CAPACITANCE AND IMPEDANCE OF IRIDIUM ELECTRODE IN MOLTEN ALKALI CHLORIDES

Abstract

Сapacitance of the iridium electrode was studied by the electrochemical impedance spectroscopy with variation of the main physical and chemical parameters: electrical potential, temperature, radius of the alkali cation. The influence of the signal frequency used in AC electrochemical methods on the capacitance value and the shape of the curve was also checked. Capacitance of the iridium electrode was obtained in molten sodium, potassium and cesium chlorides in the temperature range 1093–1123 K and the frequency range of the AC signal 3 · 100–3 · 104 Hz in the entire accessible range of electrical polarization. The obtained capacitance curves have two main minima with a maximum between them. One of these minima (cathodic one) was identified as the classical potential of minimum capacitance. A decrease in the signal frequency and the temperature of experiment, as well as an increase in the cation radius in the NaCl–KCl–CsCl order, leads to the appearance of one or two additional minima in the potential region between the main minima. The depth of these intermediate minima increases and their potential shifts in the positive direction with an increase in the radius of the alkali metal cation of the salt electrolyte. The calculated values of the capacitance of the electrical double layer and the adsorption capacitance were obtained by the method of equivalent electrical circuits. One of the additional minima obtained by direct measurement of the dependence of the electrode capacitance on the potential at a high AC frequency corresponds to the calculated capacitance of the double layer. The other additional minimum obtained at a low AC frequency corresponds to the calculated adsorption capacitance.