UK scientists almost double hydrogen storage capacity of organic polymer

Cardiff scientists exploring the safe storage of hydrogen to power vehicles as an environmentally friendly alternative to petrol have succeeded in developing an organic polymer capable of storing 3 per cent hydrogen by weight. The figure is almost double the amount of hydrogen the group’s preliminary polymers could store last year (1.7 per cent), and offers hope of producing an organic polymer in the future capable of storing enough hydrogen to successfully power a vehicle.

The team, consisting of by Professors Neil McKeown from the School of Chemistry together with Peter Budd of the University of Manchester and David Book from the University of Birmingham, knows that in order to make hydrogen a viable alternative to petrol, a material which can store hydrogen at a weight of over six per cent is required. This figure is estimated by the American Department of Energy (DOE) as the minimum required to make a fuel tank for hydrogen to power a vehicle for 300 miles. The International Energy Agency (IEA) has set a target of 5% reversible mass loading for a realistic storage system.

The Engineering and Physical Sciences Research Council funds the project focusing on the physisorption of hydrogen on the large and accessible surface of a microporous material. This technique offers the attractive possibility of safe hydrogen storage with an energy efficient release for consumption. However, physisorption relies on the very weak interactions between the microporous material and hydrogen molecules, therefore, the mass loadings are generally low. Thus, the challenge is set to make a microporous material of appropriate structure and chemical composition to help reach the DOE/IEA targets.

In order to obtain a polymer that can store useful quantities of hydrogen we need to make a much more porous material, but one in which the holes are very small so as to fit snugly the small hydrogen molecules. - Professor Neil McKeown

Previously, polymers have not been investigated as materials for the storage of hydrogen because most polymers have enough conformational and rotational freedom to pack space efficiently and are therefore not microporous:
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However, the recently developed polymers of intrinsic microporosity (PIMs) do possess significant microporosity and preliminary hydrogen sorption results are encouraging with significant quantities adsorbed. Most importantly, the chemical composition of PIMs can be tailored via synthetic chemistry. Therefore, the adventurous primary objective of the scientists' work is to prepare novel PIMs in a form that demonstrate hydrogen loadings equal to or in excess of the IEA 5% benchmark at moderate pressures and 77 K.

Professor McKeown and his team are investigating a number of promising methods to enhance pororosity as they attempt to build on their current success and produce a material that can store and release hydrogen safely and effectively. They are also collaborating with Professor Kenneth Harris within the School of Chemistry to develop other types of hydrogen storage materials.

Image: Molecular model of organic polymer. Credit: Cardiff University.

References:
Cardiff University: Developing green fuel - August 27, 2007.

Engineering and Physical Sciences Research Council: Polymer-based hydrogen storage materials - project overview.