Power Grid
Hydrogen is considered the technology of the future. But hydrogen-powered cars are still rare on our roads because there is a lack of efficient tanks. Organic polymers could help here.
Fossil fuel resources are finite and burning them has a negative impact on the environment. Alternatives are therefore in great demand. A suitable and environmentally friendly fuel would be hydrogen - but unfortunately there are problems with the implementation: So far there are hardly any technical possibilities to construct safe and efficient hydrogen tanks for cars. Microporous storage materials, such as zeolites or organometallic compounds, which have numerous cavities that are suitable for storing hydrogen and which can release it again when required, are a possible solution.
British researchers from Cardiff, Manchester and Birmingham have now ventured a new approach: Neil McKeown and his colleagues have developed a purely organic polymer that is able to absorb appreciable amounts of hydrogen.
The molecular chains in most organic polymers are flexible enough to form densely packed structures. Therefore, there are no cavities inside in which substances could be adsorbed. Consequently, chemists constructed polymers from interconverting five- and six-membered hydrocarbon rings.
In the created molecule, two five-membered rings meet at defined points in such a way that kinks and distortions occur in the rigid macromolecular structures. The distorted molecules cannot form densely packed layers – gaps and spaces appear. These "polymers with intrinsic microporosity" (PIMs) have an internal surface area of more than 800 square meters per gram - the equivalent of three tennis courts.
In reproducible synthesis steps, the researchers succeeded in producing chemically homogeneous materials with a uniform pore size distribution of 0.6 to 0.7 nanometers. These ultra-small pores can absorb and release between 1.4 and 1.7 percent hydrogen. Depending on the choice of starting building blocks, insoluble networks or soluble polymers are obtained, which can be processed and shaped like conventional plastics.
In order for the PIMs to be able to store enough hydrogen and be suitable for technical applications, they have to be further optimized. "There are numerous ways to produce customized PIMs by adapting the synthesis methods and further processing of the polymers," emphasizes McKeown. The chemist hopes that by 2010 it will be possible to produce PIMs that will then be able to store up to 6 percent hydrogen.© Angewandte Chemie
