Quasireversible reactions cyclic voltammetry

By using cyclic voltammetry, Schiffrin and coworkers [26, 186, 187, 189] studied electron transfer across the water-1,2-dichloroethane interface between the redox couple FefCNls /Fe(CN)6 in water, and lutetium(III) [186] and tin(IV) [26, 187] diphthalocyanines and bis(pyridine)-me50-tetraphenylporphyrinato-iron(II) or ru-thenium(III) [189] in the organic solvent. An essential advantage of these systems is that none of the reactants or products can cross the interface and interfere with the electron transfer reaction, which could be clearly demonstrated. Owing to a much higher concentration of the aqueous redox couple, the pseudo-first order electron transfer reactions could be analyzed with the help of the Nicholson-Shain theory. However, though they have all appeared to be quasireversible, kinetic analysis was restricted to an evaluation of the apparent standard rate constant o. which was found to be of the order of 10 cm s [186, 189]. Marcus [199] has derived a relationship between the pseudo-first-order rate constant for the reaction (8) and the rate... 

Compton and co-workers also studied the kinetics and mechanism of the HER in [C2mim][N(Tf2)] using cyclic voltammetry at Au, Mo, Ni, Ti, and Pt electrodes [63]. In this study, H[N(Tf2)] was added to the RTIL as the proton source and the CVs recorded are shown in Fig. 17.5. The HER at Pt began at about —0.25 V vs. Ag/Ag and some oxidation of H2 was visible during the reverse sweep (the reaction was quasireversible). However, use of each of the other metals resulted in an irreversible HER with negligible H2 oxidation visible during the reverse sweeps. 

In contrast to classical cyclic voltammograms, AC-cyclic voltammograms have a clear baseline, which is advantageous for quantitative measurements. By extending AC linear sweep voltaimnetry by a reverse scan, AC cyclic voltammetry is obtained. If the surface concentrations of the electroactive species are the same at the same potential for forward and reverse scans, the peaks for forward and reverse scans are expected to be identical. If the DC process is not fully reversible, the surface concentrations of the electroactive species are different at a given DC potential for forward and reverse scans— that is, for quasi-reversible systems a displacement of the jjeaks for forward and reverse scan can be observed. This displacement can be used to derive kinetic parameters of the electrode reaction. For instance, the derivation (1, p. 393] (or Eq. 5-33) for sluggish one-step heterogeneous quasireversible and/or... 

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