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Suppressing diborane production during the hydrogen release of metal borohydrides: The example of alloyed Al(BH$_4$)$_3$

Aluminum borohydride (Al(BH$_4$)$_3$) is an example of a promising hydrogen storage material with exceptional hydrogen densities by weight and volume and a low hydrogen desorption temperature. But, unfortunately, its production of diborane (B$_2$H$_6$) gases upon heating to release the hydrogen restricts its practical use. To elucidate this issue, we investigate the properties of a number of metal borohydrides with the same problem and find that the electronegativity of the metal cation is not the best descriptor of diborane production. We show that, instead, the closely related formation enthalpy is a better descriptor and we find that diborane production is an exponential function thereof. We conclude that diborane production is sufficiently suppressed for formation enthalpies of $-$80 kJ/mol BH$_4$ or lower, providing specific design guidelines to tune existing metal borohydrides or synthesize new ones. We then use first-principles methods to study the effects of Sc alloying in Al(BH$_4$)$_3$. Our results for the thermodynamic properties of the Al$_{1-x}$Sc$_x$(BH$_4$)$_3$ alloy clearly show the stabilizing effect of Sc alloying and thus the suppression of diborane production. We conclude that stabilizing Al(BH$_4$)$_3$ and similar borohydrides via alloying or other means is a promising route to suppress diborane production and thus develop viable hydrogen storage materials.