Charge metal-surface interphase

The simplest model of the structure of the metal-solution interphase is the Helmholtz compact double-layer model (1879). According to this model, all the excess charge on the solution side of the interphase, qs. is lined up in the same plane at a fixed distance away from the electrode, the Helmholtz plane (Fig. 4.4). This fixed distance xH is determined by the hydration sphere of the ions. It is defined as the plane of the centers of the hydrated ions. All excess charge on the metal, qM, is located at the metal surface. 

Figure 1. Schematic picture of the metal/solution interphase in the case of nonspecific (a) and specific (b) anionic adsorption, x = 0, x = P and x = d are die electrode surface plane, the plane of closest approach for the specifically adsorbed anions, and that for the nonspecifically adsorbed ions. Curve 1 represents the potential-distance profile. In (b), curve 1 results from the combination of curve 2, expressing die contribution from the charge density as of the specifically adsorbed anions, and curve 3, expressing die contribution from die charge density Om on the metal. The potential difference, ft1 — d> across die inner layer is the same in (a) and (b). (Reprinted from Ref.7 with permission from the Am. Chem. Soc.)...

One such properly is the capacitance, which is observed whenever a metal-solution interphase is formed. This capacitance, called the double layer capacitance, is a result of the charge separation in the interphase. Since the interphase does not extend more than about 10 nm in a direction perpendicular to the surface (and in concentrated solutions it is limited to 1.0 nm or less), the observed capacitance depends on the structure of this very thin region, called the double layer. If the surface is rough, the double layer will follow its curvature down to atomic dimensions, and the capacitance measured under suitably chosen conditions is proportional to the real surface area of the electrode. 

This two-step profile of the potential across the interphase is also characteristic for certain types of modified electrodes in which the metal surface is coated with a film, whose thickness significantly exceeds the atomic scale. Such systems represent a much more complicated type of electrified interfaces, since the distribution of charged species depends crucially on the specific properties of the film. In most cases of sufficiently thick films, the profile of the Galvani potential across the interphase possesses a plateau inside the bulk film separating two potential drops at its interfaces with the electrode and the solution [16]. 

Gouy-Chapman (GC), that is, to consider the whole interphase as consisting of two layers, a compact (or inner or Helmholtz ) and a diffuse one. The former corresponds to the aforementioned hypothesis of an ion-free layer of the solvent at the metal surface. The diffuse layer is located between the compact one and the bulk solution and the whole counterion charge is distributed inside this region in accordance with the GC theory. 

The charge on each side of the interphase is the sum of the charge of the different charged components. For a metal electrode and on the metal side, using the simple jellium model, this is the difference between the positive metal kernels and the electrons. This difference represents the asymmetry of the electron density on the metal surface. 

At the metal/liquid interphase, the conversion from electronic to ionic conduction occurs. The electrode metal is the source or sink of electrons, and electron transfer is the key process whereby the electrode exchanges charges with the arriving ions, or ionizes neutral substances (a second mechanism of charge transfer is by oxidation of the electrode metal the metal leaves the surface as charged cations and enters the solution). Without electron transfer, there is no chemical electrode reaction, no DC electrode current, and no faradaic current. In the solution at the electrode surface, the electric double layer is formed as soon as the metal is wetted. Electron transfer takes place somewhere in the double layer. 

Resistance to current flow also occurs as a consequence of solid corrosion product buildup on the metal surface. This phenomenon is most pronounced in environments containing H2S. Iron sulfide is a semiconductor whose conducting properties depend on the nature of the environment. It had been observed [39] that the anodic and cathodic polarization curves on iron sulfide covered electrodes are linear rather than exponential. In this case, the current flow is entirely controlled by the charge transfer across the interphase (not interface) consisting of FeS. The polarization admittance (1/Rp) becomes... 

A second possibility which must now be considered is that in layer 1 (of figure 1), because of the proximity of the metal surface some abnormal positions become possible for the metal cations (as in figure 5). They may be positions which are simply closer to the electrode than the normal ones but in addition a fraction (3) (X) of the cation charge (z e) may be neutralized by partial electron transfer from the metal. This is still the blocking case with equilibrium between metal cations in the bulk of the metal and the different metal cations in the bulk of the electrolyte. Thus the interphase might be that between Pt and Agi+Rbl5 at potentials between 0 mV and 600 mV anodic to Ag/Ag . ... 

