Electrochemical transient techniques for determination of uranium and rare-earth metal separation coefficients in molten salts

The main step in the pyrometallurgical recycling process of spent nuclear fuel is a molten salt electrorefining. The knowledge of separation coefficients of actinides (U, Np, Pu and Am) and rare-earth metals (Y, La, Ce, Nd and Gd) is very important for this step. Usually the separation coefficients are evaluated from the formal standard potentials of metals in melts containing their own ions, i.e. values obtained by potentiometric method. Electrochemical experiments were carried out at 723-823 K in order to estimate separation coefficients in LiCl-KCl eutectic melt containing uranium and lanthanum trichlorides. The electrochemical behaviour of UCl₃ in LiCl-KCl melt was studied by different electrochemical methods. The diffusion coefficients of U(III) were determined by linear sweep voltammetry, chronopotentiometry and chronoamperometry. The standard rate constants of charge transfer for electroreduction of uranium, U(III) + 3e⁻ → U, were calculated by the impedance spectroscopy method. The values of constants testify that electroreduction of U(III) to U is mainly controlled by the rate of charge transfer. La(III) discharge on uranium electrode was also investigated. It was shown that for the calculation of uranium and lanthanum separation coefficients it is necessary to determine the voltammetric peak potentials of U(III) and La(III), their concentration in the melt and the kinetic parameters relating to U(III) discharge such as transfer and diffusion coefficients, and standard rate constants of charge transfer.     

Electrochemistry of ytterbium (III) in molten alkali metal chlorides

This work presents the electrochemical study of Yb(III) ions in molten alkali metal chlorides in the temperature range 723-1073 K. Transient electrochemical techniques such as linear sweep, cyclic and square wave voltammetry, and potentiometry at zero current have been used to investigate the reduction mechanism, transport parameters and thermodynamic properties of the reaction YbCl₂ + 1/2Cl₂ = YbCl₃ The results obtained show that the reduction reaction Yb(III) + e⁻ ⇔ Yb(II) is reversible being controlled by the rate of the mass transfer. The diffusion coefficient of [YbCl₆]³⁻ complex ions has been determined at different temperatures in the fused eutectic LiCl-KCl, the equimolar NaCl-KCl and the CsCl media. The apparent standard potential of the soluble-soluble redox system Yb(III)/Yb(II) has been obtained by cyclic voltammetry. The influence of the nature of the solvent on the electrochemical and thermodynamic properties of ytterbium compounds is discussed.     

Reduction process of uranium(IV) and uranium(III) in molten fluorides

This study focused on the electroreduction process of uranium cations in molten fluorides. It involved cyclic voltammetry, chronopotentiometry with and without current reversal, and square wave voltammetry.The results indicate a two-step reduction process for uranium(IV). The first step U(IV)/U(III) exchanging one electron corresponds to a soluble/soluble system and is limited by U(IV) diffusion with D_U(IV) = 1.25 ± 0.35 × 10⁻⁵ cm² s⁻¹ in LiF-NaF at 720 °C.In order to perform a thorough study of the second step U(III)/U(0) in the reduction process, the melt was chemically reduced in U(III) with U metal as reducing agent. Alternatively to the use of LiF-NaF where U metal is unstable at 720 °C, the chemical reduction of U(IV) in U(III) was performed in a LiF-CaF₂-UF₄ solution containing U metal at 810 °C. It has been confirmed that the reduction of U(III) proceeds in one step exchanging three electrons and by a diffusion controlled process with D_U(III) = 2.2 ± 0.7 × 10⁻⁵ cm² s⁻¹ in LiF-CaF₂ at 810 °C.     

