The Advanced Electron Microscopy Characterization and Structure-Property Correlation of BaMnO₃ for the Electrocatalytic Oxygen Reduction Reaction

Metal oxides represent one of the most common, diverse and richest class of materials in terms of their electronic, structural, chemical and physical properties.^[1-3] In fact, nanostructured metal oxides exhibit an infinite variety of structural motifs and morphological features that make them indispensable in the design and development of energy application devices. The atomic scale characterization and investigations using electron microscopy techniques bridges the gap between the properties and applications of such materials, and their atomic structure and crystallography. Furthermore, aiding in the finer tuning of material properties, as well as the optimization of environmentally-friendly technological devices. As a result, we combine the use of advanced electron microscopy techniques with spectroscopic analysis to reveal the atomic scale chemical and crystallographic structure of barium manganese oxide (BaMnO₃), a perovskite system.<br/><br/>BaMnO₃ nanorods were synthesized via a hydroxide composite-mediated method, which is in stark contrast to the high-temperature, high-pressure ceramic methods traditionally used to synthesize perovskites. The BaMnO₃ nanorods displayed a 2H-hexagonal crystal structure with space group P6₃/mmc and lattice parameters <i>a </i>= <i>b </i>= 5.6991 and <i>c </i>= 4.8148 Å. Here, the 2H indicates the presence of continuous chains of face-sharing MnO₆ octahedra along the <i>c</i> direction. The crystal structure was successfully characterised using x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction, and directly imaged using aberration-corrected scanning transmission electron microscopy (STEM).<br/><br/>In contrast to the single crystalline nature of the bulk, the surface of the nanorods was found to be amorphous in nature. Using electron energy loss spectroscopy (EELS) and spectrum imaging, this layer was found to consist of reduced Mn states. This was further confirmed by x-ray photoelectron spectroscopy (XPS), from which the presence of both Mn³⁺and Mn⁴⁺ states were identified. Due to the presence of mixed transition metal oxidation states, the nanorods were tested for their electrocatalytic activity in the oxygen reduction reaction (ORR). The as-synthesized nanorods displayed activity for ORR. And subsequently, the activity was found to decrease linearly with post-modification annealing temperature, owing to the decrease in the percentage of Mn³⁺states and associated oxygen vacancies. Therefore, this work offers an insight into the necessity of atomic-scale structural and chemical analyses via electron microscopy for the process of structure-property correlation, in addition to the successful application of materials.<br/><br/>By taking advantage of the perovskite system and its ability to display countless structural and chemical motifs, the number of avenues for future work are endless. Future research will investigate the atomic scale chemical and crystallographic nature of titanium-doped BaMnO₃, with subsequent structure-property correlation. Additionally, the nucleation of precious metals on the nanorods with the aim of improving electrocatalytic activity will be explored.<br/><br/><b>References</b><br/>[1] M. S. Chavali and M. P. Nikolova, <i>SN Applied Sciences</i>, 2019, <b>1</b>, 607.<br/>[2] J. A. Rodríguez and M. Fernández-García, <i>Synthesis, properties, and applications of oxide nanomaterials</i>, John Wiley and Sons, 2007.<br/>[3] C. Noguera, <i>Physics and Chemistry at Oxide Surfaces</i>, Cambridge University Press, Cambridge, 1996.