【作者】 杨莉；

【导师】 孙正明；

【作者基本信息】 东南大学 ， 材料科学与工程， 2020， 博士

【摘要】 超级电容器因其功率密度高、循环寿命长以及安全性好等优点,在储能领域中占据不可替代的地位。然而,传统超级电容器面临着能量密度低、倍率性能差等问题,同时高性能电动汽车及电子设备等对超级电容器的循环稳定性提出更高要求。开发超级电容器新型电极材料是解决上述问题的关键之一。此外,电极材料在制备过程中导电剂和粘结剂的使用,进一步降低其能量密度。因此,构筑和优化无导电剂、无粘结剂的薄膜电极材料,实现超级电容器能量密度、倍率性能和循环性能的提升,逐渐成为当前研究的重点。二维材料因比表面积大、活性位点多、离子传输路径短、机械性能优良等特点,成为超级电容器领域的研究热点。MXene是一类新型二维过渡金属碳/氮化物,兼具亲水性和导电性,其层间距、成分与表面官能团可调,且易组装成自支撑薄膜直接作为电极使用,使得MXene在发现之初就在储能领域尤其是超级电容器领域展现出巨大潜力。然而,MXene的成分可调性还有待进一步挖掘;MXene表面难以避免的官能团,会一定程度上降低其电导率;MXene的二维结构易发生塌陷和堆垛,降低离子/电子迁移速率,影响电化学性能。因此,如何克服上述问题,加速MXene在超级电容器领域的发展,是当前亟需研究的课题。本文充分利用MXene独特的性能,以d-Ti₃C₂为研究对象,从原子尺度、微纳尺度和宏观尺度对其成分、结构以及材料体系进行多尺度优化设计,以提升MXene基薄膜电极的比容量、倍率性能和循环稳定性等电化学性能。在原子尺度,制备氮掺杂d-Ti₃C₂纳米片,研究不同氮含量对d-Ti₃C₂电化学性能的影响。引入氮能够增加d-Ti₃C₂层间距、提高其导电率和赝电容,从而改善其电化学性能;结合第一性原理计算,揭示掺杂氮的存在位置及其对电化学性能的影响机制。在微纳尺度,通过“原位氧化-刻蚀”,有效利用d-Ti₃C₂纳米片表面钛原子易氧化的特点,得到由面内多孔d-Ti₃C₂纳米片组装的自支撑薄膜,实现d-Ti₃C₂微观结构的调控。面内多孔结构可缩短离子传输路径、缓解d-Ti₃C₂纳米片堆垛、提高有效比表面积、增加活性位点,从而提升d-Ti₃C₂的比容量、倍率性能和循环稳定性。在宏观尺度,采用电泳沉积法和真空抽滤法,引入异质相CNTs（1D）和rGO（2D）,分别构筑d-Ti₃C₂/CNTs与氮掺杂d-Ti₃C₂/rGO复合薄膜电极材料,优化d-Ti₃C₂基电极材料体系。异质相的引入有效缓解d-Ti₃C₂纳米片堆垛,使复合薄膜能够发挥各组分之间的协同效应,从而获得优异的电化学性能。以此为基础,在掺杂的同时原位引入造孔剂（纳米TiO₂）,然后将其刻蚀,可从原子尺度和微纳尺度实现对d-Ti₃C₂纳米片化学成分和物理结构的调控。氮掺杂增强薄膜电极的导电性和提供更高的赝电容,面内多孔结构提供更多的有效比表面积和缩短离子的扩散路径,从而增强薄膜电极的电化学性能。最后,结合掺杂、多孔和异质相的引入,对d-Ti₃C₂基电极材料体系进行原子、微纳和宏观尺度的多尺度耦合设计,即通过聚多巴胺（PDA）包覆与碳化策略,制备多孔d-Ti₃C₂/氮掺杂碳自支撑薄膜。该薄膜在1 A g^-1条件下,比容量为488.1 F g^-1;即使在100 A g^-1超高电流密度下,其比容量仍高达411.2F g^-1,容量保持率达到84.2%;在10 A g^-1下循环10000次,容量保持率为95.4%。其优异电化学性能的主要原因在于成分、结构和材料体系设计的多尺度协同效应,包括氮掺杂增强材料的导电性和赝电容,多孔结构增加有效比表面积、缩短离子扩散路径以及第二相缓解d-Ti₃C₂纳米片塌陷和堆垛,提高结构稳定性。这种多尺度的耦合设计,为其他二维材料复合薄膜的制备提供了借鉴。综上,本文从化学成分、微观结构和材料体系等层面上对MXene进行调控,构筑了MXene基薄膜电极材料,并系统研究了其在超级电容器方面的性能,实现了比容量、倍率性能和循环稳定性等方面的提升。研究结果有助于理解MXene的化学成分和微观结构对其电化学性能的影响机制,充分发挥MXene材料在超级电容器领域的优势,加速MXene的实用化进程,推动新能源技术的发展。更多还原

