The Construction Of Iron Based Electrode Materials And Study On Its Electron Transport Mechanism And Electrochemical Performances

Based on situation of low energy density of supercapacitor and the properties dismatch between positive and negative electrode materials,especially,in view of the low specific capacity and poor conductivity and stability of iron based electrode materials,a series of solutions were designed and pointed out including nanostructure design,elements doping and adjustment at phase interface,etc.,on the level from the surface modification to the body modification operations,to design and fabricate some new types of iron-based electrode materials,which showing high specific capacity,efficient electron/ions transmission and good cycle stability.These results are conducive to settle the problem of performances dismatch of negative and positive electrode materials and low energy density of supercapacitor.The main contents of our researches are shown as follow:(1)The unique Fe(OH)3/Cu(OH)2/CF hollow tublar structure nanoarray was fabricated successfully by the method of chemical oxidation and electrochemical synthesis.In order to solve of problem of instability of iron based electrode materials in alkaline electrolyte,the glucose was used as carbon source to form a thin carbon layer on the surface of electode material after the high-temperature carbonization process.Because of the existence of the thin carbon layer,which can avoid the contact with electrolyte,suppress the volume expansion and collapse of structures of the iron based electrode materials in alkaline electrolyte and improve the materials conductivity thus facilitating the rate of electron transmission,the electrode materials exhibited excellent supercapacitor performances.The specific capacity can reach up to 15 F cm-2(1250 F g-1),at the current density of 5 mA cm-2.At the same time,after 5000 cycles,the electrode can maintain 88%of its initial capacity.More importantly,when assembled the asymmetric supercapacitor with NiCo hydroxide/Cu(OH)2/CF,the energy density of supercapacitor reached up to 90.6 Wh kg-1 at the power density of 188.4 W kg-1.It is a good approach to fabricate high-performances.iron-based supercapacitor negative electrodes that using the three-dimensional hierarchitectures design and surface modification method of carbon-coated.However,considering the limitation of the complicated operations and low electrochemical performances of carbon materials,we try to find the more effective method on the level of adjustment at crystal structure,including construction of the built-in electric field,anionic/cationic modification,to fabricate a series of new types of high-performances iron-based supercapacitor negative electrode materials:(2)Using the 316L stainless steel sintering felt as the basement,a unique NiOOH modified ?-FeOOH nanosheet array was fabricated by a simple chemical oxidation process.The DFT results showed that due to adsorption of SO42-,the growth orientation of ?-FeOOH will be changed to gain the unique nanosheet structure.The nano sheet structure is beneficial to the exposure of electrochemical active sites.Meanwhile,the introduction of Ni can improve the electrical conductivity and provide more electrochemical active sites,which faciliting the electron/ions transmission in electrode matrials.These factors made the electrode materials showed the excellent performances of OER and supercapacitor negative electrode.When used as a catalyst for OER,its 10 mA current overpotential was only 256 mV,and had a low Tafel slope(45 mV@dec-1).When as a supercapacitor negative electrode material,it showed the high specific capacity of 554.17 F g-1 at 1 A g-1 current density and the good cycle stability(maintaining 82.61%of the initial capacity after 4000 cycles).(3)Considering the dissolution of Cr,Mn elements in alkaline chemical oxidation etching solution(AOES),we assumed that by choosing the Cr/Fe alloy as iron-based basement to obtain pure phase?-FeOOH nanosheet array product.At the same time,inspired by the fact of the electric field can accelerate the electron transmission.By the means of in-situ sulfuration,realizing the part of a-FeOOH transformed into Fe7S8,resulting in the formation of Fe7S8/?-FeOOH nano heterostructure.The DFT results showed that due to some charge transferred from?-FeOOH to Fe7S8 spontaneously,the inherent electric field on the boundary of nano heterostructure was built,which can promote the efficiency of electron transmission and the adsorption ability of electrode material on electrolyte,thus improving the electron/ions transform efficiency.Therefore,the obtained electrode material showed excellent supercapacitor performance of negative electrode.The specific capacity was 520.27 F g-1 at the current density of 1 A g-1.And after 5000 cycle tests,it can maintain 90.63%of its initial capacity.When assembled asymmetric supercapacitor with CNTs materials,the energy density of supercapacitor reached 35.3 Wh kg-1 at the power density of 800.8 W kg-1.(4)Based on fact that the unique space channel structure P-FeOOH and its good adsorption performance on halogen anions,we obtained different kinds of halogen anion embedded ?-FeOOH electrode materials by a spontaneous adsorption process and explored the effect of electronic structure and electrochemical performances of ?-FeOOH after the introduction of halogen anions.The DFT and experiment results showed that the halogen anion embedment can make the crystal structure of the P-FeOOH changed and improve the conductivity of the electrode materials.At the same time,due to the existence of high state of Fe in?-FeOOH,which facilitating the adsorption for Na2SO3,improving the electron/ions transmission efficiency,therefore,the electrode materials showed excellent supercapacitor performances.The specific capacity of?-FeOOH(F)was 391.9 F g-1 at the current density of 1 A g-1,nearly 10 times promotion compared with that of the initial ?-FeOOH.Meanwhile,after 2000 cycles,it can maintain 80.82%of the initial capacity.When assembled as the dual electrolyte asymmetric supercapacitor(DESC)with the NiCo hydroxide/Cu(OH)2/CF,the energy density of DESC reached 31.0 Wh kg-1 at the power density of 858.5 W kg-1.