Polythermal diagrams of aqueous boundary ternary systems involving sodium borohydride hydrolysis for hydrogen generation

The comprehension of solid-liquid equilibria involved in the quaternary system NaBH4–NaBO2–NaOH–H2O allows us to follow the evolution of mixtures during the sodium borohydride hydrolysis reaction for hydrogen generation. The addition of sodium hydroxide on saturated solutions of sodium borohydride and metaborate have been studied in a temperature range of −10 °C up to 50 °C which is the operational temperature range of a hydrogen generator for consumer electronics application. The solubility data was monitored at low temperatures by Differential Scanning Calorimetry, DSC analyses. Given the large number of solid phases that may crystallize in a stable or metastable state, the definition of a specific protocol was required in order to evaluate a ternary mixture via DSC analyses presenting a good repeatability. The nature of different solid phases present in this system was confirmed by powder X-ray analyses. Finally, two polythermal diagram outlines of aqueous boundary ternary systems involving water, sodium borohydride, sodium hydroxide and sodium borate were proposed. The results obtained in the water-rich area show that the sodium hydroxide hydrates are relatively soluble and the risk of precipitation is mainly due to the sodium borohydride dihydrate (NaBH4.2H2O) and sodium metaborate tetrahydrate (NaBO2.4H2O). The crystallization areas were defined as function of temperature and composition.