Temperature-Dependent Impedance Spectroscopy of Molten Alkali-Halide Salt Binary Mixtures

Abstract

The complex electrochemical impedance spectra of tungsten -(KCl-LiCl) interface at 700-800 • C consist of two well-separated semicircles. One at high frequencies and reflects charge transfer resistance and interfacial capacitance. The second is at low frequencies and is attributed to the electrical properties of the melt. Similar spectra are measured for tungsten -(KCl-NaCl) interface. The low-frequencies semicircle though is reduced above 750 • C. Entropy estimations from cyclic voltammetry measurements indicate an increase in the entropy of KCl-NaCl at ∼750 • C and a possible change in the inter-ionic order and interactions. Asymmetric anion polarization interactions are suggested to be involved.Molten salt mixtures are widely used as media for chemical and electrochemical synthesis. The use of molten salt eutectics, such as: KCI-NaCl, KCl-LiCl, and CaCl 2 -NaCl 1-3 is popular due to their high heat capacity, high ionic-conductivity and relatively low melting points. 4-6 Molten alkali-halide salts are featured with a wide electrochemical window 7,8 that enable the synthesis of a wide range of materials.The ionic structure of molten salts and ionic liquids and the interactions that dictate their structure 9-13 are interesting issues in the context of the structure of the electrical double layer 14,15 in these media. Additionally, it is important for synthesis, since they probably affect the crystal structure 16,17 and the shape 18,19 of the products. The structure of liquids is usually studied with molecular simulations 15,20,21 and with neutron and X-ray diffraction. 22-24 Here we employ electrochemical impedance spectroscopy (EIS) to study the electrical properties of molten alkali-halide binary-mixtures as function of temperature and to probe changes of order phenomena and interactions in molten salts. 25 This is feasible since electrical properties of molecular systems are highly sensitive to changes in collective phenomena, for example: intermolecular interactions, self-organization of molecules, microstructure and phase transitions. 15,26-29 Furthermore, interfacial phenomena in electrochemistry, such as: electrical double layers, surface films, adsorption and ionic organization close to the electrodes are frequency-related. 30-32 Those can be resolved with EIS that employ broad range of frequencies and with analysis of the relaxation times of their capacitive reactance. 29,33 In addition, the electrical response of a heterogeneous cell can vary substantially by the composition of the electrolyte, its microstructure and the interactions among the ionic species, 34 as demonstrated here. ExperimentalMaterials.-Alkali-chloride salts (anhydrous, 99%, Merck KGaA, Biolab LTD, Alfa Aesar and Sigma Aldrich) were dehydrated at 500 • C for 12 hours before the experiments and at 250 • C for few hours in the cell as part of the experiment heating program. LiCl was dehydrated at 250 • C for at least 12 hours before the experiments. 35Measurements.-The measurements were performed in alumina crucibles inside a glove box ...

The Electrochemical Behavior of Tellurium Tetrachloride in LiCl-KCl Eutectic Molten Salt at 450 ºC

“…When cations are present in the molten salt with different sizes and different charge densities, an asymmetric polarization of anions is anticipated which affects the electrostatic stability of the mixture. 32 In order to better understand the interfacial impedance behavior, the EIS data were fitted into an electrical equivalent circuit (EEC) and the values of the circuit components are summarized in Table SI (supporting information). In general, the impedance spectra showed two time constants.…”

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

“…Reproduced with permission from Ref. 71. The entropy values in Table I were calculated according to Equation 1.…”