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1.
The electrochemical behaviour of the Nd(III)/Nd(0) system has been investigated in several molten media and more particularly in LiF-CaF 2. 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 Nd 3+ reduction wave. Then, the Nd(III)/Nd(0) system has been investigated at 810 °C using solutions of NdF 3 in fluoride solvents without K + and Na + ions, such as LiF-CaF 2, 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 cm 2 s −1 at 810 °C. 相似文献
2.
The influence of the molar ratio O-Ta noted Ro on the electrodeposition of tantalum in molten fluorides at 800 °C is examined. After additions of both K 2TaF 7 and Na 2O, we observed: (i) tantalum deposit is obtained only by reduction of TaF 72−; (ii) the oxide ions react with TaF 72− to give oxifluoride species, identified first as TaOF 52− and for higher oxide content, as TaO 2F 43−; (iii) the electroreduction of TaOF 52− leads to the formation of an unstable bivalent tantalum species (TaO); (iv) the conversion of TaF 72− to oxifluoride is complete when Ro is 1. Titration of Ta by ICP and oxygen by a carbothermal-reduction technique show that for Ro>1, insoluble compounds Na 3TaO 4 and NaTaO 3 are formed. For Ro>3, these species seem to be solubilised. Electrodeposition runs on copper cathodes show that pure and coherent Ta coatings are obtained for a lower oxide content ( Ro<0.5). For higher Ro, the faradic efficiencies are low. The coatings are powdery and include the presence of Na 3TaO 4, NaTaO 3 and traces of Ta metal, arising from the decomposition of TaO. 相似文献
3.
The electrodeposition of carbon films from carbonate ions (CO 32−) in molten alkaline fluorides (LiF/NaF) was investigated in the 700-800 °C temperature range using cyclic voltammetry and chronopotentiometry. The cathodic peak in the cyclic voltammogram indicates that CO 32− ions are reduced in a one-step process: CO 32−+4e −→C+3O 2−. Deposits of amorphous carbon were obtained by potentiostatic electrolysis and analysed by several physical techniques: X-ray diffraction, Raman spectroscopy, scanning electron microscopy coupled with energy dispersive spectroscopy. 相似文献
4.
The electrochemical behaviour of SmF 3 is examined in molten LiF–CaF 2 medium on molybdenum and nickel electrodes. A previous thermodynamic analysis suggests that the reduction of SmF 3 into Sm proceeds according to a two-step mechanism: The second step occurs at a potential lower than the reduction potential of Li+ ions. Cyclic voltammetry, chronopotentiometry and square-wave voltammetry were used to confirm this mechanism and the results show that it was not possible to produce samarium metal in molten fluorides on an inert cathode (molybdenum) without discharging the solvent. The electrochemical reduction of SmF3 is limited by the diffusion of SmF3 in the solution. The diffusion coefficient was calculated at different temperatures and the values obtained obey Arrhenius’ law. For the extraction of the samarium from fluoride media, the use of a reactive cathode made of nickel leading to samarium–nickel alloys is shown to be a pertinent route. Cyclic voltammetry and open-circuit chronopotentiometry were used to identify and to characterise the formation of three alloys: liquid Sm3Ni and a compact layer made of SmNi3 and SmNi2. 相似文献
5.
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 YbCl 2 + 1/2Cl 2 = YbCl 3 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 [YbCl 6] 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. 相似文献
6.
The electrochemical behaviour of the Eu(III)/Eu(II) system was examined in the molten eutectic LiF–CaF 2 on a molybdenum electrode, using cyclic voltammetry, square wave voltammetry and chronopotentiometry. It was observed that EuF 3 is partly reduced into EuF 2 at the operating temperatures (1073–1143 K). The electrochemical study allowed to calculate both the equilibrium constant and the formal standard potential of the Eu(III)/Eu(II) system. The reaction is limited by the diffusion of the species in the solution; their diffusion coefficients were calculated at different temperatures and the values obey Arrhenius’ law. The second system Eu(II)/Eu takes place out of the electrochemical window on an inert molybdenum electrode, which inhibits the extraction of Eu species from the salt on such a substrate. 相似文献
7.
The electrochemical behaviour of the Th(IV)/Th system was examined in molten LiF-CaF 2 medium on inert (molybdenum), reactive (nickel) and liquid (bismuth) electrodes in the 810-920 °C temperature range by several electrochemical techniques. Experimental results showed that (i) thorium fluoride was reduced in a single step exchanging 4 electrons and limited by thorium ions diffusion in the solution, (ii) the oxide ions induce the precipitation of Th(IV) in the form of thorium oxide (ThO 2), in a process involving as intermediate compound a soluble oxifluoride (ThOF 2), (iii) the reduction of thorium ions on reactive (Ni and liquid Bi) electrodes yields compounds Ni-Th and Bi-Th with a potential shift of around 0.7 V (for Ni and Bi) more anodic than the reduction of Th(IV) on inert substrate. 相似文献
8.
