| In recent years,with the enhancement of people’s awareness of environmental protection,the development of clean and renewable energy has attracted much attention,and the research and development of efficient storage equipment for this kind of energy is imminent.Supercapacitors with high energy density are one of the ideal choices to achieve efficient energy storage.Compared with traditional double-layer supercapacitors,hybrid supercapacitors not only have higher power density and rapid charge and discharge,but also have higher energy density,which can well meet the requirements of efficient energy storage.Hybrid supercapacitors generally use a combination of battery positive and capacitive negative electrodes.In recent years,nickel-cobalt-based compounds have been used as cathode materials for batteries and carbon materials as cathode materials.On the one hand,Ni-Co compounds have disadvantages such as poor cycling stability and less deposition on conductive substrate,which will affect their practical application.In this paper,Ni-Co layered double hydroxides were grown in-situ on carbon cloth oxidized by nitric acid by electrochemical deposition method.Compared with the existing studies,the deposition mass per unit area of active material on carbon cloth was significantly increased and the capacitance performance was greatly improved.By controlling the composition of the reactants in the electrodeposition experiment,it was found that the structure and morphology of the products changed significantly.By adjusting the reaction time,it was found that the product mass was proportional to the reaction time.On the other hand,carbon materials also have the disadvantage of having a low specific capacitance,which is further reduced when used with organic binders.Therefore,in this paper,carbon cloth with excellent flexibility,high electrical conductivity,three-dimensional structure,mechanical stability and low cost was selected as the anode material,and its hydrophilicity and capacitance performance were enhanced through functional treatment.To explore the underlying causes,we used first principles to establish a graphite structure consistent with the experimental characterization.Density functional theory was used to calculate the adsorption energies of different graphite structures on water molecules and potassium atoms.The controlling factors affecting the hydrophilicity and electrochemical properties of carbon cloth were studied theoretically.The microcosmic mechanism and theoretical mechanism of the improvement of the performance of carbon cloth were analyzed,which provided reliable theoretical guidance for the further research and development of high-performance graphite-based carbon materials.The main contents of this paper are summarized as follows:（1）In this paper,commercial carbon cloth was functionalized by fast melting salt method for the first time to obtain nitrogen doped carbon cloth（NCC）,which solved the problems of small capacitance and poor hydrophilicity of commercial carbon cloth.The electrodes made from it have excellent areal capacitance and capacitance retention.This study provides a simple and effective method for the preparation of ultra-high capacity activated carbon cloth,a new anode material for high performance supercapacitors.On the one hand,the areal capacitance of the treated NCC is about 370 times that of the original carbon cloth,which provides a new way to improve the energy storage performance of the carbon-based material.On the other hand,after functionalization,the hydrophobicity of the carbon cloth was changed to hydrophilicity,which improved the electrochemical properties of the carbon cloth.By adjusting the reaction temperature and reaction time,it is found that they have certain influence on the morphology,quality and surface element distribution of the carbon cloth.When the reaction temperature increased from 380°C to 440°C,the nitrogen content on the surface of the carbon cloth increased,while the oxygen content decreased.The mass of carbon cloth also decreased with the increase of reaction temperature and the prolongation of reaction time.Compared with the original carbon cloth,the specific surface area became larger.The SEM results showed that the surface morphology of the NCC samples became rough with the increase of temperature.The SEM and STEM images for the ends of the carbon fibers showed that the surface of the carbon fibers presented fluffy activation layer.The corresponding element distribution mapping also clearly showed the uniform distribution of elements such as C,O and N on the surface of carbon fiber.The electrochemical properties of NCC electrodes prepared at different reaction temperatures and times in 6 M KOH alkaline electrolyte had been systematically investigated.When the reaction temperature was 420°C and the reaction time was 10min,the areal capacitance of the prepared NCC-420 electrode could reach 6.72 F cm-2 when the current density was 2 m A cm-2.Even when the current density was 32 m A cm-2,the areal capacitance could reach 3.53 F cm-2,showing excellent electrochemical performance.Compared with the experimental characterization results,we believed that the increase of specific surface area and surface nitrogen content played an important role in improving the electrochemical performance of NCC.The electrochemical performance of a single NCC-420 carbon fiber electrode was further investigated.It was found that the capacitance of the NCC-420 carbon fiber electrode could reach 132 m F cm-1 when the current density was 0.5 m A cm-1.The capacitance retention rate of the NCC-420 carbon fiber electrode was 98%after 5000 galvanostatic charge-discharge cycles,showing excellent cycling stability.The Ni-Co-S nanosheet arrays were grown in-situ on carbon cloth by electrodeposition method,and the scanning rate during the reaction was controlled.It was found that the scanning rate had a significant effect on the morphology change of Ni-Co-S.The electrochemical test of the samples prepared at different scanning rates showed that the Ni-Co-S@CC electrode prepared at a scanning rate of 3 m V s-1 had the best electrochemical performance,and the areal capacitance of the electrode reached 6.77 F cm-2 when the current density was 4 m A cm-2.We further investigated the electrochemical performance of a hybrid supercapacitor with NCC-420 as the negative electrode and Ni-Co-S-3@ACC as the positive electrode.The operating potential window was up to1.8V,and the capacitor had a very high areal and volume capacitance(4.18 F cm-2,52.25 F cm-3).The maximum volume energy density was 23.51 m W h cm-3 at a power density of 1808 m W cm-3.After 10,000 cycles,the final capacitance retention rate was92%.