Preparation And Properties Of Manganese Oxides And Their Composite For Supercapacitor

Supercapacitors,as a kind of novel energy storage device,have many advantages such as high power density,long cycle life and fast charge-discharge rate.In recent years,it's very active for the research and application of supercapacitors at home and abroad.Manganese dioxide?Mn O₂?,with low-priced and good performance,is a kind of electrode material for pseudocapacitors,but the poor conductivity and reversibility restrict its practical application.To improve the properties of MnO₂,this paper focuses on the following three aspects: 1.A layer of nanocrystalline MnO₂ was deposited on the surface of nickel substrate by one-step hydrothermal method,which was directly used as supercapacitors electrode,and then SEM,EDS,XRD,BET,XPS and other techniques were used to characterize the physical properties of the material.CV,GCD,EIS and other techniques were used to characterize the electrochemical properties of the material;2.MnO₂ with different morphologies was prepared in alkaline,neutral and acidic media by controlling the conditions of the hydrothermal reaction;3.The carbon quantum dots?CQDs?was deposited on the surface of MnO₂ electrode by electrodeposition to get the CQDs/MnO₂ composite electrode.Then,the reduced graphene oxide?rGO?was used as the negative electrode,the CQDs/MnO₂ composite used as the positive electrode,and polyvinyl alcohol-potassium hydroxide was used as an all-solid electrolyte to be an asymmetric supercapacitors.In order to study the practical application ability of CQDs/MnO₂ composite electrode,the CQDs/MnO₂//rGO/Ni all solid state asymmetric supercapacitor was prepared in this paper,and it's electrochemical properties were investigated.The experimental indicated that: 1.The MnO₂ synthesized by one-step hydrothermal method on foam nickel was a kind of nano flowers ?-MnO₂ made up of lots of nanosheets,and no binder and other additives were need here.As the electrode of supercapacitors,it's specific capacitance reached 380.2 F·g-1 under the current density of 0.2 A·g-1 in the solution of 1 mol·L-1 Na₂SO₄,of which the specific capacitance remained 94.2% after 1000 cycle numbers,which showed a good electrochemical performance.2.The morphologies of MnO₂ prepared in alkaline,neutral and acid media were nano spheres,nano flowers and nano mesh respectively,and nano mesh MnO₂ had the largest specific surface area?62.4 m2·g-1?.The electrochemical tests showed that the nano mesh MnO₂ had the best performance.It's specific capacitance reached 646.4 F·g-1 under the current density of 0.5 A·g-1 in the solution of 1 mol·L-1 KOH,of which the specific capacitance remained 86.4% after 1000 cycle numbers,which showed a good electrochemical performance.In addition,EIS tests showed a small resistance and effective ion diffusion path.3.Electrochemical measurements showed that the CQDs/MnO₂ composite electrode had the best electrochemical performance when the deposition time was 10 min.In the 1 mol·L-1 KOH solution,the specific capacitance of the electrode was about 879.0 F·g-1 under a current density of 0.5 A·g–1,which was 36.1% higher than that of the pure Mn O₂ electrode?646.1 F·g-1?.When the current density increased from 1 A·g–1 to 10 A·g–1,the retention rate of it was 61.2%,which was 14.5% higher than that of the pure MnO₂ electrode?46.7%?.The results showed that the electrochemical properties of MnO₂ composite were improved significantly after the deposition of CQDs.Electrochemical measurements showed that the device could be stably charged and discharged at 1.6 V,of which the specific capacitance was about 92.4 F·g-1 under a current density of 0.5 A·g-1 in the 1 mol·L-1 KOH solution,and the energy density was about 32.68 Wh·kg-1,corresponding to a power density of 401.53 W·kg-1.When the power density increaseed to 8.34 k W·kg-1,the energy density still reached 18.52 Wh·kg-1,which was higher than that of MnO₂ reported previously and showed that the all-solid-state asymmetric supercapacitor of CQDs/MnO₂//rGO/Ni had a good electrochemical performance.