In another approach to developing a viable on-board storage system for hydrogen in vehicles, researchers in the UK have developed a purely organic polymer with microporous structures that can adsorb hydrogen via physisorption.
Research into using a microporous material for hydrogen storage tends to focus on materials such as zeolites or activated carbons, which have many molecular-size holes suitable for the containment and release of hydrogen.
By contrast, the molecular chains in most organic polymers are so flexible that they can form tightly packed structures. This means there are no cavities inside, and thus no appreciable internal surface onto which substances could be adsorbed.
The chemists constructed polymers from interlinked five- and six-membered rings. At defined points in the molecule, two five-membered rings are connected in such a way as to provide a contorted shape to the stiff macromolecular structures. The contorted molecules cannot pack together efficiently and leave gaps and interstices.
In reproducible synthetic steps, the researchers have produced chemically homogeneous materials—“polymers of intrinsic microporosity”—with a uniform distribution of pore sizes of 0.6–0.7 nm.
These ultrasmall pores can absorb and then release between 1.4 and 1.7% hydrogen by weight at liquid nitrogen temperatures. Depending on the selection of building blocks the researchers can produce insoluble networks or polymers that are soluble in solvents and can thus be processed into useful shapes.
The current rate of storage is far below the DOE target of 6% for 2010 and 9% by 2015.
In order for the PIMs to store enough hydrogen to be useful they must be optimized further by both chemistry and polymer processing techniques. Neil McKeown of Cardiff University, one of the researchers, estimates that by 2010 they will have tailored a PIM capable of storing up to 6% hydrogen.
“Towards Polymer-based Hydrogen Storage Materials: Engineering Ultramicroporous Cavities Within Polymers of Intrinsic Microporosity”; Neil B. McKeown, Bader Ghanem, Kadhum J. Msayib, Peter M. Budd, Carin E. Tattershall, Khalid Mahmood, Siren Tan, David Book, Henrietta W. Langmi, Allan Walton; Angewandte Chemie International Edition 2006, 45, 1804, doi: 10.1002/anie.200504241
UK Royal Society of Chemistry: Stirring up High Hopes for the Hydrogen Economy