Electrochemical behavior of LaCl3 and morphology of La deposit on molybdenum substrate in molten LiCl–KCl eutectic salt

• Chronological verification by SEM of La dendritic nucleation and growth from molten LiCl-KCl.
• Evaluation of exchange current density of La(III)/La(0) on molybdenum and lanthanum substrates in LiCl–KCl melts.
• Use of Nicholson and Matsuda-Ayabe methods to determine the standard rate constant of La(III)/La(0) redox reaction.
• Electrochemical kinetics and thermodynamics of La(III)/La(0) on Mo-substrate in molten eutectic LiCl-KCl

The electrochemical behavior of LaCl3 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, ELa(III/La(0))*0, and the apparent Gibbs free energy, ΔGLaCl3*0 The activity coefficients for LaCl3, γLaCl3γLaCl3 was determined from the difference of apparent and standard Gibbs free energies, ΔGLaCl3*0−ΔGLaCl3(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.

For the first time, the transient nucleation stages were possible to record during electrodeposition from molten salt electrolytes. Specific to La electrodeposition from LiCl-KCl melts, it was clearly confirmed that the nucleation and subsequent growth lead to dendritic morphology, contradicting the theoretical prediction for 3D-semispherical growth by Scharifker-Hill model. In addition to the evaluation of standard rate constant for quasi-reversible reduction reaction of La(III) on Mo substrate, several thermodynamic La(III)/La(0) properties were determined, as well.Figure optionsDownload as PowerPoint slide