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ChemMatCARS Nuggets - Materials Science
New Shape for Metal-Organic Frameworks Offers Better Hydrogen Storage Capacity
Accomplishment
One proposed method to store hydrogen for fuel-cell vehicles applications involves utilizing metal-organic frameworks (MOFs). Now, a new type of MOF exhibits exceptionally high affinity for hydrogen molecules as well as very high hydrogen storage capacity.
The researchers designed a crystal structure whose pores would be of an optimal size for H2. The structure was expected to be a cuboctahedron—a polygon with 8 triangle and 6 square faces. Micro-crystal diffraction at ChemMatCARS confirmed the compound’s complex structure and identified two variants, whose properties were subsequently found to be quite different. The resulting crystals have the formula Cu2(5,5′-methlene-di-isophthalate) (H2O)2·DMA·2H2O, and possess square and triangular faces with edge lengths of 11 Å and 8 Å, respectively.
At 77 K and 1 atm, the more promising structure stored 3 wt% hydrogen, which is the highest reported so far under similar conditions. Its H2 adsorption enthalpy, a measure of how strongly H2 interacts with the framwork, was 12.5 kJ/mol, 20% higher than other MOFs.
Impact
An obstacle to practical use of MOFs for hydrogen storage is their weak affinity for hydrogen, so that cryogenic temperatures are requied to achieve relatively high hydrogen storage. However, a liquid-nitrogen-cooled fuel system is unrealistic for a vehicle!
Recent theory predicts that ambient-temperature storage would be possible if the heat of absorption can reach 15 kJ/mol. Previous materials came in at around 10 kJ/mol, so the new material’s value of 12.5 kJ/mol represents a significant move in the direction of ambient-temperature hydrogen storage. Future synchrotron studies will examine the structure’s properties at higher temperatures.
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Molecular arrangements in the new structure as determined by X-ray crystallography. The different views depend on which part of the molecule is visualized at the vertices.
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Two views of the extended framework formed from this structure, showing pores good for hydrogen storage. |
Principal Investigators: Xi-Sen Wang,1 Shengqian Ma,1 Paul M. Forster,2 Daqiang Yuan,1 John B. Parise,2 and Hong-Cai Zhou (corresponding author)1
(1Miami University, 2Stony Brook University)
September 2007
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