Faradic reaction

The double layer can affect the distribution of the current density. When the electrode potential changes the current that flows will be distributed, part of it will be used to charge the double-layer capacity and another part will go to the faradic reaction (see Eq. 9). For a given current, the double layer will decrease the potential of the cell acting as an internal resistance (a fraction of the current will go to the double layer). 

Clay particles can be thought of as microelectrodes possessing a compact Stern layer and a diffuse layer, which mediates faradic reactions, as depicted in Figure 2.10. As the donated (or accepted) electrons pass across the electrical double layer into and out of the bulk fluid, available species are converted into others via oxidation-reduction reactions (Grahame, 1951,1952). This effect may become significant... 

We hypothesized that the above treatise of DDL interactions in the presence of an electrical field is a viable model for the explanation of enhanced oxidation-reduction in clay-electrolyte systems. Electrolytic transformations of selected chlorinated hydrocarbons (CHCs) and polyaromatic hydrocarbons (PAHs) have been demonstrated successfully in water and wastewater (Franz, Rucker, and Flora, 2002 Pulgarin et al., 1994). There has been field and laboratory evidence that these transformations can also take place in porous media (Banarjee et aL, 1987 Pamukcu, Weeks, and Wittle, 2004 Alshawabkeh and Sarahney, 2005 Pamucku, Hannum, and Wittle, 2008). As discussed previously, faradic reactions do take place on clay particle surfaces when current pass in the pathways of the DDLs (Grahame, 1951, 1952). Hence, external supply of electrical energy can help drive favorable oxidation-reduction reactions in contaminated clays not only in the bulk fluid but also on clay surfaces, as well as on where most of the contaminants tend to reside because of adsorption or exchange. 

Capacitance Enhancement through Faradic Reactions at the Carbon-Electrolyte Interface In Aqueous Media... 

The most reported dopants which confer pseudo-capacitive properties to carbons are oxygen (Fig. 12.3) and nitrogen (Fig. 12.4). Some other studies described below have recently considered boron or phosphorous as pseudo-capacitive dopants. The nitrogenated and oxygenated functionalities can imdergo pseudo-faradic reactions, which can be pH-dependent or not, as presented in Fig. 12.10. The extent of the... 

O- and N-enriched nanocarbons demonstrate high capacitance values in KOH or H2SO4 due to the contribution of pseudo-faradic reactions. Another positive effect of doping is the broadening of the potential stability window. Composites of these carbons with carbon nanotubes withstand remarkable capacitance values at high current load. Hence, the N- and O-doped carbons open the opportunity of developing high-performance supercapacitors in aqueous electrolytes. 

Analysis of experimental data that yield a full semicircular arc in the complex plane, such as that in Figure 3. b, can provide estimates of the parameters R and Ci and hence lead to quantitative estimates of conductivity, faradic reaction rates, relaxation times, and interfacial capacitance (see detailed discussion in Section 2.2.3.3). In practice, however, experimental data are only rarely found to yield a full semicircle with its center on the real axis of the complex plane. There are three common perturbations which may still lead to at least part of a semicircular arc in the complex plane ... 

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. 

On the other hand, pseudocapacitance arises from a faradic reaction at an electrode surface as an oxidation or reduction process, which can provide higher capacitance and/or higher power capability than carbon electrodes utilized in conventional EDLCs. Electrode materials that exhibit such pseudocapacitive storage are typically organic materials such as conducting polymers and transition metal oxides [2, 4]. 

Cell or stack aging is indicated by a loss in overall performance due to diminishing capacitance, slower charging and discharging rates, and increased series resistance. In addition, these signs can also be associated with macroscopic phenomena, for example, localized detachment of electrode materials from the metallic collector through electrode swelling, gas evolution, and loss of elements involved in faradic reactions. 

The principle of EDLC operation is very simple and is based on the well-known electrical or double layer phenomenon. The device operates within a potential range in which no Faradic reactions take place, and thus the behaviour is fully capacitive. Polarisation of the electrodes in opposite directions leads to accumulation of opposite charges at the electrode-solution interfaces. The higher the electrode surface area and the polarity of the electrolyte solution and its ionic concentration, the higher the capacity and energy density of these devices. The capacitance (C) and the accumulated electrostatic energy ( ) stored are given by equations [1.22] and [1.23], respectively (Nishino et af, 1985) ... 

In a double-layer capacitor, no faradic reaction takes place between the electrode and electrolyte. In a pseudocapadtor, most of the charge is transferred at the interface or by the material near the surface of the electrode, which does involve a faradic reaction. The most interesting case is the conducting polymer, in which charging/discharging is possible in the material itself, raising the possibility of increasing the capacitance to a new level. 

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