Thermodynamics of hydrogen release in complexed borohydrides

An important aspect of successful hydrogen storage materials is their hydrogen release mechanism, which- unfortunately-for many promising materials occurs at impractically high or low temperatures. In light of recent improvements observed when complexing borohydrides with NH3, we performed an ab initio investigation into the effects of complexing the borohydrides Al(BH4)(3), Mg(BH4)(2), and Zn(BH4)(2) with new additive molecules CH4, C2H6O, and H2O, as well as NH3 as a benchmark. To find candidate ground-state structures for these borohydride complexes, we first performed an in-depth evolutionary structure search. These ground-state structures were then used to both evaluate their thermodynamic properties as well as to investigate nearby metastable states that facilitate a favorable hydrogen release using evolutionary metadynamics. We found that all of these additives markedly affect-to a varying degree-the nearby metastable states relative to the unaltered borohydride. Furthermore, we found that complexing borohydrides with CH4 and H2O results in particularly favorable properties for improving their hydrogen release. Our study not only investigates new and promising hydrogen storage materials, but also elucidates the hydrogen release mechanism and the effect of additives in processes that are difficult to characterize experimentally.