Electrochemical reduction of solutions of ReF6 in fused LiF–NaF–KF eutectic mixture

Electrolyte was prepared by introducing gaseous ReF₆ into the molten LiF–NaF–KF eutectic at 600 °C. The electrochemical properties of the solutions were studied by voltammetric techniques. The reduction of ReF₈ ^2– to Re occurred via a single irreversible step with diffusion controlled mass transfer. The diffusion coefficient of ReF₈ ^2– was 8 × 10^–10 m² s^–1 and the cathodic transfer coefficient was 0.13. Well-crystallized pure rhenium layers, up to 50 m thick, were obtained on W, Ag, graphite and vitreous carbon substrates and were examined by SEM and X-ray diffraction techniques.     

Influence of CaF2 and AlF3 on the kinetics and mechanism of the Al electrode reaction in cryolite melts with various alumina contents

Electrochemical techniques were used to study the kinetics and mechanism of the aluminium electrode reaction in two cryolite-based melts containing cryolite with either 11 wt % AlF₃ or 5 wt % CaF₂ additions and variable alumina contents at 1000 °C. A three step electrode process was observed in both melts, comprising a preceding chemical reaction followed by two charge transfer steps. The exchange current density of the cathodic reaction was found to be dependent on the concentration of aluminium fluoride. By a combination of electrochemical impedance spectroscopy (EIS) and galvanostatic relaxation methods (GRM), the exchange current density of the first (slower) charge transfer step, the Warburg diffusion impedance, the double layer capacitance of the aluminium electrode and the rate of the preceding chemical step, were evaluated in the range of 2–8 wt % alumina. The role of the two additives, AlF₃ and CaF₂, was evaluated.     

On the cathodic overvoltage on aluminium in cryolite-alumina melts—I

When a constant cathodic current is applied to an aluminium electrode in a Na₃AlF₆Al₂O₃ melt at 1010°C, the potential decreases gradually and linearly with the square root of time to more negative values. Pronounced potential oscillations occurred at cds above 2A/cm² and the evolution of sodium gas was observed at very high cds. Steady state measurements yielded straight η vs log i plots with a slope of ?0.23V/decade, the overvoltage being ?0.19V at 1A/cm². The overvoltage decreased markedly when the melt was stirred. Potential decay measurements yielded linear log time plots with slopes of 0.01–0.5V/decade. The charge-transfer resistance was determined by double pulse measurements to be 0.0031 ohm cm². Ac impedance measurements gave a similar result. The charge transfer overvoltage accounts for only about two percent of the total overvoltage, the rest is apparently diffusion controlled. The cathodic overvoltage in industrial aluminium cells is of similar magnitude as found in the laboratory investigations.     

Aluminium dissolution in the NaF-AlF3-Al2O3 system: Effects of alumina,temperature, gas bubbling and dissolved metal

Current reversal chronopotentiometry, with and without a delay time between the forward and reverse current pulses, was employed to evaluate the effects of temperature, alumina content, gas bubbling (argon and carbon dioxide) and dissolved metal on the rate of aluminium dissolution in NaF–AlF₃–Al₂O₃ molten bath. The working electrode was a tungsten wire electrode and the temperature range studied was 824–1040°C. The effect of the alumina content was determined in melts with CR=1.45 and CR=4.3 at 1029±3°C (CR = mol NaF/mol AlF₃). The experiments involving gas bubbling and dissolved metal were carried out in melts similar to industrial compositions, i.e. CR=2.4, 4.8 wt. % Al₂O₃ at 980°C. In general, the dissolution rate of aluninium increased with increasing temperature, decreased slightly with increasing alumina content in acidic melts (CR<3) but changed little in basic melts (CR>3), increased with bubbling and decreased in the presence of dissolved metal. The rate of Al dissolution is thus mass transport controlled.     

Content of sodium and lithium in aluminium during electrolysis of cryolite-based melts

The content of sodium and/or lithium in polarized and nonpolarized aluminium in contact with cryolite melts was determined in a laboratory cell. The cryolite-based melts contained 0 to 20mass% excess AlF3 and 0 to 2.5mass% LiF. The cathodic current density ranged from 0 to 0.5Acm–2. The lithium content in aluminium increases linearly with increasing concentration lithium fluoride in themelt. It also increases with increasing cathodic current density and decreasing cryolite ratio. On the other hand the sodium content decreases with increasing concentration of LiF in the melt. This effect is more notable at higher current densities.     

