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: NaBO2·4H2O (Na[B(OH)4]·2H2O), NaBO2·2H2O (Na[B(OH)4]), NaBO2·2/3H2O (Na3[B3O4(OH)4]), NaBO2·1/3H2O (Na3[B3O5(OH)2]) and NaBO2 (Na3[B3O6]), and their thermal stabilities have been studied. The boundaries of the different Liquid + Solid equilibria for the temperature range from −10 to 80 °C have been determined, confirming literature data at low temperature (20–50 °C). Moreover the following eutectic transformation, Liq. → Ice + NaBO2·4H2O, occurring at −7 °C, has been determined by DSC. The Liquid–Vapour domain has been studied by ebullioscopy. The invariant transformation Liq. → Vap. + NaBO2·2/3H2O has been estimated at 131.6 °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.

► Hydration degree of borate formed after NaBH4 hydrolysis defines the GHSC.
► A revision of the NaBO2–H2O binary phase diagram under atm. pressure is proposed.
► NaBO2·yH2O with y = 4, 2, 2/3, 1/3 and 0 were observed as stable compounds.
► Three Liquid + Solid domains define the best conditions to reach highest GSHC.
► 9.3 wt%H2 could be obtained if NaBO2·2/3H2O is formed after NaBH4 hydrolysis.