Enhanced photo-electrochemical water oxidation on MnOx in buffered organic/inorganic electrolytes†
Manganese oxide (MnOx) 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 MnOx 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.