Preparation And Electrochemical Performance Of MXene 3D Network Free-standing Electrode

Supercapacitors show great application prospects in the wearable field because of high power density,good safety and long cycle life.However,the low volume specific capacitance and energy density of traditional supercapacitors cannot meet the development trend of portable and miniaturized energy storage devices.Therefore,the development of new electrode materials with high specific capacitance has become a hot research topic.MXene has both electrical conductivity and hydrophilicity,and the layer spacing and surface functional groups are abundant and adjustable,which can be used as electrode materials without additives and can help improve specific capacitance.However,the strong van der Waals forces between MXene sheets lead to re-stack,which affects the electrolyte penetration and ion migration.Therefore,solving the above problems is the key to advance the practicalization of MXene.In this paper,we mainly focus on the composition and structure of MXene free-standing electrode,and improve the ion transport rate in the internal lateral/longitudinal direction of the electrode through strategies such as in-plane pore creation,heterogeneous atom doping and 3D network construction to alleviate MXene re-stack and finally obtain excellent electrochemical performance.The main research contents and results are as follows:（1）An in-plane porous MXene free-standing electrode（p MX）was prepared by a hydrogen peroxide-hydrofluoric acid oxidative etching process,and then treated with hydrazine hydrate to obtain a free-standing electrode with a three-dimensional network structure（3D-p MX）.The synergistic effect between the in-plane porous and 3D network structure can effectively increase the specific surface area,increase the ion adsorption/desorption active sites and transport channels,and promote ion transport;meanwhile,the MXene re-stack is relieved and the sheet spacing is increased,which facilitates the electrolyte penetration.In addition,the additive-free 3D-p MX can fully utilize the electrochemical properties of MXene.3D-p MX has a specific capacitance of 416 F g^-1 at 1 A g^-1,which is about 1.5 times higher than the performance of MXene free-standing electrode（Cs=278 F g-1）.At a high current density of 10A g^-1,its specific capacitance remains 253 F g^-1 with a retention rate of 61%.It has a more desirable cycling performance at a high current density of 10 A g^-1,with a capacitance retention rate of 83.4%after 10,000 charge/discharge cycles.（2）MXene was doped with nitrogen atoms（NMX）by hydrothermal method,and then treated with hydrazine hydrate to construct three-dimensional network free-standing electrode（3D-NMX）.The nitrogen atom doping can improve its wettability and promote ion transport;the 3D network can alleviate MXene stacking and increase ion transport channels,which finally synergistically enhance its electrochemical performance.The specific capacitance of 3D-NMX at 1 A g^-1 is 442 F g^-1,which is about 1.6 times better than the performance of MXene free-standing electrode(Cs=278 F g^-1),and the specific capacitance retention at high current density of 10 A g^-1 is 61%(270 F g^-1).After 10,000 cycles at 10 A g^-1,the capacitance retention rate was 79.6%.In addition,the effect of electrode loading on the electrochemical performance was investigated.The specific capacitance at 1 A g^-1 was 442 F g^-1,355 F g^-1 and 296 F g^-1 in order when the loading of 3D-NMX was 1.77 mg/cm^-2,3.54 mg/cm^-2 and 5.31 mg/cm^-2.