Hollow La0.5Sr0.5MnO3 nanospheres as an electrocatalyst for the oxygen reduction reaction in alkaline media
Graphical abstract
Introduction
The oxygen reduction reaction (ORR) is one of the key reactions in batteries and fuel cells, such as metal-air batteries (MABs) and alkaline fuel cells (AFCs). The sluggish kinetics of the ORR is of a critical issue, which blocks the development of these electrochemical energy storage and conversion technologies. The ABO3 perovskite oxide which possesses unique physical and chemical properties, such as variable crystal structure and the non-stoichiometric chemistry has proven to be an efficient noble-metal-free catalyst for the ORR in an alkaline solution [[1], [2], [3], [4], [5], [6]]. Generally speaking, in the ABO3 perovskite oxide, A-site is located by alkaline-earth elements and/or rare-earth elements, while B-site comprises transition-metal elements [[7], [8], [9], [10], [11]]. Studies have shown that perovskite oxide LaMnO3 has a high ORR activity [2,[12], [13], [14]]. Furthermore, in LaMnO3, oxygen vacancies may form if La is partially replaced by Sr to obtain La1-xSrxMnO3 (0 < x < 1), leading to favorable crystal and electronic structures for the ORR [[15], [16], [17], [18], [19], [20], [21]].
While La1-xSrxMnO3 exhibits high catalytic activity in the ORR, there is still a performance gap between this perovskite oxide and other noble-metal-free catalysts such as heteroatom-doped carbon and single-atom electrocatalysts [22,23]. The main reasons leading to the performance gap lie in the low specific area and large particle size of the La1-xSrxMnO3 perovskite oxide. Strategies for improving the catalytic activity of La1-xSrxMnO3 in the ORR have been reported [[24], [25], [26], [27], [28]]. For example, La0·8Sr0·2MnO3 with an urchin-like morphology and high specific surface area has been shown to exhibit significantly improved ORR activity [26]. Hollow La0·8Sr0·2MnO3 sphere has been found to possess high ORR activity involving a direct four-electron transfer pathway [27]. Porous La0·75Sr0·25MnO3 nanotubes have been reported to be an excellent electrode electrocatalyst in Li–O2 (lithium-oxygen) batteries with high rate capacities (between 9000 and 11,000 mAhg−1) and coulombic efficiency (nearly 100%). The authors ascribed that the unique structure enhanced the ORR activity by providing more catalytic sites, promoting electron transport and mass transfer [28].
The template method has been used to prepare hollow La0·6Sr0·4CoO3-δ spheres with a good ORR activity [29]. The particle size of the obtained La0·6Sr0·4CoO3-δ spheres is between ~5 and 8 μm. In this work, we designed and demonstrated a different approach to preparing hollow La0·5Sr0·5MnO3 spheres by utilizing the strong ability of citric acid in interacting with carbon spheres and metal ions under ambient temperature. This new method enables the preparation of porous hollow La0·5Sr0·5MnO3 spheres of nano-sized particles with particle sizes between 30 and 100 nm. Importantly, the hollow spherical La0·5Sr0·5MnO3 nanoparticles exhibited improved ORR performance, good stability and high selectivity towards methanol in 0.1 M KOH electrolyte.
Section snippets
Synthesis of carbon spheres
The carbon spheres used in this work were prepared by a hydrothermal method, as reported elsewhere [30]. Briefly, 4.5 g of glucose (AR, Shanghai Macklin Biochemical Co., Ltd) was firstly dissolved in 30 mL of distilled water. Then the observed solution was transferred into a Teflon-lined autoclave (50 mL) followed hydrothermal reactions at 180 °C for 4 h. Carbon spheres were obtained after centrifugation, washing with distilled water and alcohol, and oven-drying at 80 °C for 4 h.
Preparation of hollow La0·5Sr0·5MnO3 nanospheres
Firstly, 4 mmol
Results and discussion
Fig. 1 shows the FTIR spectra of carbon spheres and the HLSMN-B sample. The absorption bands seen at 1618.38 and 1701.98 cm−1 from the carbon sphere in Fig. 1a belong to the vibration of –CC and –CO, respectively. And this indicates that aromatization of glucose occurred during the hydrothermal treatment. The absorption bands at 1024.51, 1308.41 and 3415.92 cm−1 are ascribed to the –OH bond, indicating the presence of a large number of hydroxyl groups on the carbon spheres [30].
The study has
Conclusions
In summary, La0·5Sr0·5MnO3 perovskite oxide with a hollow nanospherical morphology synthesized using the template method exhibited a high oxygen reduction reaction activity in 0.1 M KOH electrolyte with positive onset potential (0.86 V), Tafel slope near to Pt/C catalyst (87 mV dec−1) and a 4-electron transfer pathway. The hollow La0·5Sr0·5MnO3 nanospheres promote the ORR activity by providing more active sites, promoting electron and mass transfer. The results presented in this paper indicate
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No.: 51972183), the Youth 1000 Talent Program of China, the World-Class Discipline Program of Shandong Province and the Taishan Scholar's Advantageous and Distinctive Discipline Program of Shandong Province. The Australian Research Council (ARC) is also acknowledged for partially supporting this study under Project FL170110001.
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