Thermodynamic and Kinetic Investigations on Pure and Doped NaBH4-MgH2 System

In this work, a thermodynamic picture of the dehydrogenation mechanism of NaBH4 in NaBH4-MgH2 mixtures with 2:1 and 1:2 molar ratio is drawn, for the first time in literature, thanks to coupled manometric-calorimetric measurements up to 580 °C. Such a new approach also allows, after the measurement of the borohydride melting enthalpy, the evaluation of the dehydrogenation enthalpy of the complex hydride in the mixtures. The thermodynamics of the 2:1 sample (where the borohydride decomposition takes place mainly in liquid state) is more favorable than that of the 1:2 mixture, where the process evolves fully in solid state. The kinetics of the systems is studied at 450 °C, the minimum temperature at which the borohydride decomposition takes place. In these conditions, the 1:2 system is kinetically favored. Several additives (fluorides; chlorides; hydroxides) have been tested as possible destabilizing/catalyzing agents. These substances react with the component hydrides upon discharging, forming stable binary and ternary compounds that do not change the macroscopic desorption pathway of the composites (separate decomposition of the component hydrides) but lead to variations in the desorption temperature and kinetics. In particular, MgF2 is found to improve the desorption kinetics of both the component hydrides and to reduce the decomposition temperature and enthalpy of NaBH4 in the 2:1 system. On the contrary, none of the tested dopants exerts any positive effect on the 1:2 system.