Determination of diffusion coefficient in high temperature molten salt using cyclic voltammetry on microwire electrode: Its validity and limitation

Abstract

• A method for the determination of diffusion coefficient ( D ) is developed. • This method is based on cyclic voltammetry at microwire electrode. • This method is useful for D determination in high temperature molten salt. • D values of Eu 3+ in LiCl-KCl melt and O 2 2– in Li 2 CO 3 -Na 2 CO 3 melt are determined. • The validity and limitation of this method is examined by numerical simulations. In this study, we developed a method for the determination of the diffusion coefficient ( D ) of electroactive species in high temperature molten salt without the knowledge of its concentration, via cyclic voltammetry at a simple unshielded microwire electrode. This method is based on the assessment of the ratio of the peak current ( i p ) of the cyclic voltammogram for its electrochemically reversible reaction at the microwire electrode, with i p at the macrowire electrode. The scan rate ( ν )-dependent contribution of radial diffusion to i p at the microwire electrode provides a linear relationship between the i p ratio and ν −0.425 , where its slope includes information on D . The theoretical evaluation with numerical simulation on this method revealed that possible underestimation in D determination could occur because of the minor contribution of radial diffusion at the macrowire electrode used in this study. D determination in the molten salt was experimentally verified with Eu 3+ in 450 °C LiCl-KCl and O 2 2– in 580 °C Li 2 CO 3 -Na 2 CO 3 . For both cases, the determined D values were similar to the reported values, and they were further cross-validated by calculating their concentrations from the D values. However, the accurate determination is limited for electrochemically reversible reaction as D is overestimated for quasi-reversible and totally irreversible reactions.