Zhou, Fengling, Izgorodin, Alex, Hocking, Rosalie K., Armel, Vanessa, Spiccia, Leone, and MacFarlane, Douglas R. (2013) Improvement of catalytic water oxidation on MnOₓ films by heat treatment. ChemSusChem, 6 (4). pp. 643-651.

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Abstract

Manganese oxides (MnOₓ) are considered to be promising catalysts for water oxidation. Electrodeposited MnOₓ films from aqueous electrolytes have previously been shown to exhibit a lower catalytic action than films deposited from ionic liquids when tested in strongly alkaline conditions. In this study, we describe a thermal treatment that converts the MnOₓ films deposited from aqueous electrolytes to highly catalytic films with comparable activity to ionic-liquid-deposited films. The films deposited from aqueous electrolytes show a remarkable improvement in the catalysis of water oxidation after heat treatment at a low temperature (120°C) for 30min. The films were characterised by using XRD and SEM, and energy-dispersive X-ray (EDX), FTIR and Raman spectroscopy, which indicate that dehydration occurs during the heat treatment without significant change to the microstructure or bulk composition. The X-ray absorption spectroscopy (XAS) results show the growth of small amounts (ca. 3-10%) of reduced Mn species (Mn^II or Mn^III) after heat treatment. The dehydration process removes structural water and hydroxyl species to result in a conductivity improvement and a more active catalyst, thereby contributing to the enhancement in water oxidation performance.

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