Preparation And Electrochemical Performance Of Carbon Cloth Self-supporting Electrodes

Recently, Faced with sustained serious energy crisis and environmental issues, it is imminent to create green, environmental, high efficient materials and technological achievements. Supercapacitors, as a new energy storage device between a secondary battery and conventional electrostatic capacitor, show great practical valuesbecause of its high power density, short charge and discharge time and a long service life, so as to be considered the next generation of energy storage and conversion devices. All along,backward production technology and low specific energy density have became two main factors which limit the development of supercapacitors, so it should be the current application of hot area of research to increase its energy density, develop new technology and means of production while maintaining high power density. Based on differences in the energy storage mechanisms, supercapacitors can be divided into electric double layer capacitor which is carbon-based material, and pseudocapacitance capacitor that relies on the reversible electrochemical oxidation and reduction reaction of the metal oxide and conductive polymers.In recent years, the researches on supercapacitor are not only to develop new electrode materials, but also the assembly and performance optimization of supercapacitor devices. The advanced wearable mobile devices and flexible electronics equipment especially the increasing demand for the energy storage devices under the human’s pursuit of efficient and simple way of life have been improved. Loading active materials or in situ growing on porous flexible substrates can eliminate the negative impacts on the performance and stability of the device capacitance which are caused by conventional tabletting and the additon of binders. This article did a systematic study focusing on commercial carbon cloth both as the collector of the flexible substrate andas a carbon source, respectively. Based on the superior conductivity and multivalent centers of the spinel materials, this article designed and synthesized a porous nickel-cobalt oxide two-dimensional sheet structure with a high specific surface area.Meanwhile, the article broke the traditional knowledge which considered carbon cloth only can be as the collector. Based on the bifunctional roles both as a flexible substrate and a carbon source of the carbon cloth, this paper prepared the porous core-shell carbon fiber carbon colth electrode by a two-step process including chemical oxidation and reduction of gamma ray irradiation, and the electrochemical properties of the material were tested. The main contents are summarized as follows:(1) Solid-state symmetrical supercapacitor based on in-situ growth of nickel-cobalt oxide two-dimensional carbon cloth electrode: we use P123 and EG composite additive for the first time to in situ grow thin mesoporous mesh nickel-cobalt oxide nanosheets on carbon cloth by a hydrothermal method. Comparisons of the electrochemical performances have been made for different morphologies of cobalt-nickel active materials which are synthesized under different additives. SEM and TEM images show that the addition of P123 and EG helps the formation of a thin mesoporous nano-dimensional film, and these nanosheets will grow intertwined to form a three-dimensional network structure. The network structure is favorable for the transportation of electrolyte ions, appearance of the fast reversible Faraday reactions and chemical and thermal stability for the electrochemical properties. The bridged mode connection of nanosheets and the superior conductivity performance of nickel-cobalt improved electrical conductivity. The in situ growth of nickel-cobalt on carbon cloth eliminated the effect of the resistance in the active material of binders to the electrolyte ions and the contact resistance of the active materials to the substrate.Performance tests show that this nickel-cobalt electrode material have a high specific capacitance(1843.3F/g) at the current density of 1 A/g. And the carbon cloth electrode prepared has good rate capability(achieve 1481 F/g when the charging current is 32 A/g) and excellent cycle lifes(capacitance remains 90% after 4000 cycles). On this basis, we have assembled a flexible and symmetrical device which can maintain good electrochemical stability under conditions of bending and twisting. Besides, the energy density ofnickel-cobalt system is greatly improved(38.3 Wh/kg) while maintaining a high power density(14 k W/kg) under the electrochemical tests.(2) We have successfully designed and synthesized the porous core-shell self-supporting carbon fiber electrode by a two-step method and applied it to the flexible supercapacitors. For this part of job, we demonstrate the double features of the carbon cloth: collector and a carbon source. Firstly, we use nitric acid, concentrated sulfuric acid, hydrogen peroxide, potassium permanganate or other strong oxidants to chemically oxidized commercial carbon cloth, which will increase the chemical properties and the wettability of carbon cloth, and getting the pore surface on carbon fibers. Meanwhile, a number of oxygen-containing functional groups are introduced on the surface of carbon fibers, and these functional groups will hinder the electron transport and induce decomposition of the electrolyte. Then we use gamma ray irradiation for the reduction of functionalized surface on carbon fibers in the condition of KOH alkaline solution, resulting in the formation of porous core-shell carbon fiber electrode active material.Impedance and TEM analysis show that this porous core-shell structure has a small resistance and is favor of the electronic transprotation of the electric double layer capacitor. Experimental results show that the porous carbon fiber carbon cloth electrode has a high loading(12 mg/cm2), an high area capacitance(702m F/cm2 at a charging current of 1 m A/cm2), excellent rate performance(still up to 610 m F/cm2 at 20 m A/cm2) and long cycle stability and reversibility(achieve the initial capacitance of 110.5% after 30,000 times of electrochemical cycling at different current densities). This discovery greatly increased the capacitance of commercial carbon cloth,a 2,018-fold increase. Further inquiry indicates that this assembled solid symmetrical supercapacitor which based on the activated carbon cloth has a very high energy density(0.556 m W h/cm~3).