Electrochemical Impedance Spectroscopy (EIS)

Electrochemical Impedance Spectroscopy (EIS) is an electrochemical technique to measure the impedance of a system in dependence of the AC potentials frequency. It is for multiple reasons quite popular.

One reason is that EIS allows separating the influences of different components that means the contribution of the electron transfer resistance, double layer capacity, etc.

Another one is that electrochemical impedance spectroscopy is very surface sensitive, which makes many changes visible that other techniques don’t see, for example changes in polymer layers due to swelling, surface changes due to protein adsorption or penetration of corrosion protection layers.

As a result electrochemical impedance spectroscopy is interesting for analytical electrochemistry, because molecules can be detected without a redox active marker.

Electrochemical Impedance Spectroscopy is used to demonstrate the presence of a particular substance in a (usually a) liquid and/or to measure its quantity using electrical waves.

A potentiostat sends these waves to the liquid to be measured and from the reaction that comes back an electrochemical researcher can read the information he needs for this purpose.

One can think of testing/measuring HIV in blood or mercury in groundwater. The advantage of electrochemical impedance spectroscopy is that the testing does not affect the fluid, the testing can be done outside the lab and the testing takes little time.

In electrochemical measurements, resistance refers to how much a system opposes the flow of direct current (DC) — it’s the simple, frequency-independent opposition described by Ohm’s law (R = V/I). By contrast, impedance is the equivalent concept for alternating current (AC) and includes not only resistance but also reactive effects from capacitive and inductive behavior. Impedance is a complex quantity composed of both magnitude and phase, meaning it can vary with the frequency of the applied AC signal.

In electrochemical impedance spectroscopy (EIS), impedance gives us a much richer picture of an electrochemical system than simple resistance — because it captures how the system responds to an AC signal across a range of frequencies, revealing contributions from double-layer capacitance, charge transfer, diffusion, and more.

Electrochemical impedance spectroscopy measures how an electrochemical system responds to an applied electrical signal over a range of frequencies. To achieve this, we use a small AC perturbation — typically a sinusoidal signal — because:

1. AC lets us probe frequency-dependent processes.
   
   Different electrochemical processes (e.g., double-layer charging, charge transfer, mass transport/diffusion) respond differently at different frequencies.

2. DC can’t distinguish these processes.
   
   A DC (constant) signal only gives one value of response, whereas AC measurements spanning multiple frequencies capture dynamic behavior.

By applying sinusoidal potentials of different frequencies and analyzing both the amplitude and the phase of the resulting current, we turn what would be a single impedance value into a spectrum that reveals mechanistic information about the system — this is the core idea behind electrochemical impedance spectroscopy.

When you apply a sinusoidal (AC) potential to an electrochemical cell, the current that flows in response does not always follow the exact same timing as the applied potential. The phase shift is the time delay between the peaks of the voltage and current waves.

• In a pure resistor, voltage and current are perfectly in phase (no phase shift).

• In systems with capacitive or kinetic processes, current can lead or lag the voltage.

This phase information is not just noise — it tells us about the energy-storage processes in the electrochemical system. For example:

• Capacitive behavior (e.g., double-layer at an electrode surface) causes a phase shift of up to –90°.

• Faradaic or charge transfer processes often show intermediate phase shifts that reveal kinetic and transport characteristics.

In electrochemical impedance spectroscopy, analyzing both the magnitude and the phase of the impedance across frequencies allows us to separate and understand different underlying electrochemical mechanisms.

Frequency is one of the most important variables in electrochemical impedance spectroscopy — it essentially determines which processes you are probing in your measurement.

• High frequencies tend to highlight fast processes such as solution resistance or charging of the electrical double layer.

• Low frequencies reveal slower processes, including mass transport, diffusion and slow kinetics.

By sweeping the AC signal over a range of frequencies, you create a frequency response of the system. When plotted — in a Bode plot (impedance magnitude and phase vs. frequency) or a Nyquist plot (imaginary vs. real part of impedance) — this data reveals how each electrochemical component behaves over timescales corresponding to different frequency ranges.

Frequency tells you which processes dominate at which timescales, making it possible to distinguish and model them in EIS.

Explore how different frequencies relate to EIS responses in the PalmSens Bode and Nyquist Plot overview.

The term impedance spectroscopy refers broadly to measuring the impedance of a system as a function of frequency — this concept is used in physics, materials science, and electrochemistry alike.

Electrochemical impedance spectroscopy (EIS) is a specific application of impedance spectroscopy that targets electrochemical systems such as electrodes in electrolytes, batteries, sensors, and corrosion interfaces. It applies the same fundamental concept (frequency-dependent impedance) but is interpreted within the context of electrochemical processes and reactions.

Put simply:

• Impedance spectroscopy = general technique measuring frequency-dependent impedance.

• Electrochemical impedance spectroscopy = the electrochemical version of this technique, using equipment like potentiostats and analyzing electrochemical interfaces.

EIS is impedance spectroscopy specialized for electrochemistry — that’s the practical difference. Dive deeper into PalmSens’s explanation of electrochemical techniques.