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 Mn^IVO species in the catalysis as compared to the Mn^VO 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).