【作者】 王成名；

【导师】 熊宇杰；

【作者基本信息】 中国科学技术大学 ， 纳米化学， 2013， 博士

【摘要】 人类正在面临着日益严重的能源危机和环境问题。低温燃料电池,是解决以上两个问题的最重要的一种技术手段,应用前景广阔。如何提高催化性能和节约应用成本,也就是如何优化催化剂的组成、尺寸、结构、形貌和晶面等,是技术应用的瓶颈和关键所在。铂Pt和钯Pd等贵金属纳米晶催化剂,是最常用的高效、稳定、耐久的燃料电池催化剂。实验测量和理论模拟都证实,发生在催化剂上的反应能量和反应速率相当敏感地依赖于裸露的表面晶面。高指数晶面,相对于其低指数的基础晶面,存有台阶和缺陷,有相对大量的低配位原子,具有更好的活性。本文重点通过选择不同表面修饰剂、控制反应动力学、设计电化置换反应、活化与外延生长、调控氧化刻蚀过程等,可控合成出具有高指数晶面的贵金属纳米晶(主要是Pt、Pd及其有关合金),优化并提升质子交换膜燃料电池反应(如甲酸氧化和氧还原反应等)性能。已经开展的研究内容如下：1.铂纳米晶枝状结构的可控合成及其电催化应用。枝状纳米晶的结构调控,可以调节对其催化活性有重要影响的两大参数——比表面积,原子在表面的台阶steps、突起ledges和缺陷kinks的数目。本工作中,我们发展出了一种简单的合成体系,通过调节体系中添加的盐酸的浓度能够调控铂纳米晶的枝状数目。在这个合成方法中,HCl通过氧化刻蚀在调节枝状数目中起到了3重调控作用：(i)晶种和纳米晶的结晶度；(ii)提供给生长位点的{111}或{100}晶面的数目：(iii)溶液中新生的铂原子的供给动力学。因此,可调的铂的枝状结构——相同化学环境下的三足、四足、六足、八足结构——可以在单一体系中简单地调节刻蚀强度而被理性合成出来。铂的枝状结构的可控性揭示,它们的电催化性能可以通过构建复杂结构来最优化。在不同的枝状结构中,与其他多足结构及商业Pt/C催化剂在甲酸氧化反应中的性能相比,铂八足结构表现出特别高的活性。本工作可以预期将为设计更为复杂的纳米结构以及在各种应用中实现独特功能作用提供新的视角。2.氧还原反应中具有高活性和高稳定性的铂-石墨烯复合结构。获得在燃料电池氧还原反应中铂电催化剂的高活性和高稳定性仍然是个重大挑战。我们开发出了一类具有高度凹面立方体(HCC)结构的铂纳米晶催化剂,具有{311}等高指数晶面和高氧还原活性。HCC纳米晶的稳定性可通过和石墨烯的组装得到显著提高。该独特的复合结构表现出进一步增强的电化学活性,比商用的Pt/C催化剂活性高出七倍。这种复合结构也在半波电位(E1/2)方面表现出了突出的高性能。在较低Pt担载量为46μb/cm2时,这种催化剂的半波电位高达0.967V,比商用Pt/C催化剂高出63mV,而且还稍高于文献中活性最高的多孔Pt-Ni催化剂的记录。本工作为通过调节负载基质的表面和界面来设计高性能电催化剂铺平了道路。3.动力学控制下各向异性生长的钯孪晶纳米结构及其在电化置换反应中的应用。五重孪晶结构是具有面心立方结构的金属纳米晶体中的重要种类,当它们的{100}面被保护起来的时候,它们可以各向异性生长成纳米线。我们开发了一种温和的方法去获得富有{311}和{611}高指数晶面的Pd瓜子状纳米晶。研究表明反应动力学是调控晶体生长模式的关键,同时选择性的包覆作用也对晶面控制具有重要作用。此孪晶结构的各向异性生长,提供了无需通过使用长链包覆剂/保护剂便可构建高指数晶面的一种新途径。这些高指数晶面的纳米晶体相比于低指数晶面纳米晶体,表现出卓越的化学活性,为与HAuCl4溶液发生的电化置换反应所证实。这个工作将为高指数晶面金属纳米晶体新合成方法的发展和催化等多领域应用等打开一扇大门。4.{730}高指数晶面Pd-Pt凹面纳米立方体的外延生长合成及电催化应用。我们开发了一种方法,通过刻蚀法对纳米立方体的表面进行选择性活化,进而很好控制其外延生长。通过在Pd立方体纳米晶晶种的角和边上进行生长,实现了Pd凹面纳米立方体的合成,在纳米晶表面形成了{730}高指数晶面和高活性位点,有利于催化应用。这种方法相较以往方法,可以防止原子生长在其他位点上并保证了纳米晶颗粒尺寸在晶种生长过程中基本维持不变。由于颗粒大小基本保持不变,活性位点和高指数晶面的出现使得纳米晶产物在甲酸氧化反应中有着更优越的电催化活性。另一方面,这种方法使得具有高催化活性的贵金属能外延生长在其他类型的较便宜的金属上,降低了昂贵材料的使用成本而又同时维持高的催化活性。在本章里,我们证实在Pd纳米晶上沉积非常有限量的Pt(Pt的质量分数只占3.3%),得到的Pd-Pt凹面纳米立方体在氧还原反应中表现了优良的催化活性。初步的研究表明,通过简单地改变表面化学状态,该合成方法同样适用于将不同材料选择性地沉积在纳米晶体表面上。5.钯纳米晶体在电催化甲酸氧化反应中的形貌效应。本章选用适当的金属前驱物、还原剂、稳定剂和保护剂,通过调控氧化刻蚀和反应动力学等,成功合成了形貌和尺寸均不相同的Pd纳米晶。经过认真的纳米粒子清洗和电极修饰组装,考察了它们在电催化甲酸氧化反应中的形貌与性能的关系。研究结果表明,Pd纳米晶样品的最大电流密度以纳米八面体(anooctahedra)<纳米线(nanowires)<纳米立方体(nanocubes)<纳米瓜子(nanotapers)<凹面纳米立方体(concave nanocubes)的顺序递增,催化甲酸氧化反应的起始氧化电位均小于0.2V。研究结果印证了Pd纳米晶催化甲酸氧化反应的催化性能在尺寸效应上主要受活性表面积的影响,扣除表面积效应后的催化性能与其尺寸没有明确关系。该系列Pd纳米晶的催化性能主要取决于其表面结构,得出Pd纳米晶催化甲酸氧化反应遵循{111}.晶面<{100}晶面<高指数晶面的性能活性顺序。综合最大电流密度和最小操作电位因素发现,Pd凹面纳米立方体和纳米瓜子具有相对较好的商用价值。更多还原

【Abstract】 Human beings are now facing the increasingly serious energy crisis and environmental problems. Low-temperature fuel cell, which is one of the most important technical means to address these two issues, has a broad application prospect. It has been the key and bottleneck of technology applications how to improve catalytic performance and save application cost by optimizing the composition, size, structure and shape of a nanocatalyst. Noble metal nanocatalysts including platinum and palladium nanocrystals, as the most efficient, stable and durable catalysts for fuel cells, are commonly used. It has been confirmed by experimental measurements and theoretical simulations that the reaction rate and energy on a nanocatalyst quite sensitively depends on the crystal facets exposed on its surface. High-index facets of a nanocrystal with more steps, defects and low coordination atoms, have a higher catalytic activity than low-index facets. This research is focused on the controllable