Jun 25, 2009
Increase in palladium-carbon contacts destabilizes hydrogen
Efficient storage of hydrogen is a key issue for the development of a hydrogen-based economy. The performance of microporous adsorbents, in particular activated carbons and metal organic frameworks, comes close to meeting some of the US Department of Energy's target levels, but only at cryogenic temperatures.
Recently, it has been found that the addition of transition metals to nanoporous carbon adsorbents could have a positive effect on hydrogen storage capacity at near-ambient temperatures.
Researchers at Oak Ridge National Laboratory in the US are investigating the mechanism by which small amounts of palladium nanoparticles (3–5 nm) embedded into high-surface-area nanoporous carbons enhance room-temperature hydrogen adsorption capacity by nearly 30% compared with the equivalent carbon without the palladium particles. The group has found that hydrogen molecules dissociate into atoms upon interaction with palladium particles. A small fraction of these hydrogen atoms intercalate in the palladium lattice, forming palladium hydride, while a larger amount become mobile and spill over to the carbon to be either chemisorbed on its surface or physisorbed in nanopores.
The scientists monitored the release of hydrogen atoms from the hydrogen-rich phase of palladium hydride using in situ high-pressure X-ray diffraction techniques. Their results revealed that the palladium hydride phase becomes unstable with the increase of palladium–carbon contacts, and hydrogen is more easily released. In addition, the researchers demonstrated that removing oxygen-containing chemical groups from the carbon surface further enhances the release of hydrogen. These findings provide new insights into the factors that may be tailored to control the hydrogen spill-over process and develop materials with enhanced hydrogen-storage properties.
About the author
Cristian I Contescu and Nidia C Gallego are research scientists in the Carbon Materials Technology Group, at Oak Ridge National Laboratory (ORNL). Vinay V Bhat is a postdoctoral fellow at ORNL under the ORNL Postdoctoral Research Associates Program administered jointly by ORISE and ORNL.