The effect of electrode surface modification and cathode overpotential on deposit characteristics in aluminum electrorefining using EMIC–AlCl3 ionic liquid electrolyte

Electrorefining of aluminum alloy was investigated using AlCl₃ and 1-ethyl-3-methyl-imidazolium chloride (EMIC) (molar ratio AlCl₃:EMIC = 1.65:1) ionic liquid electrolyte on copper and aluminum cathodes at temperature of 90 ± 3 °C and cell voltage of 1.5 V. The effect of electrode surface modification and cathode overpotential on deposit characteristics of aluminum was investigated. The surface modification of electrodes reduced the dendritic depositions of aluminum. It was also observed that cathodic overpotentials obtained from experiments using modified electrodes are significantly lower than those of unmodified electrodes. A non-dendritic deposit of aluminum was observed even after prolonged electrorefining of 25 h. Pure aluminum deposits were obtained for all experiments with the current efficiencies in the range of 94–99%.     

Preparation of tin-filled carbon nanotubes and nanoparticles by molten salt electrolysis

This article presents a molten salt electrolytic method of synthesizing tin-filled carbon nanostructures, in which a melt of LiCl with an addition of SnCl₂ is electrolyzed between two graphite electrodes. In this process, Li intercalates into the cathodically polarized graphite while Sn is deposited onto the graphite surface. The Li intercalation causes the release of graphitic layers from the graphite, which enclose the Sn deposits, to form Sn-filled MWCNTs and carbon nanoparticles. By reversing the polarity of the graphite electrodes at regular intervals during electrolysis, the process can be extended substantially until the electrodes have been completely consumed, suggesting its suitability for production at a larger scale.     

Room temperature electrodeposition of aluminum antimonide compound semiconductor

AlSb is a group III-V compound semiconductor material that is conventionally grown by high temperature processes such as Czochralski and Bridgman methods. Development of a method to synthesize AlSb at room temperature will be more economical to help modulate the electronic properties. In this investigation, a pulsed potential electrodeposition method using a room temperature molten salt system (aluminum trichloride, AlCl₃/1-methyl-3-ethylimidazolium chloride, EMIC) with an addition of SbCl₃ is discussed. The potential pulse parameters were established by carrying out cyclic voltammetry at different concentrations of SbCl₃ and with varying molar ratios of AlCl₃/EMIC. Stoichiometric AlSb deposits were obtained from an acidic AlCl₃/EMIC (1.5:1 molar ratio) melt containing 4 × 10⁻³ mol/l of SbCl₃ onto an ordered TiO₂ nanotubular template. The AlSb compound was predominantly amorphous in as-deposited condition and annealing at 350 °C for 2 h in argon transformed into crystalline phase. The AlSb deposit showed a high resistivity in the order of 10⁹ Ω-cm and a defect concentration of 10¹⁶ cm⁻³ which was attributed to presence of carbon. The deposits obtained from a basic melt (0.67:1 molar ratio of AlCl₃/EMIC) were enriched with antimony.     

Electroless plating of aluminum using diisobutyl aluminum hydride as liquid reducing agent in room-temperature ionic liquid

We investigated an electroless aluminum plating based on using AlCl₃–1-ethyl-3-methylimidazolium chloride (AlCl₃–EMIC) ionic liquid with diisobutyl aluminum hydride (DIBAH) as a liquid reducing agent. The plating film was analyzed by measurements of X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX) and glow discharge optical emission spectroscopy (GD-OES). Consequently, a thick aluminum plating film with high uniformity was prepared from AlCl₃–EMIC with DIBAH. No impurity phases were detected. Moreover, we discussed the reaction mechanism of the electroless aluminum plating.     

Stability of a boron-doped diamond electrode in molten chloride systems

Stability of a boron-doped diamond as an oxygen evolution electrode material was evaluated at 773 K in molten LiCl–KCl (58.5:41.5 mol%), LiCl–KCl (75:25 mol%), LiCl–CaCl₂ (64:36 mol%), LiCl–NaCl–CaCl₂ (52.3:13.5:34.2 mol%). In molten LiCl–KCl systems, the BDD is stable at 773 K regardless of the concentration of oxide ion and the composition of the melt. In molten LiCl–CaCl₂ and LiCl–NaCl–CaCl₂, the BDD electrode is less stable than in molten LiCl–KCl systems.     

