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In Optimizing Hydrogen Storage, Larger Pores Not Necessarily Better

A ball-and-stick view of one of the compounds. Click to enlarge. Source: Angewandte Chemie

One of the promising types of candidate materials for hydrogen storage is a class of substances known as metal organic frameworks (MOFs): three-dimensional crystalline networks held together by multiple bonds between charged metal ions and carbon-based ligands. (Earlier post.)

Researchers at the University of Nottingham in the UK report the development of a new coordination framework material that could store up to 6.07 wt.% hydrogen at 20 bar—close to the US Department of Energy’s 2010 target of 6.5 wt.% for on-board hydrogen storage for fuel-cell vehicles.

In studying the MOFs for hydrogen storage, the scientists found that larger pores do not necessarily store the most hydrogen fuel.

Comparing the amount of hydrogen that could be stored in three MOFs made of the identical material (copper combined with molecular chains of benzene rings, each carrying four carboxylic acid groups) but with different pore sizes, the researchers found that the middle-sized pores held the highest density of hydrogen. The research is published in the journal Angewandte Chemie, and featured in Nature and Chemistry World.

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.

—Professor Martin Schröder

The researchers conclude that there is an optimum pore size for any given material.

The materials currently require low temperatures to achieve the high loading of hydrogen—a limitation to application in vehicles. The researchers are working on improving this aspect of the materials.

Earlier this year, chemists at UCLA and the University of Michigan achieved hydrogen storage concentrations of up to 7.5 wt% in metal organic framework (MOF) material. The storage was achieved at a low temperature of 77 Kelvin (-196º C or -321º F). (Earlier post.)



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