介孔含氮超电容炭材料和喷墨打印Ni（OH）₂膜电极的制备及电化学性能研究

【作者】 李文荣；

【导师】 江志裕；

【作者基本信息】 复旦大学 ， 物理化学， 2007， 博士

【摘要】 多孔炭材料由于其独特的化学稳定性，良好的导电能力，高的比表面积以及相对廉价的优点，使它作为制备电化学电容器电极的极佳候选者而备受人们的关注。为了提高电容器的能量密度和功率密度，需要人们在保持炭材料具有高比表面积的同时，能够有效地扩大其孔径，从而促进双电层在其界面处的快速形成，提高其功率性能。同时，为了增加准电容以及增强电极材料对于电解质溶液的润湿性，充分地利用其孔道表面进行电荷存储，在材料表面引入氮基团是一个很好的选择。随着微电子机械系统（MEMS）和超大规模集成电路技术（VLSI）的发展，对能源的微型化、集成化提出了越来越高的要求。民用电子器件如传感器、智能卡、便携式电子设备等众多领域的迅猛发展，也对化学电源的小型化、微型化和薄膜化提出了更高的要求。薄膜电池因其良好的集成兼容性和电化学性能成为MEMS、VLSI、智能卡等能源微型化、集成化的最佳选择。近年来，人们一直在寻求制备薄膜电池的最佳制备技术。本论文分为两大部分。第一部分，制备出大孔径含氮骨架介孔炭，并对其物理性质和作为超电容电极材料的电化学性能进行详细的表征和深入的研究。第二部分，利用新颖的喷墨打印方法制备氢氧化镍薄膜电极并详细考察其电化学性能。本论文的主要研究结果如下：（1）利用三聚氰胺与甲醛之间的缩聚诱导二氧化硅胶体聚集的方法首先制得了三聚氰胺甲醛树脂／二氧化硅复合微球，然后将所得树脂／二氧化硅复合微球固化，在惰性气体保护下经过800℃炭化处理，并用HF酸溶除二氧化硅模板后成功制得了具有高比表面积和较大孔径的介孔炭微球材料。所得材料保持了高分子前驱体的部分氮原子，有利于提高其对电解质溶液的润湿性能。该制备过程简单，周期短，充分利用了现已商品化的廉价化学原料，适合该类材料的批量制备。实验中我们发现，三聚氰胺、甲醛和模板剂二氧化硅的比例关系对所制得的炭材料的比表面积有很大的影响，通过多次优化三者比例，制备出了孔径为～30nm，最大比表面积为1480m²／g的介孔炭微球，在5M硫酸电解液中，1 A／g的恒电流充放电密度下的容量为226 F／g，在5 A／g电流密度下的质量比容量可达到214 F／g，能够在很宽的电流密度区间（0.1～5 A／g）内保持住很好的电容性能。该材料具有优异的电化学性能可归因于以下几个方面：比较大的比表面积可以储存更多的电荷，合适的氮含量不仅可以增加电极表面的润湿性而且可以提供准电容增加比容量，30纳米左右的孔径能够使离子在快速迁移和扩散。（2）超级电容器的最大特点是能够快速充放电，为了达到其高功率密度的要求，急需人们在保持炭材料具有高比表面积，有效地扩大其孔径的同时，提高炭材料的导电率。因此，要求所得材料应具有一定的石墨化程度，从而降低材料的电阻。文章中采用提高炭化温度的方法来改善介孔炭材料的微观结构，使其具有一定的石墨化结构，从而提高材料的电导性，降低阻抗，提高电容器的极限功率。经过1000度高温炭化所制得的介孔炭微球材料阻抗明显降低，能够在更宽的电流密度区间（1～50 A／g，在5M硫酸中）内保持住很好的双电层电容性能。超级电容器常用的水溶液电解液有H₂SO₄和KOH水溶液两种。由于水溶液分解电压太低，大大降低了电容器的能量密度。有机电解液的分解电压大大高于水溶液，可以大大提高其能量密度。