Cyclic voltammetry. .

A cyclic voltammogram (CV) of a SiNW electrode is shown in Fig. 1.6. The charge current associated with the formation of the Li-Si alloy 

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes and the kinetics of heterogeneous electron transfer reactions and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media on the redox process. 

Analytical Electrochemistry, Third Edition, by Joseph Wang Copyright 2006 John Wiley Sons, Inc. 

Cyclic voltammetry [1,2] provides basic information on the oxidation potential of the monomers, on film growth, on the redox behavior of the polymer, and on the surface concentration (charge consumed by the polymer). Conclusions can also be drawn fi om the cyclic voltammograms regarding the rate of charge transfer, charge transport processes, and the interactions that occur within the polymer segments, at specific sites and between the polymer and the ions and solvent molecules. 

For very thin films and/or at low scan rates, when the charge transfer at the interfaces and charge transport processes within the film are fast, i.e., electrochemically reversible (equilibrium) behavior prevails, and if no specific interactions (attractive or repulsive) occur between the redox species in the polymer film, a surface voltam-mogram like that shown in Fig. 3.1 can be obtained. 

The most important features of surfaee (thin-layer) voltammograms are related as follows  

If there are interactions between the surface species, the shapes of the voltammograms change, as shown in Fig. 3.2. 

The broadening and narrowing of the surface redox waves are linked to repulsive and attractive interactions. The numbers indicated for each curve are related to the interaction parameter of the Frumkin adsorption isotherm (g) g = 0 for the absence of interaction (Langmuir isotherm), g 0 and g 0 for the repulsive and attractive interactions, respectively. 

Cyclic voltammetry Square-wave voltammetry Staircase voltammetry Linear-sweep voltammetry Fast cyclic voltammetry Rotating disc voltammetry Stripping voltammetry Hydrodynamic voltammetry Direct current (d.c.) polarography Alternating current (a.c.) polarography Pulse polarography 

The majority of the procedures listed have been applied to HPLC-ED in research publications, although they are rarely employed in routine analysis. Therefore only those of relevance to HPLC-ED are discussed here. 

The anodic currents (oxidation) are plotted in the up direction and cathodic (reduction) in the downward direction. This is different from the frequently used polarographic convention (increasingly negative potentials from left to right and cathode currents in the up direction) but as in the majority of HPLC-ED applications the analyte is oxidised it makes the figure easier to understand. At point A the bulk solution contains only the reduced form (R) of the redox couple and there is no net conversion of R into the oxidised form (O). On increasing 

If a redox system remains in equilibrium throughout the potential scan, the EC reaction is said to be reversible. In other words, equilibrium requires that the surface concentrations of O and R are maintained at the values required by the Nemst equation. Under these conditions, the following parameters characterise the cyclic voltammogram of the redox process  

The positions of peak voltage do not alter as a function of voltage scan rate 

Cyclic voltammetry (CV) is a very popular technique for studying electro-chemically active species that scans the potential and observes the corresponding current. It can quickly identify at what potential(s) the species will be electrochemically active and how many redox processes the species will present. The potential scanning rate can range from 1 to 1000 mV S , although for most studies the scanning rate is lower than 50 mV s b 

Cyclic voltammetry of Pt/C in 0.5 M H2SO4 at 25°C and 50 mV s voltage scan rate. Courtesy of Dalian Institute of Chemical Physics, Chinese Academy of Sciences. 

Ft is around 0.78 V (in this particular case). Since 0.65-0.70 V is considerably lower than 0.78 V, most of the cathode catalyst surface should be in the Ft form during the operation of a fuel cell. But at OCV, the cathode catalyst will be dominant in the FtO form. Between 0.78 and OCV, the cathode catalyst will be in both the Ft and the FtO forms. 

Studies have shown that as the Ft particle size decrease the peak position for the reduction of FtO shifts to lower potentials, indicating that FtO forming on smaller particles is more difficult to be reduced back to Ft (Maillard 2009 and Hayden 2009). This will result in a relatively higher Ft surface not to be free of oxide layer at the fuel cell cathode operating voltage, leading to lower activity towards to ORR and thus the cathode performance. 

CO oxidative stripping. Courtesy of Dalian Institute of Chemical Physics, Chinese Academy of Sciences. 

