MnO 2 samples are synthesized by an electrodeposition technique at a fixed pH and are tested as active electrode material for an electrochemical supercapacitor. The physicochemical properties of electrolytic manganese dioxide, including average Mn oxidation state, amount of incorporated water, porosity, crystal structure, morphology, and electrochemical performance, are characterized. The characterization is carried out by thermal gravimetry, composition analysis, the Barrett-Joyner-Halenda model, X-ray diffraction, scanning electron microscopy, and cyclic voltammetry methods. The correlation between these properties and the deposition conditions ͑pH͒ is studied. The results show that the pH strongly influences the properties and, consequently, the activity of MnO 2 in an electrochemical supercapacitor.Manganese dioxide ͑MnO 2 ͒ is one of the most attractive materials due to its catalytic, ion-exchange, magnetic, and electrochemical properties. 1-5 It is used especially as an electrode material in batteries and supercapacitors because of its energetic compatibility in a reversible electrochemical system. 4-7 Electrolytic manganese dioxide ͑EMD͒ can be produced by an electrolysis of an aqueous solution of MnSO 4 . The electrodeposition of EMD is overlapped by the oxidation ͑Reaction 1͒ and reduction ͑Reaction 2͒ of species on the anode and the cathode, respectively 5,6 Mn 2+ + 2H 2 O → MnO 2 + 4H + + 2e − anodic reaction ͓1͔According to the anodic/cathodic reactions, the generation of excess H + ions is an inherent part of EMD electrodeposition. It leads to a large decrease in the pH value and to a change in the electrodeposition conditions and the properties of EMD deposits. 6,7 In a previous paper, we adjusted the pH at 2 and 5 and then characterized the samples. The results confirmed that the pH of the solution has remarkable effects on the nanostructure, crystal structure, porosity, and the electrochemical performance of the EMD as a cathode in rechargeable alkaline manganese dioxide batteries. The aim of this paper is to extend previous research and to characterize the manganese dioxide samples that were prepared at fixed pH ͑2, 3, 4, 5, and 6͒. The chemical composition, micro-and nanostructures, porosity ͑surface area, pore size, and pore-size distribution͒, and electrochemical performance of MnO 2 as an active material in supercapacitors are studied by attention to pH value.
ExperimentalThe EMD samples were produced from a solution containing MnSO 4 ͑0.74 mol L −1 ͒ at a temperature of 80°C and an anodic current density of 0.8 A dm −2 . A titanium plate was used as the anode ͑area: 1 dm 2 ͒ between two Pb plates that were used as the cathodes ͑area: 2 dm 2 ͒. The pH of the solution was monitored in situ by a pH meter. Five samples were electrodeposited at fixed pH 2, 3, 4, 5, and 6 by adding NH 4 OH ͑10%͒. They are named M2-M6, as shown in Table I.The chemical composition of the samples was determined using the potentiometric titration approach of Vetter and Jaeger 8 based on the cation vacancy model propos...
