Rational Design Of Nanostructured Electrodes And Their Supercapacitance Enhancement Mechanism

Supercapacitor is an energy storage device that bridges the gap between battery and traditional dielectric capacitor,and it arouses tremendous attention due to its higher power density than batteries,higher energy density than conventional capacitors and long cycle life comparable with the conventional capacitors,fast charge-discharge process and environmental friendliness.The commonly lower energy density of supercapacitors than batteries restricts their wide potential applications.Therefore,the improvement of the energy density or specific capacitance of supercapacitors has become the hotspot of the current research.Nanomaterial has dramatically promoted the development of high-performance supercapacitor.However,the effect of various nanostructures on electrode capacitive behaviours still needs systematical investigations,and it is very challenging to controllably synthesize various delicate nanostructures for such studies and to further offer valuable knowledge for realizing high-performance supercapacitors.Aiming at the motivations discussed above,this thesis work conducts studies of the rational design,controllable synthesis,capacitive characterization,and supercapacitance enhancement mechanisms.Herein,a number of nanostructured supercapacitive materials/electrodes with high conductivity,rapid electron transfer and fast mass transport are identified,which include unary metal oxides,binary metal oxide/conductive polymer composites,ternary carbon/conductive polymer/polyacid composites,and ternary battery/capacitive hybrid composites.The main contents of this thesis are as follows:1)Four different Co₃O₄ nanostructures including 1D nanowire,2D nanosheet,3D rambutan-like and hierarchical nanostructures are synthesized by capping molecular-free hydrothermal method,and the pressure and growth position are introduced to tailor the nanostructures of Co₃O₄.All the nanostructures expose the same clean crystal planes and exhibit a‘real’electrochemical capacitance performance due to the absence of surfactant molecular.Besides,I deeply study the electrochemical capacitance mechanism of the four Co₃O₄ nanostructures by analyzing their pore structure and electrochemical impedance,and reveal the influence of nanostructures on charge storage and electrode processes.The result analysis tells that the pseudocapacitance is not simply dependent on the surface area offered by the nanostructure,but relies on the conductivity,accessibility,diffusion rate and reactivity of the electrode.1D Co₃O₄ nanowire can render the highest reactant accessibility,fastest electron transfer and largest diffusion rate,thus harvesting the highest specific pseudocapacitance among all nanostructures,shedding a light on rational design of nanostructures for high pseudocapacitive materials.2)A unique networked hierarchical porous nanostructure of Co₃O₄/PEDOT with high conductivity and high ion transport efficiency is synthesized through electrochemical polymerization method to improve the performance of unary nanomaterials.The structural analysis and electrochemical results indicate that the amount of PEDOT has a tremendous impact on the pore structure and capacitance of the binary nanomaterials,and the Co₃O₄/PEDOT will exhibit a best electrochemical capacitance performance while the binary nanocomposite happens to be a networked hierarchical porous nanostructure.Besides,three symmetric supercapacitors constructed with the networked Co₃O₄/PEDOT,Co₃O₄ and PEDOT electrodes are tested,and Co₃O₄/PEDOT supercapacitor displays a specific capacitance about 2.2 times of the sum of that of plain Co₃O₄ nanowire supercapacitor and PEDOT supercapacitor.Furthermore,the mechanism investigation is conducted through pore structure analysis.The unreasonable pore size distribution of Co₃O₄ nanowire array and the dense structure of PEDOT is the main reason that leads to the poor performance of these two unary nanomaterials,while the networked porous nanostructure of Co₃O₄/PEDOT just solves the issues of the two unary nanomaterials for improved electrochemical capacitance performance.3)Nanocomposing is an important approach to improve the electrode performance,but it also has disadvantages restricting their wide applications.In order to achieve nanomaterials that simultaneously possess the advantages of all the constituted unary materials,some researchers directly mix or arbitrarily combine these unary materials without analyzing the interaction and growth mechanism among the unary materials.This could not only improve the performance of nanocomposites,but also hinder the performance of the unary materials.To prevent this phenomenon happening again,I construct ternary CNT/PMo/PANI and CNT/PANI/PMo core-shell nanostructures