Cyclic voltammetry is commonly used to study fuel cell electrodes and hydrogen crossover. In this technique, a linear sweep potential is applied to one electrode, while the other is held constant. The potential is cycled in a triangular wave pattern, while the current produced is monitored. The shape and magnitude of the current response provides useful quantitative and qualitative information regarding the amount of catalyst that is electro-chemically active, the double layer capacitance, hydrogen crossover, and the presence of oxide layers and contaminants. Wu et al. provide a description of this technique with example voltammograms [29]. 

Cyclic voltammetry is one of the most versatile electroanalytical techniques for the study of electroactive species (Bard and Faulkner, 19 Prentice, 1991). 

Cyclic voltammetry has the capability for rapidly observing redox behavior over a wide potential range. Cyclic voltammetry is generally used to study the electrochemical properties of an analyte in solution. 

The forward scan produces a current peak for any analytes that can be reduced through the range of the potential scanned. The current will increase rapidly, reaching peak value until the concentration of the anal) e at the electrode surface approaches zero. The current then decreases as the solution surrounding the electrode is depleted of analytes. When the applied potential is reversed, anodic current is generated as the electrode becomes 

A typical cyclic voltammogram showing reduction and oxidation current peaks. 

The cyclic voltammogram provides magnitudes of anodic peak current (ipa), cathodic peak current (ipj, anodic peak potential (Ep ), and cathodic peak potential (Epc). These parameters can be used to obtain information on the redox potential and detection of chemical reactions that precede or follow the electrochemical reaction and evaluation of electron transfer kinetics. 

In cyclic voltammetry, the potential applied to the working electrode is varied linearly (Fig. 2.1) between potentials Ex and E2, E2 being a potential more positive (for oxidation) or negative (for reduction) than the peak maximum observed for the oxidation/reduction reaction concerned. At E2, the voltage scan is reversed back to E3 or to another end potential value, E3. The application of this type of potential ramp can be done in a number of ways, varying the starting potential Eu the reverse potential E2, the end potential E3 and the scan rate. The latter is the rate that is applied to vary the potential as a function of time, commonly represented in Vs 1 or mVs 1. 

The resulting current measured while scanning the potential from Ex to E2 and back to the initial potential, Eu is shown in Fig. 2.2 for a reversible redoxsystem as a function of time, and in Fig. 2.3 in the more common way. Note that the scan rate (in this case lOmVs x) is the main relation between the way of data presentation in Fig. 2.2 and Fig. 2.3. 

The voltammogram shown in Fig. 2.3 is characterised by a peak potential Ev, a potential corresponding to the point where the measured current reaches it maximum value /p. For a reversible system, the peak current is given by  

1 Variation of the applied potential in cyclic voltammetry. Typical triangle wave between two potentials followed by an opposite triangle (.) or by another type of potential ramp ( ). 

2 Cyclic-voltammetry response of a reversible system represented as a function of time. 

For comparison we also show a cyclic voltammogram of a Au(lll) electrode (see Fig. 13.4). There is no detectable hydrogen adsorption region the hydrogen evolution reaction is kinetically hindered, and sets in with a measurable rate only at potentials well below the thermodynamic value. There is a much wider double-layer region in which other 

Simulaifnn L.carn more about cvelic voltammetry. 

FIGURE 25-24 (a) Potential versus time waveform and (b) cyclic voltammogram for a solution that is 6.0 mM in KjFeiCNjg and t. 0 M in KNOj. (From P T. Kissinger and W. H. Heineman. J. Chem. Educ.. 1983. 60, 702. Copyright 1983 Division of Chemical Education. Inc.) 

Quantitalivo information is obtained from the Randles-Sevcik equation, which at 2.5 C is 

FIGURE 25-25 Cyclic voltammogram of the insecticide parathion in 0.5 M pH 5 sodium acetate buffer in 50% ethanol. Hanging mercury drop electrode. Scan rate 200 m V/s. (From W. R. Helneman and P. T. Kissinger. Amer. Lab., 1982. no. 11.34. Copyright 1982 by International Scientific Communications. Inc.) 

The anodic peak at B arises from the oxidation of the hydroxylamine to a nitroso derivative during the reverse scan. The electrode reaction is 

The effect of charge transfer kinetics on cyclic voltammograms is apparent from the theoretical curves shown in Figs. 10 and 11. The 

Variation of cyclic voltammetry peak potential separation with the heterogeneous kinetic parameter i// 

Numerical calculations by Nicholson [26] provide a basis for the study of heterogeneous charge transfer using CV. Theoretical data indicate that both the shape of the waves and AEp depend upon a number of factors including a, k°, Ex and v. The current potential curves were derived in terms of a and a function t//, related to A by 

As long as E is 90/n mV beyond the peak, there is little effect of EK on AEP. Furthermore, AEp was observed to be nearly independent of a as long as the latter was in the range 0.3—0.7. The data are summarized in Table 12. 

