Peak current density

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

In an ideal case the electroactive mediator is attached in a monolayer coverage to a flat surface. The immobilized redox couple shows a significantly different electrochemical behaviour in comparison with that transported to the electrode by diffusion from the electrolyte. For instance, the reversible charge transfer reaction of an immobilized mediator is characterized by a symmetrical cyclic voltammogram ( pc - Epa = 0 jpa = —jpc= /p ) depicted in Fig. 5.31. The peak current density, p, is directly proportional to the potential sweep rate, v ... 

Fig. 10. Logarithmic plot of apparent limiting current density as a function of potential scan rate at a rotating-disk electrode i = apparent limiting current (or peak current) density iL = true steady-state limiting current density d/dt = potential scan rate expressed in units RT/nF oj = rotation rate (rad sec"l). [From Selman and Tobias (S10).]... 

NT Harrison, N Tessler, CJ Moss, RH Friend, and K Pichler, Peak current density and brightness from poly (p-phenylene vinyIcnc) based light-emitting diodes, Opt. Mater., 9 178-182, 1998. 

Figure 4. Formation condition for porous silicon solid line - peak current density, dotted line - current density at the maximum slope (see Figure.2).18...

One final issue remains to be resolved Of the portion of the AEpi that is due to resistance, what part is caused by solution resistance and what part is caused by film resistance To explore this issue we examined the electrochemistry of a reversible redox couple (ferrocene/ferricinium) at a polished glassy carbon electrode in the electrolyte used for the TiS 2 electrochemistry. At a peak current density essentially identical to the peak current density for the thin film electrode in Fig. 27 (0.5 mV see ), this reversible redox couple showed a AEpi of 0.32 V (without application of positive feedback). Since this is a reversible couple (no contribution to the peak separation due to slow kinetics) and since there is no film on the electrode (no contribution to the peak separation due to film resistance), the largest portion of this 0.32 V is due to solution resistance. However, the reversible peak separation for a diffusional one-electron redox process is —0.06 V. This analysis indicates that we can anticipate a contribution of 0.32 V -0.06 V = 0.26 V from solution resistance in the 0.5 mV sec control TiS2 voltammogram in Fig. 27. 

The decreased contribution due to slow electron transfer kinetics for the microtubular electrode is also attributable to the higher underlying surface area of the tubular current collector. Because the surface area is higher, the effective current density for the microtubular TiS2 is less than for the thin film TiS2, which has a conventional planar current collector. The decreased contributions of film resistance and slow electron transfer kinetics also account for the higher peak current density of the microtubular electrodes (Fig. 27). 

Detailed validation for low humidity PEFC, where the current distribution is of more interest and likely leads to discovery of optimal water management strategies, was performed most recently. Figure 35 shows a comparison of simulated and measured current density profiles at cell potentials of 0.85, 0.75, and 0.7 V in a 50 cm cell with anode and cathode RH of 75% and 0%. Both experimental data and simulation results display the characteristics of a low humidity cell the local current density increases initially as the dry reactants gain moisture from product water, and then it decreases toward the cathode outlet as oxygen depletion becomes severe. The location of the peak current density is seen to move toward the cathode inlet at the lower cell potential (i.e., 0.7 V) due to higher water production within the cell, as expected. 

Fig. 8.9. Variation of the antioxidant properties of the restructuring care NI-VEA Vital (NIVEA) disposed in a 40 pm layer peak current density (corresponding to ascorbate oxidation) as a function of the time.

Fig. 10.4. Cathodic OSWV peak current densities for the reduction of Cu2+ at MPA-Gly-Gly-His modified electrodes as a function of Cu2+ concentration. Error bars are +1 standard deviation of the current densities of four individual electrodes. Inset shows the peak current density in the region between 0 and 400 nM Cu2+. OSWVs were measured in 50 mM ammonium acetate (pH 7.0) and 50 mM NaCl at a pulse amplitude of 0.025 V, a step of 0.004 V and frequency of 25 Hz.

Perform experiments using the peptide-modified electrode as described in Section 13.4. Determine the peak current density of the target metal ion. 

An OSW voltammogram was performed and the peak current density of copper was measured. The concentration of copper was determined... 

Fig. 13.3. Rankit plot for the cathodic OSWV copper peak current-density effects of interferents from a Plackett-Burman 7-factor experimental design. The effect is the change in the copper current density upon increasing the metal-ion interferent concentration from its low level (—1, 0.2 pM) to its high level (+1, 5pM). Reprinted from Ref. [5], Copyright (2005) with permission from Elsevier.

Note that the peak current densities (Aippp = A/ppp /Ao) of microspheres and microdiscs of the same radius fulfill Aipppyphe peak = (w/4)Aippp lsc peak. 

FIGURE 3.6. (a) Cross-sectional schematics of a silicon wafer with a nanopore etched through a suspended silicon nitride membrance. SAM is formed between sandwiched Au eletrodes in the pore area (circled), (b) I(V) characteristics of a Au-2 -amino-4-ethynylphenyl-4-ethynylphenyl-5 -nitro-1 -benzenethiolate-Au (chemical structure shown below) molecular junction device at 60 K. The peak current density is 50 A/cm2, the NDR is 2400 pQ. cm2, the peak-to-valley ratio is 1030 1. [Adapted from Ref.30 Chen el al., Science 286, 1550-1552 (1999).]... 

FIGURE 5.4. I(V) Characteristics of a nanopore test bed device containing a SAM of molecule 1 at 60 K. The peak current density is 50 A/cm2 and the peak-to-valley ratio of the NDR response is 1030 1. 

ECL detection of Ru(bpy)2+ (or TBR) was conducted on a Si-glass chip with an ITO anode [727]. Through the transparent ITO anode, orange light (620 nm) was observed and recorded by a detector. It was found by cyclic voltammetry that the oxidation potential was more positive, and the peak current density was less on an ITO anode, as compared to the use of a Pt anode. In this work, Au cannot be used as an anode, presumably because of polymerization of TPA at the gold surface [727]. 

An improvement on the SL-EPR test is the double loop, or DL-EPR, test, which is shown schematically in Fig. 39. In this test, the potential is first scanned in the anodic direction from Ecoss to a point in the middle of the passive region before the scan is reversed. The ratio of the two peak current densities, L//a, is used as the degree of sensitization indicator. During the anodic sweep, the entire surface is active and contributes to the peak current. During the reactivation sweep, only the sensitized grain boundaries contribute to the passive-active transition. Thus in unsensitized specimens there is a small / and therefore a small ratio, while in heavily sensitized specimens, /r approaches /a, as shown in... 

The peak current density for a reversible linear potential sweep is given by... 

Determining the peak current density in cyclic voltammetry can sometimes be problematic, particularly for the reverse sweep, or when there are several peaks, which are not totally separated on the axis of potential. The usual way to determine the peak currents is shown in Fig. 7L. For the forward peak, the correction for the baseline is small and does not substantially affect the result. For the two reverse peaks, however, the baseline correction is quite large and may introduce a substantial uncertainty in the value of the peak current density. In fact, there is no llieory behind the linear extrapolation of the baselines shown in F/g. 7L, and this leaves room for some degree of "imaginative extrapolation." This is one of the weaknesses of cyclic voltammetry, when used as a niumtitative tool, in the determination of rate constants and reaction mechanisms. 

Fig. 7L Commonly used graphical method of extrapolating the baseline, to measure the peak current densities in cyclic voltammetry.

In this equation a is the transfer coefficient, which is obtained directly from the Tafel slope. The ratio of the peak current densities in the two regions is given by... 

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