Electrochemical double layer charge neutrality

The origin of the electrochemical double layer arises from the requirements of charge neutrality in which the surface charge on the cell surface must be balanced against the opposite charge in the water (or any other fluid in a more general case). 

Because of the short lifetime of ions in gaseous atmospheres, even at low pressure, gas-phase IR measurements are limited to adsorption of neutral molecules. Electrochemical applications of the IR method offer the interesting possibility of providing data on the adsorption properties of charged particles (Secs. 8 and 9). In the electrochemical environment the applied potential allows ionic adsorbates to be studied under energetically controllable conditions. Otherwise the electrochemical double layer offers exceptional conditions to investigate the Stark effect on vibrational transitions by setting tunable electric fields of the order of 10 V cm at the interface. This phenomenon will be discussed in Sec. 10. 

Thus, the photoisomerization of the monolayer between the 46a-state and the protonated nitromerocyanine 46b-state provides a means to control the electrical features of the electrode surface, thereby regulating electron transfer at the electrode interface. The 46a-monolayer results in a neutral electrode surface while the 46b-monolayer gives a positively charged surface, causing the formation of an electrical double-layer at the electrode interface. Photoisomerization of the command interface resulting from the different electrochemical kinetics of the soluble redox probe can also be probed by Faradic impedance spectroscopy [90]. A small electron transfer resistance is found for the system when there is an attractive interaction between the charged redox probe and the command interface. Much larger electron transfer resistances are found upon photoisomerization to the state when repulsive interactions exist. 

To define a unique solution, we must specify the corresponding boundary and initial conditions. Normally electrolyte solutions are in contact with or bounded by electrodes. An electrode in its simplest form is a metal immersed in an electrolyte solution so that it makes contact with it. For example, copper in a solution of cupric sulfate is an example of an electrode. A system consisting of two electrodes forms an electrochemical cell. If the cell generates an emf by chemical reactions at the electrodes, it is termed a galvanic cell, whereas if an emf is imposed across the electrodes it is an electrolytic cell (Fig. 6.1.1). If a current is generated by the imposed emf, the electrochemical or electrolytic process that occurs is known as electrolysis. Now whether or not a current flows, the electrolyte can be considered to be neutral except at the solution-electrode interface. There a thin layer, termed a Debye sheath or electric double layer, forms that is composed predominately of ions of charge opposite to that of the metal electrode. We shall examine this double layer in Section 6.4, but for our purposes here it may be neglected. 

It is valid for any electrochemical cell [1], where pT is the electrochemical potential of electrons in the catalyst electrode, Ep (= p) is the Fermi level of the catalyst-electrode and P is the outer (Volta) potential of the metal catalyst-electrode in the gas outside the metal/gas interface. The latter vanishes (T = 0, AO = 0) when no net charge resides at the metal/gas interface [1, 25]. Thus, the experimental Eq. 4 manifests the formation of a neutral double layer, termed effective double layer, at the metal/gas interface (Fig. 2). At the molecular level, the stability of the effective double layer, and thus the validity of Eq. 4, requires that the migration (back-spillover) of the promoting ion (O , Na " ) is fast relative to its desorption or catalytic consumption. When this condition is not met (e.g., high T or non-porous electrodes), or also when the limits of zero or saturation coverage of the promoting ion are reached (at very positive or negative AUwr), then deviations from Eq. 1 are observed [1, 25]. 

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