Ahlberg and Parker observed that AAEp, equal to AEp — (A2 p)rev, is described by the function [59] 

FIGURE 2-1 Potential-time excitation signal in cyclic voltammetric experiment. 

FIGURE 2-2 T pical cyclic voltammogram for a reversible O + ne R redox process. 

FIGURE 2-3 Concentration distribution of the oxidized and reduced fomis of the redox couple at different times dming a cyclic voltammetric experiment corresponding to the initial potential (a), to the formal potential of the couple dming the foi ward and reversed scans (b, d), and to the achievement of a zero reactant smface concentration (c). 

In the case of an irreversible charge-transfer process the rate of electron transfer is insufficient to maintain the charge-transfer process at equilibrium. The shape of the cyclic voltammogram is modified and peak positions shift as a function of scan rate (unlike the reversible case). A more detailed discussion can be found elsewhere.93 

It must be remembered that in aqueous systems the redox process occurs over the entire electrode area, whereas in solid electrolyte systems the redox process occurs only in the three-phase or charge-transfer region. The technique has been used with solid electrolyte systems for sometime to study the oxidation and reduction of metals and metal oxides in inert atmospheres,94,95 the behaviour of solid oxide fuel cell (SOFC) electrodes and has also been applied to the in-situ study of catalysts.31,32,95 

Vayenas and co-workers31,32 used cyclic voltammetry to investigate oxygen adsorption and ethylene oxidation over a Pt catalyst-electrode. A cathodic 

The amount of oxygen adsorbed in the three-phase region was found to depend linearly with the exchange current density for different catalyst-electrodes under similar conditions. This indicates that the electrocatalytic reaction takes place at the three-phase boundary. It was pointed out that for less porous electrodes the charge-transfer reaction at the two-phase boundary might become important and that under some conditions oxygen on the electrolyte surface itself might play a role. 

Cyclic voltammetry was also used under conditions of ethylene oxidation.31 The rate of carbon dioxide production was seen to vary with the potential of the cell as would be expected from a system exhibiting NEMCA. Cyclic voltammetry was used to estimate the coverage of oxygen under working conditions by comparing the cathodic oxygen reduction peak with the peak obtained in the absence of reaction. 

Fabry and Kleitz used cyclic voltammetry to study the behaviour of copper dissolved in zirconia in the temperature range of 1000 - 1200 K. More recently, van Manen and co-workers have used cyclic voltammetry between 700 and 900°C to investigate the behaviour of a number of metal/metal oxide systems consisting of Fe/Fc203, Ni/NiO, Cu/CuO, C0/C02O3. Ni/NiO was used as the reference electrode. All electrodes were prepared as tablets from physical mixtures of metal and metal oxide. All of the systems were found to be irreversible. Peaks in the cyclic voltammogrammes were ascribed to formation of oxide and metal layers which acted as diffusional barriers at the surface of the electrodes. 

The electrochemical properties of the Ni(lll) electrode characterized in UHV were studied using Cyclic Voltammetry, a fairly common, but powerful technique for electrochemical studies °2. In a voltammetry experiment the current I through the working electrode was recorded while its potential was cycled in a sawtooth pattern. The experimental I/V curve is called a voltammogram. Features in a voltammogram due to charge transfer provide useful information of electrochemical reaction 

As has been shown in section 2.1.2, the design of the electrochemical cell used in this research was significantly different from conventional voltammetric cells . This was necessitated by the requirements of compatibility with the UHV-transfer system . A sketch of the cyclic voltammetry system is shown in Fig. 2.7. The linear potential sweep and the current measurements were supplied by a RDE 3 Potentiostat (Pine 

Instrument Company). The current was recorded on a x-y recorder as a function of potential applied to the working electrode with respect to the reference electrode. In most CV experiments the potential sweep rate was 0.1 V per second. No potential was applied to the working electrode right after the contact was made between the Ni(lll) working electrode and the electrolyte to avoid possible electrochemical reactions before the potential sweeping was begun. The open-circuit potential (OCP) was always measured first, then the potential sweep was initiated from the 

A UHV-electrochemical experiment is very complicated and consists of many steps. A brief description of the steps will be helpful in clarifying the experiments. 

The Ni(lll) to be used as the working electrode was cleaned and characterized by AES, TPD and LEED. Details about sample preparation will be provided later. 

