Voltammetry slow-scan

Cyclic voltammetry shows the Cu(i)/Cu(0) couple to be highly reversible in dimethylformamide (DMF) even at very slow scan rates. Thus, dark blue solutions of the formally Cu(0) complex can be electrochemically... 

As already stated, other electrochemical techniques have been used to derive thermodynamic data, some of them considered to yield more reliable (reversible) redox potentials than cyclic voltammetry. This is the case, for instance, of second harmonic alternating current voltammetry (SHACV) [219,333], Saveant and co-workers [339], however, concluded that systems that appear irreversible in slow-scan CV are also irreversible in SHACV experiments. We do not dwell on these matters, important as they are. Instead, we concentrate on a different methodology to obtain redox potentials, which was developed by Wayner and colleagues [350-352]. 

Figure 48. Anodic stability as measured on a spinel LL-Mn204 cathode surface for EMS-based electrolytes (a) Lilm (b) LiC104 (c) LiTf. In all cases, 1.0 m lithium salt solutions were used, and slow scan voltammetry was conducted at 0.1 mV s with lithium as counter and reference electrodes and spinel LiJV[n204 as working electrode. (Reproduced with permission from ref 75 (Figure 3). Copyright 1998 The Electrochemical Society.)...

Figure 51. Cathodic and anodic stability of LiBOB-based electrolytes on metal oxide cathode and graphitic anode materials Slow scan cyclic voltammetry of these electrode materials in LiBOB/EC/EMC electrolyte. The scan number and Coulombic efficiency (CE) for each scan are indicated in the graph. (Reproduced with permission from ref 155 (Eigure 2). Copyright 2002 The Electrochemical Society.)...

Figure 56. Slow scan (10 /iV s ) voltammetry on a graphite working electrode (a) 1.0 M LiFAP in EC/DEC/ DMC (b) LiPEe in EC/DEC/DMC. Solid line pristine graphite. Dashed line after 1 week of cycling. (Reproduced with permission from ref 499a (Figure 4). Copyright 2003 The Electrochemical Society.)...

Comparable or larger errors are introduced by unwanted convective mass transport. Convection is caused by physical motion of the solution, sometimes purposefully introduced for techniques such as rotating electrode voltammetry. When a quiet solution is desired, however, convective errors may arise at longer experiment times (slow scan rates) from mechanical vibrations of the solution. Convection is a particular problem for cells inside inert-atmosphere boxes, on which fans and vacuum pumps may be operative. Convection raises the current... 

The EQCM has been most commonly used simultaneously to quasisteady state techniques like slow scan cyclic voltammetry. In this way mass changes during electrolysis can be obtained from A/(Am/.4) vs. potential curves, while A/(AmA4) vs. charge density curves allow evaluation of the number of Faraday exchanged per mole of electro-active species by use of Faraday s law of electrolysis. 

Although electrochemical characterizations have recently been performed on single intercalation particles, in most cases composite powdery electrodes containing a mixture of intercalation particles, electrically conductive additives (e.g., carbon black) and PVDF binder have also been used. In order to obtain consistent results and to reach comprehensible intercalation mechanisms in these electrodes, basic electroanalytical characterizations such as slow-scan rate -> cyclic voltammetry (SSCV), -> potentiostatic intermittent titration (PITT) (or -> galvanostatic intermittent titration, GITT), and -> electrochemical impedance spectroscopy (EIS) should be applied in parallel or in a single study. 

Obviously, therefore there must be an intermediate case in which the kinetics of both the forward and reverse electron-transfer processes have to be taken account of. Such systems are described as being quasi-reversible and as would be expected, the scan rate can have a considerable effect on the nature of the cyclic voltammetry. At sufficiently slow scan rates, quasi-reversible processes appear to be fully reversible. However, as the scan rate is increased, the kinetics of the electron transfer are not fast enough to maintain (Nernstian) equilibrium. In the scan-rate region when the process is quasi-reversible, the following observations are made. 

Electroanalyhcal techniques (also in combination with other techniques, e.g., ophcal techniques such as photometry and Raman spectrometry) can be employed to inveshgate many functional aspects of proteins and enzymes in particular. It is possible to study the biocatalytic process with respect to the chemistry of the active site, the interfacial and intramolecular ET, slow enzyme achva-tors or inhibitors, the pH dependence, the transport of tlie substrate, and even more parameters. For example, slow scan voltammetry can be used to determine the relation of ET rates or of protonation and ligand binding. In contrast, fast scan voltammetry allows the determination of rates of interfacial ET. In addition, it is also possible to investigate chemical reactions that are coupled to the ET process, such as protonation. The use of direct ET for mechanistic studies of redox enzymes was recently reviewed by Leger and Bertrand [27]. Mathemahcal models help to elucidate the impact of different variables on the enhre current signal [27, 75, 76]. 

