Cyclic voltammetry peak current ratio

Cyclic voltammetry peak currents and peak current ratios for reversible one-electron transfer as a function of E a... 

FIGURE 2.4. EC reaction scheme in cyclic voltammetry. Derivation of the rate constant from the anodic-to-cathodic peak current ratio in zone KO. In this example the scan is reversed 200 mV (at 25° C) after the peak. 

As mentioned above, the characteristic feature of processes in this kinetic region is that the peak current ratio — z°x/Zped varies from about unity to zero. Thus, a procedure for studying the kinetics would be to record values of —/°x//ped at different sweep rates and compare these with a working curve for the proposed mechanism in a way analogous to that discussed for DPSCA above. However, a problem with this approach is the difficulty of defining a baseline for the reverse sweep (see below) and, for that reason, CV suffers from some limitations when used in quantitative work. This has led to the development of derivative cyclic voltammetry (DCV) [37]. 

Figure 14.3.12 Peak current ratio v.y. scan rate for cyclic voltammetry when the reactant (A) or the product (B) is weakly adsorbed. Curve A Fi = Fq,s, PqCq = 1. Curve R F[ = Fr, PrCq = 1. Reversal potential = Ey2 (ISO/n) mV. [Reprinted with permission from R. H. Wopschall and I. Shain, Anal. Chem., 39, 1514 (1967). Copyright 1967, American Chemical Society.]...

This equation is often used to determine the formal potential of a given redox system with the help of cyclic voltammetry. However, the assumption that mid-peak potential is equal to formal potential holds only for a reversible electrode reaction. The diagnostic criteria and characteristics of cyclic voltammetric responses for solution systems undergoing reversible, quasi-reversible, or irreversible heterogeneous electron-transfer process are discussed, for example in Ref [9c]. An electro-chemically reversible process implies that the anodic to cathodic peak current ratio, lpa/- pc equal to 1 and fipc — pa is 2.218RT/nF, which at 298 K is equal to 57/n mV and is independent of the scan rate. For a diffusion-controlled reduction process, Ip should be proportional to the square root of the scan rate v, according to the Randles-Sevcik equation [10] ... 

Figure 1 displays derivative cyclic voltammograms for the oxidation of Cp Mn(CO)2(NCMe) (1 left) and Cp Mn(CO)2(Pl%3) (2 right) at a voltage sweep rate v - 0.2 V/s. Cyclic voltammetry peak potentials correspond to the intersections between the DCV curves and the base line. The reversible potentials for the oxidation of 1 and 2, taken as the midpoints between the anodic and cathodic CV peaks, are located at -0.12 and +0.22 V vs the ferrocene/ferricinium couple (Fc), respectively. The unity ratio of the cathodic to anodic derivative peak currents demonstrates the chemical reversibility of the 1/1 and 2/2- couples. 

Several methods have been utilized to determine the rate of the following chemical reaction from a series of CVs at different scan rates. The simplest involves a comparison of ip,e and i . The cathodic peak current is measured from the zero current baseline, while the anodic current baseline is established by the current at which the potential is switched. The experimental peak current ratios can then be compared to a previously calculated theoretical working curve to find the rate constant (for a first-order or pseudo-first-order reaction. Parker has emphasized the use of working curves based on derivative cyclic voltammetry, which discriminates to some degree against capacitive background current. ... 

Thin films of a composite nickel-iron (9 1 Ni/Fe ratio) and iron-free oxyhydroxides were deposited from metal nitrate solutions onto Ni foils by electroprecipitation at constant current density. A comparison of the cyclic voltammetry of such films in 1M KOH at room temperature (see Fig. 6) shows that the incorporation of iron in the lattice shifts the potentials associated formally with the Ni00H/Ni(0H)2 redox processes towards negative potentials, and decreases considerably the onset potential for oxygen evolution. The oxidation peak, as shown in the voltammo-gram, is much larger than the reduction counterpart, providing evidence that within the time scale of the cyclic voltammetry, a fraction of the nickel sites remains in the oxidized state at potentials more negative than the reduction peak. 

