Cyclic voltammetry
In electrochemistry, cyclic voltammetry (CV) is a type of potentiodynamic measurement. In a cyclic voltammetry experiment, the working electrode potential is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is reached in a CV experiment, the working electrode's potential is ramped in the opposite direction to return to the initial potential. These cycles of ramps in potential may be repeated as many times as needed. The current at the working electrode is plotted versus the applied voltage (that is, the working electrode's potential) to give the cyclic voltammogram trace. Cyclic voltammetry is generally used to study the electrochemical properties of an analyte in solution[1][2][3][4] or of a molecule that is adsorbed onto the electrode.
Experimental method
In cyclic voltammetry (CV), the electrode potential ramps linearly versus time in cyclical phases (blue trace in Figure 2). The rate of voltage change over time during each of these phases is known as the experiment's scan rate (V/s). The potential is measured between the working electrode and the reference electrode, while the current is measured between the working electrode and the counter electrode. These data are plotted as current density (j) versus applied potential (E, often referred to as just 'potential'). In Figure 2, during the initial forward scan (from t0 to t1) an increasingly oxidation potential is applied; thus the anodic current will, at least initially, increase over this time period, assuming that there are oxidable analytes in the system. At some point after the oxidation potential of the analyte is reached, the anodic current will decrease as the concentration of oxidable analyte is depleted. If the
For instance, if the electron transfer at the working electrode surface is fast and the current is limited by the diffusion of analyte species to the electrode surface, then the peak current will be proportional to the square root of the scan rate. This relationship is described by the Randles–Sevcik equation. In this situation, the CV experiment only samples a small portion of the solution, i.e., the diffusion layer at the electrode surface.
Characterization
The utility of cyclic voltammetry is highly dependent on the analyte being studied. The analyte has to be redox active within the potential window to be scanned.
The analyte is in solution
Reversible couples
Often the analyte displays a reversible CV wave (such as that depicted in Figure 1), which is observed when all of the initial analyte can be recovered after a forward and reverse scan cycle. Although such reversible couples are simpler to analyze, they contain less information than more complex waveforms.
The waveform of even reversible couples is complex owing to the combined effects of polarization and diffusion. The difference between the two peak potentials (Ep), ΔEp, is of particular interest.
- ΔEp = Epa - Epc > 0
This difference mainly results from the effects of analyte diffusion rates. In the ideal case of a reversible 1e- couple, ΔEp is 57 mV and the full-width half-max of the forward scan peak is 59 mV. Typical values observed experimentally are greater, often approaching 70 or 80 mV. The waveform is also affected by the rate of electron transfer, usually discussed as the activation barrier for electron transfer. A theoretical description of polarization overpotential is in part described by the Butler–Volmer equation and Cottrell equation. In an ideal system the relationship reduces to for an n electron process.[2]
Focusing on current, reversible couples are characterized by ipa/ipc = 1.
When a reversible peak is observed, thermodynamic information in the form of a half cell potential E01/2 can be determined. When waves are semi-reversible (ipa/ipc is close but not equal to 1), it may be possible to determine even more specific information (see electrochemical reaction mechanism).
Nonreversible couples
Many redox processes observed by CV are quasi-reversible or non-reversible. In such cases the thermodynamic potential E01/2 is often deduced by simulation. The irreversibility is indicated by ipa/ipc ≠ 1. Deviations from unity are attributable to a subsequent chemical reaction that is triggered by the electron transfer. Such EC processes can be complex, involving isomerization, dissociation, association, etc.[5][6]
The analyte is adsorbed onto the electrode surface
Adsorbed species give simple voltammetric responses: ideally, at slow scan rates, there is no peak separation, the peak width is 90mV for a one-electron redox couple, and the peak current and peak area are proportional to scan rate (observing that the peak current is proportional to scan rate proves that the redox species that gives the peak is actually immobilised).[1] The effect of increasing the scan rate can be used to measure the rate of interfacial electron transfer and/or the rates of reactions that are coupltransfer. This technique has been useful to study redox proteins, some of which readily adsorb on various electrode materials, but the theory for biological and non-biological redox molecules is the same (see the page about protein film voltammetry).
Experimental setup
CV experiments are conducted on a solution in a cell fitted with electrodes. The solution consists of the solvent, in which is dissolved electrolyte and the species to be studied.[7]
The cell
A standard CV experiment employs a cell fitted with three electrodes:
The electrodes are immobile and sit in unstirred solutions during cyclic voltammetry. This "still" solution method gives rise to cyclic voltammetry's characteristic diffusion-controlled peaks. This method also allows a portion of the
Common materials for the working electrode include glassy carbon, platinum, and gold. These electrodes are generally encased in a rod of inert insulator with a disk exposed at one end. A regular working electrode has a radius within an order of magnitude of 1 mm. Having a controlled surface area with a well-defined shape is necessary for being able to interpret cyclic voltammetry results.
To run cyclic voltammetry experiments at very high scan rates a regular working electrode is insufficient. High scan rates create peaks with large currents and increased resistances, which result in distortions. Ultramicroelectrodes can be used to minimize the current and resistance.
The counter electrode, also known as the auxiliary or second electrode, can be any material that conducts current easily, will not react with the bulk solution, and has a surface area much larger than the working electrode. Common choices are
Solvents
CV can be conducted using a variety of solutions. Solvent choice for cyclic voltammetry takes into account several requirements.[4] The solvent must dissolve the analyte and high concentrations of the supporting electrolyte. It must also be stable in the potential window of the experiment with respect to the working electrode. It must not react with either the analyte or the supporting electrolyte. It must be pure to prevent interference.
Electrolyte
The electrolyte ensures good electrical conductivity and minimizes iR drop such that the recorded potentials correspond to actual potentials. For aqueous solutions, many electrolytes are available, but typical ones are alkali metal salts of perchlorate and nitrate. In nonaqueous solvents, the range of electrolytes is more limited, and a popular choice is tetrabutylammonium hexafluorophosphate.[8]
Related potentiometric techniques
Potentiodynamic techniques also exist that add low-amplitude AC perturbations to a potential ramp and measure variable response in a single frequency (AC voltammetry) or in many frequencies simultaneously (potentiodynamic electrochemical impedance spectroscopy).[9] The response in alternating current is two-dimensional, characterized by both amplitude and phase. These data can be analyzed to determine information about different chemical processes (charge transfer, diffusion, double layer charging, etc.). Frequency response analysis enables simultaneous monitoring of the various processes that contribute to the potentiodynamic AC response of an electrochemical system.
Whereas cyclic voltammetry is not hydrodynamic voltammetry, useful electrochemical methods are. In such cases, flow is achieved at the electrode surface by stirring the solution, pumping the solution, or rotating the electrode as is the case with rotating disk electrodes and rotating ring-disk electrodes. Such techniques target steady state conditions and produce waveforms that appear the same when scanned in either the positive or negative directions, thus limiting them to linear sweep voltammetry.
Applications
This section may be too technical for most readers to understand.
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