Kinetics of Self-Hydrolysis of Concentrated Sodium Borohydride Solutions At High Temperatures

Hydrolysis of sodium borohydride (NaBH₄) over metal catalyst is being accepted as a potential technology to deliver H₂ for portable fuel cells. However, unresolved issues such as minimizing the amount of water and nature of hydration by-product limit the hydrogen storage capacities of NaBH₄. An alternative method, steam/water vapor hydrolysis, can enhance the hydrogen storage capacities without catalyst, if operating conditions can be optimized [1]. In this approach, solid NaBH₄ absorbs water and deliquesces, forming a highly concentrated viscous solution at near boiling point of water and hydrogen evolution occurs from this concentrated solution. Self or spontaneous hydrolysis also occurs even at room temperature when NaBH₄ is mixed with water and becomes significant at elevated temperatures and needs to be arrested for increasing the shelf life of the solution. Thus the knowledge of self-hydrolysis of concentrated solutions at elevated temperature is crucial for: (1) developing steam/water vapor hydrolysis technology; and (2) storage of NaBH₄-based fuel tanks. These conditions have not been studied in detail so far and the reported studies except two [2, 3] were for buffered dilute solutions of NaBH₄ (0C ).

The present study reports the kinetics of self-hydrolysis of unbuffered, unstabilized concentrated NaBH₄ solutions (10-20 wt %) at elevated temperatures (25-80 ⁰C). The evolution of metaborate formation and NaBH₄ consumption were measured in-situ by ¹¹B NMR to determine the reaction kinetic parameters. The reaction order with respect to borohydride concentration is independent of NaBH₄ conversion (%) and decreases with temperature, i.e., the reaction order decreases from first-order to 0.24 for an increase in temperature from 25 to 80 ⁰C. The activation energy is found to be NaBH₄ concentration dependent and increases with increase in NaBH₄ conversion (%). A power law kinetic model in borohydride concentration which accounts the effect of temperature on reaction order and the effect of NaBH₄ conversion (%) on activation energy has been developed to represent the self-hydrolysis rate.

[1] Beaird AM, Davis TA, Matthews MA. Deliquescence in the hydrolysis of sodium borohydride by

water vapor, Ind Eng Chem Res 2010;49(20):9596-9599.

[2] Gonçalves, A., Castro, P., Novais, A.Q., Fernandes, V., Rangel, C. and Matos, H., Dynamic

Modeling of Hydrogen Generation via Hydrolysis of Sodium Borohydride, In Proceedings of

PRES’07, 24-28 June, Ischia, Italy, (In Chemical Engineering Transactions, Vol. 12, 2007. Editor:

Jiri Klemes, AIDIC, Italy, 243.)

[3] Andrieux J, Demirci UB, Hannauer J, Gervais C, Goutaudier C, Miele P. Spontaneous hydrolysis of

sodium borohydride in harsh consitions. Int J Hydrogen Energy 2011;36:224-233.