The in-situ preparation of LiCl–KCl–ZrCl4 melt was investigated by the replacement reaction between Zr and CuCl in LiCl–KCl melt at 500 °C, and the reaction progress was also investigated by a series of electrochemical techniques, such as cyclic voltammetry, square wave voltammetry and open circuit chronopotentiometry. The electrochemical signals show that the concentration of Zr(IV) ions increases gradually and then reaches to the maximum value with the reaction time increasing from 0 to 180 min, while the concentration of Cu(I) ions decreases rapidly and drops to below the detection limit of the electrochemical tests. Meanwhile, the concentrations of Cu and Zr ions in the melt were determined over time by chemical analysis in the course of reaction. The results are in good agreement with the electrochemical tests. Finally, LiCl–KCl melts with (0.837 wt.% ∼ 1.709 wt.%) ZrCl4 are obtained, and the final concentration of Cu(I) in the melt has dropped to below 0.025 wt.% when the reaction lasted for 180 min.

•NbC powder was prepared electrochemically in molten salt.•The working temperature was lower than that of carbothermic reduction.•The reduction pathway was discussed compared to direct electro-deoxidation of Nb2O5.The niobium carbide powder was prepared via electrochemical reduction of the mixture of Nb2O5 and carbon in molten CaCl2–NaCl. The reaction pathway from the sintered precursor to the final product has been investigated. The effect of the working temperature on the reduction of the Nb2O5/C composite precursor was considered. The role of carbon during the electrochemical reduction of the composite pellet was discussed. The samples were analysed by XRD and SEM. The results indicated that the NbC powder was approximately 200 nm after the reduction. Nb2O5 was gradually reduced to Nb, and NbC was subsequently obtained by the reaction of carbon with Nb metal. In addition, Nb2O5 could spontaneously react with CaO in the melt to form a serious of calcium niobates. The participation of carbon was available for the efficiency of electro-reduction of Nb2O5.Niobium carbide powder was electrochemically prepared in molten salt, and the reduction pathway was illustrated schematically.

LiCl–KCl–ZrCl4 melt was prepared by an in situ displacement reaction between SnCl2 and Zr in LiCl–KCl melt at 773 K, and the progress of the reaction between SnCl2 and Zr was also investigated by dynamic electrochemical measurements, such as cyclic voltammetry, square wave voltammetry and open circuit chronopotentiometry. The results reveal that the concentration of Zr(IV) increases gradually and reaches a maximum value with the reaction time increasing from 0 to 210 min, while the concentration of Sn(II) decreases gradually and drops below the detection limit. In addition, the chemical analyses of Zr(IV) and Sn(II) in the melt at various times were also carried out and the results are in good agreement with those of the electrochemical measurements. Finally, LiCl–KCl–ZrCl4 melts with a low concentration of Sn(II) (<0.01 wt%) were obtained, when the reaction time was prolonged to 210 min.

ZrSiO4 powder synthesized by the sol–gel method is used to study the reaction mechanism of natural zircon mineral treated by an alkali fusion method. The reaction processes are analyzed by in situ Raman spectroscopy. Other characterization experiments using techniques, such as FTIR spectroscopy, TG-DTA and X-ray powder diffraction complement and verify the Raman spectroscopy study. The results reveal that hydroxylation–dehydration play important roles during the alkali-fusion process. Hydroxylation action breaks some bonds between Si and O in silicon–oxygen tetrahedral and releases zircon as ZrO2 and silanol groups and then dehydration makes SiO4 tetrahedron polymerize to the silicate with different lattice structure. The electron-donating ability of the O atom in molten alkali determines the number of Si–O bonds broken in SiO4 tetrahedral and the lattice structure of silicate products. Finally, with further development of the reaction, the bridge-oxygen bonds of intermediate products are substitutes for non-bridging-oxygen bonds.

The mechanism of decomposition of magnesium inosilicate (MgSiO3) during the alkali fusion process using NaOH was investigated by Raman spectroscopy in situ and X-ray diffraction analyses. The results show that the tetrahedral silica chains within MgSiO3 are gradually disrupted, and nesosilicate with the isolated SiO4 tetrahedra becomes reorganized at the beginning of the alkali fusion process. In the decomposition of MgSiO3, the two intermediates are Mg2SiO4 and Na2MgSiO4, while the final products are Mg(OH)2 and Na4SiO4. It can be concluded that this decomposition did not initiate from the cation exchange reaction in the process.

The electrochemical corrosion of the graphite electrode for vanadium redox flow battery is investigated by on-line mass spectrometry analysis. The results show that CO2 and CO form and evolve more preferably than O2 on the graphite anode, which lead to the electrochemical corrosion of the graphite electrode. Furthermore, the evolution rate of O2 is the highest one among evolved gases if the polarization potential becomes too positive. The oxidation of VO2 + on the graphite electrode in 2 M H2SO4 + 2 M VOSO4 hinders the carbon oxidation reaction and retards the electrochemical corrosion of the graphite electrode.Highlights► The corrosion of graphite electrode is investigated by on-line mass spectrometry. ► CO2 and CO form and evolve more preferably on the graphite electrode than O2. ► Evolution rate of O2 can be the highest if the potential becomes too positive. ► Oxidation of VO2 + on the graphite electrode hinders carbon oxidation reaction.