Investigation of the 1-methyl-3-ethylimidazolium chloride-AlCl3/LiAlCl4 system for lithium battery application Part I: Physical properties and preliminary chronopotentiometric study

The applicability of the 1-methyl-3-ethylimidazolium chloride — AlCl₃ system for lithium battery application was investigated. Lithium chloride was found to dissolve up to 1.59 mole ratio of LiAlCl₄/MeEtImAlCl₄ upon reaction between LiCl and AlCl₃ in the melt. Density, conductivity and viscosity of the melt upon addition of LiAlCl₄ were determined. The density was found to increase monotonically from 1280 to 1480 kg m^–3, while the conductivity decreased rapidly from the initial value of 5.6 mS to a steady plateau at 3.4 mS. The viscosity was varied from 1.46 Ns m^–2 to a small but distinct initial fall prior to rising to 2.75 Ns m^–2 when the mole ratio of LiAlCl₄ increased from zero to 1.59. The chronopotentiometric studies indicate a satisfactory electrochemical behaviour with no apparent attack of the melt by the formation of the reactive lithium alloys. 350 cycles were achieved with cycling efficiency over 90% using an optimal c.d. of 6 mA cm^–2 for lithium deposition on aluminium substrate in the melt. Prolonged cycling improved the nucleation rate but led to an increase in the internal resistance and a gradual reduction in the charge and discharge capacity.     

Al/Cl2 battery with slightly acidic NaCl electrolyte. I. Porous graphite chlorine cathodes

Al/Cl₂ cells were built using a coaxial cylindrical cell arrangement with cylindrical porous graphite chlorine electrodes and 99.5% Al anodes (cylindrical rods). The electrolyte used was a slightly acidic aqueous NaCl solution containing small amounts of In³⁺ and Hg²⁺ as additives. The cells have an open-circuit voltage of 2.6 V and a maximum power density of 55mW cm^–2 at a cell voltage of 1.6V with a Faradaic efficiency of 65–70%. A battery consisting of seven cells connected in parallel delivered a maximum power of about 15W.     

Effect of polypyrrole film on the stability and electrochemical activity of fluoride based graphite intercalation compounds in HF media

Fluoride intercalation/deintercalation cycles on commercially available high purity graphite electrodes leads to powder formation and electrode damage. Formation of polypyrrole films of optimum thickness by potential cycling on the graphite surface before fluoride intercalation leads to good mechanical stability to the electrode during intercalation/deintercalation cycles. The intercalation potential shifts by 200 mV in the positive direction. The intercalation and deintercalation charges (Q _a, Q _c) also decrease slightly. However the charge recovery ratio (Q _c/Q _a) improves significantly. Since the polypyrrole layer is compact on the graphite surface, the present study indicates that the film offers mechanical stability to the graphite film without affecting the electronic conductivity of the surface. F^– ion transport through the film also occurs with a small overvoltage.     

Aluminium electrodeposition from AlCl3-NaCl melts on glassy carbon,platinum and gold electrodes

The electrochemical deposition and dissolution of aluminium on glassy carbon, platinum and gold electrodes in chloraluminate melts have been investigated using linear sweep voltammetry and potentiostatic pulse techniques. It was shown that deposition of aluminium on the glassy carbon electrode at low overpotentials takes place by 3-D progressive nucleation and growth, with the incorporation of atoms in the crystal lattice as the rate-determining step. At overpotentials higher than –100 mV vs Al, in the melts containing more than 52 mol % of AlCl₃, diffusion of Al₂Cl ₇ ^– , takes over the control of deposition of aluminium. Alloying of platinum and gold electrodes with aluminium from the melt occurs in the underpotential region.     

Design, construction and operation of a laboratory scale electrolytic cell for sodium production using a β″-alumina based low-temperature process

Sodium metal can be produced at low temperatures (523 K) by electrolysis of sodium tetrachloroaluminate (NaAlCl₄) in a cell, which employs sodium ion conducting beta-alumina as diaphragm. A laboratory-scale electrolytic cell and associated systems were designed and constructed to study the various aspects of the energy efficient process. Graphite/reticulated vitreous carbon (RVC) was used as the anode and molten sodium as the cathode. Electrolysis was carried out at 523 K with currents in the range 1–10 A (10–125 mA cm^–2). The cathodic current efficiency was close to 100%, but the anodic current efficiency was very low (20–30%), probably due to the consumption of chlorine in the intercalation reaction of graphite and aluminium chloride. The sodium metal was analysed by AAS and found to have 5N purity. On prolonged electrolysis, the graphite anode disintegrated due to the formation of graphite intercalation compounds. RVC behaved as a better chlorine-evolving anode in the initial period of electrolysis, but its ability for chlorine evolution decreased on continuous electrolysis. The study indicated the need for effective stirring of the electrolyte with excess NaCl to avoid build up of aluminium chloride and the resultant complications in the cell.     

