Catalyst Brings Cheap Hydrogen Fuel Closer to Reality

by Robert F. Service

Fuel maker. A new catalyst generates hydrogen gas from water, even seawater.

Credit: Adapted from Karunadasa et al., nature, 464 (29 April 2010)

Hydrogen sure seems like the perfect alternative to fossil fuels. Just zap water with a bit of energy to split it into hydrogen and oxygen, and presto, you’ve wound up with a gas that can be used to power the planet—and that emits no CO₂. Ah, were it only so simple. Conventional water-splitting catalysts that make hydrogen gas are either too expensive, too frail, or too finicky to work in water alone.

Now, however, researchers report that they’ve created a new molybdenum-based catalyst that cranks out the hydrogen, is cheap to make, works in water, and is robust. The catalyst isn’t perfect, as it requires too much energy to generate hydrogen. But its unusual character offers chemists a valuable new lead for making and improving water-splitting catalysts.

Generating hydrogen gas (H₂) isn’t difficult. Platinum is adept at transferring pairs of electrons to pairs of protons to make H₂. But the precious metal is far too expensive to use for commercial hydrogen production. A cheaper model would use bulky microbial enzymes called hydrogenases that make H₂ using proteins based on nickel and iron. Researchers have made slimmer versions for years, but most either work too slowly, work only with the addition of organic acids and other additives, or quickly fall apart.

None of this was initially on the mind of Jeffrey Long, a chemist at the University of California, Berkeley. He and his colleagues had been working to combine metal atoms with organic appendages called PY5 groups in an effort to make molecules with the magnetic behavior of bulk magnets. Their studies revealed that one of their molybdenum complexes had an unusual ability to transfer electrons, a key requirement for hydrogen generation. So they tested its hydrogen-generating abilities and got several nice surprises.

In tomorrow’s issue of Nature, Long and colleagues report that not only did the compound turn out significant amounts of hydrogen, it also worked in pure water or seawater without the additional expense of additives. It’s also more durable, Long says, because the molybdenum atom in each compound is bonded to five other atoms, making it harder to knock apart and thus more stable than competing hydrogen-generating compounds using iron and nickel that form fewer links to their neighbors.

“It’s pretty noteworthy,” says Thomas Rauchfuss, a chemist who designs hydrogenase mimics at the University of Illinois, Urbana. “This thing has a lot of the attributes people are looking for,” he adds. Just not quite all of them yet. The catalyst still needs to work faster, for example, as it can’t match the pace of natural hydrogenases, and it requires a fairly high electric voltage to operate, an energetic penalty.

Long says that he and his colleagues are now tweaking their PY5 groups and trying different metals to see if this improves matters. They are also looking to couple it with solar power technology in order to provide a carbon-free source of energetic electrons that the catalyst needs to operate.