Atomic Layer-by-Layer Deposition of Pt on Pd Nanocrystals with Well-Controlled Facets

在这项研究项目中，佐治亚理工学院的夏博士和威斯康星大学麦迪逊分校的Mavrikakis博士在大分子、超分子和纳米化学(MSN)计划的支持下，开发了能够显著增强对聚合物电解质燃料电池运行至关重要的氧还原反应(ORR)的质量活性的铂基催化剂。作为一种清洁能源技术，聚合物电解质燃料电池在包括现场发电和用作交通工具和电子设备的便携式电源的应用中具有吸引力。然而，由于沉积在阴极上的铂催化剂用于缓解氧还原反应(ORR)的迟滞动力学，其成本高昂，因此大规模推广这项技术一直是一项挑战。为了满足这项技术大规模商业化的成本要求，需要将铂负载减少大约四倍。夏博士和Mavrikakis博士正在研究一种新的策略，该策略集成了化学合成和计算模型，以最大限度地提高铂基催化剂对ORR的质量比活性。他们特别感兴趣的是将铂作为只有几个原子层厚的超薄外壳沉积在钯纳米晶体上。他们通过控制钯(Pd)模板上小面的类型(从而控制表面原子的排列)，优化铂皮的厚度，最大化核中钯原子和壳中铂原子之间的电子耦合，开发出具有增强活性的ORR催化剂。除了科学和技术优势外，这项工作还有助于在材料化学、催化、表面科学、计算化学、胶体科学和能源技术等不同学科之间建立联系。它还对我们的社会产生了影响，因为它开发了用于燃料电池和催化的新型材料，在能源转换和环境保护方面发挥了作用。研究人员通过让女性、少数民族和其他代表性不足的群体参与到这个项目中来促进高等教育的多样性。通过大幅减少铂和钯在催化装置中的负载量，这项工作帮助社会实现了铂的可持续利用，铂是存在于地球地壳中的最稀有的贵金属之一。在聚合物电解质燃料电池的新型催化剂中，铂原子以保角壳层的形式沉积在预先合成的尺寸均匀、刻面可控的钯纳米晶表面。外壳厚度从一层到五层原子层都可以精确调整。研究了四种类型的钯纳米晶，包括立方体、八面体、菱形十二面体和凹面体，每个纳米晶的表面分别覆盖着一种类型的面：{100}、{111}、{110}和高指数面。这项研究涉及三种方法的独特组合：通过与钯核心的耦合来改变铂表面的电子结构(从而改变含氧物种的结合能)；通过控制钯核心上的小平面来开发最活跃的表面结构；以及用钯取代主体中的铂原子，钯是一种比铂便宜得多的金属，以节省材料成本。这项研究的成果可能包括通过多学科和协作研究加强研究生和本科生教育；深入了解金属纳米晶形成过程中涉及的非均相成核和生长机制；以及开发一种新型的ORR催化剂，与目前的商业催化剂相比，具有更高的性能(活性和耐用性)和成本比。

In this research project, Dr. Xia of the Georgia Institute of Technology and Dr. Mavrikakis of the University of Wisconsin-Madison are supported by the Macromolecular, Supramolecular and Nanochemistry (MSN) Program to develop platinum-based catalysts with significantly enhanced mass activity toward the oxygen reduction reaction (ORR) critical to the operation of a polymer electrolyte fuel cell. As a clean-energy technology, polymer electrolyte fuel cells are attractive for applications that include on-site power generation and use as a portable power source for transportation vehicles and electronic devices. However, it has been challenging to market this technology on a large scale due to the high cost associated with the platinum catalysts deposited on the cathodes for mitigating the sluggish kinetics of the oxygen reduction reaction (ORR). A reduction of roughly four-fold in platinum loading is needed in order to meet the cost requirement for the large-scale commercialization of this technology. Drs. Xia and Mavrikakis are investigating a new strategy that integrates chemical synthesis and computational modeling for maximizing the mass specific activity of a platinum-based catalyst toward ORR. They are particularly interested in depositing platinum as ultrathin skins of only a few atomic layers thick on palladium nanocrystals. They develop ORR catalysts with enhanced activity by controlling the type of facet (and thus, the arrangement of atoms on the surface) on the palladium (Pd) templates, optimizing the thickness of the platinum (Pt) skin, and maximizing the electronic coupling between the palladium atoms in the core and the platinum atoms in the shell. In addition to the scientific and technological merits, this work helps forge links between different disciplines that include materials chemistry, catalysis, surface science, computational chemistry, colloidal science, and energy technology. It also impacts on our society because it develops novel materials for both fuel cells and catalysis that play a role in energy conversion and environmental protection. The researchers promote diversity in higher education by engaging women, minorities, and other underrepresented groups into this project. By substantially reducing Pt and Pd loadings in catalytic devices, this work helps society achieve a sustainable use for platinum, one of the rarest precious metals that exists in the Earth's crust.In the new catalysts for polymer electrolyte fuel cells, platinum atoms are deposited as conformal shells on the surfaces of palladium nanocrystals pre-synthesized with a uniform size and well-controlled facets. The shell thickness is precisely tuned from one to five atomic layers. Four types of palladium nanocrystals are investigated, including cubes, octahedra, rhombic dodecahedra, and concave cubes, with each one of their surfaces covered by a single type of facet: {100}, {111}, {110}, and high-index ones, respectively. This research involves a unique combination of three approaches: modification of the electronic structure of the platinum surface (and thus, the binding energies of oxygen-containing species) by coupling with the palladium core; development of the most active surface structure by controlling the facet on the palladium core; and replacement of the platinum atoms in the bulk with palladium, a much less expensive metal relative to platinum, to save the materials cost. The outcomes of this research may include enhancement of both graduate and undergraduate education through multidisciplinary and collaborative research; a deep understanding of the heterogeneous nucleation and growth mechanisms involved in the formation of metal nanocrystals; and the development of a novel class of ORR catalysts with an improved performance (activity and durability) to cost ratio when benchmarked against the current commercial catalyst.