Comparison of the calculated and experimental scenarios for solid-state reactions involving Ca(AIH4)2

Solid-state reactions of Ca(AIH(4))(2) and various additives were investigated experimentally for hydrogen desorption. The Ca(AIH(4))(2) + Si, Ca(AIH(4))(2) + 2MgH(2), Ca(AIH(4))(2) + 2LiH, and Ca(AIH(4))(2) + 2LiNH(2) systems were chosen among reactions proposed theoretically(1) to study their hydrogen storage capacity and an appropriate reaction enthalpy. For all systems investigated, the reversible reactions proposed should have more than 6.5 mass % hydrogen capacity and reaction enthalpies in the range of 30-55 kJ/mol H-2. However, most of the experimentally observed reactions do not conform to theoretical propositions because different final products were obtained in all cases but one. Two tendencies were observed in the experiments. One is that the reaction comprises several distinct steps at different temperatures as observed in the Ca(AIH(4))(2) + Si and Ca(AIH(4))(2) + 2 MgH2 systems. In this case, Ca(AIH(4))(2) decomposes first and is followed by a reaction of the remaining compounds. The same kinetic reaction barriers are encountered here as in pure Ca(AIH(4))(2). Second, LiH and LiNH2 additions yield exothermic reactions of Ca(AIH(4))(2) and the other reactant as early as in the ball milling or annealing steps, leading to final products not considered in the reactions calculated.