A team led by Peidong Yang and Vojislav Stamenkovic made the catalysts by transforming solid polyhedral bimetallic nanoparticles of platinum and nickel into hollow nanoframes. The researchers synthesized the nanoparticles in the reagent oleylamine and then soaked the particles in a solvent, such as a hexane or chloroform, for either two weeks at room temperature or for 12 hours at 120 °C. “The solvent, which naturally contains dissolved oxygen, erodes the interior of the polyhedral structures and produces a hollow dodecahedron nanoframe,” explains Yang. Annealing these dodecahedron frames in argon gas then creates a catalytic platinum skin on the structures’ surfaces.

When encapsulated in an ionic liquid, the platinum-nickel nanoframes are more than 30 times better as catalysts – mass for mass – than platinum nanoparticles dispersed on carbon for the oxygen reduction reaction (which occurs in fuel cells), says Stamenkovic. They show a 22-fold enhancement in specific catalytic activity compared with these nanoparticles too.

“We also tested the nanoframe electrocatalysts, modified with electrochemically deposited nickel hydroxide, for the hydrogen evolution reaction (another important catalytic process, which takes place in electrolyzers) and found that they were an order of magnitude better than their conventional platinum-carbon counterparts,” he told nanotechweb.org. Our catalysts are superior thanks to their unique hollow nanoframe structure with 3D platinum-rich surfaces that can easily be accessed for catalytic reactions, he explains. They are also cheaper because much less precious metal is needed overall.

Although researchers have reported on making efficient catalysts from nanosegregated platinum skins on bulk single-crystal platinum-nickel alloys before now, these structures could not be integrated into electrochemical devices very easily – unlike the new dodecahedron nanoframes. And that is not all: in contrast with other processes for making hollow nanostructures that involve aggressive corrosive and oxidizing agents, the new synthesis technique can be performed in air and at room temperature, says Yang.

“Indeed, our technique for making these hollow nanoframes is a new but general concept in nanocatalyst design that might be readily applied to other multimetallic electrocatalysts or even gas phase ones,” he added. “We are now busy exploring such catalysts in their nanoframe form.”

The work described in this story is detailed in Science DOI: 10.1126/science.1249061.

Further reading

Zinc doping improves nanowire electrodes (Oct 2012)
InGaN nanowires make good photoanodes (Mar 2012)
Nitride nanocrystals for fuel cell catalysts (Dec 2007)