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

摘要

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 -1 V against 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 0.9muF cm~(-2) for chloride,0.6 for bromide and 0.2muF 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.