Capacitance of an Iridium Electrode in Molten Alkali Metal Halides

Abstract

Using the method of measuring the capacitance of an iridium electrode in molten salts NaCl, KCl, CsCl, NaBr, KBr, CsBr, NaI, and KI, the potential dependences of the electrode capacitance are obtained at different alternating signal frequencies and temperatures. In all these systems, the capacitance is shown to have the form of a curve with two minima and a maximum between them; at some temperatures and frequencies, an additional third minimum of capacitance is observed in the interval between the two main ones. The shape of the curve, the value of the capacitance and the position of the minima depend on the alternating signal frequency, the temperature, and the cation-anion composition of the electrolyte. An increase in the radii of the salt cation and/or anion, as well as a decrease in frequency and temperature, complicate the shape of the capacitance curve. Curve segment ΔE from the cathodic minimum potential E_min to the potential of the local maximum capacitance E_max are used to estimate the interval in which the shape of the capacitance dependence corresponds to the classical one. This segment decreases with an increase in the radii of the cation and anion of the salt, as well as with a decrease in the alternating signal frequency. An increase in the radii of the salt cations and anions also leads to an increase in the frequency dependence of both E_min, and ΔE. The paper proposes an approach according to which the maximum potential E_max of the capacitance curve of a solid metal electrode in ionic melts containing anions capable of specific adsorption can be considered as the potential for the transition from the electrostatic adsorption mechanism to the stage of chemical adsorption. The magnitude of ΔE in this case can be used to evaluate the influence of specific adsorption of halide ions on the capacitive properties of the iridium electrode.