Impedance Spectroscopy Analysis of a Thermo-Electrochemical Cell Under Operating Conditions

Low-grade (<200°C) heat is an abundant and available energy not generally exploited. In this regard, the so-called thermo-electrochemical cells have become a valid opportunity to harvest this ubiquitous source of heat. They are capable to convert heat into electricity by means of the temperature-redox potential dependency of a redox couple. In fact, they present Seebeck coefficient values in order of mV/K, much higher than solid-state thermoelectrics. In this work, a system formed by two Pt electrodes in contact with a 0.4 M ferro/ferricyanide aqueous solution has been studied by means of impedance spectroscopy under operating conditions ($\Delta$T=40 K). The main processes which govern the performance of the system have been identified, which lead to different resistances: (i) electrolyte ionic resistance, (ii) charge transfer resistance and (iii) mass transfer resistance. It was also demonstrated that the current-voltage characteristics of the device can be obtained knowing the open-circuit potential and performing a single impedance spectroscopy measurement, which determines the device dc resistance. This method can save a significant amount of time in many cases.