UK scientists exploring the safe storage of hydrogen for powering vehicles have developed an organic polymer capable of storing three per cent hydrogen by weight.

Previously, polymers have not been investigated as materials for the storage of hydrogen due to their conformational and rotational freedom to pack space efficiently. They are therefore not microporous enough to hold gas.

A consortium, from the Universities of Birmingham, Cardiff and Manchester, created polymers of intrinsic microporosity (PIMs). They say that preliminary hydrogen sorption results are encouraging with significant quantities adsorbed.

‘Most scientists interested in gas storage have been exploring the use of microporous materials, which have large, accessible, internal surfaces for storing hydrogen by physisorption, or noncovalent adsorption,’ explains Dr Peter Budd, a Senior Lecturer in chemistry at The University of Manchester. ‘Zeolites, various forms of carbon, metal-organic framework compounds, and other types of porous materials offer attractive possibilities for such storage.’ However, these materials are heavy and will add to the overall weight of the vehicle, making it less fuel efficient.

‘Our polymers are lightweight because they are composed only of light elements – carbon, hydrogen, nitrogen, and oxygen’, says Budd. ‘The materials are also chemically homogeneous, thermally and chemically stable, and can be synthesised reproducibly.’

He explains that the internal surface area of PIMs is greater than 800m2 per gramme of material.

‘The materials are composed entirely of macromolecules consisting of fused-ring subunits. The molecules have highly rigid and contorted structures that prevent them from packing together closely in the solid state. The majority of pores within the materials have diameters of less than 0.7nm.’

It is this spacious interconnected free volume that can be used to store hydrogen.

Hitting the target?

The Department of Energy has set a goal calling for practical hydrogen-storage systems to be able to store six per cent hydrogen by mass by 2010 and nine per cent by 2015. The leading material for hydrogen storage at present is activated carbon, which holds up to four per cent hydrogen by mass at 77K at safe, readily achievable pressures such as 5-10atm.

‘Zeolites can store up to 1.5% under the same conditions of temperature and pressure,’ Budd notes. ‘The PIMs that we [have] synthesised to date stores three per cent under these conditions.’

The team investigated one PIM that is soluble in organic solvents and two insoluble network PIMs. ‘The soluble PIM is prepared by the dioxane-forming copolymerisation of a hydroxylated aromatic monomer, a spirobisindane, and a fluorinated terephthalonitrile,’ Budd explains. ‘Both monomers are commercially available. The insoluble network PIMs are prepared using similar dioxane-forming reactions involving readily prepared cyclotricatechylene and chlorinated hexaazatrinaphthylene precursors.’

Cyclotricatechylene is a bowl-shaped receptor monomer that leads to enhanced ultramicroporosity. According to Budd, ‘this illustrates the control over microporosity that is possible with this class of material’.

‘Based on the evidence we have so far, insoluble network PIMs adsorb a little more hydrogen than the soluble PIM,’ Budd continues. ‘It would be very exciting to get a soluble PIM to take up more hydrogen because then you would have a solvent-processible material. This would allow fabrication of
whatever shape of storage structure that you want and it would also make it easier to prepare composite materials using coatings.

Dr David Hart, an expert in hydrogen energy and fuel cell technology at Imperial College London, UK, says that the material shows a lot of promise for storage applications. However, he believes that further tests are needed to find out if the polymer can be cycled repeatedly for the lifetime of a vehicle without degradation, and whether or not 100% of the stored hydrogen is released.

Budd says that tests are ongoing to determine these factors.