Solid–liquid equilibria in the ternary system NaBO2–NaOH–H2O thermal behavior of double salts

To increase the energy density of a hydrogen generator system, the NaBH4 concentration has to be maximized; nevertheless, NaBO2 crystallization must be avoided. Sodium borohydride decomposes spontaneously in water to produce hydrogen and by-products: hydrated sodium borate compounds. Sodium hydroxide is added to the solution to limit this self-decomposition, thus stabilizing the system. Therefore, the delimitation of the homogeneous liquid phase domain in the phase diagram quaternary system NaBH4–NaBO2–NaOH–H2O is fundamental. This work introduces a critical analysis of the aqueous boundary binary and ternary phase diagrams presented in the literature, which take part of the complete description of the system. After this critical review, showing numerous inconsistencies about the solid phases presented in the literature, it has been decided to redetermine the isothermal for the NaBO2–NaOH–H2O ternary system in a temperature range of −10 to 50 °C. The experimental results of the solubility curves at 50, 25 and 10 °C under atmospheric pressure will be presented. The solid phases (borate hydrates, hydroxide hydrates and double salts) in equilibrium with the liquid phase will be also reported.