  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.
