Preparation Of Novel Graphene Based Electrode Materials And Research On Performance Of Flexible Supercapacitors

With the increasing popularity of wearable electronic devices,flexible energy storage devices have become an important focus.Flexible supercapacitor is a kind of ideal energy storage device for wearable electronic devices due to its features of unique energy storage property,excellent stability,satety and pro-environment.The device structure and electrode material are two important influence factors for the application effect of flexible supercapacitor.In the common types of wearable electronic devices,fiber-shaped and planar flexible supercapacitors exhibit high research and application value.In electrode materials,graphene has become a kind of hotspot material in supercapacitor field due to its characteristics of large specific surface area and highly electrical conductivity.Particularly,reduced graphene oxide?RGO?is very suitable for the research and preparation of electrode materials of flexible supercapacitors by virtue of the advantages of low cost,good scalability and high yield.Meanwhile,graphene oxide?GO?as the precursor of RGO has the feature of excellent processibility,which is benefit to prepare high performance electrode materials.In this doctoral dissertation,high performance RGO electrode materials and flexible supercapacitors with fiber-shaped and planar structures were investigated for the applicaton background of flexible energy storage devices for wearable electronic devices.The main contents of this dissertation are divided into five parts and briefly discussed as follows:?1?RGO and carbon nanoparticles?CNPs?coated cotton thread?CNPs/RGO-CT?was prepared by dip-coating combined with low temperature vapor reduction.Then,the fiber-shaped flexible supercapacitor based on CNPs/RGO-CT was assembled.RGO sheets tightly coat on cotton fibers by electrostatic attraction to form the continuous electron transport channels.CNPs can further improve capacitance.The fiber-shaped supercapacitor based on CNPs/RGO-CT has a volumetric capacitance of 3.79 mF cm^-3and good knittability.The capacitive performance can well maintain under different bending angles,bending radii and bending states.?2?Adopting oxidized carbon yarn and iodine-doped RGO as substrate and electrode material to prepare fiber-shaped elelctrode?I-RGO/OCY?and further assemble into fiber-shaped flexible supercapacitor.Oxidized carbon yarn maintains well electrical conductivity,oxygen-containing functional groups located at the surface of oxidized carbon yarn can improve its electrochemical activity.Iodine doping can improve the carrier concentration of RGO,thus,RGO exhibits well electrical conductivity.The optimal electrode is I-RGO/OCY-3 with oxidation time of 3 h and electrical conductivity of 179.1 S cm^-1.The fiber-shaped supercapacitor based on I-RGO/OCY-3 exhibits specific capacitances of 1.7 mF cm^-1 and 520 mF cm^-3.Meanwhile,the capacitance of fiber-shaped supercapacitor almost linearly increases with device length increasing?0.5⁷ cm?.Furthermore,fiber-shaped supercapacitor also possesses excellently electrochemical stability and series-parallel performance.?3?An in-situ chemical reduction method for rapid preparation of RGO film was developed.The strong reducing property of hydroiodic acid and the strong volatility of acetic acid are key factors for achieving the rapid reduction and film formation of GO.Meanwhile,the inside-out dissipation of mixed acid solution forms the fluffy layered structure in RGO fim.Thanks to the high degree of reduction and iodine-doped effect,RGO film exhibits electrical conductivity of 6400⁶800 S m^-1.Due to the continuously layered structure,the fracture strength and elongation of RGO film are 106.23 MPa and1.41%,respectively.The sandwich-type flexible supercapacitor based on RGO film shows excellently electrochemical stability,especially,the capacitance has no significant decay under different bending states and bending cycles.?4?A rapid heat pressing method was developed to prepare crumpled RGO sheets by airflow disturbance,then,the crumpled RGO sheets were assembled into RGO film with fluffy layered and porous structure?f-RGOF?.Crumpled topography can effectively inhibit the restacking among RGO sheets.Therefore,f-RGOF exhibits improved specific surface area,specific capacitance and rate capability comparing to RGO film with layer-by-layer structure.Typically,f-RGOF exhibits a gravimetric capacitance of 238.4 F g^-1 at0.5 A g^-1,and high capacitance retention of 67%even up to 80 A g^-1.Meanwhile,the gravimetric capacitance and rate capability of f-RGOF have no significant decay with areal density increasing(1².6 mg cm^-2)due to the improved utilization rate.Furthermore,the flexible supercapacitor based on f-RGOF exhibits well volumetric performance and electrochemical stability,the volumetric capacitance and energy density of flexible supercapacitor are 12.5 F cm^-3 and 1.7 mWh cm^-3,respectively.?5?Combining drilling and intercalation strategies to construct the three-dimensional framework structure in RGO film?PPDA-HGF?.In PPDA-HGF,the nanopores of RGO sheets improve transport depth and speed of ion in film,the PPDA molecules covalent bonding to RGO sheets can inhibit the restacking among sheets and expand interlayer spacing,which is beneficial for ion transport between layers.The above strategies form synergistic effect to facilitate ion transport in PPDA-HGF.Thus,PPDA-HGF exhibits improved specific capacitance and rate capability comparing to conuterparts.Typically,PPDA-HGF has gravimetric and volumetric capacitances of 300 F g^-1 and 516 F cm^-3 at0.5 A g^-1,and high capacitance retention of 78.3%even up to 100 A g^-1.The volumetric capacitance and energy density of flexible supercapacitor based on PPDA-HGF can reach at 19.6 F cm^-3 and 2.7 Wh L^-1.