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

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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.