Exploring more efficient and low-cost electrocatalysts to replace platinum (Pt) is highly desired to promote the practical hydrogen production through water splitting. Herein, a facile and effective strategy is proposed to fabricate self-standing Al₃Ni₂/Ni electrode with controlled phase composition and surface morphology, which is obtained by one-step electrochemical reduction of Al³⁺ on commercially available nickel in eutectic NaCl-KCl melt. Different from previously reported approaches, uniform Al₃Ni₂ monolith catalyst can directly grow onto Ni substrate. The deposit possesses unique three-dimensional (3D) cauliflower-like morphology comprising of nano- and microparticles due to the rapid nucleation rate during molten salt electrolysis. The as-fabricated Al₃Ni₂/Ni electrode can be directly used as the cathode to catalyze Hydrogen evolution reaction (HER). Impressively, it exhibits remarkable HER activity comparable to commercial Pt, including a low overpotential of 83.4 mV for a current density of 10 mA cm⁻², a small Tafel slope of 40.7 mV dec^-1, and excellent long-term stability over 36 h of continuous HER operation in 0.5 M H₂SO₄ solution. The intrinsic catalytic ability of Al₃Ni₂ with the unique hierarchical structure of nano/microsized grains can offer multiple effects, including massive exposed active sites, enhanced charge transfer and mass transport, and fast gas releasing that synergistically contribute to improving the electrocatalytic performance of HER. This work represents a highly promising approach to the design and one-step controllable fabrication of efficient and self-standing base metal electrode for electrocatalytic hydrogen production.

In order to take full advantage of the secondary resources, in this paper, we reported a template-free process to prepare porous Co microfibers from spent lithium-ion batteries (LIBs). First, the waste LiCoO₂ powders were leached by oxalic acid at a suitable temperature, and rod-like cobalt oxalate powders were obtained. Second, the porous Co microfibers were prepared by using the cobalt oxalate as precursors through a thermal decomposition at 420 °C under nitrogen atmosphere. The prepared Co microfibers possess diameters of 1–2 μm, and each microfiber consists of small particles with size of 100–200 nm. The Co microfibers (25 wt%)/paraffin composite exhibited excellent microwave absorption performance. When the sample thickness is 4.5 mm, the reflection losses reach − 36.14 and − 38.20 dB at 4.16 and 17.60 GHz, respectively, and the effective bandwidth reaches up to 5.52 GHz. This indicates that the Co microfibers can be used as a promising microwave absorber. Therefore, this paper demonstrates a novel process to make a high value-added product through recycling from the spent lithium-ion batteries. In addition, it is advantageous to eliminate the hazard of spent lithium-ion batteries and electromagnetic radiation to environment and human health. Graphical abstract: [Figure not available: see fulltext.].

The electrochemical behavior of Dy(III) and its co-reduction process with Zn(II) on a tungsten electrode were studied in eutectic NaCl-KCl melts at 700 °C by using a series of electrochemical techniques. The results indicate that the reduction of Dy(III) to Dy(0) is a diffusion controlled quasi-reversible process through a one-step reaction of exchanging three electrons. The diffusion coefficient of Dy(III) was calculated to be 1.7 × 10-5 cm2 s-1. Furthermore, the co-reduction of Dy(III) and Zn(II) on the tungsten electrode makes Dy(III) be reduced at more positive potentials due to the formation of various Dy-Zn intermetallic compounds. The electromotive force measurements were performed to determine the thermodynamic properties of the Dy-Zn intermetallic compounds, including the activities and relative partial molar Gibbs free energies of dysprosium in the two-phase coexisting state, as well as the standard formation Gibbs energies of Dy-Zn intermetallic compounds. Finally, potentiostatic electrolysis at -2.0 V was carried out in molten NaCl-KCl-DyCl3(1.0 mol%)-ZnCl2(1.0 mol%) at 700 °C for 11 h to prepare Dy-Zn alloy. X-ray diffraction and scan electron micrograph - energy dispersive spectrometry analyses showed that the obtained Dy-Zn alloy mainly comprised of DyZn2, as well as the minor phases of Dy2Zn17, DyZn3 and DyZn.

Copper telluride is a sort of solid waste generated from copper anode slime processing, from which the recovery of copper and particularly the high-value element of tellurium always encounter technical difficulties due to the complex physicochemical properties of tellurium. In this study, an efficient and compact process for copper telluride recycling has been developed. Efficient separation of tellurium and copper from copper telluride was achieved through a pressure oxidizing alkaline leaching process under the optimal conditions of 5 mol/L for NaOH concentration, 5:1 for liquid to solid ratio, 150 °C for temperature, 0.7 Mpa for system pressure, and 2 h for reaction time, by which over 95% of tellurium was selectively dissolved in the solution and copper was enriched in the solid phase mainly in the form of copper oxides. Tellurium was subsequently recovered as TeO₂ by neutralization of the alkaline leaching solution with sulfuric acid to pH 4.5, delivering a TeO₂ precipitation efficiency of over 95%. In general, the masses of tellurium and copper balanced well in both the liquid and solid phases for each step, and the overall recovery efficiencies of Te and Cu reached as high as approximately 91% and 98%, respectively.

