Double layer charging cyclic voltammetry

The Cyclic Voltammetry Experiment. Faradaic and Double-Layer Charging Currents. Ohmic Drop... 

Double-Layer Charging in Cyclic Voltammetry. Oscillating and Nonoscillating Behavior... 

With the advent of digital implementation of electroanalytical experiments came the technique called staircase cyclic voltammetry, wherein the triangular wave is approximated by a series of small potential steps. The reason for such an approximation is partly due to the impossibility of digitally generating a pure ramp and more importantly to the realization that substantial improvements accrue from sampling the current at the end of each step, where double-layer charging has decayed away. If the steps are small, the data will fit theory based on pure ramps quite well. 

In this respect, this review provides a comprehensive survey of synthetic methods and physicochemical properties of the porous carbon materials. Furthermore, as electrochemical applications of the porous carbons to electrode materials for supercapacitor, the effects of geometric heterogeneity and surface inhomogeneity on ion penetration into the pores during double-layer charging/ discharging are discussed in detail by using ac-impedance spectroscopy, current transient technique, and cyclic voltammetry. 

Fig. 3L The optimum range of concentration and sweep rate (hatched area) for measurements in cyclic voltammetry on a smooth electrode. The double-layer-charging current i has been calculated for - 20 yiFIcm.

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. 

The retardation of the bimolecular reaction on the surface provided a more convenient time domain for observation. 2) Low temperature experiments (-78°C in n-butyronitrile) were easy to perform and allowed a key intermediate to be observed. 3) It was possible to use faster cyclic voltammetry sweep rates (up to 200 V/s) for surface couples because their peak current variation with sweep rate has the same functional dependence (ip v) as for double layer charging currents while for solution couples, ip v 2... 

Another electrochemical technique being used in protein studies is square-wave voltammetry. The usefulness of this method is often ascribed to its ability to factor out the double-layer charging current but for protein molecules confined to a film, the advantages are the increased sensitivity and additional kinetic information that can be obtained by varying the frequency and pulse amplitude. The data are more difficult to extract than cyclic voltammetry, and we will not attempt to elaborate on this aspect, although studies that have included square-wave voltammetry will be mentioned later in this chapter. 

Slow ET between redox species confined to two immiscible solvents was first observed by Guainazzi et al. (18). Several different theoretical and experimental studies of ET between redox species at the ITIES have been reported in the last several years (6-8,15,19-21). Severe experimental problems complicate extraction of the kinetic parameters from conventional electrochemical measurements at the ITIES (e.g., by cyclic voltammetry). Besides the difficulty of discrimination between ET and IT, there are also distortions from the double-layer charging current and /R-drop in the highly resistive nonaqueous solvents, and the limited potential window for studying ET in the absence of currents controlled by IT (1,16). 

DEMS allows the rate of the anodic half-reaction of formaldehyde oxidation to be measured online with a higher accuracy in comparison with cyclic voltammetry, since the contribution of the double-layer charging and the electrode surface oxidation/reduction features (for instance, oxide... 

The great advantage of the RDE over other techniques, such as cyclic voltammetry or potential-step, is the possibility of varying the rate of mass transport to the electrode surface over a large range and in a controlled way, without the need for rapid changes in electrode potential, which lead to double-layer charging current contributions. 

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 surface electrochemistry of Pt single-crystal electrodes has been exhaustively studied using cyclic voltammetry [5, 8-12, 61-67]. Phenomena of a step reconstruction and step coalescence have been observed [61]. For Pt(lll)-H20 interface, prepared by the flame annealing method, a double-layer charging has been observed only in a very narrow potential region (0.1 < E < 0.35 V (SCE) in 0.05 M H2SO4), which depends on the chemical... 

Since the early 1980s, cyclic voltammetry has been used to characterize single-crystal electrodes in terms of surface order, presence or absence of defects contaminations, and so on. In some cases the voltammo-grams have also been used for identification of adsorbates, mostly on the basis of electric charge calculation. However, since the double-layer charging and surface reactions such as UPD and anion adsorption may occur in parallel, it is difficult to break down the total voltammetric charge into all of its individual components. Therefore, interpretation from voltammetry alone, used as a tool for the adsorbed species identification, may be ambiguous. This has been, for instance, the case with the interpretation of the unusual adsorption states on the Pt(lll) electrode (73-82). 

Figure 7. Typical cyclic-voltammetry curve for anodic and cathodic directions of polarization in the case of a diffusion-controlled reversible redox process. /di is the double-layer charging current density and EpS the anodic and cathodic peak potentials. Inset shows potential E—time program applied to the electrode through a potentiostat used in a potentiodynamic configuration. The diagram is schematic.

EXPERIMENTAL PROBLEMS ASSOCIATED WITH LINEAR SWEEP AND CYCLIC VOLTAMMETRY 6.7.1 Double layer charging effects... 

In a comprehensive study on the role of Ru oxides, PtRu catalysts were synthesized by a modified Adams-Shriner method using various temperatures between 673 K and 873 K [113]. The composition and particle size (1 1 atomic ratio and 2-4 nm, respectively), were virtually independent of the synthesis temperature. However, the crystalline RUO2 content increased with temperature, at 873 K all the Ru was present as RUO2 [113]. The lowest overpotential for methanol oxidation was measured for the catalyst synthesized at an intermediate temperature (743 K), which also exhibited the highest capacitive behavior in cyclic voltammetry indicative of double layer charging due to the hydrous Ru oxide phase. Therefore, the presence of mixed phases including PtRu alloy and Ru hydrous oxide were found to be beneficial for good catalytic activity toward methanol oxidation [113]. 

Figure 21.9. Typical CV curve on Pt/C catalyst. Q and Q represent the amount of charge exchanged during the electroadsorption and desorption of H2 on Pt sites, and the filled area is the contribution of the double-layer charge [52]. (Reprinted fi om Journal of Power Sources, 105, Pozio A, De Franeeseo M, Cemmi A, CardeUini F, Giorgi L. Comparison of high surface Pt/C catalysts by cyclic voltammetry, 13-19, 2002, with permission fi-om Elsevier.)...

Features in Figure 3.16 that correspond to Pt oxide formation and reduction are labeled and explained in the pictorial legend. It should be noted for the illustrated CV that the surface processes do not involve any supplied reactants. They correspond to the transformation of interfacial water molecules into surface species and vice versa. A CV shows the amount of electronic charge withdrawn from the metal surface in anodic scan direction (oxidative current, j > 0) or transported to it in cathodic scan direction (reductive current, < 0). The electrical charge flux generated or consumed at the interface is the result of double layer charging and faradaic processes. An interesting aspect of cyclic voltammetry is that the surface is never in a steady state. The potential is continuously ramped up and down, at a constant scan rate, Vs, and between precisely controlled upper and lower potential bounds. 

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