A series of computer simulations has identified a polymer material with a very large capacity for storing hydrogen that could be exploited in fuel cells. Jisoon Ihm and colleagues at Seoul National University in South Korea have discovered that polyacetylene with titanium atoms attached to the polymer chain can hold 63 kilograms of hydrogen per cubic metre -- more than any other similar material in their survey.
A low-cost, high-capacity hydrogen-storage medium is essential for the commercialization of hydrogen fuel-cell technologies. Researchers had previously looked at carbon nanotubes, hydrogen-clathrate-hydrates and other nanostructured materials as ways of storing hydrogen, but they only work in fuel cells at low temperatures or high pressures. Now, Ihm and co-workers have shown that polymers covered with metal atoms can store a significant amount of hydrogen under more practical working conditions.
The large storage capacity is predicted because numerous hydrogen molecules are attracted to the metal atoms that lie along the polymer chain. Using a series of first-principles electronic-structure calculations, the physicists worked out how much energy the hydrogen molecules need to bind to the metal atoms. They looked at a wide combination of metal atoms (including titanium, scandium and vanadium), polymers (including polyacetylene, polypyrrole and polyaniline) and bonding sites for the hydrogen on the metal atoms.
The researchers found that a form of polyacetylene “decorated” with titanium atoms was the best. This molecule consists of a series of carbon atoms linked together in a chain by alternating single and double bonds. Each carbon atom has one hydrogen atom that can be replaced by a particular atom like titanium.
They found that up to five hydrogen molecules can be attached to each titanium atom in this particular form of polyacetylene, allowing the material to reversibly store 7.6 wt% of hydrogen, or 63 kilograms per cubic metre under practical working conditions. This value is higher than a target of 45 kilograms per cubic metre that the US Department of Energy said should be reached by 2010 (Phys. Rev. Lett. 97 056104).
“Our results will have considerable importance for experimentalists and engineers to synthesize metal-decorated polymers for hydrogen storage,” Ihm told PhysicsWeb. “Indeed, we have already begun to make some titanium-decorated polymers in collaboration with other researchers and are measuring their hydrogen-storage capacity now”.
