Realizing facile regeneration of spent NaBH₄ with Mg–Al alloy

The regeneration of sodium borohydride (NaBH4) is crucial to form a closed cycle after it either supplies hydrogen energy via a hydrolysis process or provides energy through electron transfer at the anode of direct borohydride fuel cells (DBFCs). In both of these cases, the spent fuels are NaB(OH)4 from NaBO2 aqueous solution. However, the current regeneration process from (NaB(OH)4)·xH2O to form NaBH4 by reduction reaction and calcination at high temperature with metal hydrides as reducing agents is very expensive. In this work, we developed a simple regeneration process via ball milling with Mg–Al alloys as the reducing agent for NaB(OH)4 under an argon atmosphere. Under optimized conditions, a high yield of about 72% of NaBH4 could be obtained. Mechanistic study showed that all the hydrogen atoms from NaB(OH)4 remain in NaBH4 and no additional hydrogen sources are needed for the reduction process. The inexpensive Mg–Al alloy works as a reducing agent transforming the H+ to H− in NaBH4. This approach demonstrates a ∼20-fold cost reduction compared with the method using metal hydrides. This opens the door to the commercial implementation of simple ball milling processes for the regeneration of spent NaBH4 from NaB(OH)4 with cheap reducing agents