Preparing Iron Oxide By Liquid-phase Reduction And Its Performance In Supercapacitors

High power density and long lifespan make supercapacitors a potential energy storage device,but the shortcomings of low energy density limit its wide application.In order to increase the energy density of supercapacitors while reducing their costs,iron oxide,which is cheap and easy to obtain and has a high theoretical specific capacitance,has become a hot spot in the research of electrode materials for supercapacitors.However,iron oxide has the disadvantages of poor conductivity and stability.For this reason,this paper combines iron oxide with carbon nanotubes?CNTs?,and uses a CNT network with good conductivity and large specific surface area to support iron oxide.The resulting composite material network can promote the rapid transmission of electrons and the rapid diffusion of ions,combined with oxygen vacancy modulation and element doping,to further improve the electrochemical performance of iron oxide,and promote the application of iron oxide as a supercapacitor electrode material.The main work carried out are:?1?Research on synthesis of iron oxide by liquid-phase reduction method.CNTs were grown on the carbon cloth as a substrate by chemical vapor deposition.FeCl₃solution and NaBH₄ were used as the main reagents to synthesize iron oxide on the carbon cloth/CNT substrate by liquid-phase reduction method with a loading of 0.43mg cm^-2.XRD results show that diffraction peaks appear at 30.3,35.7,43.4,53.8,57.3,and 63°,corresponding to?220?,?311?,?400?,?422?,?511?and?440?crystal planes of?-Fe₂O₃?JCPDS Card No.39-1346?.The three peaks of 359,505 and 696 cm^-1 in Raman signal also show that it is?-Fe₂O₃.The SEM and TEM morphology characterization shows that iron oxide particles?diameter about 50nm?are attached to CNTs.Electrochemical tests show that the specific capacitance of iron oxide reaches814 F g^-1(226 mAh g^-1)at 2 A g^-1.The large specific capacitance of iron oxide is attributed to the good electron transport channel and large specific surface area provided by CNTs.The energy density of an asymmetric supercapacitor assembled from iron oxide and nickel oxide?also synthesized by liquid-phase reduction?is up to 63.3 Wh kg^-1,and the corresponding power density is 1.6 kW kg^-1.?2?The effect of oxygen vacancy on the electrochemical performance of iron oxide.By changing the annealing temperature?200⁵00??to adjust the concentration of oxygen vacancies in iron oxide prepared by liquid-phase reduction method,it was found that with increasing temperature,the oxygen vacancy concentration gradually increased from 11%to 26%,and the specific capacity of iron oxide first rise and then fall.The specific capacity of iron oxide annealed at 300?is up to 301 mAh g^-1.After increasing the annealing temperature,the specific capacity decreases.EIS results show that iron oxide annealed at 300°C has the smallest equivalent series resistance.The iron oxide annealed at 300?still has a capacity of 90.6%of the initial value after 5000 cycles,showing excellent cycle stability.The iron oxide samples annealed at 300°C were assembled into a water-based symmetric supercapacitor with an energy density of 58.5Wh kg^-11 at a power density of 9.32 kW kg^-1 and a capacity of 88%of the initial value after 12,000 cycles,showing good cycle stability.?3?The effect of Sn element doping on the electrochemical performance of iron oxide.In the liquid-phase reduction process,Sn element is doped into iron oxide by adjusting the Sn/Fe ratio?0.5%to 20%?in the chloride mixed solution.XRD indicated that samples with different doping concentrations all contained peak positions of?-Fe₂O₃?JCPDS card No.39-1346?,and 2%and 5%Sn-doped samples also had FeOOH?JCPDS card No.44-1415?.The study found that the active materials of the 2%Sn-doped samples were evenly distributed along the CNTs and connected to each other,which greatly increased the mass loading.Electrochemical tests showed that the loading of 2%Sn-doped iron oxide reached 1.8 mg cm^-2,the largest specific capacitance was823 F g^-1,and the surface capacitance reached 1514 mF cm^-2.The energy density of the asymmetric supercapacitor assembled with NiO is up to 83.9 Wh kg^-1,and the corresponding power density is 0.8 kW kg^-1.