Structural entities in NaF---AlF3 melts containing alumina

An ionic structure model is developed for NaFAlF₃ melts containing alumina. The model can be described by the following dissociation equilibria,

Na⁺ is proposed to be the only cation present in the system. The model is derived from activity data for NaF and AlF₃ in melts saturated with alumina at 1285 K and the assumption of an ideal ionic mixture, with randomly distributed anions on the anion positions in the melt. The available data for the system seem to fit the model reasonably well. The entity Al₂F^?₇ will only be present in AlF₃ rich melts. Oxygen atoms in the complexes are most probably involved in bridging bonds of the type AlOAl and

. It appears that aluminium atoms are involved in 4 bonds in AlF₃-rich melts. The number of bonds changes partly towards 5 and/or 6 with increasing NaF content of the melt.     

Characterization of aluminium alloys in tetrahydrofuran media

Rest potential measurements and voltammetric scans at different rates (0.01–1 V min^–1) have been carried out on aluminium alloys in a tetrahydrofuran (THF) environment. The solvent contained either chloride, perchlorate or trifluoromethane sulfonate ions. For the Al-Mg alloys in chloride environments, the voltammograms observed for low scan rates show electrode passivation with the formation of a magnesium salt layer. For Al-Li alloys, rest potential measurements indicate the selective dissolution of lithium. The maximum dissolution current is sensitive to the lithium content in the alloy and limited by the solubility of lithium chloride. In chloride environments, the passivity breakdown potentialE _b and the protective potentialE _p have similar values and do not depend on the type of alloy. In contrast, large differences between these potentials are observed in the presence of CF₃SO ₃ ^– ions.     

A bipolar electrode cell for aluminium electrorefining

Electrorefining of aluminium was carried out at 750 °C using bipolar electrode cells with centre holes 2, 10 or 20 mm in diameter. Through the centre holes liquid electrorefined aluminium rises to the electrolyte surface. The bipolar electrode cell consists of graphite cathodes, Al–Cu–Fe–Mn or Al–Cu–Fe–Zn alloy anodes and a BaCl₂–NaCl–AlF₃–NaF electrolytic melt. The centre hole size of more 20 mm in diameter is required to continuously float up the aluminium electrodeposited onto the electrolyte surface, while the current efficiency of the cell decreases with increase of the centre hole size, from 97% at 2 mm diameter to 92% at 20 mm diameter. Aluminium of 99.97% purity precipitates at the cathode. Iron, manganese and zinc included in the alloy as impurities are hardly deposited and the concentrations of these elements in the deposit are 100, 80 and 170 ppm, respectively. In this process aluminium can be produced with an energy consumption of about 4.9 × 10³ kWh(t-Al)^–1, which is one-third smaller than that of the Gadeau process.     

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.     

Aluminium dissolution in NaF-AlF3-Al2O3 systems

The rate of dissolution of electrolytically deposited aluminium was determined by the method of current reversal chronopotentiometry at a tungsten electrode in NaF?AlF₃?Al₂O₃ melts of varying NaF/AlF₃ molar ratios or cryolite ratios (CR). The temperature was maintained at 1031±3°C and the alumina content at 4 wt%. More accurate data were obtained by introducing delay times of various lengths (at zero current) between the cathodic and anodic current pulses, compared to direct current reversal chronopotentiometry with varying forward (deposition) times. The rate of aluminium dissolution increased with increasing NaF/AlF₃ molar ratio, the curve showing an inflexion in the vicinity of CR=3. This inflexion indicates two dissolution mechanisms, one being predominant depending on the CR. The main reaction in acidic melts (CR<3) may be represented by $$2Al(l) + AlF_6^{3 - } \rightleftarrows 3Al(I)F_x^{1 - x} + (6 - 3x)F^ - $$ while in basic melts (CR>3) $$Al(l) + 3Na^ + \rightleftarrows 3Na(soln) + Al(III)$$ is the likely dominant mechanism. For 0.8?7 mol cm^?2s^?1.     

Ternary alkali carbonate composition-oxygen solubility relationship under atmospheric and pressurized conditions – a utility model for MCFC

The effect of O₂/CO₂ = 90/10 gas mixtures under pressurized conditions on the diffusion resistance during oxygen reduction was investigated in carbonate melts of variable composition ranging from the eutectics Li–K, Li–Na to ternary Li–Na–K carbonate melts. It was found that pressurization reduced the diffusion resistance of the reactant species in all compositions investigated. This was ascribed to the increase in concentration of the reactive species in the electrolyte. Reaction order plots of Warburg coefficient versus the total pressure in binary melts showed that: (i) mixed diffusion of superoxide ions and CO₂ prevails in lithium rich carbonates, and (ii) mixed diffusion of peroxide ions and CO₂ predominates in potassium rich carbonates. By means of a computerized statistical analysis of the results, a partial cubic model was found to describe the relationship between alkali carbonate compositions, pressure, and Warburg coefficient. The optimum alkali carbonate compositions are: (i) 0.283Li–0.373Na–0.344 K with _app equal to 137.68 cm²s^–0.5 at atmospheric conditions, and (ii) 0.390Li–0.355 Na–0.255 K with _app 87.53 cm² s^–0.5 and 4.2 atm under pressurized conditions.     

