One-thirtieth of an ounce of a newly developed zinc-oxide crystal has enough surface area to cover an entire football field

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.

“Gasses like hydrogen are really challenging to work with because they can’t be condense into liquids at reasonable temperatures,” said Hupp, who describes the work at “terrific.”

“It illustrates the principle that you can pull CO2 out of the air or store hydrogen.”

Matzger’s next step is to continue to refine his material so the pores are even smaller than they are right now, making the material hold even more hydrogen, carbon dioxide or methane, at lower temperature and pressures. Both Matzger and Hupp say that until materials can store more cost-efficiently however, a hydrogen-based economy is still years away.

By Eric Bland
© 2009 Discovery Channel

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Mudawar discusses a hydrogen-storage system for cars

The current technology consists of a system involving a very fine powder, known as metal hydride. This powder is able to absorb hydrogen very effectively, but, unfortunately, the entire process releases very large amounts of heat. Therefore, having a good cooling system at all refilling terminals is very important.

“The hydride produces an enormous amount of heat. It would take a minimum of 40 minutes to fill the tank without cooling, and that would be entirely impractical,” Purdue University (PU) Professor of Mechanical Engineering Issam Mudawar, who is also the leader of the new research, says.

“The idea is to have a system that fills the tank and at the same time uses accessory connectors that supply coolant to extract the heat. This presented an engineering challenge because we had to figure out how to fill the fuel vessel with hydrogen quickly while also removing the heat efficiently. The problem is, nobody had ever designed this type of heat exchanger before. It’s a whole new animal that we designed from scratch,” he adds. Mudawar is also working with Hydrogen Systems Laboratory (HSL) Manager Timothee Pourpoint, who is also a research assistant professor of aeronautics and astronautics.

As a response to these challenges, the team has created a system where the hydride is contained in small “pockets” inside a pressure chamber, where hydrogen is injected at pressure and gets quickly absorbed. “This process is reversible, meaning the hydrogen gas may be released from the metal hydride by decreasing the pressure in the storage vessel. The heat exchanger is fitted inside the hydrogen storage pressure vessel. Due to space constraints, it is essential that the heat exchanger occupy the least volume to maximize room for hydrogen storage,” Mudawar explains.

Basically, the finished cooling system relies on regular automotive coolant, which circulates inside a U-shaped tube, between the pressure chamber and the aluminum heat exchanger. The intricate construction of the exchanger ensures a smooth temperature absorption when the hydrogen hits the metal hydride. “As newer and better metal hydrides are developed by research teams worldwide, the heat exchanger design will provide a ready solution for the automobile industry,” Darsh Kumar, a researcher at General Motors Corp., underlines.

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