Bottom: Platinum atoms attached to layers of lithium cobalt oxide contract when electricity is applied, boosting platinum catalytic efficiency by 90 percent. Top: Removing electrons separates the atoms and lowers efficiency by 40 percent. (1 Ångstrom = 0.1 nanometer) (Image credit: Haotian Wang)
A nanosize squeeze can significantly boost the performance of platinum catalysts that help generate energy in fuel cells, according to a new study by Stanford scientists. The team bonded a platinum catalyst to a thin material that expands and contracts as electrons move in and out, and found that squeezing the platinum a fraction of a nanometer nearly doubled its catalytic activity.
“In this study, we present a new way to fine-tune metal catalysts at the atomic scale,” said Haotian Wang, a former graduate student at Stanford now at Harvard University. “We found that ordinary battery materials can be used to control the activity of platinum and possibly for many other metal catalysts.” The new technique can be applied to a wide range of clean technologies including fuel cells that use platinum catalysts to generate energy, and platinum electrolyzers that split water into O2 and H fuel.
“Our tuning technique could make fuel cells more energy efficient and increase their power output,” said Prof. Yi Cui. “The electronic structure of a catalyst needs to match the molecule of interest in order to achieve the chemical reaction you want,” Wang explained. “You can adjust the electronic structure of a catalyst by compressing the atoms or pulling them apart.” The Stanford team introduced a novel way to compress or separate the atoms by 5 %, a mere 0.01 nanometer.
Haotian Wang and his Stanford colleagues developed a new way to fine-tune platinum catalysts in fuel cells and other clean-energy technologies. Credit: L.A. Cicero/Stanford News Service
The study focused on lithium cobalt oxide, a material widely used in batteries for cellphones and other electronic devices. The researchers stacked several layers of lithium cobalt oxide together to form a battery-like electrode. “Applying electricity removes lithium ions from the electrode, causing it to expand by 0.01 nanometer,” Cui said. “When lithium is reinserted during the discharge phase, the electrode contracts to its original size.”
For the experiment, the Stanford team added several layers of platinum to the lithium cobalt oxide electrode. “Because platinum is bonded to the edge, it expands with the rest of the electrode when electricity is added and contracts during discharge,” Cui said. Separating the platinum layers a distance of 0.01 nm had a significant impact on performance. “We found that compression makes platinum much more active,” he said. “We observed a 90% enhancement in the ability of platinum to reduce oxygen in water. This could improve the efficiency of hydrogen fuel cells.” Stretching the electrode by 5% had the opposite effect, suppressing O2 production by 40%, Wang said.
“Our technology offers a very powerful way to controllably tune catalytic behavior,” Cui added. “Now, mediocre catalysts can become good, and good catalysts can become excellent.” http://news.stanford.edu/2016/11/24/platinum-catalysts-tiny-squeeze-gives-big-boost-performance/
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