Electrolytes, double-layer

Figure A2.4.7. Hypothetical structure of the electrolyte double layer. From [15],...

Figure 1.5. Schematic representation of a metal electrode deposited on a 02 -conducting (left) and on a Na -conducting (right) solid electrolyte, showing the location of the metal-electrolyte double layer and of the effective double layer created at the metal/gas interface due to potential-controlled ion migration (backspillover).

In summary AC impedance spectroscopy provides concrete evidence for the formation of an effective electrochemical double layer over the entire gas-exposed electrode surface. The capacitance of this metal/gas double layer is of the order of 100-300 pF/cm2, comparable to that corresponding to the metal/solid electrolyte double layer. Furthermore it permits estimation of the three-phase-boundary length via Eq. 5.62 once the gas exposed electrode surface area NG is known. 

Mortari, A., Maaroof A., Martin, D. and Cortie, M.B. (2007) Mesoporous gold electrodes for measurement of electrolytic double layer capacitance. Sensors and Actuators B, 123, 262-268. 

When the potential difference across the electrode/electrolyte double-layer is not at its equilibrium value then the interface is said to be polarised. When the interface is polarised a net current will flow, the magnitude of the current being dependent upon the difference in the total anodic and total cathodic currents,... 

The amount of charge which can be stored in the electrode/electrolyte double layer is typically of the order of 15-40 pF/cm2. A large capacitance... 

Charging of the electrode-electrolyte double-layer capacitance to the new potential... 

The electrolyte double layer surface tension, charge density, and capacity... 

In this chapter the structure of the electrolyte double layer, and the consequences of adsorption on the electrode surface, are described. The effect of differences in structure and electronic distribution of different metals are indicated. The space-charge region in semiconductors is then discussed. Finally some properties of colloids are mentioned, given that they possess an interfacial region very similar to an electrode. 

Nevertheless, an important application of electrostatic models is to the interface between two immiscible electrolyte solutions. This can be viewed as two electrolyte double layers arranged back to back. In reality, however, total immiscibility never occurs and the degree of miscibility increases with the presence of electrolyte, so that corrections to the models need to be introduced. 

The crucial point is that the difference of potential available to effect electrode reactions and surmount activation barriers is not simply the difference between the Galvani potential (i.e. the Fermi energy) and the potential in solution. On the side of the solid it is the Volta potential and on the side of the solution it is the potential at the inner Helmholtz plane, where species have to reach to in order for electron transfer to be possible. Corrections to rate constants for the latter are commonly carried out using the Gouy-Chapman model of the electrolyte double layer and will be described in Section 6.9. 

The effect of the electrolyte double layer on electrode kinetics... 

The electrolyte double layer affects the kinetics of electrode reactions6. For charge transfer to occur, electroactive species have to reach at least... 

Hyperbolic functions frequently appear in electrochemical problems, for instance in the inversion of Laplace transforms. An important example of the use of the cosh function is in the expression for the differential capacity of the electrolyte double layer following the Gouy-Chapman model (Chapter 3) which has a minimum value and is symmetric around this minimum—compare Fig. 3.6 with the cosh function in Fig. A1.4. 

In electrochemical systems, the capacitance may arise due to charge redistribution at interfaces or to dielectric phenomena. The charge redistribution takes place at an electrolytic double layer. 

The unique characteristic of the effective double layer, in comparison to all classical metal/electrolyte double layers, is that it is directly accessible to gaseous reactants. Thus electrochemical promotion is catalysis in the presence of a... 

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