A Study Of Al-doped MnO₂Used For The Positive Electrode Of Asymmetric Supercapacitor

Super-capacitor has very considerable application in the field of electric car or other electronic equipment backup power supply owing to the super strong power density (2-5kW/kg). However the replacement of ordinary batteries is still faraway for the low energy density (10Wh/kg) of super-capacitor.Nano-structured MnO2electrode material is synthesized by the method of chemical co-precipitation. KMnO4and MnSO4·H2O(or C4H6MnO4-4H2O) are used as raw materials, and a certain amount of Al2(SO4)3·18H2O as well as activated carbon (AC) are added for modification.The structure of Al-MnO2is characterized by X-ray diffraction (XRD) and the morphology is recognized by scanning electron microscopy (SEM). The electrochemical behavior of MnO2electrode is studied by cyclic voltammetry (CV) and galvanostatic charge-discharge so as to ascertain the best mole ratio of Al:Mn and the he best mass ratio ofAC:Mn which turns out to be8%and18%respectively. The presence of Al3+results in a structural change from a-MnO2to δ-MnO2phase and the function of AC is to reduce the resistance of MnO2electrode.Studies of the electrochemical behavior of8%Al-MnO2electrode in0.5mol/L Na2SO4shows the specific capacitance of169F/g at the current density of1000mA/g. The8%Al-MnO2/18%AC electrode is also developed which shows a relatively low electrode resistance with the capacitor decreased22%at the same time.Asymmetric super-capacitor generally consists of two types of electrodes. For example, one is a metal oxide, and the other is a carbon material. An asymmetric super-capacitor AC/0.5mol/L Na2SO4/8%Al-MnO2is assembled and tested successfully in this paper. In order to take advantage of the largest voltage window, the mass ratio (R) between the positive and the negative electrode has been adjusted taking into account that the same charge is passed through both electrodes.Then, the performance of asymmetric cells is evaluated by varying R. It turns out that when R=1.3, the asymmetric cell shows the best electrochemical behavior. It shows an energy density of18.1Wh/kg at a power density of200W/kg with the voltage of1.7V; at the same time it keeps a very good rate performance with an energy density of10.2Wh/kg at a power density of1kW/kg based on the total mass of the active electrode materials. In addition, it also shows an energy density of20.1Wh/kg at a power density of500W/kg when the voltage is increased tol.9V which is not at the regular potential window. This asymmetric aqueous capacitor has great promise for practical applications due to high energy density at high power density.