Redox reaction faradaic process

A preliminary electrochemical overview of the redox aptitude of a species can easily be obtained by varying with time the potential applied to an electrode immersed in a solution of the species under study and recording the relevant current-potential curves. These curves first reveal the potential at which redox processes occur. In addition, the size of the currents generated by the relative faradaic processes is normally proportional to the concentration of the active species. Finally, the shape of the response as a function of the potential scan rate allows one to determine whether there are chemical complications (adsorption or homogeneous reactions) which accompany the electron transfer processes. 

The first class includes non-redox reactions like isomerisation, dimerisation or oligomerisation of unsaturated compounds, in which the role of the catalyst lies in governing the kinetic and the selectivity of thermodynamically feasible processes. Electrochemistry associated to transition metal catalysis has been first used for that purpose, as a convenient alternative to the usual methods to generate in situ low-valent species which are not easily prepared and/or handled [3]. These reactions are not, however, typical electrochemical syntheses since they are not faradaic they will not be discussed in this review. 

The formation of 2D Meads phases on a foreign substrate, S, in the underpotential range can be well described considering the substrate-electrolyte interface as an ideally polarizable electrode as shown in Section 8.2. In this case, only sorption processes of electrolyte constituents, but no Faradaic redox reactions or Me-S alloy formation processes are allowed to occur, The electrochemical double layer at the interface can be thermodynamically considered as a separate interphase [3.54, 3.212, 3.213]. This interphase comprises regions of the substrate and of the electrolyte with gradients of intensive system parameters such as chemical potentials of ions and electrons, electric potentials, etc., and contains all adsorbates and all surface energy. Furthermore, it is assumed that the chemical potential //Meads is a definite function of the Meads surface concentration, F, and the electrode potential, E, at constant temperature and pressure Meads (7", ). Such a model system can only be... 

This kind of capacitance, of faradaic and not electrostatic origin, is distinguished from the double-layer capacitance and called pseudocapacitance . It can originate when fast redox reactions take place between the electrode and the electrolyte. In this process, capacitances of 100-400 pF cm can be obtained [9] because the bulk of the electrode is involved, and not only the surface as for EDLC. 

We now use an equivalent circuit representing an electrode/solution interface where the electrode surface is covered by an electroactive monolayer. The simplest circuit is shown in Fig. 2.18. We assume that the molecules in a Langmuir monolayer undergo an n-electron transfer reaction in response to ac and that the ER signal is exclusively due to this faradaic process [69]. The faradaic process of the surface-confined species at the formal potential is represented by a series connection of a constant capacitance associated with the redox reaction of the adsorbed species Q and a charge transfer resistance Ret. where Q is written for a Nernstian process as... 

Fig. 2.18 An equivalent circuit representing an electrode/solution interface. The electrode surface is covered by a monolayer of a redox-active species. e ac potential across the faradaic unit of equivalent circuit, Ca double-layer capacitance, Rs -uncompensated solution resistance, Zf impedance representing solely the electron transfer reaction process of the monolayer, )> ac current due to the faradaic process, Z, total impedance of the whole system, ks. heterogeneous electron transfer rate constant of the monolayer of electroactive species, R charge transfer resistance, Q capacitance associated with the redox reaction of the adsorbed species.

The contribution of faradaic and double-layer capacitances in the response of DG-structured V2O5 can be estimated by comparing the CV curves of devices that use RTIL electrolyte with and without lithium salt, shown in Fig. 5.14b. Since V2O5 does not react with either of the ions of pure RTIL, the lithium-free experiment tests the EDLC response. With lithium salt added, the significant increase in capacitive current and the appearance of peak pairs indicates that redox reactions are taking place. These faradaic processes are kinetically facile and thus considered pseudocapacitive, but phase transitions may occur. Although it is difficult to distinguish between redox and intercalation pseudocapacitance, the latter is likely to be present in DG bicontinuous materials. 

Porosity effects during the charging process have long been considered in discussions of the faradaic and capacitive contributions to the current, especially in the case of electronically conducting polymers. For instance, the peaks of cyclic voltam-mograms were attributed to the faradaic process while the plateaus of the cmrent were considered to be an indication of the capacitive term [99,105,106,211-215]. However, this straightforward analogy to the metal/solution interface does not work in reality the obviously faradaic process of the redox transformation of the redox species in the surface layer does not lead to a direct current, unlike similar reactions for solute species. 

When faradaic and charging currents flow through a solution, they generate a potential that acts to weaken the applied potential by an amount, iR, where i is the total current. This is an undesirable process that leads to distorted voltanunetric responses. It is important to note that, as described by equation (6.1.1.3), the cell resistance increases with decreasing electrode radius. Thus, the ohmic drop is not reduced at microelectrodes relative to macroelectrodes because of reduced resistance. However, the capacitive or double-layer charging current depends on the electrode area or r. Similarly, for reversible redox reactions under semi-infinite diffusion control, the faradaic current depends on the electrode area. This sensitivity to area means that the currents observed at microelectrodes are typically six orders of magnitude smaller than those observed at... 

These devices are based on the measurement of either electrochemical potential or faradaic current associated with redox reactions at an electrode. They are particularly suitable for enzyme-substrate receptor systems by virtue of the ionic products often produced in such reactions. The sensing membranes of the ion-selective electrodes previously described have been combined with semiconductor devices for miniaturization, low-impedance output, signal amplification, and capability of on-chip processing. The ion-sensitive field effect transistor (ISFET) is based on replacement of the conventional transistor gate with the ion-... 

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