Concentrated water-alkaline solutions of sodium and potassium borohydrides are used as a fuel and as a hydrogen source in hydrogen power engineering, including low-temperature fuel cells, where borohydrides are converted into metaborates. The performance of such mixtures is determined by the solubility of their components, negative temperatures being especially critical. The solubility in the ternary systems NaBH 4 + NaOH + H 2 O, KBH 4 + KOH + H 2 O, NaBO 2 + NaOH + H 2 O, and KBO 2 + KOH + H 2 O was studied by means of isothermal saturation at -10 °C. The compositions of the equilibrium solid phases and of mixtures corresponding to nonvariant equilibrium points were determined; the solubility diagrams of the systems were plotted. The presence of a range of homogeneous solutions and fields of crystallization of ice, anhydrous, and hydrated forms of the salts and hydroxides is characteristic of all four systems at the said temperature. The composition of crystalline hydrates depends on the ratio of the components in the mixture and on the cation nature. Of the borohydride systems, the best solubility was observed in that with NaBH 4 , whereas in the metaborate systems it was that with KBO 2 .

Concentrated water-alkaline solutions of sodium and potassium borohydrides are used as a fuel and hydrogen source in hydrogen power engineering, including low-temperature fuel cells; borohydrides convert into metaborates. The performance of fuel mixtures is determined by their solubility in water. The solubility in the ternary systems NaBOwas studied by means of isothermal saturation within the range (-10 to 50) °C. The composition of equilibrium solid phases and those of mixtures corresponding to nonvariant equilibrium points were determined; the solubility diagrams of these systems were constructed. The systems with sodium and potassium ions are considerably different in relation to the temperature influence on the solubility of the solid components. The presence of a range of homogeneous solutions and fields of crystallization of hydrated and anhydrous forms of the metaborates and hydroxides is characteristic for both systems at all investigated temperatures. The composition of crystalline hydrates depends on the component ratio in the mixture and the cation nature.