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Impedance spectroscopy (more formally, Electrochemical Impedance Spectroscopy, EIS) is a measurement technique used to study how a system responds to a small alternating electrical signal over a range of frequencies.
Impedance spectroscopy is a method in which an alternating voltage (or current) is applied to a material or device across many frequencies, and the resulting current (or voltage) response is measured to determine the system’s impedance as a function of frequency, revealing information about its electrical, chemical, and physical processes.
It asks the question, “How does this system resist and store electrical energy at different time scales?”
Because different frequencies probe different processes, impedance spectroscopy can separate:
High frequencies → bulk material properties (ionic/electronic conductivity)
Mid frequencies → interfaces and grain boundaries
Low frequencies → diffusion, reactions, and degradation processes
Impedance Spectroscopy
Batteries and fuel cells
Corrosion studies
Sensors and semiconductors
Polymers and biological systems
Nyquist and Bode Plots
Nyquist and Bode Plots
Impedance (Z) is a complex number that describes the resistance, capacitance, and chemical reactance of an electrochemical system. Plots, like Nyquits (a) are useful in visualizing the real and imaginary parts of the complex number (Z), where the real part (X-axis) illustrates the resistance and the imaginary (Y-axis) illustrates the capacitive/inductive parts of the system.
Bode plots (b and c) are useful in contextualizing at which frequencies these system elements are observed.
Equivalent circuit models can be used to validate the electochemical procresses observed by impedance and tested against experimental data as residual plots (d)
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