Nyquist plot

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Electrochemical impedance spectroscopy

Measuring a cell’s impedance across a range of frequencies, from kilohertz to millihertz, with a small alternating signal. The spectrum separates ohmic, interface and diffusion processes by their time scale and is read on a Nyquist plot.

Impedance

The frequency-dependent generalisation of resistance, with a magnitude and a phase. A cell’s impedance separates ohmic, charge-transfer and diffusion contributions that a single DC resistance lumps together.

Randles circuit

The classic equivalent circuit for one electrode interface: electrolyte resistance in series with a double-layer capacitance that shunts the charge-transfer resistance and a Warburg diffusion element. It is the template for reading an impedance spectrum.

Warburg impedance

The impedance of diffusion, which appears as a 45° line at the low-frequency end of a Nyquist plot. Its slope and onset frequency give a route to the diffusion coefficient.

References & further reading

Further reading on this topic

These sources provide broader context; they are not presented as citations for every statement in this entry.

1. Modeling of Galvanostatic Charge and Discharge of the Lithium/Polymer/Insertion Cell

   Marc Doyle, Thomas F. Fuller, John Newman · Journal of The Electrochemical Society · 1993

   Topic background · Porous-electrode transport and insertion-cell modeling; foundational work rather than a specification for a particular commercial cell.

2. Quantities, Units and Symbols in Physical Chemistry, 3rd edition, 2nd printing

   IUPAC · 2011

   Topic background · Physical-chemistry notation, thermodynamics, electrochemistry and transport quantities.