Nano MnO₂/CB Electrode Material For Electrochemical Capacitor

With the development of new clean energy sources, electrochemical capacitors（ECs） as an important energy storage device have many potential applicationprospects. ECs are environmental friendly, and have nearly infinite long cycle life, butlow energy density compared to batteries constrains its potential applications in manyplaces, which has attracted many interests in improvements of the energy densityamong investigators.I intend to increase the energy density of ECs in two aspects.One of them targets on enhancing the specific capacitance of electrode material.MnO₂as a metal oxide material has relative high capacitance, also cheap and green.But low conductivity makes the electrode material have low activity and poor cyclicperformance. So in this paper I used carbon black （CB） as the matrix, which washighly conductive and with large surface area, and deposited nano MnO₂on itssurface to obtain a composite. HRTEM results showed that nano wire-like MnO₂dispersed uniformly on the surface of CB with highly connectivity. Electrochemicalresults demonstrated that186F/g specific capacitance could be obtained at a scan rateof2mV/s from CV curves, and the capacitive retention rate was91%after1000cycles at10mV/s. From the observation of electrode before and after1000cyclesusing FESEM, it could be seen that some of MnO₂fell off from the electrode and thestructure of the electrode material changed. EIS characterization was furtherconducted to research the difference of electrode before and after1000cycles fromkinetics.The other aspect aims at improving the operation voltage of ECs. Asymmetriccapacitors use two different materials as the electrodes. As these two differentmaterials have different potential ranges operating reversibly, it could effectivelyincrease the operation voltage if combining these two electrodes. So in this paper, Iassembled an asymmetric capacitor using the MnO₂/CB composite and a commercialactivated carbon as electrodes. The electrochemical windows were confirmed by theirCV curves respectively, and the mass ratio between the positive and the negativeelectrodes were calculated based on the above. Under the mass ratio of2/3, the galvanostatic charge–discharge performance of the asymmetric capacitor at differentoperation voltages were discussed. The results showed that charge–discharge curvescould still keep symmetric when operation voltage was as high as2.0V, and theenergy density was31.9Wh/kg.1.9V was further chose as the operation voltage toresearch the galvanostatic characteristics at different current densities. The Ragoneplot showed that compared with the symmetric capacitors, the energy density of theasymmetric capacitor improved a lot, which could achieve27.5Wh/kg when thepower density was50W/kg.