The reduction and oxidation processes on platinum and glassy carbon electrodes in molten LiCl-KCl eutectic containing UCl 3 were investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) in the temperature range 660-780 K. Two redox peaks have been observed in the cyclic voltammograms corresponding to the two redox reactions U(IV)/U(III) and U(III)/U which are found to be reversible and quasi-reversible, respectively. The reduction potentials of U(IV)/U(III) and U(III)/U are −0.325 and −1.490 V versus reference electrode (0.1 mol% AgCl in the LiCl-KCl), respectively at 700 K. Chronopotentiometric measurements confirm the three-electron transfer during the reduction of U(III) to U metal. Electrochemical impedance spectroscopy data at various potentials were interpreted either as diffusion, adsorption or reduction process by nonlinear fit of the impedance data to the simulated equivalent circuits. 相似文献
9.
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 UCl 3 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. 相似文献
10.
The fluoroacidity of several alkaline fluoride media was studied by monitoring the concentration of electroactive species which is decreasing vs. time due to a gas species release, such as silicon fluorides, as indicated by the reaction: SiF 4 + xx− = SiF 4(g) + xF −. This article relates the Si(IV) reaction study to define a relative fluoroacidity scale by studying the silicon ions stability in different melts. Electrochemical techniques allow the measurement of SiF 4 + xx− concentration evolution and thus the reaction rate constant to be calculated at different temperatures and for several fluoride media. The article shows that the free F − content depends on the fluoride mixture and that the rate values are correlated with the fluoroacidity allowing a qualitative estimation. Then a fluoride solvents fluoroacidity scale was proposed, scaling the different eutectic melts from basic melt to acidic one: NaF–KF < LiF–KF < NaF–MgF 2 < NaF–CaF 2 < LiF–NaF < LiF < LiF–CaF 2. 相似文献
11.
The electrochemical behavior of uranyl nitrate in 1-butyl-3-methylimidazolium chloride at glassy carbon working electrode has been investigated in the temperature range 343-373 K by transient electrochemical techniques such as cyclic voltammetry, chronopotentiometry and square wave voltammetry. Influence of bulk concentration of uranium and temperature on the electroreduction and transport properties of U(VI) in bmimCl has been examined. Diffusion coefficient ( D) and the energy of activation ( Ea) of U(VI) in bmimCl has been estimated and is of the order of ∼10 −8 cm 2/s and 54 kJ/mol, respectively. Reduction of U(VI) takes place through an irreversible single step two-electron transfer to UO 2 deposit at glassy carbon working electrode. Thermal analysis of the uranium oxide indicated the entrapment of nearly 5% of electrolyte, bmimCl, during electrodeposition, which decomposes in the range 553-653 K. 相似文献
12.
为开发环境友好型铋提取技术,在700℃下采用循环伏安、方波伏安和计时电位等方法研究了NaCl?KCl熔盐体系中Bi(III)在玻碳电极上的电化学行为。在–0.3 V (vs. Ag/AgCl)电位下以玻碳电极为工作电极对NaCl?KCl?BiCl3进行恒电位电解。结果表明,Bi(III)在NaCl?KCl熔盐体系中的还原反应是一步得到3个电子的准可逆反应Bi3++3e?=Bi,起始还原电位为0.05 V (vs. Ag/AgCl),该反应受扩散控制。Berzins-Delahay方程和Sand方程计算的700℃下Bi(III)在熔盐中的扩散系数分别为0.83×10–5和1.0×10–5 cm2/s。阴极产物为致密纯金属Bi,不含杂质。 相似文献
13.
The electrochemical nucleation of carbon in molten alkaline fluoride media is investigated using cyclic voltammetry and chronoamperometry in the 670-750 °C temperature range. Chronoamperometric results show that the deposition process involves progressive nucleation with diffusion-controlled growth of the nuclei, and scanning electron microscopy shows that the shape of the nuclei is hemispherical. The influence of the temperature and the overpotential on the nuclear site densities is also considered. 相似文献
14.
The electrochemical nucleation of tantalum in molten alkaline fluoride media is investigated using chronoamperometry in the 670-750 °C temperature range to optimize the operating conditions for preparing tantalum coatings for anode materials. Chronoamperometric results show that the electrodeposition process involves progressive nucleation with diffusion-controlled growth of the nuclei, which was confirmed by scanning electron microscopy. The influence of the temperature and the overpotential on the nucleation site densities is considered.Once the deposit has been obtained, plotting the roughness of the tantalum coatings as a function of the current densities reveals a minimum at about 80 mA/cm 2. This minimum is considered by the authors as a consequence of the progressive nucleation. 相似文献
15.
