Investigation on the Decomposition Enthalpy of Novel Mixed Mg(1–x)Znx(BH4)2 Borohydrides by Means of Periodic DFT Calculations

Abstract

The combination of Mg(BH4)2 and Zn(BH4)2 compounds has been theoretically investigated as a possible mixed borohydride prone to give an enthalpy of decomposition around 30 kJ/molH2, that is, suitable for a dehydrogenation process close to room temperature and pressure. The total energy of pure compounds and solid solutions has been computed by means of periodic DFT calculations. To generate the Mg(1–x)Znx(BH4)2 solid solutions, the α-phase of Mg(BH4)2 (space group P6122) has been considered in which Mg2+ ions have been progressively replaced with Zn2+, without lowering the symmetry of the crystalline structure. A charge density topological analysis is reported to better understand the chemical bonding in the pure and mixed metal borohydrides. The decomposition enthalpy of the mixed borohydrides according to two different reaction paths that lead to MgH2, Zn, H2, and α-B or B2H6, respectively, as products has been estimated. As regards the former, a value of about 30 kJ/molH2 has been predicted for a Mg(1–x)Znx(BH4)2 solid solution with x = 0.2–0.3.