Manganese oxide/activated carbon materials for hybrid supercapacitors

Manganese oxide in powder form was prepared by redox reaction of a Mn(II) salt with a permanganate (VII) salt and subsequent activation at 200 °C. Electrodes were prepared with manganese oxide and commercial activated carbon by a casting method. Supercapacitor tests were conducted on assemblies formed with manganese oxide based positive electrode and activated carbon based negative electrode. A porous material impregnated with 0.1 M Na2SO4 aqueous solution formed the electrolytic separator between electrodes. Electrochemical tests were carried out in a 4 cm2 test-cell with electrodes having different amounts of active materials. Ciclic voltammetry, galvanostatic charge-discharge and impedance measurements were carried out to study the electric behaviour of the electrodes and to correlate the supercapacitor performance with the characteristics of active materials. Cyclic voltammetry endurance tests evidenced a perfect stability of materials and electrolyte in the voltage window 0 - 1.6 V. A set of voltammetric curves obtained at different voltage scan rates is reported in Fig. 1. From a not optimised couple of electrodes a specific capacitance of 46 F/g of active materials in the supercapacitor was obtained and a energy density of about 16 Wh/kg was calculated. The charge-discharge curves (Fig. 2) as well as the voltammetric curves (Fig.1) evidenced a marked pseudocapacitive effect at supercapacitor voltage values upper 0.8 V determined by an increase of oxidation state of manganese at the positive electrode. To discriminate the effective capacitive contribution of the single electrode to the performance of supercapacitor the charge-discharge curves for each electrode were also collected. The so-obtained curves evidenced a marked pseudocapacitive effect occurring at positive electrode over 0.9 V (vs. SCE) while the negative electrode showed only double layer capacitance. Referring to the same couple of electrodes above mentioned and by using the individual charge-discharge curves specific capacitances of 290 F/g and 134 F/g were calculated for the positive and negative electrode, respectively. Electrochemical impedance spectroscopy measurements were utilised to determine internal resistance of studied supercapacitors at different voltages. The results of these measurements were correlated to the chemical species that form at the electrodes during supercapacitor operation.