由于本文制备的介孔材料孔径在30 nm左右，可以很好的适应较大半径的有机电解液溶剂化离子，即使在大电流充放电条件下，溶液中的离子也可以在大孔径中迅速迁移，从而在提供高能量密度的同时满足高功率的需要。电化学测试表明，在有机电解液中经1000度炭化所得材料在0.5A／g电流密度下的质量比容量可达到159 F／g，在20A／g高充放电电流密度下的质量比容量仍可保持在130 F／g。在其工作区间1.8V-3.8V（vs Li）之间，经过1000圈充放电循环前后的交流阻抗显示，高频区半径基本没有变化，并推算出比电容法拉值变化不大，说明氮基团非常稳定。作为双电层电容器的电极材料，它具有比一般商业化活性炭材料更高的质量比容量和大电流充放电性能，尤其是在大电流和含有较大离子半径的有机体系中的表现比商业化活性炭材料更加优越。（3）氮基团对介孔炭材料的电化学性能影响明显。炭化温度不同将导致氮含量的差别，炭化温度从800℃上升到1000℃，含氮量从14.9％降低到8.9％，低温炭化样品的比容量高于高温炭化样品，但是功率性能却差于高温样品。我们发现：a．即使在快速的充放电速度下，氮基团仍然能够很快地提供准电容：b．炭化温度的高低是影响电极阻抗的首要因素。在还原铁粉保护下炭化得到的介孔炭材料表面氮含量明显增加。随着炭化温度的升高，氮基团中的氮的存在状态发生明显的变化。炭化温度越高，N-6向N-5，N-Q，N-X转化的比例越大，相应提供的电化学准电容降低，导致比电容整体下降。在比表面积类似的情况下，得出了含氮大孔径介孔炭比电容与氮基团的含量成线性关系的结论（在5M硫酸中）。在KOH体系中，氮基团也能够产生准电容，对比电容做出贡献。在小电流密度充放电条件下（小于1A／g），在硫酸溶液和氢氧化钾溶液中的质量比容量相差不大，在大电流充放电密度下，含氮介孔炭材料在KOH中的质量比电容远小于在硫酸中的比电容。这是因为氮基团与H₃O⁺的反应远远比与K⁺的反应容易，导致在快速充放电条件下，氮基团在KOH中不能够很快地提供准电容。对该制备过程及所得材料性能的系统研究有望加深人们对于高功率双电层电容器所需炭电极材料微观结构的理解，并最终根据实际需求实现对材料的设计合成。该材料在酸性、碱性和有机体系中均表现出良好的电化学特性。（4）利用喷墨打印技术制备薄膜电极需要使用纳米尺寸的电极材料。本文利用水热法合成得到了用作镍电池正极材料的纳米氢氧化镍颗粒。采用喷墨打印方法的关键是纳米粒子在分散体系中的稳定性。怎样使具有电化学活性的氢氧化镍钠米材料在水体系中十分均匀地稳定分散，是工艺成功的关键步骤。本文通过联合采用空间位阻型聚合物分散剂和湿法球磨工艺成功地解决了喷墨打印技术中的墨水制备问题，成功地制备出了氢氧化镍薄膜电极，建立了方便快速的喷墨打印制备薄膜电极的方法，并对薄膜电极的形貌和电化学性能进行了深入的研究。用喷墨打印方法在金箔上制备了厚度仅为700 nm的薄膜氢氧化镍电极，在电位区间为0.2～0.65 V vs Hg／HgO、电流密度约为4.16 A／g时的可逆放电容量约为260mAh／g，氢氧化镍薄膜电极具有高倍率放电的主要原因是：在纳米氢氧化镍颗粒中尤其是经过球磨之后的纳米氢氧化镍颗粒中，质子扩散比较容易，只有700nm厚的薄膜和较大的氢氧化镍颗粒的比表面积也是重要因素。这种直接喷墨打印出的氢氧化镍薄膜电极的充放电稳定性非常好，循坏重放100圈后基本没有容量衰减。更多还原

