Electrode Optimization And Study Of Energy Storage Mechanism For Carbon-Based Flexible Micro-Supercapacitors

The Internet of Things is a rapidly growing concept which uses the Internet to interconnect physical devices,forming an expansive network called"Internet of Everything".Our lives have become more and more enriched and convenient than ever before because of the rise of small electronics,such as smart door locks,smoke detectors,air pollution monitors,health trackers and miniature wearable smart devices.It is crucial to develop micro flexible energy devices for power supply to facilitate the seamless integration of people and electronic technology.However,to meet the energy performance requirements of energy storage and conversion,as well as the miniature,flexible,wearable,integrable and implantable characteristics,it is essential to focus on the researches based on flexible micro energy storage devices.Micro-supercapacitors（MSCs）have emerged as a promising candidate because plenty of merits,such as ultra-high power density,long-lasting cycle life,safety and environmental friendliness.It is expected that MSCs will one day replace micro-batteries and become the mainstream energy source for micro-electronic devices equipment.However,micro-supercapacitors are still facing the limitation of insufficient energy density,which greatly limits its practical application.In this thesis,we will focus on how to solve the limitation of the energy density of MSCs,Specifically,we will explore the design of MSC electrodes using carbon materials,focusing on optimizing the electrode structure,adjusting electrode materials from the perspective of energy storage mechanisms.We aim to realize the optimal combination of the Electric Double Layer Capacitance（EDLC）mechanism and pseudocapacitive mechanism.We will cover a range of research topics,from mechanism design to experimental verification,in order to achieve MSCs with high performance.The specific research contents are as follows:（1）By using bacterial cellulose biomass fibers modified with organophosphorus pesticide macromolecules（ultrafine nanofiber network with a diameter of 20-100nanometers）,a highly interconnected three-dimensional carbon nanofiber network was constructed with high specific surface area.This conductive framework was used to confine pseudocapacitive material of Co₃O₄ nanoparticle,which was employed as cathode materials for MSCs.During the modification process of bacterial cellulose nanofibers,the organic macromolecules stretched the dense nanofiber membrane network,which greatly increased the specific surface area of the nanofiber network.At the same time,plenty of functional groups are introduced,which provide more active sites on the surface for adsorbing Co²⁺,realizing the final CNFs/Co₃O₄ compound.The compound has a uniform and consistent structure.In addition,the three-dimensional CNFs network also serves as the high-speed conductive framework for Co₃O₄nanoparticles,which not only provides an efficient and stable channel for electron transport,but also effectively buffers the capacity fading caused by the volume change of Co₃O₄ during charging and discharging.The resulting CNFs/Co₃O₄ composite exhibits a high specific capacitance of 785 F g^-1.The assembled CNFs/Co₃O₄//AC asymmetric MSC exhibits an extremely high areal capacitance of 607.5 m F cm^-2 at a current density of 0.5 m A cm^-2,as well as a maximum power density of 0.418 m W cm-2 and a high energy density of 0.216 m Wh cm^-2.（2）The improved Hummer method was used to chemically exfoliate graphite to prepare graphene oxide.Then the reduced graphene oxide（r GO）aerogel with three-dimensional interconnected nanosheets sponge structure was obtained by hydrothermal reduction and freeze-drying method.The r GO aerogel is pressed into a high-load graphene paper through a piston-like device,and then a MSC device with a planar interdigitated structure was prepared by an efficient and convenient laser engraving method.By characterization of Raman spectroscopy,Fourier transform infrared spectroscopy,and nitrogen adsorption and desorption,it is noted that the obtained sponge graphene aerogel was highly reduced with a hierarchical structure and pore distribution which is suitable for electrochemical double layers.The prepared device exhibited an ultrahigh area specific capacitance of 569.5 m F cm^-2 at a current density of 0.5 m A cm^-2,which is the state-of-the-art performance in the field of graphene-based MSCs.A high capacitance retention rate of 98.8%was realized after 20000 cycles at current density of 20 m A cm^-2.The device also exhibited excellent mechanical properties:there was no attenuation of capacitance after folding tests at bending angle of 0 to 180°;98.4%capacitance retention after 2000 cycles bending test at 90°.In order to meet the requirements of integration in practical applications,we also conducted series-parallel tests.（3）As we all know,graphene oxide will undergo a violent deoxidation reaction（"deflagration"reaction）during the laser reduction process,leading reduced graphene oxide（r GO）films into brittle and irregular internal structure which is harmful to the applications.We proposed a novel pre-reduction strategy,which specifically resolves the violent deoxidation reaction during laser reduction.The self-assembled r GO（PGO）skeleton structure obtained in the pre-reduction process endows the GO a regular and uniform pre-internal structure.During the laser reduction,benefit from the this kind of pre-frame,the whole structure will not be destroyed under a strong deoxidation reaction.Furthermore,a uniform and highly regular porous structure was obtained after laser reduction.Compared with PGO before laser reduction,the expansion rate of Lr PGO is more than 70 times.More importantly,the quasi-solid-state Lr PGO-MSCs directly prepared by laser engraving technology can provide a high areal capacitance of 88.32m F cm^-2 and a high areal energy density of 12.26μWh cm^-2.Besides,the Lr PGO-based MSCs have excellent flexibility,which ensures stable performance after 5,000 bending tests.（4）In order to further enhance the storage capacity of micro-supercapacitors,we introduce a fast and reversible electrolyte anion redox couple into the energy storage system.We chose the redox couple of bromine（Br^-/Br⁰）as the capacity source.Because compared with chlorine redox couple（Cl^-/Cl⁰,1.36 V vs SHE）and iodine redox couple（I^-/I⁰,0.54 V vs.SHE）,the redox potential of bromine（Br^-/Br⁰,1.08 V vs.SHE）is within the voltage window of the aqueous electrolyte（1.23 V vs.SHE）with a properly high reaction potential.We used cheap graphite paper as the electrodes of micro-energy storage devices.Due to the excellent conductivity of graphite,we do not need to introduce additional metal conductive current collectors.By anion intercalation to activate graphite electrodes,we found that compared with the energy storage mechanism of adsorbing the oxidized Br⁰ on the surface of unactivated graphite,the activated graphite electrodes can realize the storage mechanism of intercalation of Br into graphite,which effectively reduces the loss of Br⁰ and improves Coulombic efficiency.The assembled graphite-based Zn-Br micro energy storage devices can achieve high areal capacity(0.27 m Ah cm^-2)and can be cycled for 360 cycles without attenuation.