Herein, we demonstrated that carbon-supported platinum (Pt/C) is a low-cost and high-performance electrocatalyst for polymer electrolyte fuel cells (PEFCs).

Zinc-air batteries (ZABs) have several challenges that hamper large-scale commercialization because of cost, charge/discharge performance of the cathode catalyst. To overcome these problems, perovskite oxides such as LaSr3Fe3O10 (LSFCO) catalysts, were prepared using the complex polymerization synthesis method to investigate the relationship between catalyst activity and calcination temperature. The particle diameter of the prepared LSFCO catalysts and the crystalline structure of the perovskite samples were increased with increasing calcination temperature. In the case of 1400ºC, the highest catalytic activity of the LSFCO catalyst was derived from CV analyses and the constant current charge/discharge measurement for ZABs. It is suggested that the high catalytic activity of the LSFCO catalyst can be obtained by providing a high calcination temperature with larger particles.

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: S. Ren, X. Duan, S. Liang, M. Zhang, H. Zheng, J. Mater. Chem. A 8, 6144-6182 (2020). T. Takeguchi et.al. J. Am. Chem. Soc. 135, 11125-11130 (2013). Figure 1

Polymer electrolyte fuel cell (PEFC) has tremendous interests due to its attractive properties such as low-temperature operation and high power density. For PEFC, Pt nanoparticles (NPs) loaded on carbon support (Pt/C) is widely used as the cathode, in which large amount of costly Pt is required to accelerate the oxygen reduction reaction (ORR) kinetics of PEFC [1]. However, NPs catalysts have some drawbacks such as low durability, and low catalytic activity due to aggregation. Moreover, high cost and limited availability of this precious metal hinder the large-scale commercialization. To overcome these challenges, development of low Pt cathode catalyst with high catalytic activity and durability for ORR in PEFC is highly desirable. Herein, Pt-alloy nanowires (NWs) catalyst was prepared with Co. Addition of a second metal into Pt to form Pt-metal alloy NWs catalyst can improve the electronic and geometric parameters of Pt metal, such as Pt-Pt interatomic distance and surface structure, and consequently enhances the ORR activity and stability of the catalyst [2]. To prepare the Pt-Co NWs/C catalyst, a simple and efficient method was used. For synthesis of Pt-Co NWs, 19.6 mg Pt(acac)2, 25.7 mg Co(acac)2, 135.0 mg glucose, 60.0 mg PVP (mw. 40000) and 1.8 mg W(CO)6 were mixed with 2 mL 1-octadecene (ODE) and 3 mL oleylamine (OAm). After sonication for 60 min, the mixed solution was further heated up to 140 ºC for 6 hours. Next, the Pt-Co NWs was obtained by centrifugation (9,000 rpm for 10 min), and subsequently a facile selective precipitation process was employed to purify NWs. For preparation of Pt-Co NWs/C catalysts, the ethanol reduction method was used. Pt-Co NWs/C catalyst was obtained by centrifugation, then washed with ethanol, and dried overnight. Subsequently, annealing treatment was performed under a mixing atmosphere of hydrogen and argon with an annealing temperature of 450 ºC for 12 h. After that, XRD, TEM, and EDX measurements were performed to examine the structures, morphology, and composition of the catalysts. Then, tree-electrode cell was used to evaluate the electrochemical active surface area (ECSA) estimated from hydrogen adsorption/desorption peak using cyclic voltammetry (CV). Current-voltage (I-V) measurements of the single cell (Pt loading of 0.1 mg-Pt cm−2-MEA) were performed at cell temperature of 80 ºC, a scan rate of 50 mV s−1, and a voltage range of 0.2-1.2 V. The accelerated durability test (ADT) was performed. Fig. 1 shows the XRD patterns of Pt-Co NWs/C catalyst, and compared with commercial Pt-Co/C and Pt/C catalysts. For Pt-Co NWs/C catalyst, some diffraction peaks were observed at ca. 2θ = 39.7º, 46.1º, and 67.4º assigned to (111), (200), and (220) reflections of Pt. The peak of Pt-Co NWs was slightly shifting to higher values as compared with the peak of standard Pt (JCPDS#88-2343). The shift in the peak position can be attributed to the formation of alloy crystal. Fig. 2 shows the TEM image of Pt-Co NWs. The nanocrystal exhibits a uniform morphology with a diameter of ca. 1.5 nm and a length of 64 nm. The chemical composition of the as-prepared catalyst is examined by EDX, which is consistent with the reported literatures. The area-specific activity of Pt-Co NWs/C estimated from the mass activity (0.80 V) divided by ECSA is ca. 0.70 mA cm−2, and the corresponding value of Pt-Co/C is ca. 0.13 mA cm−2, as shown in Fig. 3. Comparison of I-V polarization and power density curves obtained with Pt-Co NWs/C and commercial Pt-Co/C catalysts are shown in Fig. 4. Pt-Co NWs/C exhibits low cell voltage (0.37 V) than that of the commercial Pt-Co/C catalyst (0.46 V) at 1.0 A cm–2. Maximum power densities (Pmax) of Pt-Co NWs/C and Pt-Co/C are 0.39 W cm–2 and 0.59 W cm–2, respectively. However, the reduction of Pmax after ADT is suppressed for Pt-Co NWs/C catalyst. The Pmax losses for the Pt-Co NWs/C and Pt-Co/C are 3.4% and 20.9%, respectively. Therefore, it can be concluded that the lifespan of a PEFC prepared with Pt-Co NWs/C catalyst under steady-state operation can be very long. The Pmax could be achieved by enhancing ECSA, since the area-specific activity of Pt-Co NWs/C is higher than that of commercial catalysts. Increasing activity of Pt-Co NWs/C would be a future task. Acknowledgement The authors would like to thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support of this work. References: F. Godínez-Salomon, R. Mendoza-Cruz, M. J. Arellano-Jimenez, M. Jose-Yacaman, C. P. Rhodes, ACS Appl. Mater. Interfaces, 9, 18660-18674 (2017). V. Č olić, A. S. Bandarenka, ACS Catal. 6, 5378-5385 (2016). Figure 1