Platinum-based electrocatalysts with core-shell nanostructures.
Platinum-based electrocatalysts with core-shell nanostructures.
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DOI：
10.1002/anie.201005868
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发表时间：
2011-03
影响因子：
--
通讯作者：
Hong Yang
中科院分区：
文献类型：
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作者：
Hong Yang
在过去的几年里，汽车用低温氢燃料电池的发展取得了巨大的进步，燃料电池驱动的汽车行驶的总里程在2009年超过了百万英里大关。新的电池组件继续快速降低燃料电池的成本、体积和重量。推动这一快速发展的动力之一是质子交换膜燃料电池(PEMFCs)活性阴极催化剂的研制取得了重大进展。提高用于氧还原反应(ORR)的电催化剂的活性和耐久性仍然是开发用于汽车应用的下一代PEMFC的关键研究领域。由于仍然没有可行的替代铂族金属(PGM)制成的催化剂，对低成本、高活性和耐用的燃料电池电极的需求提高了设计标准。在这方面，多组分纳米结构可以在设计高活性和耐久性的ORR催化剂方面发挥重要作用。2]核壳或核壳状纳米结构是一种方便地将多功能构建到金属纳米颗粒的电催化剂中的方法，金属纳米颗粒的典型平均直径在2到5 nm之间。随着核壳纳米粒子的日益复杂，在新型催化剂的设计中，结构-性质关系的研究变得比以往更加重要和必要。Sun和他的同事最近报道了一种多金属核壳纳米粒子的合成，这是最近几个关于多金属核壳纳米粒子的合成和电催化性能的最新研究。以油胺和油酸为封端剂，在十八烯中合成了球形Pd/Au和Pd/Au/FePt核壳纳米粒子。用高分辨透射电子显微镜和像差校正大角度环状暗场扫描电子显微镜(HAADF-STEM)对其结构进行了表征。对于Pd/Au双金属核壳纳米粒子，核壳纳米粒子的大小相当均匀，总直径约为7 nm。此外，采用类似的溶液相合成法也可以制备出11 nm Pd/Au/FePT多金属核壳纳米粒子。通过改变反应条件，这些双金属或多金属核壳纳米粒子的尺寸也可以很容易地在一定范围内调节。到目前为止，关于合成尺寸在10 nm以下的明确的多金属核壳纳米粒子的报道仍然相对少见。然而，原则上，合成多金属核壳纳米颗粒或核壳状纳米结构，包括树枝晶、颗粒对颗粒、树莓或花朵，通常在热力学上是受青睐的。这种合成是可能的，因为第二金属组分在现有纳米颗粒种子或核心上的非均相成核具有比均相成核更低的临界能垒，即总的过剩自由能。根据总的过剩能量(这在很大程度上与表面和界面能项有关)和由于界面晶格失配而产生的应变能，形成了三种不同类型的纳米结构，即逐层、岛上润湿层和岛生长模式(图1)。当界面结构未知或不能很好地定义，或者纳米结构的形状很重要时，通常使用基于形态的通用描述，例如覆盆子、纳米花、树枝状、颗粒对颗粒或核壳纳米颗粒。在溶液相合成和使用封端配体的情况下，金属核心可以有序或无序的形式存在，并且可以由金属合金形成(图1)。金属或金属合金的非均相沉积是通过三种生长模式中的一种来形成核壳或核壳状纳米结构的。那么，为什么尺寸小于10纳米的多金属核壳纳米粒子的报道相对较少呢？除了合成过程中的内在挑战外，一个重要的原因可能在于难以详细表征多金属核壳纳米粒子。杨和他的同事此前曾报道过形状可控的铂/钯核壳纳米粒子。利用铂立方晶种可以制备出纳米铂/钯纳米立方体、立方体和八面体。高分辨率电子显微镜图像显示[*]纽约大学罗切斯特·加维特大厅206号，罗切斯特·加维特厅杨H教授(14627)传真：(+1)585.2731348Email：hongYang@che.rochester.edu主页：http://www.che.rochester.edu/~hongyang/
The development of low-temperature hydrogen fuel cells for automotive applications has witnessed tremendous progress over the past several years, and the total distance driven by fuel-cell-operated vehicles exceeded the million-mile mark in 2009. New cell modules continue to reduce the cost, volume, and weight of fuel cells at a rapid pace. One driving force for this fast development lies in the great progress made in making active cathode catalysts for proton exchange membrane fuel cells (PEMFCs). Improvement of the activity and durability of electrocatalysts for the oxygen reduction reaction (ORR) remains a key research area for the creation of future generations of PEMFCs for automotive applications. As there is still no viable alternative to the replacement of catalysts made of Pt group metals (PGMs), the need of low cost, highly active, and durable fuel-cell electrodes heightens the design criteria. In this regard, multicomponent nanostructures can play important roles in the design of ORR catalysts with high activity and durability. 