Synthesis And Electrochemical Properties Of MXene-based Supercapacitor Electrode Materials

Two-dimensional(2D)transition metal carbides,nitrides and carbonitrides(MXenes)have great application prospects in the field of supercapacitors due to its good hydrophilic surfaces,high chemical stability,and tunable interlayer as well as high electrical conductivity.However,the MXene without optimized is a micron sized particle,resulting in a small specific surface area.Although it exhibits excellent cycling performance and superior rate performance,however,the low specific capacitance limits the applications.Recently,the heterostructure with 3D interconnected network microstructures assembled by nano-materials with high specific capacitance and micron-scaled MXene can improve its specific capacitance,and even can make use of the synergetic effect in heterostructure to achieve the effect of "1+1>2".Therefore,it is a great significance to systematically study the preparation of MXene-based heterostructure and the application in the field of supercapacitor.We take MXene-based heterostructure as the main research object,describes its preparation in detail,systematically test its phase composition,microstructure and supercapacitor performance,and analyze the electrochemical energy storage mechanism.Firstly,2D multi-layered V4C3 MXene has been synthesized by selectively etching Al from V4AlC3 and it shows a high capacitance of 209 F g-1 at 2 mV s-1,and excellent cyclic performance with capacitance retention rate of 97.23%after 10000 cycles in 1 M H2SO4 electrolyte(10 A g-1).The high specific capacitance of V4C3 MXene is not only due to their wide interlayer spacing(0.466 nm),large specific surface areas(31.35 m2 g-1)and pore volumes(0.047 cm3 g-1),and good hydrophilicity but also attributed to the abundant valence states of vanadium+2,+3,+4).The high rate performance and excellent cycling stability of V4C3 MXene electrode are mainly attributed to the high electronic conductivity.Secondly,for improve the specific capacitance of V4C3 MXene,we fabricate the NiCoAl-LDH/V4C3 MXene heterostructure.NiCoAl-LDH nanosheets uniformly grow onto micron-scaled MXene sheets to form a NiCoAl-LDH/V4C3 MXene heterostructure with three-dimensional interconnected porous network microstructures by a hydrothermal method.NiCoAl-LDH/V4C3 MXene heterostructure electrodes show an excellent specific capacity of 627 C g-1 at 1 A g-1 in 1 M KOH,much higher than that of V4C3 MXene(152 C g-1).However,the NiCoAl-LDH/V4C3 MXene heterostructure is a typical battery-type electrode.Due to the limitation of the electrochemical energy storage mechanism of the battery-type electrode,it exhibits poor cycling stability.The capacity retention after 3000 cycles is only 82.7%,which is much lower than that of the capacitive electrode materials;its rate performance is also at a disadvantage compared to the capacitive electrode.Thirdly,the capacitive 1T-MoS2 electrode with high specific capacitance was fabricate,which laid the foundation for the construction of capacitive 1T-MoS2/MXene heterostructure.We report the supercapacitor performance and intrinsic electrochemical storage mechanisms in three types of multilayered MoS2 nanosheets including 2H-MoS2,MoS2-CTAB and highly ambient-stable pure 1T-MoS2 nanosheets by hydrothermal synthesis,CTAB-intercalated hydrothermal synthesis and magneto-hydrothermal synthesis,respectively.We reveal that layer spacing enhancement leads to obvious improvement in specific capacitance(20 F g-1 increased to 173 F g-1)and the enhanced hydrophilicity as well as metallic characteristic can further improve the specific capacitance(173 F g-1 increased to 320 F g-1).Furthermore,Ex-situ XRD tests show the expansion of interlayer spacing in charging of 1T-MoS2 electrodes,suggesting the advantages of multilayered nanosheets used as electrodes for supercapacitors,due to the enhanced capacitance from intercalation.1T-MoS2 electrode respectively delivers a high specific capacitance of 379 F g-1,358 F g-1 and 320 F g-1 at 1 A g-1 in 1 M Li2SO4,1 M H2SO4 and 1 M Na2SO4 electrolytes,showing very stable cycling ability(capacitance retention is close to 100%after 10 000 cycles).Fourthly,1T-MoS2/Ti3C2 MXene heterostructure with 3D interconnected networks was constructed by magneto-hydrothermal synthesis,and the supercapacitor performance and electrochemical storage mechanisms are investigated.1T-MoS2/Ti3C2 MXene heterostructure as supercapacotor electrode shows a high specific capacitance of 386.7 F g-1at 1 A g-1,even at 50 A g-1,the specific capacitance still reaches 207.3 F g-1,which is much higher than that of 1T-MoS2(113.0 F g-1)and Ti3C2 MXene(14.9 F g-1)at 50 A g-1,showing excellent rate performance.In addition,the outstanding capacitance retention remain 96.8%for the 1T-MoS2/Ti3C2 MXene electrode after 20000 cycles.Due to the high conductivity of Ti3C2 MXene,the rapid charge and discharge characteristics of the heterostructure are realized;Due to the high specific capacitance of 1T-MoS2,the high specific capacitance characteristics of the heterostructure are realized;More importantly,the specific capacitance of heterostructure achieves the effect of "1+1>2" due to the synergistic effect.