Edible crystals could store hydrogen fuel
Mix together some sugar, a generous dose of alcohol, a dash of salt and a splash of water. It sounds like a recipe for an interesting night, but this mixture could one day be used to make a crystalline material for storing hydrogen in fuel-cell cars.
A simple tank is impractical for storing the amount of hydrogen needed to give a fuel-cell car a decent range: the tank would either need to be chilled to around -250 °C to hold the hydrogen in liquid form, or be enormous if a compressed gas is used.
So researchers have experimented with storing hydrogen inside molecular “cages”, made from chains of carbon, oxygen and hydrogen atoms linked by metal ions. These so-called metal-organic frameworks (MOFs) only bond weakly with the hydrogen atoms they enclose, so the gas can be recovered simply by heating the material slightly.
Until now most MOFs have been made from organic molecules derived from oil such as benzene compounds, which can be toxic or difficult to produce. Now Fraser Stoddart of Northwestern University in Evanston, Illinois, and colleagues have found a way to make MOFs using simple, readily available materials.
Stoddart’s molecular cages are cubical, and each side is made of gamma-cyclodextrin, a ring of glucose molecules. Produced by bacteria, this sugary starch is used as an additive in many foods and pharmaceuticals – making Stoddart’s MOFs edible.
Like crackers
“This is something you can make in your kitchen. You can eat it,” Stoddart says. He hasn’t tasted it himself, but others in his lab say the taste is reminiscent of crackers.
The key to creating the cages is the symmetry of the glucose rings, the team found. In the presence of alcohol and water, they aggregate into cubes held together by ions of potassium or a similar metal.
When the alcohol and water are removed, the resulting material is both stable and spacious, boasting more than 1300 square metres of surface area – equivalent to about six-and-a-half tennis courts – per gram. Roomier MOFs have previously been made, but Stoddart says his is the first made from inexpensive, off-the-shelf ingredients.
Biochemical inspiration
Neil Marsh, a biochemist at the University of Michigan in Ann Arbor, says MOFs made of biological chemicals could have uses beyond fuel storage.
Understanding their structure could enable researchers to engineer better biologically inspired materials, such as enzymes used to produce biofuel. “I think some of the intellectual ideas that come out of MOFs are very appealing in the design of biological systems,” he told New Scientist.
