Formula omitted] couple in LiCl-KCl eutectic: Anodic polarization of thorium and electrochemical impedance spectroscopy study at tungsten, cadmium and thorium electrodes

Abstract

Cyclic voltammetry and electrochemical impedance spectroscopy were used to investigate electrochemical behaviour of thorium at inert tungsten, liquid cadmium and thorium electrodes in LiCl-KCl eutectic. Equilibrium potentials of Th4+|Th couple were measured from the cyclic voltammograms at thorium electrode in LiCl-KCl-ThCl4 electrolyte and compared with theoretically estimated equilibrium potential using ΔfGThCl4,LiCl−KCl∘, ΔfGThCl4,liq.∘ and γThCl4 from literature sources as input. It was observed that deviation of estimated values from measured data of −1.437 V at 773 K could be ascribed not only to different experimental conditions employed but also to difference in ΔfGThCl4,liq.∘ values taken from various thermochemical data compilations. Considering all experimental uncertainties and errors in linear fitting of electromotive force data from literature sources, theoretical equilibrium potential of thorium dissolution was estimated at −1.462 ± 0.065 V (vs.Ag+|Ag as ref.) for xThCl4 = 3.86 × 10−3 at 773 K. Complex impedance plots were recorded for inert tungsten, liquid cadmium and thorium electrodes at various applied potentials in the frequency and temperature range of 10 Hz-10 kHz and 698–798 K, respectively. They were further validated using Kramers-Kronig transforms and interpreted using equivalent circuit model consisting of melt resistance (Rs), charge transfer resistance (Rct) and constant phase elements (Q) as circuit components. Mechanism of anodic dissolution of thorium in LiCl-KCl eutectic melt was proposed, which was based on semi-infinite Warburg diffusion at lower frequencies having slope greater than unity and double layer capacitance at higher frequencies due to partial accumulation of Th4+ ions at the electrode-electrolyte interface. Heterogeneous rate constant estimated from Rct was used to address reversibility of Th4+|Th couple using Matsuda-Ayabe criteria for anodic (thorium electrode) and cathodic polarization (inert tungsten) cases, which was found to be quasi-reversible in the scan rate range 10–100 mV/s and temperature range 698–798 K.