Annealed Mn-Fe binary oxides for supercapacitor applications

Mn-Fe binary oxide, which is a promising electrode material for use in supercapacitors, is prepared by anodic deposition in mixed manganese acetate and iron chloride plating solution. Tailoring the material characteristics and thus the electrochemical performance of the oxide is attempted by annealing (up to 700 °C in air). The thermal properties of the as-deposited oxide are examined by thermogravimetric and differential thermal analyses. Glancing angle X-ray diffraction (GAXRD), scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS) are used to examine the crystal structure, surface morphology, and chemical state, respectively. Dehydration, organic-matter decomposition, oxidation, surface morphology reconstruction, and crystallization of the oxide as a function of annealing temperature are investigated. The 100 °C-annealed oxide, evaluated by cyclic voltammetry at a potential sweep rate of 5 mV s^-1, shows an optimum specific capacitance of 280 F g^-1. Cyclic stability of the oxide electrode can also be improved by post-heat treatment. However, the binary oxide loses its pseudocapacitive capability at the annealing temperature of 500 °C, at which point the formation of crystalline (Mn-Fe)₂O₃ occurs.