Computational electrochemo-fluid dynamics modeling in a uranium electrowinning cell

A computational electrochemo-fluid dynamics model has been developed to describe the electrowinning behavior in an electrolyte stream through a planar electrode cell system. Electrode reaction of the uranium electrowinning process from a molten-salt electrolyte stream was modeled to illustrate the details of the flow-assisted mass transport of ions to the cathode. This modeling approach makes it possible to represent variations of the convective diffusion limited current density by taking into account the concentration profile at the electrode surface as a function of the flow characteristics and applied current density in a commercially available computational fluid dynamics platform. It was possible to predict the conventional current–voltage relation in addition to details of electrolyte fluid dynamics and electrochemical variables, such as the flow field, species concentrations, potential, and current distributions throughout the galvanostatic electrolysis cell.     

Development of an anode structure consisting of graphite tubes and a SiC shroud for the electrowinning process in molten salt

In a molten chloride salt-based electrolysis, chloride evolution at an anode needs to be considered in terms of potential fluctuation, capture, and corrosion problems. Here, we demonstrate an anode structure consisting of graphite tubes and a SiC shroud to be applied to the electrowinning process. A large surface area as well as high corrosion resistivity was achieved through the use of inert graphite tubes. The Cl₂ (g) capture was enhanced by the employment of a porous SiC shroud. It also allows an efficient contact of the electrode surface to the LiCl–KCl eutectic melt for an anodic evolution. No significant effects of the use of a SiC shroud on the anode overpotential and cell potential were found during the U deposition test.     

Automated chromatographic separation coupled on-line to ICP-MS measurements for the quantification of actinides and radiostrontium in soil samples

The uranium (III) ions behaviour in fused 3LiCl–2KCl eutectic versus the Cl^?/Cl₂ reference electrode in the temperature range of 723–823 K on the liquid cadmium electrode by transient electrochemical techniques on the tungsten or molybdenum electrodes was studied. The mechanism of electrochemical reduction on cadmium cathode and the influence of temperature, cathode current density and the duration of electrolysis were studied. The activity coefficients and the base thermodynamic properties of uranium in fused U–Cd/3LiCl–2KCl system were calculated.     

Electrode processes of uranium ions and electrodeposition of uranium in molten LiCl-KCl

In the first part, LiCl-KCl-UCl₃ and LiCl-KCl-UCl₃-UCl₄ molten salts were prepared, which were studied employing cyclic voltammetry and chronopotentiometry techniques, respectively. It was determined that the reduction of U(IV) to uranium metal takes two steps. Firstly, U(IV) is reduced to U(III). Then, the reduction of U(III) to uranium metal occurs in a step with a global exchange of three electrons. Cyclic voltammetry studies indicated that at low sweep rates, the reduction of U(III) to uranium is reversible. However, a mixed control of both diffusion and electrontransfer is observed as the sweep rate increases. The diffusion coefficient of U(III) and the formal potential of U(III)/U versus Ag/AgCl reference electrode in these two salt systems were calculated respectively. In second part, based on the data of the electrode processes of uranium ions, electrodeposition of uranium metal was carried out. Uranium deposits were prepared adopting a 304 stainless steel electrode in the molten LiCl-KCl-UCl₃ and LiCl-KCl-UCl₃-UCl₄, respectively by employing suitable electrolytic techniques. The morphology of the deposits and the cross-section of the cathode were investigated by SEM. It was determined that at the beginning of the deposition process, uranium product alloys with stainless steel and forms a thin layer, and then uranium begins to grow adhering to the layer.     

Temperature-Dependent Electrochemical Properties and Electrode Kinetics of Na3V2(PO4)2O2F Cathode for Sodium-Ion Batteries with High Energy Density

Phosphate cathode materials are practical for use in sodium-ion batteries (SIBs) owing to their high stability and long-term cycle life. In this work, the temperature-dependent properties of the phosphate cathode Na₃V₂(PO₄)₂O₂F (NVPOF) are studied in a wide temperature range from −25 to 55 °C. Upon cycling at general temperature (above 0 °C), the NVPOF cathode retains an excellent charge/discharge performance, and the rate capability is noteworthy, indicating that NVPOF is a competitive candidate as a temperature-adaptive cathode for SIBs. Upon decreasing the temperature below 0 °C, the cell performance deteriorates, which may be caused by the electrolyte and Na electrode, based on the study of ionic conductivity and electrode kinetics. This work proposes a new breakthrough point for the development of SIBs with high performance over a wide temperature range for advanced power systems.     