【Abstract】 Recently, many efforts have been made to search supercapacitor electrode materials that can be used practically as high power sources to supply large pulsed current. Among various available candidates, microporous activated carbons are mostly investigated due to their large surface areas, good electric conductivity, excellent chemical stability and relatively low cost. The key factors that dictate the selection of carbon materials for supercapacitor electrodes are the following: high specific surface areas for charge storage, suitable surface functional groups to enhance the capacitance by additional faradaic redox reaction and improve the wettability of carbon surface, and large pore size to facilitate the ions diffusion with a high speed.With the development of microelectromechanical systems （MEMS） and very large-scale integration （VLSI）, there is an increasing requirement in the miniaturization and integration of power sources. The reduction in size and power requirement of electronic devices is the major driving force behind the development of thin-film batteries. Applications focus on the improvement of existing consumer and medical products, such as smart cards, sensors, portable electronic devices, as well as on the integration with electronic chips and microelectromechanical systems. With better integration compatibility and electrochemical performance, thin-film battery becomes the optimal choice for miniaturization and integration of MEMS and VLSI power.This thesis includes two major parts. Firstly, nitrogen-contained mesoporous carbon spheres were fabricated and used for supercapacitors. Their structure, morphology and electrochemical behaviors were investigated in great detail. Secondly, Ni（0H）2 thin-film electrodes were successfully fabricated by a novel and facile route of ink-jet printing technique. Their electrochemical performances were also investigated.The main results are as follows.（1） we demonstrate a facile polymerization-induced colloid aggregation method to synthesize a kind of mesoporous carbon spheres containing in-frame incorporated nitrogen using melamine-formaldehyde resin as a carbon precursor. The obtained MCS materials simultaneously possess the following characteristics: high specific surface areas contributed mainly by mesopores with uniform pore size, and suitable quantity of nitrogen on the surface of the materials. The precursors used in this simple process are commercially available and very cheap, which will be favorable in the preparation of MCS on a large scale. Their surface areas can be varied from 765 to 1480 m2/g with adjusting the ratio among melamine resin, formaldehyde and silica. As the electrode material for supercapacitor in 5 mol/L H₂SO₄, the MCS products present excellent specific capacitance as 226 F/g. Its specific capacitance can still remain 214 F/g at 5 A/g. The superior electrochemical performance of MCS is associated with the following characteristics: high specific surface area （～1480 m2/g） contributed mainly by the mesopores, uniform pore size as large as 29 nm and moderate content of nitrogen.（2） One of the most important characters of supercapacitor is high power density, which demand carbon materials used for supercapacitor possess good conductivity. Here we enhanced the carbonization temperature to 1000℃to obtain mesoporous carbon spheres with good graphitized nanostructures. The obtained carbons have good specific capacitance and rate capability as an EDLC electrode when constantly charged/discharged over a wide loading current range （1-50 A/g）.Generally, the electrolyte can be classified as aqueous and organic medium. In the aqueous solutions, the operating voltage region is restricted to be ca. 1.23 V due to the thermodynamic electrochemical window of water. The electrical energy accumulated in supercapacitor can be significantly enhanced by the selection of organic medium where the decomposition potential window of the electrolyte can reach to 2 - 4.2 V. The carbon material product presents a high specific capacitance as 159 F/g at 0.5 A/g in organic electrolyte in the potential range of 1.8V to 3.8V （vs Li）. The high specific capacitance of the carbon material is believed to be associated with its suitable nitrogen content that can afford pseudocapacitanc as well as the high specific surface area. From Nyquist we conclude that the double layer capacitance is 122 F/g before cycling and after 1000 cycles it still kept on 127 F/g. This phenomenon indicates that the MCS particles can be used for 1000 cycles without aggregation, and the nitrogen functional groups may be very stable.（3） Nitrogen groups can influence the electrochemical performance of carbon materials greatly. Variation in carbonization temperature can result in the MCS materials with different nitrogen content and graphitized nanostructures. Increasing the temperature from 700 to 1000℃, the nitrogen content decrease from 14.9 to 8.9 wt.%. The lower carbonization temperature, the higher the specific capacitance and the poorer the power performance. We conclude that: a. even at a high loading current density, the nitrogen can afford pseudocapacitanc at the same. And b. high-carbonized temperature will lead to the decrease of the equivalent series resistance.The amounts of nitrogen increase by the protection of Fe powder in the carbonization process. The results indicate that the N-6 has been chemically transformed into nitrogen species with higher binding energies through the condensation reaction during the carbonization process. The higher the nitrogen content, the higher the specific capacitance when the specific surface areas of carbon materials are similar.In KOH electrolyte, the significant presence of pseudocapacitive interactions is clear as well as in H₂SO₄. And we conclude form the experiments that the faradaic interactions between H₃O⁺ and the nitrogen functionalities are stronger than those of K⁺ and that they determine the overall capacitive performance.（4） The key procedure for the ink-jet printing process is to obtain the stability of nano-sized materials in the dispersion system. The stable Ni（OH）₂ "inks" containing binder were successfully prepared by employing both wet ball-milling technology and steric polymeric dispersant. The morphology, structure, and electrochemical performance of Ni（OH）₂ thin film electrodes were investigated by scanning electron microscopy （SEM）, cyclic voltammograms （CV）, galvanostatic charge-discharge and electrochemical impedance measurements. SEM images show uniform distribution of as-printed Ni（0H）2 thin film electrodes. The thickness of thin film electrodes were about 0.6μm by the cross-sectional profile of SEM observation. Galvanostatic charge-discharge shows that the capacity of Ni（OH）₂ film is about 260 mAh/g and stably retained after 100 cycles at a high current density of 4.16 A/g. The high charge/discharge rate capability can be attributed to the following reasons: easy proton diffusion in the nano-sized particles of Ni（OH）₂ especially followed by the ball-milling process, very thin film and high surface area of nano-Ni（OH）₂ particles. The ink-jet printing method shows the convenient, feasible, and inexpensive property for fabricating the Ni（OH）₂ thin films.更多还原