| In recent years,due to the depletion of fossil fuels and the pollution caused by these traditional fuels,there is an urgent need for clean new energy.Owing to their advantages of long-term stability,fast charging and discharging rate and high power output,supercapacitors have been intensively studied and widely used for many applications.The major electrode materials of supercapacitors are divided into three types:carbon-based materials,transition metal oxides and conductive polymer materials.These materials have their own advantages.Therefore,using two or more materials of them as composite electrodes to construct asymmetric supercapacitors having high performances is a hot research direction.In this thesis,Manganese dioxide/Activated Carbon(MnO2/AC)composites were prepared by using a simple co-precipitation and a direct reduction methods,and the resultant composites were further modified with carbon nanotubes(CNTs).The excellent properties of the composites were obtained by controlling the preparation process,reaction time,AC and CNTs doping ratio during the synthesis process of composites,and by optimizing the proportion of conducting agents Super P and CNTs during the preparation process of electrodes.The composite material was used as the positive electrode and the CNTs modified AC was used as the negative electrode to assemble the proposed asymmetric supercapacitors.Energy density,rate performance and cyclic stability of the asymmetric supercapacitors were investigated by regulating the positive-negative capacity ratio(or mass ratio):1.?a-MnO2 was prepared through the reaction between Potassium Permanganate(KMnO4)and Manganese Acetate Tetrahydrate(Mn(CH3COO)2 · 4H2O)and AC-MnO2 composite was prepared by a coprecipitation method.?-MnO2 showed a high specific capacitance of 266 F/g in a neutral Potassium Sulfate(K2SO4)electrolyte,indicating its well-defined pseudocapacitive behavior in the electrolyte used;2.AC-MnO2 composites were prepared by the direct reduction of KMnO4 with AC.Further introduction of CNTs in this system resulted in the formation of MnO2-AC-CNTs composites.By optimizing the synthesis process and adjusting the doping ratio,MnO2-AC-CNTs composites with excellent conductivity were prepared.EIS impedance of the the optimized composite was greatly reduced,which was 5 times smaller than that of the materials without CNTs.This composite displayed a specific capacitance of 129 F/g at 0.2 A/g and retained this capacitance at 78.6%at 6 A/g;3.Asymmetric supercapacitors were assembled by using MnO2-AC-CNTs as the positive and AC as the negative electrodes.First,CNTs were used to replace the conventional conductive agent of Super P during the preparation of both the positive and negative electrodes in order to further improve the rate capability for these electrodes.When tested in 0.5 M K2SO4 electrolyte,the MnO2-AC-CNTs positive electrode showed an improvement in capacitance retention from 78.6%to 95.16%,and the AC negative electrode from 70%to 75%,indicating the significance of CNTs in enhancing the rate capability for both positive and negative electrodes.The asymmetric supercapacitor assembled using the above positive and negative electrodes in a Swagelok format showed a capacitance of 38 F/g at 0.2 A/g and retained this capacitance at 72%when the current density increased to 50 A/g.After cycled at a current density of 4 A/g for 10,000 cycles,this capacitor still maintained 78%of its initial capacitance,clearly showing its good cycle stability. |
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