Adsorption impedance Capacitance

Adsorption impedance — The current flowing in an electrochemical system splits into two parts at an interface the charge either transfers across, (-> faradaic current) or gets accumulated at the two sides of the boundary (- non-faradaic or - charging current) the related impedance elements are called - Faraday impedance and non-Faraday impedances, respectively. The latter element is an essentially capacitive element its lossy character is related to the slow kinetics of - adsorption- related processes involved. 

All three elements can scarcely be identified from impedance spectra of systems with adsorption for example, in many cases the adsorption resistance is masked by the diffusional element. In addition, whenever electrodes of inhomogeneous surfaces (e.g., poly crystalline electrodes) are measured, the resistance-diffusional element - capacitance terms of the adsorption impedance are smeared out yielding a CPE-like impedance. 

The effect of adsorption of electroactive species in ac methods is taken into account by modification of the equivalent circuit representing the electrode reaction (44, 64-68). This is usually accomplished by adding an adsorption impedance in parallel with the Warburg impedance and double-layer capacitance. Expressions for this impedance have been suggested for reversible (65, 66) and irreversible (67, 68) systems, but the complexity of the resulting analysis has limited the application of these techniques. 

Impedance/capacitance measurements of gas adsorption systems in thermodynamic equilibrium ... 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

Variations of resistance with frequency can also be caused by electrode polarization. A conductance cell can be represented in a simplified way as resistance and capacitance in series, the latter being the double layer capacitance at the electrodes. Only if this capacitance is sufficiently large will the measured resistance be independent of frequency. To accomplish this, electrodes are often covered with platinum black 2>. This is generally unsuitable in nonaqueous solvent studies because of possible catalysis of chemical reactions and because of adsorption problems encountered with dilute solutions required for useful data. The equivalent circuit for a conductance cell is also complicated by impedances due to faradaic processes and the geometric capacity of the cell 2>3( . 

The capacitance-potential dependences of Cd(OOOl) in dilute solutions of Cl04, N02, and NOs" were also studied [6]. A weak specific adsorption of anions increasing in the order Cl04 < N02 < N03 was observed. The adsorption of halides on the Cd(OOOl) single crystal electrode was studied [7], and was found to increase in the sequence Cl < Br < 1 [8]. Analysis of the impedance data does not point to the specific adsorption of Cl ions, and shows that the surface excess (T) of halide ions changes with potential and increases from Br to 1 (Fig. 1) [7]... 

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. 

Moisture analyzers include a large variety of designs listed here. The list includes their inaccuracies (1) electrolytic hygrometer (2-5% FS), (2) capacitance (3% FS), (3) impedance (3% FS), (4) piezoelectric (10% AR or 2 ppm by volume), (5) heat of adsorption, (6) infrared (0.5-1% FS), (7) microwave (for a 1-15% moisture range, error is within 0.5%, less if corrected for density), (8) Karl Fischer titrator (0.5-1% FS), (9) drying oven (0.5-1% FS), (10) dipole, (11) cavity ring down, (12) fast neutron (0.2% in solid s density corrected), and (13) radio-frequency absorption (5 ppm). 

Many electrode processes are more complex than those discussed above. Besides this, the impedance of an interface is dependent on its microscopic structure which, in the case of a solid electrode, can have an important influence. Impedance measurements can be used to study complicated corrosion phenomena (Chapter 16), blocked interfaces (i.e. where there is no redox process nor adsorption/desorption), the liquid/liquid interface2425, transport through membranes26, the electrode/solid electrolyte interface etc. Experimental measurements always furnish values of Z and Z" or their equivalents Y and Y", or of the complex permittivities e and e" (e = Y/icoCc, Cc being the capacitance of the empty cell). In this section we attempt to show how to... 

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 ... 

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