【作者】 刘莹；

【导师】 谢二庆；

【作者基本信息】 兰州大学 ， 凝聚态物理， 2018， 博士

【摘要】 超级电容器作为重要的储能器件,具有功率密度大、充放电速度快、循环稳定等优势,在很多领域（如军事、混合动力汽车、电子移动设备等）有广阔的应用前景。如何在不降低功率密度和循环稳定性前提下提高超级电容器能量密度和倍率性能是其面临的主要挑战。本论文从提高电极材料导电性能出发,采用静电纺丝技术制备了纳米碳纤维,重点研究了不同前驱体制备多孔纳米碳纤维及对其电化学性能的影响。碳纤维不仅作为支架负载活性材料,还作为良好的导电通道增强电子在复合材料中的传输。这种一维结构也便于活性物质和电解液离子充分反应,从而提高电荷存储能力。具体研究内容如下:细菌纤维素具有超大的长径比可以得到高比表面积的纳米碳纤维,而它丰富的表面官能团,可以吸附大分子撑开致密的纤维,再利用冻干法保持纤维素的疏松的状态,最后经过碳化得到直径20-30 nm的超细纳米碳纤维。实验通过吸附不同分子量大小的有机物调节碳纤维比表面积,最大可达589.2 m² g^-1。电化学测试结果显示其比电容高达509 F g^-1(0.5 A g^-1),对称器件的能量密度可以达到7.7 Wh kg^-1。和普通碳纤维相比这种超细碳纤维比表面积增大,能量密度显著提高。但是纤维直径变细不仅导电性降低,影响了材料的倍率性能;还影响了其对活性材料的负载。为制备可控的纳米纤维,将聚丙烯腈（PAN）作为前驱体,利用静电纺丝法制备了直径大小可控的纳米碳纤维。为提高碳纤维的比表面积和导电性,在纺丝溶液中加入硝酸钴,既作为造孔的模板还能在碳化过程起到催化非晶碳转化成石墨碳的作用。实验中调控碳化温度来调节模板纳米颗粒大小,在碳化温度800 ^oC的样品比表面积最大468.9 m² g^-1,比电容可达104.5 F g^-1,循环稳定性良好（2000次充放电后比电容仅损失6%）;还具有良好的柔韧性,在500次弯折后比电容保持89.4%。这种导电性能良好的多孔纳米碳纤维可用作良好的支撑骨架负载活性材料。实验设计镶嵌式的一维混合结构,将活性物质纳米颗粒嵌入多孔纳米碳纤维。在前面工作的基础上将静电纺丝溶液中的硝酸钴和聚丙烯腈转化成Co₃O₄活性物质和纳米碳纤维,再通过水热处理将复合材料纤维中的Co₃O₄转化为硫化钴。在实验中调节硝酸钴含量得到各项性能最佳的样品CoS_x/C-0.4。对比Co₃O₄/C的化学性能发现CoS_x/C比电容更大(0.5 A g^-1电流密度下全电极比电容496.8 F g^-1)。和其他金属硫化物相比,CoS_x/C表现出良好的循环稳定性（超2000次循环充放电比电容保持89%）和高倍率性能(电流密度100 A g^-1时比电容保持66.1%)。这种碳包覆型的一维结构起到了提高导电性、防止活性物质团聚、提高活性物质结构稳定性的作用。但是实验中对复合材料的热处理破坏了材料的机械性能。为保持多孔碳纤维的柔韧性,实验调整了一维材料结构,利用电沉积法在碳纤维表面均匀垂直生长一层Co-Ni氢氧化物纳米片,活性物质和碳纤维形成核壳结构。这些薄层纳米片和碳纤维为离子和电子提供了开放和连续的通道,有利于电解液快速扩散与活性物质充分反应。调节电沉积时间得到适量的活性物质生长密度,Co-Ni氢氧化物/多孔纳米碳纤维表现出高比电容1503.5 F g^-1(5 A g^-1)和稳定的循环特性（2000次充放电后保留77.5%的电容）。工作还对比普通碳纤维和多孔纳米碳纤维负载的Co-Ni氢氧化物活性物质电容特性,证明多孔碳纤维在复合电极材料中是非常理想的导电骨架。更多还原

