Mechanisms of hydrogen storage in alanates; a first principles approach
02 / 2004 - 02 / 2008
Research on storage of hydrogen in alanates. Hydrogen is a clean fuel: in a car, burning hydrogen only results in water, and the greenhouse gas CO2 is not produced. Alanates are compounds that contain aluminum, (stored) hydrogen, and a metal like sodium or lithium. The focus of our investigation will be on NaAlH4. This material is very promising, because: (i) the reversible hydrogen content is 5.6 wt%, above the target of 5 wt% specified by the international energy agency, and (ii) in contrast to compounds containing magnesium, NaAlH4 can release hydrogen at temperatures close to 353 K, the temperature at which a socalled PEM fuel cell would extract energy from hydrogen in, for instance, a car. The problem now standing in the way of the the large-scale use of NaAlH4 is a problem of kinetics: the speed at which hydrogen release from or uptake in NaAlH4 takes place is too low with the current state of the art. Catalysts like titanium and zirconium are known to enhance this speed, but not yet enoug h. Better catalysts are needed. These could be obtained if the mechanisms of the reactions through which hydrogen is released or taken up were known, and if the roles played by the known catalysts were understood. Our research is aimed at this understanding. Using three new methods, the technical details of which are described in the research proposal, we will simulate the motion of the atoms that give rise to the solid state reactions through which hydrogen is stored and released. In the simulations, the motion of the atoms will be based on accurately calculated interatomic forces. Our research may point the way to faster loading and unloading of hydrogen from NaAlH4, which could make this material applicable in on-board storage of hydrogen in cars.