Gas induced bath circulation in aluminium reduction cells

Gas induced bath circulation in the interpolar gap of aluminium cells was studied in a room temperature physical model and by computer simulation. The circulation velocity increased with increasing gas formation rate, increasing angle of inclination and decreasing bath viscosity, while it was less affected by anode immersion depth, interpolar distance (in the normal range), and convection in the metal. A typical bath velocity near the cathode was 0.05 m s^–1. The flow velocity decreased with decreasing bubble size. The results were fitted to a simple semi-empirical expression, and the velocities measured in the model experiments were in good agreement with the findings of the computer simulation.Nomenclature A Surface area (m²) - c _D Drag coefficient (l) - c _pr Concentration of 1-propanol (ml/1000 ml) - d _e Equivalent diameter of gas bubble (m) - F Faraday constant (96 487 C mol^–1) - g Acceleration due to gravity (9.82 m s^–2) - g Gravity component along anode surface (m s^–2) - h Vertical dimension of gas-filled layer (m) - H Anode immersion depth (m) - i Current density (A m^–2) - k Turbulent energy (m² s^–2) - P Pressure (N m^–2) - q Gas formation rate (m³ s^–1 m^–2) - R Universal gas constant (8.314 J mol^–1 K^–1) - t Time (s) - U Liquid velocity parallel to anode surface (m s^–1) - U _b Bubble velocity parallel to anode surface (m s^–1) - U _rel Relative velocity between bubble and liquid (m s^–1) - V Liquid velocity perpendicular to anode surface (m s^–1) - x Distance from centre of anode (m) - y Vertical distance from cathode (m) - Y Interpolar distance (m) - Angle of inclination referred to the horizontal (deg.) - Dissipation rate of turbulent energy (m² s^–3) - Volume fraction of liquid (1) - v Kinematic viscosity / (m² s^–1) - Dynamic viscosity (kg m^–1 s^–1) - _t Turbulent viscosity (kg m^–1 s^–1) - Density of liquid (kg m^–3) - /g9 Kinematic surface tension (m³ s^–2) - Bubble void fraction (1) Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.     

Influence of magnesium and aluminium ions on the copper a.c. deposition into aluminium anodic oxide film nanotubes

The influence of Mg²⁺ and Al³⁺ ions on a.c. deposition of copper nanowires into aluminium anodic oxide film (AOF) nanotubes has been studied using cyclic voltammetry and d.c. plasma emission spectrometry. From the analysis of copper quantities deposited into the Al AOF nanotubes (m _Cu), 0.02 M MgSO₄ concentration was found to be optimal for Cu(II) solutions. Moreover, it was shown that Mg²⁺ and Al³⁺ ions not only prevent the breakdown of the barrier layer of AOF, but change the rate of copper deposition and modify the shape of the m _Cu against pH plots depending on the a.c. voltage applied. From the analysis of the quantities of magnesium (m _Mg) incorporated into the Al AOF nanotubes, presumably in the form of Mg(OH)₂, the m _Mg against pH dependences were determined in MgSO₄ and MgSO₄ + CuSO₄ solutions. An increase in m _Mg from 30 g dm^–2 to 1 mg dm^–2 at pH 1.5 and from 6 g dm^–2 to 16 g dm^–2 at pH 7.0 was found under the same a.c. treatment conditions from MgSO₄ solutions without and with Cu²⁺ ions, respectively, indicating the incorporation of Mg(OH)₂ into the Al AOF nanotubes to be lower up to about one hundred times in the case of Cu deposition. Based on the experimental results, it was suggested that incorporation of the Mg(OH)₂ particles into the Al AOF nanotubes occurred simultaneously with growing copper nanowires under a.c. bias is insignificant, if the pH of the CuSO₄ + MgSO₄ solution is 2.5.     

Pulse electrodeposition of titanium on carbon steel in the LiF–NaF–KF eutectic melt

Electrodeposition of titanium was carried out in the K₃TiF₆–LiF–NaF–KF melt using both direct (DC) and unipolar pulse current (PC) techniques. Dense and smooth titanium coatings were obtained by PC plating at 750 °C whereas DC plating led to rough and dendritic deposits. The best results were obtained using a 100C cm^–2 pulse charge and a cathodic current density of 50 and 75mA cm^–2. The cathodic current efficiency was in the range 60–65%. The titanium deposits obtained under such conditions behaved similarly to CP-titanium in NaCl and HNO₃ solutions at room temperature.     

