Interphase electrode-solution electrical field

In the electrode-solution interphase, the adsorption of these substances is also affected by the influence of the electric field in the double layer on their dipoles. Substances that collect in the interphase as a result of forces other than electrostatic are termed surface-active substances or surfactants. 

When charges are separated, a potential difference develops across the interface. The electrical forces that operate between the metal and the solution constitute the electrical field across the electrode/electrolyte phase boundary. It will be seen that although the potential differences across the interface are not large ( 1 V), the dimensions of the interphase region are very small (—0.1) and thus the field strength (gradient of potential) is enormous—it is on the order of 10 V cm. The effect of this enormous field at the electrode/electrolyte interface is, in a sense, the essence of electrochemistry. 

Electrodic reactions that underlie the processes of metal deposition, etc., cannot be understood without knowing the potential difference at the electrode/solution interface and how it varies with distance from the electrode. The ions from the solution must be electrically energized to cross the interphase region and deposit on the metal. This electrical energy must be picked up from the field at the interface, which itself depends upon the double-layer structure. Thus, control over metal deposition processes can be improved by an increased understanding of double layers at metal/solutioii interfaces. 

As a result of the availability of charge carriers, all the potential difference between two electrodes is dropped across the two interphases for an electrolyte solution and not across the bulk solution phase. When a current passes across the solution, there is a possibility that a potential difference will develop due to the finite conductivity of the solution. In most electroanalytical experiments this is very small compared to the interfacial potential difference and always results in a comparatively weak electric field (small potential dropped across a large distance). This matter will be dealt with beginning in Chapter 6. 

Experimentally it was early recognized that an electrode/solution interphase behaves as a capacitor [74]. This is because the negatively charged surface of the cathode generates a very strong electrical field that tends to attract positive ions from the solution, as sketched in Fig. 14a. The positive layer thus formed exerts, on the solution side, an electrical field of opposite direction that attracts negative ions from the solution. 

In the particular case of an electrode being in contact with a solution, at the interphase a new arrangement of solvent dipoles, ions in the solution and electrons in the electrodes is obtained. Equal and opposite charge concentrations arise on each side of the contact surface and consequently an electrical field is built up (fig. 1.2). 

The surface films discussed in this section reach a steady state when they are thick enough to stop electron transport. Hence, as the surface films become electrically insulating, the active electrodes reach passivation. In the case of monovalent ions such as lithium, the surface films formed in Li salt solutions (or on Li metal) can conduct Li-ions, and hence, behave in general as a solid electrolyte interphase (the SEI model ). See the basic equations 1-7 related to ion transport through surface films in section la above. The potentiodynamics of SEI electrodes such as Li or Li-C may be characterized by a Tafel-like behavior at a high electrical field and by an Ohmic behavior at the low electrical field. The non-uniform structure of the surface films leads to a non-uniform current distribution, and thereby, Li dissolution from Li electrodes may be characterized by cracks, and Li deposition may be dendritic. The morphology of these processes, directed by the surface films, is dealt with later in this chapter. When bivalent active metals are involved, their surface films cannot conduct the bivalent ions. Thereby, Mg or Ca deposition is impossible in most of the commonly used polar aprotic electrolyte solutions. Mg or Ca dissolution occurs at very high over potentials in which the surface films are broken. Hence, dissolution of multivalent active metals occurs via a breakdown and repair of the surface films. 

When solute molecules and ions are introduced into the polar liquid, a more complicated situation is encountered. The solute molecules will be either attracted or repulsed when reaching the interphase. When the solute consists of large molecules or ions, they will also be polarized in the electric field of the interphase. Thus the structure of the solution will be modified in the interphase region and some species may become adsorbed on the electrode as a result of this modification. Can this adsorption be treated in the same way as adsorption of gases on metals There is one important difference between gas adsorption and solute adsorption on metals— in the gas-metal case the reaction is... 

A word of explanation may be needed here. It was said in Section B of Chapter 3 that migration of ions under the influence of electric field is not an important mass transfer mode in fairly concentrated solutions. Here we say that the field of the interphase changes the concentrations of ions. Are these statements not contradictory That this is only an apparent contradiction becomes obvious when we remember that the thickness of the diffusion layer is commonly ten thousand times that of the double layer. Thus it is true that species move most of the distance from the bulk of the solution to the electrode surface in the gradient of forces other than electrical. At the same time the concentration at the electrode surface in the presence of field differs from that in the absence of one. 

From the preceding discussion it is clear that to understand electrode reactions and to predict their behaviour at various conditions, we must know the potential distance curve in the interphase and its dependence on the solution constituents and potential of the electrode we should know the influence of very strong electric fields on the reactant species and also the extent of adsorption of various species on the electrode. All this information could be derived readily had we known the microstructure of the interphase, i.e. the coordinates and velocities of all molecules present in the interphase region and the dependence of these coordinates and velocities on potential and other thermodynamic variables. This information, however, can never be obtained because of the interaction between the particles and our measuring device, an interaction which is formulated in Heisenberg s uncertainty principle. The next best data are the macrostructure, i.e. the concentration... 

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