Electrocatalytic water oxidation at low energy cost by a highly active and robust calcium–manganese oxide thin film sintered on an FTO electrode with ethyl methyl imidazolium triflate ionic liquid†
Abstract
We report a simple and novel method involving an ethyl methyl imidazolium triflate ionic liquid (EMI OTf) to prepare highly mechanically stable crystalline α-Mn2O3 and calcium-incorporated manganese oxide (CaMn-oxide) thin film water oxidation electrocatalysts (WOCs) on a fluorine-doped tin oxide (FTO) electrode (FTO/EMI α-Mn2O3 and FTO/EMI CaMn-oxide). Electrochemical measurements showed that the incorporation of calcium into manganese oxide promotes water oxidation at a very low energy cost, similar to that observed in natural photosystem II (PSII). The low energy cost is due to the involvement of the MnIVO species in the catalysis as compared to the MnVO species that has been previously detected in calcium-free α-Mn2O3. Moreover, the FTO/EMI CaMn-oxide electro-catalyst showed unprecedentedly high activity (TOF ≈ 29.8 s−1 at η = 0.57 V) and robustness (tested for 40 h) for water oxidation in a neutral potassium phosphate (KPi) buffer solution (0.1 M, pH 7.0). The mechanism of water oxidation by FTO/EMI CaMn-oxide was discussed and applied for water oxidation by the μ-oxido-CaMn4O5 cluster of photosystem II (PS II).