Electroactive layers cyclic voltammetry

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

The concentration profile of fixed oxidized and reduced sites within the film depends on the dimensionless parameter Dcjr/d2, where r is the experimental timescale, i.e. RT/Fv in cyclic voltammetry, and d is the polymer layer thickness. When Dcix/d2 1, all electroactive sites within the film are in equilibrium with the electrode potential, and the surface-type behavior described previously is observed. In contrast, Dcjx/d2 <3C 1 when the oxidizing scan direction is switched before the reduced sites at the film s outer boundary are completely oxidized. The wave will exhibit distinctive diffusional tailing where these conditions prevail. At intermediate values of Dcjr/d2, an intermediate ip versus v dependence occurs, and a less pronounced diffusional tail appears. 

After a description of how to control the sweep experiment and its two forms, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) (where the sweep direction is inverted at a certain, chosen potential), the voltammetric waveshape obtained for slow and fast electrode reactions is analysed. Recent advances in these topics are considered. Finally, the type of curve obtained from linear sweep in a thin-layer cell is presented thin-layer cells are important because they permit almost 100 per cent conversion of the electroactive species, and show differences in relation to electrochemical behaviour in a normal-sized cell. 

Limited diffusion — applies to a thin film or a -> thin layer cell. Limited diffusion causes a decrease of the current to zero at long times (i.e., the - Cottrell equation will then not any more be followed - chronoamperometry) or at the voltammetric waves (- cyclic voltammetry) because there is not an infinite reservoir of electroactive species. (See also - electrochemical impedance spectroscopy.)... 

Therefore the electrochemical response with porous electrodes prepared from powdered active carbons is much increased over that obtained when solid electrodes are used. Cyclic voltammetry used with PACE is a sensitive tool for investigating surface chemistry and solid-electrolyte solution interface phenomena. The large electrochemically active surface area enhances double layer charging currents, which tend to obscure faradic current features. For small sweep rates the CV results confirmed the presence of electroactive oxygen functional groups on the active carbon surface. With peak potentials linearly dependent on the pH of aqueous electrolyte solutions and the Nernst slope close to the theoretical value, it seems that equal numbers of electrons and protons are transferred. 

Surface excesses of electroactive species are often examined by methods sensitive to the faradaic reactions of the adsorbed species. Cyclic voltammetry, chronocoulometry, polarography, and thin layer methods are all useful in this regard. Discussions of their application to this type of problem are provided in Section 14.3. In addition to these electrochemical methods for studying the solid electrode/electrolyte interface, there has been intense activity in the utilization of spectroscopic and microscopic methods (e.g., surface enhanced Raman spectroscopy, infrared spectroscopy, scanning tunneling microscopy) as probes of the electrode surface region these are discussed in Chapters 16 and 17. 

Another popular mode for transmission experiments involves a thin-layer system (9, 10, 13, 18) like that shown in Figure 17.1.2. The working electrode is sealed into a chamber (e.g., between two microscope slides spaced perhaps 0.05-0.5 mm apart) containing the electroactive species in solution. The chamber is filled by capillarity, and the solution within it contacts additional solution in a larger container, which also holds the reference and counter electrodes. The electrolytic characteristics of the cell are naturally similar to those of the conventional thin-layer systems discussed in Section 11.7. One can do cyclic voltammetry, bulk electrolysis, and coulometry in the ordinary way, but there is also a facility for obtaining absorption spectra of species in the cell. 

The high sensitivity of the ER method benefits bioelectrochemists in the detection of the redox reaction of the electron transfer proteins. Even for an adsorption monolayer of proteins, the superficial density of the electroactive center is much smaller than that of small molecules, especially when the molecular weight of the protein is several kilo-Daltons. The redox reaction of adsorbed protein buried in the double-layer charging current in the voltammogram can be detected by the ER method. Ikeda and coworkers succeeded in the clear observation of the ER spectrum and ER voltammogram of a heme c in an adsorbed protein (alcohol dehydrogenase) of ca. 140 kD containing hemes and PQQ, while direct redox reaction could not be detected by cyclic voltammetry [90]. 

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. 

Cyclic Voltammetry in a Thin Layer of Redox-Containing Electroactive Species... 

Cyclic voltammetry (CV) has found widespread application in investigating and characterizing modified electrode processes. " Characterizing modifying layers under conditions of thin layer behavior has received particular attention. In the absence of diffusional limitations and under conditions of complete oxida-tion/reduction of electroactive centers, thin layer/surface-type behavior prevails. The ideal model for voltammetric behavior under such conditions was considered, and the following features are characteristic ... 

Cyclic voltammetry (CV) was carried out on PANi-CNTs composite fibers as part of the mechanical actuator testing (Figure 8). The resolved CV ensure ion and charge transfer required for actuation performance. To improve the conductivity of fibers, very thin Pt layer coated around the fibers. Much improved CVs were obtained with the Pt covered libers. The influence of CNTs on the electroactivity of Pt covered PAni fibers in IM HCl aqueous solution is illustrated in. It can be clearly seen that the addition of CNTs enhanced the amplitude of the redox peaks, indicating a higher electro-activity compared to neat PANi fibers. Peak potentials also shifted with the addition of CNTs which can be attributed to interaction between PAni and CNTs. Three oxidation... 

Other redox-active polyelectrolyte films were prepared from ferrocene-derivatized polly(allylamine) and poly(vinyl pyridine) as well as an osmium complex of poly(vinyl pyridine) [44-46]. These films were synthesized to mediate electron transfer between the electrode and a charged enzyme that was a constituent of the polyelectrolyte film. In the case of ferrocene-derivatized poly(allylamine) or polyfvinyl pyridine), cyclic voltammetry of the bound ferrocene moiety showed small peak splittings (<50 mV at a scan rate of 50 and 20 mV s respectively) [45, 46). The amount of electroactive material increased with the number of deposited layers, but the first layer contained significantly more electroactive ferrocene than the later layers in the poly(allylamine) system [46]. 

Abstract Clay-modified electrodes can provide an efficient method for studying the porosity of negatively charged, layered stuctures, i.e., swollen clay films. Diffusion transport processes of electroactive solute probes within hydrophilic and hy-drophobized montmorillonite clay films have been studied. Cyclic voltammetry was performed in a three-electrode cell. Results regarding film permeability, the structure of the porous aerogel-hydrogel, the effect of layer thickness as well as the role... 

When diffusion layers overlap by a large amount, an overall planar response will be expected, but with a characteristic area equivalent to the total array surface area rather than just the electroactive surface area. Hence, the Case 4 current will be (1/ ) times larger than the Case 1 current. This will occur when X(jiff d where d is the separation of the individual microdiscs. Therefore, Case 4 behaviour arises at t 0.1 s. This will therefore be the dominant behaviour for cyclic voltammetry at normal scan rates at this particular array. With chronoamperometry, short timescales are accessible and so Case 3 behaviour may also be observed. 

The pioneering work of Delamar et al. [14] demonstrated the coating of glassy carbon by an organic layer of 4-nitrophenyl by the electrochemical reduction of (4-nitrophenyl)diazonium tetrafluoroborate (typical concentration 1-10 mM) from an ACN solution. Cyclic voltammetry shows two reduction phenomena in the first scan. The first peak centered at —0.04 V versus SCE is irreversible and corresponds to the diazonium reduction, whereas the second one is reversible and appears at a more cathodic potential (—1.20 V vs SCE) that is related to the electroactivity of the nitro group. A second voltammogram recorded on the same electrode shows... 

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