Construction And Energy Storage Performance Of Transition Metal Oxide/Nitride Asymmetric Supercapacitor

Supercapacitors(SCs)have the advantages of fast charging,large specific power and long cycle life as a new energy storage system.But there are certain limitations in the application process because the energy density of SCs is lower than rechargeable batteries.According to the equation of energy density,the most effective approach to improve energy density of SCs is to increase cell voltage(V)and specific capacitance of electrodes(C).Thus,the selection of voltage window and specific capacity of the positive and negative electrode materials of SCs have a great significance to the construction of SCs with high energy density.Transition metal oxides have been concerned due to low cost,no pollution and abundant reserves.For example,manganese oxides are applicable positive electrode material with high theoretical capacity and wide positive voltage window.But they have the poor electrical conductivity.Transition metal nitrides exhibit excellent electrochemical performance due to high conductivity,high capacity and wide negative voltage window,but the stability is poor.Herein,nano-oxide electrode materials were constructed on carbon cloth,oxide and nitride electrodes were prepared by carbon coating and nitriding treatment,respectively,to improve their conductivity and stability.Based on this,asymmetric oxide/nitride SCs were constructed and researched the performance.The main contents are as follows:(1)Using melamine foam as a template,compressible three-dimensional porous reduced graphene(3DRG)was prepared.The vanadium oxide nanowires array was grown on the three-dimensional porous graphene through hydrothermal reaction,and then the vanadium nitride nanowires array coated with nitrogen-doped carbon layer(3DRG-VN@NC)was obtained through nitriding treatment after PDA coating.This unique 3D porous electrode material can be prepared as desired macroscopical shape and has excellent compressibility.The 3DRG-VN@NC electrode obtains a high specific capacitance of 1518 mF/cm2 at a current density of 2 mA/cm2 and good cycling stability with 87%capacitance retention after 10000 cycles,and the electrochemical behavior remains unchanged under 50%compressive deformation.(2)MnCo2O4.5 nanowire arrays were successfully synthesized on carbon cloth by hydrothermal,PDA was subsequently coated on the surface of MnCo2O4.5 nanowire arrays.The Co/MnO nanowire arrays coated with nitrogen-doped carbon(Co/MnO@NC)was prepared by further heat treatment,and the capacitance performance was investigated at different annealing temperature.The electrode material obtained at 600? improves the surface and internal conductivity of MnO due to the carbon layer wrapped on the outside and the embedded Co metal nanoparticles,so that it has the best capacitive performance.Consequently,the Co/MnO@NC show a high specific capacitance of 747 F/g at 1 A/g and good cycling stability with 93%capacitance retention after 5000 cycles.The asymmetric supercapacitor was assembled with positive electrode of Co/MnO@NC and negative electrode of VN@NC,it shows a high energy density of 48.15 Wh/kg and 27.9 Wh/kg at power density of 0.96 kW/kg and 18.95 Wh/kg,respectively.The cycling stability is also outstanding with 82%retention after 2000 cycles.