Design Of Electron Transport Channels In Carbon Nanofiber/carbon Nanotube@MnO₂ Composite Electrodes

With electronic products being more and more portable and wearable,energy storage devices,as one of their main components,has begun to focus on the electrochemical and mechanical stabilities of the devices.Recently,as a new kind of energy storage devices,supercapacitors have attracted intense attention from scientific research and industry,due to their high power density,super-high cycling stabilities,as well as safety and environmental-protection characteristcs.However,the reported researches indicate that the energy densities of supercapacitors are still far less than those of the commercial lithium-ion batteries.Therefore,how to improve the energy density and flexibility of the supercapacitors is the focus of the current research on their wearable devices.In this thesis,the authors have ultilized the excellent electrical and mechanical properties from carbon nanofibers?CNFs?and carbon nanotubes?CNTs?to build two types of carbon-based electron transport channels,highly improved the utilization efficiency of pseudocapacitive materials?MnO₂?,and accordingly enhanced the electrochemical performances of the assembled devices.Moreover,the authors have systematically investigated the optimization of preparation parameters for CNF/CNT supports,studied the effect of CNT hierarchical structures on the energy storage of their composite pseudocapacitive electrode materials,and selected the optimal composite electrode materials for device packaging and electrochemical tests.The main research contents are as follows:?1?CNF@MnO₂ composite electrode materials by one-step methodIn order to improve and simplify the preparation process of carbon-based pseudocapacitive composite materials,this thesis presents a one-step electrospinning method for the preparation of CNF/MnO₂ supercapacitor electrode materials,i.e.,to directly incorporate MnO₂ nanoparticles or manganese acetylacetonate powder into the N,N-dimethylformamide?DMF?solution of the precursor polyacrylonitrile?PAN?for electrospinning.This method greatly simplifies the preparation process ofcomposite materials,and also provides a potential program for large-scale industrial preparation.Our research indicates theincorporated manganese acetylacetonate could be decomposed into MnO₂ materials during the process of carbonizing PAN nanofibers into CNFs.The MnO₂ materials are of granular crystal and evenly dispersed on the surface of CNFs,which will be conducive to the full contact and reaction with ions in the electrolyte.Electrochemical tests showed that the obtained CNF/MnO₂ electrode materials show a specific capacitance of 220 F g-1.However,as for the flexibility testing,the CNF/MnO₂ electrode material show poor flexibility because of the low quality of CNFs,which still required further work to improve it.?2?CNF/CNT@MnO₂ composite electrode materialsIn order to improve the flexibility of CNF skeletons,this thesis presents a type of electron transport channel of hierarchical CNFs/CNTs,i.e.,to grow CNTs hierarchies on the surface of CNF by chemical vapor deposition?CVD?.First,to incorporate Iron?III?acetylacetonate,Fe?acac?3 powder into the DMF solution of PAN,followed by electrospinning;and then,the obtained polymer nanofibers were carbonized into a type of Fe-doped CNFs;finally a layer of CNT herarchies were grown on the CNFs under catalysis of Fe ion after introducing carbon source into furnace.Results showed that the prepared CNFs/CNTs composites have high specific surface area,high electrical conductivity,and greatly improved mechanical flexibility.After modified with MnO₂ pseudocapacitor materials,the CNF/CNT@MnO₂ electrodes can output their maximum specific capacitance of 472.3 F g-1,good rate capability?the specific capacitance still remained 45% when current density increases to 30 times?,and good cycle stability?the specific capacitance remains 95% after 1500 cycles?.?3?Optimization of CNF/CNT@MnO₂ composite electrode materialsTo further investigate effect of CNT hierarchical structure on the electrochemical performances of CNF/CNT@MnO₂ composite electrode materials,this thesis systematically design various experimental parameters to control the microstructure of CNF/CNT frameworks and their electronic and mechanic behaviours,i.e.:?i?to tune the diameter of CNT s skeletons via changing the mass ratios of PAN in precursor solution;?ii?to tune the density of CNT hierarchies via changing the massratios of Fe?acac?3,and?iii?to tune the lengths of CNT hierarchies via their growth time?flowing time of carbon sourface,C2H2?.Results showed that the microstructure of CNF/CNT frameworks impose a great influence on the electrochemical performances of CNF/CNT@MnO₂ composite electrodes.The optimal preparation parameter is as follows: the content of PAN is 8 wt.%,the content of Fe?acac?3 is 6-8wt.%,and the growth time of CNTs is 25 min.At the end,the optimal CNF/CNTs@MnO₂ composite electrodes were assembled into a type of symmetric supercapacitors,whose can output energy density of 19.11 W h kg-1,and their maximum power density can reach 25,000 kW kg-1.At the same time,the assembled devices also showed excellent flexibility and cycling stability.