Investigation on the electrode process of the Mn(II)/Mn(III) couple in redox flow battery

The Mn(II)/Mn(III) couple has been recognized as a potential anode for redox flow batteries to take the place of the V(IV)/V(V) in all-vanadium redox battery (VRB) and the Br₂/Br⁻ in sodium polysulfide/bromine (PSB) because it has higher standard electrode potential. In this study, the electrochemical behavior of the Mn(II)/Mn(III) couple on carbon felt and spectral pure graphite were investigated by cyclic voltammetry, steady polarization curve, electrochemical impedance spectroscopy, transient potential-step experiment, X-ray diffraction and charge-discharge experiments. Results show that the Mn(III) disproportionation reaction phenomena is obvious on the carbon felt electrode while it is weak on the graphite electrode owing to its fewer active sites. The reaction mechanism on carbon felt was discussed in detail. The reversibility of Mn(II)/Mn(III) is best when the sulfuric acid concentration is 5 M on the graphite electrode. Performance of a RFB employing Mn(II)/Mn(III) couple as anolyte active species and V(III)/V(II) as catholyte ones was evaluated with constant-current charge-discharge tests. The average columbic efficiency is 69.4% and the voltage efficiency is 90.4% at a current density of 20 mA cm⁻². The whole energy efficiency is 62.7% close to that of the all-vanadium battery and the average discharge voltage is about 14% higher than that of an all-vanadium battery. The preliminary exploration shows that the Mn(II)/Mn(III) couple is electrochemically promising for redox flow battery.     

Preparation of Co-Sn alloys by electroreduction of Co(II) and Sn(II) in molten LiCl-KCl

Co-Sn alloys were prepared by an electrochemical route in molten LiCl-KCl between 400 and 550 °C. The Sn(IV)/Sn(II), Sn(II)/Sn(0) and Co(II)/Co(0) redox couples were studied by cyclic voltammetry and/or chronopotentiometry over the temperature range. The diffusion coefficient values of Co(II) ions were measured. For example, it was found that the D_Co(II) values deduced from chronopotentiometry range from D_Co(II) = 1.65 × 10⁻⁵ cm² s⁻¹ at 400 °C to 4.95 × 10⁻⁵ cm² s⁻¹ at 550 °C. The standard potential of the Co(II)/Co(0) redox couple in molten LiCl-KCl was measured at 400 °C: vs Cl₂/Cl⁻. Finally, Co-Sn alloys were prepared in potentiostatic mode. The influence of the temperature of molten LiCl-KCl, the applied potential and the deposition time on the morphology and the composition of the Co-Sn alloys were also investigated. For T > 450 °C, the following tendency has been observed: the more negative the potential, the higher the Sn content in the deposited alloy. Thus, depending on the operating conditions, pure CoSn or CoSn₂ can be prepared.     

Redox behaviour of cerium oxychloride in molten MgCl2-NaCl-KCl eutectic

The electrochemical behaviour of cerium oxychloride in MgCl₂-NaCl-KCl ternary eutectic was investigated by cyclic voltammetry at 823 K. The cyclic voltammogram of UO₂Cl₂-CeOCl in MgCl₂-NaCl-KCl eutectic shows two peaks during the cathodic sweep as well as anodic sweep. The reduction of UO₂²⁺ is by a single step two-electron transfer and that of CeO⁺ is by a single step one-electron transfer. The reduction of CeO⁺ was found to be quasi-reversible.The reduction potentials of UO₂²⁺/UO₂ and CeO⁺/CeO versus Ag(I)/Ag reference electrode at 823 K are 0.103 and −0.299 V, respectively. The diffusion coefficient of CeO⁺ at 823 K is in the range of (1.7-1.9) × 10⁻⁵ cm² s⁻¹. The cyclic voltammogram for 0.015 mol% CeOCl shows an additional peak during the anodic sweep at −0.056 V, which is being attributed to monolayer dissolution of CeO at the glassy carbon working electrode. Electrochemical impedance data of 0.015 mol% CeOCl in MgCl₂-NaCl-KCl eutectic at the open circuit potential was fitted to a Randles cell from which the heterogeneous rate constant was estimated. X-ray photoelectron spectroscopy was used to confirm that the oxidation state of cerium in the eutectic is +3.     

