Preparation And Electrochemical Properties Of Mxene-Based Electrode Materials

With the growing demand for wearable electronic devices and clean energy,the development of flexible supercapacitors has been promoted.Pseudocapacitive materials have been favored by researchers due to their higher theoretical specific capacitance compared to double-layer carbon materials.However,most current pseudocapacitive materials have poor electronic conductivity and cannot simultaneously meet the requirements of high energy density and high-power density.The key development direction of supercapacitors is to develop new electrode materials with high specific capacitance,wide working potential window,and high-rate performance and electrochemical cycling stability.MXenes,a new type of two-dimensional（2D）transition metal carbide and/or nitride,are considered to be promising pseudocapacitive flexible electrode materials due to their metallic conductivity（≥20,000 S cm-1）,surface redox active groups,good mechanical flexibility,and ultra-high volumetric capacitance(～1,500 F cm^-3).However,their mass specific capacitance is lower due to irreversible anodic oxidation,and the high stacking height of layers(-4 g cm^-3)and narrow pseudocapacitive potential window limit their large-scale application.Therefore,this paper focuses on the preparation of MXenes-based materials with high electrochemical stability,high specific capacitance,and high-rate performance.By combining the different redox potentials of different transition metals in MXenes and designing the atomic and microscale structure of MXenes,an MXenes-based electrode with enhanced anodic oxidation stability,shorter ion transport path,increased reactive sites,and widened pseudocapacitive potential window is constructed to achieve long cycling life,high energy density,and high-power density of flexible supercapacitors.Using the abundant surface functional groups of Ti₃C₂ MXene,effective hybridization at the molecular level was achieved by self-assembly with MnO₂ nanobelts with a larger work function,thus adjusting the work function of Ti₃C₂.Specifically,a MnO₂/Ti₃C₂ hybrid cathode material with a 1D-2D alternating stacking structure was prepared by mixing colloidal solutions of Ti₃C₂ nanosheets and MnO₂ nanobelts.The hybrid cathode exhibited higher stability than pure Ti₃C₂ during anodic oxidation and synergistic effects between the high specific capacitance of MnO₂ at positive potentials and the high conductivity of Ti₃C₂,providing a high specific capacitance of 315 F g^-1 at 10 mV s^-1 and good rate capability of 166 F g^-1 at 100 mV s^-1.Experimental characterization and firstprinciples calculations showed that the high stability of the MnO₂/Ti₃C₂ hybrid cathode was mainly due to the increased work function induced by charge transfer at the Ti₃C₂-MnO₂ heterojunction interface.Furthermore,a flexible quasi-solidstate asymmetric supercapacitor（ASC）was assembled using MnO₂/Ti₃C₂ as the cathode and alk-Ti₃C₂ after alkalization as the anode.The single ASC had a high voltage output of 1.9 V,contributing to a high energy density of 16.1 Wh kg^-1 and a high-power density of 3,538 W kg^-1.After 10,000 cycles,the initial capacitance retention of the ASC was 99.8%,demonstrating excellent electrochemical stability.A facile one-step chemical etching method was reported for the synthesis of highly crystalline TiVC MXene,a bimetallic solid solution,and the electrochemical properties of TiVC thin film electrodes in acidic and neutral electrolytes were explored.The TiVC film electrode exhibited a high specific capacitance of up to 405 F g^-1(1,558 F cm^-3)and excellent rate performance in H₂SO₄ electrolyte,which exceeded the specific capacitance of the monometallic V₂C electrode and Ti₃C₂ hydrogel electrode.Furthermore,it was found that like Ti₂C and V₂C,TiVC was not stable in cyclic voltammetry tests in H₂SO₄ electrolyte,and its capacitance decayed rapidly.In addition,the electrochemical properties of the TiVC film electrode in neutral LiCl electrolytes with different concentrations were investigated,and the TiVC electrode exhibited significantly different specific capacitances in electrolytes with different concentrations.The TiVC electrode delivered a high specific capacitance of up to 198 F g^-1(762 F cm^-3)in 5 M LiCl electrolyte with high H₂O molecule activity,and retained 88%of its initial capacitance after 10,000 cycles at 10 A g^-1,which was significantly better than the monometallic d-V₂C and d-Ti₂C MXene electrodes.In contrast,the TiVC electrode showed a limited specific capacitance(25 F g^-1)in 19.8 M LiCl,mainly due to the significantly reduced activity of H₂O molecules and the strong Coulombic interaction between Cl-and Li+hindering Li+insertion between the TiVC layers.The importance of the activity of H₂O molecules for charge storage of MXene electrodes in aqueous electrolytes was confirmed from the point of view of electrolyte.Considering that the electrochemical performance of MXenes can be affected by regulating their atomic structure,and that various single-metal MXenes have different redox potentials,a novel and highly stable bimetallic solid solution MXene Ti₂VC₂ was synthesized by a mild method,and its local structure,surface chemistry,electrical,and optical properties were systematically investigated.As a supercapacitor electrode material,the electrochemical performance of the prepared multilayer Ti₂VC₂ clay and Ti₂VC₂ film electrodes was tested in acidic electrolytes.These electrodes provided a specific capacitance of up to 2,000 F cm^-3 or 520 F g^-1(at 2 mV s^-1)and retained over 90%of the initial capacitance after 20,000 cycles at a current density of 5 A g^-1,surpassing the electrochemical performance of most advanced supercapacitor materials.In addition,the charge storage mechanism of Ti₂VC₂ was explored through in-situ and ex-situ characterization methods,confirming that its pseudocapacitive mechanism is a combination of hydrated H+intercalation and surface redox reactions.By utilizing the synergistic effect of the different redox potentials of titanium and vanadium atoms in Ti₂VC₂,nearly constant current response was achieved within a wide potential window（0.7 V）.Furthermore,to achieve high specific capacitance and high-rate performance of Ti₂VC₂ electrodes,Ti₂VC₂ hydrogels and 3D macroporous Ti₂VC₂ film electrodes were designed,respectively.The prepared Ti₂VC₂ hydrogel electrode showed excellent specific capacitance of up to 2,670 F cm^-3(at 2 mV s^-1),which was mainly attributed to the exposure of more active sites.The macroporous Ti₂VC₂ electrode with shorter ion transport channels exhibited excellent rate handling capability,with a specific capacitance of 478 F g^-1(0.478 F cm^-2)at a scanning rate of 5 mV s^-1 and 233 F g^-1(0.233 F cm^-2)at 1 V s^-1.This work provides a practical route toward future rational design on the atomic scale for high-performance pseudocapacitive materials.