Probing Order Phenomena and Interactions in Molten Salt Binary Mixtures with Impedance Spectroscopy and Cyclic Voltammetry

Abstract

Temperature-dependent electrochemical impedance spectroscopy and cyclic voltammetry are employed to study changes in the order and interactions in molten alkali-halide salt binary-mixtures. Nyquist plots of KCl-LiCl, KCl-NaCl and KBr-NaBr equimolar mixtures at 700°C are characterized by a small semicircle at high-frequencies and a larger low-frequencies semicircle. Two peaks with well-separated time constants are present in the Bode phase plots. Those are referred to the interfacial and double-layer properties at high frequencies and to the bulk electrical properties away from the electrode at low frequencies. Along the measured temperatures (700–800°C), the KCl-NaCl mixture is featured with a low-frequencies peak only below ∼750°C, which diminishes at higher temperatures, as may point on a dispersion of the ionic structure. The entropies of the mixtures are calculated from the change of the electrochemical windows with temperature. The entropy of KCl-NaCl increases drastically along heating, implying on a drastic change in the order and interactions in the melt. In contrast, the KX-LiX (X = Cl, Br, I) entropies do not change. It is suggested that the stability of the ionic structure of the molten salt mixtures with temperature is a result of collaborative asymmetric polarization interactions among the different ions in the melt.