| The increasing consumption of fossil fuels and the deteriorating environment have strongly promoted renewable energy such as solar and wind energy.However,renewable energy is volatile and intermittent and requires efficient energy conversion/storage devices to ensure the power network's stable and secure operation.As one kind of advanced energy storage technology,supercapacitors have received more attention in various energy storage equipment fields.However,the limited energy density is still the primary reason restricting their wide application.Therefore,exploring new electrode materials with high energy and power density has become the goal of scientific research in this field.MXene materials have become very competitive due to their high energy and power density.Since the interlayers provide sufficient spaces for the insertion and extraction of ions,they have good electrochemical performance,exceptionally high redox activity,and fast storage of alkali metal ions.Ti3C2Tx is a typical representative of MXene materials with high chemical or electrochemical stability and is the first and most systematically studied MXene material.However,its low capacity,accompanied by slow electrochemical kinetics and interlayer self-stacking,collapse,and oxidation,seriously hinder their practical application in supercapacitors.Because of the problems mentioned above of Ti3C2Tx as a supercapacitor electrode material,in this study,a polydopamine(PDA)coating was coated on the HF-etched multilayer Ti3C2Tx layers surface,which would effectively prevent Ti3C2Tx from being easily oxidized during a simple liquid phase compound reaction in an aqueous solution.Based on this,to further improve the self-stacking problem of Ti3C2Tx,carbon nanotubes(CNTs)and manganese dioxide(MnO2)are in-situ grown on the surface of Ti3C2Tx layers utilizing the excellent secondary reaction platform of PDA coating.Then,polymethyl methacrylate(PMMA)was used as a binder to prepare efficiently dispersed Ti3C2Tx-CNTs slurries and achieve a high-quality load of 40 mg cm-2 using the dip-coating method,which guides the further practical application of Ti3C2Tx-based supercapacitors.The specific research contents are as follows:(1)The in-situ polymerized PDA was used as the coating to realize the protection of HF-etched multilayer Ti3C2Tx.The effects of different additive proportions of dopamine hydrochloride in the reaction process on the microscopic morphology,composition,and electrochemical performance of PDA-coated Ti3C2Tx were compared and analyzed,successfully controlling and optimizing the effective protection of Ti3C2Tx using the PDA coating.When the mass concentration ratio of dopamine hydrochloride to Ti3C2Tx is 1:2,the oxidation of Ti3C2Tx in the water environment can be effectively avoided;the catechol in the PDA structure can bind Ti atoms on the Ti3C2Tx surface through strong coordination bonds,enhancing the matrix's structural stability.Besides,this Ti3C2Tx@PDA composite can continue to be used as a matrix material,using the PDA coating's secondary reaction platform on its surface for further structural optimization design to construct supercapacitor electrode materials with more excellent electrochemical performances.(2)The in-situ growth of CNTs on the surface of Ti3C2Tx layers can be realized by a simple pyrolysis method,using the optimized Ti3C2Tx@PDA as the matrix material,Co nanoparticles reduced by the PDA coating as the catalyst,and urea as carbon sources.The effect of different urea addition ratios in the precursor on the composition,morphology,structure,and supercapacitor property of Ti3C2Tx@CNTs composites were compared and analyzed,and the controllable growth of CNTs on the surface of Ti3C2Tx layers can be realized.The research results show that the vertically grown CNTs can effectively expand the interlayer spaces while overcoming the self-stacking problem of Ti3C2Tx layers,exposing more redox reaction active sites,and providing a fast channel for the transport of electrolyte ions.The highest area energy density of the symmetric device assembled based on the optimized Ti3C2Tx@CNTs-6.0 composite electrode is 9.4?Wh cm-2,and the highest area power density is 5444.0 ?W cm-2.This strategy of in-situ growing CNTs on Ti3C2Tx to construct a three-dimensional network structure gives a significant reference for other MXene materials to improve the self-stacking problems.