Preparation And Electrochemical Behavior Of Ti₃c₂t_x Electrodes For Outstanding Volumetric Capacitance

MXenes,a new family of two-dimensional materials,exhibit extreme high volumetric capacitance as a supercapacitor electrode due to their unique physicochemical properties and high density,which provides an possibility to be used in compact energy storage,such as new energy vehicles,portable devices,and flexible wearable equipment.With the increasing attentions on compact energy storage from science and industry,new challenge to electrochemical property has emerged for MXenes.Therefore,the present trend for MXenes research is how to enhance their electrochemical property by changing microstructure.In this paper,preparetion of electrode with outstanding volumetric capacitance by designing microstructure of MXene?Ti₃C₂T_x?,which investigated by characterizing the morphology and microstructure using XRD?SEM?XPS?BET,and evaluated electrochemical behavior in three-electrode system and symmetric electrochemical cell.The 3D macroporous MXenes film and aerogel prepared by liquid nitrogen rapid freezing.The 3D macroporous Ti₃C₂T_x film is consisted of the macropores with regular alignment parallel to the film surface,and interconnected nano walls between adjacent macropores,presenting a 3D network structure with high flexibility.The Ti₃C₂T_x aerogel is constructed by the macropores with irregular shape and higher pore diameter,and the thinner pore walls.Those macropores can effectively prevent 2D nanosheets from restacking through the sever deformation of nanosheets during rapid cooling,which increases the specific surface area of the Ti₃C₂T_x film and Ti₃C₂T_x aerogel respectively by 5 and 22 times,enhancing electrochemical property by improving double-layer capacitance.The electrochemical measurement shows that,under the condition of 3 M H₂SO₄ electrolyte and current density of 1 A g^-1,3D macroporous Ti₃C₂T_x film and aerogel respectively show impressive volumetric capacitance of 1355 and 1293 F cm^-3,which are both higher than that of most MXenes materials.A symmetric supercapacitor device based on 3D macroporous Ti₃C₂T_x film exhibits excellent volumetric energy density up to 32.2 W h L^-1,which is far above that of most MXenes,carbons,polymers,and oxides etc.The electrode based on multiscaled Ti₃C₂T_x was prepared by“accordion”Ti₃C₂T_x nanoparticles intercalate Ti₃C₂T_x nanosheets.In the multi-scaled structure,the 3D Ti₃C₂T_x nanoparticles are uniformly distributed between the interconnected 2D Ti₃C₂T_x nanosheets,showing a typical sandwich-like hybrid structure.The introduction of Ti₃C₂T_x nanoparticles,which has little effect on chemical composition and layered structure of the2D nanosheets,can efficiently hinder the nanosheets restacking and maintain high interlayer space,providing fast transport paths for electrolyte ions.In addition,the nanoparticles play an important role of enlarging specific surface area through inducing2D nanosheets to crumple and form porous structure,facilitating increase of surface active spots.The addition of 10 wt.%Ti₃C₂T_x nanoparticles enables the specific surface area increased by about 1.5 times.The electrochemical test results reveal that,under 1 M H₂SO₄ electrolyte and current density of 1 A g^-1,the multi-scaled Ti₃C₂T_x electrode?Ti₃C₂T_x-10?,which has 10 at%addition of Ti₃C₂T_x nanoparticles,exhibits an excellent volumetric capacitance of 1377 F cm^-3 which is higher than that of pure Ti₃C₂T_x and most previously reported MXenes,which also has high rate performance and cycling stability.The energy density of the all-solid symmetric supercapacitor device assembled with the Ti₃C₂T_x-10 and the PVA/H₂SO₄ electrolyte reaches 17.4 Wh L^-1.The surface modified MXene electrode with outstanding volumetric capacitance prepared by modification of alkalinizing and annealing.The microstructure characterization and analysis imply that,the alkalinizing and annealing treatment removes a large number of surface terminal groups such as-F and-OH etc,resulting more redox-active spots exposed on nanosheet surface,and the treated Ti₃C₂T_x electrode maintains a layered microstructure with high spacing.At the same time,annealing increases the ordering level of Ti₃C₂T_x crystalline and raises its conductivity,facilitating the fast transportion of electrons in electrode.The electrochemical test results indicate that,under the condition of 1 M H₂SO₄ electrolyte and a current density of 1 A g^-1,the surface modified Ti₃C₂T_x film achieves an ultrahigh volumetric capacitance up to 1805 F cm^-3,of which is among the highest values reported for MXenes.The capacitance and the rate performance of the surface modified Ti₃C₂T_x film is easily regulated by changing its thickness under control of loading amount of Ti₃C₂T_x nanosheet suspension.With decreasing film thickness,volumetric capacitance and capacitance retention are gradually increased.A symmetric supercapacitor device based on surface modified Ti₃C₂T_x electrodes shows an outstanding volumetric energy density of 38.8 WhL^-1.