Minimizing water utilization in hydrolysis of sodium borohydride: The role of sodium metaborate hydrates

Hydrogen can be obtained from hydrolysis of chemical hydrides, making this an interesting option for hydrogen storage. The gravimetric and volumetric energy densities attainable from hydrolysis of chemical hydrides depend in large measure on the amount of water required for the process. The steam hydrolysis of sodium borohydride produces hydrated metaborate byproducts in which the degree of hydration of the metaborate is closely tied to the amount of water in the reaction. Experimental characterization of these sodium metaborate byproducts indicates that the primary byproduct of steam hydrolysis is NaBO2·2H2ONaBO2·2H2O under a variety of reaction conditions. A group contribution method is used to estimate the heats and free energies of formation of a variety of hydrated metaborates. This information will be useful in establishing the preferred (stable) hydration state of the byproducts, and in distinguishing these from metastable products.