Development of Catalyst with Optimum Calcination Temperature for the Application of Zinc-Air Battery
Authors: Kato Yoshiki , Rahman Md Mijanur , Saikawa Masato , Awata Rina , Ono Yuji , Batnyagt Garavdorj , Delgertsetsega Byambasuren , Takeguchi Tatsuya 
Published: July 2021

Zinc-air batteries (ZABs) have been considered as the next-generation electrochemical energy-storage devices due to the high theoretical energy density. It has a theoretical specific energy density of 1086 W h kg−1, which is 2.5 times higher than that of Li-ion battery [1]. Moreover, the appropriate operating voltage of ZAB is ca. 1.65 V, which is not too high to induce the decomposition of water in electrolyte. However, the actual voltage drop may occur (less than 1.40 V) depending on the applied current density. Takeguchi et al. [2] reported that the origin of this drop is mainly due to the high overpotential. As a result of increased overpotential, the performance of charge/discharge reaction and stability decrease. In order to improve the kinetics reaction and stability of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), non-precious metals as electrocatalysts need to be used more efficiently. Perovskite oxides are one such promising family of multifunctional materials, which possess catalytic activity toward the OER/ORR, especially LaSrFeCoO is considered as cost-effective catalyst for application in rechargeable ZABs [2]. However, its electrocatalytic activities and stability remain poorly understood. Herein, we prepared LaSr3Fe1.5Co1.5O10-δ (LSFCO) catalyst using different calcination temperatures due to investigate the effects of catalytic performance. The effects of calcination temperature on structural properties and its catalytic performance were investigated using different characterization methods, such as SEM, EDX, XRD, charge-discharge and so on.
For preparation of LSFCO perovskite oxide as cathode catalyst, complex polymerization synthesis method was used. High-purity La(NO3)3 × 6H2O (99%), Sr(NO3)2 (98%), Fe(NO3)3 × 9H2O (99%), Co(NO3)2 × 6H2O (98%), C6H8O7 (99%), and distilled water were used as stirring materials. The mixture was kept in the oil bath at 120 ºC with the constant stirring at 300 rpm till the gels were completed. Then, the gels were dried at 200 ºC for 1 h. Finally, the LSFCO powders were obtained 1200, 1300, 1350, and 1400 ºC for 2 h each. Besides, electrolyte solution was prepared by mixing of 26.4 g of KOH (85%), 3.0 g of ZnO (99%) and approximately 100 mL of distilled water.
Fig. 1 shows the XRD patterns of LSFCO perovskite catalysts synthesized with different calcination temperatures ranging from 1200-1400 ºC. As shown in Fig.1, almost similar diffraction peaks are observed for all of the samples. All of the samples exhibit the obvious peaks, and no other unknown impurity peak was detected. It can be ascribed to the well-crystallized perovskite structure. To elucidate the temperature effects on the surface morphology of as-prepared LSFCO perovskite catalysts, SEM observation was performed. Fig. 2 shows the SEM image of LSFCO. As shown in Fig (a)-(d), the average grain size of LSFCO increases with increasing calcination temperature level. The estimated size of the particles are ca. 1.5 µm, 5.0 µm, 7.0 µm, and 12 µm for the LSFCO perovskite catalysts obtained at 1200 ºC, 1300 ºC, 1350 ºC and 1400 ºC, respectively. The chemical composition of the as-prepared catalyst is examined by EDX, which is consistent with the reported literatures. Fig. 3 shows the galvanostatic charge-discharge profiles of ZABs prepared with LSFCO perovskite oxide as cathode catalyst and polished zinc foil as an anode at 1.0 mA/cm2 under O2. As shown in Fig. 3, the LSFCO perovskite oxide catalysts prepared at calcination temperatures of 1300 ºC, 1350 ºC and 1400 ºC exhibit the charge-discharge voltage gaps of ca. 1.50, 1.70 and 1.50 V, respectively. Low charge-discharge voltage gaps, i.e. the low overvoltage of the device can be attributed to its efficient bifunctional O2 activation. On the other hand, the higher energy efficiency of ca. 27.2% was achieved in the case of LSFCO perovskite oxide catalyst prepared at 1300 ºC, while it is 11.8% and 22.3% in the case of 1350 ºC and 1400 ºC, respectively. Therefore, it can be concluded that the LSFCO catalyst prepared at 1300 ºC is the most promising bifunctional catalyst for both OER and ORR because of its overall superior catalytic activity, high energy efficiency and stability compared with LSFCO catalysts prepared at 1350 ºC and 1400 ºC, respectively. Finally, this approach with simple synthetic route is highly suitable for commercialization of ZABs operated in an ambient air environment.
Acknowledgement:
The authors would like to thank the Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Japan for financial support of this work.
References:

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Figure 1