Magnesium borohydride Mg(BH4)2 has attracted great attention in the hydrogen storage community due to its high gravimetric capacity (14.9 wt% H2) and relatively small reaction enthalpy (≤ 50 kJ/mol H2).1, 2 However, sluggish kinetics and the formation of stable intermediate compounds result in poor cyclability, high hydrogenation pressure and high dehydrogenation temperature, hampering its application for onboard hydrogen storage. Despite extensive research efforts, the reaction pathways and intermediate thermodynamics, as well as the associated kinetic energy barriers, are not fully understood. Likewise, microstructural features, which play a key role in determining hydrogen diffusion kinetics, internal mechanical stress distributions, and cyclability, have been largely unexplored. To investigate the kinetic pathways of phase evolution involving multiple intermediates in the Mg-B-H system, we have developed a multiscale modeling approach that integrates theoretical nucleation modeling and atomistic calculations. Ab initio molecular dynamics and experimental thermochemical data for H2 gas are employed for computing the temperature- and pressure-dependent formation free energies of bulk, including anharmonic phonons, while the particle size- and morphology-dependent energies are derived from the density functional theory calculations. The predicted thermodynamic values are used to parameterize a nucleation model that can describe how chemical species evolve to a condensed phase and determine the associated phase transformation barrier. A parametric study of phase nucleation barriers shows that different values lead to different microstructural morphology and phase expression. The relationship between experimental conditions and the preferred kinetic pathways will be discussed, motivating engineering guidance for improving the overall kinetics of the Mg-B-H hydrogen storage system.

Modeling of thermodynamics and nucleation kinetics in the Mg-B-H system

2019 Spring National Meeting

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