Platinum normally plays a crucial role in fuel cells and in the production of hydrogen. Now a group of researchers has shown how to get the same kind of reactivity using a metal–nickel–that is a thousand times less expensive than platinum. The group– research scientist Vincent Artero, and Alan Le Goff and Serge Palacin at the Commissariat à l’Énergie Atomique near Paris–used nickel-based compounds chemically bound to carbon nanotubes.
Platinum is typically used in the water-splitting process because it’s such an effective catalyst. “The problem with platinum is that it’s a very expensive metal and there is not enough of it on Earth to sustain a worldwide hydrogen economy,” says Artero.
Electrodes made using the new catalyst would be about 20 percent cheaper than those made of platinum, Artero says. Given that the platinum makes up roughly a third of the cost of fuel cells, this could have a significant impact on the price of fuel cell technology.
The new compounds are based on a type of enzyme called hydrogenase. Normally found in bacteria and algae that live in anaerobic (or oxygen-free) conditions, these enzymes are used by these organisms as a catalyst to metabolize hydrogen, says Artero. “They use exactly the same process as fuel cells to stay alive,” he says.
In recent years, researchers have shown great interest in using molecular chemistry to try to mimic the structure of these natural catalysts. Because the active components of these molecular compounds are as reactive as platinum but consist of nickel or iron instead, they promise to be much cheaper to use.
However, until now, synthetic hydrogenese molecules–such as those developed by Daniel DuBois at Pacific Northern National Laboratories in Richland, WA–have only been demonstrated in solution form. To be of practical use, the molecules need to be bound to an electrode, rather than floating in a liquid.
By modifying the nickel-based active components of these compounds, Artero and colleagues found a way to attach the molecules to carbon nanotubes. “The nanotubes have two advantages–they are very good electron conductors, and they have a very high specific surface area,” Artero says. This means it’s possible to load a great deal of the catalytic material onto its surface, he says.
In tests reported in this week’s issue of the journal Science, the group showed that this modified catalyst was effective and stable for performing the reaction repeatedly.
“This work represents a significant advance in the application of molecular electrocatalysts for hydrogen production and oxidation,” says DuBois. It shows, he says, that these highly reactive molecular catalysts can operate efficiently under conditions that may be practical for electrolyzers and fuels cells. “This is an important step toward moving bio-inspired catalysts from conception to practice.”
Nate Lewis, a professor of chemistry at Caltech, agrees. “This is an important step toward the development of a full system that splits water from sunlight,” he says. But Lewis notes that finding a way to attach the catalysts to a surface so that they can be used in an electrode is just one piece of the puzzle.
John Turner, a research fellow in energy sciences at the National Renewable Energy Laboratory in Golden, CO adds that, “the major barrier for hydrogen production from water and fuel cells for transportation is not hydrogen catalysis, but oxygen catalysis.”
Nickel-based catalysts are already used in large multi-megawatt commercial electrolyzers, but these catalysts are much less efficient than platinum ones and tend to be very large–typically around 10 square meters.
Turner notes that the current produced by Artero’s catalyst is still orders of magnitude less than can be achieved with platinum. Artero says this can be fairly easily remedied. He notes that the nanotubes used in his team’s experiments received only a low loading of the catalytic material. Increasing this should boost the current: “It’s a gap that we can fill,” he says.