Faradaic effect

In the case of a Faradaic effect, all oxygen which is transported electrochemically through the electrolyte reacts at the anode (A = 1). A reaction exhibits the NEMCA effect when A > 1. When A > 1, the reaction is termed electrophobic (the catalytic reaction is promoted by a positive current or overpotential), while a reaction accelerated by a negative current or potential is termed electrophilic. 

RCSUB Corrects cell admittance arrays for the non-faradaic effects ohmic resistance and double-layer capacitance. Uses data provided by the FITl-FITALL sequence and the formula... 

In single-chamber reactor, the A value would be possible to exceed the unity. This means that there is additional hydrogen from the gas phase that accompanies the electrochemical hydrogen in the same reaction. This phenomenon is called non-Faradaic electrochemical modification of catalytic activity (NEMCA). The concept of the NEMCA effect is different from Faradaic effect. In NEMCA, an electrode will serve as a catalyst for two simultaneous processes, chemical processes and electrochemical processes. 

Due to the phase resolution of the electrochemical response of the electrode the non-faradaic effects (the equivalence of the charging ... 

Coulometric sensors are very similar to the amperometric devices. The gas species is dosed by the faradaic effect, but, in this case, the diffusion flux between the chamber and the analyzed atmosphere is negligible dining the measurement. The current pumps the internal partial pressure (PjJ from the external value (Pout)> which corresponds to equilibrium condition, to zero or from zero to P t. The electroactive gas is supposed to be an ideal and so its partial pressure is equal to... 

Figure 7.16 shows the results of an oscillographic measurement on PbO in a time range in which the interfacial process can be observed and the bulk process appears as a jump. The mechanistic interpretation of the bulk resistance was largely the subject of the previous chapters. The source of bulk capacitance has been discussed briefly above. We now wish to address interfacial layer parameters in more detail. First we neglect the influence of electrical capacitances by referring to the steady state and concentrate on the resistive parameters. Similarly, when we will discuss capacitive effects later on (Section 7.3.3) we will ignore Faradaic effects. The combination of both will, in a linear response approximation, be performed by a parallel... 

The dependence of the C,E curves for a solid metal on the method of electrode surface preparation was reported long ago.10 20 67 70 219-225 in addition to the influence of impurities and faradaic processes, variation in the surface roughness was pointed out as a possible reason for the effect.10 67,70 74 219 For the determination of R it was first proposed to compare the values of C of the solid metal (M) with that of Hg, i.e., R = C-M/c-Hg 10,74.219-221 data at ff=0 for the most dilute solution (usually... 

Despite the surprise caused by the first literature reports of such large non-Faradaic rate enhancements, often accompanied by large variations in product selectivity, in retrospect the existence of the NEMCA effect can be easily rationalized by combination of simple electrochemical and catalytic principles. 

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. 

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.

Figure 9.26 shows the steady state effect of applied current I on the induced changes, ArH2(=rH2 -r 2) and Ar0(=ro-io )> in the rates of consumption of H2 and O respectively, where the superscript o always denotes open-circuit conditions. The dashed lines in Fig. 9.26 are constant Faradaic efficiency, A, lines. The maximum measured A values are near 40 at low current densities. This value is in excellent qualitative agreement with the following approximate expression which can predict the magnitude of A in NEMCA studies ... 

The rate of ammonia production was enhanced by more than 1100% in the nitrogen rich regime (Figs 9.33 and 9.34), upon potential application of -IV between the working electrode and the Ag reference electrode. The extent of the NEMCA effect depends strongly on the kinetic regime of the reaction. Very pronounced non-faradaic behavior is observed in the regime 0.33

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The steady-state effect of positive current on ArH2 and Ar0 is shown in Fig. 10.2. The faradaic efficiency A exceeds 20 (2000%) fow low currents. Fig. 10.3 shows the corresponding effect of catalyst potential UWR=Urhe on rH2 and r0, together with the dependence of I on E. 

On the basis of experimental findings Heinze et al. propose the formation of a particularly stable, previously unknown tertiary structure between the charged chain segments and the solvated counterions in the polymer during galvanostatic or potentiostatic polymerization. During the discharging scan this structure is irreversibly altered. The absence of typical capacitive currents for the oxidized polymer film leads them to surmise that the postulated double layer effects are considerably smaller than previously assumed and that the broad current plateau is caused at least in part by faradaic redox processes. 

Makharia et al., 2005]. These spectra display a major loop in the Z" versus Z plot that cuts the Z axis at some frequency in the range 0.1-1 Hz, followed by an inductive loop that cuts the Z axis again at a much lower frequency. This frequency response of the interfacial faradaic process likely reflects variations of ORR current in response to a cychc potential perturbation, originating from two effects of the potential on ORR rate, which are well resolved by their different response times. A relevant expression describing this behavior is likely of the form... 

The control experiment in pure supporting electrolyte (dotted lines in Fig. 13.2) shows a sharp faradaic current spike, which is mainly due to pseudocapacitive contributions (adsorption of (bi)sulfate and rearrangement of the double layer) plus oxidation of adsorbed Hupd (dotted lines in Fig. 13.2a), but no measurable increase in the CO2 partial pressure (m/z = 44 current) above the background level (dotted lines in Fig. 13.2b). Therefore, a measurable adsorption of trace impurities from the base electrolyte can be ruled out on the time scale of our experiments. Moreover, this experiment also demonstrates the advantage of mass spectrometric transient measurements compared with faradaic current measurements, since the initial reaction signal is not obscured by pseudocapacitive effects and the related faradaic current spike. 

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