Hierarchical Co3O4 nanorods anchored on nitrogen doped reduced graphene oxide: a highly efficient bifunctional electrocatalyst for rechargeable Zn–air batteries

Abstract

Zn–air batteries are amongst the most promising energy storage technologies due to high theoretical energy density for which their practical application is tied to development of low-cost, effective bifunctional catalysts. Herein, a highly efficient bifunctional electrocatalyst was synthesized by hybridizing hierarchical spinel Co₃O₄ nano-rods with N-rGO. A rational design of the nano-hybrid was realized through optimizing catalytic activity of the pure Co₃O₄ NRs followed by their grafting onto N-rGO nanosheets. The optimized hybrid (N-rGO/Co₃O₄ NRs) showed an excellent bifunctional (ORR/OER) catalytic activity with ΔE = E_j=10 − E_1/2 as small as 0.78 V, outperforming state-of-the-art noble-metal catalysts (e.g. PtRuC). Rechargeable Zn–air batteries assembled with a N-rGO/Co₃O₄ NRs hybrid delivered a specific capacity of 875 mA h g_Zn⁻¹ (corresponding to an exceptional energy density of 1115 W h kg_Zn⁻¹), a peak power density of 47 mW cm⁻² and a stable cycling stability compared to Zn–air batteries based on PtRuC commercial catalyst. Outstanding electrochemical performance of the hybrid ORR/OER catalyst is credited to the hierarchical nature of Co₃O₄ NRs, optimized Co³⁺/Co²⁺ ratio, particle agglomeration prevention and superior electrical conductivity resulting from the hybridization with N-rGO. Rational design of atomic-scale interfaces in the spinel metal oxide-carbon hybrid structures demonstrated here provides new insights for the designing and fabrication of high-performance bifunctional non-precious electrocatalysts for rechargeable Zn–air batteries.