борогидриды

A study of the solubility in the ternary systems NaBH₄–NaOH–H₂O, KBH₄–KOH–H₂O, NaOH–NaBO₂–H₂O и KOH–KBO₂–H₂O is of special fundamental and practical interest. The first two systems are used as a fuel and as a hydrogen source in hydrogen power engineering that includes the low-temperature fuel cells; borohydrides are converted into metaborates. Hence, the last two systems represent the discharged by-product. The performance of such mixtures is determined by the solubility of their components, negative temperatures being especially critical. Therefore, in the present work the solubility in the ternary systems was studied by means of isothermal saturation at – 10°C. The compositions of the equilibrium liquid and solid phases and compositions of the eutonic and peritonic equilibria, and the coordinates of the homogeneous solution ranges have been determined. The best solubility of the solid components is characteristic of systems NaBH₄–NaOH–H₂O and KBO₂–KOH–H₂O.

The paper studies the influence of temperature (50–100°C) and alkalinity (C OH – = 2.33–9.53 M) of aqueous solutions on the hydrolysis (self-destruction) kinetics of borohydride ions BH4 – . Characteristic peculiarities of the kinetic curve have been established and formulae to approximate the temperature-concentration dependence of the hydrolysis rate are proposed. An increase in temperature leads to an increase in the rate constant k of borohydride hydrolysis, and the temperature dependence of k satisfactorily obeys Arrhenius' equation. The influence of solution alkalinity on the borohydride hydrolysis rate was explored. Within the temperature range studied, the k = f(C OH -) curve consists of two fragments, each with the prevalence of one of two different mechanisms (paths) of borohydride hydrolysis. In highly-alkaline aqueous solutions, non-catalytic hydrolysis mainly occurs, whose rate is determined by temperature, being p�-independent. At lower alkalinity, the hydrolysis rate sharply increases due to catalysis by � + ions. A power dependence of k on the � + concentration has been found; the point where the mechanisms are switched is determined by temperature.

The study of the anodic oxidation of borohydride-ion BH₄^– on catalytically active nickel electrode by methods of potentiostatic inclusion, galvanostatically inclusion, cyclic voltammetry and infrared spectroscopy with Fourier transformation. The composition of some intermediates of the process of electrochemical oxidation of BH₄^– and the mechanism of decomposition of borohydride, which includes the following basic stages: BH₄^– → BH₃(OH)^– → BH₂(OH)₂^– → BH(OH)₃^– → B(OH)₄^–, is determined. The regularities of the kinetics of the electrochemical oxidation of BH₄^– – ion Ni-electrode is determined. The diffusion coefficient BH₄^– ion in aqueous solution at a temperature of 25°C, measured by electrochemical methods, ranges from 5.3·10^–5 to 1.6·10^–5, the average value of 2·10^–5 cm²/s.