Voltammograms double-layer capacitance

Figure . Double-layer capacitance against potentials on Au( 100) in IM HCIO4 at 10 mV s with a change in cathodic limit potentials . Solid curve for n = -0.05 V, broken curve for j = -O.I8 V, and dotted curve for = -0.35 V(SCE). The cyclic voltammogram is given by the broken curve. (From Ref. 37.)...

We have used voltammetric measurements in the absence of the electroactive species to quantitatively evaluate this heat-sealing procedure. The magnitude of the double layer charging current can be obtained from these voltammograms [25,68-70], which allows for a determination of the fractional electrode area (Table 1). This experimental fractional electrode area can then be compared to the fractional pore area calculated from the known pore diameter and density of the membrane (Table 1). In order to use this method, the double layer capacitance of the metal must be known. The double layer capacitance of Au was determined from measurements of charging currents at Au macro-disk electrodes of known area (Fig. 6, curve A). A value of 21 pF cm was obtained. 

In order to confirm this behavior, the cyclic voltammograms obtained at a planar electrode in CV and SCV (for A = 5 mV) for a Nemstian charge transfer process at different values of the scan rate are shown in Fig. 5.11. The effect of the ohmic drop and charging current has been considered by including an uncompensated resistance Ru = 0.1 K 2 and a double-layer capacitance Cdi = 20pFcm 2. 

When the impedance versus the reciprocal of the square root of the frequency is plotted, the slope gives (Rf/Cdi)U2, where Rp is the ionic resistance of the anode catalyst layer. The double-layer capacitance can be obtained from the cyclic voltammogram. According to the definition of capacitance, the capacitance of the double layer equals the current divided by the scan rate (Cdt = TV-1). The ionic resistance can be obtained from the capacitance of the double layer. 

Figure 5 represents an ideal reversible one-electron transfer process in the absence of drop or capacitative charging current, although in real experiments contributions to the response from both these terms are unavoidable. Figure 6 shows the effect of uncompensated resistance for both transient and steady-state voltammograms, whilst Fig. 7 shows the influence of double layer capacitance on a cyclic voltammetric wave. Note that for steady-state voltammetric techniques only very low capacitative charging... 

Care should always be taken when interpreting the results of cyclic voltammetric experiments to ensure that the effects of the double-layer capacitance and uncompensated solution resistance are considered (see Section 2). Peak currents should be corrected for the baseline capacitative charging current (for example by running a background voltammogram in the absence of the electroactive species), and as the charging current is... 

The double-layer capacitance is another useful electrochemical method for probing the integrity of molecular assemblies in contact with electrolyte solutions. If a cyclic voltammogram is recorded in a solution of electrolyte only, a non-Faradaic current results, and the double-layer capacitance, Cji (or differential capacitance), is measured as half the width of the voltammogram charging envelope given by [30, 48]... 

The electrochemical stability of PtO-OTS in aqueous saline solution was tested by measuring cyclic voltammograms of PtO and PtO-OTS electrodes in 0.1-M KCl, pH 6.8 (Figure 4). The electrode capacitances that correspond to the cyclic voltammograms are shown in Figure 5. These capacitances were measured at 1000 Hz. The OTS layer can be described as a planar capacitor in series with the PtO and double-layer capacitances. Formation of the OTS layer decreased the capacitance however, the PtO beneath the OTS layer was still reduced when the potential was swept below 0.3 V. This observation again is evidence that the OTS layer on PtO is somewhat porous. The onset potential of PtO reduction in aqueous solution is pH-dependent. 

Fig. 8.16 (a) Cyclic voltammogram of horse heart cytochrome c adsorbed onto an MPA-modified Au electrode. Scan rate is 50 mVs . The dashed line corresponds to the double-layer capacitance, (b) Total charge pass during oxidation and reduction... 

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. 

The double-layer capacitance for glassy carbon in the potential region from —500 to 1000 mV varies from 30 to 40 pF/cm (geometric area), while that for diamond ranges from 3 to 7 pF/cm [22,28,33,38]. The voltammogram for glassy carbon, shown in Fig. 9, shows some evidence for characteristic oxidation and reduction peaks centered at 350 mV... 

The equivalent circuit corresponding to an uncomplicated electrochemical reaction (i.e., a one-step CT process) is shown in Figure 15.1. An important advantage of ac voltammetry is that it allows relatively easy evaluation of the solution resistance ( J and double layer capacitance (C4). These elements can be separated from the and components, which together make faradaic impedance. Without simplifying assumptions, the analysis of faradaic impedance even for a simple ET reaction is rather complicated (9). The commonly used assumptions are that the dc and ac components of the total current can be uncoupled, and the dc response is Nemstian because of the long dc time scale. The latter assumption is reasonable because ac voltammetry is typically used to measure fast electrode kinetics. The ac response of the same electrochemical process may be quasi-reversible on the much shorter ac time scale. Quasi-reversible ac voltammograms are bell-shaped. 

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]. 

The electrochemical techniques have a nominal time resolution only in the millisecond domain. This is a rather severe handicap relative to spectroscopic probes that now routinely access the femtosecond (I0 s) regime. The two main obstacles for decreasing the time window of electrochemical techniques are both related to the uncompensated resistance, / The time constant of the electrochemical cell, R Cd (where Cj is the double-layer capacitance at the WE/solution interface), for nominal values of / u and Cd O and 20 fiF, respectively) is in the microsecond domain. The cell therefore effectively acts as a low-pass filter with respect to the applied voltage [34]. Distortion of the applied voltage results in a distorted voltammogram. 

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