Correlation between structural stability of LiBH4 and cation electronegativity in metal borides: an experimental insight for catalyst design

Abstract

Nanosized metal borides MB_x (M = Mg, Ti, Fe, Si) are found to play an important role in enhancing the hydrogen storage performance of LiBH₄ in this work. The hydrogen storage behavior and mechanism of these modified systems are investigated through TPD-MS, XRD, FTIR and SEM characterization methods. By introducing these metal borides into LiBH₄ through ball milling, the systems display three dehydrogenation stages disclosing their similarity and distinction. The 1^st stage starts at 190 °C, the 2^nd stage ranges from 280 °C to 400 °C and the 3^rd stage ends at 550 °C with a peak at round 440 °C similar to that of pristine LiBH₄. Distinguishing features exist at the 2^nd stage revealing the effectiveness of MB_x in an order of MgB₂ < TiB₂ < FeB < SiB₄. Significantly, reversibility up to 9.7 wt% is achieved from LiBH₄ with assistance of SiB₄. The catalytic effect of MB_x is influenced by the Pauling electronegativity of M in MB_x and the interfacial contact characteristic between LiBH₄ and MB_x. The larger electronegativity leads to an enhanced catalytic effect and consequently lower temperature at the major stage. In contrast to the components in the solid state, the molten LiBH₄ promotes a catalytic effect due to a superior interfacial contact. These results provide an insight into designing high-performance catalysts applied to LiBH₄ as a hydrogen storage material.