Electrode and Redox Potentials of Molybdenum and Stability of Molybdenum Chloro-Species in Alkali Chloride Melts

V A Volkovich,A B Ivanov,T R Griffiths,B D Vasin,D S Maltsev,R V Kamalov

doi:10.1149/2.0511708jes

Abstract

Molybdenum electrode and Mo(III)/(IV) redox potentials were measured in fused alkali chlorides. Experiments were performed in individual salts (LiCl, NaCl, KCl, RbCl and CsCl) and in several binary and ternary eutectic or low-melting mixtures between 633 and 1173 K (depending on the melting point of the solvent salt). Formal standard electrode potentials E*Mo/Mo(III) and redox potentials E*Mo(III)/Mo(IV) in respect to Cl−/Cl2 couple and Gibbs free energy change of formation of molybdenum(III) chloride in alkali chloride melts were calculated. Electronic absorption spectra of Mo(III) ions were recorded, and spectroscopic parameters of MoCl63− complex ions determined. High temperature spectroscopy measurements were used to study the stability of Mo(III) chloro-species in fused chlorides and the reaction rates of Mo(III) ions disproportionation were also determined. Diffusion coefficients of molybdenum ions in LiCl-KCl-CsCl and NaCl-CsCl eutectic based melts were determined from the results of electrochemical measurements.

Highlights

Molybdenum electrode and Mo(III)/(IV) redox potentials were measured in fused alkali chlorides
Pyrochemical reprocessing of spent nuclear fuels (SNFs) is another field where molten salts can be employed, and this is a feasible option for treating irradiated U-Mo based metallic fuels
Stability of Mo(III) ions in chloride melts.—Electronic absorption spectroscopy offers a convenient tool for monitoring processes involving Mo(III) species in fused salts

Summary

JES FOCUS ISSUE ON PROGRESS IN MOLTEN SALTS AND IONIC LIQUIDS

Electrochemistry of molybdenum in chloride melts first attracted attention several decades ago but no systematic study of the electrode potentials has been performed, with the exception of the earlier studies by Ryzhik and Smirnov.[9,10,11,12,13,14,15,16] They determined molybdenum standard electrode potentials in melts based on individual alkali chlorides (LiCl, KCl, CsCl) and in the LiCl-KCl eutectic mixture. To determine molybdenum redox potentials, the potentiometric measurements were performed in the melts containing the products of reaction of molybdenum dioxide (or mixtures of molybdenum and molybdenum dioxide) with hydrogen chloride. In this case glassy carbon rods were used as working electrodes and with silver chloride electrode as the reference. The design of the counter electrode allowed separating any counter electrode reaction from the bulk of the electrolyte

Results and Discussion

Mo concentration

Conclusions