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