Study On New Type Of Negative Electrode Material For High Performance Water System Asymmetric Supercapacitor

Supercapacitors, also called as electrochemical capacitors, have attracted considerable attention as energy storage devices for various applications such as portable electronics and electric vehicles, which can be attributed to their desirable characteristics of high power density, fast rates of charge and discharge, excellent cycling stability, and low maintenance cost. However, for practical applications in various electronic devices, the energy density of current supercapacitors need to be further improved without sacrificing the power density and cycle life. According to the equation of energy density E=1/2CV2, the specific energy of supercapacitors can be increased by increasing the output voltage (V) and/or the specific capacitance. An effective way to increase the operation voltage is to use ionic liquids or organic electrolytes, which, however, suffer from the poor ionic conductivity, short cycle life, and toxicity, making them undesirable in practical applications. A more desirable strategy is to construct asymmetric supercapacitors using environmentally friendly aqueous electrolytes with fast ion transport. A promising alternative is fabricating asymmetric supercapacitors using aqueous electrolytes with low cost and environmental benignity.For asymmetric supercapacitors, different negative and positive electrode materials having well-separated potential windows are coupled to maximize the output voltage. Many researchers chose transition metal oxides as cathode materials and carbon-based anode materials to assemble asymmetric supercapacitors. For neutral aqueous electrolyte, MnO2-based materials have been developed as cathodes for asymmetric supercapacitors because of their excellent electrochemical performance in the positive voltage window. However, most of asymmetric supercapacitors used activated carbon as anode material and the energy density could not be increased further. For building high-energy density asymmetric supercapacitors, developing anode materials with large specific capacitance remains a great challenge.Here, my first work is to synthesized MnFe2O4/graphene nanocomposite by a hydrothermal method and demonstrated as a promising anode material for asymmetric supercapacitors. A 1.8 V asymmetric supercapacitor has been fabricated using MnFe2O4/graphene as anode and MnO2/carbon nanotube as cathode in 1 M Na2SO4 electrolyte. The asymmetric supercapacitor exhibits an energy density of 25.9 Wh kg-1 at a power density of 225 W kg-1 and an energy density of 18.1 Wh kg-1 at a power density of 14.4 kW kg-1. In addition to high energy density and power density, the asymmetric supercapacitor also exhibits good cycling stability with 90% of initial capacitance retained after about 4500 cycles.Although Fe2O3 has been considered as a promising anode material for asymmetric supercapacitors, the specific capacitance of the Fe2O3-based anodes is still low and cannot match that of cathodes in the full cells. In my second main work, a composite material with well dispersed Fe2O3 quantum dots (QDs,～2 nm) decorated on functionalized graphene nanosheets (FGS) has been prepared by a facile and scalable method. The Fe2O3 QDs/FGS composites exhibit a large specific capacitance up to 347 F g-1 in 1 M Na2SO4 between-1 and 0 V vs. Ag/AgCl. An asymmetric supercapacitor operating at 2 V is fabricated using Fe2O3 QDs/FGS as anode and MnO2/FGS as cathode in 1 M Na2SO4 aqueous electrolyte. The Fe2O3/FGS//MnO2/FGS asymmetric supercapacitor shows a high energy density of 50.7 Wh kg-1 at a power density of 100 W kg-1 as well as excellent cycling stability and power capability. The facile synthesis method and superior supercapacitive performance of the Fe2O3 QDs/FGS composites make them promsing as anode materials for high-performance asymmetric supercapacitors.