Enhanced photo-electrochemical water oxidation on MnOx in buffered organic/inorganic electrolytes

Abstract

Manganese oxide (MnO_x) materials have been widely studied as electro-catalysts for the water oxidation reaction. Although the electronic structure of MnO₂ (band gap ∼2 eV) indicates it could be a promising photo-anode material for solar water splitting, so far only quite small photocurrents have been obtained from manganese oxides. Here, we show that the photo-electrochemical water oxidation performance of MnO_x films can be significantly improved by using buffered aqueous electrolytes containing amine ionic liquids. The buffered conditions to maintain a constant proton activity and the amine salt are both crucial for achieving high photocurrents. Photocurrents as high as 4.5 mA cm⁻² were obtained at 1.0 V vs. SCE (η = 540 mV) in Bi buffered n-butylammonium nitrate (BAN) electrolyte at pH 9. The incident photon-to-current efficiency (IPCE) was found to be greater than 3% at 400 nm and 4% at 370 nm. Both H₂O₂ and O₂ are produced simultaneously in this system, with the potential subsequent decomposition of the H₂O₂ to form oxygen. An acceleration of the decomposition of H₂O₂ under illumination is proposed to explain the photocurrent improvement.