Investigation of deceleration causes in gold electrowinning from aqueous cyanide solutions on porous carbon electrodes

Deceleration of gold electrowinning from model cyanide solutions on porous cathode of graphitized carbon felt was investigated. It was established that calcium ions present in the solution did not negatively affect the electrolysis rate. The main reason of the termination of gold electrowinning on the graphitized cathode was the corrosion of the stainless steel anode resulting in penetration into the cathode chamber of the electrolyzer of CrO₄²⁻ anions further reduced to Cr(III) and probably Cr(II) compounds. X-ray Photoelectron Spectroscopy investigation of the passivated carbon cathode showed that the film formed thereon consisted of Cr(III) compounds containing cyanide and hydroxy ligands. This film covers the active sites on the surface of the porous carbon cathode preventing the deposition thereon of ad-ions of gold(I). Inert aqua and hydroxo complexes of chromium(III) stronger impede the rate of gold(0) deposition on the porous carbon cathode than chromium(III) cyanide compounds. This fact originates presumably from the requirement of lower cathode potentials for the reduction of the former to labile chromium(II) complexes than those necessary for the thermodynamically stable anions [Cr(CN)₆]³⁻.     

Cost-effective solid oxide fuel cell prepared by single step co-press-firing process with lithiated NiO cathode

A cost-effective cell fabrication process was developed for intermediate temperature solid oxide fuel cells (IT-SOFCs). Co-doped ceria Ce_0.8Gd_0.05Y_0.15O_1.9 (GYDC) was synthesized by carbonate co-precipitation method. Lithiated NiO was prepared by glycine-nitrate combustion method and adopted as cathode material for IT-SOFCs. Single cell was fabricated by one-step dry-pressing and co-firing anode, anode functional layer (AFL), electrolyte and cathode together at 1200 °C for 4 h. The cell presented decent performance and an overall electrode polarization resistance of 0.54 Ω cm² has been achieved at 600 °C. These results demonstrate the possibility of using lithiated NiO as cathode material for ceria-based IT-SOFCs and the development of affordable fuel cell devices is encouraged.     

Electrode process of Nd-Fe alloy formed on iron electrode in molten chlorides

The cyclic voltammetry, convolution voltammetry and chronopotentiometry were used to study the electrode process of Nd (III) reduced on iron electrode in molten NaCl-KCl-NdCl₃ from 700 to 850°C. The electrodeposited products were analysed by X-ray diffraction. The results indicate that the intermetallic compound Fe₂Nd forms first, and then the metallic neodymium deposits when Nd (III) is reduced on iron electrode. The Nd-Fe alloys rich in neodymium can be obtained by electrolysis with iron cathode in molten chlorides. The Nd-Fe alloys are composed of Fe₂Nd and Nd.     

Electrochemical procedures in the treatment of spent nuclear fuel

The use of an electrochemical process for U/Pu partitioning has demonstrated a good performance and is a safe alternative for nuclear facilities. Its great advantages are the lack of introduction of foreign ions into the process and, especially, the minimization of the waste volume generated. For the introduction of electrochemical U/Pu partitioning in the 2nd Pu purification cycle, preliminary studies were carried out with a single mixer-settler unit. Based on the results, an 8-stage electrolytic mixer-settler (M-S MIRELE) was designed. Titanium was MIRELE's housing material (cathode) and platinum the anode, insulated with PTFE. The Pu recovery was higher than 99%, indicating the efficiency of this equipment.     

