Seebeck coefficients of cells with molten carbonates relevant for the metallurgical industry

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.