Research On The Design And Fabrication Of Transition Metal Oxides And Carbons Electrodes For The Performance Of Supercapacitors

The increasing demands of energy and the environmental pollutions have become two major issues in modern society.Reducing fossil fuel consumption and greenhouse gas emissions has become global objectives being recognized as an imperative for the sustainable development of economy and society.Developing high efficiency electrochemical energy storage devices is considered as an attractive route to keep the balance between energy resources and environment.Among the energy storage systems,supercapacitors?also known as electrochemical capacitors or ultracapacitors?are regard as one of promising devices due to their fast charge/discharge rate,long cycling stability,high power density and environmentally friendly.However,the energy density of supercapacitors is still relatively low,which is related to the specific capacitance of electrode materials and the operating voltage of assembled systems,restricting their further applications.Recently researches show that the rational design of material structure can improve the utilization of electroactive materials,leading to high specific capacitance.Meanwhile,the fabrication of asymmetric systems can enlarge the operating voltage of supercapacitors.Therefore,in this thesis,we mainly enhance the energy density of supercapacitors through the rational design of the structure of electrode materials and the fabrication of asymmetric systems.Firstly,asymmetric supercapacitor was fabricated using Ni?OH?₂/graphene?Ni?OH?₂/GNS?as positive material and activated carbon?AC?as negative material,respectively,aiming to improve the energy density of supercapacitors.Ni?OH?₂/GNS nanocomposite with decreased grain was prepared by the assistance of NaCl saturated solution,which can provide abundant nucleation sites for the formation of Ni?OH?₂ by the precipitate of NaCl crystal.A specific capacitance of 1683 F/g can be obtained for Ni?OH?₂/GNS nanocomposite in KOH solution as an electrode material for supercapacitors.Simultaneously,AC prepared by activating polyaniline/graphene oxide shows a specific capacitance of 442 F/g.As for the assembled asymmetric supercapacitor,maximum specific capacitance of 136 F/g and high energy density of 48.4 Wh/kg as well as good cycling stability?88%capacitance retention after 1000cycles?can be achieved,which shows a certain significance in the synthesis of small-sized electrode materials and the improvement of supercapacitors energy density.In order to further enhance energy density,Fe₃O₄/GNS nanocomposite with high specific capacitance was synthesized using an electrochemical transformation route.Fe₃O₄/GNS nanocomposite was synthesized by electrochemical transformation of iron/graphene in alkaline solution,which prepared by pyrolysis of ferric ions adsorbed onto polyaniline nanosheet/graphene oxide hybrid material.A novel face/face structure can be observed through microstructure characterization,which is beneficial for electron transportation during electrochemical process.The maximum specific capacitance of 717 F/g can be obtained in 6M KOH electrolyte for electrode material.More importantly,the assembled asymmetric supercapacitor using Ni?OH?₂/carbon nanotubes as positive electrode and iron/graphene as negative electrode,can operate at a voltage range of 0-1.7 V and shows acceptable cycling performance with 78%capacitance retention after 2000 cycles as well as a noticeable ultrahigh energy density of 137 Wh/kg,which greatly enhances the energy of supercapacitors.To enhance the conductivity of electrode materials,small-sized nanocomposites with nanosized FeOOH and MnO₂ anchored on the surface of conductive graphene/carbon nanotubes networks were prepared,which can make full use of the excellent conductivity of graphene?GNS?and carbon nanotubes?CNTs?.Electron can rapidly transport in the overall electrodes due to the intimate contact between metal oxides and conductive substrates,thus,enhancing their electrochemical performances.As a result,the specific capacitances of 267F/g and 230 F/g can be obtained for iron-based and manganese-based composites,respectively.Moreover,theas-fabricatedasymmetricsupercapacitor,using FeOOH/GNS/CNTs and MnO₂/GNS as the negative and positive electrodes,respectively,exhibits wide operation voltage?0-1.7 V?and superior energy density?30.4 Wh/kg?as well as excellent cycling stability?89%capacitance retention after 1000 cycles?,providing an important guideline for the improvement of energy density and stability of supercapacitors.Finally,functionalized porous carbon with three-dimensional interconnected pores?micropores,mesopores and macropores?has been successfully synthesized through heat-treatment of KOH-soaked soybeans,which owns inherent hydration process,resulting in the permeation of KOH into soybeans.As an electrode material for supercapacitor,it exhibits a high gravimetric capacitance of 425 F/g as well as an ultrahigh volumetric capacitance of468 F/cm³ with respects to its highly packing density?up to 1.1 g/cm³?.Notably,it also delivers excellent life stability with 91%capacitance retention after 10000 cycles.Moreover,the as-fabricated symmetric supercapacitor delivers high volumetric energy density of 28.6Wh/L in neutral electrolyte,higher than that in alkaline electrolyte?13.2 Wh/L?,which provide a meaningful route for large-scale production of high volumetric performances and life stability activated carbons for supercapacitors.