Electrochemistry of ErCl3 and morphology of erbium electrodeposits produced on Mo substrate in early stages of electrocrystallization from LiCl–KCl molten salts

Abstract The electrochemistry of ErCl 3 dissolved in molten LiCl–KCl eutectic salt was studied in the temperature range 683–813 K by using inert electrodes, Mo as the working electrode, and high density graphite as the counter electrode. The erbium redox reaction was evaluated with respect to its major thermodynamic, kinetic and initial electrocrystallization properties. The reduction of Er(III) ions to Er(0) metal occurred in a single reaction step. For each temperature, the equilibrium potential of Er(III)/Er(0) redox couple was determined by using open-circuit chronopotentiometry, with subsequent calculation of the apparent standard potential, E E r ( I I I ) / E r ( 0 ) * 0 , and the apparent Gibbs energy, Δ G E r C l 3 * 0 . The activity coefficients for ErCl 3 , γ E r C l 3 , was determined from the difference of apparent and standard Gibbs energies of formation, Δ G E r C l 3 * 0 − Δ G E r C l 3 ( S c ) 0 . According to temperature dependence of diffusion coefficient, determined at five different temperatures from the Sand's equation plots, the diffusion of Er(III) ions required the activation energy of 40.8 ± 1.4 kJ·mol −1 . The exchange current density of Er(III)/Er(0) redox reaction, at three different temperatures, was evaluated by linear polarization of Mo electrode coated with erbium. The reaction rate constant determined by convolutive voltammetry shows that Er(III)/Er(0) redox system is quasi-reversible per Matsuda-Ayabe criteria. This study clearly shows that the actual mechanisms of erbium nucleation and growth are not even close to 3D-hemispherical mechanisms proposed in the literature on the basis of popularly used mathematical model of Scharifker-Hill. Actually, on molybdenum substrate, erbium nucleates by clustering of adatoms into morphology resembling dendrites organized into circular shapes, as was found for lanthanum. What distinguishes erbium from lanthanum is the growth stage. For erbium, the growth continues by filling-in the empty space in a circle. Further growth of either individual, or merged circles, is peripheral. The net result is a smooth, densely packed, very thin film of erbium.