New Pseudo-capacitive Electrode Materials For Supercapacitors

The supercapcacitor electrode materials can be traditionally divided into carbon-based materials,metal oxide materials and conducting ploymers.Among them,metal oxides and conducting ploymers are assigned to the pseudocapacitive material.In general,pseudo-capacitive materials have some disadvantages such as high cost,low utilization and poor cycling performance,which restrict their practical application.With development of eco-friendly electric vehicles,it is necessary to find new supercapacitive electrodes with high performances.The electrochemical performance of pseudocapacitive electrode materials is related to their crystal structure,component,morphology,size,dimentionm,electrolyte system and so on.In fact,new pseudo-capacitive electrode materials with a novel structure can have fast ionic migration veiocity,good electrochemical reactivity and high utilization of the active materials.In this disertation,a series of new-type pseudocapacitive materials were digsigned and syntheisized,and their crystal structure,component,size,dimention,morphology and physicochemical features were revealed.Futhermore,these new pseudocapacitive materials are used as supercapacitor electrodes and the relationships between the supercapacitive performances and intrinsic characteritics of materials have also been investigated in detail.1.Hollow V2O5 spheres constructed from plate-like particles were synthesized via a solvothermal method,and then were characterized by XRD,SEM,TEM and so on.Furthermore,an aqueous electrolyte system containing lithium salt was successfully constructed.It was found that hollow V2O5 spheres exhibited a big tremendous pseudocapacitance effect by the CV test.The electrochemical results indicated that V2O5 electrode in such a test system has a high pseudocapacitance value,but the cycling performance was poor.To improve the cycling stability of the V2O5 electrode,V2O5-PPy composite was prepared by the polymerization of pyrrole with V2O5 at a low temperature.The results indicated that this composite material not only had a higher pseudocapacitance value,but alao exhibited a better cycling stability.The improved electrochemical performance might be attributed to the insoluble PPy coating.When the hollow V2O5 sphere were composited with PPy,the insoluble PPy coating on the surface can avoid the direct contract between the electrolyte and active material,and thus suppress the dissolution of V2O5 during the cycling process.In addition,the charge transfer resistance was smaller after the composition with PPy.The smaller charge transfer resistance means a faster electron transfer rate,thereby also benefiting good supercapacitive performance.2.Nanostructured porous MnO2 film on Ni foam substrate was firstly fabricated via a CV electrodeposited route without any additive.The obtained MnO2 film had a 3D network structure and a high mass loading from 6 to 18 mg cm-2.We also investigated the effects of deposition time on their structure,composition,morphology and so on.The electrochemical tests denmonstrated that the MnO2 electrode with a high mass loading exhibited a good supercapacitive performance,which might be due to its intrinsic characteristics of nanostructured porous MnO2 film on Ni foam substrate including high mass loading,3D network,porous structure,direct deposition on Ni foam substrate and so on.These factors ensured fast electrolyte diffusion,good charge conductivity,and improved utilization of active materials,resulting in high capacitance,good rate capability and cycling stability.3.Layered titanate H2Ti3O7 nanotubes were synthesized by using a simple hydrothermal method.These nanotubes have unique layered structure,large surface area,and were used as a supercapacitor electrode for the first time.The electrochemical tests denmonstrated that this material exhibited large pseudo-capacitance,perfect rate performance,excellent cycling stability and high columbic efficiency.In fact,these ideal pseudo-capacitive behaviours may arise from the intrinsic characteristics of H2Ti3O7 nanotubes.Interlayer space may provide a transport path for the electrolyte penetration and thin thickness could effectively reduced the distance of Li+ ions intercalation/deintercaiation.In addition,its large surface area might enhance the surface redox reaction and generate a large number of sites for Li+ ions storage.4.A layered structure Ni-based MOF was synthesized by a simple thermal reaction process using p-benzenedicarboxylic acid and NiCl2 solution as precursors.Two kinds of Ni-MOF with different exposed facets were obtained by means of adjustment of reaction time and then were firstly used as an electrode material for supercapacitors.The electrochemical results denmonstrated that the Ni-MOF electrode with the largest exposed(100)facet exhibited better performance than that with the largest exposed(020)facet.It can be found that the performance advantage for the material with a(100)facet could be attributed to its favorable interspaces for the electrolyte diffusion,which was exactly a rate controlling step during the pseudocapacitance storage process.5.Layered structural Zn-doped Ni-MOFs material wase synthesized based the Ni-MOF reaction system using an inorganic zinc salt as an additive.This material with different Zn contents exhibited a spherical morphology.Using this kind of Zn-doped Ni-MOF material used as a supercapacitor electrode material,it exhibited high specific capacitance,good rate capability and good cycling stability.These excellent electrochemical properties might be related to its intrinsic characteristics.The enlarged layered interspace may employ a facil pathway for the electrolyte diffusion and fully satisfy the needs for the redox reaction on the electrode surface,thus improving the utilization of the active material.