Admittance electrolytes

By tradition, electrochemistry has been considered a branch of physical chemistry devoted to macroscopic models and theories. We measure macroscopic currents, electrodic potentials, consumed charges, conductivities, admittance, etc. All of these take place on a macroscopic scale and are the result of multiple molecular, atomic, or ionic events taking place at the electrode/electrolyte interface. Great efforts are being made by electrochemists to show that in a century where the most brilliant star of physical chemistry has been quantum chemistry, electrodes can be studied at an atomic level and elemental electron transfers measured.1 The problem is that elemental electrochemical steps and their kinetics and structural consequences cannot be extrapolated to macroscopic and industrial events without including the structure of the surface electrode. 

Figure 48. Kenjo s ID macrohomogeneous model for polarization and ohmic losses in a composite electrode, (a) Sketch of the composite microstructure, (b) Description of ionic conduction in the ionic subphase and reaction at the TPB s in terms of interpenetrating thin films following the approach of ref 302. (c) Predicted overpotential profile in the electrode near the electrode/electrolyte interface, (d) Predicted admittance as a function of the electrode thickness as used to fit the data in Figure 47. (Reprinted with permission from refs 300 and 301. Copyright 1991 and 1992 Electrochemical Society, Inc. and Elsevier, reepectively.)...

The reduction of Cd(II) ions on DME was also investigated in 1 M perchlorate, fluoride and chloride solutions using dc, ac admittance, and demodulation methods [27]. It was found that in the perchlorate supporting electrolyte, the reduction mechanism is also CEE, and that the rate constant of the chemical step is quite close to the value characteristic for fluoride solutions. The theories available at present could not be applied to the Cd(II) reduction in chloride solution because of the inapplicability of the Randles equivalent circuit. 

This work attempts to model a semiconductor/molten salt electrolyte interphase, in the absence of illumination, in terms of its basic circuit elements. Measurement of the equivalent electrical properties has been achieved using a newly developed technique of automated admittance measurements and some progress has been made toward identification of the frequency dependent device components (1 ). The system chosen for studying the semiconductor/ molten salt interphase has the configuration n-GaAs/AlCl3 1-... 

Linear sweep voltammetry, capacitance-voltage and automated admittance measurements have been applied to characterize the n-GaAs/room temperature molten salt interphase. Semiconductor crystal orientation is shown to be an important factor in the manner in which chemical interactions with the electrolyte can influence the surface potentials. For example, the flat-band shift for (100) orientation was (2.3RT/F)V per pCl" unit compared to 2(2.3RT/F)V per pCl" for (111) orientation. The manner in which these interactions may be used to optimize cell performance is discussed. The equivalent parallel conductance method has been used to identify the circuit elements for the non-illum-inated semi conductor/electrolyte interphase. The utility of this... 

Membrane capacitance and conductance were measured using the Wayne-Kerr admittance bridge and measurements were made between 100 Hz and 20 KHz using a PAR lock-in amplifier model 124. One of the results obtained is shown in Figure 1. The dotted curve shown in this figure indicates measured values at high frequencies and the downward slope arises from the presence of electrolyte solutions between the membrane and electrodes. 

In ac impedance measurement at ITIES, admittance due to the transfer of supporting electrolyte ions is significant even in the middle of the potential window, as was first suggested and treated quantitatively by Samec et al. [35]. This imposes a difficulty in accessing double layer capacitance from the admittance, particularly when the transfer of supporting electrolyte ions is not reversible. There is no straightforward way to deconvolute the admittance ascribable to double layer capacitance and that ascribable to ion transfer admittance [30]. A nonlinear least-squares... 

If the electrolyte resistance Re is removed from the expression for the admitteince, the admittance is simplified to... 

Figure 16.10 Real and imaginary parts of the electrolyte-resistance-corrected admittance on a logarithmic scale as a function of frequency for Re = 10 Ocm, R = 100 flcm, and C = 20 f F/cm. The blocking system of Table 16.1(a) is represented by dashed lines, and the reactive system of Table 16.1(b) is represented by solid lines. Characteristic frequencies are noted as /rc = (2nRC) and fc = 2TiReC) a) real part of admittance and b) imaginary part of admittance.

