Study of alloying in LiCl-KCl eutectic: development of liquid thin film bismuth macro- and microelectrodes

Abstract

The work within this thesis focuses on the study of alloy formation using an active liquid metal electrode for fundamental analysis and for the extraction and separation of the lanthanides and actinides in a pyroprocessing system. The electrochemical work herein is performed in a molten salt of lithium chloride and potassium chloride at its eutectic point (LKE). This salt is a likely candidate for pyroprocessing due to its relatively low melting point and resistance to degradation on exposure to high levels of radiation. The active electrode material under examination is bismuth due to its propensity to alloy with other elements, its relatively low melting point, high density and non-toxicity. The alloying processes studied are those of bismuth-lithium and bismuth-cerium. Lithium is the limiting reduction reaction defining the negative solvent limit in LKE. As a result, understanding the processes that would occur if the electrode were to be pushed to such negative potentials is of significant importance. Cerium is a commonly-used surrogate for plutonium, which is an element of relatively high concentration in waste nuclear fuel and is of significant interest to the nuclear international community in waste fuel recycling. This work examines the alloying processes in terms of which intermetallic compounds are formed and by what mechanisms. This is achieved through the use of co-deposition on a macro tungsten rod, employing a number of electrochemical techniques to extract pertinent information. Lithium electrodeposition and alloying with bismuth (at the negative solvent limit) was found to form BiLim alloy with increasing m at more reducing potentials, followed by the deposition of near pure lithium. Mixing of these two then gave rise to specific bismuth-lithium alloys and the apparent ejection of a lithium metal fog into the molten salt, which resulted in the chemical reduction of Bi3+ and the loss of the bismuth electrodeposition current. When electrodepositing cerium on, and alloying with, bismuth, the formation of intermetallic compounds is governed by potential with a maximum BiCem stoichiometry of m = 1 with equimolar Bi3+ and Ce3+. However, at concentrations of cerium greater than that of bismuth, alloys much richer in cerium were also deposited at more negative potentials. There is evidence that deposited cerium may also escape into solution and chemically react with Bi3+. In-house microelectrodes are also developed and used for this purpose, both through co-deposition and direct alloy formation on a liquid bismuth thin-film microelectrode. This work demonstrates that these devices provide a richness of information due to their highly beneficial microelectrode properties. A means of controllably depositing bismuth from an aqueous plating bath, without dendrite formation, on both platinum and tungsten microelectrodes was devised. This was followed by electrodeposition of bismuth films on these devices in LKE. Platinum was found to be an active electrode material, alloying with bismuth, while tungsten remained inert. Nonetheless, both electrode types produced characteristic microelectrode behaviour, which was successfully used to determine the diffusion coefficient of bismuth in LKE. A comparison of bismuth-cerium and cerium alloying on a thin film liquid bismuth microelectrode found that the latter indicated the formation of BiCe2 where only BiCe had been seen previously during co-deposition in an equivalent salt. This is thought to be due to the thin film liquid bismuth microelectrode configuration with enhanced Ce3+ mass transport. This response was also used to calculate the diffusion coefficient of cerium inside the bismuth film, which was found to be slightly slower than for Ce3+ in LKE.