Preparation Of Three Dimensional Ti₃C₂T_x Based Materials And Their Applications For Supercapacitors

Due to environmental problems and the depletion of fossil fuels,new renewable clean energy research and application are developing rapidly.Therefore,it is essential to develop an efficient energy storage system.As a representative electrochemical energy storage device,supercapacitors have attracted more and more attention because of their fast power transmission,excellent rate performance,and well cyclic stability.The performance of supercapacitors mainly depends on electrode materials.It is of great significance to develop novel and practical electrode materials for the development of supercapacitors.At present,new types of electrode materials have received more and more research and attention.The two-dimensional(2D)layered early transition metal carbide/carbonitride family(MXene)recently provided researchers with a new choice.MXene’s high conductivity and excellent mechanical properties make it a promising supercapacitor candidate.As a representative of MXene,Ti3C2Tx has conductivity up to 10000 S/cm,abundant surface functional groups(-F,-OH,and-O),and variable oxidation number of Ti.These properties enable it to have excellent electrochemical performance in alkaline,neutral and acidic electrolytes.However,in practical applications,the active surface of Ti3C2Tx cannot be fully utilized due to the layer aggregation and stacking caused by the 2D material properties of Ti3C2Tx.To solve the problem,researchers have reported various approaches,among which constructing 3D structures based on 2D materials is a strategy that can effectively avoid the layer stacking problem of 2D material.This thesis aims to build a 3D Ti3C2Tx structure with different methods to prevent the stacking of Ti3C2Tx nanosheets.The electrode of the supercapacitor was made from the prepared 3D Ti3C2Tx materials,and their application performance in the aqueous electrolyte was studied.The main content of this thesis is divided into the following parts:(1)Controllable preparation of 3D flower-like T13C2Tx microspheres and their application in supercapacitors3D flower-like Ti3C2Tx microspheres(FMXMSs)were assembled in situ using the W/O emulsion-assisted method.Polyethyleneimine(PEI),as a large molecule,could effectively stabilize the emulsion system.Ethylenediamine(EDA),as a weak crosslinking agent,contains abundant amino groups,which could effectively induce Ti3C2Tx nanosheets to assemble in spherical droplets and form N-Ti bonds,and finally form 3D flower-like Ti3C2Tx microspheres after removing water in the droplets.The formation of a strong N-Ti bond makes FMXMSs possess a stable microsphere structure.By adjusting the water-oil volume ratio,the size of emulsion droplets can be controlled,thus the structure and size of microspheres can be controlled.The 3D microsphere has a large specific surface area(108.3 m2/g),which is conducive to exposing the active surface of Ti3C2Tx and provides an effective ion diffusion pathway.As the supercapacitor electrode in 1 M KOH electrolyte,the specific capacitance of FMXMSs is 224.6 F/g at a scan rate of 5 mV/s and 193.7 F/g at a current density of 0.5 A/g.In addition,due to its solid spherical structure,the FMXMSs also show excellent long-term cycle stability,with no specific capacitance decay after 5000 charge and discharge cycles.The results show that W/O emulsion-assisted assembly is an effective strategy for preparing MXene microspheres with high electrochemical properties.(2)Synthesis of 3D Ti3C2TX MXene-Carbon black microspheres and their supercapacitor performance3D TbC2Tx-carbon black(Ti3C2Tx-CB)microspheres were prepared by a simple and efficient gas-phase spray drying method.In the process of gas-phase spray drying,CB particles and Ti3C2Tx nanosheets were encapsulated in aerosol microdroplets and formed a microsphere structure after drying.The CB particles with conductivity and high specific surface could prevent the secondary stacking of Ti3C2Tx nanosheets and fill the microspheres to prevent the collapse of the structure and make the composite have a high specific surface area.The obtained three-dimensional microsphere structure has a large specific surface area(319.5 m2/g),which is conducive to utilizing the Ti3C2Tx active surface.In 1 M KOH electrolyte,the specific capacitance of Ti3C2Tx-CB microspheres is up to 335.5 F/g at 5 mV/s and 291.6 F/g at 0.5 A/g.After a high current density charge-discharge cycle,the capacitance restores to 90.8%of the initial value.After charging and discharging 4000 times at 2 A/g current density,the capacitance retention rate reaches 74.1%.When Ti3C2Tx-CB is applied in the symmetric buckle supercapacitor,the specific capacitance of the supercapacitor is 72.7 F/g when the scan rate is 10 mV/s.When the current density is 0.5 A/g,the specific capacitance is 33.4 F/g.The capacitance retention was 93.0%after 4000 cycles at 2 A/g.When the power density of a symmetric ultracapacitor is 363.0 W/kg,the energy density is 10.1 Wh/kg.Even if the power density reaches 3.2 kW/kg,the energy density can still get 9.0 Wh/kg.The results show that this method is an effective strategy for preparing high capacitance Ti3C2Tx matrix composites and is expected to achieve mass production.(3)Preparation of 3D porous Ti3C2Tx/SWCNTs freestanding film and its application for supercapacitorsWith good flexibility,3D porous Ti3C2Tx/SWCNTs self-supported films(pTi3C2Tx/SWCNTs)were prepared by simple vacuum filtration.A 3D interconnected porous network structure was obtained using PS microspheres of uniform size as templates and SWCNTs as conductive interlayers.The heat treatment process to remove the template plays a positive role in the change of surface characteristics of Ti3C2Tx.The specific surface area of the film is 67.3 m2/g,which is much higher than Ti3C2Tx film.In the 1 M H2SO4 electrolyte,pTi3C2Tx/SWCNTs were used as independent electrodes,with a specific capacitance of 546.8 F/g at 1 A/g.The rate performance is up to 87.2%(1 A/g-10 A/g).After charging and discharging 5000 times at 5 A/g,the capacitance retention rate reaches 92.2%.When pTi3C2Tx/SWCNTs were applied in a symmetric buckle supercapacitor,the specific capacitance was 116.5 F/g at 1 A/g.The rate performance is up to 82.7%from 1 A/g to 10 A/g.After a long cycle test at 5 A/g,the capacitance can still maintain 85.2%after 5000 cycles of charge and discharge.The energy density of the capacitor reaches 10.4 Wh/kg,and the power density reaches 400.0 W/kg at 1 A/g.The energy density of the capacitor can still get 8.6 Wh/kg at 10 A/g,and the power density can reach 4.0 kW/kg.The results show that this strategy’s flexible porous composite film has great application potential in flexible wearable supercapacitors.