Faradaic oxidation

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

Examples of electroactive NP materials discussed in the review include Ti02, Mn02, iron oxides, other metal oxides, hydroxides and oxyhydroxides and Prussian Blue. We use the term electroactive N Ps to refer to the faradaic electroactivity in such materials and to distinguish them from NPs comprised of metals (such as Au, Ag, Pt, Co, etc.) or semiconductors (such as CdS, CdSe, etc.). This distinction is based on the ability of many electroactive NPs to undergo faradaic oxidation or reduction of all of the metal (redox) centers in the NP. This is in contrast to the behavior of many metal and semiconductor NPs for which oxidation or reduction is fundamentally an interfacial, double-layer process. This deflnition is somewhat arbitrary, since the smallest metal and semiconductor NPs behave molecularly, blurring the distinction... 

In Appendix A, calculations show a status, for fuel cell isothermal Faradaic oxidation, of a high vacuum of reactants relative to a high concentration of product. That calculated status cannot even be approached in the laboratory, for lack of adequate semi-permeable membranes and circulators (concentration cells). The equilibrium fuel cell of Figure A.l is dead-ended, whereas the air-breathing open-ended design must have both of its electrodes swept by a parallel flow, with an inlet and an... 

In the present system with the copper-2% zinc electrodes, all three processes of protein adsorption, charge transfer, and Faradaic oxidations and reductions are possible. The peaks observed in the anodic and cathodic processes are related, respectively, to oxidations and reductions of the electrode. Copper oxides, chlorides, basic chlorides, phosphates, etc., as well as zinc products, are probable compounds for these electrochemical reactions. Increased Faradaic processes and charge transfer processes with protein solutions are factors for increasing the j-U profiles at U s less than +0.3 V. Since the sweep rate is a constant here, the capacitance of the double layer must increase for the protein solutions, if the increase in j is not all due to Faradaic processes One analog of the electrical double layer capacitance incorporates three capacitors in series (44). Hence... 

Although the applied potential at the working electrode determines if a faradaic current flows, the magnitude of the current is determined by the rate of the resulting oxidation or reduction reaction at the electrode surface. Two factors contribute to the rate of the electrochemical reaction the rate at which the reactants and products are transported to and from the surface of the electrode, and the rate at which electrons pass between the electrode and the reactants and products in solution. 

Residual Current Even in the absence of analyte, a small current inevitably flows through an electrochemical cell. This current, which is called the residual current, consists of two components a faradaic current due to the oxidation or reduction of trace impurities, and the charging current. Methods for discriminating between the faradaic current due to the analyte and the residual current are discussed later in this chapter. 

The residual current, in turn, has two sources. One source is a faradaic current due to the oxidation or reduction of trace impurities in the sample, i . The other source is the charging current, ich> that is present whenever the working electrode s potential changes. 

F r d ic Current. The double layer is a leaky capacitor because Faradaic current flows around it. This leaky nature can be represented by a voltage-dependent resistance placed in parallel and called the charge-transfer resistance. Basically, the electrochemical reaction at the electrode surface consists of four thermodynamically defined states, two each on either side of a transition state. These are (11) (/) oxidized species beyond the diffuse double layer and n electrons in the electrode and (2) oxidized species within the outer Helmholtz plane and n electrons in the electrode, on one side of the transition state and (J) reduced species within the outer Helmholtz plane and (4) reduced species beyond the diffuse double layer, on the other. 

The electrochemical stability range determines the usefulness of nonaqueous electrolytes for electrochemical studies as well as for applications. It indicates the absence of electrochemical oxidation or reduction of solvent or ions, and of faradaic current... 

In most cases, oligomers are initially generated in solution,61-64 but most rapidly precipitate onto the electrode surface and/or couple with adsorbed chains, and become oxidized 62,63,65 As a result, an oxidized (p-doped) polymer film is deposited on the electrode surface with, in most cases, high faradaic efficiency. Since ca. 0.3 electrons are required to dope the film to the polymerization potential, the overall polymerization + deposition process consumes ca. 2.3 electrons per monomer unit. 

S. Neophytides, D. Tsiplakides, P. Stonehart, M.M. Jaksic, and C.G. Vayenas, Non-Faradaic Electrochemical enhancement of H2 oxidation in alkaline solutions, J. Phys. Chem. 100, 14803-14814 (1996). 

S. Bebelis, and C.G. Vayenas, Non-Faradaic Electrochemical Modification of Catalytic Activity 1. The case of Ethylene Oxidation on Pt, J. Catal. 118, 125-146 (1989). 

Figure 4.22. Effect of the rate of O2 supply to the catalyst electrode on the increase in the rate of C2H4 oxidation on Pt deposited on YSZ.1,4 Dashed lines are constant faradaic efficiency, A, lines. Reprinted from ref. 4 with permission from Academic Press.

Figure 4.22 shows the steady-state effect of current, or equivalently rate, I/2F, of O2 supply to the catalyst on the rate increase Ar during C2H4 oxidation on Pt/YSZ. According to the definition of A (Eq. 4.19), straight lines passing from the (0,0) point are constant faradaic efficiency A lines. 

For A l, the Faradaic efficiency A has, as already noted, an interesting physical meaning50 For oxidation reactions it expresses the ratio of the reaction rates of normally chemisorbed atomic oxygen on the promoted... 

Figure 4.53. Effect of temperature on the faradaic efficiency, A, values measured in electrochemical promotion (NEMCA) studies of C2H4 oxidation on various metals.30 Reprinted with permission from Academic Press.

T.I. Politova, G.G. Gal vita, V.D. Belyaev, and V.A. Sobyanin, Non-Faradaic catalysis the case of CO oxidation over Ag-Pd electrode in a solid oxide electrolyte cell, Catal. Lett. 44, 75-81 (1997). 

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