Bulk Faradaic processes

The relation between E and t is S-shaped (curve 2 in Fig. 12.10). In the initial part we see the nonfaradaic charging current. The faradaic process starts when certain values of potential are attained, and a typical potential arrest arises in the curve. When zero reactant concentration is approached, the potential again moves strongly in the negative direction (toward potentials where a new electrode reaction will start, e.g., cathodic hydrogen evolution). It thus becomes possible to determine the transition time fiinj precisely. Knowing this time, we can use Eq. (11.9) to find the reactant s bulk concentration or, when the concentration is known, its diffusion coefficient. 

A voltammetric sensor is characterized by the current and potential relationship of an electrochemical cell. Voltammetric sensor utilizes the concentration effect on the current-potential relationship. This relationship depends on the rate by which the reactant (commonly the sensing species) is brought to the electrode surface (mass transfer) and the kinetics of the faradaic or charge transfer reaction at the electrode surface. In an electrochemical reaction, the interdependence between the reaction kinetics and the mass transfer processes establishes the concentration of the sensing species at the electrode surface relative to its bulk concentration and, hence, the rate of the faradaic process. This provides a basis for the operation of the voltammetric sensor. 

When performing potentiometric measurements with a traveling probe, one needs to take into account the effect of ohmic drop. Whereas this effect is the basis of SRET measurements, it becomes a nuisance in potentiometric SECM applications. If the substrate is an electrode involved in a Faradaic process, the current flowing between the substrate and the counterelectrode leads to potential gradients in solution. The tip will be sensitive to the potential distribution, and this may overcome the signal due to the concentration change for the ion of interest. This is particularly pronounced if the reference electrode associated to the tip is located far away in the bulk and of course if the solution conductivity is low. To remedy this situation some researchers have used double barrel electrodes where one channel acts as the ion-sensitive element and the other acts as a reference electrode (81,82). In the life sciences intracellular measurements are usually carried out in this way. Alternatively, it is possible to subtract the ohmic drop from the tip... 

Suppose we now consider a series of step experiments in the anthracene solution discussed earlier. Between each experiment the solution is stirred, so that the initial conditions are always the same. Similarly, the initial potential (before the step) is chosen to be at a constant value where no faradaic processes occur. The change from experiment to experiment is in the step potential, as depicted in Figure 5.1.3a. Suppose, further, that experiment 1 involves a step to a potential at which anthracene is not yet electroactive that experiments 2 and 3 involve potentials where anthracene is reduced, but not so effectively that its surface concentration is zero and that experiments 4 and 5 have step potentials in the mass-transfer-limited region. Obviously experiment 1 yields no faradaic current, and experiments 4 and 5 yield the same current obtained in the chronoamperometric case above. In both 4 and 5, the surface concentration is zero hence anthracene arrives as fast as diffusion can bring it, and the current is limited by this factor. Once the electrode potential becomes so extreme that this condition applies, the potential no longer influences the electrolytic current. In experiments 2 and 3 the story is different because the reduction process is not so dominant that some anthracene cannot coexist with the electrode. Still, its concentration is less than the bulk value, so anthracene does diffuse to the surface where it must be eliminated by reduction. Since the difference between the bulk and surface concentrations is smaller than in the mass-transfer-limited case, less material arrives at the surface per unit time, and the currents for corresponding times are smaller than in experiments 4 and 5. Nonetheless, the depletion effect still applies, which means that the current still decays with time. 

The above geometry (termed external reflection) is the only one usable in practice for bulk metal electrodes (see Fig. 6.2 a) [7j. It has also been used for semiconducting electrodes [19]. Because of the non-negUgible absorbance of electrolytes in the IR, it requires the use of thin-layer cells (1-10 pm), which may be impractical in the presence of Faradaic processes (high series resistance of the electrolyte, non-uniform accessibility of the electrode) or if dynamic information is of interest (large response time of the cell). 

A preferred mass transfer condition is total diffusion, which can be described by Pick s law of diffusion. Under this condition, the cell current, a measure of the rate of the faradaic process at an electrode, usually increases with increases in the electrode potential. This current approaches a limiting value when the rate of the faradaic process at the electrode surface reaches its maximum mass transfer rate. Under this condition, the concentration of the detecting species at the electrode surface is considered as zero and the flux is governed by diffusional mass transfer. Consequently, the limiting current and the bulk concentration of the detecting species can be related by... 

The situation should then be favourable if the ion electric carriers would take part in the Faradaic process at the inner botmdary inside the film. The ions could pass across the film discharging at the metal/film interface or, vice versa, liberating in the Faradaic process at this interface, and passing through the film into the bulk of the electrolyte. The processes at the negative electrode of a lithium battery (Li metal or Li/C intercalate compound) could be an example of such situation. 

This draft represents a simplified model when an anodic film of low-solubility intermediates is typical. The film conductivity is regarded to be both ionic and metallic (see Sect. 4.1). The Faradaic process at the inner junction (that is, metal/ film interface) is bound up with the ionic current component giving rise to the film growth. The metallic part of the conductivity causes oxidation of low-valence intermediates at the outer junction (film/electrolyte), both transported from the bulk by the flux /a and the film s constituents. 

The current-potential characteristics of electroactive species can be directly affected by the mass transfer of detecting oxygen species in this case in the kinetics of the faradaic or charge transfer reaction at the electrode surface [4]. This mass transfer can be accomplished through (a) an ionic migration as a result of an electric potential gradient, (b) a diffusion under a chemical potential difference or concentration gradient, and (c) a bulk transfer by natural or forced convection [4]. The rate of the faradaic process in an electrochemical cell can be influenced by the electrode reaction kinetics and the mass transfer processes. 

At = const, when the adsorption isotherm is congruent with respect to the electrode potential and the surface coverage is low, the effective standard rate constant may be determined with the posterior correction for the double-layer effects. Besides, in some cases (for instance, in the case of M j M"" electrode), faradaic elements do not depend on the bulk concentration of M"" [18]. This effect may be useful in studies of Cjj in the presence of faradaic process. At last, no surface area of the electrode is required to obtain the parameter k (see Eq. (5.56)). Dependence of directly measured capacitance (unrelated to the surface unit) versus direct current (instead of i) may be used for this purpose. 

In the bulk electrolyte current is carried only by means of ions. If a direct current is imposed upon a chemical cell, chemical reactions will occur at the electrodes in accordance with Faraday s laws. If an alternating rather than a direct current is used, the Faradaic reaction that takes place on one half-cycle is reversed on the following half-cycle. There are still flows of current, however, and such currents, which do not produce chemical changes in materials, are called non-Faradaic current. One of these is the current due to the current-carrying ability, or conductance, of ions. Thus measurements of ionic conduction are normally made by ac techniques to avoid complications due to the Faradaic processes taking place at the electrodes. 

The results have been compared with the earlier proposal of a dual-pathway mechanism for Cl oxidation, and, together with previous experimental and theoretical results, summarized in a comprehensive reaction scheme that explicitly includes also the (reversible) exchange between adsorbed species, dissolved product species in the catalyst layer, and similar species in the bulk electrolyte. The traditional dualpathway mechanism, where both the direct and indirect pathways lead to CO2 formation, has beenextended by adding a third pathway that accounts for formation and desorption of incomplete oxidation products. In the mechanistic discussion, we have focused on the role in and contribution to the Ci oxidation process of the formation/desorption and re-adsorption plus further oxidation of incomplete oxidation products. This not only leads to faradaic currents exceeding that for CO2 formation, but may result in additional COad and CO2 formation, via adsorption and oxidation of the incomplete oxidation products. 

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