High-voltage asymmetric supercapacitors operating in aqueous electrolyte

The performance of supercapacitors based on different materials with pseudocapacitive properties such as several conducting polymers (ECPs), amorphous manganese dioxide (a-MnO₂), and activated carbon is reported. Composite electrodes of high resiliency and good electronic conductivity were obtained by mixing the active materials with carbon nanotubes. The various limitations of all the above-mentioned materials, when used as negative and positive electrodes in traditional symmetric systems, are shown. It is demonstrated that a successful application of ECPs and a-MnO₂ in supercapacitor technologies is possible only in an asymmetric configuration, i.e. with electrodes of different nature for positive and negative polarizations. Several types of asymmetric capacitors were developed by combining ECPs, a-MnO₂, and activated carbon and characterized in aqueous electrolyte by galvanostatic charge–discharge, cyclic voltammetry, and impedance spectroscopy. The best device considering the specific energy and power is the asymmetric supercapacitor using a-MnO₂ and poly(3,4-ethylenedioxythiophene) (PEDOT) for the positive and negative electrodes, respectively. It has an operating voltage of 1.8 V, which is attributed to different operating potentials of both electrodes, and good electrochemical stability in neutral aqueous electrolyte. According to the voltage value, the energy density of the asymmetric capacitor at a current density of 250 mA/g is found to be 13.5 W h/kg, which is about ten times more than for a symmetric capacitor based on PEDOT in an aqueous medium. The asymmetric capacitor provides two times higher power than a symmetric capacitor based on activated carbon in organic electrolyte, and is thus extremely promising for the development of environmentally friendly systems. PACS 82.45.Wx; 82.45.Yz; 82.47.Uv; 82.35.Cd