B. Børresen

Electrodeposition of magnesium from halide melts—charge transfer and diffusion kinetics

Electrodeposition of magnesium from molten MgCl2 and MgCl2-MgF2 (78-22 mol%) mixtures was studied by electrochemical techniques. The process was found to be quasi-reversible with a cathodic electrochemical rate constant for the total charge transfer reaction of about 10−3 cm/s at 780 °C. Underpotential deposition of an adsorbed layer of reduced Mg(II)-species was observed during cyclic voltammetry and potential step measurements. The electrode capacitance was determined from galvanostatic pulses and found to depend on the applied current density. This has been explained by a pseudo capacitance due to the formation of an adsorbed layer. At potentials positive to nucleation a diffusion controlled process was identified. The determined concentration of the diffusing species, as well as the fact that Mg(II) species are the only cations in the system, indicate that the process is diffusion of dissolved metal (the product) from the electrode interface to the electrolyte.

Iridium oxide-based nanocrystalline particles as oxygen evolution electrocatalysts

Iridium-based oxides are highly active as oxygen evolving electrocatalysts in PEM water electrolyzers. In this work XRD reveals that Ir-Sn oxides contain a single rutile phase with lattice parameters between those of pure IrO2 and SnO2. Addition of Ru leads to the synthesis of a core-shell type material due to the strong agglomeration of Ru colloids during the preparation procedure. The shell of this material consists of an Ir-Sn-Ru oxide deficient in Ru relative to the bulk. This leads to a decrease in the surface noble metal concentration (as found by XPS), which in turn results in a significant reduction in electrochemically active surface area. Polarization analysis indicates that the addition of Ru can influence the rate-determining step or mechanism by which oxygen is evolved. In a PEM water electrolysis cell, small additions of Sn do not significantly reduce the operating performance, however larger additions cause a performance loss due to a reduction in active surface area and increased ohmic resistance. When a pure IrO2 anode is used, a cell voltage is 1.61 V at 1 A cm−2 and 90°C.

Oxoacidity reactions in equimolar molten CaCl2-NaCl mixture at 575 °C

Abstract The behaviour of an yttria-stabilized zirconia electrode in the CaCl2-NaCl melt has been investigated at 575 °C. The response of the electrode is Nernstian and permits its use in the titration of Mg(II) with oxide ions. The solubility product of MgO was also determined. We have calculated the equilibrium constant of the carbonate ion dissociation as well as those corresponding to the systems HCl H 2 O and Cl 2 O 2 , finding that molten CaCl2-NaCl is more acidic than the LiCl-KCl eutectic melt and the BaCl2-CaCl2-NaCl (23.5:24.5:52 mol%) melt. On the other hand, the mixture can be purified with respect to oxide ions by gaseous reagents such as HC1 and Cl2.

Kinetics and mechanism of the magnesium electrode reaction in molten magnesium chloride

Using electrochemical impedance spectroscopy (EIS) and relaxation method with galvanostatic perturbation (RM) the kinetics and mechanism of the magnesium electrode reaction in pure molten M9Cl2 have been determined at several temperatures. A three-step electrode process has been found, the high frequency process being pure charge transfer with the low frequency process showing mixed charge transfer-diffusion character. The low frequency step has also been treated as a preceding chemical reaction followed by charge transfer. On the basis of the corresponding exchange current densities and Warburg diffusion impedance, a mechanism of the overall electrode reaction in this melt is proposed.

Electrodeposition of Magnesium from CaCl2 ­ NaCl ­ KCl ­ MgCl2 Melts

Electrodeposition of magnesium from calcium chloride-based melts was studied on metallic (tungsten and molybdenum) and glassy carbon electrodes. Pure CaCl 2 melt and different alkali chloride solvents including binary CaCl 2 -NaCl, equimolar composition, and ternary CaCl 2 -NaCl-KCl melts with compositions 35:55:10 mol %, containing different MgCl 2 concentrations, were used. In the case of the binary mixture, the operating temperature was both above and below the melting point of metallic magnesium (923 K) so that solid and liquid deposits were obtained. Underpotential deposition of calcium is an important process in melts containing CaCl 2 due to formation of Ca-Mg alloys, which affects the magnesium electrodeposition process. Three-dimensional magnesium deposits that grow macroscopically on the studied substrates were found. This influences determination of the diffusion coefficient of Mg(II) ions. Formation of a magnesium monolayer on both electrodes, and especially on tungsten, was a stable process in the ternary mixture containing MgCl 2 , which was related to the presence of 10 mol % KCl. Moreover, in this melt a remarkable pseudopassivation process was found at high overpotentials.