2] Core–shell or core–shell-like nanostructures are a convenient way to build multifunctionality into the electrocatalysts of metallic nanoparticles, which have a typical average diameter between 2 and 5 nm. With the increasing complexity of core–shell nanoparticles, the study of structure–property relationships becomes even more important and essential than before in the design of new catalysts. Sun and co-workers recently reported a synthesis of multimetallic core–shell nanoparticles, which is the latest of several recent investigations into the synthesis and electrocatalytic properties of multimetallic core–shell nanoparticles. Spherical Pd/Au and Pd/Au/FePt core–shell nanoparticles were synthesized in octadecene using oleylamine and oleic acid as the capping agents. The structures were characterized by high-resolution transmission electron microscopy (TEM) and aberrationcorrected high-angle annular dark-field scanning TEM (HAADF-STEM). The core–shell nanoparticles were fairly monodisperse in size and had overall diameters of about 7 nm for Pd/Au bimetallic core–shell nanoparticles. Furthermore, 11 nm Pd/Au/FePt multimetallic core–shell nanoparticles were also readily produced by using a similar solution-phase synthesis. The size of these bimetallic or multimetallic core– shell nanoparticles can also be easily tuned within a certain range by changing the reaction conditions. To date, reports on the synthesis of well-defined multimetallic core–shell nanoparticles with sizes below about 10 nm are still relatively uncommon. In principle, the synthesis of multimetallic core–shell nanoparticles or core–shelllike heterogeneous nanostructures including dendrite, particle-on-particle, raspberry, or flower, however, is usually thermodynamically favored. The synthesis is possible as the heterogeneous nucleation of second metal component on the existing nanoparticle seed or core has a lower critical energy barrier, that is, the overall excess free energy, than the homogenous nucleation. Depending on the overall excess energy, which is largely related to the surface and interfacial energy terms, and the strain energy because of lattice mismatch at the interface, three different major types of nanostructures form, namely, layer-by-layer, island-on-wetting layer, and island growth modes (Figure 1). When the interfacial structures are not known or cannot be well defined, or the shape of the nanostructure is important, a generic description based on the morphology, such as raspberry, nanoflower, dendrite, particle-on-particle, or core–shell nanoparticle, is often used. In the solution-phase synthesis and with the use of capping ligands, metallic cores can exist in ordered or disordered forms, and can be formed from metal alloys (Figure 1). Heterogeneous deposition of a metal or metal alloy on the core occurs through one of the three growth modes to form core–shell or core–shell-like nanostructures. So why have relatively few multimetallic core–shell nanoparticles with sizes less than 10 nm been reported? Besides the intrinsic challenges in the synthesis, one important reason perhaps lies in the difficulty of characterizing multimetallic core–shell nanoparticles in detail. Yang and coworkers have previously reported shape-controlled Pt/Pd core–shell nanoparticles. Pt/Pd core–shell nanocubes, cuboctahedra, and octahedra could be made by using Pt cube seed crystals. High-resolution TEM (HRTEM) images revealed [*] Prof. Dr. H. Yang Department of Chemical Engineering University of Rochester Gavett Hall 206, Rochester, NY 14627 (USA) Fax: (+ 1)585-273-1348 E-mail: hongyang@che.rochester.edu Homepage: http://www.che.rochester.edu/~hongyang/