Redox Behavior of Moisture in LiCl-KCl Eutectic Melts: A Cyclic Voltammetry Study

Abstract

In order to develop a flow sheet for the purification procedure of a LiCl-KCl eutectic mixture with the underlying aim of avoiding Cl₂ handling in an engineering-scale laboratory, the present work focuses on electrochemical investigations on vacuum dried eutectic mixtures LiCl-KCl + H₂O and vacuum dried LiCl-KCl + H₂O mixtures using cyclic voltammetry at 500°C. Complicated voltammetric features were observed that were attributed to the cathodic reduction of H₂O to form OH⁻, adsorption of OH⁻ on tungsten electrode, and cathodic reduction of OH⁻ to form O²⁻. The onset of cathodic current around −0.45 V (versus Ag${ }^{+}|$Ag as reference) was due to both cathodic reductions of H₂O and HCl, the latter being formed in melt due to high-temperature hydrolysis of LiCl, although it had limited solubility as compared to that of H₂O. Cyclic voltammograms also showed an anodic peak at around −0.30 V attributable to the adsorption of O²⁻ on the tungsten electrode.

A total of 19 different LiCl-KCl eutectic melts subjected to various vacuum drying conditions and moisture content were investigated in this work using cyclic voltammetry. The LiCl-KCl + H₂O mixtures were prepared to simulate conditions when there is an ingress of moisture in LiCl-KCl mixtures during long storage to determine whether the mixture can be purified. A larger composition range of moisture in the LiCl-KCl eutectic mixtures was used that not only helped in the attribution of cathodic reduction peaks to reactions mentioned above but also served as references to investigate the influence of vacuum drying of moisture-containing eutectic mixtures.

A two-point criterion consisting of cathodic onset Li${ }^{+}|$Li potential and residual cathodic current density estimated from cyclic voltammograms in the potential region −1.5 to −2.0 V at 500°C was used to quantify the purity of the eutectic melts. Former data were compared with the theoretical equilibrium potential of −3.637 V for the Li${ }^{+}|$Li couple in LiCl-KCl eutectic melt at 500°C and those obtained from cyclic voltammograms of chlorinated melt. Cathodic reduction potentials for the above reactions were then compared with literature data where they were measured against Li-Al or Ni${ }^{2+}|$Ni reference electrodes in LiCl-KCl melts. Although reduction of HCl at Pt electrode in LiCl-KCl eutectic melts was known to be reversible from literature, it was not found to be so in the present work where a tungsten working electrode was used. The LiCl-KCl eutectic mixtures vacuum dried at 300°C were found to be closer in purity to those of chlorinated melt in that the onset Li${ }^{+}|$Li potential and residual cathodic current density were similar. A lower residual cathodic current density for LiCl-KCl + 2 wt% H₂O vacuum dried at 300°C was also achieved with the onset Li${ }^{+}|$Li potential quite close to the theoretical equilibrium potential of LiCl (−3.637 V) in the eutectic melt.

Keywords:

• Pyroprocessing
• moisture
• cyclic voltammetry
• theoretical equilibrium potential
• residual cathodic current density

Nomenclature

$a_{\mathrm{K}\mathrm{C}\mathrm{l},liq.}$ =	=	activity of KCl(liq.) in LiCl-KCl eutectic melt
$a_{\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{l},liq.}$ =	=	activity of LiCl(liq.) in LiCl-KCl eutectic melt
$E_{\mathrm{A}{\mathrm{g}}^{+}|\mathrm{A}\mathrm{g}(\mathbf{P}\mathbf{y}\mathbf{r}\mathbf{e}\mathbf{x})\mathbf{v}\mathbf{e}\mathbf{r}\mathbf{s}\mathbf{u}\mathbf{s}\mathrm{L}\mathrm{i}\mathrm{A}\mathrm{l}}$ =	=	equilibrium potential of Ag${ }^{+}|$Ag couple (in Pyrex glass) against Li-Al reference electrode
$E_{\mathrm{A}{\mathrm{g}}^{+}|\mathrm{A}\mathrm{g}(\mathbf{V}\mathbf{y}\mathbf{c}\mathbf{o}\mathbf{r})\mathbf{v}\mathbf{e}\mathbf{r}\mathbf{s}\mathbf{u}\mathbf{s}\mathrm{L}\mathrm{i}\mathrm{A}\mathrm{l}}$ =	=	equilibrium potential of Ag${ }^{+}|$Ag couple (in Vycor glass) against Li-Al reference electrode
$E_{\text{AgCl}}^{°}$ =	=	standard electrode potential of AgCl in LiCl-KCl eutectic melt (versus Cl${ }^{-}|$Cl2)
$E_{\text{LiCl},liq}^{°}.$ =	=	theoretical equilibrium potential of pure LiCl in liquid reference state (versus Cl${ }^{-}|$Cl2)
$E_{\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{l},melt}^{eq,liq.}$ =	=	theoretical equilibrium potential of LiCl(liq.) in LiCl-KCl eutectic melt (versus Cl${ }^{-}|$Cl2)
$E_{{\text{NiCl}}_{\text{2}}}^{°}$ =	=	standard electrode potential of NiCl2 in LiCl-KCl eutectic melt (versus Cl${ }^{-}|$Cl2)
$F$ =	=	Faraday constant (96 500 C ·mol−1)
$R$ =	=	gasconstant(8.31452 J · K−1 · mol−1)
$T$ =	=	temperature (K)
$x_{\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}$ =	=	mole fraction of AgCl in LiCl-KCl eutectic melt
$x_{\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}^{\mathbf{P}\mathbf{r}\mathbf{e}\mathbf{s}\mathbf{e}\mathbf{n}\mathbf{t}\mathbf{W}\mathbf{o}\mathbf{r}\mathbf{k}}$ =	=	mole fraction of AgCl in LiCl-KCl eutectic melt used in present work
$x_{\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{l}}$ =	=	mole fraction of LiCl

Greek

${\mathrm{\Delta }}_{\mathrm{f}}{G}_{\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{l},melt}^{liq.}$ =	=	Gibbs energy of formation of LiCl(liq.) in LiCl-KCl eutectic melt
${\Delta }_{\text{f}}{G}_{\text{LiCl},liq.}^{°}$ =	=	Gibbs energy of formation of pure LiCl in liquid reference state
${\gamma }_{\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{l},liq.}$ =	=	activity coefficient of LiCl(liq.) in LiCl-KCl eutectic melt
${\gamma }_{\mathrm{K}\mathrm{C}\mathrm{l},liq.}$ =	=	activity coefficient of KCl(liq.) in LiCl-KCl eutectic melt