Distortions in Electrochemical Impedance Spectroscopy Measurements Using 3‐Electrode Methods in SOFC. I – Effect of Cell Geometry

Abstract

The use of a reference electrode (RE) is necessary to independently measure the overpotential of each electrode in solid oxide fuel cells (SOFC). This type of set‐up, known as the 3‐electrode (or 3‐terminal) configuration, can give erroneous results if the RE does not effectively separate the potential of the two active electrodes. In this work, calculations and experiments were performed to verify the effectiveness of the 3‐electrode configuration used in electrochemical impedance spectroscopy (EIS) measurements for studying the kinetics of anodes and cathodes in SOFC. Initially, a theoretical analysis of the impedance distortions in relation to the electrode geometry and configuration is presented and the main causes of distortions are elucidated. Then, this analysis is corroborated by experimental results obtained using two specially designed cells. Calculations and experiments reconfirm that configurations characterised by electrodes of equal area and symmetrical placement do not produce EIS distortions when the electrodes have similar area‐specific polarisation resistances and time constants. Moreover, distortions can be low even in considerably misaligned configurations when electrodes are small and relatively inactive.

Three electrode configuration measurements of electrolyte-diffusion barrier-cathode interface

“…13), 14) The reference electrode should be located at the area where the potential distribution is uniform and constant. 15), 16) In addition, the reference electrode should be positioned at a distance greater than the triple thickness of the electrolyte from the edge of the working and counter electrode, 14), 17) while its size should not be too large.…”

An experimental study on the placement of reference electrodes in alkaline polymer electrolyte membrane fuel cells

“…A reference electrode (RE) is required to measure the individual anode and cathode potentials in fuel cells but the applicability of the commonly used three electrode configuration 2 alongside thin solid electrolytes is still being debated. The main concern is that the overpotentials and impedance responses (impedance spectra) of the anode or cathode are a function of RE position and the degree of misalignment of the active electrodes of the membrane electrode assembly (MEA); there have been extensive theoretical studies, involving simulation methods [5][6][7][8][9][10][11][12][13].…”

“…The situation is worse when trying to separate the impedance responses (on introduction of a.c. signals) of an anode and a cathode from the whole cell response, as the potential distributions in thin electrolytes are frequency dependent [9,[11][12].…”

“…The prior simulation studies [11] show that the impedance of a half-cell does not distort significantly when the active electrodes of a MEA are well aligned and the active electrodes of the MEAs have the same electronic properties i.e. with the same electronic resistance (R) and capacitance (C).…”

“…with the same electronic resistance (R) and capacitance (C). However, when the active electrodes of the MEAs have different R and C characteristics, the impedance is distorted even when the electrodes are precisely aligned [9,12]; Cimenti et al has discussed the simulation results on this topic in detail [11][12]. Five different cell geometries are commonly encountered in the literature [14], with pellet geometry and microfabricated microelectrode arrays being the two that are generally acknowledged to allow isolation of the impedance of working electrode; these geometries are, however, very different to the planar structure encountered in fuel cells.…”