Electrochemical properties were examined to gain more quantitative insight into the redox properties of this system. Cyclic voltammetry on bis(dithiazole) 23 in acetonitrile (with Pt electrodes and 0.1 M -Bu4NPF6 as supporting electrolyte) reveals a reversible oxidation wave with i/2(°x) = 0.93 V and a second, irreversible oxidation process 

A listing for known 1,2,3-dithiazoles of the half-wave potentials Ei/2(ox) of the first and second oxidation as well as the cathodic peak potential (Epc) for the reduction process is given in Table 1. 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

This technique is extremely useful experimentally as the resulting peakshaped signal provides a direct fingerprint of the features of the reduction and oxidation processes. Analysis of the position and shape of the peaks can give important information about the nature of the electrochemical process taking place and about the chemical species themselves. 

At time t = tgwitch, the potential reaches Ey and the potential sweep 

Assuming the kinetics of the electron transfer are fast relative to the rate of mass transport, Nernstian equUibrinm is attained at the electrode surface throughout the potential scan, and the Nernst equation therefore relates 

Schematic showing the distribution of particles (a) 0, (b) 1, (c) 5 and (d) 50 arbitrary time units after a potential pulse is applied to the electrode. White dots are the starting species, A, and black dots are the reduced species, B. Concentration profiles over the same space are also shown. 

There are basic electroanalytical characterization techniques that are consistently used to evaluate performance characteristics of BFCs. Standard electroanalytical techniques include linear sweep voltammetry, cyclic voltammetry, amperometry, and both galvanostatic and potentiostatic coulometry [1-5]. 

Voltammetry is a common electroanalytical technique for characterization of enzyme-modified electrodes. In cyclic voltammetry (CV), a potential window is scaimed in the forward and reverse directions while the resulting current is measured. This technique is useful for determining the reduction potential of the enzyme or coenzyme and for determining the overpotential for the system, which, in turn, corresponds to efficiency. Using this technique, detailed information about the catalytic cycle of the system can be determined including electron transfer kinetics, reaction mechanisms, current densities, and reduction potentials [6,7]. 

Before discussing the voltammogram obtained with the triangular waveform of figure 16.3, which is simply a plot of the observed current intensity versus the applied potential, it is useful to describe some experimental details of a cyclic voltammetry experiment [335-337] and to recall some basic theory of dynamic electrochemistry [180,332], A typical cell (figure 16.4) consists of 

Let us now suppose that the waveform of figure 16.3 is applied to study the reversible oxidation of a species R to R in a given solvent. The reaction occurs at the working electrode (anode), and /i°(R/R ) is the standard potential of the R/R- couple. Because the standard potential of the reference electrode in our cell is known accurately relative to the standard potential of the SHE (E° = 0 by definition), we can write the cell reaction and the Nernst equation as 

Note that we had to multiply the cell potential by — 1 because the net equilibrium 16.15 does not follow the accepted convention, that is, the SHE is not the anode. Equation 16.16 becomes 

It is important to stress that the activity coefficients (and the concentrations) in equation 16.18 refer to the species close to the surface of the electrode, and so can be very different from the values in the bulk solution. This is portrayed in figure 16.6, which displays the Stern model of the double layer [332], One (positive) layer is formed by the charges at the surface of the electrode the other layer, called the outer Helmholtz plane (OHP), is created by the solvated ions with negative charge. Beyond the OHP, the concentration of anions decreases until it reaches the bulk value. Although more sophisticated double-layer models have been proposed [332], it is apparent from figure 16.6 that the local environment of the species that are close to the electrode is distinct from that in the bulk solution. Therefore, the activity coefficients are also different. As these quantities are not 

The formal potential is the quantity determined from the analysis of a volta-mmogram, but the true thermodynamic quantity (the standard potential) can be derived by obtaining 0/(R/R ) for different bulk concentrations (c) and extrapolating to c = 0 (unit activity coefficients). The procedure is, however, seldom adopted in practice, (R/R-) is identified with the standard potential. The lower the concentration of the electroactive species, the better the assumption. 

