Hydrolysis of the Borohydride Anion BH4-: A 11B NMR Study Showing the Formation of Short-Living Reaction Intermediates including BH3OH.

In hydrolysis and electro-oxidation of the borohydride anion BH4-, key reactions in the field of energy, one critical short-living intermediate is BH3OH-. When water was used as both solvent and reactant, only BH3OH- is detected by 11B NMR. By moving away from such conditions and using DMF as solvent and water as reactant in excess, four 11B NMR quartets were observed. These signals were due to BH3-based intermediates as suggested by theoretical calculations; they were DMF·BH3, BH3OH-, and B2H7- (i.e., [H3B-H-BH3]- or [H4B-BH3]-). Our results shed light on the importance of BH3 stemming from BH4- and on its capacity as Lewis acid to interact with Lewis bases such as DMF, OH-, and BH4-. These findings are important for a better understanding at the molecular level of hydrolysis of BH4- and production of impurities in boranes synthesis.

Hydrogen generation from a sodium borohydride-nickel core@shell structure under hydrolytic conditions.

Sodium borohydride (NaBH4) is an attractive hydrogen carrier owing to its reactivity with water: it can generate 4 equivalents of H2 by hydrolysis (NaBH4 + 4H2O → NaB(OH)4 + 4H2). Since using NaBH4 in the solid state is the most favorable way to achieve a high gravimetric hydrogen storage capacity (theoretical maximum of 7.3 wt%), we have investigated the possibility of developing a core@shell nanocomposite (NaBH4@Ni) where a metallic nickel catalyst facilitating the hydrolysis is directly supported onto NaBH4 nanoparticles. Following our initial work on core-shell hydrides, the successful preparation of NaBH4@Ni has been confirmed by TEM, EDS, IR, XRD and XPS. During hydrolysis, the intimately combined Ni0 and NaBH4 allow the production of H2 at high rates (e.g. 6.1 L min-1 g-1 at 39 °C) when water is used in excess. After H2 generation, the spent fuel is composed of an aqueous solution of NaB(OH)4 and a nickel-based agglomerated material in the form of Ni(OH)2 as evidenced by TEM, XPS and XRD. The effective gravimetric hydrogen storage capacity of nanosized NaBH4@Ni has been optimized by adjusting the required amount of water for hydrolysis and an effective hydrogen capacity of 4.4 wt% has been achieved. This is among the best reported values.