Capacitance and Impedance of an Iridium Electrode in Molten Alkaline Metal Bromides

Abstract—The adsorption activity of the iridium electrode in molten sodium, potassium, and cesium bromides is studied by the capacitance and electrochemical impedance methods in a temperature range of 1033–1123 K and at the alternating signal frequency from 3 × 10⁰ to 1 × 10⁴ Hz in the whole available range of electric polarization. Two main minima on the capacitance curves corresponding to the ranges of increasing cathodic and anodic currents are observed in all systems under study for both the direct measurement of the electrode capacitance and calculation from the impedance spectra. The calculated capacitances obtained by the equivalent-circuit method in the potential range more positive than the cathodic capacitance minimum coincide with the iridium electrode capacitance obtained for the potential sweep rate at an alternating signal frequency of 10 kHz for the capacitance of the electric double layer and 10 Hz for the adsorption capacitance, respectively. An additional (intermediate) minimum that disappears with increasing temperature is observed in molten sodium bromide at the lowest temperature of the temperature range under study. In molten potassium and cesium bromides, this intermediate minimum is observed at all temperatures under study, and its position on the capacitance curve depends on the alternating signal frequency. An increase in the cation radius in the NaBr–KBr–CsBr series results in the narrowing of the potential range in which the behavior of the system in the anodic range is described by the Gouy–Chapman–Stern model. A decrease in the alternating signal frequency and an increase in the experimental temperature also lead to this range narrowing. The narrowing is assumed to be associated with the enhancement of halide ion adsorption from the melt on the iridium electrode surface upon the potential shift to the positive values from the point of the cathodic capacitance minimum.