Electrodissolution of aluminium thin film microband electrodes

This paper discusses the electrodissolution of aluminium thin films as microband electrodes (length = 5 × 10^–3 m) in terms of mass transfer determined by voltammetry and a.c.-impedance techniques as a function of bandwidth (20 to 2000 nm) in 0.1m NaOH solution. The anodic polarization curves of the aluminium microband electrodes show that current density is enhanced with decreasing bandwidth. The ac impedance response suggests that a steady-state diffusion layer appears the more markedly, the smaller the bandwidth. The anodic polarization curves are analysed on the basis of the combined Butler-Volmer high field approximation and the semi-cylindrical diffusion field approximation. As a result of the analysis, the electrodissolution proceeds by a mixed kinetic-mass transfer controlled reaction. The analysis also makes it possible to distinguish the semi-cylindrical diffusive mass transfer contribution to the electrodissolution from the kinetic contribution, i.e., mass transfer index linearly diminishes with decreasing bandwidth. The increased current density is attributable to the decreased mass transfer contribution, i.e., the more predominant semi-cylindrical diffusive mass transfer as compared to laminar diffusive mass transfer.Nomenclature k _a anodic kinetic constant - k _c cathodic kinetic constant - F Faraday constant - _s kinetic transfer coefficient for anodic reaction - _c kinetic transfer coefficient for cathodic reaction - c ^s surface concentration - V anodic polarization - D diffusion coefficient - d diffusion layer thickness - z number of electrons transferred - l length of microband electrode - w bandwidth of microband electrode - r radius of cylinder     

Cycling behaviour of lithium-aluminium alloys formed on various aluminium substrates as negative electrodes in secondary lithium cells

The cycling efficiencies of lithium were examined on various metal substrates using different methods. The efficiency was found to be strongly dependent on the evaluating method and on the alloying process of lithium with the metal substrates. The electrochemical behaviour of the Li–Al alloys formed on several kinds of thin Al substrates were investigated in 1m propylene carbonate solution of LiClO₄ at room temperature. It was found that the cycling behaviour was dependent on the alloying rate of lithium with the Al substrate, and the electrochemically etched Al substrate, having a microstructure of a considerably preferred (100) orientation and a larger effective surface area, gave excellent cycling behaviour, showing a high cycling efficiency of 9085% at a high current density of 7 mA cm^–2.     

High-rate plating of aluminium from the bath containing aluminium chloride and lithium aluminium hydride in tetrahydrofuran

An electrolyte for the high-rate plating of aluminium from the tetrahydrofuran solutions of aluminium chloride and lithium aluminium hydride has been developed. A smooth and coherent deposit of aluminium has been obtained at the current density of 18 A dm^–2 without stirring, whereas the conventional diethyl ether solvent bath allows good plating up to 5 A dm^–2 under the same condition. The current densities applicable are increased with an increase in the molar fraction of aluminium chloride in tetrahydrofuran. The conductivity of the plating solution was measured at various molar ratios of aluminium chloride to lithium aluminium hydride. A plateau region of the conductivity curve plotted against the molar ratio is consistent with the composition of the plating bath giving a good plating. The plateau region is enlarged with an increase in total aluminium concentration.     

Influence of different anions on the behaviour of aluminium in aqueous solutions

The influence of chloride, sulfate and perchlorate anions on the behaviour of native oxide layers on aluminium is investigated using electrochemical techniques. Due to its influence on the open circuit potential and the cathodic side of the polarization curve the oxygen concentration has been carefully controlled. Two kinds of attack on a commercially pure aluminium (99.5 wt %) have been observed. In all the investigated 0.5 M Cl^–, 0.5 M ClO^– ₄ and 0.5 M SO^2– ₄ aqueous solutions the metal is corroded around the iron and silicon containing precipitates, but only in Cl^– and ClO^– ₄ solutions is crystallographic pitting observed. Comparison with high purity aluminium (99.99 wt %) shows that pitting corrosion is not influenced by the presence of impurities in the aluminium alloys, but by the presence of anions in solution. The pH and/or oxygen concentration determine whether or not the pitting potential coincides with the corrosion potential.     

Electrical conductivity of low-melting electrolytes for aluminium smelting

To determine the electrical conductivity of low-melting electrolytes (AlF₃-rich, e.g. NaF/AlF₃ molar ratio = 1.2), a tube-type cell was used, applying ac-techniques with a sine wave signal with small amplitude in the high frequency range.One melt tested contained 55 mol% NaF and 45 mol% AlF₃ with and without addition of 2 wt.% alumina. Another melt tested contained 55 mol% KF and 45 mol% AlF₃ with and without addition of 2 wt.% alumina. The electrical conductivity data in the molten system can be described by a simple equation of the Arrhenius type:

κ=A e^−B/T