Electrochemical intercalation of aluminium chloride in graphite in the molten sodium chloroaluminate medium

Aluminium chloride intercalation in graphite was studied by anodic oxidation of compacted graphite (rod) and graphite powder electrodes in sodium chloroaluminate melt saturated with sodium chloride at 175 °C. The studies carried out by employing both galvanostatic and cyclic voltammetric techniques had shown that the intercalation reactions take place only beyond the chlorine evolution potential of +2.2 V vs. Al on both the electrodes. The extent of intercalation reaction was directly related to the anodic potential and probably to the amount of chlorine available on the graphite anodes. In the case of graphite powder electrode, a distinctly different redox process was observed at sub-chlorine evolution potentials and this was attributed to the adsorption of chlorine on its high surface area. This finding contradicts a report in the literature that the intercalation reactions occur at potentials below chlorine evolution in the chloroaluminate melt.     

Novel graphite/TiO2 electrochemical cells as a safe electric energy storage system

A graphite/TiO₂ full cell has been developed as a new safety energy storage system using a highly safety process. The crystal structures of the anatase TiO₂ electrode have been investigated with respect to the performance of the electrodes. Due to the large anion intercalation into the graphite positive electrode, the possible charging potential can be raised to around 5.3 V against the Li/Li⁺ electrode, which is a higher charging voltage than lithium-ion batteries (maximum voltage is around 4.3 V vs. Li/Li⁺). In situ XRD measurements have been carried out on both the cathode and anode electrodes of the graphite/TiO₂ cell during the charge process to elucidate the intercalation mechanism.     

Exploration of molten hydroxide electrochemistry for thermal battery applications

The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag⁺/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH^– = H₂O + O^2– where H₂O is the Lux-Flood acid (oxide ion acceptor) and O^2– is the Lux–Flood base. This reaction dictates the minimum H₂O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li₂O, Na₂O or K₂O. Thermodynamic calculations suggest that Li₂O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li₂O_(s) + 2 OH^– = 2 LiO^– + H₂O whereas Na₂O acts as a Lux–Flood base, Na₂O_(s) = 2 Na⁺ + O^2–. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH^– to yield oxygen, that is 2 OH^– 1/2 O₂ + H₂O + 2 e^–. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H₂O] [O^2–]) and the reduction of Na⁺ or K⁺ in basic melts. The direct reduction of OH^– to hydrogen and O^2– is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system.     

Anodic behaviour of carbon materials in NaCl saturated NaAlCl4 fused electrolyte at low temperatures: A cyclic voltammetric study

The anodic behaviour of compacted graphite, graphite powder, glassy carbon and reticulated vitreous carbon electrodes in basic sodium chloroaluminate melt in the temperature range 428–573 K was studied using cyclic voltammetry. Chlorine evolution (> + 2.1 V vs Al) alone was the predominant reaction on the compact glassy carbon and fresh RVC electrodes. On compacted graphite, chlorine-assisted chloroaluminate intercalation was found to be a competitive process to the chlorine evolution. At high sweep rates, intercalation/deintercalation near the graphite lattice edges occur faster than chlorine evolution. Subsequent intercalation, however, is a slow process. Chlorine evolution predominates at higher temperatures and at higher anodic potentials. On graphite powders, a more reversible free radical chlorine adsorption/desorption process also occurs in the potential region below chlorine evolution. The process occurs at the grain boundaries, edges and defects of the graphite powder material. Intercalation/deintercalation processes are mainly responsible for the disintegration of graphitic materials in low-temperature chloroaluminate melts. Repeated intercalation/deintercalation cycles result in the irreversible transformation of the electrode surface and electrode characteristics. The surface area of the electrode is increased substantially on cycling. Electrode materials and operating conditions suitable for chlorine generation, intercalation/deintercalation and chlorine adsorption/desorption and power sources based on these processes are identified in this work.     

Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueous solutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hydrochloric acid over an electrode potential range of –0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen evolution and chromium(II) formation. At electrode potentials more negative than –0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occurred simultaneously. At electrode potentials –0.8 to –1.2 V vs SCE, the electrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous carbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl₃, allowed the exchange current density and Tafel slope for hydrogen evolution, and for the reduction of Cr(III) to Cr(II), to be determined. The chromium(III) reduction on vitreous carbon and graphite electrodes can be predicted by the extended high field approximation of the Butler–Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).     

