Controlled hydrogen release from metastable hydrides

Metastable hydrides are an interesting class of hydrogen carrier since many offer high volumetric and gravimetric hydrogen densities and rapid hydrogen release rates at low temperatures. Unlike reversible metal hydrides, which operate near equilibrium, metastable hydrogen carriers rely on kinetic barriers to limit or prevent the release of hydrogen and can be prepared in a stabilized state far from equilibrium. Despite the advantage of low temperature hydrogen release, this type of one-way thermolysis reaction can be difficult to control since the hydrogen release rate varies with temperature and composition. Here we developed a kinetic rate equation from a series of isothermal measurements, which describes the relationship between temperature, hydrogen release rate and composition for aluminum hydride. This equation is necessary to thermally control the rate of hydrogen release throughout decomposition. This equation was used to run a fuel cell at a controlled rate of ∼1 wt%/hr. Although the equation established in this paper relates specifically to aluminum hydride, the method used is applicable to other metastable hydrides.