Phase Diagrams of the Ternary Systems NaBH₄ + NaOH + H₂O, KBH₄ + KOH + H₂O, NaBO₂ + NaOH + H₂O, and KBO₂ + KOH + H₂O at −10 °C

Abstract

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 .

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

“…Figure 3 shows the superposition of isothermal sections of the ternary system NaBH4-H2O-NaOH at -10 °C determined by Churikov et al [12] and at 0, 18 and 30 °C determined by Mikheeva and Breitsis [14] also illustrated by Shang and Chen [21]. The graphic on Figure 3 highlights the solid phase domains of different ternary invariant points of each isotherm.…”

“…In a previous work, the solubility limit of the hydrolysis's by-products, NaBO2.yH2O, has been investigated and the experimental results for the solubility limit in the ternary system NaBO2-NaOH-H2O under ambient pressure at 10, 25 and 50 °C have been presented [11]. In this present work, polythermal diagrams of the two aqueous boundary ternaries NaBH4-NaOH-H2O (corresponding to 0 % of hydrolysis reaction conversion) and NaBO2-NaOH-H2O (100 % of conversion) will be proposed from critical analysis of the description of several isotherm sections of each system [11][12][13][14] and will be completed by DSC analysis with the intent to give more precision about invariant transformations of the system. The third aqueous boundary ternary system NaBH4-NaBO2-H2O cannot be experimentally evaluated because of the spontaneous decomposition of NaBH4 in water, i.e.…”

Solution Structure of Alkaline NaBH4 Solution: A sight From X-ray Scattering

“…TheNaBH 4 ·2H 2 O was synthesized according the ternary phase diagram of NaBH 4 -NaOH-H 2 O [7] and examined by both element analysis and X-ray powder diffraction. Density, d, of all the solutions was measured using a graduated pycnometer(≈15cm 3 ) with a reproducibility of (0.05%) in a thermostat (GDH-1015W, safe, China, ±0.01K).…”

Solid–Liquid Equilibrium in the Aqueous System Containing the Borates of Potassium, Rubidium, and Magnesium at 348 K

“…To understand the thermodynamics behavior of the system mentioned above, some phase equilibria have been performed, such as the ternary system K + // BO 2 − , OH − ·H 2 O at 263 K, 2 Li + , K + // borate·H 2 O 3 and Rb + , Cl − // borate·H 2 O 4 at 348 K, and K + , Rb + (Mg 2+ ) // Cl − ·H 2 O at 298 K, 5 323 K 6 and 348 K; 7 the quaternary system Li + , K + , Rb + // borate·H 2 O 8 and Li + , K + , Mg 2+ // borate·H 2 O 9 at 348 K. Commonly, the crystalloid form of the borates is affected by the coexist ions. To date, the stable phase equilibrium of the system K + , Rb + , Mg 2+ // borate· H 2 O in aqueous solution at 348 K has not been reported.…”