|SEM micrograph of hypercrosslinked polyaniline at magnification of 15000x. Click to enlarge.|
Researchers at Lawrence Berkeley National Laboratory and the University of California, Berkeley have developed an entirely new type of nanoporous material—hypercrosslinked polyaniline—that shows promise as a potential storage medium for hydrogen.
The new materials have a permanent porous structure and specific surface areas exceeding 630 m2 g-1. The researchers, led by Frantisek Svec, found that the best adsorbent of the materials had a hydrogen storage capacity of 2.2 wt% at 77 K and 3.0 MPa.
Hypercrosslinked polyanilines exhibit a remarkably high affinity for hydrogen, according to the researchers, which results in enthalpies of adsorption as high as 9.3 kJ mol–1 (exothermic), in sharp contrast with hypercrosslinked polystyrenes and metal–organic frameworks which have significantly lower enthalpies of adsorption, typically in the range of 4–7 kJ mol–1.
The team made the new materials by adding small molecular crosslinkers to commercial polyanoiline that had been swollen in an organic solvent. The hypercrosslinking reaction results in a rigid, mesh-like structure.
The current materials are still from practical hydrogen stores. Even the current best-performer at 77K still falls far short of Department of Energy targets for storage system capacity. And, said Andrew Cooper, who studies hydrogen storage polymers at the University of Liverpool (UK), in a report on the work in Chemical Technology:
With what you’d have to change in structure to achieve room temperature hydrogen storage, it’s arguable whether you could still call it the same material. The key advance with this work is the new approach to make porous polymers.
The Berkeley team is currently trying different crosslinkers and different reaction conditions, to increase the material’s capacity.
Jonathan Germain, Jean M. J. Fréchet and Frantisek Svec, “Hypercrosslinked polyanilines with nanoporous structure and high surface area: potential adsorbents for hydrogen storage” J. Mater. Chem., 2007, 17, 4989 - 4997, DOI: 10.1039/b711509a