Template-free synthesis of mesoporous manganese oxides with catalytic activity in the oxygen evolution reaction

Abstract

Porous manganese carbonate was obtained via solvothermal synthesis using ethanol and urea. The manganese carbonate was subsequently used as a precursor to synthesise mesoporous manganese oxides via thermal treatments at three various temperatures. X-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS) results shows that γ-MnO₂ is synthesised at 380 and 450 °C while Mn₂O₃ is produced at the annealing temperature of 575 °C. X-ray absorption spectra show that γ-MnO₂ converts completely to Mn₂O₃ after annealing over the 450–575 °C range. The oxides obtained at 380 °C and 450 °C possess extremely high specific surface area, which is of interest for catalytic applications. The oxides were investigated as electrocatalysts for the oxygen evolution reaction; the oxide prepared at the lowest annealing temperature was found to be the optimum catalyst with an overpotential of 427 ± 10 mV at a current density of 10 mA cm⁻², normalised by the geometric area. The improved catalytic activity was related to the presence of defect-rich and highly porous manganese dioxide at the lowest annealing temperature.