Achieving a better understanding of the interaction of hydrogen with metal surfaces is gaining new significance in the drive to develop a hydrogen storage material that functions at convenient rates and conditions for mobile power applications. Hydrogen must bind tightly enough to the material to remain bound at ambient temperatures and pressures but not so tight that it is released below about 200 °C. In addition, further study is needed on the role that kinetic barriers may play in limiting hydrogen uptake and release rates.

Since kinetic barriers may limit hydrogen uptake and release rates, we are investigating the effects of the use of small amounts of transition (or rare earth) metals with partially filled d (or f) orbitals on hydrogen sorption (desorption and absorption). While transition or rare earth metals typically have low barriers for hydrogen sorption, their high atomic weight limits their use in hydrogen storage materials. We are looking at whether use of small amounts of these metals (just a few atomic percent) still provides this favorable quality. It is possible that transition metals that are coordinated by the lighter first and second row elements lose their ability to easily form or break H-H bonds. For example, NiAl(11), which as a 50-50 ratio of the two species at the surface, has virtually no dissociation barrier for H₂ on pure Ni surfaces, while the barrier is 0.72 eV on NiAl(110).

Our focus is to develop an atomistic model of how 2-4% of titanium added to NaAlH₄ transforms this hydride into a reversible hydride with usable kinetics for hydrogen storage. Our current model assumes that some fraction of the Ti is present at the surface of NaAlH₄ or its main decomposition products Al, NaH, or Na₃AlH₆. The Ti at the surface reduces the barriers for H₂ dissociation and recombination, and thus improves the speed for hydrogen uptake and release. We are also investigating the long range transport of Al atoms, which has to occur during formation and decomposition. This might be aided by the formation of weakly bound AlHx or alane species.

In testing the elements of our model, we are applying thermal desorption spectroscopy, low-energy electron diffraction, low-energy electron microscopy, low-energy ion diffraction, and scanning tunneling microscopy. Our goal is to closely link the well-controlled vacuum and theory experiment and "real world" high-pressure observations to establish the significance of our model for applications. Ultimately, we seek to base the development of improved hydrogen storage materials on our fundamental understanding of the physics and chemistry at the atomic level.

Contact:
Roland Stumpf
rrstump@sandia.gov
(925) 294-6114