Electrochemical behavior of LaCl_3 and morphology of La deposit on molybdenum substrate in molten LiCl-KCl eutectic salt

摘要

The electrochemical behavior of LaCl_3 dissolved in molten LiCl-KCl eutectic salt was studied in the temperature range of 693-823 K by using inert electrodes, Mo as the cathode, and high density graphite as the anode. Cyclic voltammetry, chronopotentiometry and square wave voltammetry were used to determine major kinetic parameters. The standard reaction rate constant of the order ≈ 10~(-3) cm s~(-1), determined by Nicholson method, placed the redox reaction of lanthanum in the quasi-reversible range per Matsuda-Ayabe criteria for practical concept of electrochemical reversibility. Sand's equation was used to determine the diffusion coefficient of La(III) ions at four different temperatures. The effect of temperature on diffusion coefficient obeyed the Arrhenius law, according to which the activation energy for diffusion of La(III) ions was 33.5 ± 0.5 kJ mol~(-1). The exchange current density of La(III)/La(0) redox reaction, evaluated at three different temperatures by linear polarization method on Mo and La substrates, was consistently somewhat higher on the later. For each temperature, the equilibrium potential of La(III)/La(0) redox couple was determined by using open circuit chronopotentiometry, with subsequent calculation of the apparent standard potential, E_(La(III/La(0)))~(*0), and the apparent Gibbs free energy, ΔG_(LaCl_3)~(*0) The activity coefficients for LaCl_3, γ_(LaCl_3) was determined from the difference of apparent and standard Gibbs free energies, ΔG_(LaCl_3)~(*0) - ΔG_(LaCl_3(SC))~0. The nucleation mechanism of lanthanum deposition on a molybdenum substrate according to the electrochemical model of Scharifker-Hill indicated the instantaneous nucleation with three-dimensional growth of the hemispherical nuclei. Contrary to this, the SEM studies of electrode surface morphology as a function of electrodeposition time clearly showed that La nucleation and growth follows the mechanisms responsible for dendritic growth. For the first time, the transient dendritic morphology events were possible to record, which is the major contribution of this work.