Thermodynamic Properties of Hafnium Chlorides Dissolved in the NaCl-KCl Melt Obtained by Electrochemical Transient Techniques
Authors: Kuznetsov Sergey A. 
Published: February 2020

The electrolysis of molten salts is an effective method for production of hafnium. The knowledge of thermodynamic properties and electrochemistry of hafnium provides data for optimization of electrolytic process.
In previous studies, the thermodynamic properties of HfCl2 and HfCl4 in an equimolar mixture NaCl-KCl were determined by EMF method. However, in these investigations oxide materials were used in the construction of the electrochemical cell and the reference electrode.
Our experiments showed that oxide materials interacted with the NaCl-KCl-HfCl2, NaCl-KCl-HfCl2-HfCl4 and NaCl-KCl-HfCl4 melts.
EMF measurements are unsuitable for the abovementioned systems due to reactions of the next types:
HfCl4 + O2- ↔ HfOCl2↓ + 2Cl- (1)
2HfCl2 + O2- ↔ HfOCl2↓ + Hf + 2Cl- (2)
This reaction leads to decreasing of the concentration Hf(IV) and Hf(II) complexes and to change the ratio of lower and higher oxidation states. In addition, measurements of equilibrium potentials in melts containing Hf(II) are complicated by their tendency toward disproportionation:
2Hf(II) ↔ Hf(IV) + Hf (3)
The reaction of disproportionation of the type (2) occurs due to corrosion of oxide materials because compounds of hafnium have a high affinity to oxygen.
For the elimination of the abovementioned difficulties for the determination of thermodynamic properties hafnium in molten systems electrochemical transient techniques was used.
In our study, cyclic voltammetric curves and impedance spectra were recorded at different electrodes (glassy carbon, several metals) with respect to a glassy carbon quasi-reference electrode. The glassy carbon ampoule served as the counter electrode. While the potential of this quasi-reference electrode does not constitute a thermodynamic reference, the use of this electrode was preferred in order to avoid any contact between the melt and oxygen-containing material as used in classical reference electrodes. A Ag/NaCl-KCl-AgCl (2 wt.%) reference electrode was used in order to obtain more reliable potential values. At the final stage of each experimental set, this reference electrode was immersed in the melt for a short time for determination of the potential peaks, the melt being no longer used after these measurements.
In this study, the thermodynamic properties of hafnium chloride solutions in an equimolar mixture NaCl-KCl were obtained with utilization of this approach.
The electroreduction of HfCl4 in the NaCl-KCl melt was studied in the temperature range 973-1123 K by cyclic voltammetry and impedance spectroscopy methods. It was determined that electrochemical reduction of Hf(IV) in the NaCl-KCl melt occurs via two successive stages involving transfer of two electrons at each stage. The formal standard potentials of E
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Hf(II)/Hf, E
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Hf(IV)/Hf and the formal standard redox potentials of E
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Hf(IV)/Hf(II) were determined from the cyclic voltammetry and the values of the standard rate constants of charge transfer obtained by impedance spectroscopy method. The free Gibbs energy changes for the reaction HfCl2(sol.) + Cl2(g.) ↔ HfCl4(sol.) and the equilibrium constants of metal-salt reaction Hf(IV) + Hf ↔ 2Hf(II) were calculated. The thermodynamic properties of the dilute solutions of hafnium di-and tetrachloride formation in the NaCl-KCl melt were determined.