Free tool
EIS Nyquist & Bode plot tool
Paste impedance data from your potentiostat and get a Nyquist plot with equal axes, a Bode plot, and the high-frequency resistance read off automatically. No Excel templates, no Python, nothing uploaded.
Runs entirely in your browser. Your data is not uploaded anywhere.
Reading a Nyquist plot in 30 seconds
Each point is one frequency. High frequencies sit on the left, low frequencies on the right. Where the curve meets the real axis on the left is the ohmic (high-frequency) resistance. The first semicircle reflects charge-transfer resistance in parallel with double-layer capacitance; its width is roughly Rct. A 45° line at low frequency points to diffusion (Warburg) behaviour, and in PEM fuel cells a second low-frequency arc usually means mass-transport losses.
Equal axis scaling matters: Excel stretches the axes independently, which turns semicircles into ellipses and makes arcs hard to compare between runs. This tool always plots Z′ and −Z″ on the same scale.
When you outgrow a single plot
Comparing EIS at several current densities, tracking HFR over a durability test, or producing the same report for every stack is where a one-off plot stops being enough. The EIS and polarization curve guide shows how to do that in Vizalyze without scripting.
FAQ
What data format does the Nyquist plotter accept?
Any comma, semicolon or tab separated text with three columns: frequency, real impedance Z′ and imaginary impedance Z″. Files with frequency, |Z| and phase also work. Header names are detected automatically; exports from most potentiostats (Gamry, BioLogic, Zahner, Autolab) can be pasted after removing the instrument preamble.
Do I need to flip the sign of the imaginary part?
No. Electrochemists plot −Z″ on the y-axis so capacitive arcs appear above the axis. If most of your imaginary values are negative, the tool flips them for you and says so.
How is the high-frequency resistance (HFR) calculated?
HFR is read where the curve crosses −Z″ = 0 on the high-frequency side, using linear interpolation between the two points either side of the crossing. If there is no crossing (inductive tail missing), the highest-frequency Z′ is used. For PEM fuel cells and electrolyzers this approximates membrane plus contact (ohmic) resistance.
Does this fit an equivalent circuit?
No. The arc width shown is a quick read from the data, not a fitted Rct. For equivalent-circuit fitting, overlaying many runs and building a repeatable report, use the Vizalyze desktop app.
Is my data uploaded?
No. Parsing and plotting run in your browser; the data never leaves your computer.