synthesis of noble metal nanocrystals with high index facets (mainly Pt, Pd and related alloys) by selecting different capping agents, controlling reaction kinetics, designing galvanic displacement reaction, modulating activation and epitaxial growth, employing oxidation etching process, and further optimizes the catalytic performance of some important reaction such as formic acid oxidation and oxygen reduction reaction in the proton exchange membrane fuel cells. Related research works are summarized as follows:1. Control over the branched structures of platinum nanocrystals for electrocatalytic applications. Structural control of branched nanocrystals allows tuning two parameters that are critical to their catalytic activity:the surface-to-volume ratio, and the number of atomic steps, ledges, and kinks on surface. In this work, we have developed a simple synthetic system that allows tailoring the numbers of branches in Pt nanocrystals by tuning the concentration of additional HC1. In the synthesis, HC1plays triple functions in tuning branched structures via oxidative etching:(i) the crystallinity of seeds and nanocrystals;(ⅱ) the number of{111} or {100} faces provided for growth sites;(ⅲ) the supply kinetics of freshly formed Pt atoms in solution. As a result, tunable Pt branched structures-tripods, tetrapods, hexapods, and octopods with identical chemical environment-can be rationally synthesized in a single system by simply altering the etching strength. The controllability in branched structures enables to reveal that their electrocatalytic performance can be optimized by constructing complex structures. Among various branched structures, Pt octopods exhibit particularly high activity in formic acid oxidation as compared with their counterparts and commercial Pt/C catalysts. It is anticipated that this work will open a door to design more complex nanostructures and to achieve specific functions for various applications.2. A unique platinum-graphene hybrid structure for high activity and durability in oxygen reduction reaction. It remains a grand challenge to achieve both high activity and durability in Pt electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Here we develop a class of Pt highly concave cubic (HCC) nanocrystals, which are enriched with high-index facets and exhibit high ORR activity. The durability of HCC nanocrystals can be significantly improved via assembly with graphene. The unique hybrid structure displays further enhanced specific activity, which is7-fold greater than the state-of-the-art Pt/C catalysts. Strikingly, it exhibits impressive performance in terms of half-wave potential (E1/2). The E1/2of0.967V at the Pt loading as low as46μg cm-2, which stands as63mV higher than that of the Pt/C catalysts, is slightly superior to the record observed for the most active porous Pt-Ni catalyst in literature. This work paves the way to designing high-performance electrocatalysts by modulating their surface and interface with loading substrates.3. Anisotropic growth of palladium twinned nanostructures controlled by kinetics and their unusual activities in galvanic replacement. Five-fold twinned structures are a class of important members in the family of metallic nanocrystals with face-centered cubic (fCC) structures, which can anisotropically grow into nanowires when their{100} facets are protected. In this work, a facile synthetic approach has been firstly developed to synthesize