Carbon-based composite materials show promise in electrocatalytic applications for the production of low-cost, high-efficiency fuel cells. Itaru Honma at the Japanese National Institute of Advanced Science and Technology (AIST) and colleagues1 have now demonstrated catalytic composites formed from clusters of platinum atoms stabilized on sheets of graphene. This material shows surprisingly high catalytic activity and low deactivation rate for the oxidization of methanol in comparison with standard platinum/carbon-black catalysts.

Graphene is a carbon mono-layer structure in which the atoms are arranged in a honeycomb pattern. This form of carbon has attracted considerable attention in many fields for its unique electronic and mechanical properties.

Fig. 1: Transmission electron microscopy image of sub-nanometer platinum clusters on graphene (2 wt% platinum).

The researchers prepared their graphene/platinum composites by a simple route in which a platinum precursor salt was mixed with graphene nanosheet powder in solution. After drying, their composites contained 20% by weight of platinum. Transmission electron microscopy (Fig. 1) revealed the platinum to be finely dispersed on the graphene sheets, with many clusters being less than 0.5 nm in diameter, and the largest being just 2 nm wide.

The composite material showed increased activity for oxidizing methanol compared to platinum on carbon black — the conventional catalyst used in fuel cells. The new material was also 'poisoned' much more slowly by carbon monoxide than the comparable carbon-black-supported catalyst. The researchers say that the increased activity was due to the small size of the clusters; the clusters on carbon black are generally 2 nm in size and larger.

“We synthesized platinum clusters less than 1 nm in size that were thermally stable with high electrocatalytic activities,” says Honma. “This kind of innovative electrode can only be formed by using graphene to stabilize the sub-nanometer-size platinum clusters.”

The new catalyst has useful applications. “The amount of platinum needed for fuel cells can be reduced enormously, and applications to other catalytic systems is also promising,” says Honma.