【作者】 张鹏；

【导师】 谢二庆； 潘孝军；

【作者基本信息】 兰州大学 ， 物理学·凝聚态物理， 2017， 博士

【摘要】 超级电容器（又称电化学电容器）作为一种新型绿色的储能器件,因其具有充放电速度快、循环寿命长、功率密度高、安全可靠等优点而备受研究者的广泛关注。作为超级电容器的核心部件,电极对超级电容器的能量存储性能有着关键性的影响,因此提升电极材料的电化学性能已成为目前超级电容器领域的研究热点。其中,过渡金属化合物由于其较高的比容量和良好的电化学反应活性,是非常具有发展潜力和研究价值的电极材料。高性能的超级电容器电极不仅要求材料本身具有优异的电化学活性,还应满足电荷在储能过程中可以实现快速的转移和传输。而传统的超级电容器电极的制备涉及到导电剂、粘结剂等额外添加剂的引入,导致制备工艺复杂,同时其电极结构的无序性大大限制了储能过程中电荷在电极内部的传输效率。针对以上问题,本论文以钴镍基金属氧化物为研究对象,将电极材料直接生长在电流收集极上,制备了多种单元和二元钴镍基金属氧化物,形成多孔、有序且自支撑的阵列结构,并系统地研究了不同制备参数对钴镍基氧化物本身性质及结构的影响,进而如何影响材料的电化学性能。研究内容主要分为以下几个方面:（1）首先我们对单元金属氧化物Co₃O₄的制备和电化学性能进行了系统性研究,利用水热法在泡沫镍基底表面生长Co₃O₄纳米线,通过改变前驱物浓度、反应温度、退火温度等实验参数来实现电极材料结构的优化,并得到了排列有序性良好且孔隙率较高的Co₃O₄纳米线阵列结构。结果表明,前驱物浓度对纳米线阵列的疏密有重要影响,进而影响电极内部的电荷传输效率,浓度过高会导致纳米线出现严重的团聚和粘连现象,而浓度过低时纳米线无法实现良好的自支撑;反应温度对材料的晶体结构有重要影响,在90°C得到的产物为CoO,随着反应温度的升高电极材料转变为Co₃O₄;退火温度对电极材料的结晶性和形貌影响较大,随着退火温度的升高,材料结构由纳米线向纳米片转化。通过优化条件,Co₃O₄纳米线阵列电极在1 A/g的电流密度下获得了842 F/g的质量比电容,充放电速率提高15倍后电容保持率为70.3%,经过5000次循环测试后电容保持率为81.4%。（2）由单元金属氧化物Co₃O₄进一步拓展到二元金属氧化物,制备出了CoMoO4纳米片阵列,NiCo₂O₄纳米线和纳米片阵列,NiMoO4超薄纳米片结构。通过调节前驱物浓度、反应温度和表面活性剂等实验参数,对CoMoO4纳米片阵列的形貌结构及电化学性能进行优化,在1 A/g电流密度下获得了912 F/g的质量比电容,当充放电速率提高15倍后电容保持率为74.3%,表现出优于Co₃O₄电极的倍率特性。对于NiCo₂O₄,我们通过改变表面活性剂制备出了纳米线和纳米片两种形式的阵列结构,并研究两种不同电极结构内部电荷传输的差异。测试结果表明,NiCo₂O₄ NWAs和NiCo₂O₄ NSAs电极均表现出优异的电化学性能,在2 mA/cm2电流密度下面积比电容分别达到了1.62和1.47 F/cm2（对应的质量比电容分别为898和923 F/g）,充放电速率提高15倍时电容保持率分别为76.8%和85.7%,经过5000次循环测试后电容保持率分别为83.4%和88.5%。NiCo₂O₄NSAs电极结构中片与片之间形成的三维交联导电网络,可以为电子提供更多的传输路径,同时片与片之间的相互支撑提高了电极的结构稳定性,从而使其展现出更优异的倍率和循环性能。然而对于NiMoO4超薄纳米片结构,虽然相互连接形成了多孔结构,但是由于其有序性较差,未能表现出较好的倍率性能,在充放电速率提高15倍的条件下电容保持率仅为60.7%。（3）对电极结构内部空间的高效利用是提升电极材料电化学性能的有效途径。针对前面NiMoO4电极材料遇到的问题,我们对电极结构进一步优化,将NiMoO4作为次级结构修饰在原有纳米阵列的表面,制备了多种核壳异质结构:Co₃O₄-NWA@NiMoO4、Ni Co₂O₄-NWA@NiMoO4、NiCo₂O₄-NSA@NiMoO4,其中原本的纳米阵列支架可以作为电子转移的“高速公路”。这种设计不仅大大提高了电极上活性物质的空间利用效率,而且使NiMoO4纳米片的超薄特性得到发挥。结果表明,核壳异质结构使电极材料的电化学性能均大幅提升,尤其是NiCo₂O₄-NSA@NiMoO4电极,在2 mA/cm2电流密度下质量比电容和面积比电容分别达到了1941 F/g和7.29 F/cm2,当充放电速率增大30倍时电容仍能保持84.1%,表现出非常优异的倍率特性,所组装的非对称电容器能量密度达到了52.6 Wh/kg。这些结果为探究钴镍基高性能电极材料提供了一定的参考。更多还原

