Construction Of Cellulose/MXene Flexible Composite Electrode And Its Supercapacitor Performance Investigation

Flexible supercapacitors have attracted much attention due to the rapid development of portable electronics and wearable devices.Ti₃C₂T_x,a new two-dimensional nanomaterial with many advantages including ultra-high area specific capacitance,outstanding electrical conductivity,good mechanical properties,excellent hydrophilicity,and easy dispersion,is suitable for being used as electrode material for supercapacitors,and has aroused great interest of researchers in the field of energy storage.Cellulose,a kind of natural biomass with high hydrophilicity,high specific surface area,and high mechanical flexibility,can be used as a cheap and high-quality flexible substrate in the field of flexible supercapacitors.In allusion to the application limitation of Ti₃C₂T_x in energy storage field caused by its stackability,this thesis prepared a series of flexible electrode materials with excellent electrochemical properties using green renewable cellulose as the flexible component and two-dimensional layered Ti₃C₂T_x as the active component,respectively,and explored the application potential of these materials in the field of asymmetric supercapacitors.Specific research contents are as follows:1.Using electrostatic interaction and ion intercalation to control the microstructure of materials.Flexible MFC/Ti₃C₂T_x composite membranes were prepared by K⁺-induced electrostatic self-assembly and rapid vacuum filtration using D-Ti₃C₂T_x（prepared through Li F+HCl etching of Ti₃Al C₂ and subsequent ultrasonication）as the active component and MFC as the flexible skeleton respectively.The intercalation effect of K⁺and short-time filtration could effectively increase the interlayer spacing of Ti₃C₂T_x nanosheets,which reduced the diffusion resistance of electrolyte ions and improved the accessibility of electroactive sites,resulting in the significant improvement of electrochemical performance.The prepared composite had ultrahigh specific areal capacitance(2168 m F cm^-2 at 2 m A cm^-2),excellent large-current charge/discharge capability,and wonderful cycling stability（over 99%capacitance retention after 10000 cycles）,and an asymmetric supercapacitor using this composite as negative electrode and MFC aerogel/chitosan-derived N-self-doped carbon sheet/polyaniline（MFCA/N-CS/PANI）as positive respectively electrode possessed excellent energy storage capacity(a power density up to 4220μW cm^-2 when the energy density is of36.63μWh cm^-2)and good cycle stability（86.4%retention after 2000 cycles）.The composite with higher mass loading also exhibited excellent electrochemical performance,which meant a good potential for practical application.2.Using-20℃freezing and freeze-drying method to transform D-Ti₃C₂T_x into laminate freeze-dried Ti₃C₂T_x（F-Ti₃C₂T_x）with a large interlayer spacing.The stress-free preparation process and the escape of internal fine ice crystals during freeze-drying increased the interlayer spacing of F-Ti₃C₂T_x,making its internal active site highly accessible to electrolyte ions.Two kinds of composites obtained by combining F-Ti₃C₂T_x with MFC fiber and MFC aerogel respectively both possessed high specific areal capacitance(1635 and 1508.2 m F cm^-2respectively,at 2 m A cm^-2)and excellent cycling stability（more than 99%retention after 10000cycles）.And aerogel-based composite had better folding ability but a little lower capacitance.The asymmetric supercapacitor using aerogel-based composite as negative electrode and MFCA/N-CS/PANI as positive electrode respectively had a power density up to 4190.4μW cm^-2 when the energy density is of 26.8μWh cm^-2,and a retention rate of 81.3%after 2000 cycles,showing high charge storage capacity and good cycling stability.3.Improving the electrochemical properties of F-Ti₃C₂T_x by optimizing its structure and reconstructing its surface functional groups,respectively.F-Ti₃C₂T_x derived from D-Ti₃C₂T_xwith a much lower concentration has a larger specific surface area,which could improve the effective utilization of active sites.KOH treatment combined with heat treatment can remove most-F terminations on the surface of Ti₃C₂T_x and introduce some oxygen-containing groups,resulting in more active sites.Different composites were prepared by combining each optimized F-Ti₃C₂T_x with MFC fiber and MFC aerogel respectively.Compared with the corresponding composites in the previous chapter,the composites prepared in this chapter all had higher specific areal capacitance（improvements range between 24%to 33%）and the same excellent cycling stability（more than 99%retention after 10000 cycles）.The asymmetric supercapacitors using an aerogel composite as negative electrode and MFC/acetylene black（ACB）/N-CS as positive electrode respectively showed excellent electrochemical properties,including outstanding charge storage capacity(a power density high up to 12787.3μW cm^-2 when the energy density is of 65.73μWh cm^-2)and excellent cycling stability（91.5%retention after 2000cycles）.4.Treating the homogeneous dispersion of cellulose nanofiber（CNF）and D-Ti₃C₂T_x by orientation freezing and freeze-drying and subsequent tabletting to obtain self-supporting composite with oriented lamellar structure.Due to the extrusion action of ice crystals and desolvation effect during freezing+freeze-drying process,surface oxygen-containing groups of CNF can form hydrogen bond network with the surface terminations of Ti₃C₂T_x（-OH,-O,and-F）,thus improving the integrity and continuity of composite.The intercalation of CNF and escape of water vapor significantly increased the interlayer spacing of Ti₃C₂T_x,which was conducive to the transport of electrolyte ions between layers.The prepared self-supporting composite achieved an utralhigh specific areal capacitance up to 2183 m F cm^-2(2 m A cm^-2)and excellent cycling stability（more than 99%retention after 10000 cycles）.The asymmetric supercapacitor using the prepared composite as negative electrode and MFC/ACB/N-CS as positive electrode respectively had excellent cycling stability（90.8%retention after 2000 cycles）and outstanding charge storage capacity(a power density high up to 12819.5μW cm^-2 when energy density is of 70.8μWh cm^-2).