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

Sodium alanate NaAIH(4) is a very suitable material for solid-state hydrogen storage owing to its relatively high hydrogen capacity (7.5 wt.% H-2) and moderate temperatures for reversible H-2 release/uptake (100-150 degrees 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 H-2 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 H-2 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 H-2 sorption when using this mixture compared to NaAIH(4) milled with TiCl3 or CeCl3 only. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.