Highly active screen-printed electrocatalysts for water oxidation based on β-manganese oxide

Abstract

A versatile screen-printing method is applied for the preparation of efficient water oxidation catalysts based on a nanostructured β-MnO₂ material prepared by a redox-precipitation method, and commercial β-MnO₂. The catalyst films were tested for activity in water oxidation over a range of neutral to alkaline pH. The onset of water oxidation in case of the nanostructured MnO₂ films is found at an overpotential (η) of 300 mV at pH 13.6 (1.0 M NaOH), with current densities reaching 10 mA cm⁻² at η = 500 mV. The screen-printed MnO₂ (nano) is one of the most active manganese oxide-based catalysts reported to date, despite consisting mostly of the common pyrolusite (β-MnO₂) phase, so far generally found inactive in water oxidation. The films prepared from commercial β-MnO₂ were found to be moderately active, with an onset of water oxidation at η = 500 mV (pH 13.6), and currents up to 5 mA cm⁻² at η = 800 mV. At pH 6, the two samples exhibit similar activity and also match or surpass the performance of recent benchmark manganese oxides. X-ray absorption spectroscopy (XAS) studies suggest that the crystal phase is unchanged after prolonged electrochemical cycling. Scanning electron microscopy (SEM) analysis indicates very little corrosion of the surface morphology after prolonged catalyst operation at alkaline pH. However, high-resolution transmission electron microscopy (HRTEM) analysis shows the formation of a small amount of an amorphous phase on the surface of the nanorods after oxygen evolution over 12 hours in alkaline media.