Multi-harmonic ElectroThermal Spectroscopy (METS) for a non-invasive and spatially resolved determination of electrochemical processes in an electrochemical cell*

Lock-in detection techniques like the 3-omega method have been used for determining spatially resolved thermal properties. In electrochemistry, Electrochemical Impedance Spectroscopy (EIS), another lock-in technique, is ubiquitously used for studying transport resistance and charge transfer resistance. However, EIS lacks spatial information and cannot differentiate mechanisms of the charge transfer resistance at the electrode. In this work, we present a lock-in detection technique based on thermal signatures of electrochemical processes. The processes are differentiated from the harmonics and the phase of the heat generated when an Alternating Current (AC) is passed through the cell. The spatial resolution is achieved from the frequency dependent thermal penetration depth. We name this method Multi-harmonic ElectroThermal Spectroscopy (METS). A four-point probe RTD sensor acting as a METS sensor can be employed on the exterior of the cell to non-invasively probe spatially resolved electrochemical information. In summary, METS provides a unique capability to directly probe and differentiate electrochemical processes with spatial information.