(283c) A Computational Study of the NaBH4 Hydrolysis Reaction for Hydrogen Production

NaBH₄ is a promising hydrogen storage material due to its high hydrogen content (10.7 wt%), relative safety, and low cost. The NaBH₄ hydrolysis reaction: NaBH₄ + 4H₂O → NaBO₂·2H₂O + 4H₂ has found application in niche markets such as portable energy generation. The rate of hydrolysis reaction without catalyst is very low in aqueous solution, so a great deal of work has been focused on catalyst design and application.^1,2 Additionally, the aqueous hydrolysis reaction also suffers from low system hydrogen storage capacity due to excess water used as solvent for NaBH₄ and byproducts. Recently, Matthews and coworkers demonstrated that steam hydrolysis of NaBH₄ can be used to release hydrogen with both fast kinetics and high extent of reaction without the use of any catalyst and with reduced water consumption.^3-5 Despite a long history of research and development, the basic mechanism of NaBH₄ hydrolysis reaction is not understood on a molecular level. We have performed first-principles density functional theory calculations to investigate the elementary reaction steps in aqueous NaBH₄ hydrolysis. Detailed understanding of these steps would be useful for reaction condition optimization, catalyst design, and ultimately for precise control of the reaction in practical applications. Representative minimum energy pathways have been characterized from the family of mechanisms that exist in the real system by using the conventional nudged elastic band method⁶ and the solid-state nudged elastic band method.⁷ We identified energy barriers and reaction intermediates for each elementary step. Taken together, these steps provide a full picture of the NaBH₄hydrolysis reaction at the molecular level.

References

                (1)          Schlesinger, H. I.; Brown, H. C.; Finholt, A. E.; Gilbreath, J. R.; Hoekstra, H. R.; Hyde, E. K. Journal of the American Chemical Society 1953, 75, 215-219.

                (2)          Liu, B. H.; Li, Z. P.; Suda, S. Journal of Alloys and Compounds 2006, 415, 288-293.

                (3)          Aiello, R.; Sharp, J. H.; Matthews, M. A. International Journal of Hydrogen Energy 1999, 24, 1123-1130.

                (4)          Marrero-Alfonso, E. Y.; Gray, J. R.; Davis, T. A.; Matthews, M. A. International Journal of Hydrogen Energy 2007, 32, 4717-4722.

                (5)          Marrero-Alfonso, E. Y.; Gray, J. R.; Davis, T. A.; Matthews, M. A. International Journal of Hydrogen Energy 2007, 32, 4723-4730.

                (6)          Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. Chem. Phys. 2000, 113, 9901-9904.

                (7)          Sheppard, D.; Xiao, P.; Chemelewski, W.; Johnson, D. D.; Henkelman, G. J. Chem. Phys. 2012, 136, 074103-074108.