Spectroscopic study of a magnetically rotating arc between opper electrodes in contaminated argon

The effects of N₂ and CO contaminants in atmospheric-pressure argon on an arc rotating between two concentric copper electrodes has been studied using optical spectroscopy of copper lines. The axial temperature of the magnetically driven arc in Ar + %N₂ was determined to be around 10,000 K for arc currents of SO to 200 A. The diffusion process of the copper vapor from the cathode was also studied. A copper density maximum 1 mm from the cathode along the arc column was found in Ar + %N₂. Removal of the contaminated cathode surface layers by the arc when contaminant injection in the plasma gas was stopped was found to be a slow process with a time scale depending on the type of the gas contaminant. The presence of gas contaminant in the electrode material controls the cathode erosion mechanism and the overall arc behavior in the transition between a contaminated to a pure argon arc.     

Temperature effect on the recovery of SO2-Poisoned GC/Nano-Pt electrode towards oxygen reduction

The SO₂ poisoning of Pt nanoparticle (n-Pt) modified glassy carbon (GC/n-Pt) electrode and the recovery of its activity for the oxygen reduction reaction (ORR) were studied using cyclic voltammetry at ambient (25 °C) and elevated (70 °C) temperatures. Recovery of the GC/n-Pt electrode by cycling the potential within the ORR range (1.0 to 0.2 V (standard hydrogen electrode)) in 0.1 M H₂SO₄ was not effective at 25 °C, but at 70 °C the onset potential of the ORR was almost the same as that at the fresh GC/n-Pt electrode. For the two different temperatures used here, the recovery on cycling the potential between 0.4 and 1.7 V was efficient. However, the number of cycles and the amount of charge required for the recovery at 70 °C were the smallest, which is of great interest for the proton exchange membrane fuel cell performance. The recovery using such a wide potential range at 70 °C resulted in an enhancement of the electrocatalytic activity of the GC/n-Pt electrode over a non-poisoned (bare) GC/n-Pt electrode.     

Application of hollow fiber supported liquid membrane for the separation of americium from the analytical waste

Americium from analytical solid waste containing U and metallic impurities was separated using hollow fiber supported liquid membrane (HFSLM) technique impregnated with DHOA–TODGA from nitric acid medium. An aliquot of 5 g of the solid waste containing Am (19.95 mg) as minor actinide and of U (2,588 mg), Fe (1,360 mg), Ca (1,810 mg) and Na (3,130 mg) as major impurities was processed. The feed solution obtained after the dissolution of the residue in ~4 M HNO₃ was passed through HFSLM module. In the first stage using 1 M DHOA–dodecane U was recovered while Am and other impurities were left in the raffinate. In the second stage, 0.5 M DHOA + 0.1 M TODGA/dodecane was used for the separation of Am from other impurities. Though, majority of the elements were separated in this cycle, Ca was co extracted along with the americium. CMPO extraction chromatographic technique was used for further separation of americium from Ca. Significant decontamination factors were achieved in this three step separation process with respect to U, Fe, Na and Ca with ~77 % recovery of americium.     

Construction of a new solid-state U(VI) ion-selective electrode based on polypyrrole conducting polymer

In this study, a potentiometric sensor based on a pencil graphite electrode (PGE) coated with polypyrrole doped with uranyl zinc acetate (termed PGE/PPy/U) have been prepared for potentiometric determination of uranyl in aqueous solutions. Electropolymerization reaction for preparing of U(VI) sensor electrode was carried via applying a constant current of 1.0 mA on PGA working electrode in a solution containing 8.0 mM pyrrole and 0.8 mM ZnUO₂(CH₃COO)₄ salt. The constructed electrode displayed a linear and near Nernstian response (22.60 ± 0.40 mV/decade) to U(VI) ions in the concentration range of 1.0 × 10^?6–1.0 × 10^?2 M. A detection limit of 6.30 × 10^?7 M and a fast response time (≤12 s) was observed during measurements. The working pH range of the electrode was 4.0–8.0 and lifetime of the sensor was at least 60 days. The electrode revealed good selectivity with respect to many cations including alkali, alkaline earth, transition and heavy metal ions. The introduced uranyl electrode was used for measurement of U(VI) ion in real samples without any serious inferences from other ions.     

