Economic and effective regeneration of LiBH4 from its direct hydrolytic products using Al-rich alloys

Abstract

The hydrolysis of lithium borohydride (LiBH₄) is very promising for on-site hydrogen generation to supply hydrogen since it has appealing features such as a high hydrogen capacity (13.9 wt%, water included), fast hydrogen evolution rate, and mild reaction conditions. The large-scale application of LiBH₄, however, is restricted by its difficult regeneration from stable hydrolytic products (LiBO₂·xH₂O), which requires expensive reducing agents such as MgH₂, high energy-consuming dehydration process for LiBO₂·2H₂O, and additional hydrogen input. Herein, we develop a facile method to directly regenerate LiBH₄ from LiBO₂·2H₂O by ball milling with low-cost Al-rich alloys as reductants and Li₂O as additives, balancing efficiency and economy well. Introducing Mg (or Ca) into Al to form Al-rich alloys optimizes the mechanical brittleness and chemical reducibility, which facilitate the crushing, collision, and reaction of reactants and the conversion of H⁺ to H⁻. The specific mechanism study illustrates that Mg in Al₃Mg₂ preferentially reacts and the in-situ derived Al with fresh surface and fine size is highly active for reductive regeneration of LiBH₄. Besides, a little addition of Li₂O suppresses the formation of “boron sink”, [B₁₂H₁₂]⁻, further increasing the yield of LiBH₄. This work evidences the regeneration potential of Al-based reducing agents on metal borohydrides.