Preparation And Performance Of Free-standing MXene-based Film Electrodes For Supercapacitors

Two-dimensional transition metal carbides(MXene)have promising applications as supercapacitor electrode materials due to its metal-like electrical conductivity,hydrophilic surface,abundant surface groups,and excellent mechanical properties.However,the van der Waals forces between MXene layers can lead to serious stacking phenomena in its lamellae,which hinders the electron and ion transport and reduces the electrochemical performance of the material,severely limiting its practical applications.In this thesis,with the objective of obtaining high performance MXene based self-supporting film electrodes for supercapacitors,the surface modification treatment of MXene was carried out by high temperature annealing method,and the influence of annealing temperature on the capacitive performance of MXene film was investigated;the structure of MXene film electrodes was regulated by alkaliinduced and templating methods,the MXene/CNTs film electrode and MXene@Se composite film electrode with Two-dimensional(3D)porous structure were prepared.(1)The surface modification treatment of MXene was carried out by hightemperature annealing,and the influence of different annealing temperatures(300,350,400,450 and 500?)on the surface groups and electrochemical properties of MXene films were investigated.The results show that the high temperature annealing treatment can significantly reduce the content of fluorine-containing functional groups on the surface of MXene films,which in turn exposes more surface active sites and improves the electrochemical properties of the materials.In particular,the content of fluorine-containing functional groups on the surface of MXene film after high-temperature treatment at 400? is only 1.35%,which shows the best electrochemical performance.It exhibited a specific capacitance of 343.3 F g-1 at 1 A g-1,which was significantly higher than that of the MXene film without high-temperature annealing treatment(287.0 F g-1).As the current density increased to 500 A g-1,the specific capacitance still remained 142.8 F g-1.(2)A MXene/CNTs film electrode with 3D porous structure were prepared through a NaOH-induced transformation of MXene nanosheets into 3D flocculent precipitation,vacuum filtration and followed high temperature annealing route.Compared with the pure MXene film,MXene/CNTs film and Alkali-inducted MXene film,the 3D porous MXene/CNTs film has the following advantages:the construction of 3D structure can suppress the stacking of MXene sheets and promote the ion/electron transport;the high temperature annealing treatment reduces the content of fluorine-containing functional groups on the MXene surface and exposes more CNTs as interlayer particles can effectively avoid the stacking of MXene lamellae and promote the effective utilization of MXene surface active sites.Thanks to this unique structure,the 3D porous MXene/CNTs film exhibited excellent electrochemical performance.Its specfic capacitance reached up to 401.4 F g-1 at a current density of 1 A g-1 with 3 mol L-1 H2SO4 as the electrolyte,and it still maintained a high specific capacitance of 336.2 F g-1 at a high current density of 1000 A g1,the retention rate is as high as 83.8%,making it a promising electrode for supercapacitors with both a high specific capacitance and outstanding rate capability.(3)A MXene@Se composite film electrode with 3D porous structure was prepared by theramal treatment of a MXene/SeO2/GO composite film.In this process,SeO2 and GO undergo carbothermal reduction reaction at high temperature,and the CO2 gas produced will escape between MXene layers and flush out the lamellar stacking structure of MXene to form a large number of mesopores.In addition,the appropriate amount of Se can act as interlayer particles to support the layer spacing of composite film electrode,and then the MXene@Se composite film with 3D porous structure can be constructed,which can effectively promote the ion/electron transport and improve the electrochemical performance of the electrode.Being used as a supercapacitor electrode,the MXene@Se composite film prepared with an initial MXene content of 70 wt%showed the best electrochemical performance.It exhibited a specific capacitance of 323.8 F g-1 at a current density of 1 A g-1.When the current density increased to 500 A g-1,it still maintained a specific capacitance of 243.6 F g-1 with a capacity retention rate of 75.2%,showing an excellent rate capability.