H-trap

The chassis and body panels of a car may one day store the vehicle’s fuel, doing away with the bulky tank altogether. That’s one possible consequence of research into new ways to contain hydrogen.

The elusive gas’s ability to escape from most materials is one of the barriers hindering the development of a hydrogen economy. Currently reinforced pressure vessels are used to contain it but methods to attach hydrogen molecules to porous solids are being investigated because they could make storage and transportation significantly easier.

‘The greatest obstacle to the development of hydrogen-powered cars is the lack of a system for safe, efficient and convenient on-board storage of hydrogen,’ said Prof Neil McKeown of CardiffUniversity, who is working with Dr Peter Budd of ManchesterUniversity to trap the gas in porous polymers.

Organic polymers had not been investigated as materials for storage of hydrogen until McKeown’s development of polymers of intrinsic microporosity (PIMs) in 2000. He came up with the idea of a rigid polymer with a molecular structure that does not pack easily and so has a very large surface area.

The materials are composed entirely of macromolecules consisting of fused-ring sub-units. The majority of pores within the materials have diameters less than 0.7nm, giving a large internal surface of 800m2 per gram – equivalent to the area of three tennis courts.

The next step was to see if the new materials could hold helium. ‘We’ve done some preliminary tests which are encouraging because they showed that significant quantities of the gas can be adsorbed,’ said McKeown. The hydrogen is actually ‘physisorbed’ on to the PIM, meaning it is held in place by weak forces, not by covalent bonds, so it is easy to remove.

Early results show that these ultra-small pores can adsorb and then release 1.4-1.7 per cent of the hydrogen, if cooled by liquid nitrogen to 77K and kept at low pressure of about five atmospheres.

The International Energy Authority (IEA) has set a target of five per cent reversible mass loading for a realistic storage system. So the challenge faced by McKeown and Budd is to make a microporous material of appropriate structure and chemical composition to help reach this target.

Zeolites, various forms of carbon, metal-organic compounds and other types of porous materials are being investigated by other groups but have not been shown to be superior to the PIMs in the proportion of hydrogen they can hold. Zeolites can store up to 1.5 per cent under the same temperature and pressure.

Depending on the selection of building blocks, the researchers can produce insoluble networks or polymers that are soluble and can thus be processed into useful shapes like common plastics.

‘Most importantly, the chemical composition of PIMs can be tailored through synthetic chemistry. We could process them differently. There are tricks we’d like to play to bump up the quantity of hydrogen that could be adsorbed,’ said McKeown.

General Motors has helped the researchers informally with information, advice and some meas- urement tasks but the project is too immature to attract full commercial partners. So McKeown and Budd have received £115,000 of funding from the EPSRC to support their investigations for three years.

‘We hope that we will have optimised the materials by then and be closer to commercial applications,’ said McKeown. If they succeed in creating a better hydrogen trap then the world of commerce will quickly beat a path to their door.