Highly hydroxylated carbon fibres as electrode materials of all-vanadium redox flow battery

A highly effective hydroxylated-functionalization of carbon fibres for use as electrodes of all-vanadium redox flow battery (VRFB) was developed. Carbon paper made of carbon fibres was hydroxylated ultrasonically with mixed acids (H₂SO₄/HNO₃, V_H2SO4/V_HNO3 = 3/1) in a Teflon-lined stainless steel autoclave for different time at 80 °C. The structure, composition, and electrochemical properties of the treated samples for positive and negative electrodes of VRFB were characterized with Fourier transformation infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectrometry, scanning electron microscopy, X-ray diffraction, cyclic voltammetry, electrochemical impedance spectroscopy, and cell charge and discharge tests. The content of hydroxyl group changes from 3.8% for the untreated sample to 14.3% for the carbon paper treated in mixed acids for 10 h. The highly hydroxylated sample shows its high activity toward the redox reactions of V(II)/V(III) and V(IV)/V(V). The VRFB using the carbon paper treated for 8 h as the electrodes exhibits excellent performance under a current density of 10 mA cm⁻². The average voltage efficiency reaches 91.3%, and the average energy efficiency reaches 75.1%. The mechanisms for the high hydroxylation of the carbon fibres with the mixed acids and the high activity of the treated sample toward the vanadium redoxs are discussed.     

Direct electrochemistry and biocatalytic activity of hemoglobin entrapped into gellan gum and room temperature ionic liquid composite system

A new composite film of microbial exocellular polysaccharide-gellan gum (GG) and room temperature ionic liquid (IL) 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIMPF₆) was firstly used as an immobilization matrix to entrap proteins and its bioelectrochemical properties were studied. Hemoglobin (Hb) was chosen as a model protein to investigate the composite system. UV-vis spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the composite film. The obtained results demonstrated that the Hb molecule in the film kept its native structure and showed its good electrochemical behavior. A pair of well-defined, quasi-reversible cyclic voltammetric peaks appeared in pH 7.0 phosphate buffer solutions (PBS, 0.1 M), with the formal potential (E°′) of −0.368 V (vs. SCE), which was the characteristic of Hb Fe(III)/Fe(II) redox couples. The Hb-IL-GG-modified electrode also showed an excellent electrocatalytic behavior to the reduction of hydrogen peroxide (H₂O₂). Therefore, this kind of composite film as a novel substrate offers an efficient strategy and a new promising platform for further study on the direct electrochemistry of redox proteins and the development of the third-generation electrochemical biosensors.     

The characterization and bioelectrocatalytic properties of hemoglobin by direct electrochemistry of DDAB film modified electrodes

Direct electrochemistry of hemoglobin can be performed in acidic and basic aqueous solutions in the pH range 1-13, using stable, electrochemically active films deposited on a didodecyldimethylammonium bromide (DDAB) modified glassy carbon electrode. Films can also be produced on gold, platinum, and transparent semiconductor tin oxide electrodes. Hemoglobin/DDAB films exhibit one, two, and three redox couples when transferred to strong acidic, weak acidic and weak basic, and strong basic aqueous solutions, respectively. These redox couples, and their formal potentials, were found to be pH dependent. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ deposition of DDAB on gold disc electrodes and hemoglobin deposition on DDAB film modified electrodes. A hemoglobin/DDAB/GC modified electrode is electrocatalytically reduction active for oxygen and H₂O₂, and electrocatalytically oxidation active for S₂O₄²⁻ through the Fe(III)/Fe(II) redox couple. In the electrocatalytic reduction of S₄O₆²⁻, S₂O₄²⁻, and SO₃²⁻, and the dithio compounds of 2,2′-dithiosalicylic acid and 1,2-dithiolane-3-pentanoic acid, the electrocatalytic current develops from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in neutral and weakly basic aqueous solutions. Hemoglobin/DDAB/GC modified electrodes are electrocatalytically reduction active for trichloroacetic acid in strong acidic buffered aqueous solutions through the Fe(III)/Fe(II) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in weak acidic and basic aqueous solutions. The electrocatalytic properties were investigated using the rotating ring-disk electrode method.     

Direct electrochemistry and electrocatalysis of heme-proteins immobilized in porous carbon nanofiber/room-temperature ionic liquid composite film

The combination of porous carbon nanofiber (PCNF) and room-temperature ionic liquid (RTIL) provided a suitable microenvironment for heme-proteins to transfer electron directly. Hemoglobin, myoglobin, and cytochrome c incorporated in PCNF/RTIL films exhibited a pair of well-defined, quasi-reversible cyclic voltammetric peaks at about −0.28 V vs. SCE in pH 7.0 buffers, respectively, characteristic of the protein heme Fe(III)/Fe(II) redox couples. The cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the modified electrode. The heme/PCNF/RTIL/CHIT films were also characterized by UV-vis spectroscopy, indicating that heme-proteins in the composite film could retain its native structure. Oxygen, hydrogen peroxide, and nitrite were catalytically reduced at the heme/PCNF/RTIL/CHIT film modified electrodes, showing the potential applicability of the films as the new type of biosensors or bioreactors based on direct electrochemistry of the redox proteins.     

