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2 Polish Journal of Chemistry

2 2004

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The Influence of teh Experimental Setup Upon the Modelling of the Impedance Spectra in Molten Salts

Kisza A., Kaźmierczak J., Meisner B.

Polish Journal of Chemistry

2004

Vol. 78 / nr 9

1235-1244

To settle the frequently raised problem of the influence of cell geometry on the value of the evaluated by the electrochemical impedance spectroscopy (EIS) double layer capacitance, in this paper two types of cells have been tested in two and three electrodes experiments in molten salts. In the first type cell, the working silver electrodes were immersed directly in the molten NaCl - KCl eutectic mixture at 700°C, while in the second type cell, the working silver electrodes were placed inside quartz tubes connected to the bulk melt by a small hole. The only effect of the quartz tube was the desired increase of the electrolyte resistance from 0.7_ to 5.5 _, which is important in lowering the ac measuring current of the Solartron 1260 Phase Gain Analyser. The evaluated values of the double layer capacitance were to experimental uncertainty the same in both cells. It can be thus concluded, that the frequently raised in aqueous electrolytes problem of the influence of the cell geometry upon the values of the evaluated double layer capacitance, in molten salts does not lead to any significant errors. This is presumable due to the fact that the conductivity of molten salts is usually two order higher than aqueous electrolytes, and the corresponding ohmic drops are much smaller.

The Double Layer Capacitance of the Liquid Lead Electrode in Molten NaX-KX (X=Cl, Br, I) Eutectic Mixtures

Kisza A., Kaźmierczak J., Meisner B.

Polish Journal of Chemistry

2004

Vol. 78 / nr 4

561-573

Using the Electrochemical Impedance Spectroscopy, the double layer capacitance of the liquid lead electrode in the molten NaX-KX eutectic mixture has been evaluated at several temperatures at the rest potential, and in the potential range from 0 to -1Vagainst reversible lead electrode in the same melt. At no applied potential the liquid lead electrode in the studied eutectic melts can be treated as an "ideally polarizable" electrode. It has been found that the double layer capacitance at the rest potential is very small of the order of 0.9_F cm-2 for chloride, 0.6 for bromide and 0.2 _F cm-2 for iodide melts. As expected its value is decreasing with the increase of the temperature. At the far negative end of the potential range (-1.0 V), the double layer capacitance is decreased by the faradaic process (formation of the alkali metal - amalgam in liquid Pb).With the increase of the potential, the rate of this process decreases and the double layer capacitance increases.With a further increase of the potential, the alkali metal cations in the primary ionic shell are substituted by the halide anions, decreasing the charge density in the primary ionic shell, which results in the decrease of the capacitance toward some small value at the "potential of zero charge". Still further increase of the potential creates a new double layer, composed mainly of halide ions and the charge density increase is accompanied by the increase of the double layer capacitance. When the anodic dissolution of the metallic lead starts, another decrease of the double layer capacitance is observed. The evaluated values are much lower than the previous results, obtained in early sixties of the 20-th century, which has been obtained by the neglecting of the frequency dependence of the measured capacitance.