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

Graphical A cost-effective way to produce NaBH 4 with high yield by ball milling hydrated borax (Na 2 B 4 O 7 ⋅ 10H 2 O) with cheap Al or Al 88 Si 12 at ambient condition is presented. An effective strategy was developed to facilitate mass transfer during the reaction by introducing NaH to enable the formation of loose NaAlO 2 instead of dense Al 2 O 3 on Al surface, and by using Si as a milling additive that prevents agglomeration and also breaks up passivation layers. A significant obstacle in the large-scale applications of sodium borohydride (NaBH 4 ) for hydrogen storage is its high cost. Herein, we report a new method to synthesize NaBH 4 by ball milling hydrated sodium tetraborate (Na 2 B 4 O 7 ⋅ 10H 2 O) with low-cost Al or Al 88 Si 12 , 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 NaAlO 2 instead of dense Al 2 O 3 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 Na 2 B 4 O 7 ⋅ 10H 2 O serves as a hydrogen source for NaBH 4 generation. Considering the low cost of the starting materials and simplicity in operation, our studies demonstrate the potential of producing NaBH 4 in a more economical way than the commercial process.