MnO₂ Decorated On Carbon-based Materials Applied In Electrode For Supercapacitors

Supercapacitors have been one of the most promising energy storage devices due to their excellent properties such as long cycle life,high power density and fast charging/discharging rate.However,the lower energy has greatly hampered their practical application.Improving the capacity of supercapacitors is a solution to salve the problem.The capacity has a significantly raise via increasing the surface area and electrical conductivity of the electrode materials.Carbon based nanomaterials have been considered as excellent candidate for electrode materials owing to their high surface area and superior electrical conductivity.Various carbonaceous electrode materials,graphene,owing to its additional flexibility and machinability has received significant scientific and technological attention in the past few years.The strong aggregation tendency of graphene sheets leads to severe decrease of active surface area and the ion diffusion rate,resulting in lower capacity and charge-discharge rates.Transition metal and metal oxide are frequently used to decorating on electrode materials for supercapacitors.MnO₂ has attracted much attention for its environmentally friendliness,good reversibility,low coat and high theoretical capacitance.In the thesis,focusing on carbon material decorated by MnO₂ to improve the capacitance of supercapacitor.Through a simple vacuum filtration method,carbon spheres,carbon nanotubes and hollow carbon spheres spaced graphene nanosheets and provide electrically conducting channels within the electrode for the fast electron transfer and electrolyte-ion penetration during charge-discharge process.To enhance the electronic conductivity of electrode materials,silver ions are doping on the electrode materials.The main contents of this paper are as follows:1.MnO₂/carbon sphere/graphene electrodes are prepared by vacuum filtration method and applied in supercapacitor.In the unique structure,carbon spheres spaced sandwich-like porous structures can provide plenty of paths for electrolyte-ion penetration.After decorating with high performance MnO₂,MnO₂/carbon sphere/graphene films can deliver a gravimetric capacitance of 319.3 F g^-1 and a volumetric capacitance of 277.8 F cm^-3,much higher than graphene and Carbon sphere/graphene films.More importantly,the electrodes also exhibit high gravimetric and volumetric energy densities of 29.4 Wh kg^-1 and 25.6 Wh L^-1,respectively,as well as excellent cycling stability after 5000 charge-discharge cycles.It is proved that the flexible material prepared by this method can provide a new idea for the modification of the electrode material of supercapacitor.2.Free-standing graphene/carbon nanotubes/MnO₂ films are fabricated through a simple vacuum filtration route.In the unique structure,carbon nanotubes separate the graphene nanosheets and provide convenience for electrolyte ions entering the film and increase the reaction surface area.Combining high performance MnO₂ with graphene/carbon nanotubes can help to enhance the specific capacity of the compounds.It is found that the MnO₂ nanoparticles are grown on the surface of graphene and carbon nanotubes,yielding 44%increase in gravimetric capacitance.After testing the structure and electrochemical performance of the material,the material is used as the cathode of the supercapacitor,and the activated carbon is used as the negative electrode to form a supercapacitor device.The device can light a red LED.3.Using SiO₂ as template to prepare hollow carbon spheres.Then using an vacuum filtration method to synthesis graphene/hollow carbon spheres/MnO₂.After investigating microstructure and characterization of the materials,it is found that graphene supported by hollow carbon spheres can obviously improve the electrochemical performance.4.Carbon hollow microtubes decorated with Ag nanoparticles and amorphous MnO₂ was rationally designed and fabricated by the Kirkendall effect.The Ag/C/MnO₂composites were characterized by the structure,composition and electrochemical properties,and it was found that the composite s had good electrochemical properties.By comparison,it is found that Ag particles can improve the electrochemical performance.When assembled as an aqueous hybrid supercapacitor device with the C/Ag/MnO₂ hybrids and activated carbon as the positive and negative electrodes,respectively,the device exhibits a high energy density of 16.6 W h kg^-1 at a power density of 1920 W kg^-1.The present work provides an excellent scheme for high performance asymmetric supercapacitor applications.