Electrolyte impedance

FIGURE 2.20 Change of anode/electrolyte impedance spectra with respect to change in anodic current. (From van Herle, J. et al., Proceedings of the Fifth International Symposium on Solid Oxide Fuel Cells, 97(40) 565-574, 1997. Reproduced by permission of ECS-The Electrochemical Society.)... 

Figure 2. Equivalent circuit of a bacteriorhodopsin membrane that includes the circuit parameters of the inert supporting structure and the access impedance of the measuring system. Re is the access impedance, which includes the input impedance of the measuring device, the electrode impedance, and the electrolyte impedance between the membrane and the electrodes. Rm and Cm are the resistance and the capacitance of the membrane, C p is the chemical capacitance, Rp is the internal resistance of the photoelectric voltage source, Ep(U, which is a function of the illuminating light power, and Rs is the transmembrane resistance encountered by the dc photocurrent. (Reproduced from reference 19.

The simple electrode model in Figure 3-19 illustrates the ac impedance theory following Lamarre and Melville (1992). The capacitive Zc and resistive Zr (due to the electrolyte) impedances are in parallel with the inductance being ignored. The following definitions and limitations together with Equations 3.7 and 3.15 give the complex impedance Z of a liquid-gas probe ... 

Porous Electrode. Most battery electrodes comprise an open stmcture consisting of small particles compressed together, as shown in Figure 4.5.6. This structure does not have well-defined pores (such as cylindrical) but rather an irregular network of interconnected space between particles filled with electrolyte. The absence of well-defined pores complicates ab initio deduction of electrolyte impedance however, the frequency dependence of the impedance of porous materials is well described by the ladder-network approach originally proposed by de Levie [1963] for cylindrical pores. See also Section 2.1.6 for treatment of various geometries of porous electrodes. 

Impedance is typically measured in a two-electrode configuration where the electrolyte is compressed between two blocking (steel, platinum) or nonblocking Li-electrodes (Qian et al. [2(X)2]). Analysis of electrolyte impedance in the presence of electrode impedance is complicated and usually assumes that the electrolyte is responsible for the highest frequency region of the spectrum, about IkHz. To improve confidence in the conductivity estimation, measurements with several layer thicknesses should be performed. To remove the effect of the electrode impedance in a test setup, four-electrode measurements have also been proposed (Bruce et al. [1988]). Typically, two pseudoreference electrodes made of Li-foil strips are pressed through a cavity in the middle of circular main electrodes to the surface of the polymer electrolyte under test. 

Ferris, C. D., 1963, Four-electrode electronic bridge for electrolyte impedance determinations, Rev. Sci. Instr. 34(1) 109-111. 

Figure 10.6 illustrates a cell for use in four-electrode measurements of electrolyte impedance. The working electrodes are platinum. The sensing electrodes are also platinum, but are platinized. They are recessed from the main cell by salt bridges. It is possible to obtain four sample sizes by changing the relative positions of the two sensing electrodes. A detailed description of this cell appears in Schwan and Ferris (1968). 

Figure 10.6. Recessed-electrode cell for electrolyte impedance measurements.

Keywords Dielectric spectroscopy Electrolyte Impedance spectroscopy Ion conductor Ion dynamics Polyelectrolyte complex... 

The polarization free resistance of the melt was measured with an ac Wheatstone type bridge using an input frequency from 0.5 to 10 kHz. The schematic diagram of the apparatus and the experimental procedure were described in detail elsewhere [13,14]. The electrical conductivity k was calculated by use of the cell constant and the real electrolytic impedance. The molar conductivity A of the mixture melt was evaluated from the following equation [15] ... 

Figure 13 shows typical complex impedance spectra of the three types of polymer electrolytes sandwiched between lithium electrodes at an oscillation level of 0.5 V. The profiles of the spectra were two neighboring arcs. The low-frequency arcs (right-hand side arcs) and the high-frequency arcs (left-hand side arcs) corresponded to the loci of the charge transfer impedance and the bulk electrolyte impedance, respectively. The bulk resistance (/ .0 and the charge transfer resistance (R ) are shown in Fig. 13. The ionic conductivities at 50°C for the polymer electrolytes (1), (2), and (3) were approximately 10 to 10 S cm (depending on the LiC104 concentration (see Table 1), 10 S cm, and 10 S cm , respectively. The values, corresponding to the electrode reaction, were about lO" Cl, irrespective of the kinds of polymer electrolytes. These values were similar to those found in the poly(propylene oxide) networks with dissolved lithium salts [93] and to those found in the poly(ethylene succinate) with dissolved lithium salts [88,94], but were considerably higher than those found in poly(p-propiolactone)-LiC104 complexes [95].

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