Thermoelectric Powers of Cells With NaF-AlF₃-Al₂O₃ Melts

Abstract

A thermodynamic description of the Peltier heat at the aluminum and the oxygen electrode in the system NaF–AlF₃–Al₂O₃ is given. The thermoelectric power in melts with molar ratios n_NaF/n_AlF3 from 3.0 to 1.0, saturated with alumina are measured. Seebeck coefficients for molten fluoride electrolytes saturated with alumina, electrolytes that are relevant for aluminum electrowinning electrolysis cells, are reported. The results allow determinations of Peltier heats of aluminum, oxygen and carbon electrodes in NaF–AlF₃ electrolytes saturated with alumina. For molar ratios of n_NaF/n_AlF3 between 2.6 and 1.2, there is a Peltier heating of the aluminum cathode. This heating is in the same order of magnitude as the electrolyte Joule heat, when the current density is 0.7 A cm⁻². For molar ratio n_NaF/n_AlF3 equal to 1.0 the Peltier effect at the aluminum electrode approaches zero. From theoretical considerations we expect a drop also for molar ratio 3.0. For the anode we report a Peltier cooling that is larger than the heat produced by the anodic overvoltage, in melts with NaF/AlF₃ molar ratio between 2.6 and 1.2 saturated with alumina.