Intermediate phases during decomposition of metal borohydrides, M(BH₄)_n (M = Na, Mg, Y)

Ideal materials for hydrogen storage for on-board vehicular applications need to be reversible, store hydrogen with high gravimetric and volumetric density, and operate at near ambient conditions. Although metal borohydrides meet the high-density requirements, the presence of intermediate phases during decomposition adversely affects their reversibility. A fundamental understanding of these phases is, therefore, important before metal borohydrides can be used for practical applications. Recent experiments indicate that the decomposition of NaBH₄ yields Na₂B₁₂H₁₂ as the intermediate phase, while decomposition of Mg(BH₄)₂ and Y(BH₄)₃ yields Mg(B₃H₈)₂ and Y(B₃H₈)₃, respectively. To understand why the intermediate phases are different for Na, Mg, and Y borohydrides, we have carried out systematic first-principles calculations based on density functional theory to investigate the relative stability of M[BH₄]_n, M[B₃H₈]_n, and M₂[B₁₂H₁₂]_n (M = Na, Mg, and Y; n is the valence of metal atom). The agreement between our theoretical results with available experiments provides a fundamental understanding of the origin of these intermediate phases and sheds light on the preferred metal borohydride for reversible hydrogen storage.