Absorbing step

A class of materials developed at UCLA could bring the ‘hydrogen economy’ a step closer by solving a problem that has so far eluded scientists: bulk storage of the gas.

If cars and other vehicles are to run on hydrogen, there must be a safe, easy way of keeping the volatile gas, which can be filled and emptied quickly, and preferably doesn’t need extreme cooling or pressurisation.

The US Department of Energy has estimated that any hydrogen storage material must be able to store at least 6.5 per cent by weight of hydrogen. Previous research has found materials that can manage concentrations of around 2.5 per cent, but anything above that has, until now, proved impossible.

UCLA chemist Omar Yaghi, who specialises in materials that can absorb gases, now believes he has the key to solving the problem. Yaghi, who has been working with highly porous substances called metal-organic frameworks (MOFs), claims to have found a material that can absorb 7.5 per cent of its weight in hydrogen, albeit at low temperatures.

The team hopes these materials could be used not only for hydrogen cars but to power laptops, mobiles, digital cameras and other electronic devices. MOFs have scaffold-like structures of linked rods that can form pores of different sizes, but they store hydrogen by adsorption on to the rod components of the structure. Yaghi, who has been designing MOFs since the 1990s, is enthusiastic about their potential. ‘We have a class of materials in which we can change the components nearly at will; there is no other class where one can do that.’

MOFs can have very simple components, such as zinc oxide and terephthalate, a plastic most familiar from soft drinks bottles. Changing the structure of the starting materials allows Yaghi to make a wide variety of structures, including nested shapes, with one scaffold inside sitting inside the pores of another; and interpenetrating scaffolds.

‘We can make polymers inside the pores with well-defined and predictable properties,’ Yaghi added.

Last year Yaghi designed a MOF based on copper compounds and an organic acid that could store almost 2.5 per cent hydrogen at 77K, the temperature of liquid nitrogen. The new structure stores almost three times that amount at the same temperature.

‘Our idea was to create a material with pores that attract hydrogen, making it possible to stuff more hydrogen molecules into a smaller volume,’ he said.

The fact that the MOF can only store hydrogen at an inconveniently low temperature is still a stumbling block for the technology; at higher temperatures the weak adsorption bonds between the hydrogen and the polymer rods break down. But Yaghi believes that modifying the organic components of the material will strengthen the bonds and make storage possible at more accessible temperatures in the 0-45 Deg C range.

‘A decade ago people thought methane would be impossible to store, but that problem has largely been solved by our MOF materials,’ he commented. ‘Hydrogen is a little more challenging than methane, but I am optimistic.’