A similar response is obtained from the cyclic voltammetry (CV) of [Ru(NH3)5(dmso)]3+ at slow scan rates (<100 mV s-1) (Fig. 3C). However, at higher scan rates, since the scanning rate of CV is faster than the rates of the linkage isomerizations, both complexes display reversible cyclic voltammo-grams for [Ru(NH3)5(dmso)]2+ at 1 V s-1, a couple is observed at 0.97 V, while for [Ru(NH3)5(dmso)]2+ at 20 V s-1, a couple is observed at 0.07 V (Fig. 3B and 3D). This linkage isomerization scheme is illustrated in Fig. 5. 

Cyclic voltammetry studies reveal striking differences between complex 13 and the analogous complex [HFe(depp)(dmpm)(CH3CN)f (17) in which the NMe group of 13 has been replaced by a methylene group. At normal scan rates the Fe " couple is reversible for complex 17, but irreversible for 13. Scan rate dependence measurements and potential step experiments indicated that this difference in behavior arises from a rapid transfer of the proton of the Fe hydride to the N atom of the pendant base with a rate constant of 1.1 x 10 s at room temperature. This proton transfer results in an irreversible Fe " " couple at low scan rates. A similar process cannot occur for 17, and the Fe " " couple remains reversible, even at slow scan rates in the presence of an external base. These results indicate that pendant bases in the second coordination sphere can facilitate the coupling of electron and proton transfer reactions. 

When the formal potentials of the adsorbed and non-adsorbed redox couples are similar, a single wave is obtained where the contribution of the electron transfer involving the adsorbed species increases with the scan rate. Thus, there is a transition from diffusional-shaped voltammograms at slow scan rates to adsorptive-shaped at fast scan rates. To understand this behaviour and illustrate the characteristics of adsorptive voltammograms let us consider the response in cyclic voltammetry of a monolayer of species A that undergoes a one-electron, fully reversible electron transfer ... 

This classification scheme may be extended to a fifth case [3]. In Case 5, the experimental time scale is very long (very slow scan rate) so that diffusion to the entire substrate is convergent, giving sigmoidal voltammetry. This behaviour is most readily observed experimentally when the supporting substrate itself is of microscale and the electroactive particles are of sub-micron size. 

With very fast scan rate cyclic voltammetry, an upper limit of the scan rate where standard theory prevails is given by the condition that the diffusion layer becomes equal in size to the diffuse layer (see Fig. II. 1.8). It has been estimated that this limit occurs at a scan rate of 1-2 x 10 V s [45]. In the other extreme, at very slow scan rates, natural convection is known to affect the shape of experimental cyclic voltammograms. 

Cyclic voltammetry can be used directly to establish the initial redox state of a compound if data analysis is applied in a careful manner [70]. In Fig. II.1.15, simulated and experimental cyclic voltammograms are shown as a function of the ratio of Fe(CN)g and Fe(CN) present in the solution phase. It can clearly be seen that the current at the switching potential, ix,a or fyc is affected by the mole fraction /Hred- Employing multi-cycle voltammograms at slow scan rates is recommended. Quantitative analysis of mixed redox systems with this method may be based on the plot shown in Fig. II. 1.15d. 

Very informative and reliable data are available from complementary steady-state voltammetric techniques, such as rotating disc voltammetry or slow scan rate microelectrode voltanunetry. Preliminary data may be obtained even by simply stirring the solution phase during the course of a cyclic voltammetric experiment. In Fig. II. 1.16... 

Figure 5A shows the sigmoidal-shaped responses that characterize steady-state mass transfer in slow scan-rate cyclic voltammetry. In contrast, as illustrated in Figure 5B, at short experimental timescales (high scan rates), peaked responses similar to those observed at conventional macroelectrodes are seen. 

The activity and the stability of the added OER catalysts were evaluated via quasisteady state polarization measurements. Slow scan cyclic voltammetry (CV) was performed from 0.7 V to the upper voltage limit which was sequentially increased in increments of 0.05 V from 1.45 to 1.65 V. Three consecutive scans for each upper potential limit were recorded. In Fig. 22.8, some of the characteristic polarization scans that best illustrate the OER on Ru and Ir overcoated on Pt-NSTF are presented. Up to the first voltage limit of 1.45 V, only the added Ru produced a substantial... 

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