Electrode potential dependence, of the

Figure 3.3.10 (A) The electrode potential dependence of the Gibbs free energy reaction pathway of the ORR. While the overall reaction has elementary steps that are energetically uphill at +1.23 V (red pathway), all elementary steps become downhill at +0.81 V (yellow pathway) (i.e. at an overpotential of approximately -0.42 V. At this point, the reaction is not limited by kinetics anymore. (B) The experimentally observed current-potential (j-E) relation of the ORR is consistent with the computational conclusions from (A) between +1.23 V and +0.81 V the j-E curve shows an exponential behavior, while at electrode potentials below +0.81 V, the ORR reaction rate becomes oxygen mass-transport limited, which is reflected by a flat ( j-E) profile. Figure adapted with permission from [19]. 

Figure 7-25. Measurements of the single molecule conductance of 6V6 molecular wires performed under electrochemical control in 0.1 M phosphate buffer solution. The electrode potential dependence of the conductance at constant tip-sample bias (Ut = 0.2 V) of a single 6V6 molecule is shown. " ...

The electrode potential dependence of the C NMR spectra of CO adsorbed on Pd powder provides essential additional evidence for alterations to the metal/solution interface due to the application of an electric field." Samples consisted of Pd powder with an average particle size of about 8 nm, produced by a gas condensation technique. Pd powder was cleaned in 0.1 M NaOH through multiple cycles of potential controlled reduction and oxidation. An adlayer of CO was produced from CO saturating the supporting electrolyte. The cleaned powder and electrolyte were introduced into a sealed vessel and stirred under COgas. 

Comparing the results of EC-NMR and IR investigations, we find that the potential dependence of C NMR shift and the vibrational frequency of adsorbed CO are primarily electronic in nature, and originate from changes in the f-LDOS. C NMR results show that CO adsorbed on Pt, either directly from CO gas or from methanol oxidation, have the same electronic properties. That is, the chemisorbed product (surface CO) from CO solutions and from methanol decomposition is the same. The electrode potential dependence of the C NMR spectra of CO adsorbed on Pt and Pd nanoparticles provide direct evidence for electric field induced alterations in the E/ -LDOS. In relation to fuel cell catalysis, EC-NMR investigations of Pt nanoparticles decorated with Ru show that there exist two different kinds of CO populations having markedly different electronic properties. COs... 

Figure 10.6 The electrode potential dependence of the integrated band intensity of terminal H observed In 0.01 M H2SO4 O, 0.5 M H2SO4 ( ), 3 M H2SO4 (o) and 1 M HCIO4 (d)-The data points shown by (+) were taken from Ref [10] and multiplied by a factor of 8.5 to compensate for the difference in sensitivity between SEIRAS and IRAS. Reprinted from Ref. [15] with permission from Elsevier.

It is the electrode potential dependence of the latter step that is responsible for the extreme slow-down in adsorption velocity at cathodic potentials. 

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

This relation shows that the lifetime of PMC transients indeed follows the potential dependence of the stationary PMC signal as found in the experiment shown in Fig. 22. However, the lifetime decreases with increasingly positive electrode potential. This decrease with increasing positive potentials may be understood intuitively the higher the minority carrier extraction (via the photocurrent), the shorter the effective lifetime... 

Thus, the temperature coefficient of Galvanic potential of an individual electrode can be neither measured nor calculated. Measured values of the temperature coefficients of electrode potentials depend on the reference electrode employed. For this reason a special scale is used for the temperature coefficients of electrode potential It is assumed as a convention that the temperature coefficient of potential of the standard hydrogen electrode is zero in other words, it is assumed that the value of Hj) is zero at all temperatures. By measuring the EMF under isothermal conditions we actually compare the temperature coefficient of potential of other electrodes with that of the standard hydrogen electrode. 

The form of the kinetic equation depends on the way in which the surface potential X varies with electrode potential E. When the surface potential is practically constant, the first factor in Eq. (14.24) will also be constant, and the potential dependence of the reaction rate is governed by the second factor alone. The slope b of the polarization curve will be RT/ F (i.e., has the same value as that found when the same reaction occurs at a metal electrode). When in another case a change in electrode potential E produces an equally large change in surface potential (i.e., E = x + const), while there is practically no change in interfacial potential. Then Eq. (14.24) changes into... 

In general, the potential dependence of the current is determined by both the potential dependence of the concentrations of the reacting particles near the electrode surface and the potential dependence of the reaction rate constant itself (i.e., the probability of the elementary reaction act per unit time, W). 

The surface concentration Cq Ajc in general depends on the electrode potential, and this can affect significantly the form of the i E) curves. In some situations this dependence can be eliminated and the potential dependence of the probability of the elementary reaction act can be studied (called corrected Tafel plots). This is, for example, in the presence of excess concentration of supporting electrolyte when the /i potential is very small and the surface concentration is practically independent of E. However, the current is then rather high and the measurements in a broad potential range are impossible due to diffusion limitations. One of the possibilities to overcome this difficulty consists of the attachment of the reactants to a spacer film adsorbed at the electrode surface. The measurements in a broad potential range give dependences of the type shown in Fig. 34.4. 

