Electrode, capacitance

Met. = Method. EC = electrocapillary method, PC = electrode capacitance measured with a pulse method, T = ten-sammetry. See also list of symbols. 

As mentioned above, the electrode material comprises two types of pores - NP and TC, NP being mainly responsible for the electrode capacitance value. On the other hand, TC, which contribute far less to the... 

Interaction parameters for polymer blends, 20 322 in surfactant adsorption, 24 138 Interaortic balloon pump, 3 746 Intercalated disks, myocardium, 5 79 Intercalate hybrid materials, 13 546-548 Intercalation adducts, 13 536-537 Intercalation compounds, 12 777 Intercritical annealing, 23 298 Interdiffusion, 26 772 Interdigitated electrode capacitance transducer, 14 155 Interesterification, 10 811—813, 831 Interest expense, 9 539 Interface chemistry, in foams, 12 3—19 Interface metallurgy materials, 17 834 Interfaces defined, 24 71... 

In a CV measurement, the current output always contains two components the Faradaic current, /F, due to the reaction of the redox species and the capacitive charging current, /c, which results from the charging of the electrode double layer and the diffusion layer. (This diffusion layer contains all charged and polar species in the solution and therefore differs from that of the redox species.) While /F changes linearly with vm as determined by diffusion, Ic is directly proportional to v as shown below, where CD is the total electrode capacitance and q the added capacitance charge ... 

This means that with increasing scan rate or lowering the solution concentration, the effect of lc will increase. Because a peak-shaped CV can only be obtained at a sufficiently high scan rate, the effect of electrode capacitance charging limits the CV application in low-concentration solutions. SWV has been developed to overcome this problem and to increase the quantitative accuracy of voltammetric techniques. The concentration for recording a SW voltammogram can be as low as a hundredth of that for recording a CV. 

Unlike in CV, pairs of current measurements are made on each period of the square wave. These are at time forward late in the forward pulse, named /forward, and frevere in the reverse pulse, named Iteveix- Both tformad and Reverse are much greater than the time for fully charging the electrode capacitance, so that only the Faradaic current is recorded. With calculation of /net, the difference between /forward and /reverse, SWV presents three types of peak-shaped I-E relations. Figure 65 displays the SWV of a reversible one-electron reduction process. 

Yang, L. J., Ruan, C. M., and Li, Y. B. (2003). Detection of viable Salmonella typhimurium by impedance measurement of electrode capacitance and medium resistance. Biosens. Bioelectron. 19,495-502. 

Adsorption of cyanide anions can be affected by adsorption of cations. In the solutions containing nonspecifically adsorbed anions, the nature of alkali metal cations was found to influence the measured value of the electrode capacitance at potentials more negative than —0.6 V (versus standard hydrogen electrode (SHE)). At < —l.OV adsorption of CN ions was enhanced in the presence of Li+ and Na+ cations, and inhibited in the presence of Cs+ ions [81]. A combined SERS and density-functional theory has been applied to study cyanide adsorption at Au electrode [82]. The authors have arrived at the conclusion that the polarity of Au—CN bonds falls between that of Au—Cl and Au—Br surface bonds. The binding strength for three different gold surfaces decreased in the order ... 

When one considers a distance scale much smaller than 1 pm, surface roughness also is an issue to observed electrode behavior. The ratio of the microscopic surface area to the projected electrode area is usually designated the roughness factor, and can vary from 1.0 to 5 or so for typical solid electrodes, or much higher for porous electrodes. Capacitance, surface faradaic reactions, adsorption, and electrode kinetics all depend on microscopic area. For example, double-layer capacitance increases with roughness such that the apparent capacitance (C°bs) is larger than the value for a perfectly flat electrode (Cflat) as shown in Equation 10.1 ... 

Two general approaches have been used in low-temperature studies. In the first, the uncompensated resistance, electrode capacitance, diffusion coefficient, and kinetic and thermodynamic parameters describing the electrode reaction are incorporated in a master model, which is treated (usually by some form of digital simulation) to calculate the expected voltammetric response for comparison with experiment [7,49]. 

R.M. Guijt, E. Baltussen, G. van der Steen, H. Frank, H. Billiet, T. Schalkhammer, F. Laugere, M. Vellekoop, A. Berthold, L. Sarro and G.W.K. van Dedem, Capillary electrophoresis with on-chip four-electrode capacitively coupled conductivity detection for application in bioanalysis, Electrophoresis, 22 (2001) 2537-2541. 

