Extremely positive effects of a mesoporous structure in La0.6Ca0.4CoO3 on the cycle life of the Zn–air battery (>1000 cycles).

Highly porous and uniform sphere shape Fe powder with a nano-porous structure can be prepared successfully with a spray pyrolysis method and applied for solid oxide Fe-air rechargeable battery, which is consisted of solid oxide reversible cells using an H 2 O/H 2 redox mediator. The prepared Fe powder modified with both Cr 2 O 3 and Ce 0.6 Mn 0.3 Fe 0.1 O 2 shows a high redox reaction rate as well as cycle stability by uniform distribution. A discharge capacity of 1100 mAh⋅g -Fe −1 with almost 100% coulombic efficiency was achieved at initial cycles and that larger than 800 mAh⋅g -Fe −1 was sustained over 20 cycles at 873 K. After the reduction of Fe powder with H 2 flow so-called chemical charge, a discharge capacity of 900 mAh⋅g -Fe −1 was recovered and stably sustained for the following 20 cycles (40 total cycles), and discharge capacity may be sustained further longer cycles at 823 K. The round-trip efficiency for charge and discharge was about 90%, which can be assigned to the high reversibility of Fe redox with nano-porous structure prepared by spray pyrolysis method.

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 LaCoO3 electrode with hard template method is effective for increasing cycle performance and decrease in overpotential of air electrode for zinc-air battery. NiCo2O4 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 NiCo2O4 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 NiCo2O4 spinel for electrode. 8M KOH aqueous solution at 313K was used for electrode and constant current of 20 mA/cm2 was applied by battery charge discharge equipment. Single phase of NiCo2O4 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 76m2/g and average pore diameter was 2.12 nm. Therefore, it is considered that NiCo2O4 oxide with mesoporous structure was successfully prepared. ORR/OER activity of mesoporous NiCo2O4 was measured. It was found that NiCo2O4 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 NiCo2O4 (Ni0.8Fe0.2Co2O4, Ni0.65Fe0.35Co2O4) are successfully prepared. At beginning of charge/discharge cycle, spray pyrolysis synthesized NiCo2O4 shows slightly larger overpotential for ORR and OER comparing with that of NiCo2O4 prepared with hydrothermal method. However, after some cycles, ORR potentials are increased and OER potentials are decreased, so mesoporous NiCo2O4 shows superior electrochemical performance which is almost the same with that of hydrothermal NiCo2O4. Moreover, cycle stability is much higher than that of hydrothermal NiCo2O4 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 NiCo2O4 based spinel oxide at 673 K is the most active and stable to ORR/OER. Therefore, this study reveals that Fe doped NiCo2O4 with mesoporous structure is highly active and stable air electrode catalyst for Zn-air battery.

A rechargeable Zn-air battery is expected as a large capacity battery alternative to the Li ion battery; however, short cycle life is the current issue. In this study, it was found that the addition of 3,3-diaminodipropylamine was effective for preventing the formation of a Zn dendrite in the anode resulting in a long cycle life with more than 300 cycles and a discharge capacity larger than 700 mAh/g (>85% Zn used for discharge) when Ni 0.8 Fe 0.2 Co 2 O 4 and 8 M KOH-H 2 O were used for the air electrode and electrolyte, respectively. The increased cycle stability could be assigned to the change in the solvated structure of Zn 2+ in the electrolyte by the strong interaction between 3,3-diaminodipropylamine and solvated water, which is suggested by Raman and Fourier transform infrared spectroscopic analyses. Ab initio molecular dynamics simulations suggest that the amine is strongly interacted with OH of solvated water molecules and changes the coordination number of Zn 2+ , and as result, the size of the solvated Zn 2+ cluster was increased, resulting in enhanced Zn 2+ mobility in the electrolyte for the uniform deposition of Zn on the anode during charge by increased mobility.

There is a strong demand for rechargeable battery with large energy density and capacity because of increase in performance of mobile equipment such as mobile phone, laptop personal computer (PC) and also electric vehicle. At present, Li ion battery (LIB) have been widely used for such purpose. However, there are several issues such as the insufficient capacity and safety required to solve. Zinc-air batteries have been used commercially as a non rechargeable battery at present, however, recently, there is strong interest for development of rechargeable Zn-air battery because of its theoretical large capacity, low cost, and safety. However zinc-air rechargeable battery still has limits in cycle file and energy efficiency. In particular, increase in air electrode performance is strongly required. In this study, nickel cobalt spinel oxide, NiCo2O4, was studied as reversible air electrode for Zn-air battery. In addition, effects of dopant on oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were studied. NiCo2O4 based spinel oxide is active to ORR and OER, however, surface area is still insufficient by the preparation of conventional hydrothermal method. Therefore, application of splay pyrolysis method for preparation of NiCo2O4 oxide is also studied for increasing ORR/OER activity and stability. NiCo2O4 based oxide was prepared by hydrothermal synthesis method and partial substitution was performed by using Mn, Fe, and Cu. 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. Air electrode performance was measured by using gas diffusion layer, and PTFE and graphitic carbon were mixed with NiCo2O4 spinel for electrode. 8MKOH aqueous solution at 313K was used for electrolyte and a constant current of 20 mA/cm2 was applied by battery charge discharge equipment. Various spinel oxides were successfully prepared by hydrothermal synthesis method from XRD measurement. It was found that ORR and OER activity of MnCo2O4 and NiCo2O4 was reasonably high among the spinel oxides prepared. ORR activity is slightly higher on MnCo2O4, however, NiCo2O4 shows much longer cycle stability of ORR/OER. In addition, elution of Mn was observed in case of MnCo2O4, in contrast, almost no change in composition was observed for NiCo2O4 after cycle measurement. Therefore, NiCo2O4 shows reasonable activity to ORR and OER and high chemical stability. Effects of dopant on NiCo2O4 on ORR and OER activity and doping Cu and Fe is effective for increasing ORR potential and decreasing OER potential. Although the most superior performance of air electrode was observed on Cu doped one, elution of Cu is obviously observed. Therefore, from chemical stability, Fe doped NiCo2O4 is the most active to air electrode catalyst. The optimization of Fe amount in Ni site was also studied and it was found that Ni0.65Fe0.35Co2O4 is the most active and stable air electrode catalyst. On this electrode, overpotential for ORR reaction is around 0.25V at 20mA/cm2, 313 K and almost 500 cycles of ORR/OER which is 1 h ORR and OER each. In addition, introduction of mesoporous structure was achieved by splay pyrolysis method and increase in surface area is also effective for increasing stability to ORR/OER activity. This work is based on results obtained from a project, "Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING2)", JPNP16001, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).