Catalysis of H2 sorption in NaAlH4: General description and new insights

Sodium alanate NaAlH4 is a very suitable material for solid-state hydrogen storage owing to its relatively high hydrogen capacity (7.5 wt.% H2) and moderate temperatures for reversible H2 release/uptake (100-150 °C) compatible with PEM fuel cells. These temperatures are obtained by adding a dopant. TiCl3, ScCl3 or CeCl3 are known to be efficient dopants but the details of the catalytic mechanism are not fully understood yet. In this work, the H2 sorption of NaAlH4 doped with TiCl3, ScCl3 or CeCl3 is systematically studied. The doped samples were prepared in one step by reactive ball milling of NaH and Al with 4 mol.% dopant in an H2 atmosphere. The efficiencies of the dopants are different for desorption and absorption, indicating that different catalytic mechanisms and rate limiting steps are taking place during both steps and these are described here. TiCl3 is more efficient for desorption and CeCl3 for absorption. ScCl3 is less efficient than the first two for all reactions. A mixture of TiCl3 and CeCl3 is thus added to NaAlH4 to act at the same time on the desorption and the absorption processes. The results show enhanced overall performances for H2 sorption when using this mixture compared to NaAlH4 milled with TiCl3 or CeCl3 only.