Yield faradic

Polcaro et al. (2003, 2005) verified that during the oxidation of organic compounds, such as phenol, diuron, 3,4-dichloroaniline, and triazines, the crucial point to obtain high Faradic yields is the rate of mass transfer of the reactant toward the electrode surface (Fig. 2.11). Thus, they developed an impinging cell that enabled them to obtain high mass-transfer coefficients (e.g., 10-4 m s-1). With this cell, at a current density of 150Am-2, they achieved a Faradic yield of 100%, up to the almost complete disappearance of the organic load. 

Fig. 8.3 Trend of TOC circles), dissolved carbon attributable to cyclic intermediates (squares), both normalised with respect to TOC0, and faradic yield (triangles) as a function of y...

However, although the oxidation process was ascribed to either direct reaction on the electrode surface, or mediated by peroxodisulphate and other inorganic reagents electrogenerated at the anode surface, the linear decrease of the Faradic yield down to zero in the investigated range of concentration was interpreted as an indication of a process under diffusive control. This leads to the conclusion that the oxidative degradation of the compounds essentially occurred at the electrode interface. 

The degradation of anionic surfactants was also considered by other authors and compared with that of cationic surfactants (Lissens et al. 2003). Although mineralization to C02 was obtained in both cases, the electrochemical oxidation of the cationic surfactant was found to proceed at higher rate and higher Faradic yield than that of anionic surfactant (Fig. 8.7). 

When the waste contains more complex molecules such as compounds refractory to oxidation with OH radicals, as well as in the presence of inorganic ions which can be precursor of long-life oxidants, the Faradic yield cannot be calculated by (8.3) and different alternatives have been proposed. Faouzi and co-worker (Faouzi et al. 2006) proposed a comparison between electrochemical oxidation at BDD anodes and Fenton and Ozone treatments for the removal of dyes a specific parameter OCC (oxygen-equivalent chemical-oxidation capacity) was proposed which is defined as the kg of 02 equivalent to the quantity of oxidant used in each AOP to treat 1 m3 of wastewater. As highlighted by the authors, the parameter OCC may only give information on the chemical efficiency of the oxidants, but it does not give any information related to the real cost of the treatment, as the oxidants can... 

These modified electrode having the generic formula [Ru (bpyRR)(CO)2Cl] , display outstanding electrochemical activity towards the reduction of carbon dioxide to either (i) carbon monoxide, 100 % faradic yield in water at -1.2 V vs SCE, bpy = 2,2 -bipyridine, R = H (ii) or formate, 95 % faradic yield in aqueous electrolyte at -1.2 V vs Ag/Ag, R = isopropyl esters groups. 

In the Westinghouse electrochemical step, hydrogen is released at the cathode interface. In the numerical study, bubble generation is assumed to be localized in the first row of fluid cells neighbouring the electrode. In this special zone analogue to a boundary layer, the rate of gas production is assumed equal to the rate of the reduction process. It is modelled by the source term S2 of the dispersed phase mass-balance equation, assuming a 100% Faradic yield. 

In modern oells, the supplied electric power between the cell terminals and the fact of regulating the inter-eleotrode distance both lead to a current intensity of about 500 000 A and to a voltage of about 4 V for each cell, including a 1.7 V ohmic drop within the electrolyte, with an energy yield value of about 0.5 and a faradic yield close to 0.95. The difference to 1 comes mainly from a chemical shuttle , which is caused by the process of metal aluminium being dissolved in the electrolyte, and then followed by a reoxidation process by dissolved CO2 in the vioinity of the anode. 

The key parameters that govern the teohnological developments are current, faradic yield, energy consumption, the kinetios of alumina dissolution and, from more recent analysis, temperature. Moreover, in a bid to avoid CO2 emissions, researchers are looking into replacing carbon anodes with dimensionally stable anodes which should produce dioxygen. However this would oause the energetio yield to decrease. 

The faradic yield of a given redox half-reaction is the ratio between the variation of the amount of a given species in the medium used and the corresponding variation that should be expected from Faraday s law ifthe totality of the current was used for the halfreaction to progress ... 

This notion of faradic yield is most often used for quantifying the competition between several reactions occurring simultaneously at a given interface. It represents the fraction of the actual current used for the half-reaction in question. The faradic yield (either anodic or cathodic) of a particular half-reaction is therefore inferior to 100%. 

In the end, the faradic yield obtained for nickel deposition is therefore97%. 

If the faradic yield for a given working point of the electrochemical system is close to 100% at both electrodes, then the two redox half-reactions are called main reactions. In such a case, writing the balanced results of these two half-reactions corresponds to the overall conversion of reactants to products within the system. 

In most cases however, since there Is an excess of active material at the negative electrode, the faradic yield of... 

For a given potential, the construction of the overall current-potential curve from the different contributions of each electroactive species allows to view the concept of faradic yield as shown below. 

The overall current-potential curve (represented by a dashed and dotted line) is the sum of the two grey curves. As for the working point shown in figure 2.24, the current required for the reduction of Ni " ions, 7i represents about 25% of the overall current, /= I + h. This therefore means that the faradic yield for hydrogen production by reduction is equal to about 75%. 

