How to read a cyclic voltammogram — and diagnose a cell from it
Cyclic voltammetry is the first measurement most people run on a new electrode material, and the most misread. This is how to interpret the curve like an electrochemist: what the shape means, what the numbers tell you, and where it quietly lies to you.
A cyclic voltammogram (CV) plots current against a potential that is swept up and then back down. The forward sweep produces an oxidation (anodic) peak, the reverse sweep a reduction (cathodic) peak — together the characteristic "duck" loop. The separation between the two peaks reports the reversibility of the electron transfer (about 57–59 mV for a reversible one-electron couple at 25 °C), and the peak height grows with the square root of scan rate when the reaction is diffusion-controlled.
What a CV actually measures
You apply a potential to the working electrode and sweep it linearly from a start value up to a vertex, then reverse and sweep back — a triangular waveform in time. You record the resulting current. As the potential reaches the value where your species oxidizes, current rises, peaks, then falls as the electrode surface depletes of reactant faster than diffusion can replenish it. Reverse the sweep and the product you just made gets reduced back, producing the mirror peak below the axis. The plot is current (y) versus potential (x) — not versus time — which is why the trace doubles back on itself into a loop.
How to read it well
1 · Peak separation = reversibility
Measure the gap between the anodic peak potential (Epa) and the cathodic peak potential (Epc). For a fast, reversible one-electron transfer at room temperature, that gap (ΔEp) sits near 57–59 mV. If you see 90, 150, 300 mV, the electron transfer is sluggish — quasi-reversible or irreversible — and the gap widens further as you speed up the scan. This single number is the fastest read on electrode kinetics you have.
2 · Peak height vs √(scan rate) = diffusion control
Run the CV at several scan rates and plot peak current against the square root of scan rate. A straight line through the origin means the process is diffusion-controlled (Randles–Sevcik). Curvature, or a line that misses the origin, points to adsorption, thin-layer behavior, or a coupled chemical step. This is the test that separates "my species is diffusing freely to the electrode" from "something is stuck to the surface."
3 · Peak current ratio = chemical stability
For a clean reversible couple the return peak should be about as tall as the forward peak (ipa/ipc ≈ 1). If the return peak is missing or stunted, the species you generated reacted away before you could sweep back — an EC mechanism, a following chemical reaction. The ratio is your readout of intermediate stability.
Common ways a CV misleads you
- Uncompensated resistance (iR drop) widens peak separation and can fake irreversibility. A "sluggish" couple is sometimes just a resistive cell — check before you blame kinetics.
- Capacitive background from double-layer charging scales linearly with scan rate and inflates apparent peak current at fast scans. Subtract the baseline before quoting peak heights.
- Too-fast scan rates outrun the instrument and the kinetics, distorting the loop. If ΔEp grows steadily with scan rate, that is a kinetics signature, not a glitch.
- A drifting reference electrode shifts every potential. Peaks that march over repeated cycles are often the reference, not the chemistry.
Where this gets slow by hand
One CV is quick to eyeball. The problem is volume: a formulation screen produces hundreds of curves across scan rates, states of charge, and cell builds. Manually picking peaks, computing ΔEp and the Randles–Sevcik slope, baseline-correcting capacitance, and cross-checking the reference for every file is where days disappear — and where subtle, slow drifts get missed because no one is comparing curve 3 to curve 300.
From a folder of cycler files to a diagnosis
Niobia ingests raw potentiostat exports, finds the anodic and cathodic peaks, computes ΔEp and the peak-current ratio, and fits peak current against √(scan rate) to extract the Randles–Sevcik slope and a diffusion coefficient — automatically, across every file in the run. It baseline-corrects the capacitive background, flags curves where iR drop is masquerading as slow kinetics, and watches for reference-electrode drift across a series. The result isn't another plot to read; it's a verified read on reversibility, diffusion, and intermediate stability — reconciled against the same diffusion coefficient measured by GITT and EIS.
Frequently asked
What does a cyclic voltammogram show?
It plots measured current against the applied potential as that potential is swept up and back. The forward sweep drives an oxidation (anodic) peak and the reverse sweep a reduction (cathodic) peak, producing the characteristic duck-shaped loop. Peak positions and heights encode the electrode kinetics and the amount of electroactive species.
What is the peak separation in a reversible CV?
For an electrochemically reversible one-electron couple at 25 °C, the anodic and cathodic peaks are separated by about 57–59 mV. A separation much larger than this indicates sluggish (quasi-reversible or irreversible) electron-transfer kinetics — or uncompensated cell resistance, which should be ruled out first.
Why does CV peak current scale with the square root of scan rate?
Because the reaction is diffusion-controlled. The Randles–Sevcik relationship makes peak current proportional to the square root of scan rate for a freely diffusing species, so peak current plotted against √(scan rate) is a straight line through the origin when diffusion dominates. Deviation points to adsorption or a coupled chemical step.