Kinetics and modeling study of a Mg(BH4)2/Ca(BH4)2 destabilized system

Borohydrides such as Mg(BH4)2 and Ca(BH4)2 continue to attract attention as potential hydrogen storage materials because of their high hydrogen content. In this study the desorption kinetics of Mg(BH4)2, Ca(BH4)2 and a 5Mg(BH4)2/Ca(BH4)2 mixture of the two were compared in the two-phase region at the same temperature and at a constant pressure thermodynamic driving force. The rate of hydrogen desorption from the two-phase region in the 5Mg(BH4)2/Ca(BH4)2 mixture was faster than that from either of the constituents. This indicated that Ca(BH4)2 was able to serve as a destabilizing agent for Mg(BH4)2. The results also showed that the desorption rates from the two-phase region were much faster than those from the single phase region. Modeling studies showed that the rate of hydrogen release from Mg(BH4)2, during the first 80% of the reaction, is diffusion controlled while in Ca(BH4)2 the reaction rate is phase boundary controlled. In the mixture the rate appears to be under the mixed control of both processes.

► A 5Mg(BH4)2/Ca(BH4)2 mixture releases H2 faster than either constituent.
► The rate of H2 release from Mg(BH4)2 is diffusion controlled.
► The rate of H2 release from Ca(BH4)2 is phase boundary controlled.
► The rate of H2 release from a 5Mg(BH4)2/Ca(BH4)2 mixture is under the mixed control of both processes.