Faradaic pathway

Traditionally, all the noncapacitive charge-injection mechanisms are grouped together into the Faradaic pathway. This includes both the reversible and irreversible chemical reactions. They are grouped together because the distinction between the two can be unclear in some instances. The reversible oxida-tion/reduction reaction plays an important role for charge injection and it is part of what gives each metal its unique electrochemical properties. 

Conversion of the m/z = 44 ion current into a partial faradaic reaction current for formaldehyde oxidation to CO2 (four-electron reaction) shows that, under these experimental conditions, formaldehyde oxidation to CO2 is only a minority reaction pathway (dashed line in Fig. 13.6a). Assuming CO2 and formic acid to be the only stable reaction products, most of the oxidation current results from the incomplete oxidation to formic acid (dotted hne in Fig. 13.6a). The partial reaction current for CO2 formation on Pt/Vulcan at 0.6 V is only about 30% of that during formic acid... 

In the original proposal of the dual-pathway mechanism (for formic acid oxidation, see [Capon and Parsons, 1973a, b, c] for methanol oxidation, see [Parsons and VanderNoot, 1988 Jarvi and Stuve, 1998 Leung and Weaver, 1990 Lopes et al., 1991 Herrero et al., 1994, 1995]), both pathways lead to CO2 as the final product, as illustrated in the reaction scheme depicted in Fig. 13.8a [Jarvi and Smve, 1998]. In this mechanism, desorption of incomplete oxidation products was not included. The existence of a direct reaction pathway for methanol oxidation, following the dual-pathway mechanism, was justified by the observation of a methanol oxidation current at potentials where COad oxidation is not yet active [Sriramulu et al., 1998, 1999 Herrero et al., 1994, 1995]. The validity of this interpretation was questioned, however, by Vielstich and Xia (1995), who claimed that CO2 formation is observed only with the onset of COad oxidation and that the faradaic current measured at lower potentials is due to the formation of the incomplete oxidation products formaldehyde and formic acid. The latter findings were later confirmed by Wang et al. [2001], Korzeniewski and Childers [1998], and Jusys et al. [2001, 2003]. In more... 

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. 

Figure 16. Equivalent circuit (a) and a simulated Nyquist plot (b) for the charge transfer pathway illustrated in Figure 15. The capacitance C represents that of the space-charge layer and the parallel branch components represent the Faradaic charge transfer process. Refer to the original work for further details. (Reproduced with permission from Ref. [84).)...

As mentioned in the introduction, the electrical nature of a majority of electrochemical oscillators turns out to be decisive for the occurrence of dynamic instahilities. Hence any description of dynamic behavior has to take into consideration all elements of the electric circuit. A useful starting point for investigating the dynamic behavior of electrochemical systems is the equivalent circuit of an electrochemical cell as reproduced in Fig. 1. The parallel connection between the capacitor and the faradaic impedance accounts for the two current pathways through the electrode/electrolyte interface the faradaic and the capacitive routes. The ohmic resistor in series with this interface circuit comprises the electrolyte resistance between working and reference electrodes and possible additional ohmic resistors in the external circuit. The voltage drops across the interface and the series resistance are kept constant, which is generally achieved by means of a potentiostat. 

It is important to stress one major difference between ac and dc modes. In the ac mode a conducting substrate participates in the conduction process and reduces the solution resistance observed. On the contrary, an insulating substrate blocks the ionic pathway and increases the solution resistance observed. In the dc mode both conducting (provided the applied potential is sufficiently small that no Faradaic process occurs on the substrate) and insulating substrates block the conduction pathway and increase the resistance between the tip and the auxiliary electrode (see Fig. 5). 

Analyzing Figure 4.24 a number of intriguing questions emerge (i) is the faradaic dual pathway mechanism completely parallel or under certain conditions (e.g., catalyst or co-catalyst, potential, temperature) an interconversion between the labile and strongly adsorbed intermediates, [Fi] and [Fn] respectively, can occur (as exemplified by Equation 4.20) (ii) what is the contributions of the non-faradaic, thermocatalytic, pathway during formic acid electrooxidation on various electrocatalysts (e.g., [Hn] = [Fn] = COad) , (iii) what catalyst compositions favor the pathway of least kinetic resistance to oxidation (i.e., via the intermediate [Fi]) , (iv) what is the role of Had and under what conditions OH d plays a role in the overall reaction scheme ... 

Contaminant adsorption (competitive in mixtures with preferential adsorption of the largest-affinity contaminant), contaminant decomposi-tion/electrochemical reaction intermediates production, O reduction reaction pathway modification (atop Oj adsorption favored rather than bridged Oj, electric double layer structure change induced by cation insertion in iono-mer, Pt oxide modification including kinetics, changes in proton activity) or contaminant deposition reduces the catalyst area, increases the reduction reaction overpotential, decreases faradaic efficiency, and increases product selectivity (increased HjO contaminant production) Pt particle dissolirtion acceleration by adsorbed S on Pt from SOj or other soirrces decreasing iono-mer ionic conductivity... 

Fig. 4 (a) Oxygen reduction reaction pathways in the absence Qeft) and in the presence (right) of a contaminant. The presence of a contaminant affects the surface coverage G, oxygen reduction reaction product selectivity (j), and faradaic efficiency 0. (b) Platinum dissolution rate in the absence (top) and in the presence (bottom) of sulfur contamination either from the air intake or from the carbon support. The increased platinum dissolution rate favors a decrease in ionomer ionic conductivity. 

Alternatively, the second barrel can be filled with the electrolyte and used in the same manner as in an ion conductance microscope [50]. It is possible to relate the solution conductance between tip and counter electrode to the normalized tip-to-substrate distance L. If the conductance is measured by a dc technique and the surface is impermeable to ions, a negative feedback effect (decrease of conductance as the tip approaches the surface) is observed. If an ac technique is used and the substrate is metallic, a positive feedback effect can be observed irrespective of whether the substrate is biased or not because the lowest impedance pathway for the current is through the metal via the metal-solution interface. In fact, after normalizing the conductance G(L) by the value with the tip far from the surface G , the distance dependence of the conductance is identical to that for faradaic currents in feedback SECM with a redox mediator ... 

Bocarsly and co-workers [149,150] reported the mechanistic pathway for the electrochemical conversion of CO2 to methanol. They described the selective conversion of CO2 to methanol at a p-GaP semiconductor electrode, catalysed by pyridinium ions, where the reaction was driven by light energy, to yield Faradaic efficiencies near 100%, at potentials well below the standard potential. At illuminated electrodes, cathodic currents of --20 mA/cm could be maintained, without an applied bias. At metal electrodes, formic... 

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