Interfacial Potential Drop

The dipole density profile p z) indicates ordered dipoles in the adsorbate layer. The orientation is largely due to the anisotropy of the water-metal interaction potential, which favors configurations in which the oxygen atom is closer to the 

Solving the one-dimensional Poisson equation with the charge density profile p z) leads to the electrostatic (dipolar) potential drop near the interface according to 

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The ET reaction between aqueous oxidants and decamethylferrocene (DMFc), in both DCE and NB, has been studied over a wide range of conditions and shown to be a complex process [86]. The apparent potential-dependence of the ET rate constant was contrary to Butler-Volmer theory, when the interfacial potential drop at the ITIES was adjusted via the CIO4 concentration in the aqueous phase. The highest reaction rate was observed with the smallest concentration of CIO4 in the aqueous phase, which corresponded to the lowest driving force for the oxidation process. In contrast, the ET rate increased with driving force when this was adjusted via the redox potential of the aqueous oxidant. Moreover, a Butler-Volmer trend was found when TBA was used as the potential-determining ion, with an a value of 0.38 [86]. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

DPSC was recently used to examine whether DMFc transfers across the water-DCE interface at typical interfacial potential drops encountered in SECM studied [86]. For this measurement, the DCE phase contained 10 mM DMFc and 0.1 M THAP, while the aqueous phase contained 0.01 M NaC104 and 0.1 M NaCl. In general, DPSC data obtained for tip-interface separations of 4.4/rm to 14.6/rm were found to be in close agreement with the theoretical predictions for no transfer of electrogenerated DMF across the interface on the SECM time scale. Typical forward and reverse step data for a tip-interface distance of 4.4/am and = 0.371 s, are shown in Fig. 26. 

In electrode kinetic studies, reactant concentrations are, in general, in the millimolar range and double layer contributions for such low ionic concentrations may become very important. If excess of inert or supporting electrolyte is used, the relative variation in the ionic concentration at the double layer due to the electrochemical reaction is at a minimum at high concentration of an inert z z electrolyte, most of the interfacial potential drop corresponds to the Helmholtz inner layer and variations of A02 with electrode potential are small (Fig. 3). In addition, use of supporting electrolyte prevents the migration of electroactive ionic species from becoming important and also reduces the ohmic overpotential. 

The interfacial potential drop at the nonpolarizable ITIES was controlled by varying the concentration of either the cation or the anion of the ionic liquid in the aqueous phase. The kinetics of interfacial ET followed the Butler-Volmer equation, and the measured bimolecular rate constant was much larger than that obtained at the water-1,2-dichloroethane interface. In the second publication, Laforge et al. [112] developed a new method for separating the contributions from the interfacial ET reaction and solute partitioning to the SECM feedback. 

Typical values of transfer coefficients a and ji thus obtained are listed in Table 4 for single crystal and polycrystalline thin-film electrodes [69] and for a HTHP diamond single crystal [77], We see for Ce3+/ 41 system (as well as for Fe(CN)63 /4 and quinone/hydroquinone systems [104]), that, on the whole, the transfer coefficients are small and their sum is less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, a reaction proceeding via the valence band has transfer coefficients a = 0, / =l a + / = 1 [6], Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials fabricated by advanced technologies. Departure from the ideal semiconductor behavior is likely because the interfacial potential drop is located in part in the Helmholtz layer (due e.g. to a high density of surface states), or because the surface states participate in the reaction. As a result, the transfer coefficients a and ji take values intermediate between those characteristic of a semiconductor (0 or 1) and a metal ( 0.5). 

Figure 19. The electronic structure of an n-type semiconductor/electrolyte solution interface under conditions of free electron depletion at the surface. Shown are the conduction and valence band edges as a function of the distance from the surface. The interfacial potential drop is distributed over a region in the solid (depletion region, width 4c) and the molecular Helmholtz layer at the liquid side (not shown). The interfacial capacitance is represented by a series connection of the capacitance of the depletion layer (Csc) and the Helmholtz layer (Csoi).

The distribution of the interfacial potential drop over the semiconductor and the depletion layer is an important problem in the field of electrochemistry. It is often strongly related to the kinetics of interfacial electron transfer [15]. 

Also, in the case of more complex electrochemical processes, these conditions may hold. Consider, for instance, the electrochemical equilibrium between Ag and Ag+ (aq) (Eq. 80). The hydrated silver ion is, finally, outside the double layer its chemical potential does not depend on the electrode potential. Since a change of the electrode potential only affects the interfacial potential drop and (see Section 4.5), it is reasonable to assume that the chemical potential of the silver atoms in the silver phase is also independent of the electrode potential. Hence, also in the case of Eq. 80, the polarization V — leads to a change of only. In contrast, if products or reactants are chemisorbed on the electrode, their chemical potential depends on the electrode potential. If the conditions presented by Eqs. 106 are valid, it also holds that... 

Figure 29. Calculated current-potential characteristics for direct (dashed lines, 0/cm ) and surface state mediated electron transfer between an -type semiconductor electrode and a simple redox system. The plots show the transition from ideal diode behavior to metallic behavior with increasing density of surface states at around the Fermi-level of the solid (indicated in the figures). This is also clear from the plots below, which show the change of the interfacial potential drop over the Helmholtz-layer (here denoted as A(Pfj) with respect tot the total change of the interfacial potential drop (here denoted as A(p). Results from D. Vanmaekelbergh, Electrochim. Acta 42, 1121 (1997).

As a result of the high rate of the hydrogen oxidation process at platinum in contact with this ionomeric medium, the interfacial potential drop at a well-humidified H2 anode in a PEFC operating at 80 °C at 1 A/cm has been usually considered negligible. It should be remembered, however, that this would not be the case when ... 

Girault (lb) pointed out that the apparent potential dependence of the ET rate may be attributed to the change in concentration of the reactants near the interface rather than to activation control. This model, further developed by Schmickler (9), postulates that the rate constant is essentially potential-independent because the potential drop across the compact part of the double-layer at the ITIES is small. In this model, the ET rate dependence on the interfacial potential drop is only due to the diffuse layer effect similar to Frumkin effect at metal electrodes. 

FIG. 3 Different models of interfacial ET. (A) Aqueous and organic redox species are separated by the sharp interfacial boundary. (B) Interfacial potential drop across a thin ion-free layer between redox reactants. (C) ET reaction occurs within a nm-thick mixed solvent layer. No potential drops between reactant molecules. 

The driving force for an ET reaction at the ITIES consists of two components, i.e., the difference of standard potentials of redox mediators and the interfacial potential drop [Eq. (14)]. The dependence of kf on AE° was studied for reactions between ZnPor+ in benzene and the series of similar cyanide complexes in water (26) ... 

The majority of electrochemical problems can be solved without separating the emf into absolute potentials . However, it should be mentioned that the problem concerning the structure of the interfacial potential drop becomes the topical problem for the modern studies of electrified interfaces on the microscopic level, particularly, in attempts of testing the electrified interfaces by probe techniques [92-94]. The absolute scales are also of interest for electrochemistry of semiconductors in the context of calibrating the energy levels of materials. This problem is related to another general problem of physical chemistry - the determination of activity coefficient of an individual charged species. 

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