Study On Preperation And Electrochemical Characteristics Of MnO₂Composite Electrode Material For Aqueous Supercapacitors

In this thesis, MnO₂composite electrode materials were prepared by ball millingthe mixture of MnO₂powder, Multi-Walled Carbon Nanotubes （MWCNTs） andacetylene black. The structure and morphology of the composite electrode materialswere studied by X-ray Diffraction （XRD） and Scanning Electron Microscope （SEM）methods. The electrochemical performances of MnO₂composite electrodes wereevaluated by cyclic voltammetry（CV） and galvanostatic charge-discharge in0.5mol/LNa2SO4solution. The properties of asymmetry supercapacitor using activatedcarbon（AC） as the negative electrode were inverstigated in0.5mol/L Na₂SO₄solution,too. In addition, a asymmetry supercapacitor with large-capacity was assembled andstudied. The results showed as follows:1. The conductive network of the composite electrode was enhanced with theMWCNTs adding. The power performance of the electrode was improved with theincreasing of MWCNTs content. The values of Rct （reflective of electrochemicalresistance）showed the following arranging rules:MnO₂/MWCNTs5>MnO₂/MWCNTs10>MnO₂/MWCNTs15>MnO₂/MWCNTs20>MnO₂/MWCNTs30. Based on10000cycles stability test at300mA/g, the composite electrode with different contents ofMWCNTs had good capacitance stability. When the MWCNTs content is greater than10%, Equivalent series resistance（ESR） of the composite electrode can stay stable for10000cycles or more.2. Charge-discharge test at the current of5mA showed that the cell capacity ofAC-MnO₂//MWCNTs asymmetry supercapacitor （2cm×2cm） was between3.35F and5.06F, corresponding its capacity varying from15.8F/g to19.8F/g. When changed tolarge current （16C） test, the cell’s capacity was between3.25F and3.75F and the chargeefficiency remained above95%.3. MnO₂composite electrode materials with bi-component of MWCNTs andacetylene black adding exhibited three-dimensional conductive network and the porestructure. The ratio of5HS100（G350）+5MWCNTs in the composite materials showedthe highest capacity and smallest charge transfer resistance. Based on10000cycle’sstability test at300mA/g, the composite electrode with different contents of MWCNTs/acetylene black had good capacitance stability. The ratio of5HS100（G350）+5MWCNTs showed the largest capacity of78.6F/g and79.1F/g. ESR remained thelowest, fluctuating between3Ω and4.5Ω. However, the other six samples stayedbetween4.5Ω to6Ω.4. Charge-discharge test at small current（5mA） showed that capacity of the supercapacitor cell （2cm×2cm） with dual MWCNTs/acetylene black adding was between20.38F and20.19F. When changed to large current （16C） test, the cell’s capacity keptabove3.0F. After10,000cycles test, the cell had a minimum of ESR of5000mΩ and6000mΩ respectively.5. A large super capacitor of800F eventually obtained. The ESR was112mΩ and energy density was26Wh/kg. The charge-discharge efficiency of the cell was over95%.