Evaluation\nof MnO_<i>x</i>, Mn₂O₃,\nand Mn₃O₄ Electrodeposited\nFilms for the Oxygen Evolution Reaction of Water

Abstract

Different manganese oxide phases\nwere prepared as thin films to\nelucidate their structure–function relationship with respect\nto oxygen evolution in the process of water splitting. For this purpose,\namorphous MnO_<i>x</i> films anodically deposited\non F:SnO₂/glass and annealed at different temperatures\n(to improve film adherence and crystallinity) were tested in neutral\nand alkaline electrolytes. Differential electrochemical mass spectroscopy\nshowed that the anodic current correlated well with the onset of the\nexpected oxygen evolution, where in 1 M KOH, the anodic current of\ncrystalline α-Mn₂O₃ films was determined\nto onset at an overpotential (η) of 170 mV_RHE (at <i>J</i> = 0.1 mA/cm²) with current densities of ca.\n20 mA/cm² at η = 570 mV_RHE. Amorphous\nMnO_<i>x</i> films heated at 573 K (MnO_<i>x</i>-573 K) were found to improve their adherence to\nF:SnO₂/glass substrate after heat treatment with a slight\ncrystallization detected by Raman spectroscopy. The onset of water\noxidation of MnO_<i>x</i>-573 K films was identified\nat η = 230 mV_RHE (at <i>J</i> = 0.1 mA/cm²) with current densities of ca. 20 mA/cm² at η\n= 570 mV_RHE (1 M KOH). The least active of the investigated\nmanganese oxides was Mn₃O₄ with an onset at\nη = 290 mV_RHE (at <i>J</i> = 0.1 mA/cm²) and current densities of ca. 10 mA/cm² at η\n= 570 mV_RHE (1 M KOH). In neutral solution (1 M KPi), a\nsimilar tendency was observed with the lowest overpotential found\nfor α-Mn₂O₃ followed by MnO_<i>x</i>-573 K and Mn₃O₄. X-ray photoelectron\nspectroscopy revealed that after electrochemical treatment, the surfaces\nof the manganese oxide electrodes exhibited oxidation of Mn II and\nMn III toward Mn IV under oxygen evolving conditions. In the case\nof α-Mn₂O₃ and MnO_<i>x</i>-573 K, the manganese oxidation was found to be reversible\nin KPi when switching the potential above and below the oxygen evolution\nreaction (OER) threshold potential. Furthermore, scanning electron\nmicroscopy (SEM) images displayed the presence of an amorphous phase\non top of all manganese oxide films here tested after oxygen evolution.\nThe results indicate that structural changes played an important role\nin the catalytic activity of the manganese oxides, in addition to\noxidation states, a large variety of Mn–O bond lengths and\na high concentration of oxygen point defects. Thus, compared to Mn₃O₄, crystalline α-Mn₂O₃ and MnO_<i>x</i>-573 K are the most efficient\ncatalyst for water oxidation in the manganese–oxygen system.