Exchange Current Measurements in KCl-LiCl Eutectic Melt

Abstract

Exchange currents have been measured of some liquid and solid metal electrodes (Cd, Zn, Pb, Bi, Ag, Ni, Pt) and one redox system (V~+/V ~*) in a KC1-LiC1 eutectic melt at 450~ Measurements over a hundredfold concentration range permitted calculation of the heterogeneous rate constant and of the transfer coefficient. Two relaxation methods were used: the double pulse method as developed by Gerischer and Krause and the voltage step method as described by Vielstich and Delahay. In cases where both methods were applied independently to the same system the results were in good agreement. The purity of the eutectic melt was improved considerably by filtration and by displacement of heavy metal ion impurities with metallic magnesium. The evaluation of voltage step measurements was revised by allowing for the effect of the finite charging time of the double layer. Agreement was not found with the theory of the double pulse method given by Matsuda, Oka, and Delahay, perhaps because of some additional process, other than diffusion, taking place in a period of time comparable to the duration of the first pulse. On the basis of the reaction of platinum with cadmium at 450~ it is possible to account for the observations made earlier in this laboratory by C. H. Liu and tI. C. Gaur on the emf and polarization behavior of cadmium-plated platinum electrodes. In recent years the increasing importance of fused salts in many fields of science and technology has incited a considerable number of investigations of their properties. Molten salts have found much interest as media for electrochemical processes in connection with their applications in metallurgy, analytical and synthetic chemistry and, more recently , in high-temperature galvanic cells. Relatively few studies, however, have been made of the rates of electrode processes in molten salts, because such electrode processes can be expected to be very fast and their study would therefore require elaborate techniques. The relaxation methods developed recently for the study of fast reactions in aqueous solutions are most suitable for fused salt studies. Doubts have sometimes been expressed as to whether these methods would be able to avoid the interference of diffusion as a rate-determining process, since diffusion in molten salts (1) is often not faster than in aqueous solutions at room temperature. It is possible, however, to distinguish between diffusion controlled and charge transfer controlled reactions even in fused salts, and diffusion effects can be eliminated under the proper experimental conditions. The first successful application of a relaxation method to electrode reactions in fused salts was made by Randles and White (2). In a Iow melting nitrate eutectic the rate of the reaction NV § q-2e-+ Hg ~ Ni (Hg) was evaluated from a-c impedance measurements. Measurements were attempted on several other metal ion-metal amalgam reactions, but they were found to be too rapid for the method. Laitinen and Osteryoung (3), about the same time, sought to use the same a-e technique with platinum 1 Present address: Gebr. B6hler and Co. AG, Diisseldorf-Oberkas-sel, Germany. 2 Present address: Shell Development Co., Emeryville, California. electrodes in the KC1-LiC1 eutectic. Direct interpretation of the measurements was not possible because of complicating experimental conditions which caused a very large frequency dispersion of the resistance and capacitance measurements. Under more amenable experimental conditions, primarily with a melt prepared from dry salts, Laitinen and Gaur (4) were able to make more refined measurements of the same type. Although the frequency dispersion of the measurements on platinum microelectrodes was still present, the data were interpretable by the introduction of a correction for adsorption of the reducible ion (5). The correction was necessary to remove the "inverted" character of the reaction impedance, in which the resistive component is smaller than the capacitative component. While there is no direct evidence that this is a valid correction, the resulting exchange currents for the cadmium and zinc couples appear to have reasonable values, as will be confirmed in this paper. Hill (6) measured the impedance behavior of silver electrodes using AgNO~ as the solute in a tern-ary nitrate melt, of silver-plated tungsten electrodes in LiC1-KC1 eutectic containing dissolved AgC1, and of tungsten electrodes in Ti(III)-Ti(II) systems in the LiC1-KC1 eutectic. In all cases the residual capacity and resistance of the solvent were markedly frequency dependent Many of the faradaic impedance plots were "inverted." Interpretation of a-c impedance measurements requires the elimination of the double layer capacity. The residual capacity has, in general, been found to be a function of frequency in fused salts. The other relaxation methods, potential step and current step, are made at relatively very short times, which corresponds to high frequencies in the a-c method, 546) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.129.182.74 Downloaded on 2015-06-09 to IP