Electrode ionic double layer

Specific adsorption may involve short-range, strong interactions due to the overlapping of the electronic orbitals of the adsorbate and the electrode and ionic species or dipoles in the electrolyte. These will be considered in Sect. 6.1 together with the effect of changes of the structure of the interfacial region on electrode kinetics (double layer effects [3,5]). 

The structure of the double layer can be altered if there is interaction of concentration gradients, due to chemical reactions or diffusion processes, and the diffuse ionic double layer. These effects may be important in very fast reactions where relaxation techniques are used and high current densities flow through the interface. From the work of Levich, only in very dilute solutions and at electrode potentials far from the pzc are superposition of concentration gradients due to diffuse double layer and diffusion expected [25]. It has been found that, even at high current densities, no difficulties arise in the use of the equilibrium double layer conditions in the analysis of electrode kinetics, as will be discussed in Sect. 3.5. 

It is well known that solid-state LECs exhibit a significant response time since electroluminescence can only occur after the ionic double-layers have been built up at the electrode interfaces [79,82]. Since in this case only the PFg anion is mobile, the double-layers are formed by accumulation and depletion of PFg at the anode and cathode, respectively. The LEC device with 45 started to emit blue-green light at a bias of 5 V after several minutes. The electroluminescence spectrum, as shown in Fig. 36 (trace a), is very similar to the photoluminescence spectrum recorded for a spin-coated film on glass and of a solution of the complex. For comparison, the electroluminescence... 

The electrochemistry of ionic liquids is different in some essential features from the electrochemistry of aqueous electrolytes. Particularly for electrodeposition, which involves charge transfer from the electrolyte to the electrode, the double layer on the electrode is of great importance. In general the cathode is negatively charged for the electrodeposition of metals and therefore coated with a (Helmholtz-) layer of cations at least 0.5 nm thick but the metal species in most ionic liquids is anionic (for instance AlCh ). This makes the metal deposition process complicated, for more details we refer to Chapter 2. 

It is usually believed that high frequency capacitance obtained from impedance spectroscopy can represent ionic double layer capacity. However in general this is not the case for platinum electrode, this is the reason why one arrow in Fig. 1 (in the left) is crossed. In contrast to Cf measured under equilibrium conditions (by means of isoelectric potential shifts), non-equilibrium impedance response can contain a contribution from Ah (and/or Ao, surface concentration of oxygen-containing species). These contributions are determined by Ah and Ao potential derivatives and their free electrode charge derivatives, and in general can be either positive, or negative. 

To check further on the extent to which electrode reactions on the basal plane may be impeded by semiconductor effects, the Fe(CN)6 -Fe(CN)6 redox couple has been examined and found to have an exchange current on the basal plane which is 1/3 of that on the edge plane. This difference may be caused by a difference in the ratio of true-to-apparent surface area or ionic double-layer effects (different point of zero charge) as well as semiconductor effects but is certainly far less than the two orders of magnitude difference in the exchange current densities for the O2 reduction on the basal and edge planes. 

The experimental methods available for determination of the surface concentration of adsorbed intermediates formed in charge-transfer processes have been reviewed recently by Gileadi and Conway. They have been divided into two groups (a) methods in which the charge needed to form or desorb the surface species is directly measured and b) methods in which the capacity at the interface (i.e., the sum of the ionic double-layer capacity and the adsorption pseudocapacity) is measured as a function of potential and integrated to obtain the charge. These methods will not be discussed further here. The experimental methods described below pertain to the determination of adsorption of uncharged species on solid electrodes. 

For a smooth electrode, the double-layer capacitance shows nofrequency dependence up to ionic relaxation frequencies. 

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

The electrical double layer is the array of charged particles and/or oriented dipoles that exists at every material interface. In electrochemistry, such a layer reflects the ionic zones formed in the solution to compensate for the excess of charge on the electrode (qe). A positively charged electrode thus attracts a layer of negative ions (and vice versa). Since the interface must be neutral. qe + qs = 0 (where qs is the charge of the ions in the nearby solution). Accordingly, such a counterlayer is made... 

For a metal/solution interface, the pcz is as informative as the electron work function is for a metal/vacuum interface.6,15 It is a property of the nature of the metal and of its surface structure (see later discussion) it is sensitive to the presence of impurities. Its value can be used to check the cleanliness and perfection of a metal surface. Its position determines the potential ranges of ionic and nonionic adsorption, and the region where double-layer effects are possible in electrode kinetics.8,10,16... 

Very simply these equations are valid as long as ion backspillover from the solid electrolyte onto the gas-exposed electrode surfaces is fast relative to other processes involving these ionic species (desorption, reaction) and thus spillover-backspillover is at equilibrium, so that the electrochemical potential of these ionic species is the same in the solid electrolyte and on the gas exposed electrode surface. As long as this is the case, equation (5.29) and its consequent Eqs. (5.18) and (5.19) simply reflect the fact that an overall neutral double layer is established at the metal/gas interface. 

The operation of a double-layer capacitor is tied to a displacement of electrolyte ions. In a fully charged capacitor, anions accumulate as counterions in the solution layer next to the positively charged electrode while the eoncentration of the cations decreases. At the negative electrode, the opposite situation is seen. During discharge, the ionic concentrations level out to the bulk solution values by migration and diffusion. 

Previously, we have proposed that SFG intensity due to interfacial water at quartz/ water interfaces reflects the number of oriented water molecules within the electric double layer and, in turn, the double layer thickness based on the p H dependence of the SFG intensity [10] and a linear relation between the SFG intensity and (ionic strength) [12]. In the case of the Pt/electrolyte solution interface the drop in the potential profile in the vicinity ofelectrode become precipitous as the electrode becomes more highly charged. Thus, the ordered water layer in the vicinity of the electrode surface becomes thiimer as the electrode is more highly charged. Since the number of ordered water molecules becomes smaller, the SFG intensity should become weaker at potentials away from the pzc. This is contrary to the experimental result. 

Despite these arguments and the conceptual attractiveness of the procedure which is sketched in Fig. 1 convincing evidence for the relevance of a particular gas phase adsorption experiment can only be obtained by direct comparison to electrochemical data The electrode potential and the work function change are two measurable quantities which are particularly useful for such a comparison. In both measurements the variation of the electrostatic potential across the interface can be obtained and compared by properly referencing these two values 171. Together with the ionic excess charge in the double layer, which in the UHV experiment would be expressed in terms of coverage of the ionic species, the macroscopic electrical properties of the interracial capacitor can thus be characterized in both environments. 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

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