EXPERIMENTAL AND THEORETICAL-ANALYSIS OF A THERMOGALVANIC UNDIVIDED FLOW CELL WITH 2 AQUEOUS-ELECTROLYTES AT DIFFERENT TEMPERATURES

The electrochemical and thermal performance of a new thermogalvanic undivided flow cell using two aqueous electrolytes maintained at different temperatures between platinum electrodes is described. Measurements on the Fe(CN)6(3-)/Fe(CN)6(4-) redox system in NaOH medium yield a thermoelectric effect between -1.1 and -1.4 mV/degree depending on the composition of the electrolytes. The influence of different parameters (temperature gradient, concentration of the electroactive species, fluid velocities and channel thickness) on the power output are determined. The experimental results show that electrical power is largely limited by charge and mass transfer overvoltages, but that it is possible to maintain the temperature gradient between the electrodes, i.e. the thermal boundary layers developing at the interface between the electrolytes do not reach the electrodes. A model based on the generalized Butler-Volmer kinetic equation is found to be in very good agreement with the experiments in terms of the current-voltage relationship and power output. This model is used for the prediction of the maximum electrochemical performance. From a practical point of view the efficiency of these devices remains very low due to the high thermal flux between the hot and cold electrolytes.