Altering the chemical state of boron towards the facile synthesis of LiBH₄ via hydrogenating lithium compound-metal boride mixture

Abstract

Boron sources in forms of SiB4/FeB/TiB2 were used to react with LiF/LiH under hydrogen atmosphere to investigate their effectiveness for synthesizing LiBH4, a promising hydrogen storage material. Fourier transform infrared (FTIR) study revealed the formation of Bsingle bondH bond vibrations in these hydrogenated systems, and it demonstrated the generation of LiBH4. When using FeB and TiB2, few amounts of Bsingle bondH bonds were formed in the hydrogenated samples either reacting with LiH or LiF. When utilizing SiB4, the formation of Bsingle bondH bonds was promoted for both systems mixing with LiH and LiF. The results imply that a stepwise process of LiBH4-x→LiBH4 possibly took place during the hydrogenation process. Importantly, SiB4single bondLiH system exhibited the best hydrogenation performance. At moderate conditions of 250 °C and 10 MPa H2, LiBH4 was successfully synthesized from this system. A facile synthesis pathway, SiB4(s) + 4LiH(s) + 6H2(g) → 4LiBH4(s) + Si(s), having a ΔrHm of −65 kJ/mol H2, was proposed. This study supports that the chemical state of boron in the reactant is an important factor affecting the generation of LiBH4. A hydrogenation reaction between SiB4 and CaH2 or MgH2 may be also applicable for synthesizing Ca(BH4)2 or Mg(BH4)2, which are regarded as potential hydrogen storage materials.