Deliquescence Behavior of Sodium Borohydride and Steam

Sodium borohydride as an effective, cheap and stable material for hydrogen storage has been studied widely. Despite prior studies [1-5] on the steam hydrolysis of NaBH₄, the mechanism for the release of hydrogen by water vapor/ NaBH₄ contact has not been well understood. Recently, Beaird et al. [2] established that the initial step in the pathway is deliquescence of water onto NaBH₄, and she reported the deliquescence relative humidity (DRH) based on visual observation of the change in appearance of NaBH₄ crystals during reaction. A basic study of the phase behavior during the water vapor hydrolysis reaction is necessary should the reaction pathway be exploited in for future engineering designs. Therefore, in this presentation we build on prior DRH measurements by developing a theoretical model and utilizing this, with experiments and simulation, to accurately predict the deliquescence behavior of steam on sodium borohydride. Macroscopically, we describe deliquescence in the bulk limit where the theory is well established. Microscopically, we have also used first principles density functional theory to examine the initial stages of deliquescence at the atomic level. We have performed ab initio molecular dynamics at 300 K and 350 K using a slab model of the NaBH4(001) surface covered by water. The simulations indicate that the initial stages of deliquescence happen very rapidly, with water penetrating the (001) surface on the picosecond time-scale. Deliquescence is known to initiate clearly with increasing relative humidity on reaching free energy equality between the initial (salt particle plus vapor) and final (fully dissolved solution drop plus vapor) states until deliquescence. Additional experiments utilize in situ visualization, as described by Beaird et al. [2].

[1] R. Aiello, et al., "Production of hydrogen from chemical hydrides via hydrolysis with steam," International Journal of Hydrogen Energy, vol. 24, pp. 1123-1130, 1999.

[2] A. M. Beaird, et al., "Deliquescence in the Hydrolysis of Sodium Borohydride by Water Vapor," Industrial & Engineering Chemistry Research, vol. 49, pp. 9596-9599, 2010.

[3] H. Liu, et al., "Vapor phase batch hydrolysis of NaBH4 at elevated temperature and pressure," International Journal of Hydrogen Energy, vol. In Press, Corrected Proof.

[4] E. Y. Marrero-Alfonso, et al., "Hydrolysis of sodium borohydride with steam," International Journal of Hydrogen Energy, vol. 32, pp. 4717-4722, 2007.

[5] E. Y. Marrero-Alfonso, et al., "Minimizing water utilization in hydrolysis of sodium borohydride: The role of sodium metaborate hydrates," International Journal of Hydrogen Energy, vol. 32, pp. 4723-4730, 2007.