【作者】 覃远航；

【导师】 张新胜；

【作者基本信息】 华东理工大学 ， 化学工程， 2011， 博士

【摘要】 直接乙醇和甲酸燃料电池作为小功率便携式电子设备未来潜在的动力源在近年来越来越引起人们的关注。目前,乙醇和甲酸氧化催化剂的低活性及高成本是阻碍直接乙醇和甲酸燃料电池商业化的主要因素。因此,开发高效的乙醇和甲酸氧化催化剂以提高催化活性从而降低贵金属的负载量具有重要意义。采用大比表面积的载体材料锚定贵金属催化剂是提高催化剂性能的常用方法。作为一种新型碳材料,纳米碳纤维(carbon nanofiber, CNF)因为其独特的电子及结构特性,例如高的电导率及中孔结构,作为催化剂载体材料具有巨大的潜力。而且,CNF的织构及结构特性,例如直径,边面原子与基面原子的比例等都是可调控的,这为调控负载贵金属在CNF上的沉积和CNF与负载贵金属的相互作用提供了途径。本研究的目的在于以CNF为催化剂载体,选择合适的合成方法开发高效的乙醇和甲酸氧化电催化剂。同时,也研究了CNF作为催化剂对氧的电催化还原性能。目前得到的主要研究结果如下：(1)采用电泳沉积技术制备了由相互缠绕的CNF形成的具有开孔结构的CNF膜。研究了采用脉冲电沉积法负载在CNF膜上的Pd纳米粒子对乙醇在碱性溶液中的电催化氧化性能。循环伏安测试表明,电泳沉积联合脉冲电沉积技术制备的这种新型催化剂对乙醇在KOH溶液中的电化学氧化具有良好的催化活性。这种高的催化活性可归因于高度分散在具有三维网状结构的CNF膜上的Pd纳米粒子可以为乙醇氧化提供大的可接触电化学活性面积及CNF膜的结构和电子特性。(2)分别采用传统的粉末工艺及电泳沉积联合脉冲电沉积的工艺制备了两种Pd/CNF修饰的玻碳(glassy carbon, GC)电极：Pd/CNF/GC-C和Pd/CNF/GC-E,研究了电极制备方法对电极的乙醇催化氧化性能的影响。场发射扫描电镜及高分辨透射电镜的表征结果表明Pd纳米粒子在两个电极中都比较均匀地分散在CNFs上,X-射线衍射结果表明Pd/CNF/GC-E电极中Pd纳米粒子的粒径要稍大于Pd/CNF/GC-C电极中Pd纳米粒子的粒径。循环伏安测试表明Pd/CNF/GC-E电极对乙醇在碱性溶液中的电化学活性要高于Pd/CNF/GC-C电极,尽管Pd/CNF/GC-C电极有较高的Pd载量及较小的粒径。Pd/CNF/GC-E电极比Pd/CNF/GC-C电极的性能要好,归因于Pd催化剂在前者的利用率比后者高,这通过对两个电极电化学活性面积的测试得到证实。另外,计时安培测试表明Pd/CNF/GC-E电极对乙醇电催化氧化的稳定性要好于Pd/CNF/GC-C电极。(3)制备了板式、鱼骨式和管式CNF负载的Pd电催化剂,考察了CNF的微结构对电催化剂催化乙醇氧化活性的影响。在同样的制备条件下,Pd纳米粒子在板式、鱼骨式和管式CNF上分别呈现均匀致密、松散随意和聚集的分散状态。这种分散状态的差异可能是CNF微结构的不同造成的。管式CNF表面暴露的主要是基面原子,其与负载Pd纳米粒子的相互作用较弱,在这种较弱的相互作用下Pd粒子倾向于聚集。板式和鱼骨式CNF表面暴露的主要是高能端面原子,其可以锚定负载的Pd纳米粒子从而防止Pd粒子的聚集。电化学测试的结果表明鱼骨式CNF负载的Pd纳米粒子对乙醇氧化具有最好的催化活性,而板式CNF负载的Pd纳米粒子对乙醇氧化具有最好的催化稳定性。(4)分别以柠檬酸钠和硼氢化钠为稳定剂和还原剂合成了CNF负载的Pd催化剂。发现通过改变合成溶液的pH值可以很方便地调节负载Pd纳米粒子的粒径和分布。随着合成溶液的pH值从3.2增至6.0,CNF负载的Pd纳米粒子在CNF上的分散变得越来越均匀,平均粒径从5.8 nm降至3.6 nm。然而,当合成溶液的pH值继续增至6.5时,CNF负载的Pd粒子的平均粒径增大,而且Pd/CNF催化剂中有PdO的生成。对Pd/CNF催化剂的电化学测试表明在溶液的pH值为6时合成的Pd/CNF催化剂对甲酸氧化具有最好的催化活性及稳定性,这可归因于其小的粒径及均匀的粒径分布。(5)分别以柠檬酸钠和硼氢化钠为稳定剂和还原剂合成了CNF负载的PdAu催化剂,XRD测试的结果表明合成溶液中四氢呋喃的添加有助于Au与Pd合金化程度的提高。电化学测试的结果表明在适当的Pd/Au质量比下,合金化程度高的PdAu/CNF催化剂对甲酸电催化氧化的活性及稳定性均高于合金化程度低的PdAu/CNF催化剂及Pd/CNF催化剂。PdAu/CNF催化剂对甲酸的催化活性受到多重因素的影响,只有当催化剂的粒径及合金化程度同时兼顾时PdAu/CNF催化剂才有较优的催化性能。(6)CNF在混酸(浓硫酸+浓硝酸)和氨水中的超声处理分别在CNF表面成功地引入了含氧和含氮基团。循环伏安测试的结果表明含氧基团的引入使得CNF对氧的电催化还原活性有显著提高,而含氮基团的引入使得CNF的氧电催化还原活性有进一步提高。鱼骨式CNF对氧的电催化还原性能要好于相应的管式CNF。以上的研究表明CNF的微结构及表面性质都对CNF的氧电催化还原活性具有影响,而后者的影响更显著。更多还原

【Abstract】 Recently, interest in the development of direct ethanol and formic acid fuel cells has considerably increased because they are considered as promising future power sources for small portable electronics. However, the low activity and high cost of the ethanol and formic acid oxidation electrocatalysts are still major obstacles impeding the commercialization of direct ethanol and formic acid fuel cells. Therefore, it is of great importance to develop highly efficient ethanol and formic acid oxidation electrocatalysts which serve to improve the electrocatalytic activity and decrease the amount of noble metal catalyst required.The support materials with large surface area are often employed to anchor noble metal catalysts so as to improve the performance of catalysts. As a novel type of carbon material, carbon nanofibers (CNFs) are promising electrocatalyst supports due to their unique electrical and structural properties such as high electrical conductivity and well-developed mesopores. What’s more, the texture and microstructure of CNFs, such as the diameter and the ratio of edge atoms to basal atoms, are tunable and