（2）Ni-Co layered double hydroxides were grown in situ on carbon cloth by electrochemical deposition method,and the synthesis parameters such as reactant composition and reaction time were obtained to effectively regulate the morphology,structure and properties of the products.The cathode materials with excellent electrochemical performance were obtained,and the cycling stability was significantly improved.Before the synthesis,the carbon cloth was oxidized with nitric acid to obtain acid-activated carbon cloth（ACC）,which improved its hydrophilicity and was conducive to the deposition and growth of active substances.In the electrodeposition process,the composition of reactants and reaction time were controlled,and it was found that the composition of reactants and reaction time had certain influence on the structure,morphology and mass distribution of the products.When the reactants contained only nickel acetate and cobalt acetate,the resulting product was cobalt metal,because the acetate solution was weakly alkaline and the cobalt metal did not dissolve.Ni Co（N）-LDH was synthesized only when the reactant solution contained acidic cobalt nitrate and nickel nitrate,and when the Ni/Co nitrate and acetate coexisted,the resulting Ni Co（NA）-LDH formed a uniform and compact three-dimensional mesh structure on the carbon cloth.Because nitric acid was a strong electrolyte,and acetic acid was a weak electrolyte.The concentration of free moving ions in nitrate solution was much higher than that in acetate solution,which resulted in the conductivity of acetate solution being lower than that in nitrate solution.In the electrodeposition process,the current was maximum when the electrolyte contained only nitrate ions.When the electrolyte contained only acetate ion,the current was the lowest.When the two ions coexisted,the current was between them.So this regular three-dimensional grid is formed by using a mixture of Ni/Co nitrates and acetate as reactants during electrodeposition,so as to achieve control free mobile ions in the electrolyte concentration,adjust the response current and ion movement speed,finally control reaction process to achieve the purpose of adjusting the product morphology.SEM results showed that the size and pore size of Ni Co（NA）-LDH nanosheets increased with the increase of reaction time.XRD and TEM images also confirmed the low crystallization and layered structure of Ni Co（NA）-LDH,which enabled it to have more active sites and ion channels.By comparing the electrochemical properties of ACC,Co@ACC,Ni Co（NA）-LDH@ACC and Ni Co（NA）-LDH@ACC electrodes in 6 M KOH alkaline electrolyte,it was found that the Ni Co（NA）-LDH@ACC electrode had the best electrochemical performance.When the mass load of the active material was 16 mg cm-2 and the current density was 10 m A cm-2,the areal capacitance and mass specific capacitance of Ni Co（NA）-LDH@ACC electrode could reach 27.3 F cm-2 and 1709 F g-1.It was worth mentioning that even when the mass load reached 26 mg cm-2,the mass specific capacitance of the material barely changed,indicating that the electrochemical properties of the material were not affected under the condition of such a large mass deposition,and the material was highly involved in the energy storage process.The kinetic behavior of charge storage at Ni Co（NA）-LDH@ACC electrode in KOH electrolyte was investigated.The results showed that the charge storage process of Ni Co（NA）-LDH@ACC electrode was controlled by both capacitance and diffusion,and diffusion control accounts for a large proportion.In addition,the hybrid supercapacitor assembled with Ni Co（NA）-LDH@ACC as the positive electrode and AECC as the negative electrode could achieve an energy density of 2.24 m W h cm-2at a power density of 3.71 m W cm-2,and had good cycle stability（104%after 10,000cycles）.The coulomb efficiency remained around 100%during the whole process.The hybrid supercapacitor device composed of 2 cm fibers could keep the digital electronic watch running for 1 hour.A hybrid supercapacitor with an area of 2 cm2 could even drive an electric fan when charged to 1.7 V,fully demonstrating the potential value of such hybrid supercapacitors in practical applications.（3）According to the experimental characterization of carbon cloth,a carbon cloth model based on graphite structure was established.Density functional theory was used to study the adsorption properties of different graphite structures on water molecules and potassium atoms.The controlling factors that affect the hydrophilicity and electrochemical properties of carbon cloth are theoretically revealed,which provides theoretical support at atomic level for analyzing and guiding experiments.The adsorption energy of water molecules on the surface of graphite with the presence of epoxy group,hydroxyl group and defects was studied.It was found that the presence of oxygen-containing groups and defects significantly improved the hydrophilicity of graphite surface.When epoxy and hydroxyl coexisted,the adsorption energy increased obviously.The combination of the defects with epoxy groups showed excellent adsorption properties,and the theoretical calculation results were in good agreement with the experimental results.The structure of B,N,S and P doped graphite and its adsorption energy with water molecules were simulated.It was found that B and N doped graphite had little effect on the planar structure,while S and P doped graphite would lead to the structure protrusion plane around the doping site,resulting in structural deformation.The calculation of adsorption energy showed that all the doping could enhance the adsorption of water molecules,and the S-doped graphite had the highest adsorption energy.It was concluded that the S-doped graphite structure might have a potential application prospect in the field of energy storage.Combining with the experimental characterization results of nitrogen-doped carbon cloth,a nitrogen-doped graphite model was established,and the adsorption energy of potassium atom for nitrogen-doped pyrrole（N-5）and pyridine（N-6）was mainly discussed.The results showed that the adsorption energy of potassium atom was enhanced obviously with the increase of doping amount of N-5 and N-6,especially the change of adsorption energy caused by N-6 was much greater than that of N-5.This also proved that N-6 doping played an important role in improving the capacitance of nitrogen-doped carbon cloth,which was consistent with the experimental conclusion.The adsorption energy of potassium atom on graphite surface was studied with the presence of epoxy group,hydroxyl group and defect.It was found that the adsorption energy of potassium atom on graphite surface was obviously enhanced with the presence of epoxy group and defect.In the presence of hydroxyl group,due to the interaction between hydroxyl group and K atom,the position of hydroxyl group on the graphite surface changed significantly,thus affecting the stability of the graphite surface structure.By contrast,the structure of epoxy group on the graphite surface was more stable. |
PDF Full Text Request