Design Of Supercapacitor Materials Based On Fe₂O₃ Anode/NiMoO₄ Cathode And Their Composites Performance With Mxene

Supercapacitors with their benefits of high power density,stability and safety,quick charging and discharging response time,easy maintenance and environmental friendliness have been applied to many fields such as industrial applications,transportation,military defense,intelligent communication,etc.However,the low energy density of current commercial supercapacitors prevents them from being widely used as independent power sources like secondary batteries.At present,the research hotspots and main challenges of improving the energy density of supercapacitors focus on finding and preparing electrode materials with excellent electrochemical performance.Recently,transition metal oxides are known as favorable contenders for excellent electrode materials owing to their abundant reserves,high theoretical specific capacity,low price and toxicity.However,transition metal oxides are inflicted with the disadvantages of poor conductivity and bad cycling stability.Thus,this paper focuses on the structural design and production ofα-Fe₂O₃,Ni Mo O₄,MXene and their composites,and discusses the electrochemical performance of above materials for the purpose of ameliorating above problems.The research of this paper could be the succeeding four parts:（1）Facet controlled preparation,oriented aggregation behavior and supercapacitor performance ofα-Fe₂O₃ nanocrystalsThe high-index crystal facets with high surface energies of metal oxides always grow rapidly and occupy only a small part of the surface and even disappear from the final crystals.Unlike most methods that use surfactants to control the growth of crystal facets,a simple hydrothermal method was reported prepareα-Fe₂O₃ nanosheets with both low-index{001}facets and relatively high-index{012}facets exposed,rhombic shapedα-Fe₂O₃ nanocrystals with high-index{104}facets exposed and bipyramid shapedα-Fe₂O₃nanoparticles with high-index{116}facets exposed in this paper.In addition,the role of particle morphology on the directionality of OA using above three hematite particle morphologies with various exposed facets was addressed for the first time.All three particle types formed single crystal or twinned chain-like structures along the[001]direction driven by the attractive interactions between{001}facets and repulsive interactions between other pairs of hematite faces.Moreover,simulations of the potential of mean force for iron species confirms that the formation of one-dimensional chains is a result of the attachment of independently nucleated particles and does not follow the near-surface nucleation pathway.These results highlight the role of repulsive interactions between crystal faces in face selectivity during the OA process.In addition,by investigating the supercapacitor performance of above threeα-Fe₂O₃ nanocrystals,it is found that{001}facets exposedα-Fe₂O₃ nanosheets present high conductivity and small energy diffusion barrier,thus improving the electron and ion transmission speed.As a result,theα-Fe₂O₃ nanosheets electrode exhibits a specific capacity of 91.6 C/g at 1 A/g.（2）Controlled preparation and supercapacitor performance of multilayer MXeneBased on the results ofα-Fe₂O₃ crystal facets enginneering in last chapter,it is confirmed thatα-Fe₂O₃nanosheets with{001}facets exposed have high electrochemical activity.However,the electrochemical performance ofα-Fe₂O₃ are still unsatisfied,so highly conductive materials（MXenes）are sought to further improve its performance.Herein,the controllable preparation of multilayer Ti₃C₂T_x was successfully achieved by hydrothermal method,and its performance for supercapacitors was studied in this paper.The results show that phosphorus and sulfur atoms are not suitable for the hydrothermal introduction,while nitrogen atoms can partially replace the original carbon atoms in the Ti₃C₂T_x lattice,so that the N-doped Ti₃C₂T_x sample exposes more carbon defects.The introduction of nitrogen not only increases the lamellar spacing of Ti₃C₂T_x,but also effectively reduces the content of non-electrochemically active fluorine functional groups on the surface of Ti₃C₂T_x,and effectively improves the interlamellar ion migration rate and electron conductivity of N-Ti₃C₂T_x electrode in KOH electrolyte.As a result,the N-Ti₃C₂T_x electrode displays a specific capacity of 70.58 C/g at 0.2 A/g in company with87.9%capacity retention（10000 cycles）.（3）Preparation and supercapacitor performance ofα-Fe₂O₃/MXene anode materialsAiming at the problems of the limited surface area,poor electrical conductivity and poor cycle stability ofα-Fe₂O₃,and the problems of limited space and restacking of multilayer Ti₃C₂T_x sheets.A strategy was designed to use Ti₃C₂T_x as conductive substrate to makeα-Fe₂O₃nanosheets self-assemble on different forms of Ti₃C₂T_x by van der Waals forces.Theα-Fe₂O₃/Ti₃C₂T_x anode materials were successfully prepared and their difference of electrochemical properties for supercapacitors were measured.It shows that the existence of MXene not only advances the electrical conductivity of the composite compared with that of pureα-Fe₂O₃,but also inhibits the volume expansion problem ofα-Fe₂O₃ during electrochemical reaction.The existence ofα-Fe₂O₃ nanosheets also effectively avoids the stacking of Ti₃C₂T_x lamellae,thus improving the rate performance and cyclic stability of the composites.Based on the synergistic effect betweenα-Fe₂O₃ and Ti₃C₂T_x,theα-Fe₂O₃/Ti₃C₂T_xelectrode（the mass ratio of Fe₂O₃ and Ti₃C₂T_x is 1:1）represents a specific capacity of 285.4C/g at 1 A/g alongside with good cycling stability（80.5%,5000 cycles）.（4）Preparation and supercapacitor performance of Ni Mo O₄/MXene cathode materialsThe mechanism ofα-Fe₂O₃/Ti₃C₂T_x composites in previous chapter is further applied to the preparation of cathode materials.In this paper,the Ni Mo O₄/Ti₃C₂T_x composites with 2D/2D heterostructure were fabricated via a simple hydrothermal method by introducing nickel source and molybdenum source to adsorb on the Ti₃C₂T_x nanoflakes.It is certified that the connected porous structure can fully exhibit electrochemically active sites while greatly shortening the transport path of electrolyte ions to accelerate their diffusion rate.The excellent conductivity and hydrophilicity of Ti₃C₂T_x effectively improve the conductivity and wettability of the composite electrode,while Ni Mo O₄ not only provides high electrochemical reactivity but also avoids the restacking problem of Ti₃C₂T_x nanoflakes.Based on the synergistic effect between Ni Mo O₄ and Ti₃C₂T_x,the Ni Mo O₄/Ti₃C₂T_x electrode presents a specific capacity of 545.5 C/g at 0.5 A/g alongside with attractive cycling stability（10000 cycles,68.0%capacity retention）.In addition,an asymmetric supercapacitor was assembled by using Ni Mo O₄/Ti₃C₂T_x as cathode material and Fe₂O₃/Ti₃C₂T_x as cathode material to investigate the practical application potential of two composite materials.The results show that the ASC device demonstrates an energy density of 41.89 Wh/kg and a good cycling stability（77.1%,10000 cycles）.