Breaking the Passivation: Sodium Borohydride Synthesis by Reacting Hydrated Borax with Aluminum

A significant obstacle in the large-scale applications of sodium borohydride (NaBH₄ ) for hydrogen storage is its high cost. Herein, we report a new method to synthesize NaBH₄ by ball milling hydrated sodium tetraborate (Na₂ B₄ O₇  ⋅ 10H₂ O) with low-cost Al or Al₈₈ Si₁₂ , instead of Na, Mg or Ca. An effective strategy is developed to facilitate mass transfer during the reaction by introducing NaH to enable the formation of NaAlO₂ instead of dense Al₂ O₃ on Al surface, and by using Si as a milling additive to prevent agglomeration and also break up passivation layers. Another advantage of this process is that hydrogen in Na₂ B₄ O₇  ⋅ 10H₂ O serves as a hydrogen source for NaBH₄ generation. Considering the low cost of the starting materials and simplicity in operation, our studies demonstrate the potential of producing NaBH₄ in a more economical way than the commercial process.