The mechanism of water oxidation by MnO2 electrodes was investigated at various pH values, using in situ UV-vis absorption spectroscopy. The surface Mn3+ formed as an intermediate species during water oxidation showed a d-d transition band with a peak at 510 nm at pH 6, while the peak shifted to 470 nm at pH 13. This shift of the absorption peak is attributed to the deprotonation of hydroxyl ligands of Mn3+ from OH− to O2−. Interrelation between the protonation state of Mn3+ and the O2 evolution activity of MnO2 electrocatalysts is discussed.
2014 The Electrochemical Society of Japan