Optimization of cobalt/nitrogen embedded carbon nanotubes as an efficient bifunctional oxygen electrode for rechargeable zinc–air batteries

Abstract

The design of efficient, durable and affordable catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is indispensable in regenerative energy conversion and storage systems, such as water splitting, fuel cells and rechargeable metal–air batteries. Here we present a high-performance bifunctional catalyst with cobalt and nitrogen embedded carbon nanotubes (CNCNs), which exhibits high electrocatalytic activity towards the ORR and OER. For comparison, iron and nickel embedded carbon nanotubes (FeNCNs and NiNCNs) are also investigated to reveal the electrochemical performance upon transition metal doping. In the half-cell testing, FeNCN-44 and CNCN-100 show better electrochemical activities towards the ORR, while the compositionally optimized CNCN-44 outperforms other prepared catalysts during the OER including commercial RuO₂ and Pt/C. The CNCN-44 exhibits a low oxidation overpotential of 0.38 V and stability with negligible deactivation over extended operation, which make it the best catalyst in overall bifunctional performance. We also test the suitability and durability of CNCN-44 as the oxygen electrode for rechargeable Zn–air batteries. The homemade battery exhibits a high open-circuit potential of 1.45 V and stable charge–discharge durability over 12 hours in 6 M KOH electrolyte at 10 mA cm⁻², which make our compositionally optimized CNCN-44 a promising candidate as a cathode material for rechargeable zinc–air batteries.