【作者】 王昕；

【导师】 朱雪斌；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2020， 博士

【摘要】 二维(2D)过渡金属碳/氮/碳氮化物(MXenes)具有良好的亲水性、高的化学稳定性、层间距可调以及高的电子电导率等优点,使其在超级电容器领域具有广阔的应用前景。通常,未经优化的MXene属微米级颗粒,比表面积较小,虽然具有高的循环稳定性以及优异的倍率性能,但是比电容值较低。近期,利用高比电容纳米材料对微米级MXene进行复合,制备的异质结构,可以显著提高其比电容值,甚至还可以利用异质结构中的协同效应,达到“1+1>2”的效果。因此系统地开展MXene基异质结构的制备及其在超级电容器领域的应用研究,将具有重要的意义。本论文以MXene基异质结构为主要研究对象,对其制备方法进行了详细描述,对其物相组成、微观结构、超级电容器性能进行了系统表征与测试,并对其电化学储能机理进行了讨论与分析。论文的主要内容如下:1、通过在HF中刻蚀MAX相V4AlC3,成功制备了多层的V4C3 MXene。该材料作为超级电容器电极时,在1摩尔每升(M)H2SO4电解液中比电容值达到209 F g-1(扫描速率2 mV s-1),经过10000次充放电后,电容保持率为97.23%(10Ag-1)。V4C3MXene的高比电容不仅得益于其大的层间距(0.466nm)、大的比表面积(31.35 m2 g-1)和良好的亲水性,还得益于钒元素丰富的价态(+2,+3,+4)。此外,V4C3 MXene电极的高倍率性能和良好的循环稳定性主要归功于其高的电导率。2、制备层状双氢氧化物(Layered Double Hydroxides,简写为LDHs)与V4C3 MXene的异质结构,提高了 V4C3MXene的比容量。通过水热法成功制备了具有三维互连网络微结构的NiCoAl-LDH/V4C3 MXene异质结构,其作为超级电容器电极时,在1 M KOH电解液中表现出电池型电极的行为,比容量可达到627 C g-1(电流密度 1 A g-1),远高于 V4C3 MXene(152 C g-1)。但是,NiCoAl-LDH/V4C3 MXene异质结构属于典型的电池型电极,由于电池型电极本身的电化学储能机理限制,材料表现出较差的循环稳定性,3000圈后容量保持率仅为82.7%,远低于电容型的电极材料,其倍率性能与电容型电极相比也处于劣势。3、制备高比电容的电容型1T-MoS2电极,为构建电容型的1T-MoS2/MXene异质结构奠定基础。分别通过水热法、十六烷基三甲基溴化铵(cetyltrimethylammonium bromide,简称为CTAB)插层水热法以及磁水热法(H=9T)制备了多层的2H-MoS2、MoS2-CTAB和1T-MoS2纳米片,研究了它们的超级电容器性能,并揭示了电极的电化学存储机理。结果表明,层间距的增加有利于比电容的提升(20 F g-1增加到173 F g-1),亲水性和电导率的增强可以进一步提高比电容值(173 F g-1增加到320F g-1)。非原位XRD测试结果表明,1T-MoS2电极在充电时,其层间距在不断增大,说明多层纳米片作为超级电容器电极时可以提供插层型赝电容。1T-MoS2电极在1 M Li2SO4、H2SO4、Na2SO4电解液中,比电容值分别高达379 F g-1、358 F g-1与320 F g-1,并具有优异的循环稳定性,10000次循环后容量保持率接近100%。4、通过磁水热法构建了 1T-MoS2/Ti3C2 MXene异质结构,并研究了其超级电容器性能及电化学储能机理。该异质结构在作为超级电容器电极时,在1A g-1下比电容值达到386.7 Fg-1,特别是在50 A g-1的大电流密度下比电容仍高达207.3 F g-1,远高于 1T-MoS2(113.0Fg-1)与 Ti3C2MXene(14.9 F g-1),表现出优异的倍率性能。此外,在20000次充放电后电容保持率为96.8%,循环稳定性也十分出色。由于Ti3C2 MXene高导电性从而实现了该异质结构的快速充放电特性;由于1T-MoS2高的比电容从而实现了该异质结构的高比电容特性;更为重要的是,由于协同效应,该异质结构的比电容值达到“1+1>2”的效果。更多还原

【Abstract】 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.更多还原