The characteristic frequency evident as a peak for the imaginary part of the complex-capacitance in Figures 16.12(b) and 16.13(b) has a value corresponding exactly to fc = 27tReC) only for the blocking system. As found for data presentation in admittance format, the presence of a Faradaic process confounds use of graphical techniques to assess this characteristic frequency. Like the admittance format, the complex capacitance is not particularly well suited for analysis of electrochemical and other systems for which identification of Faradaic processes parallel to the capacitance represents the aim of the impedance experiments. It is particularly well suited for analysis of dielectric systems for which the electrolyte resistance can be neglected. 

Theoretical analysis of the admittance behavior of passive film/electrolyte junction based on the theory of amorphous semiconductor Schottky barriers... 

FIGURE 4.22 Admittance hodographs in the electrical circuit at the different magnitudes of R( / Rj 1 0 2 0.1 5 1.0 4 10 and 5 100. (From Zhuiykov, S., In-situ diagnostics of solid electrolyte sensors measuring oxygen activity in melts by developed impedance method, Meas. Sci. Technol. 17 (2006) 1570-1578. With permission.)... 

Figure 18.2 depicts the electrode-polymer-electrolyte configuration that is used in the SmartSense system. Admittance in this type of electrode system is due to contributions from the interface and bulk film regions. For the electrodes under consideration, the polymer film thickness is of the order of 100 nm and the electrode separation is 0.05 cm for these dimensions, the impedance through the film itself should be in excess of 10 The measured impedance at 10 Hz, however, is of the order of 10 Q, implying that the predominant current path is across the polymer-electrolyte interface and through the electrolyte. 

As shown in figure 18.2, the electrode-polymer system is symmetric, which allows a half-cell analysis. To simplify the analysis, three assumptions are made at the outset (i) the electrolyte is highly conducting so that solution resistance is negligible (ii) oxidation occurs primarily at the polymer-electrolyte interface and (iii) the Pt-polymer interface admittance does not change substantially during an assay. Pt is the electrode material of choice because of its inertness in the system and its ability to form an ohmic contact with conducting polymers... 

Given the nature of the polymer and the conduction pathway, a simple homogeneous model cannot be applied to thin conducting polymer film-electrolyte systems [27,28,31]. For thin films (< lOOnm) with pore sizes estimated to range from 1 to 4 nm, the porous surface-electrolyte interface will dominate the electrical and physical properties of the sensor. Since the oxidation of the porous surface occurs first, the interface properties play a major role in determining device response. To make use of this information for the immunosensor response, the appropriate measurement frequency must be chosen to discriminate between bulk and interface phenomena. To determine the optimum frequency to probe the interface, the admittance spectra of the conducting polymer films in the frequency range of interest are required. 

Admittance spectra have been used to characterize conducting polymer-electrolyte interfaces for poly aniline [32], polypyrrole [33], and poly thiophene... 

Greszczuk et al. [252] employed the a.c. impedance measurements to study the ionic transport during PAn oxidation. Equivalent circuits of the conducting polymer-electrolyte interfaces are made of resistance R, capacitance C, and various distributed circuit elements. The latter consist of a constant phase element Q, a finite transmission line T, and a Warburg element W. The general expression for the admittance response of the CPE, Tcpr, is [253]... 

J.E. BAUERLE, Study of solid electrolyte polarization by a complex admittance method , J. Phys. Chem. Solids, 30, p. 265,1969. 

Data interpolated from points of 20-Hz resolution. Admittance maximum (at resonant frequency). Afis the shift in center frequency with respect to the value on initial exposure to H2O (corrected for viscous load of electrolyte). Column A Data for first exposure to electrolyte without electrochemical cycling Column B Open circuit data after electrochemical cycling at each concentration. Gravimetric surface coverage 20 x 10 mol cm. ... 

With a dry electrode plate, the moisture buildup and admittance increase in the SC start at the moment of electrode onset. With a hydrogel, admittance may increase or decrease. With wet gel or a liquid, the initial admittance is high, and with strong contact electrolytes the admittance will further increase for many hours and days (Figure 4.20). Because the outer layers of SC may be wet or dry according to the ambient air, it will not be possible to find a general contact medium that just stabilizes the water content in the state it was before electrode onset the onset of the electrode will generally influence the parameters measured. 

With dry skin, the admittance may be less than 1 pS/cm at 1 Hz. A typical admittance of more than 100 pS/cm is possible when the SC is saturated by electrolytes and water. The conductivity is very dependent on water content and is believed to be caused, for example, by protons (H" ") and charged, bound proteins that contribute only to AC admittance. 

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