Impedance Phenomena in Molten Salts

Abstract

Impedances have been measured of polarized platinum micro-electrodes in the molten salt systems, LiC1-KC1 at 400~ NaNO~-KNO. at 270~ NaBr-A1Br8 at 225~ and PbCL at 530~ Frequency dispersion of both the observed resistance and capacitance was evident in all cases, but apparent double layer capacities obtained by extrapolation to infinite frequency are in reasonable accord with some previously reported data. They are of magnitudes consistent with a simple Helmholtz model although the anodic values tend to be high, perhaps because of specific adsorption. Some measurements of faradaic impedance have also been made, but these were less successful and the exchange current for the Pt/Pt ~ system is high.Metal~melt interfaces are met with in four connections:(i) undergoing steady cathodic electrolysis either as part of an electroanalytical study or as part of an electrowinning process;(ii) at equilibrium in a melt which may or may not contain a significant concentration of the metal ions, e.g., at the internal surface of a metal vessel containing molten salts;(iii) approximately at equilibrium but subject to a small applied voltage changing periodically with time; and (iv) undergoing steady anodic electrolysis where in many cases the metal dissolves smoothly to form ions of its lowest stable valence state.In each case, the composition of the metal-melt interface and the degree of reactivity of the metal should be explicable in terms of the kinetic parameters (particularly exchange currents) of the various feasible electrode reactions. The structure of the melt in contact with the metal is an important related property of such systems, and some conclusions with regard to this may be drawn from values of double layer capacities at metal-melt interfaces.Until recently, only the a-c impedance method has been applied to the measurement of exchange currents and double layer capacities in melts. Double layer capacities of 15-20 ~F cm -~ were observed at carbon electrodes in cryolite-alumina melts (1), and impedance measurements at carbon electrodes in halide melts were interpreted in terms of a slow heterogeneous anode reaction such as C1 + C1 -* C12 (2).Randles and White (3) found that the capacitypotential curves for mercury in molten alkali nitrates, perchlorates, and bisulfates were shallow parabolas with minimum values of ,~20 #F cm -~. In the case of nitrates, a very high concentration of water increased the capacity and produced some semblance of the "hump" found in the curves for aqueous solutions. The rate constant for the discharge of Ni ~ from nitrates was obtained by a straightforward application of the faradaic impedance theory.Hill (4) measured the impedance of Ag in dilute solutions of AgNO3 in molten LiNO3-NaNO~-KNO~, of Ag in solutions of AgC1 in LiC1-KC1, of W in LiC1-KC1 and W in solutions of TiCL and TiCI~ in LiC1-KC1. In all cases the residual capacity and resistance of the solvent were markedly frequency dependent, the double layer capacities obtained by extrapolation to infinite frequency being 300, 500, ...

A Potential-Induced Transformation in the Double Electrical Layer on the Rhenium Electrode in Alkali Chloride Melts

“…At more positive potentials, the capacitance decreases sharply and reaches a minimum value, after which it begins to grow again. The experimental curves resemble those obtained for platinum and gold electrodes in the alkali halide melts [19][20][21]24,25,38]. Such an anomaly was not observed on the silver and copper electrodes [25,26].…”

“…This phenomenon might be explained by the behavior of salt phase ions at the electrode surface in the region of anode potentials. The unusual state of anions was first mentioned in [18][19][20][21] focused on the capacitance of a double electric layer on solid Pt and Au electrodes in molten alkali chlorides. This "unusual state" was recorded as a maximum on the anode branch of the capacitance curve.…”

Electrochemistry and Reactions in Molten Li[sub 2]SO[sub 4]-Na[sub 2]SO[sub 4]-K[sub 2]SO[sub 4]

“…An electrolysis beaker was supported on glass wool in a glass envelope, itself suspended by its ground glass flange in a copper cooling jacket. A head for electrodes and connections to vacuum and argon lines fitted the flange (18). The method of compartmenting an electrolyte and details of the remaining equipment are described elsewhere (19).…”

Electrochemical Formation of Divalent Samarium Cation and Its Characteristics in LiCl–KCl Melt

“…36−39 To consider the effect of the surrounding LiCl-KCl melt, a polar continuum solvation model was adopted with a dielectric constant (ε) of 2.3741, because it is known that the dielectric constant of ionic liquids ranges from 2 to 3. 40 We performed a natural bonding orbital (NBO) analysis using NBO 3.1 41 equipped within the Gaussian 09 program package and a natural transition orbital (NTO) analysis to assign UV−vis spectrum signals.…”

“…The complexes were energetically optimized by Becke 3-parameter exchange and Lee–Yang–Parr correlation functionals with the Coulomb-attenuating method (CAM-B3LYP) . Time-dependent DFT calculations for [SmCl 6 ] 4– were performed to obtain a simulated UV–vis spectrum with the same basis sets and functionals. − To consider the effect of the surrounding LiCl-KCl melt, a polar continuum solvation model was adopted with a dielectric constant (ε) of 2.3741, because it is known that the dielectric constant of ionic liquids ranges from 2 to 3 . We performed a natural bonding orbital (NBO) analysis using NBO 3.1 equipped within the Gaussian 09 program package and a natural transition orbital (NTO) analysis to assign UV–vis spectrum signals.…”