【Abstract】 Supercapacitors have an irreplaceable position in the field of energy storage due to their high power density,long cycle life and good safety.However,traditional supercapacitors have a low energy density and inferior rate performance,and meanwhile,the cycle stability need to be improved because of the demand for high-performance electric vehicles/electronic equipment.Developing new electrode materials for supercapacitors is one of the keys to solving the above problems and improving the performance of supercapacitors.In addition,the use of conductive agents and binders in the electrode materials further reduces the energy density of supercapacitors.Therefore,it is the heart of current research to construct and optimize film electrode materials without conductive agents and binders to improve the energy density,rate performance and cycle stability of supercapacitors.Owing to the large specific surface area,multiple active sites,short ion transport path and excellent mechanical properties,two-dimensional materials have become a research hotspot in the field of supercapacitors.MXenes are a new type of two-dimensional transition metal carbides/nitrides with combination of hydrophilicity and conductivity.Their layer spacing,composition and surface functional groups are tunable.Furthermore,they can be easily assembled into free-standing films and directly used as electrodes without any conductive agents and binders.All these characteristics make MXenes great potential in the field of energy storage especially supercapacitors since their discovery.However,the composition tunability of MXenes needs to be further explored;the unavoidable functional groups on the surface of MXenes reduce their conductivity;the two-dimensional structure of MXenes is prone to collapse and re-stacking,slowing ion/electron migration rate and affecting the electrochemical performance.Therefore,overcoming the above problems and accelerating the development of MXenes in supercapacitors are an urgent research topic.Making full use of MXenes’unique characteristics combination,the composition,structure and composite of MXene based film electrodes（taking d-Ti₃C₂ as a typical representative of MXene family）were optimized from the atomic/micro-nano/macroscopic scale to improve the electrochemical properties,such as specific capacitance,rate performance and cycle stability.At the atomic scale,nitrogen-doped d-Ti₃C₂ nanosheets were successfully prepared by hydrothermal method,and the effects of different nitrogen contents on the electrochemical performance of d-Ti₃C₂ were investigated.The introduction of N increases the spacing of d-Ti₃C₂ layer,the electrical conductivity and the psudocapacitance.Combining first principles calculation,the position of doped N and its influence mechanism on electrochemical performance of N-doped d-Ti₃C₂ nanosheets were revealed.At the micro-nano scale,taking advantage of the susceptibility of titanium atoms on the surface of d-Ti₃C₂ nanosheets,the in-plane porous structure of d-Ti₃C₂ nanosheets was constructed by in situ oxidation-etching process,realizing the optimization of microstructure.The in-plane porous structure shortens the ion transport path,mitigates d-Ti₃C₂ nanosheets re-stacking,and increases the effective specific surface area and the active sites,so as to improve its specific capacitance,rate performance and cycle stability.At the macroscopic scale,CNTs（1D）and rGO（2D）heterogeneous phases were introduced to construct d-Ti₃C₂/CNTs and nitrogen-doped d-Ti₃C₂/rGO films by electrophoretic deposition and vacuum filtration,respectively,optimizing the d-Ti₃C₂-based film electrodes.The introduction of the heterogeneous phases effectively alleviates the re-stacking of d-Ti₃C₂ nanosheets,exerts synergetic effects between the components,thereby obtaining excellent electrochemical performance.Subsequently,the pore forming agents（nano-TiO₂）were in situ introduced during the nitrogen doping process,and then etched off to realize the regulation of chemical composition and physical structure of d-Ti₃C₂ nanosheets from the atomic and micro-nano scales.The modification of composition and structure of d-Ti₃C₂ enhances the conductivity and psedocapacitance,provides more effective specific surface area and shortens diffusion path of ions,thereby improving the electrochemical performance.Finally,combining the introduction of doping,porous structure and heterogeneous phases,d-Ti₃C₂ based electrode was conducted by the multi-scale regulation to improve its electrochemical performance.Via the polydopamine（PDA）coating and carbonization strategy,we prepared a porous d-Ti₃C₂/nitrogen-doped carbon free-standing film with a specific capacitance of 488.1 F g^-1 at1 A g^-1;even at ultra-high current density(100 A g^-1),its specific capacitance still can reach as high as 411.2 F g^-1,with the capacitance retention rate of 84.2%;After 10,000 cycles at 10 A g^-1,the capacitance retention rate is 95.4%,showing good cycling performance.The excellent electrochemical performance is ascribed to the synergetic effect from composition,structure and material designing,including the enhanced conductivity and psedocapacitance by the nitrogen-doping,the more effective specific surface area and shorter the ion diffusion path from the porous structure and the alleviation of the collapse and re-stacking of d-Ti₃C₂nanosheets from the heterogeneous phase introduction.This multi-scale design provides a potential for the film preparation of other two-dimensional materials.In summary,MXenes were tuned from the chemical composition,microstructure and material design,to construct MXene based film electrodes,achieving the improvements of specific capacitance,rate performance and cycle stability.The results are beneficial to understand the effects of MXenes’chemical composition and microstructure on their electrochemical performance,give full play to the advantages of MXenes in supercapacitors,accelerate the practical application of MXenes,and promote the development of new energy storage technologies.更多还原