a new structure of palladium nanocrystals, which are palladium nanotapers potentially enclosed by high-index facets. We have revealed that the reaction kinetics holds the key to tuning the growth mode of the nanocrystals while the selective capping effect makes a contribution to facet control. The anisotropic growth of twinned structures here provides a new approach for constructing a high-index surface without the need to use long-chain capping agents. These palladium nanotapers exhibit superior chemical activities compared to their low-index counterparts, as proven in the galvanic replacement. It is anticipated that this work opens a door for the development of new synthetic methods for metallic nanocrystals with high-index facets for various applications such as catalysis.4. Synthesis and eletrocatalytic applications of Pd-Pt concave nanocubes with {730} high-index facets by epitaxial growth methods. A method has been developed for controlled epitaxial growth on cubic nanocrystals by selectively activating their surface via etching. Pd concave nanocubes were produced via seeding growth on their corners and edges, formulating high-index facets and highly active sites for catalysis. This method offers a better capability of preventing atomic addition on undesired locations and maintaining particle size in the seeding process, as compared with the previous technique. With the particle size well maintained, the products fully exhibit superior electrocatalytic performance enabled by active sites and high-index facets in formic acid oxidation. Another contribution of this work is to enable the growth of a noble metal with high catalytic activities on another type of cheaper metal, which greatly reduces the usage of expensive materials while retaining high catalytic activity. In this work, we have demonstrated the deposition of a very limited amount of Pt (only3.3wt%.) on Pd nanocrystals towards high electrocatalytic activities in oxygen reduction reaction. Preliminary studies demonstrate that the synthetic strategy can be also applied to the controllable deposition of a different material on the faces of a nanocrystal by simply altering surface conditions.5. Structural effects of palladium nanocrystal electrodes on electrocatalytic oxidation reaction of formic acid. Palladium nanocrystals with various shapes and sizes were controllably synthesized by altering oxidative etching and reaction kinetics in the presence of appropriate metal precursors, reducing agents, stabilizers and capping agents, based on our previous works. Upon cleaning nanoparticles and assembling them onto electrodes, we systematically investigated the relationship between nanocrystal structures and catalytic performance in the oxidation of formic acid. It demonstrates that the maximum current densities of Pd nanocrystals increase in the order of nanoctahedra<nanowires<nanocubes<nanotapers<concave nanocubes. The onset potentials of all the samples are below0.2V. The results prove that the electrocatalytic performance of Pd nanocrystals is not significantly dependent on their sizes after being normalized by active surface area. Instead, the catalytic activity is mainly determined by surface structures:the activities of various Pd facets in the oxidation of formic acid should be in the order of{111}<{100}<high-index facets. Among various Pd nanostructures, concave nanocubes and nanotapers exhibit much better electrocatalytic performance.更多还原