Evaluation of MnO_x, 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 MnO_x 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 mV_RHE (at J = 0.1 mA/cm²) with current densities of ca. 20 mA/cm² at η = 570 mV_RHE. Amorphous MnO_x films heated at 573 K (MnO_x-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 MnO_x-573 K films was identified at η = 230 mV_RHE (at J = 0.1 mA/cm²) with current densities of ca. 20 mA/cm² at η = 570 mV_RHE (1 M KOH). The least active of the investigated manganese oxides was Mn₃O₄ with an onset at η = 290 mV_RHE (at J = 0.1 mA/cm²) and current densities of ca. 10 mA/cm² at η = 570 mV_RHE (1 M KOH). In neutral solution (1 M KPi), a similar tendency was observed with the lowest overpotential found for α-Mn₂O₃ followed by MnO_x-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 MnO_x-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 MnO_x-573 K are the most efficient catalyst for water oxidation in the manganese–oxygen system.