Suitability of III-V [XH₄][YH₄] materials for hydrogen storage: A density functional study

In the search for novel hydrogen storage media, the III-V hydridic material [NH₄][BH₄] is a natural candidate. It can store a high wt % of hydrogen and has a favorable volumetric density. Unfortunately it was found to decompose slowly at room temperature. It is of interest to consider chemically related materials, such as the series of [XH₄][YH₄] ionic solids (X=B,Al,Ga and Y=N,P,As). Even if the wt % of hydrogen in the heavier congeners is necessarily lower, they might offer superior material properties, notably higher (but not too high) stability. We have therefore performed a first-principles investigation of the cohesive energies of the XH₄YH₄ solid-state materials. In addition we have analyzed the bond character and energy within the building blocks of these materials, the XH₄⁻ and YH₄⁺ molecular ions, including a comparison to the AH₄ molecules (A=C,Si,Ge). The calculations have been performed within the density functional framework employing plane waves for the bulk materials and Slater-type functions for the molecules. A detailed study of the electronic structure reveals that the hydrides of the light (second period) elements, BH₄⁻, CH₄, and NH₄⁺, exhibit the strongest and shortest X-H bonds. This is caused by Pauli repulsion effects of the hydrogen substituents with the larger cores of the heavier (third and fourth period) elements. The important consequence is that in the crystals, where the ionic hydrides retain their identity and charge, the distance between the negative and positive ions is larger in the heavier systems, hence less Madelung stabilization and a smaller cohesive energy. Moving from [NH₄][BH₄] to heavier congeners thus does not seem to be a promising route to obtain more suitable materials for hydrogen storage. Other types of chemical variation (different substituents) on the [NH₄][BH₄] building blocks may prove more advantageous.