Interfadal potential

Next, we consider the interface M/S of a nonpolarizable electrode where electron or ion transfer is in equilibrium between a solid metal M and an aqueous solution S. Here, the interfadal potential is determined by the charge transfer equilibrium. As shown in Fig. 4-9, the electron transfer equilibrium equates the Fermi level, Enn) (= P (M)), of electrons in the metal with the Fermi level, erredox) (= P s)), of redox electrons in hydrated redox particles in the solution this gives rise to the inner and the outer potential differences, and respectively, as shown in Eqn. 4-10 ... 

For the hydrogen electrode, the interfadal potential between the electrode metal and the hydrogen gas film is determined by the electron transfer equilibrium and the interfacial potential between the hydrogen gas film and the aqueous... 

In recent investigations, it appears that the interfadal potential between a metal electrode and an aqueous solution somehow survives after the electrode is taken out of the aqueous solution and into ultra high vacuum or an inactive gas phase [Wagner, 1993]. This circumstance is referred to as emersion . As shown in Fig. 4—26, the electrode potential E m of the emersed electrode is... 

Fig. 6-40. An interfadal potential, distributed to Msc in the space charge layer and to in the compact layer as a function of the concentration of surface states, D . [From Chandrasekaran-Kainthla-Bockris, 1988.]...

The maximum electrical potential in the compact layer A < - includes a dipolar potential which is shown schematically as a narrow region at the sharp interface. A dipolar layer can be located not only in the compact layer but can also occupy part of the diffuse layer. The amplitude and sign of isPpg can differ from the total interfadal potential. Figure 4 illustrates four possibilities for potential distribution at ITIES. Generally, the dipolar potential depends on the total interfadal potential A <. ... 

The interfadal potential difference consists of the sum of the potential drops ... 

The potential and concentration distributions described for the system with no kinetic limitations to interfadal reactions are constrained by the rates of generation and mass transfer in the semiconductor. More generally, kinetic limitations to interfadal reactions are compensated by the increased interfadal potential and concentration driving forces required to allow passage of electrical current. In contrast to the results shown as curve c in Fig. 4, the surface concentration of holes under kinetic limitations to interfadal reactions can increase with increasing current density. The presence of these limitations may be inferred from experimental data by inflection points in the current-potential curve. 

Fig. 5-8. An interfadal double layer model (triple-layer model) SS = solid surface OHP = outer Helmholtz plane inner potential tt z excess charge <2h = distance from the solid surface to the closest approach of hydrated ions (Helmluritz layer thickness) C = electric capacity.

Fig. 6-13. Potential created at a contact interface between metal M and a<)ueoii8 solution S (a) before contact, (b) after contact, (c) charge-induced and dipole-induced potentials X = surface potential at free surfaces gdip = potential due to an interfadal dipole gun = potential due to an interfadal charge = potential across an interfadal compact layer.

The surface potential, Xm> due to the interfadal dipole of the electron tailing away from the metal surface is given as a function of the excess or defidt of metal electrons in Eqn. 5-27 ... 

The potential Ma across the compact layer is a function of the interfadal chaiges ou and = z eFaa shown in Eqn. 5—43 ... 

The clean surface of metals in vacuum sustains a surface lattice transformation, as described in Sec. 6.1. Similarly, an interfadal lattice transformation takes place also on metal electrodes in aqueous solutions. In general, the interfadal lattice transformation of metal electrodes is affected by both the electrode potential and the ionic contact adsorption. 

Fig. 6-96. Change in differential capacity of an interfadal double layer leading or not leading to interfadal lattice transformation in anodic and cathodic potential sweeps for a gold electrode surface (100) in perchloric add solution Ey = critical potential beyond which the interfadal lattice transforms from (5 x 20) to (1 x 1) E = critical potential below which the interfadal lattice transforms from (1 x 1) to (5 x 20) Ejm = potential of zero charge VacE = volt referred to the saturated calomel electrode. [From Kolb-Schneider, 1985.]...

Fi . 5-37. Degree of transformation of interfadal lattice (1x1) - (5x20) observed as a function of cathodic potential for a gold electrode with a plane (100) in perchloric add solution A(total) = total interface area A(5x20) s interface area of (5x20) supper lattice. [From Kolb-Schneider, 1988.]... 

Equation 5-82 indicates that the potential AjtH across the compact layer depends linearly on the solution pH. This potential Atv includes both the potential due to the interfadal charge oh (= [-OHj (b)] — [-0 (a)]) and the potential 4 due to the interfadal dipole, 4 >h = 4o + as shown in Fig. 5-51. 

Fig, 6-61. Potential across a compact layer without adsorbed ions on semiconductor electrodes Oh = interfadal charge of dissociated hydroxyl group os = excess charge at OHP on the solution side d dip = potential of a compact layer due to interfadal dipole = potential of a compact layer due to interfadal charge. 

Fig. 6-53. Interfadal charges, electron levels and electrostatic potential profile across an electric double layer with contact adsorption of dehydrated ions on semiconductor electrodes ogc = space charge o = charge of surface states = ionic charge due to contact adsorption dsc = thickness of space charge layer da = thickness of compact la3rer.

Fig. 5-56. Capacity Csc of a space charge layer and capacity Ch of a compact layer calculated for an n-type semiconductor electrode as a function of electrode potential Ct = total capacity of an interfadal double layer (1/Ct = 1/ Csc+ 1/Ch). [From Gerisdier, 1990.]...

Figure 9-1 illustrates the energy barrier to the transfer of metallic ions across the electrode interface these energy barriers are represented by two potential energy curves, and their intersection, for surface metal ions in the metallic bond and for hydrated metal ions in aqueous solution. As described in Chaps. 3 and 4, the energy level (the real potential, a. ) of interfadal metal ions in the metallic bonding state depends upon the electrode potential whereas, the energy level (the real potential, of hydrated metal ions is independent of the electrode potential. 

Fig. 9-1. Potential energy profile for transferring metal ions across an interface of metal electrode M/S py. = metal ion level (electrochemical potential) x = distance fiom an interface au. = real potential of interfacial metal ions = real potential of hydrated metal ions - compact layer (Helmholtz layer) V = outer potential of solution S, curve 1 = potential energy of interfadal metallic ions curve 2 = potential energy of hydrated metal ions.

The point at which the straight line of (tph) versus Eintersects the coordinate of electrode potential represents the flat band potential. Equation 10-15 holds when the reaction rate at the electrode interface is much greater than the rate of the formation of photoexcited electron-liole pairs here, the interfadal reaction is in the state of quasi-equilibrium and the interfadal overvoltage t)j, is dose to zero. 

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