Electrochemical behavior of europium (III) in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide

N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyNTf₂) was synthesized and characterized by CHNS elemental analysis, ¹H and ¹³C NMR and IR spectroscopy. Europium tris[bis(trifluoromethylsulfonyl)imide] (Eu(NTf₂)₃) was prepared and studied for the electrochemical behavior of Eu(III) in BMPyNTf₂ at glassy carbon and stainless steel working electrodes at 298-373 K by cyclic voltammetry, chronopotentiometry and chronoamperometry. Cyclic voltammogram of Eu(III) in BMPyNTf₂ consisted of a quasi-reversible cathodic wave at −0.45 V (vs. Fc/Fc⁺, 373 K), which could be attributed to the reduction of Eu(III) to Eu(II) and an irreversible wave at −2.79 V (vs. Fc/Fc⁺) due to reduction of Eu(II) to Eu(0). The diffusion coefficient of Eu(III) in BMPyNTf₂ was determined to be in the range of ∼10⁻⁷ cm² s⁻¹ by various electrochemical methods and the charge transfer rate constant was determined to be ∼10⁻⁵ cm s⁻¹ by cyclic voltammetry.     

Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on manganese oxides supported on Vulcan carbon (Mn_yO_x/C). The oxides were prepared by thermal decomposition of manganese nitrate at different conditions. The oxides were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical studies were conducted using cyclic voltammetry (CV) and steady state polarization measurements carried out with a thin layer rotating ring/disk electrode. XRD results showed that the manganese oxide prepared at 200 °C in air is formed by a major phase of β-MnO₂ and the polarization curves indicated the highest activity for this material. In situ XANES evidenced the occurrence of a redox process involving Mn(II)/Mn(III) and Mn(III)/Mn(IV) in the range of potentials of the CV measurements. The electrocalytic activity was related to the occurrence of a mediation process involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen and by a disproportionation reaction of the HO₂⁻ species in the Mn_yO_x sites. In situ XANES results showed that the Mn(IV) species is MnO₂ and the Mn(III) is most likely MnOOH.     

Preparation and characterization of osmium hexacyanoferrate films and their electrocatalytic properties

Osmium hexacyanoferrate films have been prepared using repeated cyclic voltammetry, and the deposition process and the films’ electrocatalytic properties in electrolytes containing various cations have been investigated. The cyclic voltammograms recorded the deposition of osmium hexacyanoferrate films directly from the mixing of Os³⁺ and Fe(CN)₆³⁻ ions from solutions containing various cations. An electrochemical quartz crystal microbalance, cyclic voltammetry, and UV-visible spectroscopy were used to study the growth mechanism of the osmium hexacyanoferrate films. The osmium hexacyanoferrate films showed a single redox couple, and the redox reactions included “electron transfer” and “proton transfer” with a formal potential that demonstrates a proton effect in acidic solutions up to a 12 M aqueous HCl solution. The electrochemical and electrochemical quartz crystal microbalance results indicate that the redox process was confined to the immobilized osmium hexacyanoferrate film. The electrocatalytic reduction of dopamine, epinephrine, norepinephrine, S₂O₃²⁻, and SO₅²⁻ by the osmium hexacyanoferrate films was performed. The preparation and electrochemical properties of co-deposited osmium(III) hexacyanoferrate and copper(II) hexacyanoferrate films were determined, and their two redox couples showed formal potentials that demonstrated a proton effect and an alkaline cation effect, respectively. Electrocatalytic reactions on the hybrid films were also investigated.     

