3 February 2012
Fuel-Cell Catalysis: A single layer of bimetallic nanoparticles on graphene speeds oxygen reduction
A new process for assembling iron-platinum nanoparticles on graphene could lead to robust, effective catalysts for fuel cells (J. Am. Chem. Soc., DOI: 10.1021/ja2104334). The resulting materials made by Brown University chemist Shouheng Sun display up to nine times higher catalytic activity in laboratory tests, and are more stable, than commercial platinum catalysts.
Cathodes in polymer-electrolyte-membrane fuel cells, the type of fuel cells being studied to power cars, are made of amorphous carbon sheets embedded with platinum nanoparticles. The nanoparticles speed up the reduction of oxygen to water. “The problem is that gradually the carbon begins to degrade and then the catalyst begins to move around and lose activity,” says Jun Liu, a materials scientist at Pacific Northwest National Laboratory, who was not involved in the new work.
Replacing amorphous carbon with graphene—a one-atom-thick sheet of carbon—is an approach researchers have been experimenting with to stabilize the catalyst. Graphene is mechanically robust and its flat, crystalline structure bonds strongly with the platinum nanoparticles, keeping them in one spot, explains Sun. He and others have also been trying to find nanoparticle alloys of platinum that would cost less and last longer than the pure metal.
“The key for catalysis is you want the nanoparticles to be uniform, with the same size and shape,” Sun says. However, when researchers have used the common technique of growing nanoparticles directly on graphene surfaces using chemical vapor deposition, they had little control over the particle structure. Others have tried to deposit solutions of nanoparticles on graphene and then remove the solvent, but this method resulted in multiple layers of nanoparticles. Creating a monolayer of nanoparticles should enhance catalysis, Sun hypothesized, by making the surface of every particle available for the chemical reaction.
Sun and his graduate student Shaojun Guo found that the trick was to make separate nanoparticle and graphene solutions in solvents that do not normally mix, and then force them to mingle by exposing them to ultrasound waves. Sun and Guo disperse graphene in a polar solvent, dimethylformamide, and surfactant-coated iron-platinum nanoparticles in a nonpolar solvent, hexane. The surfactant keeps the particles from clumping together, and they assemble on the graphene surface. Heating the material to 100 °C bonds the nanoparticles to graphene.
The researchers confirmed using electron microscopy that they had created a single layer of nanoparticles. Compared to commercial platinum catalysts on amorphous carbon, the material shows 5.9 to 8.8 times higher current density, a measure of catalytic activity, in an oxygen reduction reaction. The material is also durable: Its activity does not decrease after the reaction runs 10,000 times.
“This is good work,” Liu comments. “The challenge will now be to demonstrate the improvement in more realistic testing conditions and devices, and to scale up the process for mass manufacture.” He calls the use of an alloy of iron and platinum “unique” and says the nanoparticles may prove more cheaper and more stable than all-platinum particles
Sun and Guo are now trying to understand the mechanism for the nanoparticles’ self-assembly and to make other catalysts using various metal nanoparticles.