(3)Ti3C2Tx@MnO2 composites modified by MnO2 with various morphologies were prepared via the hydrothermal method and the liquid phase co-precipitation method,using Ti3C2Tx@PDA as the matrix material and KMnO4 as manganese sources.The possible growth and morphological transformation mechanism of MnO2 on Ti3C2Tx@PDA surface were discussed by elemental composition,microscopic morphology,phase structure analyzing,and the electrochemical performance of Ti3C2Tx@MnO2 composites was also studied.PDA can directly reduce[MnO4]-to MnO2 and fix it uniformly on the substrate's surface.It naturally grew into ?-MnO2 nanosheets in a mild environment;it grew into ?-MnO2 nanowires under the guidance of polyethylene glycol(PEG)surfactant;with the aid of cetyltrimethylammonium bromide(CTAB),it grew into?-MnO2 nanoflowers along the periphery of CTAB micelles;also,it grew into rice-like ?-MnO2 nanorods under high temperature and pressure of hydrothermal reaction.The loading of MnO2 with different morphologies would introduce pseudocapacitances;simultaneously,the self-stacking of Ti3C2Tx layers would be improved.The effects of MnO2 morphologies on the electrochemical performance of Ti3C2Tx@MnO2 composites were systematically studied.By comprehensive comparison,Ti3C2Tx@?-MnO2 NSs and Ti3C2Tx@a-MnO2 NWs have lower charge transfer resistances,higher cycle stability,and better electrochemical performances,which can effectively improve the specific capacitance of Ti3C2Tx in positive potential ranges and broaden its application as a positive electrode material for energy storage.(4)Based on the excellent electrochemical performance of Ti3C2Tx@?-MnO2 composites with MnO2 nanosheets,the growth and actual loadings of MnO2 on Ti3C2Tx layers surface can be optimized,thus the practical design of the structure of Ti3C2Tx@?-MnO2 composite materials can be realized.The influence of the introduction of MnO2 nanosheets on the phase structure,electrochemical activity,and charge storage mechanism of Ti3C2Tx was intensely discussed.The kinetic process was also analyzed to explore the energy storage mechanism of composite electrodes deeply.The results show that the optimized Ti3C2Tx@?-MnO2-0.06 electrode has good cycle stability,excellent area specific capacitance,and its capacitance is composed of diffusion control contribution and non-diffusion control contribution.The Ti3C2Tx@?-MnO2-0.06//AC asymmetric supercapacitor device assembled based on this electrode has a maximum area energy density of 110.4 ?Wh cm-2 in the operating voltage range of 0-1.8 V,and the highest area power density is 9493 ?W cm-2.It also can provide energy for commercial LEDs,indicating that Ti3C2Tx@?-MnO2-0.06 composite has enormous potential for energy storage applications.This strategy achieves the modification of Ti3C2Tx layers using MnO2 nanosheets.Such a simple method can be extended to other MXene members for energy storage research and provides ideas for the effective structural design of other MXene materials.(5)A simple dip-coating process was developed to design and prepare Ti3C2Tx-based electrodes with a high active mass load(40 mg cm-2),using PMMA as the binder,isopropanol-water(4:1)as the solvent,and CNTs as conductive additive.The effects of the contents of CNTs on the electrochemical performance of Ti3C2Tx-CNTs composite electrodes were studied,and various current collectors' influences on the active material's mass loadings and electrochemical performances were also explored.The results show that the optimized Ti3C2Tx-CNTs-20 electrode using nickel foam as the current collector can achieve higher active mass load and better electrochemical performance.The Ti3C2Tx-CNTs-20//MnO2-CNTs-20 asymmetric device assembled based on this electrode can achieve a maximum area energy density of 440.3 ?Wh cm-2 with a maximum area power density of 55652 ?W cm-2,and can provide continuous power for the LED group.This simple dip-coating method is universal for other MXene-based electrodes to achieve high active mass loadings. |
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