Analysis of Intermediates and Products from the Dehydrogenation of Mg(BH4)2

Theoretical prediction of nanosizing effects and role of additives in the decomposition of Mg(BH4)2

The energetic transition towards renewable resources is one of the biggest challenges of this century. In this context, the role of H2 is of paramount importance as a key source of energy that could substitute traditional fossil fuels. This technology, even if available in several manufactures, still needs to be optimized at all levels (production, storage and distribution) to be integrated on a larger scale. Among materials suitable to store H2, Mg(BH4)2 is particularly interesting due to its high content of H2 in terms of gravimetric density. Nanosizing effects and role of additives in the decomposition of Mg(BH4)2 were studied by density functional theory (DFT) modelling. Both effects were analyzed because of their contribution in promoting the decomposition of the material. In particular, to have a quantitative idea of nanosizing effects, we used thin film 2D models corresponding to different crystallographic surfaces and referred to the following reaction: Mg(BH4)2 → MgB2 + 4H2. When moving from bulk to nanoscale (2D models), a remarkable decrease in the decomposition energy (10-20 kJ mol-1) was predicted depending on the surface and the thin film thickness considered. As regards the role of additives (Ni and Cu), we based our analysis on their effect in perturbing neighboring borohydride groups. We found a clear elongation of some B-H bonds, in particular with the NiF2 additive (about 0.1 Å). We interpreted this behavior as an indicator of the propensity of borohydride towards dissociation. On the basis of this evidence, we also explored a possible reaction pathway of NiF2 and CuF2 on Mg(BH4)2 up to H2 release and pointed out the major catalytic effect of Ni compared to Cu.

First-Principles Elucidation of Initial Dehydrogenation Pathways in Mg(BH4)2

Complex borohydrides such as Mg(BH₄)₂ offer one of highest capacities to chemically store hydrogen for onboard applications; however, it suffers greatly from kinetic constraints that prevent realization of full capacity and reversibility. Understanding these kinetic limitations solely from experiments is extremely challenging due to the unusual complexity of various competing elemental reaction steps involved during the de/rehydrogenation reaction. This work aims to map out the energetics associated with initial dehydrogenation of Mg(BH₄)₂ from first-principles simulations and to identify the preferred reaction pathways. Our calculations suggest the rate-limiting step during BH₄^--B₃H₈^- conversion is the formation of the B₂H₇^- intermediate. We further emphasize and clarify that the B₃H₈^- and H^- intermediates, formed during initial Mg(BH₄)₂ decomposition, appear as molecular species that are embedded in the Mg-BH₄-Mg matrix as evidenced in the nuclear magnetic resonance measurements and not as bulk MgH₂ and Mg(B₃H₈)₂ as previously assumed in theoretical predictions of the thermodynamics.

Thermal stability and structural studies on the mixtures of Mg(BH₄)₂ and glymes

Coordination complexes of Mg(BH₄)₂ are of interest for energy storage, ranging from hydrogen storage in BH₄ to electrochemical storage in Mg based batteries. Understanding the stability of these complexes is...