Adsorption capacitance

The intercept of this line is a function of cKci-. Thus, this theory predicts that the Oo/pH curves at various concentrations should all tend to become parallel lines far from the pHpzc. This is indeed the usual experimental observation. The slope of the surface charge measurements. This had already been noted by Smit (21). The intercept of the straight line predicted by Equation 37 can be used to find cKCi-, provided that as and cdl are known. 

For SiC>2, we have only considered sources for silica suspensions which were non-porous, such as Ludox (39), pyrogenic silica (40), heat-treated BDH silica (22), or ground quartz (41). The data from these sources at 0.1M concentration has been collected in Figure 7. The data of the various researchers is quite consistent, in spite of the differences in origin of the suspensions, and the different electrolytes used. The slope of the points above pH 7 shows that the adsorption capacitance for cations is very large for both sodium and potassium ions, around 200 pF/cm2. Such a capacitance corresponds to a distance of 0.25.X, when using the dielectric constant of immobilized water molecules. The equilibrium constant for adsorption is low, however, since both KNa+ and Kk+ lie between 0.1 and 0.01 dms/mol. A possible interpretation of these results is as follows there is little specific attraction between SiC>2 and alkali cations,... 

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

To appreciate that a majority of non-faradaic currents are caused by the effects of adsorption, capacitance and the electrode double-layer, or by competing side reactions such as solvent splitting. 

Fig 29. A simple equivalent circuit for the artificial permeable membrane. Physical meaning of the elements C, membrane capacitance (dielectric charge displaceme-ment) R, membrane resistance (ion transport across membrane) f pt, Phase transfer resistance (ion transport across interface) Zw, Warburg impedance (diffusion through aqueous phase) Ctt, adsorption capacitance (ion adsorption at membrane side of interface) Cwa, aqueous adsorption capacitance (ion adsorption at water side of interface). From ref. 109. 

The detailed derivation leads to an impedance function, having an equivalent circuit where serially to the adsorption capacitance there are a resistance and a -> Warburg impedance [1]. Important points are as follows ... 

The adsorption capacitance is related to the potential dependence of the equilibrium surface excesses ... 

The simulated complex-plane impedance diagram is shown in Figure 4.27b. As can be seen in the figure, this ladder structure is characterized by two semicircles with two time constants, r, = RclCd] and r2 = R3C2, accounting for the two-step reaction. The element C2 symbolizes the adsorption capacitance, and r2 represents the relaxation of the adsorbing process. 

Meads equilibrium surface concentration on native substrate mol cm actual concentration of species i at the interface S/El concentration of species i bulk concentration of species i adsorption capacitance (adatom pseudo-capacitance) double layer capacitance... 

The difference in behavior of pyrolytic graphite, examined in Ref. 2, and the glassy carbon investigated in this more recent work, is probably done to the different electrocatalytic surfaces that these materials present. Thus, impedance measurements we have made indicate little adsorption pseudocapacitance for adsorbed Br species at glassy carbon whereas, at pyrolytic graphite, an adsorption capacitance is measurable. The results depend on edge or basal-plane exposure. 

Figure 4.26 Equivalent circuit used for the simulation of a metal deposition process on a stepped surface (Ref. , p. 35). electrolyte resistance, Cdi double layer capacitance, charge transfer resistance, ad-atom charge transfer resistance, ad-atom surface diffusion resistance, R ad-atom incorporation resistance, ad-atom adsorption capacitance, and step half-width. (Reproduced with permission from Ref. [59], 1996, Wiley-VCH.)...

Gerischer made an experimental attempt to measure the crystallization overvoltage of Ag deposition in a chronopotentiometric experiment. In this experiment two results were obtained. From the slope at t -> 0 the capacitance was determined. This capacitance was much larger than the double-layer capacitance and was interpreted as adsorption capacitance Qd- The ad-atom concentration was calculated from the adsorption capacitance... 

The in-phase and out-of-phase AC current components are measured as a function of the frequency. The data are analyzed in terms of an equivalent circuit (Fig. 8) containing the uncompensated resistance, the interfacial capacitance, and a series RC circuit for the faradaic current component. Laviron has derived expressions for the adsorption resistance Ra and adsorption capacitance Ca, assuming that a is 0.5 [217]. At r] = 0V, the expressions are particularly simple (Eqs 12 and 13) ... 

Fig. 8 Equivalent circuit for an electroactive SAM. Ru is the uncompensated resistance, CPE is the constant phase element, Ra is the adsorption resistance and Ca is the adsorption capacitance.

Simultaneous analysis of Am(E), k/( ) and adsorption capacitance C E) variations (Fig. 4e-g) shows that the Pb UPD is reversible in the range between around -0.7 V and -0.88 V. There is, however, a significant irreversible component at more positive potentials (Fig. 4e-g). While CJ E) and Am(E) give the information about the quantity of the adsorbate reduced or oxidized at each step, k/(Q) dependences characterize changes in the adsorbate-adsorbate and adsorbate-substrate interactions at different potentials. 

At electrode potentials that are the boundaries between the regions of adsorption and desorption mass transport effects play an important role. As in the case of redox reactions, the mass transfer Warburg term appears in series with the adsorption capacitance C representing the change of the double layer due to adsorption. We will not go into... 

In addition, results by Armstrong and Mason (23) show similar shapes of capacitance-voltage curves for RbAg I Pt at 22° and 95°C and for Agl Pt at 200°. It seemed worthwhile, then, to attempt to develop a simple theoretical "adsorption capacitance" model and compare it with available data. 

If Tq is the fixed concentration of adsorption sites on the electrode, each of which is always occupied by an anion or cation, then the "adsorption capacitance" is given by... 

Fig. 6 Adsorption capacitance (in reduced units) as function of voltage with respect to PZC at two temperatures, from first-order theory (Eq. 14).

Adsorption capacitance and diffusion impedance from electrochemical impedance spectroscopy... 

In Figure 1.3.8 one observes an adsorption capacitance with a maximum peak in the range of CO-CO2 evolution and an adsorption coverage that goes to one in the range of the limiting cd for the same process [3]. 

Figure 1.3.8 Polarisation curve, fractional coverage of adsorbed CO and Cl and adsorption capacitance for a graphite anode in NaCTCaCl2-CaO (40 60) with l.6mol% CaO...

Electrodes where charge transfer occurs via a diffusion controlled adsorption step, has a characteristic adsorption distance La = (9F/dC)E near electrode, it represents the extent to which an electroactive species is adsorbed on the surface from the volume to create surface excess (F). Surfaces with large La causes enhanced rate of reaction due to onset of superdiffusive behavior in high frequency or short time which crosses over to intermediate adsorption kinetics controlled subdiffusion in multiscale nanostructures or rough electrodes [22], The intermediate crossover frequency between super-and sub- diffusion is con While at lower frequency coAd D / L, response cross over to adsorption capacitance. 

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