Scientists design outstanding catalysts by controlling the composition and shape of these tiny plate-like structures on the nanoscale.
A team from the Center for Functional Nanomaterials and universities in China and California designed a new fuel cell catalyst. The catalyst is a platinum-lead core wrapped in a platinum shell and shaped like a tiny hexagonal plate. The catalyst’s shape and composition dramatically enhance the oxygen evolution reaction. The reaction is vital for fuel cells. The catalyst’s design also provides stability during operation. Detailed studies show that tensile strain — stress built into the catalyst by its shape — is key to high performance.
These new catalysts exceed the 2020 US Department of Energy performance targets by 10 times and thus could produce fuel cells with higher power and greatly extended number of cycles. The results point toward a new strategy of using strain, specifically bi-axial strain, for enhancing catalysts in fuel cells.
Electric vehicles powered by proton exchange membrane fuel cells, which use hydrogen gas as fuel and produce water as exhaust, have entered the U.S. vehicle market in the past year. These vehicles operate with higher efficiency than gasoline-powered automobiles and have zero air pollution. A number of challenges impede further growth of fuel-cell powered vehicles, including the high cost of platinum electrocatalysts used to drive the oxygen reduction reaction, which produces energy inside the cell.
Designing materials having compressive surface strains creates catalysts to enhance the oxygen reduction reaction. Typically, such surface strain is induced in a core/shell catalyst design, where a metallic core (e.g., nickel, cobalt, iron) is surrounded by a platinum shell. This work reports on a class of catalysts — platinum-lead (Pt-Pb) cores surrounded by a platinum shell. The catalyst is shaped like plates that exhibit large tensile strains. The stable Pt(110) facets enhance the nanoplate catalytic mass activity for the oxygen reduction reaction, reaching nearly 10 times higher than the 2020 DOE target for fuel cell performance of 0.44 A/mg. These materials are among the most efficient bimetallic catalysts ever reported. The intermetallic core and uniform four layers of the Pt shell of the PtPb/Pt nanoplates render these catalysts highly stable — they can undergo 50,000 voltage cycles with negligible decay in catalytic performance, and no apparent changes in structure or chemical composition. High-resolution electron microscopy studies show that a bi-axial strain in the (110) plane of the platinum shell is key to high performance.
Center for Functional Nanomaterials, Brookhaven Laboratory
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This work was financially supported by the National Key Research and Development Program of China (contract 2016YFB0100201), the National Natural Science Foundation of China (contracts 21571135 and 51671003), the Ministry of Science and Technology (contract 2016YFA0204100), start-up funding from Soochow University and Peking University, and the Young Thousand Talented Program and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Part of electron microscopy work was performed at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under contract DE-SC0012704. The work at California State University Northridge was supported by the U.S. Army Research Office via the Multidisciplinary University Research Initiative grant W911NF-11-1-0353.
L. Bu, N. Zhang, S. Guo, X. Zhang, J. Li, J. Yao, T. Wu, G. Lu, J.Y. Ma, D. Su, and X. Huang “Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis [External link] .” Science 354, 1610 (2016). [DOI: 10.1126/science.aah6133]