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Cyclic Voltammetry

Cyclic Voltammetry. Probing electron transfer mechanisms. CV. Application of a reversing linear sweep If an electroactive species is present we obtain a Faradaic current, summed to the background charging current I = I C +I f = AC d u +I f. LSV at planar electrodes.

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Cyclic Voltammetry

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  1. Cyclic Voltammetry Probing electron transfer mechanisms

  2. CV Application of a reversing linear sweep If an electroactive species is present we obtain a Faradaic current, summed to the background charging current I = IC+If = ACdu+If

  3. LSV at planar electrodes ip = -(2.69x105) n3/2ACD1/2v1/2 at 25ºC

  4. Scan Rate (reversible)

  5. CV E° = (Epa + Epc)/2 DEp = Epa - Epc = 59mV/n

  6. DL effect

  7. More than one component?

  8. Non-reversible systems

  9. Irreversible For irreversible processes peaks are reduced in size and widely separated. Totally irreversible systems are characterized by a shift of the peak potential with the scan rate: Ep = E° - (RT/anaF)[0.78 - ln(ko/(D)1/2) + ln (anaFn/RT)1/2] where a is the transfer coefficient and na is the number of electrons involved in the charge-transfer step. Thus, Ep occurs at potentials higher than E°, with the overpotential related to k° and a. The peak current, given by: ip = (2.99x105)n(ana)1/2ACD1/2n1/2 is still proportional to the bulk concentration, but will be lower in height (depending upon the value of a). Assuming a = 0.5, the ratio of the reversible-to-irreversible current peaks is 1.27

  10. Quasi-reversible For quasi-reversible systems (with 10-1 > k° > 10-5 cm/s) the current is controlled by both the charge transfer and mass transport. The shape of the cyclic voltammogram is a function of the ratio k°/(pnnFD/RT)1/2. As the ratio increases, the process approaches the reversible case. For small values of it, the system exhibits an irreversible behavior. Overall, the voltammograms of a quasi-reversible system are more drawn out and exhibit a larger separation in peak potentials compared to a reversible system.

  11. Mechanistic complications part 1:  The EC mechanism

  12. The ECE mechanism

  13. Catalytic

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