As is well known, oxidation and/or reduction of ionic species in solution can occur on electrolysis. This is the basis of electroplating and, in reverse, the mechanism of action of many batteries. Controlled potential electrolysis as a preparative method was met in Section 4.2.3. Cyclic voltammetry is a method of studying such oxidation-reduction processes in detail. It is a method that has gained much in popularity in recent years since among other things it enables the measurement of thermodynamic redox potentials. The word cyclic in the name refers to the fact that if in the measurement A B is an electrolytic oxidation, in the measurement it is immediately afterwards followed by B A in an electrolytic reduction. This might sound rather like a pointless exercise but it, in fact, enables the fate of B to be probed immediately after it is formed. If it is found that there is less B to be reduced than there was B formed, experiments can perhaps be designed to 

These equations assume that the initial scan direction is positive as normally will be the case when studying an oxidation process. In (28) and (29) it is also assumed that the scan rate is the same in both the initial and reverse sweep directions which need not always be the case (the scan rate may be increased in the reverse scan in order to outrun homogeneous chemical steps associated with species formed by heterogeneous electron transfer in the forward scan). 

In cyclic voltammetric experiments, the sole form of mass transport to the electrode surface is diffusion, and in the case of large (millimetre dimensions) electrodes the diffusion of material to the electrode occurs in the single dimension perpendicular to the electrode surface. As will be discussed in Section 5 the situation is more complex for electrodes of smaller dimensions. 

Initially a simple reversible one-electron oxidation process is examined [see (19)], such as the oxidation of ferrocene to the ferricinium cation in acetonitrile/0.1 M (C4Hg)4NC104 (Sharp et al, 1980 Kadish et al, 1984). In (19), initially only A is present in solution. At the usual macrodisc electrode (radius in the millimetre range), material reaches the electrode by linear diffusion which is perpendicular to its surface (x-direction), and the concentrations of A and B may be obtained as a function of time by solving Fick s second law of diffusion as applied to species A and B, (30) and (31). 

However, the problem is subject to a number of boundary conditions which are defined in Table 4 (the symbols are described in the appendix). 

The time variation of the electrode potential is given by (28) and (29). Details of the solution of (30) and (31) are beyond the scope of this review, although a general approach for solving voltammetric problems is discussed in Section 7. 

A preliminary electrochemical overview of the redox aptitude of a species can easily be obtained by varying with time the potential applied to an electrode immersed in a solution of the species under study and recording the relevant current-potential curves. These curves first reveal the potential at which redox processes occur. In addition, the size of the currents generated by the relative faradaic processes is normally proportional to the concentration of the active species. Finally, the shape of the response as a function of the potential scan rate allows one to determine whether there are chemical complications (adsorption or homogeneous reactions) which accompany the electron transfer processes. 

Voltammetric techniques involve perturbing the initial zero-current condition of an electrochemical cell by imposing a change in potential to the working electrode and observing the fate of the generated current as 

Without any doubt, cyclic voltammetry is the most popular voltam-metric technique used in the field of inorganic chemistry. Unfortunately, the power of the technique is frequently overestimated in that simple cyclic voltammetric measurements rarely allow one to gain complete electrochemical information. As we will discuss, it must be always coupled with complementary techniques. 

Cyclic voltammetry belongs to the category of voltammetric techniques based on a linear potential sweep chronoamperometric technique. It certainly constitutes the most useful technique for a preliminary determination of the redox properties of a given species. 

The experiment is carried out under stationary conditions (i.e. the solution is kept unstirred) in order to ensure that the mass transport is purely diffusive. 

With the scanning of the potential, concentration waves extend into the electrolyte. The concentration gradients at the electrode surface go through maxima with characteristic current peaks in the current potential diagram. The Randles-Sevcik equation describes the dependence of peak currents on concentration, Cq and scan rate, v 

All cyclovoltaimnetric experiments were performed with HEKA PG 284 (Germany) potentiostat under argon using a standard three-electrode arrangement of a platinum wire as woildng electrode, a platinum coil as counter electrode, and a Ag/AgCl as reference electrode. The scan rate used was 100 mV/s. 

Frank Marken, Andreas Neudeck, Alan M. Bond 

Although one of the more complex electrochemical techniques [1], cyclic voltammetry is very frequently used because it offers a wealth of experimental information and insights into both the kinetic and thermodynamic details of many chemical systems [2]. Excellent review articles [3] and textbooks partially [4] or entirely [2, 5] dedicated to the fundamental aspects and applications of cyclic voltammetry have appeared. Because of significant advances in the theoretical understanding of the technique, today, even complex chemical systems such as electrodes modified with film or particulate deposits may be studied quantitatively by cyclic voltammetry. In early electrochemical work, measurements were usually undertaken under equilibrium conditions (potentiometry) [6] where extremely accurate measurements of thermodynamic properties are possible. However, it was soon realised that the time dependence of signals can provide useful kinetic data [7]. Many early voltammetric studies were conducted on solid electrodes made from metals such as gold or platinum. However, the complexity of the chemical processes at the interface between solid metals and aqueous electrolytes inhibited the rapid development of novel transient methods. 