Kinetics of aluminium deposition from aluminium chloride—alkali chloride melts

The kinetics of aluminium deposition from NaClAlCl₃ and NaClKClAlCl₃ melts (c_AlCl3 < 0.4 mol%) was studied by linear sweep voltammetry and potential step amperometry. The reduction of AlCl₃ on tungsten and aluminium electrodes was found to be diffusion controlled. The diffusion coefficients of AlCl₃ were: 3.5 × 10^?5 cm² s^?1 at 820°C in NaClAlCl₃, 2.7 × 10^?5cm²s^?1 at 825°C, and 2.1 × 10^?5cm²s^?1 at 705°C in KClNaClAlCl₃. The rate constant for AlCl₃ reduction at these conditions was found to be in the order of 0.2 cm s^?1, in good agreement with extrapolated literature data.     

Lewis acidity dependency of the electrochemical window of zinc chloride-1-ethyl-3-methylimidazolium chloride ionic liquids

Negative ion fast atom bombardment mass spectra (FAB-MS) recorded for ZnCl₂-1-ethyl-3-methylimidazolium chloride (ZnCl₂-EMIC) ionic liquids with various compositions indicate that various Lewis acidic chlorozincate clusters (ZnCl₃⁻, Zn₂Cl₅⁻ and Zn₃Cl₇⁻) are present in ZnCl₂-EMIC ionic liquids depending on the percentage of ZnCl₂ used in preparing the ionic liquids; higher ZnCl₂ percentage favors the larger clusters. Cyclic voltammetry reveals that the potential limits for a basic 1:3 ZnCl₂-EMIC melt correspond to the cathodic reduction of EMI⁺ and anodic oxidation of Cl⁻, giving an electrochemical window of approximately 3.0 V which is the same as that observed for basic AlCl₃-EMIC ionic liquids. For acidic ionic liquids that have a ZnCl₂/EMIC molar ratio higher than 0.5:1, the negative potential limit is due to the deposition of metallic zinc, and the positive potential limit is due to the oxidation of the chlorozincate complexes. All the acidic ionic liquids exhibit an electrochemical window of approximately 2 V, although the potential limits shifted in the positive direction with increasing ZnCl₂ mole ratio. Underpotential deposition of zinc was observed on Pt and Ni electrodes in the acidic ionic liquids. At proper temperatures and potentials, crystalline zinc electrodeposits were obtained from the acidic ionic liquids.     

Comportement electrochimique d'une electrode de graphite en milieu tetrachloroaluminate de lithium—carbonate de propylene

The electrochemical behaviour of the system LiAl/Propylene-carbonate—LiAlCl₄/graphite has been investigated.Such a system can store energy and give it back at a potential of 4V. At the charging state, the AlCl^?₄ ion is oxidized into chlorine which saturates the solution.Graphite is not oxidized before the evolution of chlorine in propylene carbonate solution: no intercalation compound was evidenced.     

Comparison of pH 6 potentials, alkaline potentials and initial open circuit voltages of electrolytic MnO2

The alkaline battery industry typically reports three electrolytic MnO₂ (EMD) potentials: alkaline potentials, pH 6 potentials, and initial open circuit voltages (IOCV). These measurements differ with the electrolyte, the reference electrode, and the cathode composition. Despite such physical differences, theoretical relationships exist between the electrolytic potentials that are verifiable by experiment. The calculated difference (alkaline potential – pH 6 potential) is 0.785 V, which compares favourably with the experimental value of 0.795 ± 0.003 V. Another difference (alkaline potential – IOCV) depends on carbon-induced EMD reduction, which varies with EMD type and graphite:EMD ratio. After determining the carbon effect experimentally and graphically estimating [ZnO₂ ^2–], (alkaline potential – IOCV) was calculated as –0.045 V. This is roughly 60 mV from the experimental value of +0.017 V. Our analysis shows that when the differences in electrolyte and cathode compositions, and reference electrodes, are accounted for, the three EMD potentials are equivalent.     

Electrochemical intercalation of potassium into graphite in KF melt

Electrochemical intercalation of potassium into graphite in molten potassium fluoride at 1163 K was investigated by means of cyclic voltammetry, galvanostatic electrolysis and open-circuit potential measurements. It was found that potassium intercalated into graphite solely between graphite layers. In addition, the intercalation compound formed in graphite bulk in molten KF was quite unstable and decomposed very fast. X-ray diffraction measurements indicate that a very dilute potassium-graphite intercalation compound was formed in graphite matrix in the fluoride melt. Analysis with scanning electron microscope and transmission electron microscope shows that graphite was exfoliated to sheets and tubes due to lattice expansion caused by intercalation of potassium in molten KF.