Ce(III)/Ce(IV) in methanesulfonic acid as the positive half cell of a redox flow battery

The characteristics of the Ce(III)/Ce(IV) redox couple in methanesulfonic acid were studied at a platinum disk electrode (0.125 cm²) over a wide range of electrolyte compositions and temperatures: cerium (III) methanesulfonate (0.1–1.2 mol dm⁻³), methanesulfonic acid (0.1–5.0 mol dm⁻³) and electrolyte temperatures (295–333 K). The cyclic voltammetry experiments indicated that the diffusion coefficient of Ce(III) ions was 0.5 × 10⁻⁶ cm² s⁻¹ and that the electrochemical kinetics for the oxidation of Ce(III) and the reduction of Ce(IV) was slow. The reversibility of the redox reaction depended on the electrolyte composition and improved at higher electrolyte temperatures. At higher methanesulfonic acid concentrations, the degree of oxygen evolution decreased by up to 50% when the acid concentration increased from 2 to 5 mol dm⁻³. The oxidation of Ce(III) and reduction of Ce(IV) were also investigated during a constant current batch electrolysis in a parallel plate zinc–cerium flow cell with a 3-dimensional platinised titanium mesh electrode. The current efficiencies over 4.5 h of the process Ce(III) to Ce(IV) and 3.3 h electrolysis of the reverse reaction Ce(IV) to Ce(III) were 94.0 and 97.6%, respectively. With a 2-dimensional, planar platinised titanium electrode (9 cm² area), the redox reaction of the Ce(III)/Ce(IV) system was under mass-transport control, while the reaction on the 3-dimensional mesh electrode was initially under charge-transfer control but became mass-transport controlled after 2.5–3 h of electrolysis. The effect of the side reactions (hydrogen and oxygen evolution) on the current efficiencies and the conversion of Ce(III) and Ce(IV) are discussed.     

Electrochemistry of TiCl4 in 1-butyl-2,3-dimethyl imidazolium azide

Electrochemical behaviour of titanium tetrachloride solutions in 1-butyl-2,3-dimethyl imidazolium azide (BMMImN₃) at 65 °C has been examined. Ti(IV) reduction was studied with chronopotentiometry and cyclic voltammetry methods in melts with different concentrations of TiCl₄. According to IR spectra, Ti(IV) exists in form of a hexaazidotitanate complex. The electrochemical reduction of this complex was found to proceed irreversibly to Ti(III) species only. Diffusion coefficients of Ti(IV) in this ionic liquid at temperature 65 °C were calculated based on the chronopotentiometry measurements at different TiCl₄ concentrations (D_Ti(IV) = 1.3 ± 0.6 × 10⁻⁶ cm² s⁻¹).     

Electrochemical behavior of Fe(II) in acetamide-urea-NaBr-KBr melt and magnetic properties of inductively codeposited Nd-Fe film

The electroreduction of Fe(II) and Nd(III) in MCl_x-acetamide-urea-NaBr-KBr were studied by cyclic voltammetry and chronoamperometry. The reduction of Fe(II) to Fe is an irreversible process, the value of αn_α of the electrode reaction was calculated to be 0.31 and the diffusion coefficient of Fe(II) was calculated to be 9.53 × 10⁻⁷ cm² s⁻¹ at 343 K. Nd(III) cannot be reduced alone in urea melt, but Nd-Fe can be codeposited by induced codeposition. The composition of Nd-Fe film varies with the Nd(III)/Fe(II) molar ratio, at the potential of −1.25 V the maximum content of Nd in Nd-Fe film is 60.4 wt%. The morphology of Nd-Fe film was investigated by SEM and AFM. Nd-Fe film comprises of nanoparticles with the size about 100-200 nm. X-ray diffraction (XRD) shows it is amorphous. After heat-treatment at 1173 K the crystal Nd₂Fe₁₇ phase can be formed. The magnetic properties of the Nd-Fe films were determined using hysteresis loops, at 5 K the coercive field H_c of Nd (62.6 wt%)-Fe amorphous film is 1225 Oe, the remanent magnetization M_R and the saturation magnetization M_S are 5.15 and 15.80 emu g⁻¹, respectively.     

