Storing and sorting hydrogen with porous materials
Hydrogen produces no carbon dioxide when it is used as a fuel, which makes it a promising option for clean energy. The catch is that it is a very light gas that takes up a lot of space. Storing it usually means very high-pressure tanks or extreme cold.
Holding hydrogen in a sponge
Porous materials offer another option. Their internal surfaces can hold hydrogen molecules, packing in more gas at lower pressures. We work with metal-organic frameworks (MOFs), which are crystals full of molecule-sized pores.
MOFs are usually made using large amounts of solvent and long heating times. We tried a simpler method: ball milling, which makes the material by grinding the ingredients together in a mill. Using a statistical approach to find the best conditions, we made a copper-based MOF that stored 3.2% of its weight in hydrogen at 20 bar and −196 °C (77 K). That was close to the conventionally made material and about 10% above earlier studies.
The difference was in the making. Production took minutes and used less than 2% of the energy of the conventional method.
Molecular trapdoors
Hydrogen also has a heavier form called deuterium, which is hard to separate because it behaves so similarly. We studied zeolites, porous minerals whose pores are guarded by metal atoms that act like tiny trapdoors.
At −196 °C, these materials took up about twice as much deuterium as ordinary hydrogen. Deuterium also moved through them more slowly. That difference suggests a possible way of separating the two.
Why it matters
Cheaper and cleaner storage materials could support a future hydrogen economy, and better separation could make hydrogen isotopes easier to produce. Both areas need further development before they can be used at scale.
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