Introduction to Cyclic Voltammetry

Summary

Cyclic voltammetry (CV) is an electroanalytical technique in which a potentiostat sweeps an electrode’s potential back and forth while measuring the resulting current. Plotting current against potential produces a cyclic voltammogram.

The video explains the potential waveform and its adjustable parameters, then uses a three-electrode setup and ferrocyanide to illustrate what happens at the electrode and how those events appear in the data.

Main Ideas and Concepts

The Potential Waveform

In classical CV, the potential changes linearly with time in one direction, reaches a switching potential, and then sweeps back. The experiment starts at an initial potential and ends at a final potential. It may include one or more cycles.

• The scan rate, also called the sweep rate, describes how quickly the potential changes.
• The initial, switching, and final potentials, as well as the number of cycles, can be adjusted for the system being studied.
• A CV experiment has at least two sweep segments. A single segment is called linear sweep voltammetry.

The Three-Electrode System

The example uses a glassy-carbon working electrode, a platinum-wire counter electrode, and a silver/silver-chloride reference electrode. All three are immersed in an aqueous electrolyte containing ferrocyanide.

• The potentiostat controls the working-electrode potential relative to the reference electrode.
• The electrolyte supports electrical conductivity. In this example, its ions are treated as electrochemically inert.

The Electrical Double Layer

Applying a positive bias to the working electrode attracts negatively charged electrolyte ions toward its surface. These ions and the surrounding solvated ions form layers near the electrode, including the inner and outer Helmholtz planes and a diffuse layer.

Together, the charged electrode surface and nearby ionic layers form the electrical double layer. Its structure behaves like a capacitor: opposite charges are separated but not neutralized.

In a solution containing only supporting electrolyte, the CV can look roughly rectangular. The charging current is related to the double-layer capacitance and scan rate.

Faradaic and Non-Faradaic Current

• Faradaic current results from electron transfer at the electrode, such as the oxidation or reduction of ferrocyanide.
• Non-Faradaic current arises from charging and discharging the electrical double layer, without electron transfer to a redox-active molecule.

The measured current can contain both contributions at once.

Ferrocyanide Example: What the CV Shows

1. Sweep toward more positive potential: Ferrocyanide near the electrode loses an electron and is oxidized to ferricyanide. This electron transfer produces Faradaic current.
2. Oxidation peak: The current rises as more ferrocyanide is oxidized, then reaches a peak. The peak corresponds to a high rate of electron transfer from molecules near the electrode.
3. Current decay: Ferrocyanide near the surface becomes depleted. Molecules farther away must diffuse toward the electrode before reacting, so the current falls.
4. Reverse sweep: After the switching potential, the potential sweeps back in the negative direction. At sufficiently negative potential, ferricyanide gains an electron and is reduced back to ferrocyanide, producing a reduction peak.
5. Interpretation: The forward and reverse portions of the voltammogram show the oxidation and reduction processes, while diffusion and double-layer charging also influence the measured current.

Understanding the physical processes—electron transfer, diffusion, and double-layer charging—helps explain the shape of the voltammogram. The video notes that a detailed mathematical treatment is beyond the scope of this conceptual introduction.

Speakers and Sources Mentioned

• Speaker: One unnamed narrator/presenter speaking on behalf of Pine Research Instrumentation; no other speakers are heard.
• Educational references: Electrochemical Methods: Fundamentals and Applications by Allen Bard and Larry Faulkner; Wikipedia’s article on the electrical double layer; and a separate Pine Research video about how a potentiostat works.