Revision of the NaBO2-H2O phase diagram for optimized yield in the H2 generation through NaBH4 hydrolysis

The binary phase diagram NaBO2-H2O at ambient pressure, which defines the different phase equilibria that could be formed between borates, end-products of NaBH4 hydrolysis, has been reviewed. Five different solid borates phases have been identified: NaBO2 center dot 4H(2)O (Na[B(OH)(4)]center dot 2H(2)O), NaBO2 center dot 2H(2)O (Na[B(OH)(4)]), NaBO2 center dot 2/3H(2)O (Na-3[B3O4(OH)(4)]), NaBO2 center dot 1/3H(2)O (Na-3[B3O5(OH)(2)]) and NaBO2 (Na-3[B3O6]), and their thermal stabilities have been studied. The boundaries of the different Liquid + Solid equilibria for the temperature range from 10 to 80 degrees C have been determined, confirming literature data at low temperature (20-50 degrees C). Moreover the following eutectic transformation, Liq -> Ice + NaBO2 center dot 4H(2)O, occurring at -7 degrees C, has been determined by DSC. The Liquid-Vapour domain has been studied by ebullioscopy. The invariant transformation Liq. -> Vap. + NaBO2 center dot 2/3H(2)O has been estimated at 131.6 degrees C. This knowledge is paramount in the field of hydrogen storage through NaBH4 hydrolysis, in which borate compounds were obtained as hydrolysis reaction products. As a consequence, the authors propose a comparison with previous NaBO2-H2O binary phase diagrams and its consequence related to hydrogen storage through NaBH4 hydrolysis. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.