Faradic process

In chronoamperometry, which is employed in a few electronic tongue systems (Cortina et al., 2008 Han et al., 2004), the potential is kept constant, while the current variations, resulting from faradic processes occurring at the electrode, are monitored as a function of time. 

The SG/TC mode of SECM was also applied by Martin et al. [86] to study the oxidation of DMPPD. The generator was a 2-mm2 substrate electrode, and the collector was a 25-pm diameter Pt disk electrode. The substrate potential was stepped from 0 V versus Ag quasi reference electrode, where no Faradic process took place, to +500 mV, where the oxidation of DMPPD was diffusion controlled. The tip potential was held at 0 V, at which the oxidized form of DMPPD could be reduced at a diffusion controlled rate. After the tip-substrate separation was found from the positive feedback current-distance curve, the rate constant was obtained from the current transient at the tip. The feedback and SG/TC modes were also used to study the reduction of... 

Figure 7 shows the detection principle of an impedimetric IME biosensor for bacterial detection [8]. It is based on measurements of electrochemical faradic impedance in the presence of [Fe(CN)6] " as a redox probe. When a bare interdigitated microelectrode is immersed into an electrolyte solution containing the redox couple and a small-amplitude AC potential (5 mV) is applied to the electrode, the faradic process of oxidation and reduction of the redox couple occurs, and then electrons are transferred between the two sets of array electrodes through the redox couple (Fig. 7a). When antibodies are immobilized onto the electrode surface (Fig. 7b) they form a layer that can inhibit the electron transfer between the electrodes, and thus an increase in the electron transfer resistance can be expected. If bacterial cells attach to the antibody-modified electrode surface (Fig. 7c), the intact cells can create a further barrier for...

PM-IRRAS can be implemented using modem commercially available FT IR spectrometers. Indeed, most modern surface IR studies simply use these instruments in conjimction with some form of subtraction. In subtractively normalized FTIR or potential difference IR, a single beam FTIR spectrum Sf is collected at a reference potential E, (at which no Faradic processes occur) and then at successively higher or lower potentials Eg scans Ss are obtained. Then plots (usually overlaid) of AR/R = (Ss — Sr)/Sr are obtained for a range of working potentials. Unlike EMIRS or PM-IRRAS, there is no necessity for the electrode process imder study to be reversible. The method is very populaq capable of investigating... 

The theoretical treatments referenced above all suffer from a major deficiency. The nonlinear term of interest in corrosion (the electron transfer process) is contained within a circuit comprising other linear (electrolyte resistance) and nonlinear (double-layer capacitance and diffusional impedance) terms. Since the voltage dropped across nonlinear circuit elements cannot be considered to linearly superimpose, we cannot use the equivalent circuit method to isolate the impedance terms of interest. Properly, one must solve for the system as a whole, including diffusional and double-layer terms, and identify the harmonic components associated with the faradic process of interest. 

Direct conductometric sensors Conductometric monitoring systems should be constructed with regard to the need to minimize faradic processes that cause a concentration polarization of electrode surfaces and with regard to removing the influence of double-layer charging. The... 

Curve A is the —/ d vs t transient following the application to the Au disc of a potential step from +0.8 V to +0.4 V, where a monolayer of Hg deposits. Curve B is plot of /r/A during this experiment when the Hg/Au ring is held at 0.0 V, a value where the reduction of Hg " is mass transport controlled. For a simple reaction the two transients would coincide and the difference corresponds to Hg2 removed at the disc by a non-faradic process, i.e. adsorption of Hg2. Reproduced with permission from W. G. Sherwood S. Bruckenstein, / Electrochem. Soc., 125,(1978), 1098. 

Two types of processes occur at the electrode. Faradic processes involve charge transfer across the metal-solution interface, and nonfaradic processes involve transient current flow due to double layer charging [31], Impedance due to faradic and nonfaradic processes is denoted as Zp and Z p, respectively. Their frequency dependence is [20, 31] ... 

It can be seen from the aforementioned equations that faradic processes dominate over low frequencies due to inverse square root dependence. To estimate biofilm conductivity, we were interested in charge transport across the gap rather than charge transfer across the electrodes. (The latter describes a one-step tunneling process [19].) Hence, it was necessary to eliminate faradic reactions involving charge transfer. In the case of anode biofilms, a low-frequency cut-off of 100 mHz was used to eliminate any possible contributions from electrochemical reactions occurring at very low frequency [31, 46, 48]. Electrolytes with different concentrations were tested to verily this frequency range. 

However, the cycle life is a major limitafion because the material phase may be changed over time through faradic processes, leading to a much shorter cycle life than that possible with double-layer capacitance. This statement holds true for many other redox materials including transition metal candidates, providing variable results among studies of the same active material. 

Cao [1] believed that if diffusion was not involved in the Faradic process for reaction sequence, the admittance Y (or impedance Z) for the electrode could be expressed as ... 

If there are two surface state variables, Xi and X2, which controlled the electrode processes and the two variables X and X2 were independent of each other, then it can be shown [1,2] that the admittance of the faradic and non-faradic processes is ... 

At an anodic potential, the current of the Faradic process could be expressed... 

Kinetic studies of some electrochemical reactions in nematic solvents have been made, as well as studies of those solvents dissolved in more polar solvents. It was noticed early that under dc excitation of dynamic-scattering cells,the currents remained relatively constant and the cells continued to operate for orders of magnitude longer than would be expected for a simple faradic process, i.e., where an electrochemical reaction was controlled by the net charge passed through the cell. 

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