Synthesis of Mesoporous NiCo₂O₄ By Spray Pyrolysis Method for Air Electrode of Zinc-Air Rechargeable Batteries

Abstract

Recently, rechargeable zinc-air batteries have been attracted attention as promising energy storage devices because of their theoretical large capacity, low cost, and safety. However zinc-air battery still has limits in cycle file and energy efficiency. In particular, increase in air electrode performance is strongly required. Therefore, variety of materials, mainly perovskite oxide, have been studied for air electrode, however, activity and stability are still not high enough. In this study, nickel cobalt spinel oxide was synthesized by spray pyrolysis method in order to increase in surface area and formation of mesoporous structure. In our previous study, it was found that introduction of mesoporous structure in LaCoO 3 electrode with hard template method is effective for increasing cycle performance and decrease in overpotential of air electrode for zinc-air battery. NiCo 2 O 4 based spinel oxide is active to oxygen reduction (ORR) and oxygen evolution reaction (OER), however, surface area is still insufficient by the preparation of conventional hydrothermal method. Therefore, it is expected that introduction of mesoporous structure into NiCo 2 O 4 spinel oxide will increase ORR/OER activity and stability. On spray pyrolysis method, precursor solvent turned into nano-sized droplet by ultrasonic wave vaporizer, and the droplets are transferred to the tubular electric furnaces by air as carrier gas. The solvent evaporates from the droplets resulting in the formation of mesopore and after calcination, the fine powder is gathered in filters. Nitrate solution was used as precursor, and air at 3 L/min was used as oxidant and carrier gas in this work. Air electrode performance was measured by using gas diffusion layer and PTFE and graphitic carbon was mixed with NiCo 2 O 4 spinel for electrode. 8M KOH aqueous solution at 313K was used for electrode and constant current of 20 mA/cm 2 was applied by battery charge discharge equipment. Single phase of NiCo 2 O 4 spinel phase was obtained from XRD measurement and the SEM image of the sample was spherical with many meso size pore. The estimated BET surface area was 76m 2 /g and average pore diameter was 2.12 nm. Therefore, it is considered that NiCo 2 O 4 oxide with mesoporous structure was successfully prepared. ORR/OER activity of mesoporous NiCo 2 O 4 was measured. It was found that NiCo 2 O 4 shows reasonable activity to ORR and OER reaction. On the other hand, partial substitution of Ni with Fe is effective for increasing ORR/OER activity. Two different compositions of iron added NiCo 2 O 4 (Ni 0.8 Fe 0.2 Co 2 O 4 , Ni 0.65 Fe 0.35 Co 2 O 4 ) are successfully prepared. At beginning of charge/discharge cycle, spray pyrolysis synthesized NiCo 2 O 4 shows slightly larger overpotential for ORR and OER comparing with that of NiCo 2 O 4 prepared with hydrothermal method. However, after some cycles, ORR potentials are increased and OER potentials are decreased, so mesoporous NiCo 2 O 4 shows superior electrochemical performance which is almost the same with that of hydrothermal NiCo 2 O 4 . Moreover, cycle stability is much higher than that of hydrothermal NiCo 2 O 4 and ORR/OER more than 500 cycles was stably sustained and so introduction of mesoporous structure is effective for increasing cycle stability of air electrode. Effects of calcination temperature during splay pyrolysis were also studied and it was found that calcination of NiCo 2 O 4 based spinel oxide at 673 K is the most active and stable to ORR/OER. Therefore, this study reveals that Fe doped NiCo 2 O 4 with mesoporous structure is highly active and stable air electrode catalyst for Zn-air battery.