Synthetization And Electrochemical Performance Of Novel Transition Metal Compounds As Supercapacitor Electrode Materials

Due to its large specific capacity,transition metal oxides have become one of the most widely used materials in the field of supercapacitor electrodes.However,the low conductivity and poor stability make it unable to meet the requirement of society gradually.It is a common method to solve these problems by compounding other materials,which can produce a synergistic effect and exert their strengths respectively.However,to make breakthroughs fundamentally,researchers must also innovate the nature of materials.In order to solve the problems of transition metal oxides in supercapacitor electrode application,it is necessary to find new alternative that satisfy the conditions of high electrical conductivity and high cycle stability.At present,transition metal fluorides,layered carbides have behaved excellent performance in the other fields of electrochemical energy storage.Therefore,this thesis is based on these two types of materials and writes about their electrochemical properties improved by regulating the microstructure or combining them with other materials.At the same time,their energy storage mechanisms are studied in the three electrode system and flexible capacitor system to provide ideas for future research on supercapacitors.The main research contents of this paper are as follows:1)The fluorine atoms in FeF3 were partially replaced by oxygen atoms via solvothermal method and then FeOF nanorods were produced successfully.Meanwhile,the structure of the FeOF nanorods was characterized and their electrochemical performance was also tested.During the reaction of FeF3 with 1-propanol,FeOF is formed firstly.If the reaction time is too long,the iron element will be reduced and FeF2 will be obtained.The morphology also changes from blocks to rods,and finally to small particles.In terms of electrochemical performance,the replacement from fluorine atoms to oxygen atoms can not only maintain the fast charge transfer properties between interfaces,but also reduce the internal resistance of the materials effectively,leading to increased theoretical specific capacity,reduced polarity of the materials,and controlable growth of crystal grains.So the actual specific capacity would be increased from 617 F·g-1 to 894 F·g-1.2)To acquire FeOF/Ni(OH)2 core-shell nanocomposites,FeOF nanorods were used as the cores,and Ni(OH)2 nanosheets were used as the shells coated on the surface of FeOF nanorods by a simple chemical precipitation method.From the structural characterization,it is exhibited that Ni(OH)2 nanosheets can grow on the surface of FeOF nanorods homogeneously.The successful combination of the two materials also ensures the synergistic effect in electrochemical performance.As shells,the large specific surface area of Ni(OH)2 nanosheets can provide more active sites for electrochemical reactions,decreasing the mass transfer resistance of FeOF nanorods.And as the cores,the high conductivity of FeOF nanorods can accelerate the internal charge transfer and increase the specific capacity.The specific capacitance of FeOF/Ni(OH)2 composite can reach as high as 1452 F·g-1 at a current density of 1 A·g-1.Furthermore,all-solid-state flexible asymmetric supercapacitors based on FeOF/Ni(OH)2 nanocomposites as the positive electrode,activated carbon as the negative electrode and KOH/PVA as the electrolyte can show high power density,high energy density,and long cycling lifespan.3)The precursor V2AIC was synthesized using a solid phase method of high temperature sintering.Then the precursor was etched by hydrofluoric acid to remove the Al atom layer and layered V2C materials were obtained.Subsequently,TMAOH was used to adjust the interlayer spacing of V2C.Being intercalated for different periods,V2C-in with extended interlayer spacing and V2C-ex with few layers or monolayer structures can be obtained.Then the electrochemical performance of V2C,V2C-in and V2C-ex is compared.Although V2C-ex has the highest specific capacity at low current densities,V2C-in is the best when we take the overall electrochemical performance into consideration such as rate capacity and cycling stability.That is,V2C-in has the highest rate capacity and the best cycling stability.Because the interlayer spacing of V2C-in is suitable both for the rapid transmission of ions and the stability of the lattice.And the V2C-in//AC all-solid-state flexible supercapacitor using V2C-in as the positive electrode,activated carbon as the negative electrode and KOH/PVA as the electrolyte can also exhibit good cycling stability.