In 1873, Gabriel Lippmann (1845-1921 Nobel prize, 1908) performed extensive experiments of the electrocapiUary behavior of mercury and established his equation describing the potential dependence of the surface tension of mercury in solutions. In 1853, H. Helmholtz, analyzing electrokinetic phenomena, introduced the notion of a capacitor-like electric double layer on the surface of electrodes. These publications... 

Figure 5.8 shows the potential dependence of the relative phase difference between and X . The relative phase was changed by about 180° at 200 mV, which is close to the pzc for a Pt electrode in HCIO4 electrolyte solution [52,5 3]. This orientation change is most probably associated with a change in sign of the charge at the Pt surface. This clearly demonstrates that the orientation of water dipoles flips by 180° at the pzc. 

Equation (3.3) gives the potential dependence of the reaction free energy of Reaction (3.2). Since this reaction equilibrium defines the standard hydrogen electrode potential, we now have a direct fink between quite simple DFT calculations and the electrode potential. In a similar way, we can now calculate potential-dependent reaction free energies for other reactions, such as O - - H" " + e OH or OH - -+ e HzO. 

Lopez-Cudero A, Cuesta A, Gutierrez C. 2005. Potential dependence of the saturation CO coverage of Pt electrodes The origin of the pre-peak in CO-stripping voltammograms. Part 1 Pt(lll). J Electroanal Chem 579 1-12. 

For semiconductor electrodes and also for the interface between two immiscible electrolyte solutions (ITIES), the greatest part of the potential difference between the two phases is represented by the potentials of the diffuse electric layers in the two phases (see Eq. 4.5.18). The rate of the charge transfer across the compact part of the double layer then depends very little on the overall potential difference. The potential dependence of the charge transfer rate is connected with the change in concentration of the transferred species at the boundary resulting from the potentials in the diffuse layers (Eq. 4.3.5), which, of course, depend on the overall potential difference between the two phases. In the case of simple ion transfer across ITIES, the process is very rapid being, in fact, a sort of diffusion accompanied with a resolvation in the recipient phase. 

Figure 25a, as an example, shows the potential dependence of the single-junction conductances of 44-BP measured in 0.1 M HCIO4 solution (pH 1) in —0.10 V < E < 0.90 V in a semi-logarithmic representation. The values of L, M, and H decrease with more positive electrode potentials, and follow nearly the same trend for each family. The single-junction conductances decrease by a factor of 3-5 upon potential excursion towards positive values in the accessible potential region. A similar trend is also observed for electrolytes with variable pH ranging between 1 and 10, as... 

Typical plots for the coverage and the capacity as a function of the electrode potential are shown in Fig. 4.16. Note the pronounced maxima in the capacity near the potentials where the substance is desorbed. Equation (4.18) can be improved by allowing for the potential dependence of the two capacities Co and Ci, and for a shift in.the pzc with adsorption, but little is gained in physical insight. 

Potential dependency of the enzyme activity of the FDH/PP/Pt electrode is shown in Fig. 28. The dependency was investigated by adding 5mM fructose and the resulting current response was compared. 

This result is quite in contrast to the common expectation that the electrode potential changes the activation barrier at the interface which would result in a temperature independent transfer coefficient a. Following Agar s discussion (30), such a behavior indicates a potential dependence of the entropy of activation rather than the enthalpy of activation. Such "anomalous" behavior in which the transfer coefficient depends on the temperature seems to be rather common as recently reviewed by Conway (31). 

The latter discussion confirms the results of the potential dependence of the current in that the activation barrier for the hydrogen evolution reaction is, at least on copper and silver, not affected by the electrode potential. This behavior is, on the other hand, connected with the observation of straight lines in a Tafel plot. It would be premature to come up with a comprehensive model that would explain this behavior more experimental work is necessary to substantiate and quantify the effects for a larger variety of systems and reactions. A few aspects, however, should be pointed out. 

Evidence that H20 species also interact with the Ag electrode independent of adsorbed anions comes from the potential dependence of the i/(0H) intensity as compared with the i/(Ag-X) (X-Cl", Br") intensities. The normalized intensities of the i/(Ag-X) (X-Cl", Br") vibrations in 0.1 M KC1 and 0.1 M KBr are shown in Figure la, and the corresponding intensities of the v(0H) vibration shown in Figure lb. The observation that the intensity of the i/(0H) vibration reaches a maximum at more negative potentials than the i/(Ag-X) (X-Cl", Br") vibrations has been interpreted as indication that the H20 molecules can become maximally adsorbed on the surface when the positive charge has decreased to allow partial desorption of the anions.(21) Obviously, the potential at which this occurs depends on the strength of interaction of the anion with the electrode. 

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