Cyclic voltammetry, oxidation potential

In this section, we will present and discuss cyclic voltammetry and potential-step DBMS data on the electro-oxidation ( stripping ) of pre-adsorbed residues formed upon adsorption of formic acid, formaldehyde, and methanol, and compare these data with the oxidative stripping of a CO adlayer formed upon exposure of a Pt/ Vulcan catalyst to a CO-containing (either CO- or CO/Ar-saturated) electrolyte as reference. We will identify adsorbed species from the ratio of the mass spectrometric and faradaic stripping charge, determine the adsorbate coverage relative to a saturated CO adlayer, and discuss mass spectrometric and faradaic current transients after adsorption at 0.16 V and a subsequent potential step to 0.6 V. 

Aliphatic ketones are oxidised in both acetonitrile [1,2] and trifluoracetic acid [3] at potentials less positive than required for the analogous hydrocarbons. The oxidation process is irreversible in both solvents and cyclic voltammetry peak potentials are around 2.7 V V5. see. Loss of an electron from the carbonyl oxygen lone pair is considered to be the first stage in the reaction. In acetonitrile, two competing processes then ensue. Short chain, a-branched ketones cleave the carbon-carbonyl bond to give the more stable carbocation, which is then quenched by reaction with... 

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. 

Panizza, M. and Cerisola, G. (2003b) Influence of anode material on the electrochemical oxidation of 2-naphthol. Part 1. Cyclic voltammetry and potential step experiments. Electrochim. Acta 48, 3491-3497. 

The redox potential for the dye can shift upon adsorption from solution due to coulombic or stronger covalent interactions with the solid substrate. This potential change can amount to several hundreds of millivolts. While n-type semiconductors cannot be used generally to measure oxidation potentials of adsorbed dye sensitizers by conventional cyclic voltammetry, reduction potential i°(S/S ) is often more accessible. Assuming oxidation and reduction potentials of the dye ground state on the surface are linked by a constant relation ... 

The combination of the high sensitivity of SEIRAS and a rapid-scan FT-IR spectrometer enables the spectral collection simultaneously with electrochemical measurements such as cyclic voltammetry and potential-step chronoamperome-try. The time-resolved measurement can give some information on electrode kinetics and dynamics, as has been shown in Fig. 8.24. Figures 8.25 and 8.26 represent another example of millisecond time-resolved ATR-SEIRAS study of current oscillations during potentiostatic formic acid oxidation on a Pt electrode [123]. At a constant applied potential F of 1.1 V, the current oscillates as shown in Fig. 8.25 a. Synchronizing with the current oscillations, the band intensities of linear CO and formate oscillate as shown in Fig. 8.26 (and also in Fig. 8.25 c). 

Another comphcation arises from the fact that, in contrast to conventional electrodes, CP-coated electrodes often undergo changes in their oxidation state and structure in the course of metal electrodeposition. This is usually the case when driving metal-ion reduction by means of the most fiequently applied electrochemical techniques - cyclic voltammetry, multistep potential procedures, repetitive square-wave potential (or pulse potentiostatic) techniques, and galvanostatic reduction (Figure 7.3). In spite of the difficulty in... 

In cyclic voltammetry, a potential range is swept in forward and reverse direction once or several times. Figure 5.17 schematically shows the cyclic voltammogram of an electrode reaction involving dissolved species such as for example Fe " and Fe " ions. In the anodic sweep Fe is oxidized to Fe, in the reverse direction theFe " is reduced yielding a cathodic current. In simple cases, the shape of the voltammogram and the difference between the potential maxima allow one to identity the reaction mechanism [8], but in corrosion, these criteria are usually of limited usefiilness and shall therefore not be further discussed here. 

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. 

If we check all the polymer films, after synthesis, in the background solution by cyclic voltammetry (or potential steps), choosing adequate potential limits (or anodic and cathodic potentials), the electrical charge needed to oxidize or reduce the polymer film can be obtained. The variation of the redox charges in polymer films obtained at different polarization times, after substituting Rp from equation (10.2), 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. 

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. 

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). 

Figure 12-8 summarizes the information available as far as the HOMO/LUMO positions of the compounds is concerned. Being inferred from oxidation/rcduction potentials measured by cyclic voltammetry in polar solution and from HOMO/ LUMO gaps, respectively, absolute values should be viewed with some caution. 

For PPV-imine and PPV-ether the oxidation potential, measured by cyclic voltammetry using Ag/AgCl as a reference are ,M.=0.8 eV and 0.92 eV, respectively. By adopting the values 4.6 eV and 4.8 eV for the work functions of a Ag/AgCl and an 1TO electrode, respectively, one arrives at zero field injection barriers of 0.4 and 0.55 eV. These values represent lower bounds because cyclic voltammetry is carried out in polar solvents in which the stabilization cncigy of radical ions exceeds that in a polymer film, where only electronic polarization takes place. E x values for LPPP and PPPV are not available but in theory they should exceed those of PPV-imine and PPV-ether. 

Cyclic voltammetry is most commonly used to investigate the polymerization of a new monomer. Polymerization and film deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potentials below the onset of monomer oxidation. A nucleation loop, in which the current on the reverse scan is higher than on the corresponding forward scan, is commonly observed during the first cycle.56,57 These features are all illustrated in Fig. 3 for the polymerization of a substituted pyrrole.58... 

One of the most problematic issues, still to be fully resolved, is the dependence of the degree of oxidation on potential, as measured most commonly by cyclic voltammetry at low scan rates. There is currently no accepted model to describe the shape of the curve and the hysteresis between anodic and cathodic scans. The debate over whether the charge has a significant component due to a polymer/solution double layer is still not fully resolved. 

Fig. 18. pH dependence of the oxidized Rieske fragment from bovine heart mitochondria (ISF). (a) Redox potential determined by cyclic voltammetry. The line was fitted to the data points, giving = 7.6 and pi a, x2 = 9-2. (b) CD intensity of the oxidized... 

The coordination of redox-active ligands such as 1,2-bis-dithiolates, to the M03Q7 cluster unit, results in oxidation-active complexes in sharp contrast with the electrochemical behavior found for the [Mo3S7Br6] di-anion for which no oxidation process is observed by cyclic voltammetry in acetonitrile within the allowed solvent window [38]. The oxidation potentials are easily accessible and this property can be used to obtain a new family of single-component molecular conductors as will be presented in the next section. Upon reduction, [M03S7 (dithiolate)3] type-11 complexes transform into [Mo3S4(dithiolate)3] type-I dianions, as represented in Eq. (7). 

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