In the present study, a novel process for effective recoveries of Te and Cu from copper telluride from anode slime processing in copper smelters was proposed. The process consists of two hydrometallurgical steps of atmospheric alkaline leaching for Cu and Te separation, and TeO₂ precipitation with H₂SO₄ for Te recovery. The effects of NaOH concentration, liquid to solid ratio, temperature, H₂O₂ to Cu₂Te mole ratio, and reaction time on the dissolution behavior of tellurium were investigated. A Te leaching efficiency of about 91% was obtained under the optimal experimental conditions. The results of thermodynamic and kinetic analysis indicate that a lower temperature is favorable for the dissolution of Te, and mass transfer inside the solid particle is the rate-determining step. In addition, a mechanochemical-assisted leaching was conducted, by which Te leaching efficiency was enhanced to approximately 93% with ball milling at 180 rpm for 5 h. After Te leaching, H₂SO₄ was utilized to adjust the pH value of the Te-containing alkaline leach solution to 4.5 for TeO₂ precipitation. The crystallization of TeO₂ can be completed by reacting for 1 h and the overall tellurium recovery has reached nearly 90%.

Waste aluminate phosphor is a valuable secondary resource of rare earth elements (REEs). However, Ce and Tb in aluminate green phosphor can hardly be extracted by direct leaching in an inorganic acid. Therefore, Na₂CO₃ assisted roasting is adopted to decompose the stable spinel structure of Ce_0.67Tb_0.33MgAl₁₁O₁₉ in the present work and to achieve the transformation of REEs to simple oxides. Based on the thermodynamic calculations, systematic experiments of thermal decomposition have been conducted. The thermal decomposition behavior, phase evolution, valence state change, variations in micro and macro morphology of the green phosphor during Na₂CO₃ assisted roasting were examined by using TG-DSC/MS, XRD, XPS, SEM/EDS analyses. The results indicated that the green phosphor began to react with solid Na₂CO₃ at 800 °C, and the reaction was dramatically accelerated with temperature rising above 851 °C. At about 1000 °C, Ce_0.67Tb_0.33MgAl₁₁O₁₉ could completely decomposed into CeO₂, Tb₂O₃ and MgO by roasting in an equivalent mass of Na₂CO₃ for 2 h, while α-Al₂O₃ was hardly attacked in roasting. The decomposition mechanism of Ce_0.67Tb_0.33MgAl₁₁O₁₉ in molten Na₂CO₃ could be depicted by the unreacted shrinking core model, and the reaction rate constant was estimated at approximately nanometers per second. The synergistic effect of cation-oxoanion ensures the successful extraction of CeO₂ and Tb₂O₃ from the green phosphor via Na₂CO₃ assisted roasting method. The converted CeO₂ and Tb₂O₃ can be extracted by using chlorination roasting and separated from non-REE residues. According to these investigations, a new efficient process technology is proposed for sustainable recycling of waste phosphor.

The electrochemical behavior of yttrium and its co-deposition with aluminum were investigated by several transient electrochemical techniques on a tungsten electrode at 973K in NaCl-KCl eutectic melts. The results reveal that the reduction of Y(III) in NaCl-KCl-YCl₃ melts is a one-step process with three-electron exchanged and the reaction is a quasi-reversible diffusion-controlled process at low scan rates (0.05∼0.5 V/s). The calculated diffusion coefficient is approximately 2.8 × 10−⁵cm²/s. After AlCl₃ was introduced into the melts, cyclic voltammetry and open circuit chronopotentiometry showed the formation of two Y-Al intermetallic compounds, indicating that under-potential deposition of yttrium occurred on tungsten electrode covered with liquid Al. The electromotive force was measured at 973K to determine the thermodynamic properties of Y-Al intermetallic compounds, such as the activity of Y in the two-phase coexistence state, relative partial molar Gibbs energies, as well as the standard Gibbs energies of Y-Al intermetallic compounds. Finally, potentiostatic electrolysis was conducted to prepare Y-Al alloys from molten NaCl-KCl-YCl₃ (1.5 mol%)-AlCl3 (1.5 mol%) by the co-reduction method. The cathodic alloys were characterized using X-ray diffraction (XRD) and scan electron micrograph (SEM)-energy dispersive spectrometry (EDS) and the results indicated that the obtained alloys were mainly composed of YAl₂, as well as YAl₃ and YAl phases. The Y-rich phase intermetallic compound YAl, formed in the later period of electrolysis just when the concentration of AlCl₃ is fairly low.

The electrochemical behavior of Nd(III) and its coreduction with Mg(II) were investigated on a molybdenum electrode at 1023 K in eutectic NaCl-KCl melts. The results indicate that the reduction of NdCl₃ in NaCl-KCl melts is a one-step process with three electrons exchanged, and the reaction is an irreversible diffusion-controlled process at low scan rate with the calculated diffusion coefficient of about 6.8 × 10^-5 cm² s^-1. After MgCl₂ is introduced to the melts, the reduction of Nd(III) takes place at a more positive potential value due to the formation of Mg-Nd intermetallic compounds through electrochemical co-deposition and chemical reduction of Nd(III) ions by preferentially deposited Mg. The solid intermetallic compounds of Mg₃Nd, Mg₂Nd, and MgNd observed in open circuit chronopotentiometry curves are transformed to thermodynamically more stable Mg-rich phases of Mg₁₂Nd and Mg₄₁Nd₅ when potentiostatic electrolysis at -2.10 V. It has been confirmed by X-ray diffraction and SEM-EDS microscopy that the cathodic deposits are composed of Mg, Mg₁₂Nd, Mg₄₁Nd₅, and Nd. The present results confirm that it is an effective method for recycling of neodymium from waste NdFeB magnets by means of electrochemical formation of Mg-Nd alloys from NaCl-KCl-MgCl₂-NdCl₃ melts.