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Battery electrode material boosts platinum catalyst’s performance with a light squeeze

By Prachi Patel December 14, 2016

Researchers have come up with a novel way to compress a platinum catalyst by a fraction of a nanometer to boost its performance. By coating platinum on a battery electrode material that expands and contracts, they are able to squeeze platinum crystals by 0.01 nm, which nearly doubles its catalytic activity.

The technique could be used to design next-generation catalysts. It could also make fuel cells more energy efficient, productive, and affordable. Fuel cells convert the chemical energy in a fuel to electrical energy directly through a chemical reaction. “Platinum is demonstrated to be the best catalyst for fuel cells,” says Haotian Wang, an applied physicist at Harvard University who conducted the work while at Stanford University. “But platinum is very expensive and we need to increase its activity per mass which can reduce the amount we need to use.”

Scientists have known for years that applying strain to a catalyst strongly affects its activity. “The distance between atoms shifts when you expand or compress the catalyst material,” Wang explains. This changes the electronic structure on the surface, affecting how the catalyst binds with chemical reactants.

One way to create such a strain is to make nanoparticles with copper cores and platinum shells. Because the crystal lattice of copper is smaller than that of platinum, it has a squeezing effect on the platinum. “But the problem is you cannot rule out the alloying effect, electron transfer between the metals, which also affects electronic structure,” he says.

So Wang, Stanford materials science and engineering professor Yi Cui, and their colleagues came up with a new way to squeeze the Pt catalyst. They deposited 5-nm-diameter platinum particles on a lithium cobalt oxide substrate. LCO, a common electrode material for lithium-ion batteries, contains lithium ions sandwiched between cobalt-oxygen slabs. As the lithium ions move in and out of the material during discharging and charging, respectively, the material expands and contracts.

Using transmission electron microscopy, the researchers saw that the volume changes of the LCO pull and squeeze the platinum by 5%, which translates to a distance of 0.01 nm. When the catalyst is compressed, its oxygen reduction capacity increases by 90%, and when it is pulled apart that capacity decreases by 40%. Wang says that the researchers now plan to pursue the technique with other catalysts.

Battery electrode material boosts platinum catalyst’s performance with a light squeeze
Platinum atoms attached to layers of lithium cobalt oxide contract when electricity is applied (bottom), decreasing the layer spacing by 1.3 ‎Å, and boosting platinum catalytic efficiency by 90%. Removing electrons separates the atoms (top) and lowers efficiency by 40%. Credit: Haotian Wang

Using battery electrode materials for controlling the strain in catalytic metal nanoparticles is novel, says Manos Mavrikakis, a professor of chemical and biological engineering at the University of Wisconsin-Madison. “Isolating and controlling the effect of strain in metal nanoparticles has been a challenge,” he says. “One can easily envision that using battery materials for controlling strain in catalytic nanoparticles, and through the development of a computationally derived materials database on how strain affects the activity of catalytic reactions, we can eventually get to the realm of strain-engineered catalytic materials with fine-tuned electronic structure and reactivity.”

“The major advantage is that the strain is precisely tunable and reproducible,” adds Younan Xia, a professor of biomedical engineering and chemistry at the Georgia Institute of Technology.

Read the abstract in Science.