1 / 16

Potential Step

Potential Step. The working electrode is a microelectrode , and the solution volume is large enough that the passage of current does not alter the bulk concentrations of electroactive species. Such circumstances are known as small A/V conditions.

odette-rasmussen
Télécharger la présentation

Potential Step

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Potential Step • The working electrode is a microelectrode, and the solution volume is large enough that the passage of current does not alter the bulk concentrations of electroactive species. Such circumstances are known as small A/V conditions. • Controlled potential microelectrode techniques: the working electrode potential may be constant or it may vary with time in a predetermined manner, and the current is measured as a function of time or potential.

  2. Schematic experimental arrangement for controlled potential experiments

  3. Potential step Resulting i-t curve Signal

  4. Concentration profiles for various times into the experiment

  5. Chronoamperometry: current is recorded as a function of time • Ei is chosen to be at a constant value where no faradaic processes occur • E2 is a potential at which surface concentration of elctroactive species goes nearly to zero, therefore, consider E2 to be in the “mass transfer limited” region. • The flux of electroactive species is proportional to the concentration gradient at the electrode surface. • The zone of electroactive species depletion is thickened with time, therefore, the current declines with time.

  6. A series of step experiments • When the step potential is in the open circuit potential region, no faradaic current is yielded. • When the step potential is in the mass-transfer-limited region, surface concentration of elctroactive species is zero, but the zone of electroactive species depletion is thickened with time, therefore, the current declines with time. • When the step potential is in the mix control region, since the difference between the bulk and surface concentration is smaller than in the mass-transfer-limited case and the currents are smaller than the latter.

  7. Sampled-current voltammetry • Plot the sampled current i(τ) vs. the potential to which the step takes place • dc polarography: voltammetry at the dropping mercury electrode

  8. Double potential step chronoamperometry • The forward step, that is, the transition from E1 to E2 at t=0, is exactly the chronoamperometry experiment • For a period τ, it causes a buildup of the reduction product in the region near the electrode. After t= τ, the potential returns to E1, where only the oxidized form is stable at the electrode. A large anodic current flows as it begins to reoxidize, then the current declines as the depletion effect sets in

  9. Chronocoulometry: enhance noise on the signal • The usual observation in controlled potential experiments are currents as functions of time or potential. In some experiments, it is useful to record the integral of the current versus time. Since the integral is the amount of charge passed, these methods are coulometric approaches. The most prominent examples are chronocoulometry and double potential step chronocoulometry.

  10. Several special cases are easily identified: • Large-amplitude potential step: i is independent of E • Small-amplitude potential changes: current and potential are linked by the linearized i-ηrelation • Reversible electrode process: consistent with the Nernst equation • Totally irreversible electrode process:

  11. Approximate forms of the i-η equation • No mass transfer effects, the equation is known generally as Butler-Volmer equation • Linear characteristic at small η, the –η/i has dimensions of resistance and often called the charge transfer resistance, Rct: • Tafel behavior at large η, for large values of η, one of the bracketed terms becomes negligible, that is

  12. Potential step under diffusion control • Cottrell equation: • Instrumental and experimental limitations: (1) Potentiostatic limitations, (2) limitations in the recording device, (3) limitations imposed by Ru and Cd, (4) limitations due to convection: the available “time window” for Cottrell measurements lies approximately between 20 μsec and 200 sec.

  13. Limitations imposed by Ru and Cd Note that for a potential step input, an exponentially decaying current is obtained, with a time constant, τ=RsCd, hence, the current for charging the double-layer capacitance drops to 37% of its initial value at t=τ

  14. Concentration profiles for various times into the experiment

  15. Chronocoulometry • At t=0, the potential is shifted to E2, which is sufficiently negative to enforce a diffusion-limited current. The following equation describes the chronocoulometric response:

  16. Effects of the additional components on chronocoulometric response • Where Qdl is the capacitive charge and nFAΓquantifies the faradaic component given to the reduction of Γof adsorbed species:

More Related