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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Electrochemical Behavior of Telluride Ions (Te2−) in Molten LiCl− Li2Te Solution at 650 °C Timothy Lichtenstein,† Nadia A. Elbaar,† Takanari Ouchi,‡ and Hojong Kim*,† †
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Materials Science and Engineering, The Pennsylvania State University, 406 Steidle Building, University Park, Pennsylvania 16802, United States ‡ Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ABSTRACT: The electrochemical behavior of Te2− ions was investigated in the LiCl−Li2Te binary on glassy carbon electrodes at 650 °C as a means to understand the fundamental thermodynamic and mass transport properties of Te2− ions. Cyclic voltammetry and constant-potential electrolysis confirmed an electrochemically reversible, two-electron soluble−insoluble reaction of Te2−/ Te(l). The formal potential for the Te2−/Te(l) reaction was determined to be 1.744 V vs Li+/Li(l), and the diffusivity of Te2− ions was about 0.44−1.25 × 10−5 cm2 s−1. The low value for diffusivity relative to those of other cations in molten salts suggests the possibility of forming complex ions such as [Li8Te]6+ due to strong chemical interactions with Li+ ions in the electrolyte. The anodic polarization of Te(l) indicated that the formation of TeCl2(g) and the cathodic polarization of Te(l) involved the codeposition of Li metal into the Te(l). The results of this work provide essential knowledge in developing electrochemical processes for separation of tellurium as well as in mitigating the degradation reactions with structural materials.
1. INTRODUCTION Tellurium is a metalloid that forms diverse compounds (e.g., CdTe, Li2Te, TeCl2, TeCl4, TeO2) with various valence states (−2, 0, +2, and +4) and exists as both anions and cations in solutions.1−4 The multiple valence states of tellurium in solutions allow for unique electrochemical behavior of telluride ions; for example, the CdTe compound dissolved in a molten salt solution (CdCl2−KCl) was successfully recovered as pure Cd and Te metals by electrolysis at 500 °C, which was demonstrated by Bradwell et al. as a methodology to recycle tellurium from thin-film solar cell applications.3 In their work, the liquid Te metal (Tm,Te = 450 °C) was deposited anodically and identified as the insoluble product of the anodic reaction (Te2− → Te(l) + 2e−). In addition, dissolved Te2− ions in molten salts are known to result in the rapid degradation of structural materials, such as Pt anodes in electrolytic cells5 and Ni-based alloy containers in molten salt reactors,6−9 by forming brittle telluride compounds (e.g., Pt + Te2− → PtTe + 2e−). To prevent the corrosive attack from Te2− ions, the redox conditions of the solution should be controlled so that Te2− ions are stabilized in the molten salt phase without the formation of telluride compounds, along with suitable materials (alloy) selection. Considering the electrochemical nature of these degradation reactions, the mitigation strategies will require the fundamental thermodynamic and kinetic properties of Te2− ions including the redox potentials of various electrochemical transition reactions, charge transfer kinetics, and diffusivity. Overall, understanding the electrochemical properties of soluble−insoluble Te2−/Te transition is essential for develop© XXXX American Chemical Society
ing electrochemical processes for the recovery and separation of tellurium as well as for selecting suitable redox control buffers for reliable operation of structural components in molten salts containing tellurium fission products. In this work, the electrochemical behavior of Te2− ions was investigated by cyclic voltammetry at 650 °C using an inert glassy carbon electrode in LiCl−Li2Te solutions at various compositions of dissolved Li2Te. On the basis of the detection of characteristic waves, constant-potential electrolysis was conducted to verify the soluble−insoluble (Te2−/Te) transition reaction through the identification of electrolysis products. In each electrolyte composition, the anodic peak potentials and currents were carefully analyzed to examine the reversibility of the Te2−/Te reaction, and were used to evaluate the formal potential, the number of electrons transferred in the reaction, as well as the diffusivity of Te2− ions. The electroanalytical relations for these analyses were based on the theoretical results derived by Berzins and Delahay for the reversible deposition of an insoluble substance but were modified to account for the anodic deposition of Te metal. Using a liquid Te electrode, this work further elucidated the distinct electrode processes during the cathodic sweep in contrast to the anodic sweep, originating from strong chemical interactions between lithium and tellurium metals. Received: October 31, 2018
