Syntheses Of High Mass Loading Transition Metal Oxide Materials For Supercapacitors

Supercapacitor is a significant new type of energy storage device,which has attached tremendous attention due to their superior power density,fast charge-discharge rate and long working lifespan.Recent cutting-edge research has been focusing on pseudocapacitors,with the storage charge through not only electrical double layer but also redox reaction,which achieve satisfactory electrochemical performance.Pseudocapacitive-type materials mainly include transition metal oxides,conductive polymers,nitrides,sulfides,etc.Due to the poor electrical conductivity and structural stability of Pseudocapacitive-type materials,pseudocapacitors still face problems such as poor stability and rate capability.In this paper,we report the preparation of high mass loading transition metal oxide materials（vanadium oxide and manganese oxide）with excellent performance using electrochemical technology,and study their practical application in supercapacitor.The main findings of the paper are as follow:（1）Different compounds have different electrochemical conversion potentials.Vanadium shows multiple stable oxidation states in its oxides,which allows vanadium oxides available for charge storage in a wide potential window.In this paper,a mixed-valence vanadium oxide（VO_x）with high mass loading was grown on the surface of functionalized carbon cloth with three-dimensional structure by electrochemical techniques.The charge storage potential window was expanded by means of multiple oxidation states of vanadium.The functional carbon cloth provides good conductive support for the growth of VO_x,effectively promotes the rapid transport of electrons and ions.The as-synthesized electrode achieves a remarkable capacitance of 1.35 F cm^-2 at10 m A cm^-2.To further solve the problem of VO_x dissolution,a layer of molybdenophosphate is coated on the surface of VO_x,which effectively improves the cycle stability.The obtained VO_x@Mo PO_x/FCC electrode exhibits an areal capacitance of 3.95 F cm^-2 at 10 m A cm^-2,and great durability with 91%retention after 8000 cycles.In addition,VO_x@Mo PO_x/FCC was used as both positive and negative electrodes to assemble a symmetric supercapacitor.The operating voltage is up to 2.2 V and the volume energy density is up to 7.56 m Wh cm^-3.（2）Manganese oxide is the most widely studied transition metal oxide material with great potential.In this paper,high mass loading manganese oxide material with excellent electrochemical performance was prepared by adjusting the ratio of electroplate electrolyte.The electroplate electrolyte was determined to be 0.2 mol L^-1manganese acetate and 2 mol L^-1 ammonium acetate.Then,the optimal sweep speed of 50 m V s^-1 was determined by controlling electrodeposition parameters.The as-synthesized manganese oxide electrode achieves an areal capacitance of 2.12 F cm^-2 at1 m A cm^-2.After 12,000 cycles of charge-discharge tests,the capacitance retention rate of the manganese oxide electrode is over 85%,showing excellent cycling stability.Then,an asymmetric supercapacitor device was assembled by using the high mass loading manganese oxide material as the positive electrode and partially exfoliated graphite foil as negative electrode.Our device exhibits a volume energy density of 9.6m Wh cm^-3a power density of 74 m W cm^-3.