Middle-sized holes best for storing hydrogen
Painstaking study paves the way for gas-holding materials.
Bigger isn't always better. That's the message from a team of scientists trying to squeeze as much hydrogen as possible into a sponge-like material: bigger pores, they find, don't store the most hydrogen fuel.
The work gives a theoretical boost to attempts to cram hydrogen into a small space so that it can practically be used as fuel. Fuel cells, which run on hydrogen and oxygen, are a potentially environmentally friendly way to power vehicles, producing only water as a waste product.
But hydrogen fuel needs to overcome a number of stumbling blocks before it can replace our oil-based economy. Not the least of these is how to safely store enough hydrogen fuel for cars to cover a reasonable distance before their supplies must be replenished.
One possible solution is to pack hydrogen into porous materials, which soak up the gas like a sponge. Martin Schrder and his colleagues at the University of Nottingham, UK, have been investigating so-called metal organic frameworks (MOFs) molecular scaffolding filled with tiny cylindrical pores that hydrogen gas can be forced into.
"The idea up to this point has been to increase the pore volume, so as to fit in more gas," says Schrder. That makes intuitive sense: the bigger the cylinders, the more their capacity, and the greater the inside surface area available for hydrogen to attach to. But it has been hard to prove. Actually measuring the amount of hydrogen, as opposed to air or water, in the matrix is surprisingly difficult.
Now a painstaking study has quantified the amount of hydrogen that can be stuffed into three MOFs made of identical material but with different pore sizes holes of 6.5, 7.3 and 8.3 angstroms in diameter. The middle-sized pores could hold the highest density of hydrogen, Schrder's team reports in Angewante Chemie1: 43.6 grams of hydrogen per litre. That's 4.7 grams per litre better than the little holes, and 2.5 better than the large ones.
Pack it in
"In a very small tube, the hydrogen gas molecules all see the wall and interact strongly with it," Schrder explains. "But in a larger tube, the molecules see less of the wall and more of each other: that interaction is weaker, so they don't pack together as closely." The researchers conclude that there is an optimum pore size for any given material.
The US Department of Energy (DOE) has set a series of advisable targets that a hydrogen-fuelled vehicle should meet in order to be economically viable: by 2010, the storage system's capacity will need to be greater then 6% hydrogen by weight, for example. Schrder's team shows that their frameworks reach this requirement, and come close to the DOE's volume-density target of 45 grams per litre. That's impressive, says Matt Rosseinsky, a chemist at the University of Liverpool, UK.
But the way isn't yet clear for these materials to win the competition for best hydrogen-storage technique. Several strategies are being investigated, each with their own advantages and disadvantages and the competition is fierce.
Current storage methods include freezing the hydrogen to store it as a liquid, or compressing it in a tank at high pressure, both of which take a lot of energy, and may not be particularly safe for the general public if light-weight cars carry them.
The alternatives include porous materials, including nanotubes or MOFs such as those Schrder is investigating. The problem with MOFs is that they store hydrogen best at exceedingly low temperatures (77 K); not ideal for commercial cars.
Another option is 'metal hydrides', in which hydrogen is taken up by very strong chemical bonds into the structure of metals such as lithium, sodium and magnesium. "In that case, it's hard to get the hydrogen out again when you need it," says Martin Jones, a researcher in hydrogen storage at the University of Oxford, UK.
So far, summarizes Mark Thomas, a chemist at the University of Newcastle, UK, it's impossible to pinpoint the best way of storing hydrogen, given the competing variables of capacity, pressure, temperature, cost and safety. "The jury's still out," he says. But Schrder is upbeat about his results: "MOFs really are a viable alternative technology to other materials currently being investigated for hydrogen storage."
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- Schrder M., et al. Angew. Chem. Intl Edn, doi: 10.1002/anie.200601991 (2006).