Preparation and regeneration of metal borohydrides for high-density hydrogen supply: Progress, challenges, and perspectives

Metal borohydrides have high theoretical hydrogen production/storage densities (>10 wt%) and offer an efficient real-time hydrogen supply for small and medium-sized mobile/portable electronics through catalytic hydrolysis under mild conditions. They are also suitable for large-scale applications. However, their practical applications are limited because of by their complex synthesis, high cost, low purity, and poor reversibility. Numerous strategies have been proposed to overcome these limitations, including increase in the purity and reduction of the synthesis cost of these materials by optimizing the boron/hydrogen source, reducing agent, and reaction methods used. This review summarizes recent progress in the preparation and regeneration of single-cation (e.g., NaBH4, LiBH4 and Mg(BH4)2) and dual-cation borohydrides. Ouyang developed a practical, facile, and low-cost method for regenerating MBH4 (M=Na, Li) by ball milling of MBO2·xH2O and cheap Mg/MgH2. In this method, the direct hydrolysis of MBH4 produces MBO2·xH2O, and the hydrogen needed to regenerate MBH4 comes from the crystallization water of MBO2·xH2O. This method has the potential to overcome the prevailing challenges of decreasing the synthesis cost and increasing the reversibility of metal borohydrides.