Effect of Adsorption of Halogenide Ions on the Capacity of Iridium Electrode in Molten Salts

The adsorption activity of the iridium electrode in molten sodium, potassium and cesium chlorides and bromides, sodium and potassium iodides was estimated from the dependence of the shape and position of the capacitance curve on the cation-anion composition of the electrolyte, the AC frequency and temperature. The capacitance curves have a complex shape with two main minima and a maximum between them in the entire studied ranges of temperature, frequency and electric polarization. In some cases, one or two additional minimа appear on the curves between the main minima. The position of both the minima and maxima of the capacitance depends on the radius of the salt cation and anion. The dependence of the position of the main maximum of the capacitance on the radius of the cation and anion is presented at T=1093 K in the AC frequency range of 3·10⁰–1·10⁴Hz. According to this approach, the potential of the capacitance maximum of a solid metal electrode in ionic melts with specifically adsorbing anions can be considered as the critical potential of the adsorption phase transition. The segment of the capacitance curve ΔEfrom the cathodic capacitance minimum, which corresponds to the classical minimum capacitance, to the capacitance maximum can be considered as a range of potentials in which the change in the properties of the double electric layer obey the Gouy-Chapman-Stern model. With a further shift from the critical potential to the positive direction, the electrostatic adsorption mechanism turns into chemical adsorption, with the transfer of part of the anions from the salt phase to the electrode and the formation of complex compounds. The value ofΔEcan help to estimate the effect of specific adsorption of halide ions on the shape of the capacitance curve.