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Short answer
Impedance spectroscopy measures how a system resists alternating current across a range of frequencies. In its electrochemical form (EIS), a small sinusoidal signal is applied to a cell and the response is recorded as a complex impedance, with a magnitude and a phase, at each frequency. Reading the resulting plots, usually Nyquist and Bode, lets you separate processes such as solution resistance, charge transfer and diffusion.
What is electrochemical impedance spectroscopy?
Electrochemical impedance spectroscopy is, in Metrohm Autolab’s words, a widely used multidisciplinary technique for characterizing the behavior of complex electrochemical systems. It is employed for batteries, catalysis and corrosion processes, and more recently for semiconductor interfaces and ion diffusion across membranes.
Impedance is the generalisation of resistance. Gamry Instruments notes that, like resistance, impedance measures a circuit’s ability to resist the flow of electrical current, but it is not limited by the properties of an ideal resistor: Ohm’s law holding at all levels, no dependence on frequency, and voltage and current in phase. Real cells break these rules, which is why the frequency dependence carries information.
Electrical versus electrochemical impedance spectroscopy
People also search for “electrical impedance spectroscopy”. The underlying idea, measuring complex impedance as a function of frequency, is the same. This page covers the electrochemical case, an electrode in contact with an electrolyte, as described in the Gamry and Metrohm application notes, so everything below applies to that case.
How a measurement works
The measurement applies a small-amplitude sinusoidal excitation to the system and records the response, which may be current, voltage or another signal. In potentiostatic EIS, a low-amplitude sine wave is superimposed on a DC polarisation voltage and the current response is phase shifted relative to the voltage. In electrohydrodynamic (EHD) impedance spectroscopy, the measured signal is instead the rotation speed of the working electrode, so the response does not have to be a current.
The amplitude is kept small on purpose. Gamry states that in normal EIS practice a small AC signal of 1 to 10 mV is applied, because electrochemical cells are not linear: doubling the voltage will not necessarily double the current. With a small perturbation the cell response is pseudo-linear, and the higher-order terms of the response are negligible to a first approximation (Metrohm). The result is a complex impedance Z, with magnitude and phase, that depends on frequency.
Metrohm gives the typical electrochemical frequency range as 100 kHz to 0.1 Hz. Measuring at each frequency in turn is what makes a spectrum take time, which matters for the checklist below.
Reading Nyquist and Bode plots
The same data can be drawn two ways, and each answers different questions.
| Plot | What is drawn | What it is good for | Caution (from the sources) |
|---|---|---|---|
| Nyquist | Imaginary against real impedance; each point is the impedance at one frequency (Gamry) | A quick overview and qualitative interpretation (Metrohm); a semicircle is characteristic of a single time constant (Gamry) | Axes must be isometric so the shape is not distorted; the plot carries no frequency information unless some frequencies are labelled on the curve (Metrohm) |
| Bode | Log frequency on the x-axis; impedance magnitude and phase on the y-axis (Gamry) | Metrohm calls it a more complete way of presenting the data because frequency is explicit | Frequency is read directly from the axis |
Gamry adds that Nyquist plots often contain several semicircles, and often only part of one.
Equivalent-circuit elements and what they stand for
To get numbers out of a spectrum, a model is fitted to the data and the fit returns estimates of the model parameters (Gamry). The simplest widely used model is the Randles cell, which Gamry describes as one of the most common cell models.
| Element | Physical origin per the sources | Where it shows up |
|---|---|---|
| Solution resistance | The solution resistance term of the Randles cell | High-frequency real-axis intercept of the Randles Nyquist plot |
| Double-layer capacitor | The electrical double layer at the electrode/electrolyte interface | In parallel with the charge-transfer resistance in the Randles cell |
| Charge-transfer (polarization) resistance | Charge-transfer resistance across the interface | Low-frequency intercept is solution resistance plus polarization resistance; the simplified Randles plot is always a semicircle |
| Warburg impedance | Diffusion | Small at high frequency, larger at low frequency; phase angle of 45 degrees |
For coatings, Gamry describes an intact coating as a resistor in series with a coating capacitance that changes as the coating absorbs water. A failed coating adds pore resistance and a double-layer capacitance, giving two time constants. Gamry also notes that researchers disagree on assigning impedances to physical processes in that failed-coating model, so a good fit is not proof of a mechanism.
A checklist: can you trust the spectrum?
Most of the useful guidance in the manufacturer notes concerns data quality rather than theory. Use it as a pre-interpretation check.
- Small signal. Was the excitation in the 1 to 10 mV range Gamry describes? Harmonics of the excitation frequency in the response indicate non-linearity.
- Steady state. The system must stay at a steady state for the whole time a spectrum takes, which Gamry says is often many hours. Drift is a common cause of problems. Gamry lists adsorption of solution impurities, growth of an oxide layer, build-up of reaction products, coating degradation and temperature changes as causes, and warns that standard analysis tools may give wildly inaccurate results on a system that is not at steady state.
- Data quality test. Gamry notes the Kramers-Kronig relations can be used to evaluate data quality.
- Plot construction. Are Nyquist axes isometric, and are key frequencies labelled (Metrohm)?
- Model humility. Does the equivalent circuit correspond to an independently known physical process, or is it only a curve that fits?
Common misunderstandings
- “Impedance is just resistance.” Resistance is frequency independent with voltage and current in phase; impedance is complex and frequency dependent (Gamry).
- “A larger excitation gives a better signal.” Cells are not linear, so a large signal can break the linear assumption the analysis depends on (Gamry).
- “Every Nyquist plot is one semicircle.” Plots often contain several semicircles, often only partly visible (Gamry).
- “A good fit identifies the mechanism.” Model-to-process assignments can be disputed (Gamry, failed coatings).
- “Impedance spectroscopy is the same as impedance counting.” Automated cell counters use impedance to count particles, a different use; see the automated cell counter guide.
Where it connects to other lab topics
A spectrum is a frequency-domain signal, so the ideas in signal processing apply to how it is acquired and checked. For another electrochemical measurement, see the guide to the pH meter; for how instrument records are documented, see the guide to a calibration certificate.
FAQ
What is electrochemical impedance spectroscopy?
It is a technique that applies a small-amplitude sinusoidal signal to an electrochemical system, measures the response across a range of frequencies (typically 100 kHz to 0.1 Hz), and expresses it as a complex impedance. It is used to characterize batteries, catalysis and corrosion processes, among others.
What is electrical impedance spectroscopy?
The phrase is used for the same general idea of measuring impedance against frequency. The Gamry and Metrohm notes cited here describe the electrochemical case, so this page does too.
Why is the excitation signal so small?
Because electrochemical cells are not linear. A 1 to 10 mV signal keeps the response pseudo-linear, so the analysis assumptions hold (Gamry).
What does a Nyquist semicircle mean?
A semicircle is characteristic of a single time constant. In the simplified Randles cell the plot is always a semicircle whose intercepts give solution resistance and solution plus polarization resistance (Gamry).
Sources
- Gamry Instruments, “Basics of Electrochemical Impedance Spectroscopy”, application note, gamry.com/App_Notes/EchemImpedanceSpecs.htm, checked 2026-10-29.
- Metrohm Autolab, Application Note AN-EIS-001, “Electrochemical Impedance Spectroscopy (EIS), Part 1 – Basic Principles”, metrohm.com AN-EIS-001 (PDF), checked 2026-10-29.
