Bigger is not necessarily better in hydrogen storage
FUEL CELLS, which run on hydrogen and oxygen, are a potentially environmentally friendly way to power vehicles, producing only water as a waste product.
The possible solution
One of the problems with hydrogen is safe storage 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.
University of Nottingham scientists have made a breakthrough, which could help in the development of the next generation of environment-friendly cars.
In research published in the journal Angewandte Chemie, and featured in Nature and Chemistry World, they studied materials that have a porous sponge-like structure in which to store hydrogen — and found that bigger is not necessarily better.
Bigger pores, they found, don't necessarily store the most hydrogen fuel, according to a University of Nottingham press release.
Professor Martin Schröder and his colleagues have been investigating so-called metal organic frameworks (MOFs) — molecular scaffolding filled with tiny cylindrical pores that hydrogen gas can be forced into.
Makes intuitive sense
Professor Schröder said: "The idea up to this point has been to increase the pore volume, so as to fit in more gas." 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 now the painstaking University of Nottingham study has quantified the amount of hydrogen that can be put into three MOFs made of identical material but with different pore sizes.
Surprisingly, the study showed that the middle-sized pores could hold the highest density of hydrogen.
Professor Schröder added: "In a very small tube, the hydrogen gas molecules all see the wall and interact with it. 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."
Optimum pore size
The researchers conclude that there is an optimum pore size for any given material. Schröder's team shows that their frameworks comply with the US Department of Energy (DoE)'s volume-density target of 45 gm per litre.
In fact they have achieved the highest percentage hydrogen uptake of any such material thus far reported. — Our Bureau
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