controllable, which provides a means to adjust the deposition of and the interaction with the supported noble metals.The aim of this research has been to develop highly efficient electrocatalysts for ethanol and formic acid oxidation by using CNF as the catalyst support and following certain synthesis procedures. Also, CNF as the electrocatalyst for oxygen reduction is explored. The main results of the research achieved up to now are as follows:(1) A network-like CNF film with an open porous structure formed by the open space between entangled CNFs is fabricated by electrophoretic deposition technique. The performance of the CNF film as an electrocatalyst in the presence of pulse electrodeposited Pd nanoparticles for ethanol oxidation in alkaline media has been investigated. Cyclic voltammetric analyses show that the novel electrocatalyst prepared by electrophoretic deposition in conjunction with pulse electrodeposition technique has good electrocatalytic activity and stability for ethanol oxidation in KOH solution. This is believed to be due to the high dispersion of Pd nanoparticles on the CNF film with a three-dimensional network structure which can provide a large number of available Pd active sites for ethanol oxidation, and to the structural and electrical properties of the CNF film. (2) Two Pd/CNF modified glassy carbon (GC) electrodes, Pd/CNF/GC-C and Pd/CNF/GC-E, are fabricated by the conventional powder type method and by the electrophoretic deposition in conjunction with pulse electrodeposition method, respectively, and the effect of electrode fabrication methods on the electrode performance for ethanol oxidation has been investigated. Field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) show that Pd particles are uniformly dispersed on CNFs at each electrode and X-ray diffraction (XRD) reveals that the average Pd particle size of the Pd/CNF/GC-E electrode is slightly larger than that of the Pd/CNF/GC-C electrode. Cyclic voltammetric analyses show that the electrocatalytic activity of Pd/CNF/GC-E electrode is better than that of Pd/CNF/GC-C electrode for ethanol oxidation in alkaline media, although the Pd/CNF/GC-C electrode has higher Pd loading and smaller particle size. This is believed to be due to the higher utilization of Pd catalyst on Pd/CNF/GC-E electrode than on Pd/CNF/GC-C electrode, which is confirmed by the electrochemically active surface area measurements. In addition, chronopotentiometric analyses show the long-term operation stability of Pd/CNF/GC-E electrode is better than that of Pd/CNF/GC-C electrode.(3) Platelet CNF (p-CNF), fish-bone CNF (f-CNF) and tubular CNF (t-CNF) supported Pd electrocatalysts are prepared and the effect of CNF microstructure on the electrocatalytic activity of the electrocatalysts for ethanol oxidation has been investigated. Under the same preparation conditions, the Pd particles display a uniform dense distribution, a loose random distribution and an agglomerated distribution on p-CNFs, f-CNFs and t-CNFs, respectively. The diffrence in distrubution of Pd particles may result from the different microstrucutures of CNF supports. It is known that the exposed surfaces of t-CNFs are dominantly occupied by basal atoms, which have a weak interaction with the supported particles. Under