Improved Oxygen Evolution and Oxygen Reduction Behavior of NiCo2O4: Revisiting the Use of Mesocarbon Microbeads

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital to the operation of rechargeable zinc-air batteries. This work demonstrates that both the intrinsic activity and the electrochemically active surface area (ECSA) of the oxide-carbon composites are key factors determining the overall catalytic activity. Here, we revisit the possible use of mesocarbon microbeads (MCMBs) as a support to improve the bifunctional activity of NiCo2O4 (NCO). Pre-treatments including oxidation and expansion of MCMB have been applied to obtain two types of modified MCMBs. The abundant functional groups on the oxidized MCMB (OMCMB) provide strong interaction between oxide and carbon, although a relatively low electrical conductivity is obtained. Compared to OMCMB, the expanded MCMB (EMCMB) with less functional groups shows a much larger surface area and presumably higher conductivity. The three-electrode tests reveal that the composite supported by OMCMB-3h (i.e., 3-h oxidation treatment) has a moderate intrinsic activity of the OER and the highest ECSA, leading to the enhanced bifunctional activities. The charge-discharge test of a rechargeable zinc-air battery at various current densities shows the cycling stability of NCO/OMCMB-3h at 150 mA cm(-2), which also can be cycled at 15 mA cm(-2) for 60 h without obvious decay.

Automated summary

This study improves the bifunctional activity of NiCo2O4 on mesocarbon microbeads (MCMBs) as a support through pre-treatment methods. Oxidized MCMB provides a strong interaction between oxide and carbon, while expanded MCMB has a larger surface area and presumably higher conductivity. The results demonstrate that OMCMB treated for 3 hours has moderate oxygen evolution reaction intrinsic activity and the highest electrochemically active surface area, resulting in enhanced overall bifunctional activity.