Cycling properties of Sc- and Ce-doped NaAlH4 hydrogen storage materials prepared by the one-step direct synthesis method

The so-called “one-step direct synthesis method” – simultaneous ball-milling (b.m.) of NaH–Al powder doping agent mixtures under H2 pressure – turned out to be a simple way for the preparation of metal-doped sodium alanate hydrogen storage materials, leading to solids with high storage capacity and excellent kinetics. The method has been published until now only for TiCl3 as a doping agent. In combination with ScCl3 or CeCl3 as doping agents, the one-step direct synthesis method delivers materials with hydrogen storage properties which come close to those required by the car industry for hydrogen supply of PEM fuel cells. With respect to the kinetics of the chemical reaction, hydrogenation rates corresponding to 3–5 min time for refuelling of a hydrogen tank can be realized, although removal of the resulting hydrogenation heat in such a short time posses a severe engineering problem. Release of all stored hydrogen in a time compatible with handling of a car is possible without additional heating devices, if instead of the current fuel cells, advanced designs on the basis of polybenzimidazole membranes are used. These fuel cells work at temperatures of 150–200 °C, so that their waste heat temperature level is sufficiently high for desorption of hydrogen from both dehydrogenation steps of the NaAlH4 system. Additionally, it has been reported that using mischmetal with 42 at.% of Ce as a dopant for NaAlH4, at 150 °C, ∼5 wt.% of hydrogen can be desorbed in ∼3 h. Moreover, in a 95 cycles de- and re-hydrogenation test the present Ce-doped NaAlH4 storage material showed stable storage properties.