Scalable synthesis of efficient water oxidation catalysts: insights into the activity of flame-made manganese oxide nanocrystals

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 μmol_O2 m^-2 s^-1) than commercial nanocrystals (0.02 mol_O2 m^-2 s^-1), and production of up to 0.33 mmol_O2 mol_Mn^-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.