Electrochemistry of terbium in the eutectic LiCl-KCl

The electrochemical behaviour of terbium at a tungsten electrode, in the eutectic LiCl-KCl molten was investigated in the temperature range 673-823 K, by means of cyclic voltammetry, chronopotentiometry and chronoamperometry. It was found that during cathodic polarization, deposition of metallic Tb from the chloride mixture onto the tungsten surface proceeds in a single step, and electrocrystallization plays an important role in the whole process. Experimental current-time transients followed the theoretical models based on instantaneous nucleation with three-dimensional growth of the nuclei whatever the applied overpotential. From chronopotentiometric measurements, the diffusion coefficient of the Tb(III) ions was determined by applying the Sand equation and modifying the immersion dept of the working electrode in stages. The validity of the Arrhenius law was also verified and the activation energy for diffusion was found to be 33.4 ± 1.5 kJ mol⁻¹.The standard apparent potential value of the Tb(III)/Tb(0) system has been determined by potentiometry at several temperatures, and the activity coefficient of Tb(III) in the molten chloride media based on a pure supercool reference state has also been calculated.     

An electrochemical biosensor based on Nafion-ionic liquid and a myoglobin-modified carbon paste electrode

An electrochemical biosensor was constructed based on the immobilization of myoglobin (Mb) in a composite film of Nafion and hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) for a modified carbon paste electrode (CPE). Direct electrochemistry of Mb in the Nafion-BMIMPF₆/CPE was achieved, confirmed by the appearance of a pair of well-defined redox peaks. The results indicate that Nafion-BMIMPF₆ composite film provided a suitable microenvironment to realize direct electron transfer between Mb and the electrode. The cathodic and anodic peak potentials were located at −0.351 V and −0.263 V (vs. SCE), with the apparent formal potential (Ep) of −0.307 V, which was characteristic of Mb Fe(III)/Fe(II) redox couples. The electrochemical behavior of Mb in the composite film was a surface-controlled quasi-reversible electrode process with one electron transfer and one proton transportation when the scan rate was smaller than 200 mV/s. Mb-modified electrode showed excellent electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) in a linear concentration range from 2.0 × 10⁻⁴ mol/L to 1.1 × 10⁻² mol/L and with a detection limit of 1.6 × 10⁻⁵ mol/L (3σ). The proposed method would be valuable for the construction of a third-generation biosensor with cheap reagents and a simple procedure.     

Neodymium(III) cathodic processes in molten fluorides

The electrochemical behaviour of the Nd(III)/Nd(0) system has been investigated in several molten media and more particularly in LiF-CaF₂. A preliminary study based both on thermodynamic and experimental data showed that it is not possible to observe the Nd(III)/Nd(0) system in LiF-KF and LiF-NaF melts; because the K⁺ and Na⁺ cation reduction waves hide the Nd³⁺ reduction wave. Then, the Nd(III)/Nd(0) system has been investigated at 810 °C using solutions of NdF₃ in fluoride solvents without K⁺ and Na⁺ ions, such as LiF-CaF₂, by cyclic voltammetry, chronopotentiometry and square wave voltammetry. Experimental results show that neodymium trifluoride is reduced in Nd(0) in a one-step process exchanging three electrons (Nd(III) + 3e⁻ → Nd(0)). The electrode process is shown to be diffusion controlled. Nd(III) diffusion coefficient is in the range of 1.1-1.3 × 10⁻⁵ cm² s⁻¹ at 810 °C.     

Electrochemistry of thorium in LiCl-KCl eutectic melts

This work presents a study of the electrochemical properties of Th chloride ions dissolved in a molten LiCl-KCl eutectic, in a temperature range of 693-823 K. Transient electrochemical techniques such as cyclic voltammetry, chronopotentiommetry and chronoamperometry have been used in order to investigate the reduction mechanism on a tungsten electrode and the diffusion coefficient of dissolved Th ions. All techniques showed that only one valence state was stable in the melt. The reduction into Th metal was found to occur according to a one-step mechanism, through a nucleation-controlled process which requires an overpotential of several 100 mV. At 723 K, the diffusion coefficient is D_Th(723 K) = 3.15 ± 0.15 × 10⁻⁵ cm² s⁻¹. EMF measurements indicated that, at 723 K, the standard apparent potential is (723 K) = −2.582 V versus Cl₂/Cl⁻, and the activity coefficient γThCl₄ (723 K) = 4.6 × 10⁻⁴ on the mole fraction scale (based on a pure liquid reference state).