Evaluation of MnO<sub><i>x</i></sub>, Mn<sub>2</sub>O<sub>3</sub>, and Mn<sub>3</sub>O<sub>4</sub> Electrodeposited Films for the Oxygen Evolution Reaction of Water RamírezAlejandra HillebrandPhilipp StellmachDiana MayMatthias M. BogdanoffPeter FiechterSebastian 2015 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<sub><i>x</i></sub> films anodically deposited on F:SnO<sub>2</sub>/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-Mn<sub>2</sub>O<sub>3</sub> films was determined to onset at an overpotential (η) of 170 mV<sub>RHE</sub> (at <i>J</i> = 0.1 mA/cm<sup>2</sup>) with current densities of ca. 20 mA/cm<sup>2</sup> at η = 570 mV<sub>RHE</sub>. Amorphous MnO<sub><i>x</i></sub> films heated at 573 K (MnO<sub><i>x</i></sub>-573 K) were found to improve their adherence to F:SnO<sub>2</sub>/glass substrate after heat treatment with a slight crystallization detected by Raman spectroscopy. The onset of water oxidation of MnO<sub><i>x</i></sub>-573 K films was identified at η = 230 mV<sub>RHE</sub> (at <i>J</i> = 0.1 mA/cm<sup>2</sup>) with current densities of ca. 20 mA/cm<sup>2</sup> at η = 570 mV<sub>RHE</sub> (1 M KOH). The least active of the investigated manganese oxides was Mn<sub>3</sub>O<sub>4</sub> with an onset at η = 290 mV<sub>RHE</sub> (at <i>J</i> = 0.1 mA/cm<sup>2</sup>) and current densities of ca. 10 mA/cm<sup>2</sup> at η = 570 mV<sub>RHE</sub> (1 M KOH). In neutral solution (1 M KPi), a similar tendency was observed with the lowest overpotential found for α-Mn<sub>2</sub>O<sub>3</sub> followed by MnO<sub><i>x</i></sub>-573 K and Mn<sub>3</sub>O<sub>4</sub>. 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-Mn<sub>2</sub>O<sub>3</sub> and MnO<sub><i>x</i></sub>-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 Mn<sub>3</sub>O<sub>4</sub>, crystalline α-Mn<sub>2</sub>O<sub>3</sub> and MnO<sub><i>x</i></sub>-573 K are the most efficient catalyst for water oxidation in the manganese–oxygen system.