Research On Electrochemical Performance And Energy Storage Mechanism Of Iron Based Supercapacitor Electrode

Iron-based oxide/hydroxide has many advantages such as high specific capacitance,stable and wide working potential window,environmental friendliness,rich mineral resources and low cost,so it has been extensively researched in the field of supercapacitors.However,iron-based materials have the disadvantages of poor electrical conductivity and large volume changes during charging and discharging processes,resulting in unstable structures.Therefore,the capacitance and cycle stability performance of iron-based materials is not very ideal.After research,it is found that the limitations of iron-based materials can be effectively improved by designing nano-microstructures,element doping,compounding with high-conductivity materials,and introducing defects and vacancies.In particular,it has been widely proved that the composite of iron-based materials with carbon materials and the preparation of reasonable nanostructures can effectively improve the conductivity and capacitance of iron-based materials.Therefore,it is of great significance to explore simple and applicable synthetic methods to prepare iron-based composites with excellent properties.In this paper,we take iron-based self-supported carbon cloth electrode as the main research object,and systematically characterize and test its phase composition,micro morphology,theoretical electrochemical activity,electrochemical energy storage performance and assembly of related supercapacitor devices,and the following meaningful research results have been achieved:(1)Firstly,the carbon cloth was pretreated with strong oxidizing acids(concentrated HNO₃ and H₂SO₄),made the surface of carbon cloth rough and had oxygen-containing functional groups,which is beneficial to the adhesion and growth of electrode materials.Furthermore,by adjusting the concentration of reactants and using a simple 48-hour hydrothermal method,a layer of porous Fe₂O₃ nanospheres was uniformly and densely grown on the surface of the activated carbon cloth.The self-supported design and porous nanostructure effectively improved the conductivity of the electrode and electrochemical performance.The BET test found that the specific surface area(S_BET)of the Fe₂O₃ electrode is 99.1 m² g^-1.Then,the surface properties of the trigonal Fe₂O₃ were analyzed by first principles calculation,and the electrochemical activity of the electrode surface was confirmed theoretically.Furthermore,using electrochemical tests confirmed that the self-supported design and porous nanostructure effectively improved the capacitance performance,conductivity and cycle stability of the electrode.In the end,the Fe₂O₃@ACC electrode showed excellent capacitance performance(2775 m F cm^-2 at 1 m A cm^-2)within(-0.8-0)V,rate performance and cycle stability(92%of initial value after 10000 cycles),and good capacitance performance at positive potential window of(0-1)V.Therefore,we assembled the Fe₂O₃@ACC electrode into a symmetrical supercapacitor,which can reach a working voltage of 1.8 V in aqueous electrolyte.(2)Then,using the strong double hydrolysis reaction between anion and cation,a self-supported carbon cloth electrode with low crystalline Fe OOH nanosheets was prepared,and the preparation time of the electrode was shortened to 24 h.Vacancies and defects are introduced into the lattice of low crystalline nanosheets,which improves the cycle life and capacitance performance of the electrode.The use of contact angle and Raman test confirmed that activated carbon cloth has better hydrophilicity and exposed the graphitized part,which promotes the growth of electrode materials and improves the performance of the electrode.These low-crystalline Fe OOH nanosheets grow vertically and cross each other to form a network structure on the carbon fiber,which will greatly increase the effective contact area between the electrode and the electrolyte,and provide abundant adsorption sites for the diffusion of electrolyte ions.Then,the surface properties of the tetragonal Fe OOH(200)lattice plane were analyzed through first-principles calculations,which proved that it has better electrochemical activity than the above-mentioned trigonal Fe₂O₃.Finally,the Fe OOH@AC electrode reached the maximum capacitance value of 4090 m F cm^-2 at 1 m A cm^-2,and the potential window was also widened to-1-0 V.The Fe OOH@AC electrode also showed excellent electrochemical performance within the potential window of(0-1)V,so we finally used it to assemble a symmetrical supercapacitor,which can reach 2 V working voltage in aqueous electrolyte.(3)Finally,since iron-based materials are generally used as negative electrodes,we used a one-step electrodeposition method to prepare a self-supported carbon cloth positive electrode with multiple layers of amorphous Mn(OH)₂ nanosheets by adjusting the deposition potential,and assembled asymmetric supercapacitor with excellent energy storage performance.By using untreated carbon cloth as a substrate for comparison,it is found that the size of the Mn(OH)₂ nanosheets grown on the surface of the unactivated carbon cloth is significantly smaller,which proves that the activation treatment can promote the growth of electrode materials.Subsequently,TEM tests confirmed the amorphous and multilayer characteristics of the Mn(OH)₂ nanosheets,which provided abundant active sites for electrolyte ions in the electrode process.The Mn(OH)₂ electrode exhibited a maximum capacitance value of 3103 m F cm^-2 within the potential window of(0-1)V at 1 m A cm^-2?And through the CV test,the charge storage behavior and the charge matching degree of the two electrodes are further explored.Finally,the above two electrodes are assembled into an asymmetric supercapacitor.The device can reach a working voltage of 2 V in 1 mol/L Li NO₃ aqueous electrolyte,and exhibits excellent charge storage capacity and cycle stability.Connecting two devices in series can charge up to 4 V and light up 72 LED lights for about 2 minutes,showing an ideal practical application prospect.