Starting at open circuit, when the electrolysis current is gradually increased, the voltage imposed across the system also increases in turn and several simultaneous half-reactions may possibly be observed at one electrode (or both of them). Remember that the currents for each half-reaction at the same interface must be added together. This results in faradic yields that are lower than 100%. When it is possible for several reactions to occur at one of the electrodes, the main half-reaction is the one that would lead to the lowest polarisation in absolute value, for the same individual current When several reactions can be envisaged at both interfaces in the whole system, then the overall reaction which results from the two main half-reactions is the one that requires the lowest imposed voltage. 

The anodic branch at the positive electrode corresponds to a single redox half-reaction, i.e., water oxidation whatever the working point is, the anodic faradic yield along this branch is 100%. For the current to flow and therefore for electrolysis to occur, the following condition must be fulfilled -i-I. 4 V/sce- This value... 

The cathodic branch at the negative electrode corresponds to one or two redox half-reactions, depending on the potential zone the reduction of Cu " ions and the reduction of protons (or of water). Here for the current to flow and therefore for electrolysis to occur, one must have the following Ec [lTtO)<-OM l/ici- If the absolute value of the current increases, then the copper electrode potential decreases. When the potential becomes lower than -0.4 V/sce (the working point is indicated by squares in figure 2.37) two half-reactions occur simultaneously and the faradic yield of each half-reaction drops lower than 100%. 

Such an overall reaction only makes sense if the anodic and cathodic faradic yields are identical and equal to 100%. 

While the open-circuit voltage of this electrolysis cell is 0.9 - (-0.04) = -1-0.94 V (the platinum electrode is chosen as the working electrode), the minimum electrolysis voltage is about 1.4 - (-0.04) = 1.44 V. Here we can finally evaluate the maximum electrolysis voltage at which the 100% faradic yield is preserved. If we neglect the polarisation and consider the anodic branch to be guasi-vertical when the current increases, then the electrolysis voltage must be kept lower than 1.4 - (-0.4) = 1.8 V. ... 

As in electrolysis mode, the main half-reaction usually offers the highest faradic yield. The current-potential curves define a potential range for each electrode such that the faradic yield remains close to 100%, as illustrated in figure 2.38. 

The cathodic branch on the positive electrode side corresponds to one, two or even three redox half-reactions depending on the potential range considered the reduction of Ag", Cu ions, or the reduction of protons (or of water). For a cathodic current to flow, one must have the following i Ag(/5 0) <-1-0.4 V ce. If the current delivered by the electrochemical system increases in absolute value, then the silver electrode potential decreases. When this potential reaches values lower than about -0.05 V/jcj, two simultaneous half-reactions occur the reduction of Ag" ions and of Cu " ions, which are represented by squares in figure 2.38. The faradic yield of the main half-reaction becomes lower than 100%. When the potential gets lower than about -0.4 V/scE three half-reactions occur simultaneously. 

While the open-circuit voltage of the cell is 0.4 - (-1.0) = 1.4 V, with the silver electrode chosen as working electrode, the maximum voltage that can be delivered by this system working as power source is about 0.4 - (-0.6) = 1.0 V. The conditions required for preserving faradic yields of 100% equate to a voltage value between - 0.05 - (-0.6)=0.55 V and 1.0 V. 

In an industrial aluminium production plant, the main cathodic reaction involves the AI(III)/AI couple with a faradic yield of 90%. The amount of aluminium produced per hour in an electrolysis cell working with a current of 300 000 A is ... 

Remember that Faraday s law produces the relationship to the current. Here, for a redox reaction with a 100% faradic yield, the slope of the concentration profile is also proportional to the current ... 

Defining and using the fggj Faradic yield proportion of current used fora given half-reaction... 

Tin dioxide, an n-type semiconductor with a wide bandgap (3.6 eV at 300 K), has been widely studied as a sensor, a (photo)electrode material and in oxidation reactions for depollution. The performance of tin(iv) oxide is closely linked to structural features, such as nanosized crystallites, surface-to-volume ratio and surface electronic properties. The incentive for carbon-dioxide transformation into value-added products led to examination of the electroreduction of carbon dioxide at different cathodes. It has been recognised that the faradic yield and selectivity to carbon monoxide, methane, methanol, and formic acid rely upon the nature of the cathode and pH. ° Tin(iv) oxide, as cathode, was found to be selective in formate formation at pH = 10.2 with a faradic yield of 67%, whereas copper is selective for methane and ethene, and gold and silver for carbon monoxide. Nano-tin(iv) oxide has been shown to be active and selective in the carboigrlation of methanol to dimethyl carbonate at 150 °C and 20 MPa pressure. The catalyst was recyclable and its activity and selectivity compare with that of soluble organotins (see Section 21.5). 

On a pure Pf/C catalyst, Rousseau et showed that the electro-oxidation of ethanol at the anode of a DEFC working at 80°C mainly led to the formation of acetaldehyde, acetic acid and carbon dioxide, with chemical yields of 47.5%, 32.5% and 20.0%, respectively. By comparing the mass yield and the faradic yields, they concluded that no other products were formed in a significant amount. This result confirms that Pt is able to break the C-C bond to some extent In situ infrared measurements on ethanol adsorption and electro-oxidation at platinum electrodes have clearly shown that the adsorbed CO species are formed from 0.3 V vs RHE at the platinum surface moreover Iwasita and Pastor found some traces of CH4 at potentials lower than 0.4 V vs RHE. Previous studies showed that the initial steps of ethanol adsorption and oxidation on Pt can follow two different modes ... 

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