Two-Dimensional Ti₃C₂ Sheets For Assembling Neat Macro-Fibers And Their Applications In Flexible Supercapacitors

Based on the recent explosive development of mobile electronic technology,flexible energy supply devices have become increasing popular.High-performance fiber electrodes are very important for flexible device,the development of which,however,is very challenging.Two-dimensional sheets are known to be excellent units for constructing macroscopic fiber electrodes due to their large aspect ratio,which give them the characteristics of easy assembly.Graphene oxide sheets with hydrophilic surfaces can be well dispersed in aqueous media,facilitating the creation of continuous fibers with good mechanical strength using scalable wet-spinning methods.The assembly of fiber electrodes using two-dimensional materials with high electrochemical activity can further enrich their application in flexible energy supply devices.Two-dimensional titanium carbide(Ti3C2)is considered to be an excellent material for assembling high-performance fiber electrodes because of its excellent electrical conductivity and electrochemical activity,and can be prepared into a stable colloidal solution.However,large grain boundaries will be generated that have a significant obstacle for constructing macroscopic neat Ti3C2 fibers due to the sheets with small lateral size,and usually additives are needed to prepare the fiber electrode,which will affect the electrical and electrochemical properties of Ti3C2 fiber.This thesis focuses on the structure and properties of the fiber and explores its application in flexible wearable supercapacitors.The main research contents are as follows:1.Two-dimensional Ti3C2 sheets for assembly neat Ti3C2 fiberWe successfully assembled Ti3C2 ribbon fibers with oriented sheet stacking using universal wet spinning technology by optimizing the lateral size of Ti3C2 sheets and colloid concentration,selecting a suitable flocculant(protonated chitosan solution)and carefully adjusting spinning parameters,although the solution did not form a liquid crystal phase.The fiber was then treated with acid to remove the interlayer chitosan and simultaneously intercalating the electrolyte into the interlayer,by which neat Ti3C2 macro-fibers with a continuous ion transport network structure between the layers was obtained.By successfully assembling the non-liquid crystal phase Ti3C2 colloid into neat Ti3C2 fibers,it provides a new strategy for the macro assembly of the two-dimensional colloidal solution and expands its application in the field of flexible energy storage.2.Characterization of neat Ti3C2 fiberThe neat Ti3C2 fibers was confirmed to have a ribbon structure with oriented sheet stacking through the characterization of X-ray diffraction,scanning electron microscope,etc.The highly oriented stacking structure is conducive to integrating the mechanical properties of the sheets and the fibers still have good mechanical flexibility(mechanical strength of 30 MPa)even after the removal of interlayer chitosan and immersion in the electrolyte.The orderly stacking of sheets can reduce the resistance between the sheets and improve the electrical conductivity of neat Ti3C2 fibers(2458 S cm-1),which is 1-2 orders of magnitude higher than that of composite fibers.In addition,the immersion of electrolyte between the layers can form a two-dimensional channel that facilitates the rapid transmission of ions,so that H+ has a high difiusion coefficient(9.2×1017 cm2 s-1)for the interlayer transmission.The ordered stacking of sheets significantly improves fiber mechanics and electrical properties,and provides advantages for neat Ti3C2 fibers in electrochemical applications.3.Electrochemical performance and application of neat Ti3C2 fiberThe electrochemical performance of neat Ti3C2 fiber was tested in an acidic electrolyte.Based on the orderly stacking of sheets endowing the fiber with high conductivity and a continuous ion transmission network structure constructed between layers,the neat Ti3C2 macro-fiber had a high specific capacity(309 F g-1)and excellent rate performance.In addition,in order to solve the problem of limited energy density of symmetric capacitors,we assembled asymmetric fiber capacitors using the neat Ti3C2 fiber as a cathode coupled with reduced graphene oxide fiber as an anode.The device had an operating voltage of 1.5 V and delivered a high volume energy density(58.4 mW h cm-3).In view of the good mechanical properties of the fiber electrodes,the device can effectively provide energy for flexible electronic components,and it still has good working stability even under bending conditions.The preparation of neat Ti3C2 fiber not only enriches the use of MXene,but also expands its application in the field of flexible energy storage.