Fig. 9.22 Coupled lumped model with electrode capacitance.

Guijt et al. [69] reported four-electrode capacitively coupled conductivity detection in NCE. The glass microchip consisted of a 6 cm etched channel (20 x 70 pm cross-section) with silicon nitride covered walls. Laugere et al. [70] described chip-based, contactless four-electrode conductivity detection in NCE. A 6 cm long, 70 pm wide, and 20 pm deep channel was etched on a glass substrate. Experimental results confirmed the improved characteristics of the four-electrode configuration over the classical two-electrode detection set up. Jiang et al. [71] reported a mini-electrochemical detector in NCE,... 

A four-electrode capacitively coupled (contactless) detector has been integrated on a Pyrex glass chip for detection of peptides (1 mM) and cations (5 mM K+, Na+, Li+). The A1 electrode (500 nm Al/100 nm Ti) was deposited in a 600-nm-deep trench and was covered with a thin dielectric layer (30-nm SiC). The other parts of the channel were covered and insulated with Si3 N4 (160 nm). To avoid gas bubble formation after dielectric breakdown, the electric field for separation was limited to 50 V/cm [145]. This four-electrode configuration allows for sensitive detection at different background conductivities without the need of adjusting the measurement frequency [328]. 

As described previously, the De Levie model is useful to describe the electrochemical ac (or dc) behavior of a porous electrode. This TLM describes the porous electrode as an interpenetrated network of RC elements, whose contribution depends on the frequency of the ac signal. In other words, the capacitance and the resistance of the EDLC change with the frequency. However, it is difficult to get from these equations the change of the capacitance with the frequency useful to characterize an EDLC electrode. This is why other approaches have been developed and focused more onto the EDLC problematic, with the aim to quantify the change of the porous electrode capacitance (or the EDLC device capacitance) with the frequency of the ac signal. 

The values of specific capacitance and cell resistance obtained with symmetric two-electrode capacitors built with a-Mn02/CNTs composite electrodes are presented in Table 8.3. The addition of nanotubes to a-Mn02 causes a drastic decrease of cell resistance and an increase of specific capacitance referred to the mass of a-Mn02 H20. However, when the specific capacitance is referred to the total mass of the composite electrode material, to be realistic, a CNTs loading higher than 10-15 wt% does not improve the electrodes performance. Therefore, 10-15 wt% of CNTs conducting additive seems to be an optimal amount both on the point of view of electrodes capacitance... 

To optimize the device volumetric capacitance density, once the DLC geometric parameters such as the cell size, the electrode thickness, and width have been fixed, the development efforts must be concentrated on the research of the carbon performance. Typical commercial carbons [18] have a capacitance density in the range of 50F/cm3. Their capacitance specific density is in the range of 100 F/g. Among the best-performing carbons available, there are those derived from metal carbide (carbide derived carbon [CDC]) [19,20], They may reach a capacitance density of 130-140F/g. At that point, to avoid confusion, it is worth mentioning the difference between carbon or electrode capacitance and DLC capacitance. The later is exactly four times smaller because of the series connection of two electrodes whose volume is half of the total electrode volume. 

To get an estimation of the time required to reach 20% of electrode capacitance loss, the coefficients X0 and p must be determined for all the operating temperatures and for all the operating... 

In a series of papers Jaeger, Hudson and coworkers investigated how the spatio-temporal behavior is affected by different model parameters, such as reaction rate, electrode capacitance, the geometry of the WE and the relative positions of all three electrodes, WE, CE and RE in the simplified N-NDR model for electrodissolution reactions [27-29]. Below, we discuss a few examples taken from these works. For a geometry similar to the one discussed above, i.e. with ring-shaped working and... 

The above-described situation is but an exception rather than the rule. Generally, the diamond electrode capacitance is frequency-dependent. In Fig. 12 we show a typical complex-plane plot of impedance for a single-crystal diamond electrode [69], At lower frequencies, the plot turns curved (Fig. 12a), due to a finite faradaic resistance Rp in the electrode s equivalent circuit (Fig. 10). And at an anodic or cathodic polarization, where Rf falls down, the curvature is still enhanced. At higher frequencies (1 to 100 kHz), the plot is a non-vertical line not crossing the origin (Fig. 12b). Complex-plane plots of this shape were often obtained with diamond electrodes [70-73],... 

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