电解制备二氧化锰强酸性电解液中气体扩散电极的稳定性与失效行为

采用气体扩散电极(GDE)代替传统析氢阴极电解制备二氧化锰(EMD),重点研究了气体扩散电极在强酸性MnSO4-H2SO4电解液中的稳定性、寿命及失效行为.结果表明:气体扩散电极在MnSO4-H2SO4电解液中重现性好、具有一定的稳定性,寿命可达400 h;平行实验表明,阳极沉积一定厚度的EMD是槽电压第一次升高的主要原因;电流密度为100 A m-2时,气体扩散电极失效前阴极过程的速度由氧的离子化反应和氧的扩散混合控制,失效后阴极过程由氧去极化和氢去极化共同组成,主要发生析氢反应;催化层聚四氟乙烯(PTFE)网络结构的破坏和镍网层的溶解是电极失效的原因之一;Pt的团聚降低了电极的电催化活性,是电极失效的主要原因;阴极失效是槽电压再次升高的主要原因.     

Composite cathode materials Ag-Ba0.5Sr0.5Co0.8Fe0.2O3 for solid oxide fuel cells

Silver-Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) cathodes were prepared in two ways. In the first method, Ag-BSCF composite powder was prepared in ethanol solution, where Ag nanoparticles serving as a component in the preparation of Ag-BSCF composite cathodes had been previously obtained via one-step synthesis in absolute ethanol using a neutral polymer (polyvinylpyrrolidone). To the best of our knowledge, this is the first study to use a Ag sol obtained by the above method for preparation of Ag-BSCF composite powder. Then, a paste containing this powder was screen-printed on a Sm_0.2Ce_0.8O_1.9 electrolyte and sintered at 1,000 °C. In the second technique, an aqueous solution of AgNO₃ was added to a previously sintered BSCF cathode, which was then sintered again at 800 °C. The oxygen reduction reaction at the quasi-point BSCF cathode on the Sm_0.2Ce_0.8O_1.9 electrolyte was tested by electrochemical impedance spectroscopy at different oxygen concentrations in three electrode setup. The continuous decrease of polarization resistance was observed under polarization ?0.5 V at 600 °C. The comparative studies of both obtained composite Ag-BSCF materials were performed in hydrogen-oxygen IT-SOFC involving samaria-doped ceria as an electrolyte and Ni-Gd_0.2Ce_0.8O_1.9 anode. In both cases, the addition of silver to the cathode caused an increase in current and power density compared with an IT-SOFC built with the same components but involving a monophase BSFC cathode material.     

A membrane electrode assembly for the electrochemical synthesis of hydrocarbons from CO2(g) and H2O(g)

We report the electrochemical reduction of CO₂ into hydrocarbons using a new electrochemical membrane reactor holding a yet unreported membrane electrode assembly comprising a copper mesh cathode and a Ti felt coated with mixed metal oxide (MMO) catalyst anode separated by a proton conductive membrane. CO_2(g) was supplied to the cathodic reduction compartment, whilst humidified N₂ was supplied to the anodic oxidation compartment. The MMO anode produces protons transported across the proton exchange membrane and electrons transported via the external circuit to the copper cathode to reduce CO_2(g). Production rates of methane, propane, propene, iso-butane and n-butane were determined as a function of cell potential at temperatures between 30 and 70 °C and relative humidity between ca. 25% and 75%. Maximum methane concentration and the current efficiency for production of hydrocarbons were 3.29 ppm and 0.12%, respectively. Whilst the observed product spectrum is desirable, such low current efficiencies require systematic optimization of the catalytic membrane system, in particular an improved cathode with an optimum contact between proton conducting membrane, electrode and catalyst is desired.     

Strategies of Removing Residual Lithium Compounds on the Surface of Ni‐Rich Cathode Materials†

Ni‐rich cathode materials have become one of the most promising cathode materials for advanced high‐energy Li‐ion batteries (LIBs) owing to their high specific capacity. However, Ni‐rich cathode materials are sensitive to the trace H₂O and CO₂ in the air, and tend to react with them to generate LiOH and Li₂CO₃ at the particle surface region (named residual lithium compounds, labeled as RLCs). The RLCs will deteriorate the comprehensive performances of Ni‐rich cathode materials and make trouble in the subsequent manufacturing process of electrode, including causing low initial coulombic efficiency and poor storage property, bringing about potential safety hazards, and gelatinizing the electrode slurry. Therefore, it is of considerable significance to remove the RLCs. Researchers have done a lot of work on the corresponding field, such as exploring the formation mechanism and elimination methods. This paper investigates the origin of the surface residual lithium compounds on Ni‐rich cathode materials, analyzes their adverse effects on the performance and the subsequent electrode production process, and summarizes various kinds of feasible methods for removing the RLCs. Finally, we propose a new research direction of eliminating the lithium residuals after comparing and summing up the above. We hope this work can provide a reference for alleviating the adverse effects of residual lithium compounds for Ni‐rich cathode materials’ industrial production.     

