[en] Electrochemical reduction of metal ions at glassy carbon and tungsten electrodes in molten KCl–LiCl eutectic was studied by linear sweep voltammetry (LSV), convolutive potential sweep voltammetry (CPSV), chronopotentiometry and chronoamperometry. The metal–metal ion systems studied consisted of Pb/Pb(II), Zn/Zn(II) and Mg/Mg(II). In the case of lead deposition, an initial nucleation stage was found to influence the shape of the voltammetric current peak. Because of this, CPSV was believed to be a more reliable method than LSV for the calculation of the diffusion coefficient of the Pb(II) ion. Such types of complications were not observed for zinc and magnesium, since the nucleation overpotentials for these metals were significantly lower than for lead. The convolution voltammograms for magnesium were difficult to interpret because of lithium co-deposition at these negative potentials. Chronopotentiometry and voltammetry yielded practically identical results for D_Pb(II). Chronopotentiometric results for reduction of Zn_(II)-ions and Mg_(II)-ions did not obey the Sand equation very well due to residual currents. Neither was determination of diffusion coefficients from single potentiostatic current transients and the Cottrell equation found to be very reliable for such determinations. LSV seemed to be the most accurate method for determining diffusion coefficients for Zn_(II) and Mg_(II) ions. Empirical expressions for the temperature dependency of the diffusion coefficient in the temperature range 400–500 °C were calculated for all three ions.

[en] The Seebeck coefficient is reported for thermoelectric cells with gas electrodes and a molten electrolyte of one salt, lithium carbonate, at an average temperature of 750 °C. We show that the coefficient, which is 0.88 mV K⁻¹, can be further increased by adding an inorganic oxide powder to the electrolyte. We interpret the measurements using the theory of irreversible thermodynamics and find that the increase in the Seebeck coefficient is due to a reduction in the transported entropy of the carbonate ion when adding solid particles to the alkali carbonate. Oxides of magnesium, cerium and lithium aluminate lead to a reduction in the transported entropy from 232 ± 12 to around 200 ± 4 J K⁻¹ mol⁻¹. This is of importance for design of thermoelectric converters.

[en] We report Seebeck coefficients of electrochemical cells with molten carbonate mixtures as electrolytes and carbon dioxide|oxygen electrodes. The system is relevant for use of waste heat and off-gases with concentration of carbon dioxide different from air, as for example in the metallurgical industry. The coefficient is −1.25 mV K⁻¹ for a nearly equimolar mixture of lithium and sodium carbonate with dispersed magnesium oxide at 750 °C, one bar total pressure and a pressure ratio of carbon dioxide to oxygen of 2:1. The value is slightly lower when sodium is replaced by potassium. The theoretical expression of the Seebeck coefficient was established using the theory of non-equilibrium thermodynamics. We used this expression to predict an increase to −1.4 mV K⁻¹ when lowering the gas partial pressures to 0.015 and 0.2 bar, respectively, for carbon dioxide and oxygen, a gas composition that can represent that of the off-gases from a silicon furnace which we are concerned with. The absolute value of the Seebeck coefficient increases by 0.2 mV K⁻¹ when the cell average temperature increases from 550 to 850 °C. The presence of a second component in the electrolyte increases the coefficient significantly above the values obtained with one component, compatible with a lowering of the transported entropy of the carbonate ion. A concentration cell, using the off-gas from the silicon furnace as anode gas and air as cathode gas, will add 0.14 V at 550°C to the absolute value of the potential. The series construction has the potential to offer a power density at matched load conditions in the order of 0.5 kW m⁻².