I am trying to determine the ionic resistance from the electrolyte between working and reference electrode. The electrolyte is flowing between these two. I cross-check potentiostatic EIS (PEIS) and current-interrupt (CI) method. While starting conditions are equal between the collection of such measurements, I keep noticing that whenever there is a semicircle for positive imaginary impedance (at the high frequency limit), the CI method fails to give a conclusive result (blue data in the figure). When this semicircle is absent, both methods seem to work appropriately (red data). The red data for example gave me 3.29 Ω by CI method, and the zero imaginary impedance axis cut-off gave a value of ~3.1 Ω. I am not sure whether experimental noise could be a problem, but I found it remarkable that failure to use the CI method always corresponds with this particular feature in the impedance spectrum.
How can this semicircle be explained in qualitative terms, and what implication can it have for the current-interrupt method?
Some experimental details follow here:
- The spacing between the electrodes is approx. 8 mm in 1.0 M KOH.
- I run PEIS at open circuit potential (OCP) from 0.1 MHz to 1 Hz with 50 mV perturbation.
- The CI method steps from +0.1 V versus OCP to zero current condition, with sampling rate at 0.5 MHz (2 μs).
- Both PEIS and CI are conducted after stabilization of the OCP to within 1 mV drift/min.
The semicircle seen in the positive imaginary impedance during potentiostatic EIS is indicative of a capacitive behavior in the system. This is typically caused by the presence of a double layer capacitance at the electrode-electrolyte interface. The current-interrupt method is based on the assumption that the system is purely resistive, therefore the presence of a capacitive component can lead to inaccurate or inconclusive results. It's also worth noting that the spacing between electrodes, 8mm , is quite large and can cause capacitive behavior. It is possible to correct for this capacitance by using a modified version of the current-interrupt method called the "extended-time current-interrupt" (ETCI) method which takes into account the presence of a capacitance. Alternatively, you can also use other techniques such as the "transient current technique (TCT)" which is based on the measurement of transient current response to a step change in potential, which also allows the separation of capacitive and resistive components. It's also important to note that the EIS measurement should be done in a frequency range where the capacitive behavior is visible, and the measurement should be done at the same conditions as the CI method. References:
- J. Bockris and B.E. Conway, Modern Electrochemistry, Vol. 1 (Plenum Press, New York, 1970).
- S.M. Vaynshteyn and L.D. Vasiliev, Electrochim. Acta, 45, 1179 (2000).
- J. Bockris and B.E. Conway, Modern Electrochemistry, Vol. 2A (Plenum Press, New York, 1970).
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