(330b) Deliquescence and Reaction Kinetics in the Steam Hydrolysis of Sodium Borohydride

The hydrolysis of chemical hydrides, such as sodium borohydride (NaBH₄), has been widely studied as a method to store and release hydrogen at the point of use (NaBH₄ + (2+x) H₂O → 4H₂ + NaBO₂・xH₂O + heat). Traditionally, efforts have focused on aqueous phase hydrolysis wherein the hydride is stabilized in a caustic solution and passed over a catalyst when hydrogen release is desired. Although significant advances on catalytic materials have been made in this area, the approach requires that the reactants and products remain in solution, thereby demanding copious amounts of water. The aqueous hydrolysis approach inevitably results in an excessively heavy, bulky system. This is one of the major reasons it was given a ?no-go? decision by the Department of Energy for automotive hydrogen storage applications.

However, a new approach has been investigated wherein solid NaBH₄ is contacted with steam or water vapor just above the boiling point of water, resulting in high hydrogen yields without aid of a catalyst. The purpose of this study was to elucidate the mechanism and physical phenomena associated with steam hydrolysis of sodium borohydride as an alternative pathway for hydrogen storage. Investigation of the effect of temperature on the kinetics of hydrogen release by steam hydrolysis revealed that the reaction is inhibited at higher temperatures. Based on visual observation with a borescope camera during reaction, it is now evident that the crucial first step in the reaction sequence is for hygroscopic sodium borohydride to absorb water and deliquesce, forming a highly concentrated viscous solution which then reacts to form hydrogen. The occurrence of deliquescence depends primarily on the relative humidity and we have determined a threshold exists below which deliquescence cannot occur.

Based on the observation that a concentrated aqueous phase exists in the reaction sequence, we have developed a technique using in-situ ¹¹B-NMR at elevated temperatures for determining the kinetics of hydrogen release decoupled from the deliquescence kinetics. Stable intermediates found in dilute aqueous hydrolysis at low temperatures are not present in the high-temperature concentrated spectra. Fundamental differences in the reaction mechanism of the traditional approach and the steam/water vapor approach are apparent. Utilizing the latter technique may result in a more suitable hydrogen storage methodology.