Converting H+ from coordinated water into H− enables super facile synthesis of LiBH4

Abstract

LiBH₄ has excellent application potential as an off- or on-board hydrogen carrier for its unparalleled hydrogen capacity (18.5 wt%). However, the difficulty in recycling “one-pass” LiBH₄ and the resulting high cost hamper its large-scale applications. Here, we report a facile and low-cost method for LiBH₄ regeneration by ball milling its hydrolysis by-product (LiBO₂·2H₂O) and Mg under ambient conditions, where expensive H⁻ stored in LiBH₄ is entirely converted from cheap H⁺ in coordinated water. This scenario is reported for the first time without hydrides employed as hydrogen sources for the reduction process. Moreover, the yields for LiBH₄ synthesized by this method may reach 40%. The cost of LiBH₄ regeneration exhibits a 5-fold reduction over a previous study using NaBH₄ as a raw material. Significantly, the as-purified product displays even better physicochemical properties than commercial LiBH₄, for example, superior hydrolysis kinetics. Overall, a one-step approach, combining hydrogen production and storage in a closed cycle, is proposed in this work. Furthermore, the formation mechanism of LiBH₄ is illustrated and the intermediate product, LiBH₃(OH), is successfully observed for the first time during the regeneration course.