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

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 Delta E = 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.