Studies On The Construction Of Iron Oxide Based Electrode Materials And Their Electrochemical Properties

Electrode materials in supercapacitors are the key components to determine the electrochemical performance of supercapacitors.Among the electrode materials,transition metal oxides/sulfides receive much attention due to their high theoretical specific capacitance,abundant reserves and environmental friendship.However,transition metal oxygen/sulfide suffers from low conductivity and poor structural stability,and so on,which limits its practical application.The above problems can be solved to some extent by compounding it with conductive polymer or carbon material.However,this still remains other problems such as poor electron transfer uniformity and weak affinity with current collectors,which makes the stability of the composite materials is not so satisfied.Thus,in this thesis,iron oxide（Fe₂O₃）was used as a functional material,constructing Fe₂O₃/polyaniline（PANI）composites and Fe₂O₃/heteroatom-doped carbon composites.Then,effects of the types and roles of oxygen vacancies and heteroatoms on the uniformity of electron transfer in composite were studied.The affinity between the electrode material and the fluid collector was improved by using polydopamine-derived carbon as the intermediate layer.Finally,symmetric and asymmetric supercapacitors were assembled,respectively,and their electrochemical performances were evaluated.The detailed researches are as follows:（1）Firstly,ferric oxide（Fe₂O₃）with oxygen vacancy defect was prepared by chemical reduction method,and the formation of oxygen vacancy was regulated by changing the chemical reduction time.Then aniline was polymerized in situ on its surface to prepare iron oxide/polyaniline composite(Fe₂O_3-x/PANI).The effects of chemical reduction time and aniline concentration on the electrochemical properties of the composite material was studied,and the area specific capacitance of Fe₂O_3-x/PANI was 492.8 m F cm^-2when the chemical reduction time was 0.5 h and the aniline concentration was 0.05 mol L^-1.Moreover,it had well cycle stability.（2）The Fe₂O_3-x/N and P co-doped carbon(Fe₂O_3-x/NPC)composites were prepared by one-step calcination method with ferric hydroxide/PANI as precursor to further improve the structural stability of Fe₂O_3-x/PANI composites.The effects of calcination temperature and time on the electrochemical properties of the composites were studied.When calcined at 450°C for2 h,the Fe₂O_3-x/NPC showed a higher area specific capacitance(605 m F cm^-2)than Fe₂O_3-x/PANI.The assembled symmetrical supercapacitor had well cycle stability with an energy density of 0.0148 m Wh cm^-2at a power density of 0.4 m W cm^-2.（3）Ni/Co-doped Fe₂O₃（Ni/Co-Fe₂O₃）was prepared by one-step calcination method with nickel/cobalt（Ni/Co）-doped hydroxy iron oxide as the precursor.Polydopamine-derived carbon was used to improve the stability of Ni/Co atoms in Ni/Co-Fe₂O₃/N-doped carbon composites（Ni/Co-Fe₂O₃/CN）.Effects of different Ni/Co ratios on the electrochemical properties of Ni/Co-Fe₂O₃/CN were studied.When the Ni/Co ratio was 3:1,the area specific capacitance of the Ni/Co-Fe₂O₃/CN was 713 m F cm^-2and exhibited well rate performance.The assembled symmetrical supercapacitor remained stable after 6000 cycles.（4）NiCo₂S₄/N-doped carbon composites were prepared by in-situ vertical growth of Ni Co₂S₄ on the surface of polydopamine-derived N-doped carbon as a carrier.Polydopamine-derived N-doped carbon layer effectively improved the affinity of Ni Co₂S₄ with the current collector and improved its cycle stability.Furthermore,an asymmetric supercapacitor was assembled by using it as the positive electrode and the Ni/Co-Fe₂O₃/CN prepared in the previous chapter as the negative electrode.The capacitor showed an energy density of 0.082 m Wh cm^-2(0.86 m Wh cm^-3)at the power density of 0.6 m W cm^-2(92.3 m W cm^-3)and could still maintain a high energy density at a high-power density of 6 m W cm^-2.In addition,it also showed a well cycle stability with the specific capacitance still maintained 80%after 6000 cycles.