Study On The Influence Of Surface/Interface Regulation On The Capacitive Properties Of "Green" Self-supporting Electrode Materials And Its Mechanism

The depletion of fossil fuels and the growing environmental problems have greatly affected the development of the"green and rich"society.The popularity of hybrid vehicles and portable wearable devices has increased the demand for"green"high-power energy.Supercapacitors（SCs）have attracted wide attention owing to their unique advantages,such as low energy consumption,high power density,and ultra-long living life.SCs fall into two groups depending on the energy storage mechanism:pseudo-capacitors and double-layer capacitors.They are derived from the electrochemical charge regulation at the double-layer and the occurrence of Faraday reaction,respectively.And either energy storage mechanism is related to the surface activity of the electrode.According to the Helmholtz equation,only substances exposed to the electrolyte can contribute to the storage of electrical energy.Accordingly,it is clearly that surface-interface modulation plays a decisive role in enhancing the energy storage performance of electrodes.Most of the electrodes used in commercially available SCs are powder,which have inherent problems such as poor contact.In contrast,self-supporting electrodes show their unique advantages.First,the stacked or interconnected 3D/2D network structure can greatly increase the contact area between electrodes and electrolyte,reducing the diffusion resistance during device operation.Secondly,there is no need to add non-conductive and non-active binders,reducing the cost of use and improving the theoretical specific capacity of the electrode.More importantly,self-supported electrodes can be used as flexible electrodes,thus finding new practical applications in high-performance energy storage devices with long-term stability.In this paper,a series of inexpensive and high performance"green"self-supporting electrodes are prepared based on surface/interface modulation strategies（morphology,defects,doping,heterostructure effects）,and the application of electrode materials in supercapacitor is further investigated systematically.The internal chemical structure of the electrode materials was analyzed by various advanced fine structure characterization techniques.The"constitutive relationship"between the electronic structure of the surface/interface and the energy storage performance of the electrode material is also illustrated through a combination of experimental analysis and theoretical calculations,providing a theoretical basis for the future commercialization of"green"and efficient self-supporting electrodes.The study is divided into five chapters:Based on the morphological structure effect,urchin-like amorphous carbon anchoring on nickel foam（UAC@NF）is synthetized,which is acted as the positive electrode.And the negative electrode is graphite carbon felt（GF500）with high temperature activation.Both of which are assembled into an asymmetric supercapacitor.The device can attain a high energy density of 0.036 m Wh cm^-3 at 0.984 m W cm^-3 and has excellent recyclability;it can maintain a capacitance retention rate of 91.4%after 10,000 cycles.Theoretical calculations show that the insertion of cobalt atoms between the amorphous carbon（UAC）and nickel foam（NF）well enhances the conductivity of the composite,greatly increasing the electron transfer efficiency and the structural stability.This study can provide a new perspective for the application of amorphous carbon materials in the energy field,which can be used to promote the commercialization of self-supporting electrode materials.Based on the defect effect,carbon quantum dots rich in oxygen vacancy defects were loaded on the surface of graphite carbon felt to obtain CDs@graphite felt-600.Due to their ultra-small size and inherent surface/edge defects,carbon quantum dots,a 0D carbon material have excellent capacitive properties.However,their poor electrical conductivity and high tendency to agglomerate pose challenges to the practical application of carbon quantum dots.By constructing a heterostructure that brings together the advantages of carbon quantum dots and graphitic carbon felt,as expected,the synthesized CDs@graphite felt-600 shows an excellent area specific capacitance that can reach 5.99 F cm^-2.In addition,a homemade button supercapacitor using CDs@graphite felt-600 as electrodes can provide an areal specific energy density of 20.7μW h cm^-2 at 150.0μW cm^-2.To determine the charge storage mechanism at the CDs@graphite felt-600 interface,the binding energy between CDs and graphitic carbon felt was calculated by density functional theory（DFT）.It provides a theoretical basis for design of the structural and performance of other carbon/metal quantum dot-based electrode materials.Based on heterojunction effect,three-dimensional porous composite electrode material,graphite carbon felt/polypyrrole with less"dead volume"（GF@PA@PPy）,was prepared by phytic acid induction.When the polypyrrole film is directly deposited on the surface of graphite carbon felt,only the uneven and fragile polypyrrole film can be obtained,and it is difficult to obtain electrode materials with high energy density.Through phytic acid induction,the volumes and categories of defects and oxygen-containing groups on the surface of carbon felt（GF）are increased.First,these functional groups improve the hydrophilicity of GF surface and can guide the preferential and uniform distribution of pyrrole monomer（Py）.More importantly,the synergy between these defects and oxygen-containing groups increases the attraction of pyrrole ring,thus promoting the formation of"perfect"polypyrrole film.Finally,the supercapacitor assembled by GF@PA@PPy-40 shows a high area specific energy density of 0.0732 m Wh cm^-2,which is superior to the current commercial PPy-based electrode.Therefore,we have a deeper understanding of the important position of oxygen-containing functional groups and defects in the polymerization process of polypyrrole film,and provide a new strategy for the construction of advanced PPy-based SCs.Based on chapters 2～4,a flexible,customizable,conductive,and environmentally friendly nanofiber composite membrane for flexible SCs is proposed,considering the high performance and ecological applications of SCs.The PLA/PDA/GO/PPy nanofiber composite film is obtained by sequentially overlaying functional coatings on a biodegradable poly（lactic acid）（PLA）nonwoven fabric.The multiple coatings on PLA provide good flame retardancy and greatly increase the capacitive performance.The optimized PLA/PDA/GO/PPy demonstrated an area specific capacitance of up to 1331 m F cm^-2.The symmetric aqueous SCs assembled using PLA/PDA/GO/PPy electrodes showed higher energy density than other nanofiber composite film-based SCs reported in the literature.This study provides a feasible path for the compatibility of more pseudo one-and two-dimensional materials on biodegradable PLA,which can be applied to more multifunctional electronic devices.Recently,lightweight elastomeric carbon materials have attracted great interest for energy storage in wearable devices.A woody elastic carbon aerogel with a three-dimensional network structure is proposed by further expanding on the basis of chapters 2～5.Using cellulose nanofibers as a backbone and chitosan as an additive,an interconnected framework is self-assembled to obtain a lightweight elastic carbon aerogel（CS/CNF-CA）by carbonization.Unlike previous studies,CS/CNF-CA is a substrate-free electrode material with a homogeneous and dense internal structure that is very stable.In fact,the carbon aerogels exhibit excellent mechanical properties,especially high compressibility（up to 80%strain）.Moreover,the assembly of CS/CNF-CA into symmetric SCs shows satisfactory electrochemical properties.These characteristics make wood-derived carbon aerogels very attractive for flexible electrode applications and highly competitive in the field of multifunctional electrode materials.