# How to check and correct the time variance of your system under EIS measurements

Latest updated: June 2, 2021

What is the problem?

For valid impedance measurements, the system under investigation should be linear, stable, causal, and stationary. EC-Lab®’s THD Quality Indicator can be used to test the linearity of the system. The THD Quality indicator, as well as the NSD (Non-Stationary Distorsion) and NSR (Noise to Signal Ratio), are also described in the BioLogic Learning center article “How to make reliable EIS measurements with your potentiostat or battery cycler

Stationarity here means two different things: steady-state and time-invariance.

Steady-state is the state of a system that is not in a transient state. For example, an R/C circuit with a definite time constant is submitted to a potential or current step. Its response will vary over time until it reaches a steady state (Fig. 1).

Figure 1: Potential response (Right) of an R/C circuit (Middle) to a current step (Left), illustrating the steady-state of a system.

Time variance is the property of a system whose parameters defining its transfer function change over time. For example, a corroding electrode whose polarization resistance changes over time, either because of corrosion or of passivation, is a time-variant system. A discharging battery also sees its various resistances (charge transfer, diffusion) evolve during a discharge (Fig. 2). Sometimes steady-state and time-invariance are difficult to distinguish.

Figure 2: A battery under operation is a time-variant system.

The effects of transient state or time-variance on EIS measurements are generally visible at lower frequencies, because the changes are usually slower than the high frequencies used in EIS, say from 1 MHz to 10 Hz. When the potentiostat measurement time is long enough to capture the time variance, its effects will be seen. In the case of a change of polarization resistance of a corroding electrode, the effects can be seen at frequencies around or below 1 Hz. What happens is that the electrochemical impedance data at lower frequencies are deformed, as seen in Fig. 3.

Figure 3: Nyquist representation of the impedance data (200 kHz – 10 mHz frequency range) obtained on a mild steel sample (undisclosed composition) after immersion in 0.1 M H2SO4. This data was recorded with a BioLogic SP-200 single-channel potentiostat.

In Fig. 3, the data shown below 32 mHz are due to the change of the polarization resistance of the system with time. As can be seen on the Non-Stationary Distorsion (NSD) factor shown in Fig. 4 (See “How to make reliable EIS measurements with your potentiostat or battery cycler” article for a definition of NSD), the impedance data seems to be affected by steady-state and time-variance effects from 1.1 Hz as it is the frequency at which the NSD increases. At 32 mHz the NSD reaches a value as small as 0.58 % and it can be seen in Fig. 3 that such a small value can be representative of the dramatic deformation of the impedance graph.

Figure 4: Non-Stationary Distorsion (NSD) of the current response from EIS measurements shown in Fig. 3. Below 1.1 Hz the NSD starts increasing.

What is the solution?

One easy way to solve this issue is to use the analysis tool in the EC-Lab® potentiostat software called Z Inst, which implements a method discovered by Z. Stoynov and B. Savoya [1]. The basic steps of this method are the following:

1. Several impedance graphs are acquired sequentially on a system that changes with time (Fig. 5a).
2. The impedance data are plotted as a function of time (Fig. 5b).
3. The data at the same frequencies are connected and interpolated
4. We obtain an impedance surface
5. By choosing cross-sections of this tunnel we can have instantaneous impedance data (Figs. 5c and d).

Figure 5: a) Nyquist representation of successive impedance graphs of a corroding electrode and b) corresponding 3D representation as a function of time, c) Nyquist representation of instantaneous impedance obtained by the “Z Inst” tool with 20 cross sections and d) corresponding 3D representation as a function of time
The impedance graphs shown in Figs. 5c and d can be considered to have been corrected from the time variance and the change of polarization resistance of the corroding electrode. ZFit can be used to determine all the relevant parameters such as the double-layer capacitance $C_{\mathrm{dl}}$, the charge transfer $R_{\mathrm{ct}}$ and the polarization $R_{\mathrm{p}}$ which is the limit at low frequencies. It should be noted that most of the impedance graphs have an inductive part at low frequencies, which shows that the reduction reaction of protons involves an adsorption step [2].

The impedance graphs shown in Figs. 5c and d can be considered to have been corrected from the time variance and the change of polarization resistance of the corroding electrode. ZFit can be used to determine all the relevant parameters such as the double-layer capacitance Cdl, the charge transfer Rct, and the polarization Rp which is the limit at low frequencies. It should be noted that most of the impedance graphs have an inductive part at low frequencies, which shows that the reduction reaction of protons involves an adsorption step [2].

References

1. Z. Stoynov, B. Savoya, J. Electroanal. Chem. 112 (1980) p. 157
2. J.-P. Diard, P. Landaud, B. Le Gorrec, C. Montella, J. Electroanal. Chem. 255 (1988) p. 1

Further information

Application notes

EIS stationarity – Electrochemistry, Battery & Corrosion – Application Note 55

EIS Quality Indicators: THD, NSD & NSR – Electrochemistry, Battery & Corrosion – Application Note 64

EIS Quality Indicators THD – Electrochemistry, Battery & Corrosion – Application Note 65

White paper

Studying batteries with Electrochemical Impedance Spectroscopy – Electrochemistry & Battery

Literature

http://dx.doi.org/10.5599/jese.725

EIS time-variance Z4D Stoynov correction Instantaneous impedance

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