In the study, which was published in Nano Letters, the researchers from the Georgia Institute of Technology described a possible new way to overcome sintering, a key cause degradation in platinum catalysts in which particles of platinum migrate and clump together. This reduces the specific surface area of the platinum and causes catalytic activity to drop.
To reduce sintering, the researchers devised a method to anchor the platinum particles to their carbon support material using selenium.
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"There are strategies out there to mitigate sintering, such as using platinum particles that are uniform in size to reduce chemical instability among them," said Zhengming Cao, a visiting graduate student at Georgia Tech. "This new method using selenium results in a strong metal-support interaction between platinum and the carbon support material and thus remarkably enhanced durability. At the same time, the platinum particles can be used and kept at a small to attain high catalytic activity from the increased specific surface area."
According to Georgia Tech, the process starts by loading nanoscale spheres of selenium onto the surface of a commercial carbon support. The selenium is then melted under high temperatures so that it spreads and uniformly covers the surface of the carbon. Then, the selenium is reacted with a salt precursor to platinum to generate particles of platinum smaller than 2nm in diameter and evenly distributed across the carbon surface.
The covalent interaction between the selenium and platinum provides a link to stably anchor the platinum particles to the carbon.
"The resulting catalyst system was remarkable both for its high activity as a catalyst as well as its durability," said Younan Xia, professor and Brock Family Chair in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University.
Because of the increased specific surface area of the nanoscale platinum, the new catalytic system initially showed catalytic activity three and a half times higher than the pristine value of a commercial platinum-carbon catalyst. Then, the research team tested the catalytic system using an accelerated durability test. Even after 20,000 cycles of electropotential sweeping, the new system still provided a catalytic activity more than three times that of the commercial system.
The researchers used transmission electron microscopy at different stages of the durability test to examine why catalytic activity remained so high. They found that the selenium anchors were effective in keeping most of the platinum particles in place.
"After 20,000 cycles, most of the particles remained on the carbon support without detachment or aggregation," Cao said. "We believe this type of catalytic system holds great potential as a scalable way to increase the durability and activity of platinum catalysts and eventually improve the feasibility of using fuel cells for a wider range of applications."
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