| Posted: Oct 04, 2017 |
Adding stress boosts performance, stability for fuel cells
(Nanowerk News) A team from the Center for Functional Nanomaterials and universities in China and California designed a new fuel cell catalyst (Science, "Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis").
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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.
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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.
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| A bimetallic nanoplate catalyst (right) with a platinum-lead (Pt-Pb) core and a thin platinum shell (left). This structure enhances the important fuel cell oxygen reduction reaction (that takes oxygen, O2, to water, H2O) through strain developed in the catalysts because of their shape and composition. (Image: Center for Functional Nanomaterials, Brookhaven National Laboratory)
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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.
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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.
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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.
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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.
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High-resolution electron microscopy studies show that a bi-axial strain in the (110) plane of the platinum shell is key to high performance.
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