Construction And Supercapacitance Of Carbon-based Multidimensional Composite Structure

In order to solve the problem of energy shortage,the development and utilization of clean energy such as wind,solar and tidal energy are increasing all over the world.At the same time,the electric vehicles and portable electronic devices are also constantly progressing and developing.Supercapacitors are a kind of energy storage devices with the advantages of high power,fast charging and discharging,and long cycle life,which have been widely concerned by researchers and even the whole world.Electrode material is the key to improve supercapacitive performance.Electrode materials with Faraday redox reaction can be combined with carbon materials represented by activated carbon to improve the specific capacitance and the overall performance under the condition of ensuring the cycle stability and rate performance.The structure of electrode materials also has a significant impact on supercapacitance.Large specific surface area and reasonable pore structure are conducive to the full exposure of active sites and the transfer of electrolyte ions,and then to improve the performance.Therefore,in order to improve the overall performance of electrode materials for supercapacitors,three parts of the research are carried out in this thesis.?1?Synthesis and supercapacitance of Co₃O₄ supported porous carbon derived from biomassBiomass derived activated carbon is a kind of excellent carbon material with a wide range of sources and low production cost,and the abundant oxygen-containing functional groups on the surface are conducive to the adhesion of other particles and the construction of composite structure.Firstly,wheat flour was used as carbon source after carbonization and activation by high temperature,the three-dimensional porous biomass derived activated carbon BPC was obtained.The precursor was prepared by precipitation method and hydrothermal method using BPC as substrate,and then calcined in air respectively,then two composites of Co₃O₄ and BPC,P-BPC and H-BPC,were obtained.By phase and structural characterization,it was proved that BPC,P-BPC and H-BPC with three-dimensional structure were successfully synthesized,and their formation mechanism was explored.The energy storage mechanism of the three samples was studied by electrochemical performance test.Compared with BPC(161.4 F g^-1),the specific capacitance of H-BPC(232.9 F g^-1)is greatly improved at the current density of 0.5 A g^-1 in 2 M KOH;the cycle stability is well,and the capacity retention is 80.35%after 1000 cycles.?2?facile fabrication of ZIF-derived graphene-based 2D Zn/Co oxide hybrid for high-performance supercapacitorsMetal organic frameworks?MOFs?with rich active sites,high porosity,large specific surface area and adjustable pore structure.So that the derivatives using MOFs as template can better preserve the pore structure,and the microporous and mesoporous can effectively improve the specific surface area and charge transfer.Therefore,MOFs drivatives have been widely used in the field of energy storage and conversion.But their Derived metal oxides have poor conductivity,that can affect the rate performance and cyclic stability.Graphene conducts electricity well and has a large specific surface area,but only provide a small electrochemical double layer capacitance because of easy reunion.The combination of the two,and design of its structure,improve the overall performance of the material.A simple solution reaction was used to synthesize the precursor?Zn/Co ZIF/GO?,which is the hybrid of zinc/cobalt dual metal zeolitic imidazolate frameworks?Zn/Co ZIF?and graphene oxide?GO?as.After calcination,Zn/Co oxide/GO?ZnCo O-G?was obtained.For comparison,Zn/Co oxide?ZnCoO?,Co oxide/GO?CoO-G?,and ZnCoC?without air oxidation?were synthesized by similar process.Through a series of characterization methods,it is proved that the synthesized ZnCoO-G contains ZnO,Co₃O₄ and other phases,and its surface has typical two-dimensional sheet structure and unique pore structure.The specific capacitance of ZnCoO-G is 711.6 F g^-1 at current density of 1 A g^-1,and 63.6%after 1000 cycles at 5 A g^-1.It shows better performance compared with ZnCoO,ZnCoC and CoO-G.ZnCoO-G//AC asymmetric supercapacitor was fabricated by using ZnCoO-G and commercial activated carbon as positive and negative electrode,respectively.It was tested in 2 M KOH with working voltage of 0-1.6 V.When the power density is 800 W kg^-1,the energy density is 18.7 Wh kg^-1,and when the power density is 9600 W kg^-1,it can still maintain11.5 Wh kg^-1.?3?Synthesis and supercapacitance of self-supporting graphene-based composite electrodeBinderless self-supporting composite electrode can provide better electrochemical performance without the use binders and conductive agents.The nickel foam?NF?was coated with GO as substrate by electrodeposition.A simple solution reaction and calcining were used to prepare the binderless graphene-based composite electrodes?Zn/Co oxide-GO/NF,O-GNF-X,where X is the electrodeposition time,minute?,and the O-NF electrodes were prepared using pristine NF by the same process for comparison.The specific capacitances of O-NF,O-GNF-2 and O-GNF-10 are 90.7 F g^-1,264.5 F g^-1,and 330.1 F g^-1,respectively,at the current density of 1A g^-1.When the current density rises to 10 A g^-1,the specific capacitance of O-GNF-10 can be kept at84.27%.After 3500 cycles at 5 A g^-1,the capacity retention rate increased to 116%.The results show that the specific capacitance of the samples after electrodeposition is higher than that of the samples without electrodeposition,and the specific capacitance is related to the deposition time of GO.