Tuning the hydrogen desorption of Mg(BH4)2 through Zn alloying

We study the effect of Zn alloying on the hydrogen desorption properties of Mg_(BH4)2 using ab initio simulations. In particular, we investigate formation/reaction enthalpies/entropies for a number of compounds and reactions at a wide range of temperatures and Zn concentrations in Mg_1-xZn_x(BH₄)2. Our results show that the thermodynamic stability of the resulting material can be significantly lowered through Zn alloying. We find, e.g., that the solid solution Mg_2/3Zn1/3(BH4)2 has a reaction enthalpy for the complete hydrogen desorption of only 25.3 kJ/mol H₂—a lowering of 15 kJ/mol H₂ compared to the pure phase and a corresponding lowering in critical temperature of 123 K. In addition, we find that the enthalpy of mixing is rather small and show that the decrease in reaction enthalpy with Zn concentration is approximately linear.