(La0.8Sr0.2)0.9MnO3–Gd0.2Ce0.8O1.9 composite cathodes prepared from (Gd, Ce)(NO3) x -modified (La0.8Sr0.2)0.9MnO3 for intermediate-temperature solid oxide fuel cells

Development of high performance cathodes with low polarization resistance is critical to the success of solid oxide fuel cell (SOFC) development and commercialization. In this paper, (La_0.8Sr_0.2)_0.9MnO₃ (LSM)–Gd_0.2Ce_0.8O_1.9(GDC) composite powder (LSM ~70 wt%, GDC ~30 wt%) was prepared through modification of LSM powder by Gd_0.2Ce_0.8(NO₃) _x solution impregnation, followed by calcination. The electrode polarization resistance of the LSM–GDC cathode prepared from the composite powder was ~0.60 Ω cm² at 750 °C, which is ~13 times lower than that of pure LSM cathode (~8.19 Ω cm² at 750 °C) on YSZ electrolyte substrates. The electrode polarization resistance of the LSM–GDC composite cathode at 700 °C under 500 mA/cm² was ~0.42 Ω cm², which is close to that of pure LSM cathode at 850 °C. Gd_0.2Ce_0.8(NO₃) _x solution impregnation modification not only inhibits the growth of LSM grains during sintering but also increases the triple-phase-boundary (TPB) area through introducing ionic conducting phase (Gd,Ce)O_2-δ, leading to the significant reduction of electrode polarization resistance of LSM cathode.     

Salt clean-up for recycling by electrolysis with a cathode-perforated ceramic container assembly

Pyroprocessing is a promising way for the recovery of actinide elements from the used nuclear fuel. Electro-refining is a key technology of pyroprocessing and the electro-refining is generally composed of two recovery steps—deposit of uranium onto a solid cathode and the recovery of actinide elements by a liquid cathode. After the electro-refining process, it is necessary to remove the solutes from the molten salt for the salt regeneration. In this study, it was attempted to clean up a molten salt with a solid cathode- perforated ceramic container assembly and a glassy carbon anode. LiCl–KCl eutectic salt was used as a medium of the electrolytic bath. Uranium and cerium were used as solutes, where uranium was used as a surrogate for the actinide elements. The initial contents of uranium and cerium in the salt were varied in the range of 0–5 wt%. Electrolysis experiments were carried out by passing a constant current between the anode and cathode at 500 °C. The solute contents were measured using ICP-AES spectroscopy. The initial cathode potential was about ?1.6 V. This value decreased with increasing time in the salt. The solutes in the saline phase were successfully recovered onto the cathode.     

An electrochemical sensor based on TiO2/activated carbon nanocomposite modified screen printed electrode and its performance for phenolic compounds detection in water samples

Herein, we reported a titanium oxide (TiO₂) modified activated carbon nanocomposite that showed advantageous characteristics in terms of electro-conductivity, catalytic activity and surface area. The designed nanocomposite was employed to modify the screen printed carbon electrode transducer surface in the construction of an electrochemical sensor. The electrode surface modification was characterised by cyclic voltammetry and impedimetric studies. The modified transducer surface was subsequently used for the detection of four phenolic endocrine disruptors, p-nitrophenol, hydroquinone, catechol and 1-naphtol. Under optimal conditions, TiO₂ modified activated carbon sensor was evaluated by differential pulse voltammetry showing a good linearity with correlation coefficients higher than 0.99. It showed, in parallel, a high sensitivity where the detection limits were 348 ng/L, 110.1 ng/L, 3.3 ng/L and 7.2 µg/L for the respective studied compounds (S/N = 3). Finally, we validated the method with river water samples, and good recovery values were obtained showing the potential application of the reported biosensor.