Determination of the diffusion coefficient in electrodeposition reactions by electrochemical impedance spectroscopy: a case study of cobalt in molten LiCl-KCl salt

Abstract

Compared to direct current (DC) techniques, electrochemical impedance spectroscopy (EIS) allows for a more accurate determination of the diffusion coefficient of metal ions in electrodeposition reactions. However, its actual application has been less attempted due to the difficulty in determining complex parameters, such as the concentration of deposited species in the electrode matrix. Here, we introduce a simplified approach to facilitate the EIS application to electrodeposition reactions, using the Co(II)/Co(0) reaction in molten LiCl-KCl salt as a case study. The working electrode surface area was determined by the photography of the electrochemical cell taken in situ. The diffusion coefficients of Co(II) in the LiCl-KCl salt, determined by EIS, were found to be (3.56 +/- 0.49) x 10-5 cm2/s and (1.61 +/- 0.12) x 10-5 cm2/s for the tungsten and liquid bismuth electrodes, respectively. The possible reasons for the observed discrepancy in the diffusion coefficients of Co(II) obtained from the two different electrodes were discussed. Impedance spectroscopy determines ion diffusivity in deposition reactions with minimal changes to the electrode area.Challenges arise due to coefficients related to the working electrode matrix.Rational approximation is possible under near-zero net current conditions.We applied a visual monitoring system to measure the electrode surface area.Liquid metal electrodes provide more reliable results than solid-state electrodes.