Evaluation of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water

Different manganese oxide phases were prepared as thin films to elucidate their structure–function relationship with respect to oxygen evolution in the process of water splitting. For this purpose, amorphous MnOx films anodically deposited on F:SnO2/glass and annealed at different temperatures (to improve film adherence and crystallinity) were tested in neutral and alkaline electrolytes. Differential electrochemical mass spectroscopy showed that the anodic current correlated well with the onset of the expected oxygen evolution, where in 1 M KOH, the anodic current of crystalline α-Mn2O3 films was determined to onset at an overpotential (η) of 170 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE. Amorphous MnOx films heated at 573 K (MnOx-573 K) were found to improve their adherence to F:SnO2/glass substrate after heat treatment with a slight crystallization detected by Raman spectroscopy. The onset of water oxidation of MnOx-573 K films was identified at η = 230 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE (1 M KOH). The least active of the investigated manganese oxides was Mn3O4 with an onset at η = 290 mVRHE (at J = 0.1 mA/cm2) and current densities of ca. 10 mA/cm2 at η = 570 mVRHE (1 M KOH). In neutral solution (1 M KPi), a similar tendency was observed with the lowest overpotential found for α-Mn2O3 followed by MnOx-573 K and Mn3O4. X-ray photoelectron spectroscopy revealed that after electrochemical treatment, the surfaces of the manganese oxide electrodes exhibited oxidation of Mn II and Mn III toward Mn IV under oxygen evolving conditions. In the case of α-Mn2O3 and MnOx-573 K, the manganese oxidation was found to be reversible in KPi when switching the potential above and below the oxygen evolution reaction (OER) threshold potential. Furthermore, scanning electron microscopy (SEM) images displayed the presence of an amorphous phase on top of all manganese oxide films here tested after oxygen evolution. The results indicate that structural changes played an important role in the catalytic activity of the manganese oxides, in addition to oxidation states, a large variety of Mn–O bond lengths and a high concentration of oxygen point defects. Thus, compared to Mn3O4, crystalline α-Mn2O3 and MnOx-573 K are the most efficient catalyst for water oxidation in the manganese–oxygen system.