To illustrate the uses, benefits, and pitfalls of cyclic voltammetry, an example of a system with complex chemical processes being disentangled step by step based on a systematic simulation procedure may be given [14] (Fig. II.l.l). Prenzler et al. studied the scan rate, pH, and concentration dependence of the reduction of a solution of polyoxoanion, [P2Wig062] (see Fig. II.l.l a), in aqueous media by cyclic voltammetry (Fig. II.l.lb, c) and determined the equilibrium constants for the protonation processes (see Fig. Il.l.ld). 

Additional disproportionation and cross-redox reactions associated with the redox system described in Fig. Il.l.ld (Eqs. II.l.l and II.1.2) are difficult to monitor directly by cyclic voltammetry but still may have subtle effects on cyclic voltammetric data. These so-called thermodynamically superfluous reactions [15] can be derived from the voltammetric data because the equilibrium constant data can be calculated from the formal potentials and protonation equilibrium constants. For the resolution of this type of complex reaction scheme, data obtained over a wide range of conditions and at different concentrations are required. 

In these equations A is reduced in a one-electron process to the product A , which in a fast equilibrium process forms B. An equilibrium constant K4 = 1000 and rate constant = 10 s for the forward direction of the chemical reaction step were also included in these simulations and it can be seen that the shape and peak-to-peak separation change characteristically with the k value. However, essentially the same set of cyclic voltammograms (only offset in potential) 

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Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

Allemand P-M, Koch A, WudI F, Rubin Y, Diederich F, Alvarez M M, Anz S J and Whetten R L 1991 Two different fullerenes have the same cyclic voltammetry J. Am. Chem. Soc. 113 1051-2... 

By analogy to additions of divalent carbon to the Cio aromatic framework, the molecule Cgi was expected to have the norcaradi-ene (II) or the cycloheptatriene (III) structure. Although an X-ray structure was not available, the UV-visible spectrum, NMR spectrum, and cyclic voltammetry supported the cycloheptatriene (III) structure. The researchers then calculated the relative molecular mechanics energies of II and III and found the cycloheptatriene structure stabilized by 31 kcal/mol with respect to the norcaradi-ene structure. Although the calculations do not confirm the structures, they provide additional supporting evidence. 

Porphyrin, octaethyl-, aluminum hydroxide complex cyclic voltammetry, 4, 399 <73JA5140)... 

Flavin adenine dinucleotide (FAD) has been electropolymerized using cyclic voltammetry. Cyclic voltammograms of poly (FAD) modified electrode were demonstrated dramatic anodic current increasing when the electrolyte solution contained NADH compare with the absence of pyridine nucleotide. 

Measurement of (R /R ) can be accomplished by cyclic voltammetry for relatively Stable species and by other methods for less stable cations. The values obtained for AG -range from 83kcal/mol for the aromatic tropylium ion to 130kcal/mol for destabilized betizylic cations. For stable carbocations, the results obtained by this method correlate with cation stabiUty as measured by pKf.+. Some of these data are presented in Table 5.3. 

There is some evidence that Cs + can be formed by cyclic voltammetry of Cs+[OTeF5] in pure MeCN at the extremely high oxidizing potential of 3 V, and that Cs + might be stabilized by 18-crown-6 and cryptand (see pp. 96 and 97 for nomenclature). However, the isolation of pure compounds containing Cs + has so far not been reported. 

Julid investigated the behavior of terfuran 22 and bis(thienyl)furan 23 by cyclic voltammetry as well as the EPR spectra of the radical cations derived from these two compounds. Condensation of the diketone 20 with sulfuric acid furnished furan 22 in 18% yield, while reaction of diketone 21 with hydrochloric acid produced 23 in 84% yield.In a related report, Luo prepared oligomeric bis(thienyl)furans via similar methodology. ... 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

Tire deprotonation of thiazolium salts (see Section II) under argon at room temperature allowed the characterization of nonfused DTDAF of types 52 and 53 by cyclic voltammetry. Their very good donor properties were confirmed by two quasi-reversible peaks of equal intensity (93CC601). It is noteworthy that upon a second scan the first oxidation peak was shifted from -0.03 to -0.04 V. Upon further scans the voltam-mogram remains unchanged. Tliis interesting feature has been observed previously with TTF analogs. It was demonstrated that the neutral form... 

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