Study Of Supercapacitor Performance Of Ti₃C₂T_x In Aqueous Acidic Electrolyte

MXene as a fast-developing family of materials have attracted tremendous interest in recent years.Ti₃C₂T_x is the most representative material in the MXene family,which is because its simple synthesis,high stability,highest capacity in acid solution and high comprehensive performance.In this thesis,we concentrate on investigations of Ti₃C₂T_xbased supercapacitors.First,we investigate the charge storage mechanisms of Ti₃C₂T_xin acid solutions.Then,environmental-friendly flexible solid-state supercapacitors with excellent performances are assembled.The main contents and innovations are as follows:1.Anion co-insertion hypothesis is proposed.6 acid electrolytes with different anion sizes are selected to verify the hypothesis.In situ X-Ray diffraction（XRD）tests,scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,X-ray photoelectron spectroscopy（XPS）and Raman spectrum comfirm:1.In acidic electrolytes,under the potential range of-0.1 V to-0.35 V,the CV curves present as rectangular shaped.There are no changes in interlayer spacings,the charge storage mechanism is electrochemical double layer capacitance（EDLC）in this range.2.A pair of broad redox peaks are presented at around-0.7 V,in corresponding with tiny shrinkage in interlayer spacing,which represents proton/hydronium intercalation mechanism originated from the electrostatic effect.3.Below-0.8 V in these acids,the CV curves show a second pair of redox peaks with higher peak heights and narrower half-peak widths.This corresponds to the anion co-intercalation/de-co-intercalation during the reduction/oxidation process.Interlayer spacing expansion induced by the big anion expansion intercalation domains beyond the shrinkage induced by electrostatic adsorption.Thus,the larger the anions are,the lower the peak potential in the cyclic voltammetric curve because larger ionic enviroment needs higher activation energy.In p-CH₃C₆H₄SO₃H,the c-lattice parameter value can change up to 16.52（?）.In the CF₃SO₃H,C₆H₅SO₃H,p-CH₃C₆H₄SO₃H,the anion sizes are larger,due to the space effect and other effects such as isotropy and anisotropy co-intercalation etc,anion co-intercalation processe are more complicated.2.With the continuous development of wearable electronic products,the market has put forward higher requirements for their comfort and endurance.Thus,researching higher capacitance flexible energy storage device is important.In the second chapter,we further apply MXene materials in flexible solid-state supercapacitors.The mechanical propertie of the self-supported Ti₃C₂T_x film is not enough for real life applications.In order to overcome the above disadvantage,Ti₃C₂T_x@CC flexible integrated electrode is fabricated by Ti₃C₂T_x supported by flexible carbon cloth substrate.The electrode material presents better dynamics than self-supported Ti₃C₂T_xfilm.Environmentally friendly methanesulfonic acid/PVA-based gel polymer electrolyte is selected as the electrolyte.To solve the problem of the low voltage of all-MXene based two-electrode supercapacitors,this work offers a new strategy of assembling all-MXene based asymmetric two-electrode supercapacitors.The rate performance of the obtained solid-state supercapacitor is much better than that prepared by self-supporting Ti₃C₂T_x film material.When the current density rises from 0.5 m A cm^-2 to 10 m A cm^-2,the capacitance retention is 80%.When the current density rises from 0.5 m A cm^-2 to 20 m A cm^-2,the capacitance retention is 59%.When the scan rate rises from 5 m V s^-1 to 50 m V s^-1,the capacitance retention is about 100%.Further rising to 100 m V s^-1,200 m V s^-1 and 300 m V s^-1,the capacitance retentions are 89%,77%and39%.Then the effect of mass loading is discussed.The highest areal capacitance reaches 1820 m F cm^-2.High energy density and power density are also obtained.After cycling for 10000 cycles at 10 m A cm^-2,the capacitance retention is 98%.After bending for 1000 cycles and with harder conditions of twisting and cutting,the CV curves remains,demonstrating good flexibility and stability.3.This chapter addresses the problems of flexible solid-state supercapacitors in the previous chapter,where the voltage range is limited by carbon cloth,the energy density and power density are not high enough.Current research on MXene solid-state supercapacitors mainly focuse on improving electrode materials.In this investigation,we have chosen to optimize the flexible solid-state supercapacitor from the perspective of electrolyte.A simple freeze-thawing method is used to fabrivate MSA/PVA electrolyte,who presents high ionic conductivity(0.2 S cm^-2).Then,a simple casting method is used to cast Ti₃C₂T_x on both surfaces of electrolyte.The carbon cloth is changed by Ti foil.Then,the flexible solid-state supercapacitor FT-SSS is obtained.MSA/MSA ion pre-intercalate into Ti₃C₂T_x layers,which reduces stacking effect of MXene and enhances accessibility of electrolyte by forming a porous and wrinkled electrode.-OH and H₂O（physisorbed to-OH）content in Ti₃C₂T_x rise,too,which contributes to pseudocapacitance performance.The as fabricated device（FT-SSS）presents outstanding capacitance and rate capability that even exceed corresponding aqueous device（LQS）.The areal capacitance increases by 37.5%and 127%at 5 m V s^-1and 1000 m V s^-1 compared with LQS.The highest energy density reaches 239μWh cm^-2,the highest power density reaches 50027μW cm^-2.After 10000 cycles,the capacitance retention is 100%.After ultralong 80000 cycles,the capacitance retention is 92%.Besides,51%capacitance is retained at-30℃.These all demonstrate excellent performances of FT-SSS.