Nanocluster acts as hydrogen super sponge

Material could store hydrogen for cars or pull carbon dioxide from air
By Eric Bland

A crystal riddled with tiny pores has the highest surface area of any material in the world, according to the University of Michigan chemists who created the material, which is detailed in the latest issue of the Journal of the American Chemical Society.

One-thirtieth of an ounce of the zinc-oxide crystal has enough surface area to cover an entire football field. Scientists say this labyrinthine material could eventually store hydrogen for cars or pull planet-warming carbon dioxide out of the air.

"It's a crystalline material like salt, or sugar," said Adam Matzger, the University of Michigan chemist who created the material. "Looking at it you would never know that it is filled with empty space, that it's full of these holes."

Each pore is tiny, only a nanometer or two in size, just large enough for two hydrogen atoms, bonded to each other, to slip into a pore and bounce around like a rubber ball.

The secret is in the bounce. It's not instantaneous. Each time a hydrogen molecule hits the wall the hydrogen sticks for a fraction of a second, the product of what's known as the London dispersion force.

Hydrogen molecules are almost always electrically neutral, with two negatively charged electrons flying around two positively charged protons. Every once in a while, however, both electrons end up on the same side of the molecule, giving the first lightest element a slight electrical charge, just enough for the gas to stick to the wall for a slice of a second. Then the electrons zoom away, and the hydrogen molecule bounces away.

The bond between the hydrogen and the wall doesn't last long, but it's long enough to reduce the pressure inside the container, one of the key stumbling blocks for a hydrogen-based economy.

Today, hydrogen has to be stored at very high pressures, very low temperatures, or a combination of the two, all of which takes a great deal of energy. In fact, it requires much more energy than the hydrogen itself has and that's one reason why hydrogen-based cars aren't feasible today.

The new material makes a hydrogen economy more feasible than it was before, but don't expect to pull up to a hydrogen filling station any time soon. Even with the new material, the hydrogen gas has to be stored at about -195 degrees C.

The good news is that the porous crystals are easy to create. Dumping white zinc salts into an environmentally-friendly solvent and drying the resulting crystals with a vacuum is all it takes to create the pore-filled crystals.

More walls means more chances for the hydrogen to find, and stick to, a wall, which is why scientists have worked to develop materials with higher and higher surface areas.

As expected, the Michigan material hold more hydrogen than any other material. Unexpectedly, the material didn't hold as much as the scientists initially calculated. More walls means more hydrogen, but, it turns out, only up to a point.

"We showed that the material with the highest surface does not necessarily have the highest hydrogen storage," said Matzger.

The new material absorbs other materials, like carbon dioxide or methane, as well. Expanding the size of the pores from a couple of nanometers to a few nanometers lets larger gases like carbon dioxide or methane into the pores and the same London dispersion forces help hold it in place.

Chemistry professor Joseph Hupp of Northwestern University, a "friendly competitor" of Matzger's, says that high surface area material are especially difficult to work with.