Preparation And Energy Storage Properties Of Iron Oxide Nanoparticles And Their Composites

Because of its high specific capacitance and long cycle life,supercapacitors are a potential new type of energy storage.However,its energy density and electrochemical stability are limited by electrode materials,and can not meet the requirements of wide application in the future.As a pseudo-capacitor anode material,Fe₂O₃has a theoretical capacitance of up to 3625 F g^-1.At present,the preparation process ofα-Fe₂O₃studied is complex and costly,and most of theα-Fe₂O₃particles prepared in the study are large in size,uneven in particle size,poor in dispersion,and serious in agglomeration,which makes the energy storage performance ofα-Fe₂O₃not fully developed.In this paper,sphericalα-Fe₂O₃nanoparticles with high dispersion and average size of 70 nm were prepared by chemical bath deposition using iron as the iron source.Iron oxide/polypyrrole composite was prepared by using polypyrrole（PPy）and iron oxide composite;Carbon coated sphericalα-Fe₂O₃composites were prepared using glucose as carbon source;And using multilayer graphene as the substrate to prepare iron oxide/multilayer graphene composites.The main research contents of this paper are as follows:（1）Spherical Fe₂O₃nanoparticles with high homogeneity were prepared by chemical bath deposition with iron as the iron source,dilute nitric acid and anhydrous sodium acetate as the reactants.The average size of Fe₂O₃spherical nanoparticles obtained was 70 nm.The specific capacitance of pureα-Fe₂O₃material obtains 211.1 m Ah g^-1at 2 A g^-1,and the multiplying power is57.1%at 10 A g^-1.After 5000 working cycles at 2 A g^-1,the capacity retention rate is only 17.4%.On the basis of sphericalα-Fe₂O₃,the composite ofα-Fe₂O₃and PPy（polypyrrole）was studied.The preparation of composite materials with different ratios of polypyrrole and iron oxide was investigated.The best composite delivers a high specific capacitance of 238.1 m Ah g^-1at a current density of 2 A g^-1,and the capacitance remained 82.9%at 10 A g^-1.（2）Study on the surface of Fe₂O₃nanoparticles coated with carbon composites.Mix glucose and spherical Fe₂O₃evenly,calcine at high temperature under nitrogen to obtain a carbon coating with core-shell structureα-Fe₂O₃composite.Through the control of annealing temperature,we find that with the gradual increasing of reaction temperature,the particle size of the product becomes larger.High temperature causes Fe₂O₃to be reduced to Fe₃O₄,Fe O and elemental Fe.The composite prepared by annealing temperature at 600℃is spherical with an average particle size of80 nm.The specific capacity of the composite reaches 165.1 m Ah g^-1at a current density of 2 A g^-1,and the capacity retention rate is 62.6%at 10 A g^-1.After 5000 cycles of charging and discharging at 2 A g^-1,the capacity retention rate reached 40.7%,which greatly improved the cycle performance compared with pureα-Fe₂O₃.Finally,an asymmetric supercapacitor was assembled with carbon-coatedα-Fe₂O₃as negative electrode and Ni（OH）₂/MLG as positive electrode.Its capacity was measured to reach 86.1 m Ah g^-1at 1 m A cm^-2,and the corresponding energy and power densities reached 90.6 Wh kg^-1and 280.5 W kg^-1.The capacity retention rate reaches 48.3%after1000 cycles measured at 4 m A cm^-2,the capacitor shows good cycle performance.（3）In this work,separately distributed uniformα-Fe₂O₃nanoparticles with diameter of about80 nm were synthesized on ultrasonically prepared multilayer graphene（MLG）（α-Fe₂O₃/MLG-3）to obtain high capacity and rate capability.For this purpose,iron powder was firstly used as a precursor in a chemical bath deposition method.Through the control of the concentration of HNO₃,a balanced dissolution-crystallization process is achieved to obtain uniform large Fe₂O₃nanoparticles on MLG..CV and EIS tests show thatα-Fe₂O₃/MLG-3 exhibits fast electric and ionic movement,which contributes to high rate performance.The composite can deliver a specific capacity of 191.1 m Ah g^-1and 160.8 m Ah g^-1at current densities of 2 A g^-1and 10 A g^-1.The composite material was used as the negative electrode,and the Ni（OH）₂/MLG composite material was used as the positive electrode to assemble the asymmetric supercapacitor.The assembled supercapacitor has a wide effective voltage window（0～1.6 V）.The specific capacity at 1 m A cm^-2is 101.5 m Ah g^-1,the energy density and power density are 115.8 Wh kg^-1and 443.3 W kg^-1,The capacity efficiency achieves 35.3%after 1000 working cycles test.