Optimization of La, Sr, Fe, Co-Doped Perovskite Oxides Particle for Bifunctional Oxygen Electrocatalysis in Alkaline Zinc-Air Battery

Abstract

Among the energy technologies developed, rechargeable alkaline zinc-air batteries (ZABs) have been extensively studied due to their high theoretical specific energy density (1086 Wh kg−1), low cost and environmentally friendly operation.[1] Moreover, ZABs are compact and lightweight because they use a lighter air cathode. However, the ZABs are still in their early stages of development, as there are still critical issues to be addressed. For example, the ZABs operating with two-electrodes must overcome the limited life, high cost and lack of bi-functional catalysts. In order to make the available rechargeable ZABs, it is urgently necessary to develop bifunctional catalysts based on metallic oxide, which can catalyze satisfactorily both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Among the different metal oxide-based bifunctional electrocatalysts, LaSr3Fe3−xMxO10−δ (M = Co, Ni, Mn) perovskite oxides have received particular attention as potential bifunctional catalysts in both the ORR and OER in metal-air batteries.[2] Although it is known that perovskite oxides have a high intrinsic activity, unoptimized particles size and shape from traditional synthesis pathways limit their practical use due to their low mass activity and low electrochemical surface area (ECSA). Thus, particle size control of perovskite oxides is necessary for higher ORR and OER activities.Here, we report the synthesis of LaSr3Fe1.5Co1.5O10−δ (LSFCO) perovskite catalysts, where the size and growth of LSFCO particles have been investigated to influence different calcination temperature in order to study the effects of electrocatalytic performance for alkaline ZABs. The effects of particle size on structural properties of the prepared samples were investigated using XRD, SEM, EDX and TEM. The electrocatalytic property was investigated by CV. For preparation of LSFCO as cathode catalyst, complex polymerization synthesis method was used. High-purity metal nitrates, citric acid (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 calcined at 1100, 1200, 1300, and 1400 ºC for 2 h each to obtain a different size of particle. For electrochemical measurements, electrolyte solution was prepared by mixing of 26.4 g of KOH (85%) and 100 mL of distilled water.Fig. 1 shows the XRD patterns of LSFCO synthesized with different calcination temperatures ranging from 1100 to 1400 ºC. As shown in Fig.1, almost similar diffraction peaks are observed for all of the samples. All samples exhibit obvious peaks, and no other unknown impurity peaks were detected. This can be attributed to the well-crystallized perovskite structure. The SEM observation was conducted to elucidate the effects of temperature on the surface morphology of the prepared catalysts. Fig. 2 shows the SEM image of LSFCO obtained at a calcination temperature of 1100, 1200, 1300 and 1400 ºC. The result indicated that the average particle size increases with the increase in calcination temperature. To investigate the chemical composition of the prepared catalysts, EDX measurement was conducted. Table Ⅰ presents the detailed physical properties obtained from the SEM and EDX analysis. The crystalline structure of LSFCO prepared at 1400 ºC was further investigated by TEM, as shown in Fig. 3. The d-spacing estimated from the TEM image is nearly close to the theoretical values determined by XRD calculations at the corresponding angles with Bragg's law (e.g. 0.273 nm at the phase of (110)) [3]. The ECSA of all samples was evaluated by measuring the double-layer capacitance (C dl) via CV in O2-saturated 4 M KOH at 1600 rpm in a typical three-electrode cell as shown in Fig. 4. Table Ⅰ also summaries the ECSA results, wherein the C dl values for all samples obtained from the current response to different scan rates. Maximum capacitance of the catalyst prepared at 1400 ºC was 0.489 μF cm−2 compared to 1300 ºC (0.305 μF cm−2) and 1200 ºC (0.167 μF cm−2). Thus, it can be concluded that the higher crystallized particle will improve OER/ORR electrocatalytic activity, which is crucial for high performance alkaline ZABs. In addition, this approach is simple and effective in enhancing catalytic activities in an ambient air environment. Acknowledgement: The authors would like to thank the ESICB, Kyoto University, Japan for financial support of this work.