Organic crystals for hydrogen storage
Scientists in the UK say they have fashioned a crystal that could store 2.5% hydrogen
by mass. ‘If we or someone else doubles [the pore sizes] twice more, then we could have the best material for hydrogen adsorption,’ says Professor Neil McKeown of Cardiff University.
The development is based on the discovery of an organic crystal that offers nearly twice the hydrogen storage capacity of any other recorded microporous organic compound.
The rigid organic crystal, 3,3´,4,4´tetra(trimethylsilylethynyl)biphenyl (TMS), was one of six studied by researchers at Cardiff as part of a UK Engineering and Physical Sciences Research Council project. They successfully removed the solvent molecules trapped in the compounds to create storage sites suitable for hydrogen.
The solvent ‘comes out on exposure to air, but removal can be speeded up by heat and vacuum’, explains McKeown. A major challenge, however, is preventing the subsequent collapse of the crystal structure, as was the case with several of the compounds investigated. The TMS crystal was rigid enough to survive the procedure, which involved being heated to 60ºC for several hours at one millibar, creating storage channels as small as four Ångstrom in diameter.
The biphenyl material can store around 0.8% hydrogen by mass at 77K/10 bar, though McKeown believes it could save up to one per cent at higher pressures – the only other reported organic compound that stores hydrogen is a dipeptide that can store 0.45% at 77K/10 bar.
Although this does not compare favourably to carbon or metal-organic framework microporous materials, which can absorb five to six per cent under the same conditions, McKeown says the important thing is that these crystals have been shown to store hydrogen at all. ‘A few years ago this would have been seen as impossible. [While] we have not solved the problem, others can now see that this is a viable approach.’
The TMS crystal was uncovered in the Cambridge Structural Database, where McKeown says he ‘was amazed by its similarity to the structure of zeolite’, with its cubic symmetry and pore size. His team remade it using organic chemistry.
While they try to improve upon the 2.5% storage capacity of the new crystal, he says the accessible metal sites on the compound could make it useful for heterogeneous catalysis.
Ulrich Eberle of German carmaker Opel has previously supported the Cardiff team with advice on what manufacturers require for hydrogen vehicles. ‘If the novel high-surface materials [being investigated at Cardiff] reach similarly high values as activated carbon, that would be interesting. But we currently have 700 bar compressed hydrogen technology inside our vehicles.
‘Every material class in the solid-state absorber area has to beat these system energy values (for example, providing higher range for the vehicle or lower cost). This is a challenging and not yet achieved target.’Materials World Magazine, 01 Oct 2009
The development is based on the discovery of an organic crystal that offers nearly twice the hydrogen storage capacity of any other recorded microporous organic compound.
The rigid organic crystal, 3,3´,4,4´tetra(trimethylsilylethynyl)biphenyl (TMS), was one of six studied by researchers at Cardiff as part of a UK Engineering and Physical Sciences Research Council project. They successfully removed the solvent molecules trapped in the compounds to create storage sites suitable for hydrogen.
The solvent ‘comes out on exposure to air, but removal can be speeded up by heat and vacuum’, explains McKeown. A major challenge, however, is preventing the subsequent collapse of the crystal structure, as was the case with several of the compounds investigated. The TMS crystal was rigid enough to survive the procedure, which involved being heated to 60ºC for several hours at one millibar, creating storage channels as small as four Ångstrom in diameter.
The biphenyl material can store around 0.8% hydrogen by mass at 77K/10 bar, though McKeown believes it could save up to one per cent at higher pressures – the only other reported organic compound that stores hydrogen is a dipeptide that can store 0.45% at 77K/10 bar.
Although this does not compare favourably to carbon or metal-organic framework microporous materials, which can absorb five to six per cent under the same conditions, McKeown says the important thing is that these crystals have been shown to store hydrogen at all. ‘A few years ago this would have been seen as impossible. [While] we have not solved the problem, others can now see that this is a viable approach.’
The TMS crystal was uncovered in the Cambridge Structural Database, where McKeown says he ‘was amazed by its similarity to the structure of zeolite’, with its cubic symmetry and pore size. His team remade it using organic chemistry.
While they try to improve upon the 2.5% storage capacity of the new crystal, he says the accessible metal sites on the compound could make it useful for heterogeneous catalysis.
Ulrich Eberle of German carmaker Opel has previously supported the Cardiff team with advice on what manufacturers require for hydrogen vehicles. ‘If the novel high-surface materials [being investigated at Cardiff] reach similarly high values as activated carbon, that would be interesting. But we currently have 700 bar compressed hydrogen technology inside our vehicles.
‘Every material class in the solid-state absorber area has to beat these system energy values (for example, providing higher range for the vehicle or lower cost). This is a challenging and not yet achieved target.’Materials World Magazine, 01 Oct 2009
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