The Synthesis Of C/MnO_x Composite And Its Supercapacitance Application

The serious energy crisis and environmental pollution have signicantly increased the demand for clean energy,leading to the attention to energy storage devices.As a novel energy storage device,supercapacitors with the advantages of high power density,long cycle life and low cost,have been widely used in many fields,such as electric vehicles,backup power and consumer electronics.The performance of supercapacitors is directly dependent on the electrode materials.Therefore,electrode materials become the research focus in this filed.The manganese oxides have a broad prospect in supercapacitance application,due to their high theoretical specific capacitance,low price and environmental friendliness.However,the poor conductivity has great influences on their electrochemical performance,which limits their practical application.There are various methods,like doping metal elements and fabricating binder-free electrode,to improve the conductivity of manganese oxides.However,the most effective approach is to combine with highly conductive carbon materials.In this thesis,we prepared various kinds of C/MnOx composites and investigated their supercapacitance performances.The major research contents and results were shown as followings.（1）Ultrathin MnO₂ nanosheets are coated on the surface of 0D（carbon sphere,XC）,1D（carbon nanotubes,CNT）and 2D（reduced graphene oxide,rGO）carbon materials.Their electrochemical performances were investigated in a typical three-electrode system with 0.5 M Na₂SO₄ aqueous solution as electrolyte.At the current density of 1 A g^-1,the pure MnO₂ electrode exhibits a specific capacitance of 99.0 F g^-1.Combining with various desimentional carbons,all the C@MnO₂ composites electrode shows an increase in the specific capacitance.At the same current density,the XC@MnO₂,CNT@MnO₂ and rGO@MnO₂ composites electrodes present a specifc capacitance of 158.1 F g^-1,120.5 F g^-1and 186.7 F g^-1,respectively.The differences of their electrochemical performances were investigated by the AC impedance spectroscopy.The CNT@MnO₂ composites electrode indicates the smallest charge-transfer resistance and largest diffusion coefficient of sodium ion（9.2′10-10 cm2 s-1）.These results suggest that MnO₂ modified with 1D carbon materials shows the best electrochemical performance,which can provide a reference for other similar composites applications.（2）Bacterial cellulose is a high-performance biomass carbon source,with an unique 3D network structure,that can absorb a large number of ions.Therefore,a facile method to fabricate composites based on biomass-derived carbon nanofibers was presented.Firstly,bacterial cellulose pellicles were immersed into KMnO₄ solution to replace the water in bacterial cellulose hydrogel with KMnO₄ solution.Then a freezedried process was conducted to keep the 3D network structure of the bacterial cellulose hydrogel.Finally,the pyrolysis process was used to obtain the composites.The particle size and content of MnO can be controlled by changing the concentration of KMnO₄ solution.Carbon nanofiber/manganese oxide-20（CF/MnO-20）composites,which are derived from 20 mM KMnO₄ solution immersed the bacterials cellulose pellicles,have the highest specific capacitance of 192.3 F g^-1 at the current density of 0.5 A g^-1.The asymmetric supercapacitors,assembled by using rGO as the negative and CF/MnO-20 as the positive electrode,exhibit an excellent energy density of 25.9 Wh kg^-1.The results indicate that the absorption-pyrolysis method to fabricate composites based on biomassderived carbon nanofibers have bright prospects in supercapacitance application.