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

Abstract

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.