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.