through self-assembly and in-situ chemical polymerization method.CNT/PMo/PANI shows an obviously better electrochemical capacitance than CNT/PANI/PMo.Besides,the pore size and structural analysis reveal these two nanomaterials with a similar BET surface area,but more mesopores and macropores in CNT/PMo/PANI nanostructure.The mechanism study reveals the intrinsic difference between these two ternary nanomaterials.In detail,PMo with oxidative property can contribute to the in-situ polymerization of PANI and establish an orderly connection between CNT and PANI.Besides,PMo is a proton-rich material which can promote the doping-dedoping process of PANI.On the other hand,PANI can accelerate the electron transfer between CNT and PMo,and protect PMo from dissolving.The analyses above demonstrate that CNT/PMo/PANI is the most rational nanostructure in this work.This work reveals the properties of each unary material,showing that rationally designing proper nanostructures is critical to deliver high capacitive performance for nanocomposite electrode.4)Nanocomposites improve the energy density of supercapacitors significantly,but the energy density of supercapacitors is still lower than that of batteries.In this work,I construct a unique ternary multilayered porous nanostructured（Bi₂S₃/CNT）/rGO electrode by combining battery-material Bi₂S₃ and capacitor-material CNT and rGO,and assemble battery-type supercapacitor with this multilayered electrode.The electrochemical results display that the electrochemical process of the Bi₂S₃/CNT electrode is controlled by diffusion processes,which belong to a typical battery behavior.While the multilayered（Bi₂S₃/CNT）/rGO electrode exhibits a surface-controlled reaction,which reveals a typical capacitive property.The structural analysis shows the monolayer in the multilayered nanostructure has a shorter thickness than the diffusion thickness of ions,which demonstrate the unique multilayered nanostructure can accelerate the ion diffusion and improve the mass transport.On the other hand,rGO connects the monolayers in the multilayered nanostructure,which improves the conductivity of the electrode.Besides,the battery-type supercapacitor constructed with this unique multilayered structure shows a power density close to supercapacitors and an energy density close to batteries,which demonstrate the multilayered nanostructure could translate the battery electrode into capacitor electrode with high energy density.Furthermore,this work sets a firm theoretic foundation for constructing battery-type supercapacitors with high power density and high energy density,and simultaneously creates a significantly approach for achieving novel batteries with high power.In brief,the works performed in this thesis project mainly investigate the effect of various nanostructures on the capacitive behaviours of the supercapacitors,especially the pseudocapacitors when they have the same chemistry.This thesis discloses a number of methods to successfully synthesize various delicate nanostructures and further explores profound effects of nanostructures on the supercapacitive behaviors.Different dimensional nanomaterials are synthesized and results show that 1D nanostructure offers the highest reactant accessibility,fastest electron transfer and largest diffusion rate to harvest the highest specific pseudocapacitance,vividly demonstrating that the physical structure plays a critical role in the pseudocapacitive behavior.A unique nanostructure can be well tailored by tuning the ratio of two components nanocomposite for an optimal porous structure,which possesses a rational ratio of micro-:meso-:macro-pore,of which the former small pores can offer a large electrode surface and high electroactivity,while the macropore can great enhance the diffusion rate for fast mass transport.Interestingly,a sequentially nanocomposed 3-components nanostructure can be tailored by the composing sequence including conductivity,pore size distribution and even the cycle stability,etc.This in fact hints that there are different physical and chemical interactions occur during the differently sequential nanocomposing.The beauty of nanostructure effect is greatly demonstrated by our delicately designed layered nanostructure,which can realize an electrode process without diffusion control for both remarkably high energy density and power density when the layer thickness is less than the diffusion thickness.It has never overestimated the nanostructure effect on electrochemical behaviors in an electrochemical system.It is convinced by both fundamental theory and experimental result that the future energy storage system can be artificially realized to become battery-type supercapacitors or capacitor-type batteries with both high energy density and high power density for broad important applications.