【Abstract】 As a new type of green energy storage devices,supercapacitors（also known as electrochemical capacitors）,have attracted much attention because of their advantages such as fast charging and discharging,long cycle life,high power density and high reliability.The electrode material has a crucial influence on the energy storage performance of supercapacitors.Therefore,the research on improving electrochemical performances of the electrode materials is the key point in the field of supercapacitors.Because of the high specific capacities and good electrochemical activities,transition metal compounds have great potential for development and research.Both the electrochemical activities of materials and charge transport channels in electrodes are very crucial to achieving high performance supercapacitor.For conventional electrodes,the addition of polymeric binder and conductive agent can result in complex manufacturing process,and the disorder of the electrode structure greatly limits the charge transport efficiency in the energy storage process.In view of the above problems,in this paper,we take cobalt nickel based metal oxides as the research objects,and a variety of cobalt nickel based metal oxides（including binary metal oxides）were grown directly on current collectors to achieve highly porous and self-supporting array structures.We systematically studied the effects of different preparation parameters on cobalt nickel oxide properties and structure,and thus affect the electrochemical properties of the electrodes.And the main contents are as follows:The preparation and electrochemical properties of Co₃O₄ were systematically studied,and Co₃O₄ nanowires were synthesized on Ni foam by a hydrothermal method.By changing the experimental parameters such as precursor concentration,reaction temperature and annealing temperature,the structures of electrode materials were optimized,and Co₃O₄ nanowire arrays with good arrangement and high porosity were obtained.The results show that the concentration of precursor has an important effect on the density of the nanowire arrays,and thus affects the charge transfer efficiency in electrode.High concentration can lead to serious agglomeration and adhesion phenomenon of nanowires,but when the concentration is too low,nanowires cannot achieve good self-support.The reaction temperature has an important effect on the crystal structure of the material.The product obtained at the reaction temperature of 90 °C is CoO,and with the increase of the reaction temperature,the electrode material changes to Co₃O₄.The annealing temperature has a great influence on the crystallinities and morphologies of the electrode materials.With the increase of the annealing temperature,the structure of the electrode material changes from the nanowire to nanosheet.The obtained Co₃O₄ nanowire arrays can deliver a specific capacitance of 842 F/g at a current density of 1 A/g,and the capacitance retention is about 70.3% when the current density increases 15 times,implying good rate capability.Moreover,the capacitance retention is about 81.4% after 5000 cycling tests.Binary metal oxides nanomaterials were also prepared as electrode materials,including CoMoO4 nanosheet arrays,NiCo₂O₄ nanowire and nanosheet arrays,and ultrathin NiMoO4 nanosheets.The morphologies and electrochemical properties of CoMoO4 nanosheet arrays were optimized by adjusting the concentration of precursor,reaction temperature and surfactant.The obtained electrode can deliver a specific capacitance of 912 F/g at a current density of 1 A/g,and a capacitance retention of 74.3% at 15 A/g,which is higher than Co₃O₄ electrode.For NiCo₂O₄ materials,two types of nanoarrays（nanowire arrays and nanosheet arrays）were prepared by changing the surfactant,and the charge transfer difference between the two electrodes was investigated.Results show that both NiCo₂O₄ NWAs and NiCo₂O₄ NSAs electrodes exhibit good electrochemical properties,and obtained high specific capacitances of 898 and 923 F/g,and areal capacitances of 1.62 and 1.47 F/cm2 at 2 mA/cm2,respectively.The two electrodes also show good rate capabilities of 76.8% and 85.7% at 15 A/g,and cycling stabilities of 83.4% and 88.5% after 5000 cycles,respectively.The uninterrupted network formed by tight interconnection between NiCo₂O₄ nanosheets is beneficial to electron transfer and structure stability of electrode,thus exhibiting better electrochemical performance.Additionally,ultrathin NiMoO4 porous nanosheets were synthesized,while the rate capability is just 60.7% due to the poor arrangement of nanostructure and need further improvement.Improving the utilization efficiency of the internal space in the electrode is an effective way to boost the electrochemical performance of electrode materials.In this work,we designed and fabricated three kinds of core-shell structure: Co₃O₄-NWA@NiMoO4,NiCo₂O₄-NWA@NiMoO4,and NiCo₂O₄-NSA@NiMoO4 hybrid electrodes,in which the pristine nanoarrays serve as highway for electron transfer.The uniform coating of ultrathin NiMoO4 nanosheets can improve the utilization efficiency without sacrificing the porosity of electrode,thus greatly enhancing the electrochemical properties of electrodes.At a current density of 2 mA/cm2,NiCo₂O₄-NSA@NiMoO4 hybrid electrodes achieve a high specific capacitance of 1941 F/g and areal capacitance of 7.29 F/cm2,which can still maintain 84.1% at 60 mA/cm2,exhibiting remarkable rate capability.A type of asymmetric supercapacitors based on NiCo₂O₄-UNSA@NiMoO4 electrodes shows a maximum energy density of 52.6 Wh/kg,suggesting its great potential for energy storage applications.Besides,this research would give a general reference for the rational design of other active materials for high-performance supercapacitors.更多还原