The electrochemical behaviour of UCl 4 (0.01 mol L −1 up to 0.05 mol L −1) in 0.1 mol L −1 TBAPF 6/DMF solution at vitreous carbon was studied, at room temperature, by cyclic voltammetry and potentiostatic techniques. The electrolytic solutions were analyzed by UV spectroscopy (UV), and the electrodeposited films were characterized by Rutherford Backscattering Spectroscopy (RBS) and X-ray diffraction (XRD). The cyclic voltammetric results, at low UCl 4 concentrations (0.01 mol L −1), point that the reduction of U(IV) to U(0) occurs in two steps involving mainly U(IV) and U(III) species. The first electron transfer reaction is quasi-reversible and the second irreversible. The diffusion coefficient of U(IV) in DMF and the charge rate constant were determined to be 4.78 × 10 −7 cm 2 s −1 and 1.93 × 10 −3 cm s −1 (at 0.02 V s −1), respectively.RBS data obtained from samples prepared at constant potential (−3.10 V) during 3 h at room temperature, indicated the presence of uranium particles deposited all over the vitreous carbon surface with aggregates in some places, confirming that the second reduction step corresponds to uranium electrodeposition. No crystallographic ordering could be seen by XRD, pointing to an amorphous character of the uranium films. 相似文献
16.
This work concerns the extraction of europium from molten fluoride media. Two electrochemical ways have been examined: (i) the use of a reactive cathode made of copper and (ii) the co-deposition with aluminium on inert electrode, leading to the formation of europium–copper and europium–aluminium alloys, respectively, as identified by SEM-EDS analysis. Cyclic voltammetry and square wave voltammetry were used to identify the reduction pathway and to characterise the step of Cu–Eu and Al–Eu alloys formation. Then, electrochemical extractions using the two methodologies have been performed with extraction efficiency around 92% for copper electrode and 99.7% for co-reduction with aluminium ions. 相似文献
17.
Electrochemical behavior of rhodium(III) chloride in 1-butyl-3-methylimidazolium chloride was investigated by various electrochemical transient techniques at glassy carbon working electrode at different temperatures (343-373 K). Cyclic voltammogram of rhodium(III) in bmimCl consisted of a surge in reduction current occurring at a potential of −0.48 V (vs. Pd) is due to the reduction of Rh(III) to metallic rhodium and a very small oxidation wave occurring at −0.1 V. Increase of scan rate increases the peak current and remarkably shifts the cathodic peak potential () in negative direction indicating the irreversibility of electroreduction of rhodium(III). The diffusion coefficient of rhodium(III) in bmimCl (∼10 −9 cm 2/s) was determined and the energy of activation (∼25 kJ/mol) was deduced from cyclic voltammograms at various temperatures. The cathodic ( τr) and anodic ( τo) transition times were measured from chronopotential transients and the ratio τo/ τr was found to be 1:7. Electrowinning of rhodium from bmimCl medium results in a deposition of metallic rhodium with lower (20-25%) Faradaic efficiency. A separation factor of rhodium from co-existing noble metal fission product palladium in bmimCl was determined during electrodeposition. 相似文献
18.
The electrochemical reduction of GdF 3 was studied in the LiF-CaF 2 eutectic (77/23 mol%) on a tantalum electrode for various GdF 3 concentrations in the 800-900 °C temperature range. A previous thermodynamic study showed that gadolinium trifluoride is reduced into metal in a one-step process:
19.
The electrochemical reduction of oxygen has been studied on gold, boron-doped diamond (BDD) and glassy carbon (GC) electrodes in a ternary eutectic mixture of acetamide (CH 3CONH 2), urea (NH 2CONH 2) and ammonium nitrate (NH 4NO 3). Cyclic voltammetry (CV), differential pulse voltammetry (DPV), chronoamperometry and rotating disk electrode (RDE) voltammetry techniques have been employed to follow oxygen reduction reaction (ORR). The mechanism for the electrochemical reduction of oxygen on polycrystalline gold involves 2-step, 2-electron pathways of O 2 to H 2O 2 and further reduction of H 2O 2 to H 2O. The first 2-electron reduction of O 2 to H 2O 2 passes through superoxide intermediate by 1-electron reduction of oxygen. Kinetic results suggest that the initial 1-electron reduction of oxygen to HO 2 is the rate-determining step of ORR on gold surfaces. The chronoamperometric and RDE studies show a potential dependent change in the number of electrons on gold electrode. The oxygen reduction reaction on boron-doped diamond (BDD) seems to proceed via a direct 4-electron process. The reduction of oxygen on the glassy carbon (GC) electrode is a single step, irreversible, diffusion limited 2-electron reduction process to peroxide. 相似文献
20.
The electrochemical behaviour of nitride ions has been studied in fluoride melts (eutectic LiF–CaF 2) by cyclic voltammetry and square wave voltammetry. The purpose of this work is to propose an alternative way for anodic reaction in molten fluorides processes. Thermodynamical analysis can be used for the evaluation of the anodic material regarding to its oxidation potential and reactivity with nitrogen. Then electrochemical investigations confirm the oxidation of nitride ions into nitrogen in a one-step process exchanging 3 electrons, and let propose several materials usable for nitrogen evolving. 相似文献
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