Preparation Of Two-Dimensional MXene Materials And Their Application In Supercapacitors

With the rapid development of economy,the rapid consumption of traditional energy and the aggravation of various environmental problems have attracted more and more public attention and panic,and the demand for new energy is becoming more and more urgent.Under the background of vigorously developing green energy,supercapacitor,which is new energy storage devices,has shone in the field of energy storage and conversion.The performance of supercapacitors often depends on their energy density.Increasing the energy density usually requires increasing the specific capacity of the existing material or increasing the specific surface area of the electrode material.In that case,increasing the specific surface area of the material,the particle size of the material is reduced,so that it has more ion adsorption sites to maintain high conductivity.Transition metal carbides and nitrides/carbonitrides（MXene）,which have high electrical conductivity,high specific surface area and excellent hydrophilicity,occupy a place in the field of high-performance electrode materials.The etched MXene usually exhibits an accordion-like morphology,and the electrical properties of MXene can be significantly changed by compounding,hybridization and modification.However,hydrofluoric acid is often used in the preparation of MXene materials,and hydrofluoric acid solution will strongly stimulate and corrode the skin.Therefore,it is necessary to develop and select a highly stable,low toxicity and environmentally friendly preparation method.In addition,MXene has a wide range of common problems in two-dimensional materials.Its easy oxidation,easy accumulation,and low theoretical specific capacity hinder the transmission of electrons and ions,thereby reducing the charge storage capacity.MnO₂and TiO₂can obtain excellent pseudocapacitance by redox reaction.By introducing MnO₂and TiO₂as spacer layers,the restacking of Ti₃C₂T_xlayers is inhibited.At the same time,Ti₃C₂T_xwith excellent conductivity can also be used as a conductive substrate for metal oxides to promote electron transfer and alleviate the volume expansion of metal oxides during charging and discharging.In this paper,the preparation method of Ti₂Al C₃was improved based on alkali-assisted hydrothermal method.In the environment of high temperature and high concentration,potassium permanganate and sodium hypochlorite were selected as oxidants to accelerate etching while ensuring etching efficiency.Through this preparation method,we can make full use of the interlayer structure of MXene,so that MnO₂and TiO₂grow in situ on the surface and interlayer of MXene,which can effectively alleviate the self-stacking problem of nanosheets.With the increase of the distance between the layers,the electrolyte can fully contact with the MXene nanosheets to form a conductive network and accelerate the electron transport,which significantly improves the performance of the MXene-based supercapacitor electrode material.（1）Ti₃Al C₂was etched by HCl/Na F mixed solution and high concentration Na OH solution,respectively.The properties of the materials prepared under the two reaction conditions were comprehensively compared by electrochemical tests to explore the influence of reaction conditions on the properties of etched materials.The electrode prepared by etching Na F/HCl mixed solution has a capacitance retention rate of 79.8%after 10000charge-discharge cycles,while the electrode prepared by etching Na OH solution has a capacitance retention rate of 81.7%.The MXene material prepared by alkali-assisted hydrothermal method does not contain electrochemically inert-F,showing better electrical properties.（2）Based on the alkali-assisted hydrothermal method,Ti₃Al C₂was used as raw material to accelerate the etching process by adding KMn O₄oxidant during the etching process.MnO₂was generated and attached to the surface and interlayer of Ti₃C₂T_x.The results show that the MXene/MnO₂composites prepared by 0.06 g KMn O₄and 12 h reaction time exhibit higher electrical properties.In the voltage window of-0.6～0.2 V,the maximum specific capacity reaches 182.75 F/g,and the capacitance retention rate still reaches 85.65%after 10000 cycles of discharge.The introduction of MnO₂increases the specific surface area of the material and reduces the stacking effect between the layers.The in-situ generated MnO₂has a stronger binding force than the later loaded MnO₂,and the adsorption is also closer,which is conducive to charge transport.（3）Based on the alkali-assisted hydrothermal method,Ti₃Al C₂was used as raw material,and 10%Na Cl O aqueous solution was added as oxidant during the etching process to accelerate the etching and generate TiO₂with excellent conductivity.When the oxidation time is 12 h and the amount of Na Cl O is 0.2 m L,the TiO₂particles generated by the oxidation of Ti element in MXene are evenly distributed on the surface and interlayer of the material,which effectively curbs the stacking problem between Ti₃C₂T_xlayers and improves the specific surface area of the material.The prepared electrode in 1 M H₂SO₄electrolyte,-0.5～0.2 V voltage window,the highest mass specific capacity can reach 321 F/g,and after 10000charge-discharge cycles,the capacitance retention rate can still reach 86.4%,which is higher than that of Ti₃C₂T_xmaterial（81.7%）.