【Abstract】 Supercapacitors as an important energy storage device with high power density,rapid charge and discharge capacity,long cycle stability have broad application prospects in many fields（such as military,hybrid electronic,mobile devices,etc）.The main challenges of supercapacitor are their relatively low energy density and rate performance.Therefore,the main research of this paper is to improve the conductivity of electrode materials.The electrospinning technology is used to prepare carbon nanofibers,and the electrochemical performance of porous carbon nanofibers prepared with different precursor is studied.Here,carbon nanofibers not only act as physical support to load the active materials,but also as a good conductive channel to facilitate the electron’s transportation in the composite materials.At the same time this kind of one dimensional structure also facilitate the contact between the active materials and the electrolyte ions,thus stored energy through the chemical reaction.The concrete research content is as follows:Bacterial cellulose（BC）is selected as precursor of carbon nanofibers,because it has a large aspect ratio to get high specific surface area（SSA）carbon nanofibers.The abundant surface functional groups（hydroxy/carboxylic groups）of BC can serves as an excellent adsorbent materials,the absorbed OPPs molecules are large enough to create loose 3D networks after freezing dry,thus,prevent the raw BC nanofibers from aggregation/conglutination during high-temperature pyrolysis,finally obtained the ultrafine carbon nanofibers with a diameter of20-30 nm.The SSA of carbon nanofibers is controlled by absorbing organic macromolecules with different molecular weight,the best sample present a high SSA(589.2 m² g^-1)and exhibit great potential as electrode materials for supercapacitors.Therefore,in three-electrode system,the sample exhibit a high specific capacitance(509 F g^-1 at 0.5 A g^-1)when compared with the carbonized pure BC(132 F g^-1 at 0.5 A g^-1).Moreover,the assembled symmetric supercapacitors is free standing and delivers an energy density of 7.7 W h kg^-1 at the power density of 325 W kg^-1.However,the small diameter of the fibers is not only reduced the conductivity,thus,decrease the rate performance of the electrode materials,but also affects the mass loading of active materials.In order to obtain controllable morphology of carbon nanofibers,we use polyacrylonitrile（PAN）as a precursor of carbon nanofibers,and then the porous carbon nanofibers（P-CNFs）were fabricated by electrospining technique combining with metal ion-assistant acid corrosion process.The resultant fibers display improved SSA and high conductivity.The cobalt nitrate hexahydrate added in the precursor solution was a key material for the formation of the unique hierarchical pore structure which acted as the porogen and catalyst.In the experiment,the template particle size is controlled by temperature,among them;the sample carbonized at 800 ^oC displayed the largest SSA(468.9 m² g^-1)and the best electrochemical property.In the three electrodes testing system,the resultant P-CNFs electrodes can exhibit a specific capacitance of 104.5 F g^-1(0.2 A g^-1),and capacitance retention of⁹4%after 2000 cycles.Furthermore,the P-CNFs also present excellent flexibility（the capacitance retaind 89.4%after 500 bending cycles）.It seems that this porous carbon nanofibers with good conductivity can used as conductive frame to support active materials.Subsequently,we designed a kind of one-dimensional（1D）hybrid structure with active materials particles embedded in porous carbon nanofibers.Based on previous work,cobalt nitrate and polyacrylonitrile were converted into Co₃O₄ and carbon,the Co₃O₄ in the composite fiber was then converted to cobalt sulfide by hydrothermal.In the experiment,the content of cobalt nitrate was optimized to obtain the best samples CoS_x/C-0.4.As a comparison,the specific capacitance of CoS_x/C nanofibers and Co₃O₄/C nanofibers were tested,at a given current density of 0.5 A g^-1,the specific capacitance of the CoS_x/C nanofibers was 496.8 F g^-1,which is 1.9 times higher than that of the Co₃O₄/C nanofibers(258.8 F g^-1).The CoS_x/C-0.4 nanofibers electrode are also compared to other cobalt sulfide based supercapacitors reported previously which showed superior rate capability of 87.0%when the current density increased from 0.5 to 10 A g^-1,and good cycling stability with over 89.0%specific capacitance remained after 2000 cycles.From these investigations,it is recognized that the carbon coating structure of the composite really enhanced their electrical conductivities,prevented reuniting of active substances and improved the structural stabilities.However,the mechanical properties of the composites were destroyed by the heat treatment.To this end,we adjusted the experimental process and used electrodeposition method to vertically grow a layer of Co-Ni hydroxide nanosheets on the surface of porous carbon nanofibers.These nanosheets and porous carbon nanofibers provide an open and continuous channel for electrons and ions to facilitate the rapid diffusion of electrolyte into the active substance.In this process,the active material growth density was adjusted by the electrodeposition time,so that the Co-Ni hydroxide/porous carbon nanofibers showed a high specific capacitance(1503.5 F g^-1 at 0.5A g^-1)and good cycling stability with over 77.5%specific capacitance remained after 2000 cycles.This work also compares the active material of the Co-Ni hydroxide with ordinary carbon nanofibers and porous carbon nanofibers,which proves that porous carbon fiber is an ideal conductive skeleton.更多还原