nonprecious-metal bifunctional catalyst of ultrafine and uniform Ni2.25Co0.75N nanoparticles anchoring on N-doped reduced graphene oxide (denoted as Ni2.25Co0.75N/NrGO) was prepared by the thermal ammonolysis of the corresponding hydroxide/graphene oxide precursor. As a result of the intimate combination of redox-active metal nitrides and electroconductive N-doped reduced graphene oxide (NrGO), the Ni2.25Co0.75N/NrGO hybrid not only exhibited high OER activity but also showed outstanding ORR kinetics and durability, comparable to commercial RuO2 and Pt/C electrocatalysts, respectively. Furthermore, Zn-air batteries assembled by using the as-prepared Ni2.25Co0.75N/NrGO hybrid electrocatalysts yielded a high power density and gravimetric energy density of 193 mW cm(-2) and 864 W h kg(-1), respectively, characteristic with a low charge/discharge voltage gap of 0.72 V and excellent cyclability up to 166 h at 10 mA cm(-2) in an aqueous system. More importantly, the ORR experiment and X-ray photoelectron spectroscopy coupled with density functional theory calculations verified that the electronic transfer from bimetallic NiCo nitride to NrGO may enhance the ability in forming O-2 adsorption and *OOH on NrGO, which is the possible origination of the ORR high activity.