Electrochemical Impedance Spectroscopy (EIS): What is it and who is it for?

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Electrochemical Impedance Spectroscopy has a reputation for being difficult to master. The terminology is dense, the learning curve is steep, and many practitioners describe it as something of a black art. This reputation is not entirely undeserved—EIS does require time and effort to understand properly. However, it remains one of the most powerful diagnostic tools available for electrolyser development and optimisation.

What is EIS?

EIS is a diagnostic technique used to analyse the performance of electrochemical systems, such as PEM electrolyser cells. The method involves applying small-amplitude sinusoidal perturbations to the system across a range of frequencies and measuring the corresponding response.

These perturbations—applied as either voltage or current—are deliberately small, typically less than 10% of the system's operating conditions. This ensures the system remains in its normal operating regime and avoids non-linear responses that would complicate interpretation.

By sweeping through different frequencies, EIS captures how the system responds at different timescales. Different electrochemical processes occur at characteristic frequencies, allowing EIS to isolate and identify specific phenomena within the electrolysis cell—from fast electronic processes to slower mass transport limitations.

Why EIS matters

Standard electrochemical measurements provide only aggregate performance data of the electrolyser. A voltage measurement at a given current density tells you the overall result, but reveals nothing about the underlying causes of that performance.

EIS provides granular insight into the individual processes that contribute to overall cell performance:

• Electrical and ionic resistance in membranes, contacts, and bulk electrolyte

• Charge-transfer kinetics at catalyst surfaces

• Mass transport limitations affecting reactant delivery or product removal

This diagnostic capability transforms troubleshooting from guesswork into targeted problem-solving. Rather than simply knowing performance has degraded, EIS identifies which specific component or process is responsible.

Essential terminology

The specialised vocabulary of EIS presents an initial barrier to entry. The following terms form the foundation of EIS analysis:

Term	Definition
Impedance (Z)	Opposition to alternating current, analogous to resistance in DC circuits but including capacitive and inductive effects
Nyquist Plot	Plot of imaginary vs. real impedance components; semicircular features correspond to distinct electrochemical processes
Bode Plot	Frequency-domain representation showing impedance magnitude and phase angle
HFR	High-frequency resistance, typically dominated by ohmic losses
Rs/Re	Solution or electrolyte resistance, reflecting membrane, contact, and bulk electrolyte performance
Rct	Charge-transfer resistance, indicating catalyst activity and reaction kinetics
Cdl	Double-layer capacitance at the electrode-electrolyte interface
Warburg (Zw)	Impedance feature indicating diffusion-limited mass transport
Equivalent Circuit	Simplified circuit model (resistors, capacitors, etc.) used to fit and interpret impedance data

The value proposition of EIS insights

EIS requires upfront investment to understand properly. The technique involves complex data interpretation and requires familiarity with electrochemical principles. However, for those working in electrolyser development, operation, or optimisation, this investment delivers substantial returns. EIS provides diagnostic capabilities that simply cannot be achieved through conventional measurements and it can be an indispensable tool for understanding electrolyser performance.

Find out more about how Endua and Pulsenics are partnering to bring the future of EIS-enabled electrolysis closer to reality.