by Robert F. Service
Battery-powered cars may be on the cusp of the mainstream auto market, but scientists and car makers still have high hopes for hydrogen fuel cell vehicles, which should refuel faster and travel longer distances between fill-ups. Hydrogen fuel cells have their own Achilles’ heel, however: They are easily poisoned by carbon monoxide (CO). Now, researchers report that they’ve created novel catalysts for fuel cell cars that strongly resist carbon monoxide contamination, potentially solving a problem that has vexed the industry for years.
Fuel cells generate electricity by combining hydrogen gas with oxygen to produce water. Although that sounds perfectly clean and green, that hydrogen is typically generated by “reforming” fuels such as natural gas, gasoline, or ethanol, which invariably introduces carbon monoxide into the hydrogen gas. If even minute quantities of carbon monoxide are present in that gas, it can poison the platinum catalysts that are key to driving the fuel cell. (In the heart of a fuel cell, CO binds tightly to platinum and prevents it from grabbing hydrogen, the first step in the reaction.) Recently, researchers at Brookhaven National Lab (BNL) in Upton, New York, and elsewhere have found that a catalyst made from platinum and ruthenium helps block CO adsorption. But because these metals are rare and expensive, scientists have continued to search for alternatives.
Chemists Héctor Abruña and Francis DiSalvo of Cornell University and their colleagues took a look at tungsten. Tungsten alloys have long been known to resist CO poisoning, but straight tungsten oxide is a poor electrical conductor, making it a bad choice for a fuel cell electrode. So the researchers added a bit of tungsten spice to titanium dioxide nanoparticles, which are good electrical conductors. They wound up with titanium tungsten oxide nanoparticles, which they then coated with a thin layer of platinum and used to fashion an electrode.
In a paper posted online this week in the Journal of the American Chemical Society, the Cornell team reports that when its new nanoparticle catalysts carried out their job with hydrogen spiked with 2% CO, their performance dropped only 5% compared with a 30% drop for commercial catalysts. Although Abruña says it’s not clear why the new catalyst is better, he suspects that during the reaction, hydroxide groups (OH–) bind to the titanium tungsten oxide close to the platinum, putting them in close proximity to react with incoming CO molecules to form CO2. But Abruña stresses that plenty of additional tests are needed to confirm this.
In any case, “it’s a nice result,” says Radoslav Adzic, a chemist at BNL who helped pioneer the platinum-ruthenium catalysts. If it succeeds in long-term tests and proves economical, he says that the catalyst could reignite interest in reforming liquid fuels, such as gasoline and ethanol, onboard cars to make the hydrogen needed to power fuel cells. And because liquid fuels pack far more energy than the same volume of gaseous hydrogen, that could allow fuel cell cars to have a longer range than even today’s gas burners.