The independence of this minimum in the C,- vs. cr curve of the nature of anions can be readily explained by remembering that at the high negative charge (cr, = — 12 x 10 Cm ) anions are repelled from the electrode and the layer closest to the metal surface is that of cations. The independence of temperature shows that the structure of the inner part of the interphase is... 

At any interface between two different phases there will be a redistribution of charge in each phase at the interface with a consequent loss of its electroneutrality, although the interface as a whole remains electrically neutral. (Bockris considers an interface to be sharp and definite to within an atomic layer, whereas an interphase is less sharply defined and may extend from at least two molecular diameters to tens of thousands of nanometres the interphase may be regarded as the region between the two phases in which the properties have not yet reached those of the bulk of either phase .) In the simplest case the interface between a metal and a solution could be visualised as a line of excess electrons at the surface of the metal and an equal number of positive charges in the solution that are in contact with the metal (Fig. 20.2). Thus although each phase has an excess charge the interface as a whole is electrically neutral. 

In Chapters 2 and 3 we have described basic structural properties of the components of an interphase. In Chapter 2 we have shown that water molecules form clusters and that ions in a water solution are hydrated. Each ion in an ionic solution is surrounded predominantly by ions of opposite charge. In Chapter 3 we have shown that a metal is composed of positive ions distributed on crystal lattice points and surrounded by a free-electron gas which extends outside the ionic lattice to form a surface dipole layer. 

The surface tension was stated (Section 6.4.5), on general grounds, to be related to the surface excess of species in the interphase. The surface excess in turn represents in some way the structure of the interface. It follows therefore that electrocapillaiy curves must contain many interesting messages about the double layer at the electrode/ electrolyte interface. To understand such messages, one must learn to decode the electrocapillary data. It is necessary to derive quantitative relations among surface tension, excess charge on the metal, cell potential, surface excess, and solution composition. 

In the mechanisms to be described in this section, one of the idealizations of electrochemistry is being portrayed. Thus, in perfectly polarizable metal electrodes, it is accepted that no charge passes when the potential is changed. However, in reality, a small current does pass across a perfectly polarizable electrode/solution interphase. In the same way, here the statement free from surface states (which has been assumed in the account given above) means in reality that the concentration of surface states in certain semiconductors is relatively small, say, less than 10 states cm. So when one refers to the low surface state case, as here, one means that the surface of the semiconductor, particularly in respect to sites energetically in the energy gap, is covered with less than the stated number per unit area. A surface absolutely free of electronic states in the surface is an idealization. (If 1012 sounds like a large number, it is in fact only about one surface site in a thousand.) A consequence of this is the location of the potential difference at the interphase of a semiconductor with a solution. As shown in Fig. 10.1(a), the potential difference is inside the semiconductor, and outside in the solution there is almost no potential difference at all. 

The strict thermodynamic analysis of an interfacial region (also called an -> interphase) [ii] is based on data available from the bulk phases (concentration variables) and the total amount of material involved in the whole system yielding relations expressing the relative surface excess of suitably chosen (charged or not charged) components of the system. In addition, the - Gibbs equation for a polarizable interfacial region contains a factor related to the potential difference between one of the phases (metal) and a suitably chosen - reference electrode immersed in the other phase (solution) and attached to a piece of the same metal that forms one of the phases. 

The first attempt to explain the capacitive nature of the interphase is credited to Helmholtz, in the middle of the nineteenth century. In his model, the interphase is viewed as a parallel-plate capacitor - a layer of ions on its solution side and a corresponding excess of charge on the surface of the metal. It should be noted here that electroneutrality must be maintained in the bulk of all phases, but not at the... 

The surface excess is an integral quantity. This has the advantage of relieving us of the need to define the boundary of the interphase. On the other hand, it cannot yield any information on the variation of the concentration inside the inlerphase. Another point to remember is that the surface excess, as defined here, can have both positive and negative values. This statement is generally correct, but its validity can most easily be seen in electrochemistry. Thus, a negative charge on the metal (q < 0) causes a positive surface excess of cations and a... 

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