Study On The Synthesis And Electrochemical Properties Of Iron-based Anode Materials For Supercapacitors

Supercapacitors?SCs?,known as a new type of energy storage system with high power density and more energy storage,have attracted extensive attention in recent years.In order to improve the energy density of SCs,assembling of asymmetric supercapacitors with wider voltage window and higher specific capacitance has become an effective solution.Recently,extensive research has been carried out on the positive electrode materials,but researchers always choose activated carbon with low specific capacitance as the negative electrode,which makes the asymmetric supercapacitors exhibit unsatisfactory electrochemical performance.Previous studies have shown that iron oxide has become another anode material with a higher theoretical capacity.Therefore,Fe₂O₃ can be a potential candidate to replace traditional carbon materials.However,the intrinsical characteristics of oxide materials,such as poor electrical conductivity,sluggish ion transport kinetics and limited specific surface area,have become the three main obstacles restricting the application of metal oxides in supercapacitors.Aiming at these problems of poor conductivity and unsatisfatory cycling stability of iron oxide materials,this article explores the effects of oxygen vacancies,phosphating treatment,carbon coating,and structural design on electrochemical performance.Three innovative supercapacitor anode materials including oxygen-deficient hollow Fe₂O₃ microcubes,FeP nanoparticles encapsulated in one-dimensional carbon-nanotube,and FeP hollow nanoparticles encapsulated in two-dimensional carbon layer were designed,respectively.In order to improve the intrinsical conductivity of Fe₂O₃,reduction treatment by Na BH4 solution was used as a simple method to introduce oxygen vacancies in Fe₂O₃,which can improve the conductivity,accelerating ion diffusion and further improve electrochemical activity.At the same time,considering the hollow structure design method,the electrode material is designed into a hollow microcubic structure,which produces high specific surface area and enhanced structural stability.Finally,the oxygendeficient hollow Fe₂O₃ microcubes are used as the negative electrode material of the supercapacitor.According to electrochemical measurements,the electrode provides a higher specific capacitance of 495 F·g^-1 at 1A·g^-1,which is double that of Fe₂O₃ electrodes without oxygen vacancies.The improvement in electrochemical performance is attributed to the improved charge-transfer kinetics caused by oxygen vacancies,and the generation of built-in electric field around the oxygen vacancies provides new active sites for electrochemical reactions.Nanometerized active materials can provide a larger specific surface area,but nanoparticles inevitably face the problem of particle agglomeration.Therefore,compounding the nanostructural active material with a highly conductive carbon material can effectively avoid the agglomeration,and also enhance the onductivity of the composite electrode.In this article,the FeP nano-particles with high electrochemical activity are encapsulated into a one-dimensional carbon-nanotube.The nanometerized particles and the hollow structure of the tubular carbon layer ensure that the electrode material has a high specific surface area.Meanwhile,the carbon layer serves as an excellent conductor,providing an ordered path for electron transportation between each nanoparticle,and further improving the conductivity of the composite material.The FeP nanoparticles encapsulated in one-dimensional carbon-nanotube has a specific capacitance of 719 F·g^-1 at 1 A·g^-1.In addition,carbon can also serve as a good buffer layer to relieve the structural stress during charging/discharging process.After 10000 charge/discharge cycles at 10 A·g^-1,it still maintain 93% of the initial capacitance.Two-dimensional carbon layer with larger area can provide a long range conductive network for electron transport and shorten the ion diffusion path between nanoparticles.In addition,the hollow nanoparticles are obtained with larger specific surface area and enhanced cycling stability.The FeP hollow nanoparticles encapsulated in twodimensional carbon layer electrode material achieves a high specific capacitance of 678 F·g^-1 at the current density of 1 A·g^-1,and the capacitance still reach 376 F·g^-1 when the current density increases 20 times,demonstrating excellent rate performance.This is attributed to the three-dimensional honeycomb carbon skeleton,which provides a longrange order conductive path for the charge transfer during the electrochemical reaction.Moreover,benefitting from the hollow structure and the buffering of the two-dimensional porous carbon layer,which reduces the stress caused by volume change during charging/discharge process,the electrode exhibits excellent cyclic stability?96% of the initial capacitance after 10000 cycles?.The excellent electrochemical properties make it a very promising anode material for supercapacitors.