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DOI: 10.1021/acs.inorgchem.8b03073 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. (a) Configuration of working (WE), reference (RE), and counter (CE) electrodes for electrochemical measurements. (b) Schematic of three-electrode electrochemical cell assembly using a glassy carbon WE, a two-phase Li−Bi (xLi = 0.65) RE, and a single-phase liquid Li−Bi (xLi = 0.15) CE for cyclic voltammetry at 650 °C in LiCl−Li2Te electrolytes. height, and 32 mm in depth) where a 1.5 mm capillary hole was drilled into the sidewall to facilitate contact with the electrolyte. The electrical contact with the Te metal was also established with a graphite rod during induction melting (Figure 1a). The reference electrode (RE) was constructed using a binary Li−Bi alloy (mole fraction, xLi = 0.65) which exhibits a two-phase behavior (liquid + Li3Bi) at 650 °C with invariant activity (Gibbs phase rule) and, thus, provides a constant electrode potential in the presence of compositional uncertainty. The two-phase Li−Bi RE was fabricated by melting Li and Bi (99.999%, Sigma-Aldrich, Product 556130) metals in a BN crucible (same dimension used for Te WE) using an induction heater, and inserting a tungsten electrical lead wire (1 mm diameter) into the molten Li−Bi alloy. Four holes (1.5 mm diameter) were drilled through the BN walls at 7 mm from the bottom of the crucible to establish a stable contact between the bulk electrolyte and the two-phase alloy (Figure 1a). The equilibrium potential of the twophase Li−Bi alloy (liquid + Li3Bi) relative to Li+/Li(l) was obtained from emf measurements by Ga̧sior and Moser at 500−717 °C and was determined to be 0.663 V vs Li+/Li(l) at 650 °C.10 Using this emf value, the WE potentials measured using the two-phase Li−Bi RE in this work are reported relative to Li+/Li(l). The counter electrode (CE) was constructed using a liquid Li−Bi alloy (xLi = 0.15) with a larger surface area (3.8 cm2) than WE (0.14− 0.18 cm2) to facilitate the electrode reactions at the WEs. The liquid Li−Bi alloy was fabricated using the induction heater by gradually adding Li into liquid Bi inside a BN crucible (25 mm outer diameter, 22 mm inner diameter, 15 mm in height, and 11 mm in depth), and a tungsten electrical lead was inserted into the liquid alloy before allowing the electrode to cool with the lead in contact (Figure 1a). 2.3. Three-Electrode Cell Assembly and Measurements. The electrodes and a thermocouple (ASTM Type K) were arranged inside an alumina crucible (60 mm in diameter and 100 mm in height; Advalue Technology, Product No. AL-2250), and then, the electrolyte was poured into the crucible (Figure 1b). The assembled cell was placed into a test chamber; the chamber was sealed inside the glovebox, loaded into a crucible furnace, and evacuated under vacuum, following a similar procedure for preparing the electrolytes. Finally, the chamber was purged with ultra-high-purity Ar gas three times and heated at 650 °C under a slowly flowing (50 mL min−1) Ar atmosphere, and the cell temperature was monitored using a data acquisition board (National Instruments, NI 9211). The electrodes were allowed to equilibrate for 5 h at 650 °C, and electrochemical measurements were conducted using a potentiostat− galvanostat equipped with a frequency response analyzer (Autolab PGSTAT302F with an FRA32 M module). In each experiment, the uncompensated solution resistance (Ru) was determined by electrochemical impedance spectroscopy (EIS) by applying 5 mV (rootmean-square potential) over a frequency range 10 mHz to 100 kHz,
2. EXPERIMENTAL SECTION Due to the hygroscopic nature of LiCl and the reactivity of Li2Te, the electrochemical cell components were prepared and assembled in an Ar filled glovebox (
