Realizing facile regeneration of spent NaBH4 with Mg–Al alloy

Abstract

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.