KOH Activation And Energy Storage Mechanism Of Three-dimensional Carbon Nanotube/carbon Nanofiber Composite Structure

Supercapacitors have been considered as a new complemented or replaced device for batteries in rapid energy storage and high power transmission because of its significant high power density,long cyclic stability and high safety.Carbon material has been one of the most eye-catching supercapacitor electrodes with its rich accessibility,nontoxicity and high chemical stability.Currently,research concerns on carbon-based electrode materials have been focused on the nano morphology controlling,porous structural optimization,surface modification and coupling with pseudocapacitance materials.Many researchers grow carbon nanotubes?CNTs?on skeletons via an in-situ chemical vapor deposition?CVD?method to improve the mechanical properties of carbon-based electrode materials.On one hand,the hierarchical structure of CNT can increase the specific surface area of the electrode,which is beneficial to energy storage.On the other hand,the in-situ preparation of CNT can greatly boost the mechanical flexibility of the electrode material and facilitate the application.However,in most cases,the tips of CNTs are almost well closed,which often limits the utilization efficiency of their tubular structure in storage of charges.In this thesis,authors proposed to open the CNT tip to enhance the electrochemical performance of composites and explored the energy storage mechanism of tip-open CNTs.The basic idea is:First,authors prepared the carbon nanotubes/carbon fibers?CNT/CNF?composite with uniform CNTs grown on CNF skeletons via CVD;And KOH activation were used to open the tips of CNTs for the preparation of tip-open CNT/CNF?t-CNT/CNF?composite structure;Finally,the charge storage ability of activated CNT/CNF were tested to analyze the energy storage mechanism.The main research contents are as follows:?1?three-dimensional CNT/CNF hybrids structure preparation and optimizationIn this thesis,authors firstly obtained the carbon nanofiber skeletons using electrospinning,and grew the CNTs hierarchical structure on the CNFs with Al-Fe catalyst via CVD method for three-dimensional CNT/CNF hybrids structure.Then,KOH activation were used to open the tips of CNTs,obtaining t-CNT/CNF composite.Finally,the electrochemical performance of this hybrid structure was tested.?i?CNTs hierarchical structure optimizationAccording to comparing the microstructure of CNT/CNF by regulating the aluminum content in the Al-Fe composite catalyst,authors found out the optimum catalyst:the mass concentration of Al salt is 5.5%.After KOH activation?700?-120min?,three-dimensional t-CNT/CNF hybrid structure were obtained.Tip-open CNT/CNF hybrid electrodes can exhibit a greatly improved specific capacitance,which is 4-5 times that of the pristine ones in 2M Li₂SO₄.(The specific capacitance increase from 9.3 F g^-1 to 43.3 F g^-1 at scan rate 5 mV s^-1.)?ii?KOH activation process optimizationIn order to further optimize the performances?including the specific capacitance and structural stability?of t-CNT/CNF hybrid structure,systematic research has been conducted on the conditions of KOH activation process in this thesis.By regulating the temperature and activated time of the KOH treatment,authors characterized the structural morphologies,electrochemical performances and surficial defects of samples from different conditions,detecting the optimum of KOH treatment:the temperature is700-750?and the activation time is 90 min in Ar.In addition,the research shows that KOH can not only open the CNTs'tips,but also generate mesopores into CNT/CNF skeletons.It is beneficial for ion diffusion and storage to provide more active sites and increase specific surface area.?2?Analysis on energy storage mechanism of three-dimensional t-CNT/CNF hybrids structureAuthors divided the capacitance into two types:it is defined that diffusion type Helmholtz type capacitance?C_H?stands for the portion of capacitance which caused by surface charge storage,and diffusion type capacitance(C_diff)for the portion caused by rapid ion diffusion/transport.Meanwhile,sweep voltammetry analysis proposed by Prof.Bruce Dunn was conducted to calculate the two types of capacitance of t-CNT/CNF structure from different conditions and to analyze the effect of KOH activation process on C_diff and C_H.According to the comparison with the activation on CNTs,the contribution of t-CNTs in the t-CNT/CNF composite structure was ascertained.In addition,authors discussed the influence of electrolyte ions on C_diff and C_H by analyzing the energy storage mechanism of three-dimensional t-CNT/CNF composite in different electrolyte.