the weak interation, particles tend to agglomerate with each other. While the exposed surfaces of p-CNFs and f-CNFs are dominantly occupied by energetic edge atoms, which may serve as anchoring sites for Pd particles and prevent the particles from agglomeration. The results of the electrochemical characterization also indicate that the microstructure of CNF significantly influences the electrocatalyst performance and f-CNF supported Pd electrocatalyst possesses the best electrocatalytic activity while p-CNF supported Pd electrocatalyst possesses the best electrocatalytic stability for ethanol oxidation in KOH solution.(4) CNF supported Pd nanoparticles are synthesized with sodium citrate and sodium borohydride served as the stabilizing and reducing agent, respectively. The size and distribution of the supported Pd nanoparticles can be readily controlled by adjusting the pH value of the synthesis solution. Analyses of the obtained Pd/CNF catalysts indicate that the supported Pd nanoparticles become more uniform in size and the average particle size is decreased from 5.8 to 3.6 nm with pH value of the synthesis solution increasing from 3.2 to 6.0. However, the particle size is increased and the PdO phase is formed in the synthesized Pd/CNF catalyst when the pH value is further increased to 6.5. Electrochemical characterization shows that the Pd/CNF catalyst synthesized at pH 6 exhibits the highest electrocatalytic activity and stability for formic acid oxidation due to its small particle size and uniform size distribution.(5) CNF supported PdAu nanoparticles are synthesized with sodium citrate and sodium borohydride served as the stabilizing and reducing agent, respectively. XRD characterization indicates that the alloying degree of the synthesized PdAu nanoparticles can be improved by the addition of tetrahydrofuran to the synthesis solution. The results of electrochemical characterization indicate that the CNF supported high-alloying PdAu catalyst with a proper mass ratio of Pd to Au possesses better electrocatalytic activity and stability for formic acid oxidation than either the CNF supported low-alloying PdAu catalyst or the CNF supported Pd alone catalyst. The electrocatalytic activity of PdAu/CNF catalyst for formic acid oxidation is affected by multiple factors, and the highly active PdAu/CNF catalyst can be obtained only by taking into account both the particle size and the alloying degree of the catalyst.(6) Oxygen-and nitrogen-containing functional groups are successfully introduced onto the CNF surface by sonochemical treatment in mix acids (concentrated sulfuric acid and nitric acid) and ammonium hydroxide, respectively. The cyclic voltammetric results show that the electrocatalytic activity of CNF for oxygen reduction is significantly improved by the introduction of oxygen-containing functional groups and is further improved by the additional introduction of nitrogen-containing functional groups. The f-CNF-based electrodes have higher electrocatalytic activity for oxygen reduction than their t-CNF-based counterparts. The above results indicate that the micro structure and the surface property of CNF both have an effect on the electrocatalytic activity of CNF for oxygen reduction, while the latter have a dominant effect.更多还原