H2/Cl2 fuel cell for co-generation of electricity and HCl

A H2/Cl2 fuel cell system with an aqueous electrolyte and gas diffusion electrodes has been investigated and the effects of electrolyte concentration and temperature on the open circuit voltage (OCV) and cell performance have been evaluated. Furthermore, the kinetics and long-term stability of Pt as electrocatalyst have been studied under various conditions. In addition, the long-term stability of Rh electrocatalyst has been evaluated. The OCV obtained showed that the Cl2 reduction is more reversible than O2 reduction. The ohmic drop was determining the cell performance at high current densities. An output power of about 0.5 W cm−2 was achieved with this system.

Anodic behavior of carbon electrodes in CaO-CaCl2 melts at 1123 K

The anodic process on glassy carbon electrodes in molten CaCl 2 containing CaO was investigated at 1123 K by cyclic voltammetry and convolution voltammetry. It was found that the oxidation sequence occurs through a consecutive two-step electrochemical reaction: O 2- (diss) + x C → C x O(ads) + 2e and O 2- (diss) + C x O(ads) → CO 2 (gas) + (x - 1)C + 2e. Both steps were found to be irreversible. Experimental data indicate that the first electrochemical step is preceded by the dissociation of a complex. The diffusion coefficient of the oxygen-containing species, the charge-transfer coefficient, and the charge-transfer rate constant were found equal to 2.0 × 10 -6 cm 2 s -1 , 0.43, and 1.8 × 10 -5 cm s - respectively

Chemical and electrochemical behaviour of chromium in molten chlorides

The chemical and electrochemical behaviour of chromium ions in the equimolar CaCl2 NaCl mixture at 550°C has been studied by electrochemical techniques (i.e. cyclic and square wave voltammetry, chronopotentiometry, chronoamperometry and potentiometry) using various substrates (Mo, W and glassy carbon) as working electrodes. Some of the results have been compared to data obtained in the eutectic LiClKCl mixture at 450°C. The stable oxidation states of chromium species were found to be the same in both melts. However, in the case of the CaCl2 NaCl melt and when using W substrates, a new electrochemical wave appears, which has been related to an enhancement of the oxidation of trace amounts of O 2 ions on the W material by the presence of Cr ions. Kinetic parameters characterising the mass and charge transfer of the electrochemical reduction and oxidation of Cr(II) ions as well as of the Cr(III) reduction process were also obtained. Chronoamperometric studies did not show evidence of nucleation phenomena in the case of the equimolar CaCl2 NaCl mixture when using any of the substrates. However, in the eutectic LiClKCl it has been shown that nucleation of metallic chromium, especially on glassy carbon substrates, controls the electroreduction process. The electrochemical behaviour of some chromium oxo-anions, i.e. Cr2O7 and CrO4 , has also been studied. The results show that both complexes lead to the same stable Cr(VI) species in the melt, whose reduction at glassy carbon electrodes implies a single one-electron process giving Cr(V) ions which are present in the melt as soluble species. Potentiometric titration of both Cr(III) and Cr(II) solutions were performed by using an oxide selective ion electrode, which consist of a membrane of ZrO2 stabilised with yttrium oxide (YSZME). The values of pKs for the different CrO compounds stable in the melt, i.e. CrO, Cr2O 2 and Cr2O3, were obtained.

Kinetics and Mechanism of the Magnesium Electrode Reaction in Molten MgCl2 ‐ NaCl Binary Mixtures

Using electrochemical impedance spectroscopy and a relaxation method with galvanostatic perturbation the kinetics and mechanism of the magnesium electrode reaction in six melts of the binary MgCl 2 -NaCl system were determined at several temperatures. A three-step electrode process was found, the high frequency one being pure charge-transfer whereas the low frequency one showing mixed charge transfer-diffusion character. The exchange current density for the high frequency step is in the range from 20 to 50 A cm -2 whereas for the low frequency step it varies from 3 to 10 A cm -2 . On the basis of the corresponding exchange current densities and structural and thermodynamic data, a mechanism of the overall electrode reaction in these melts is proposed. The course of the low frequency exchange current density with the composition, corresponds to the change of the MgCl 2 thermodynamic activity in the more di ute solutions of MgCl 2 in the melt.

The Kinetics of the Sodium Electrode Reaction in Molten Sodium Chloride

The kinetics of the sodium electrode reaction in molten sodium chloride has been investigated at several temperatures using electrochemical impedance spectroscopy (EIS) and the potential relaxation method (PRM) after galvanostatic perturbation. The electrode process exhibits a mixed charge-transfer diffusion character since sodium ions in the melt exchange with sodium in the sodium-lead alloy used as a working electrode. The standard rate constant arid the activation energy of the charge-transfer reaction as well as the diffusion impedance have been determined. The electrode is very reversible with an exchange current density that varies from about 100 A cm -2 at 820 o C to about 200 A cm -2 at 920 o C. In such a reversible system a double-layer inductance is observed