The Cyclic Voltammetric Experiment

Electrochemical measurements on polyaniline (PANI) produce a picture of the charge storage mechanism of conducting polymers which differs fundamentally from that obtained using PTh or PPy. In the cyclic voltammetric experiment one observes at least two reversible waves in the potential range between —0.2 and -)-1.23 V vs SCE. Above -1-1.0 V the charging current tends to zero. Capacitive currents and overoxidation effects, as with PPy and PTh, do not occur The striking... 

If the nonlinear character of the kinetic law is more pronounced, and/or if more data points than merely the peak are to be used, the following approach, illustrated in Figure 1.18, may be used. The current-time curves are first integrated so as to obtain the surface concentrations of the two reactants. The current and the surface concentrations are then combined to derive the forward and backward rate constants as functions of the electrode potential. Following this strategy, the form of the dependence of the rate constants on the potential need not be known a priori. It is rather an outcome of the cyclic voltammetric experiments and of their treatment. There is therefore no compulsory need, as often believed, to use for this purpose electrochemical techniques in which the electrode potential is independent of time, or nearly independent of time, as in potential step chronoamperometry and impedance measurements. This is another illustration of the equivalence of the various electrochemical techniques, provided that they are used in comparable time windows. 

The initial sweep peak current (amperes) for a reversible one-electron reduction at the electrode in the cyclic voltammetric experiment is given by... 

Cyclic voltammetry provides a convenient method of recognizing such processes provided the lifetime of the intermediate is less than a minute or so. Consider an idealized reaction pathway (14) which involves the reversible one-electron reduction of a compound M. Of primary interest in the cyclic voltammetric experiment is the ratio of the back- and forward-peak currents, ipb/ip, and the dependence of this ratio upon the scan rate, v. 

If M is unstable then ipb/fpf will be less than unity. Its magnitude will depend upon the scan rate, the value of the first-order constant k, and the conditions of the experiment. At fast scan rates the ratio ipb/ ip, may approach one if the time gate for the decomposition of M is small compared with the half-life of M-, (In 2jk). As the temperature is lowered, the magnitude of k may be sufficiently decreased for full reversible behaviour to be observed. The decomposition of M- could involve the attack of a solution species upon it, e.g. an electrophile. In such cases, ipb/ipf, will of course be dependent upon the concentration of the particular substrate (under pseudo-first-order conditions, k is kapparent). Quantitative cyclic voltammetric and related techniques allow the evaluation of the rate constants for such electrochemical—chemical, EC, processes. At the limit, the electron-transfer process is completely irreversible if k is sufficiently large with respect to the rate of heterogeneous electron transfer the electrochemical and chemical steps are concerted on the time-scale of the cyclic voltammetric experiment.1-3... 

Numerous examples could be cited in which two or more suspected products of an electrode process exhibit similar electrochemical behavior. In other instances, the species that is stable during the time required to complete the cyclic voltammetric experiment may undergo a slow chemical reaction to give the product that is isolated. These problems arise sufficiently frequently that the identification of products and the determination of the product distribution are required. 

The cyclic voltammetric experiment can give a great deal of information about the redox activity of a compound and the stability and accessibility of its reduced or oxidised forms. For a fully chemically reversible process, ipa must equal rpc, i.e. all of the material oxidised at the electrode surface on the forward scan must be re-reduced on the reverse scan (or vice versa). If this condition does not hold true, then the process may be partially reversible (rpc < ipa) or irreversible (rpc = 0). Observation of processes that are not fully reversible implies decomposition or chemical reaction of the reduced or oxidised species and the ratio of ipa to /p(. will show a strong dependence on scan rate since the reverse current is related to the lifetime of the redox-generated material. Note that processes that are chemically reversible (in the sense that the reduced and oxidised species are both stable) may not be electrochemically reversible (a term that relates to the relative rates of forward and back electron transfer). Electrochemically reversible processes are characterised by a separation between the forward and reverse potential peaks of exactly 59 mV. 

The cyclic voltammetric experiment is an interfacial electrochemical technique (i.e. monitoring of processes occurring near the electrode surface) that involves measurement of current flow as a function of applied potential. The potential is swept from a resting potential, (measured against a reference such as the saturated calomel electrode or the potential of the Fe(II)/Fe(III) couple in ferrocene), at a constant rate (usually... 

The proposed model of two-stage process is well supported by the cyclic voltammetric experiments presented in Section III.4. The fast reversible stage, attributed to formation of an electric double layer at the catalyst/gas interface via backspillover of promoters, has been discussed in detail in Section III.5. To explain the slow irreversible pretreatment. This phenomenon is called permanent electrochemical promotion or permanent NEMCA effect. The similarity between the regions of rate increase and decrease indicates that similar mechanisms are involved during current application and interruption, but the enhancement of the open-circuit rate indicates that the electrochemical promotion of the Ir02 catalyst is not reversible. This behavior of an oxide catalyst is different from that of a metal catalyst for which the electrochemical promotion is usually reversible. ... 

All the cyclic voltammetric experiments have been conducted in DMF + O.IM N t4Cl4 on glassy carbcm electrodes. The potentials are mesured versus an SCE (saturated with NaCl). 

THE CYCLIC VOLTAMMETRIC EXPERIMENT Correcting for Capacitive Current... 

The cyclic voltammetric experiments performed in Me0H/H20 (1 1) clearly demonstrated the effective inclusion of ferrocifen 4 in the cavity of Me-f)-CD, most likely via the apolar ferrocene moiety (Fig. 47.14). 

Equation 7.7 has been derived for linear sweep voltammetry the reverse scan in the cyclic voltammetric experiment is symmetrical about the zero current axis. 

B]x q at the electrode surface at any time t during the course of the cyclic voltammetric experiment may be calculated by semi-integration of the experimental voltammogram (Eqs. II.1.8 and II.1.9). 

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