Carbon films with a Ti–O–C graded interlayer were deposited on titanium by electrochemical reduction of carbonate ions in a molten LiCl–KCl–K2CO3 system. A graded interlayer, prepared by the joint process of oxidation in air and electrochemical reactions on titanium substrate in the LiCl–KCl melt, can enhance obviously adhesion between the titanium substrate and carbon films. The formation of carbon films with a Ti–O–C gradient was demonstrated by SEM and XRD, and the adhesion strength of the carbon films was measured by scratch test.Highlights►A Ti–O–C gradient was prepared on site. ►Carbon film was deposited on titanium in molten salt. ►The Ti–O–C gradient improved adhesion of carbon film to titanium substrate. ►The method to produce the Ti-O-C gradient could be extended to other metal substrates.

The graphite plate is easily suffered from corosion because of CO2 evolution when it acts as the positive electrode for vanadium redox flow battery. The aim is to obtain the initial potential for gas evolution on a positive graphite electrode in 2 mol dm−3 H2SO4 + 2 mol dm−3 VOSO4 solution. The effects of polarization potential, operating temperature and polarization time on extent of graphite corrosion are investigated by potentiodynamic and potentiostatic techniques. The surface characteristics of graphite electrode before and after corrosion are examined by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The results show that the gas begins to evolve on the graphite electrode when the anodic polarization potential is higher than 1.60 V vs saturated calomel electrode at 20 °C. The CO2 evolution on the graphite electrode can lead to intergranular corrosion of the graphite when the polarization potential reaches 1.75 V. In addition, the functional groups of COOH and CO introduced on the surface of graphite electrode during corrosion can catalyze the formation of CO2, therefore, accelerates the corrosion rate of graphite electrode.Graphical abstractThe overpotential for gas evolution on positive graphite electrode decreases due to the functional groups of COOH and CO introduced on the surface of graphite electrode during corrosion process, which can self-catalyze the oxidation of carbon atoms therefore, accelerates corrosion process.Highlights► Initial potential for gas evolution is higher than 1.60 V vs SCE. ► Factors affecting the graphite corrosion are investigated. ► Functional groups of COOH and CO introduced during corrosion process. ► The groups can self-catalyze the oxidation of carbon atoms.

CeNi4Cu alloy powders were prepared by electro-deoxidation of oxide precursors in molten KCl–LiCl at 650 °C. The reduction pathway from the mixture of CeO2 and (Ni0.8Cu0.2)O to CeNi4Cu was studied by examination of partially and fully reduced samples using XRD and SEM with EDX analyses, which were obtained by interrupting the reduction process after different times. The first stage of the reaction involved the rapid formation of Ni–Cu alloy and Ni, thereafter CeO2 was electrochemically reduced and alloyed with Ni–Cu alloy or Ni. Interconnected nodular CeNi4Ce particles with smooth surfaces about 2 μm were obtained after the heat treatment in situ in the molten KCl–LiCl.Electrochemical synthesis of CeNi4Cu alloy from the mixed oxides and in situ heat treatment in a eutectic LiCl–KCl melt. CeNi4Cu alloy powders are prepared by electro-deoxidation of the oxide precursors in molten KCl–LiCl at 650 °C, which undergo heat treated in situ under cathodic polarization. Their phase composition and morphology are characterized by XRD diffraction and SEM analysis.Research Highlights►Electrochemical synthesis of the interconnected nodular CeNi4Cu particles with smooth surfaces after heat treatment in situ in the LiCl–KCl melts;►Understanding of electrochemical reaction pathway of CeNi4Cu alloy from its oxide precursor.

Nickel coating on the carbon–polythene composite plate was prepared by electrodeposition in a nickel sulfate solution in this work. The morphology and cross-sectional microstructure of the nickel coating were examined by scanning electron microscope (SEM) and optical microscope (OM), respectively. The influence of bath temperature on the nickel deposition rate was investigated experimentally. The adhesion between the coating and the substrate was evaluated by the pull-off test. The corrosion behavior of the coating in an aqueous solution of NaCl was studied by electrochemical methods. The results showed that the nickel electrodeposition rate could reach up to 0.68 μm min−1 on average under conditions of cathodic current density of 20 mA cm−2 and bath temperature of 60 °C. It was confirmed that increasing the bath temperature up to 50 °C had a positive effect on the nickel deposit rate, while an adverse effect was observed beyond 60 °C. The adhesion strength between the nickel coating and the substrate can be more than 2.3 MPa. The corrosion potential of the bright coating in the NaCl solution was more positive than that of the dull coating, and the anodic dissolution rate of the bright coating was also far lower at the same polarization potential compared with the dull coating.