(456e) A Computational Study of Steam Hydrolysis of NaBH4

Complex hydrides are promising hydrogen storage materials and have received significant attention due to their high hydrogen storage capacity. NaBH₄ has high gravimetric (10.7 wt%) and large volumetric (115 kg of H₂ m^-3) hydrogen content. The hydrolysis reaction of NaBH₄ releases hydrogen with both fast kinetics and high extent of reaction under technical conditions by using steam deliquescence of NaBH₄. This catalyst-free reaction has many advantages over traditional catalytic aqueous phase hydrolysis. However, the reaction mechanism and kinetic barriers associated with steam hydrolysis have not yet been elucidated. We are developing a fundamental molecular level understanding of the steam hydrolysis reaction of NaBH₄ using a combination of experiments and first-principles density functional theory (DFT) calculations. We have applied finite temperature ab initio molecular dynamics (AIMD) to investigate the reaction mechanism. AIMD provides clues to elementary reaction steps. We have studied the effect of NaBH₄ concentration on the initial reaction rate, and found that high NaBH₄ concentrations lead to high initial reaction rates. The reason for this is appears to be due to a cooperative interaction between two BH₄^- groups. We have computed the free energy profiles of possible reaction pathways observed in AIMD using a cluster approach with an implicit solvent model and explicit water molecules. Explicit water is critical because proton transfer is a key step in the hydrolysis reaction.