Research On Synthesis And Supercapacitors Performance Of MXene-based Flexible Free-standing Electrodes

Supercapacitors,a kind of electrochemical energy storage device with high power density and long life,bring a lot of convenience to people’s production and life.To meet people’s demand for portable electronics,flexible supercapacitors are gradually emerging.In order to prepare high-performance flexible supercapacitors,it is particularly important to develop high-performance flexible free-standing electrodes.A new group of 2D materials known as MXene,has been widely studied in flexible free-standing electrodes for supercapacitors because of its high conductivity,excellent flexibility and pseudocapaciance.However,due to hydrogen bonds and van der Waals forces,MXene nanosheets are tightly stacked during the preparation of flexible free-standing film electrodes,resulting in reduced specific surface area and lower capacity.Moreover,few-layer MXene nanosheets usually have poor chemical stability,because nanosheets edge exposed metal atoms are easy to react with oxygen or/and water molecules resulting in forming the corresponding metal oxide or chemical degradation.This phenomenon is more serious under heating condition.Eventually,the activity of MXene is sharply declined leading to worse physical and chemical properties.This thesis chooses T₃C₂T_x MXene as the research object,and it is designed in terms of structure,component and antioxidant perspectives.As follows:（1）First,T₃C₂T_x nanosheets were pretreated with alkali to reduce the-F functional group on the surface which is no capaciance contribution.Biological chain polymer sodium alginate（SA）was selected as the intercalation reagent.SA can be inserted into the layer of T₃C₂T_x sheets by hydrogen bonding,but the excellent water dissolution of SA leads to the loss of molecules in the preparation process of film by filtration.Due to the strong chelating ability between SA and Ca²⁺,SA molecules are crosslinked into larger molecular network hydrogels and the water solubility of SA is destroyed.At the same time,Ca²⁺can serve as a bridging agent between SA molecules and T₃C₂T_x,and finally SA molecules can firmly insert into T₃C₂T_x layers to form the T₃C₂T_x-Ca-SA units.Then,these units form the final 3D cross-linked T₃C₂T_x-Ca-SA flexible free-standing films with expanded layer spacing through self-assembly.The areal capacitance of T₃C₂T_x-Ca-SA flexible free-standing electrode is 594 m F cm^-2 in 3M H₂SO₄ electrolyte,and the assembled symmetric pseudopocapacitor has excellent flexibility and higher energy density.（2）To further improve the capacitance performance of T₃C₂T_x,a 3D porous flexible free-standing T₃C₂T_x/WO₃·H₂O-AA all-seudocapacitance film was prepared by simple acid-induced electrostatic self-assembly.T₃C₂T_x nanosheets are tightly wrapped around WO₃·H₂O nanoplates.In turn,WO₃·H₂O nanoplates are firmly confined in the 3D T₃C₂T_x conductive network.The areal capacitance of T₃C₂T_x/WO₃·H₂O-AA（WO₃·H₂O=30wt%）flexible free-standing electrode is 679 m F cm^-2 in 1M H₂SO₄,and the symmetric solid-state supercapacitor based on the T₃C₂T_x/WO₃·H₂O-AA（WO₃·H₂O=30wt%）electrode has excellent flexible capacitance.At 20 m V s^-1,capacity remain 87%after 300 cycles under bending 180°.（3）In order to overcome T₃C₂T_x oxidation and introduce more redox active sites into T₃C₂T_x,a simple antioxidant method is proposed to in-situ derive T₃C₂T_x quantum dots（MQDs）.Citric acid（CA）is added to the T₃C₂T_x.Due to the reduction of CA and the interaction with the T₃C₂T_x,the surfaces and edges of T₃C₂T_xcan be passivated preventing T₃C₂T_x from oxidation.The strong interaction between CA and Ti atoms on T₃C₂T_x edges splits C-Ti bonds,and thus allows the in situ derivative of 2D T₃C₂T_x nanosheets for 0D MQDs.Since the MQDs surfaces have rich C-Ti-O_xactive sites,the areal capacitance of the prepared T₃C₂T_x/MQDs flexible free-standing electrode is 1661 m F cm^-2 in 1M H₂SO₄.The capacity retention rate of assembled flexible symmetric solid-state pseudocapacitors is 84%after 10,000 charge and discharge,indicating their excellent cycle stability.