Capacitance and Impedance of an Iridium Electrode in Molten Alkali Metal Chlorides

Abstract—The capacitance of the iridium electrode is studied by electrochemical impedance spectroscopy as a function of the main physical and chemical parametersnamely, electrical potential, temperature, and alkali cation radius. The influence of the signal frequency used in ac electrochemical methods on the capacitance and shape of a capacitance curve is also tested. Data on the capacitance of the iridium electrode are obtained in molten sodium, potassium, and cesium chlorides in a temperature range of 1093–1123 K and the ac signal frequency range from 3 × 100 to 3 ×104 Hz in the entire available electrical polarization range. The detected capacitance curves have two main minima with a maximum between them. One of the minima (cathodic) is identified as the classical potential of capacitance minimum. A decrease in the signal frequency and the experiment temperature and an increase in the cation radius in the NaCl–KCl–CsCl order result in the appearance of one or two additional minima in the potential range between the main minima. The depth of the intermediate minima increases and their potential shifts to the positive direction with an increase in the radius of the alkali metal cation of the salt electrolyte. The capacitances of the electrical double layer (EDL) and the adsorption capacitances are calculated by the method of equivalent electrical circuits. One of the additional minima obtained by the direct measurement of the potential dependence of the electrode capacitance at a high frequency corresponds to the calculated capacitance of the EDL. The second additional minimum having formed at a low frequency is reproduced in the adsorption capacitance calculation.