Search

Herein we report a molten salt electrochemical approach to synthesize two MAX phases, V 2 AlC and V 4 AlC 3 , using inexpensive V 2 O 3 , Al 2 O 3 , and graphite as raw materials. The oxides/carbon mixture is directly reduced into MAX phases under cathodic polarization at moderate temperatures (600–850 °C). The corresponding MXenes, V 2 CT x and V 4 C 3 T x , are successfully produced through the HF etching method. The optimized molar ratio of V 2 O 3 /Al 2 O 3 /C precursor for V 2 AlC synthesis is achieved, and the resulting V 2 CT x film possesses great stability in air at 30 °C for at least 10 days. Furthermore, the obtained V 2 CT x and V 4 C 3 T x exhibit excellent capacities of 468 mAh g−1 and 152 mAh g−1 after 100 cycles, respectively, at a current density of 100 mA/g, comparable to those made from conventional methods. [ABSTRACT FROM AUTHOR]

• Quantitative analysis of the electrochemical-induced thermal convection. • Confirmation the flux contribution of the electrochemical-induced thermal convection and density-driven convection. • Demonstration of small temperature difference caused by electrochemical reaction-induced thermal convection in molten salts. Understanding the natural convection in molten salt electrochemical systems is of great importance to accurately reveal the reaction kinetics and mechanism. This electrochemical reaction would cause changes in the density and temperature of salt near the electrode due to the depletion of reactant and the generation of product. However, in most studies, the temperature-induced convection effects received very limited attention in molten salts as the experiments were usually conducted in a thermostatic furnace. Herein we report the electrochemical reaction-induced convection in molten LiCl-KCl, with reactive EuCl 3 or CrCl 3 added into the system. On the basis of numerical simulations for chronoamperometry, it was proved that the thermal convection effects were not comparable to the density-driven convection even at molten salt containing 100 mM redox concentration and an electrolysis time of 100 s. Nevertheless, we observed the enhancement of density-driven convection by thermal convection in cyclic voltammetry simulation in LiCl-KCl-CrCl 3 system at 5 mV/s. Furthermore, the simulated results revealed that the temperature difference between electrode and solution ranged from 2.94 × 10−3 to ∼ 0.4 K at the applied conditions, showing the negligible change in temperature caused by electrochemical reactions. [ABSTRACT FROM AUTHOR]

Here we reported an abnormal intense natural convection in molten salts. The experimental and simulated results confirmed a thin convection–diffusion layer (ranging from 93 μm to 161 μm) attached to the electrode surface in molten salt systems. This intense natural convection can be further utilized for the concentration measurement of soluble-soluble redox couple in molten salts. [Display omitted] • The observation of intense natural convection in molten salts. • The built models confirmed the existence of a thin convection–diffusion layer in molten salt system, which can be utilized for in-situ concentration measurement during cyclic voltammetry. Herein we reported the in-situ electrochemical concentration measurement of soluble-soluble redox species (Eu3+/Eu2+ and Sm3+/Sm2+) in molten LiCl-KCl, based on the notable natural convection in molten salts. The combined simulation and experiments confirmed the presence of strong natural convection, which resulted in steady-state current in cyclic voltammetry tests at low scan rates. Interestingly, this natural convection effects offered a novel and simple way to calculate the diffusion coefficient and concentration ratio of soluble redox species. [ABSTRACT FROM AUTHOR]

Electrochemical studies of LaF 3 in molten FLiNaK salt were performed on inert working electrodes. An additional reduction reaction prior to the decomposition of FLiNaK melt was observed when LaF 3 was dissolved in and the reduction product was later recognized as lanthanum metal by SEM analysis. This phenomenon is very surprising and interesting considering the standard reduction potential of La (III) is more cathodic than that of K (I) and Na (I). The present study corrected the previous study [ 1 ] and will provide a new insight on the electrochemical and thermodynamic investigation of other lanthanides in molten FLiNaK salt. [ABSTRACT FROM AUTHOR]

