NiCo₂O₄ nanoarray on CNT sponge: a bifunctional oxygen electrode material for rechargeable Zn-air batteries

Ni- and Co-based materials have of late gained prominence over conventional noble metal-based ones as catalysts for energy devices. Here, a high performance catalyst which can facilitate both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was developed by anchoring a NiCo₂O₄ nanowire array on a carbon nanotube sponge (NCS). The three-dimensional morphology of NCS ensured efficient transport of the reactants and products on the catalyst surface, thereby improving the activity of the material. The prepared catalyst showed remarkable OER activity, requiring an overpotential of 280 mV, which is comparable to that of noble-metal catalysts. In addition to the noteworthy OER performance, the catalyst performed well with respect to the ORR. The total oxygen electrode activity overpotential of the catalyst was found to be 0.83 V, which is lower than that of commercial electrodes such as Pt/C and RuO₂. A rechargeable Zn-air battery constructed with NCS had an open circuit voltage of 1.42 V, a maximum power density of 160 mW cm^-2, and an energy density of 706 W h kg^-1. NCS exhibited bifunctional electrocatalytic activity and high stability for both the OER and ORR, proving to be a good replacement for noble metal electrodes in rechargeable metal-air batteries.