Scalable Synthesis of Efficient Water Oxidation Catalysts: Insights into the Activity of Flame-Made Manganese Oxide Nanocrystals

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Journal Article
ChemSusChem, 2015, 8 (24), pp. 4162 - 4171
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© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Chemical energy storage by water splitting is a promising solution for the utilization of renewable energy in numerous currently impracticable needs, such as transportation and high temperature processing. Here, the synthesis of efficient ultra-fine Mn3O4 water oxidation catalysts with tunable specific surface area is demonstrated by a scalable one-step flame-synthesis process. The water oxidation performance of these flame-made structures is compared with pure Mn2O3 and Mn5O8, obtained by post-calcination of as-prepared Mn3O4 (115 m2 g-1), and commercial iso-structural polymorphs, probing the effect of the manganese oxidation state and synthetic route. The structural properties of the manganese oxide nanoparticles were investigated by XRD, FTIR, high-resolution TEM, and XPS. It is found that these flame-made nanostructures have substantially higher activity, reaching up to 350 % higher surface-specific turnover frequency (0.07 μmolO2 m-2 s-1) than commercial nanocrystals (0.02 molO2 m-2 s-1), and production of up to 0.33 mmolO2 molMn-1 s-1. Electrochemical characterization confirmed the high water oxidation activity of these catalysts with an initial current density of 10 mAcm-2 achieved with overpotentials between 0.35 and 0.50 V in 1 m NaOH electrolyte.
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