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Influence of carbon nanotubes to the supercapacitive properties of the MnO2

Manganese oxide is nowadays very attractive material for supercapacitive application. As the transition metal oxide with great number of oxidation states, MnO2 has a lot of advantages when compared to other related oxides (RuO2, V2O5, IrO2 etc.). It has low toxicity, low cost, environmental friendliness and it is easy synthesised. Theoretical specific capacitance value of MnO2 with one electron exchange is 1370 F/g [1], but such high value is not achievable in practice due to its intrinsically poor electronic conductivity and dense morphology which limits the penetration of ions into the bulk of the oxide layer [2]. The utilization of MnO2 can be enhanced by preparation of composite electrode containing highly conductive materials such as conducting polymers or/and different carbon materials. The aim of this work was to enhance the MnO2 supercapacitive properties by the help of the multi-walled carbon nanotubes (CNT) which are very favourable due to high electric conductivity, high mechanical stability and a high surface area [3, 4, 5]. Manganese oxide was synthesised electrochemically from aqueous solution of 0.25 M MnCl2 at constant potential of 0.9 V during 150 s, 300 s or 450 s. In order to examine the electrochemical behaviour, the MnO2 was synthesised onto different supports such as Pt-quartz crystal electrode, CNT and oxidised CNT-EO. The CNTs were oxidised electrochemically by using cyclic voltammetry method in the potential range from -0.8 to 1.9 V during 3 cycles (CNT-EO1) or 6 cycles (CNT-EO2). Different degrees of oxidation introduce different amount of oxygen functionalities that are responsible for electron transfer activity, as well as for the CNT wettability and surface accessibility. The prepared electrodes were investigated in the 0.25 M MnCl2, 0.5 M NaCl and 0.5 M LiCl in three- electrode cell containing saturated calomel electrode as reference electrode and Pt-foil as a counter electrode. The electrochemical behaviour of the electrodes was examined by the cyclic voltammetry and the method of electrochemical quartz crystal microgravimetry. Results showed that CNT have favourable influence on the specific capacitance values of the MnO2/CNT composite electrode compared to MnO2 onto bare Pt electrode. The oxidation of the CNT resulted in increased specific capacitances of CNT-EO. Electrochemical quartz crystal microgravimetry clarified the reaction mechanism during charging/discharging process of MnO2 electrodes.

supercapacitors, MnO2, CNT, cyclic voltammetry, electrochemical quartz crystal microbalance

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