Hierarchical-Structured Ti₃C₂T_x Composite Fiber-Based Flexible Supercapacitors

With the rapid development of portable electronic devices and smart wearable industry,the development of flexible energy storage materials with high performance to power flexible electronic devices has become the focus of research in this field.Among large quantity of flexible energy storage materials,the fiber-based supercapacitor（FSC）is one of the best choice because of its good flexibility,long cycle life,high power density,small size and light weight.Among them,the structure,chemical composition and assembly method of the fiber electrodes have a close influence on their energy storage,transmission and conversion characteristics.However,it is difficult to regulate the microstructure and active components of the fiber electrode materials constructed by traditional methods in an integrated manner,resulting in blocking ion transfer and charge transfer,and low energy density,which limits the application of FSC in wearable power supply.Therefore,one of the challenges in this field is how to design the composition,micro-structure and assembly process of the electrodes,construct advanced and ordered fibrous materials,and greatly increase the energy density and mechanical strength of FSC.Recently,two-dimensional transition metal carbide/nitride（MXene）is an ideal candidate for FSC fiber electrodes due to its high conductivity,abundant active sites,good cycling performance and adjustable structure.However,due to the strong van der Waals force between MXene sheets,it is easy to re-stack,which hinders the dynamic transport and storage of ions,resulting in a decrease in energy density.In recent years,based on microfluidic control technology,the composition,assembly and fiber forming processes of building elements have been regulated in limited channels.This allows precise control of the ordered microstructure,specific surface area,and ion channels of the fiber electrodes,resulting in high performance wearable fiber energy storage materials.Therefore,this paper uses microfluidic method to prepare hierarchical MXene composite fiber-based flexible supercapacitor.The detailed work is as follows:1.Preparation and electrochemical properties of Ti₃C₂T_xand Ti₃C₂T_x/SNFs composite fibers by microfluidic technologyFirst,few layers of Ti₃C₂T_xmaterial were prepared by chemical etching of MAX phase with Li F and HCl.Preparation of Silk Nanofibers（SNFs）by Low-soluble Solvent Method.Secondly,in the T-type microchannel structure,Ti₃C₂T_xand Ti₃C₂T_x/SNFs fibers with parallel orientation were prepared by adjusting the assembly and chelation of precursors.Secondly,in the T-type microchannel structure,Ti₃C₂T_xand Ti₃C₂T_x/SNFs fibers with parallel orientation were prepared by adjusting the assembly and chelation of precursors.The results show that SNFs can improve both the tensile strength and electrochemical properties of Ti₃C₂T_xfibers.In 1 M H₂SO₄electrolyte,the Ti₃C₂T_x/SNFs composite fiber electrodes showed good volumetric specific capacitance(1169.5 F cm^-3)and Faraday reversibility,and had higher performance than pure Ti₃C₂T_xfibers(936.9 F cm^-3).The tensile strength and elongation at break of Ti₃C₂T_x/SNFs composite fibers were 115.5 MPa and 4.5%,which were higher than those of pure Ti₃C₂T_xfibers（21.4 MPa,2.8%elongation at break）.2.Preparation and electrochemical properties of NCDs-Ti₃C₂T_x/SNFs composite fibers by microfluidic technologyNitrogen-doped carbon quantum dots（NCDs）were prepared from0-phenyldiamine（o PD）and folic acid（FA）.The hierarchical-structured NCDs-Ti₃C₂T_x/SNFs composite fibers were prepared by adjusting the assembly and chelation of NCDs,Ti₃C₂T_xand SNFs in the T-type microchannel structure using microfluidic control technology.The results show that NCDs as active units can change the surface charge density of Ti₃C₂T_x,increase the layer spacing and specific surface area,reduce the adsorption energy barrier of electrolyte ions,and accelerate the kinetic transfer process.The NCDs-Ti₃C₂T_x/SNFs composite fibers constructed in 1 M H₂SO₄electrolyte exhibit super high volume specific capacitance(2218.4 F cm^-3).The tensile strength and elongation at break of NCDs-Ti₃C₂T_x/SNFs composite fibers reach 118.7MPa and 5.4%,respectively.3.Construction,Performance and Application of Solid State NCDs-Ti₃C₂T_x/SNFs//r GO Asymmetric FSCPVA/H₂SO₄electrolytes were coated on parallel NCDs-Ti₃C₂T_x/SNFs and reduced graphene oxide（r GO）fibers to construct solid NCDs-Ti₃C₂T_x/SNFs//r GO asymmetric FSC.FSC exhibits high energy density(57.9 m Wh cm^-3),good specific capacitance(1057 F cm^-3),long cycle stability（40,000 times）,large bending deformation and high and low temperature stability.The fiber electrodes are integrated into a chip-based supercapacitor to achieve stable power supply applications for devices such as lanterns,watches,high-power cars,etc.