Measurement of ionic conductivity

The ionic conductivity of a solvent is of critical importance in its selection for an electrochemical application. There are a variety of DC and AC methods available for the measurement of ionic conductivity. In the case of ionic liquids, however, the vast majority of data in the literature have been collected by one of two AC techniques the impedance bridge method or the complex impedance method [40]. Both of these methods employ simple two-electrode cells to measure the impedance of the ionic liquid (Z). This impedance arises from resistive (R) and capacitive contributions (C), and can be described by Equation (3.6-1) ... 

Figure 15. Complex plane impedance plots for polypyrrole at (A) 0.1, (B) -0.1, (C) -0.2, (D) -0.3, and (E) -0.4 V vs. Ag/AgCl in NaCl04(aq). The circled points are for a bare Pt electrode. Frequencies of selected points are marked in hertz. (Reprinted from X. Ren and P. O. Pickup, Impedance measurements of ionic conductivity as a probe of structure in electrochemi-cally deposited polypyrrole films, / Electmanal Chem. 396, 359-364, 1995, with kind permission from Elsevier Sciences S.A.)...

This equation shows that it is possible to determine the diffusion coefficient from the easier measurement of ionic conductivity. However, Da is derived by assuming that the conductivity mechanism utilizes a random-walk mechanism, which may not true. 

Figure 5.8. a Schematic diagram of a two-probe conductivity cell [9], (Reproduced by permission of ECS—The Electrochemical Society, from Xie Z, Song C, Andreaus B, Navessin T, Shi Z, Zhang J, Eloldcroft S. Discrepancies in the measurement of ionic conductivity of PEMs using two- and four-probe AC impedance spectroscopy) b Equivalent circuit of the two-probe method. 

Macroscopically, ionic condnctivity can be characterized through electrochemical and impedance analysis. Direct measurements of ionic conductivity in MD simulations are challenging based on system size and simulation duration constraints requiring umealistically large fields. 

In polymer electrolytes the electronic conductivity is minimal and the conductivity observed is due to migration of ions. Measurement of this ionic conductivity, however, is not straightforward. This aspect has been dealt with elsewhere in detail [45-47]. However, a brief description of the method of measurement of ionic conductivity is given below. 

The implication is that it is possible to determine the diffusion coefficient from the easier measurement of ionic conductivity. However, the assumption that both processes utilise exactly the same mechanism is important In general, this is not true. In such a case, the relationship is slightly different from that in Equations (7.15) and (7.16) and depends on the details of the diffusion mechanism. For vacancy diffusion in a cubic structure. 

A more advantageous method is a.c.-impedance spectroscopy, which has become a standard method for the measurement of ionic conductivities in general. The basic principles have been described many times and the interested reader may refer to the excellent review by Gabrielli. A small applied potential difference allows measurements close to thermodynamic equilibrium. The accessibility of an extended frequency range (typically 1-10" s ) allows the separation of impedance contributions from the sample itself and from the electrode/electrolyte interface using equivalent circuits to assist the interpretation of the data obtained. Unfortunately interpretation is unambiguous only for simple circuits and the different... 

The first measurements of ionic conductivity in polymer-salt complexes were carried out by Wright [14, 15]. The initial realization that these materials could be used as polymer electrolytes in battery applications, however, was by Armand [16, 17] and his seminal work has spawned a vigorous, expanding, world-wide research effort. In the remainder of this chapter, the terms polymer electrolyte and ion conducting polymer will be used interchangeably. 

Zipprich, W., and Wiemhofer, H. D. (2000). Measurement of ionic conductivity in mixed conducting compounds using solid electrolyte microcontacts. Solid State Ionics 133 699-707. 

Electrochemical quartz crystal microbalance (EQCM) studies showing that also solvation of ions is of importance for EDLC behavior [25]. Measurement of ionic conductivity of electrolytes (solvent(s) -i- salt(s) or ILs) [10, 17, 26]. 

Plesse C, Vidal F, Teyssie D, Chevrot C (2010) Conducting polymer artificial muscle fibres toward an open air linear actuation. Chem Commun 46 2910-2912. doi 10.1039/c001289k Ren X, Pickup P (1995) Impedance measurements of ionic-conductivity as a probe of structure in electrochemically deposited polypyrrole films. J Electroanal Chem 396 359-364. doi 10.1016/ 0022-0728(95)04064-U... 

Riess, 1., Kramer, S., and Tuller, H.L., Measurement of ionic conductivity in mixed conducting pyrochlores by the short circuit method, in Solid State Ionics, Balkanski, M., Takahashi, T., and Tuller, H.L., Eds., North-Holland, Amsterdam, 1992, 499-505. 

The Hebb-Wagner polarization technique has been developed either for the determination of electron and hole conductivity in ionic conductors [Hebb, 1952 Joshi Wagner, 1975 Wagner, 1957] or for the measurement of ionic conductivity in MIECs [Riess, 1996 Wiemhofer et al., 2002]. Basically, the method consists in using a reversible electrode and blocking electrodes to suppress the predominant charge carrier and thus enable measurement of the minority sp>ecies. The main limitations of the method have been reviewed [Riess, 19%] and new experimental set-ups have been proposed. 

In the bulk electrolyte current is carried only by means of ions. If a direct current is imposed upon a chemical cell, chemical reactions will occur at the electrodes in accordance with Faraday s laws. If an alternating rather than a direct current is used, the Faradaic reaction that takes place on one half-cycle is reversed on the following half-cycle. There are still flows of current, however, and such currents, which do not produce chemical changes in materials, are called non-Faradaic current. One of these is the current due to the current-carrying ability, or conductance, of ions. Thus measurements of ionic conduction are normally made by ac techniques to avoid complications due to the Faradaic processes taking place at the electrodes. 

Measurement of Ionic Conductivity. The synthesis of solvent-free metal salt complexes of polyethylene oxides prompted detailed electrical measurements with the thought that these materials might prove to be useful electrolytes, in a hydrous environment, for high energy density batteries (13-15). Many fundamental properties of these polymer electrolytes have been examined and a large literature on the subject is available (16-17). We prepared a disk of one of our polyether complexes and measured its conductivity by impedance methods. 

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