This is a case where another electrochemical technique, double potential step chronoamperometry, is more convenient than cyclic voltammetry in the sense that conditions may be defined in which the anodic response is only a function of the rate of the follow-up reaction, with no interference from the electron transfer step. The procedure to be followed is summarized in Figure 2.7. The inversion potential is chosen (Figure 2.7a) well beyond the cyclic voltammetric reduction peak so as to ensure that the condition (Ca) c=0 = 0 is fulfilled whatever the slowness of the electron transfer step. Similarly, the final potential (which is the same as the initial potential) is selected so as to ensure that Cb)x=0 = 0 at the end of the second potential step whatever the rate of electron transfer. The chronoamperometric response is recorded (Figure 2.7b). Figure 2.7c shows the variation of the ratio of the anodic-to-cathodic current for 2tR and tR, recast as Rdps, with the dimensionless parameter, 2, measuring the competition between diffusion and follow-up reaction (see Section 6.2.3) ... 

Using, for example, cyclic voltammetry, the cathodic peak current (normalized to its value in the absence of RX) is a function of the competition parameter, pc = ke2/(ke2 + kin), as detailed in Section 2.2.6 under the heading Deactivation of the Mediator. The competition parameter can be varied using a series of more and more reducing redox catalysts so as eventually to reach the bimolecular diffusion limit. km is about constant in a series of aromatic anion radicals and lower than the bimolecular diffusion limit. Plotting the ratio pc = keij k,n + km) as a function of the standard potential of the catalysts yields a polarogram of the radical whose half-wave potential provides the potential where ke2 = kin, and therefore the value of... 

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. 

Rashid and Kalvoda examined this reaction using cyclic voltammetry by measuring the current enhancement for the electro-oxidation of potassium ferricyanide on addition of the amine. Using working curves derived by Nicholson and Shain (1964) relating the ratio of the peak current measured in the presence and absence (i.e. the diffusion-controlled peak current for oxidation of ferricyanide) of the amine to the parameter kfRTInFvioT an EC mechanism, the kinetic parameter, kf, could be calculated. 

With faster scan cyclic voltammetry, a new two-electron anodic peak was detected, at more negative potentials, for the first stage of the oxidation process, with an accompanying cathodic peak on the reverse scan (11). The ratio of the forward to the reverse peak currents increased towards unity as the scan rate was raised to —200 V s 1 (Fig. 15). This behavior was attributed to the initial two-electron process being accompanied by a fairly rapid follow-up chemical reaction and was successfully analyzed in terms of an EqCi process (quasi-reversible electron transfer followed by a first-order irreversible chemical process), with a rate constant for the chemical step, k, = 250 s 1. 

In a cyclic voltammetry experiment kca, can also be calculated from either the ratio of the plateau current with the catalyst to the peak current without the catalyst... 

A useful adjunct of linear potential sweep methods is called cyclic voltammetry. Rather than stopping an oxidative voltammogram at, say, + 0.8 V, the potential is reversed and scanned backward, i.e., a triangular wave potential is applied. The oxidation product formed is present at and close to the electrode surface. With fairly rapid potential sweeps (ca. >4 V/min) it is almost completely re-reduced back to the starting material on the reverse potential sweep. Figure 14B shows a typical cyclic voltammogram for a reversible system (solid line). The ratio of forward to reverse peak currents is unity. If, however, some rapid process removes the product(s), litde or no reverse current is obtained (dotted lines of Fig. 14B). This happens if the overall oxidation is totally irreversible, or fast chemical reactions intervene. We will also see later that a peculiar property of very small electrodes can eliminate most of the reverse current in a cyclic voltammogram. 

Our studies have focused on a pentaerythiitol-basedmetaUodendruner with Ru terpyridine units (RuDen), which was synthesized and purified based on a procedure by Constable et al. [39]. The structure is shown in Figure 10. Mass spectrometry, NMR, and UV-visible spectrophotometry established the veracity of the synthesis, and the oxidation state (-1-2) of the ruthenium centers in the product was verified by EPR [40]. Cyclic voltammetry of RuDen in homogeneous solution demonstrated the reversible, one-electron oxidation of Ru . The voltam-mogram of a 1.0 mM RuDen solution in acetonitrile with 0.1 M tetrabutylammo-nium perchlorate as the supporting electrolyte showed an oxidation peak at 1.1 V versus Ag/AgCl. With a scan rate of 0.2 V s the difference between anodic peak potential and that for the corresponding reduction peak was 60 mV, and the ratio of the anodic to cathodic peak current was 1.0. 

Pt-Pd-Rh/ glassy carbon Coprecipitation method Pt Pd Rh = 4 3 I (weight ratio) Cyclic voltammetry at room temperature in 0.1 MNH3-t0.5 M H2SO4 Peak current density ca. 0.3 mA cm (y = 50mV s ) 55... 

---