Producing graphene through the electrochemical reduction of CO2 remains a great challenge, which requires precise control of the reaction kinetics, such as diffusivities of multiple ions, solubility of various gases, and the nucleation/growth of carbon on a surface. Here, graphene was successfully created from the greenhouse gas CO2 using molten salts. The results showed that CO2 could be effectively fixed by oxygen ions in CaCl2-NaCl-CaO melts to form carbonate ions, and subsequently electrochemically split into graphene on a stainless steel cathode; O2 gas was produced at the RuO2-TiO2 inert anode. The formation of graphene in this manner can be ascribed to the catalysis of active Fe, Ni, and Cu atoms at the surface of the cathode and the microexplosion effect through evolution of CO in between graphite layers. This finding may lead to a new generation of proceedures for the synthesis of high value-added products from CO2, which may also contribute to the establishment of a low-carbon and sustainable world. [ABSTRACT FROM AUTHOR]

This work studied the electrochemical behavior of iodide in molten LiF-NaF-KF mixture and the separation of iodide from the molten salt at 973 K using a graphite working electrode. The cyclic voltammetry (CV) and chronoamperometry (CA) measurements indicated an anodic passivation layer was formed on the graphite electrode in this melt and the layer blocked the charge transfer across the electrolyte/electrode interface. However, the added I− ions could pass through the passivation layer and were oxidized at the graphite electrode. The characterization of the obtained products proved that iodide ions was efficiently oxidized to I 2 gas during electrolysis. Furthermore, the graphite anode showed excellent stability without any obvious change in dimensions after electrolysis, which showed the high current efficiency of I 2 evolution reaction. Thus, the graphite electrode was possible to be served as an excellent anode to separate other anions (such as Br) from molten fluoride salts. [ABSTRACT FROM AUTHOR]

Here we report an innovative application of chronopotentiometry (CP) and potentiodynamic scan (PD) techniques to the measurement of redox potentials in a molten salt. Commonly, a stable Cl 2 /Cl− reference electrode is achieved by direct injection of chlorine gas to a graphite electrode, which requires rigorous design and operation for the high-temperature system to be studied. Instead, in situ techniques (CP and PD) were introduced to generate chlorine gas on the working graphite electrode, which then acts as the chlorine gas source. The measurements of formal potential and activity coefficient of AgCl/Ag at different AgCl mole fractions (X = 0.0039 and X = 0.0145) and temperatures (723–823 K) in molten LiCl-KCl salt were carried out using these two techniques. Moreover, these simple methods can be applied to measure the equilibrium potentials, formal potentials and activity coefficients of other metal chlorides in molten chloride salt as well as in other molten salt systems. • Cl 2 gas is produced by in-situ methods rather than by injecting Cl 2 gas. • CP & PD methods are safe and easy to operate with high accuracy and stability. • CP & PD methods can be applied in other salt systems to study the redox potential. [ABSTRACT FROM AUTHOR]

Separation of some active metal ions such as Ce3+, Eu3+, and La3+ from molten fluoride salt using electrochemical process is difficult. The currently available electrodeposition process assisted by the solid reactive electrode or liquid metal electrode faces challenges such as the isolation of the reactive electrode due to the formation of a compound, the ability to maintain a low activity value of the active metal in liquid electrode, and a complex purification process of the target metal. In the present study, we developed a new method that successfully proved the extraction of lanthanum from molten LiF salt using an inert Mo electrode with the assistance of either NaF or KF salt. In that case, the formation of LaF 6 3− in LiF-KF (or LiF-NaF) salt, instead of La3+ in LiF, positively shifts the electrodeposition potential of lanthanum by a value of about 1 V. The simplicity of this method compared to the conventional routes for the extraction of lanthanum from molten salt will result in a reduction in separation costs and this approach has a potential application for the separation of other active metals in molten fluorides or chlorides. • Electrochemical tests were performed in LiF-LaF 3 , LiF-KF-LaF 3 , and LiF-NaF-LaF 3 , respectively. • The electrodeposition of La metal in LiF-KF and LiF-NaF molten salts was realized and further confirmed by SEM analyses. • Electrodeposition of lanthanum was found to be greatly shifted to a more positive value with the addition of KF or NaF. [ABSTRACT FROM AUTHOR]