Impedance spectroscopy ionic conductivity

Impedance spectroscopy is best suited for the measurement of electronic conductivities in the range 10 -7to 10 2S cm 1.145 In principle, it is perhaps the best method for this range, but it is often difficult to interpret impedance data for conducting polymer films. The charge-transfer resistance can make measurements of bulk film resistances inaccurate,145 and it is often difficult to distinguish between the film s ionic and electronic resistances.144 This is even more of a problem with chronoamperometry146 and chronopotentiometry,147 so that these methods are best avoided. 

The intrinsic ionic conductivities of hydrated chitosan membranes investigated using impedance spectroscopy were as high as 10 S cm [232]. 

On the other hand, electrical impedance measurement of bacterial suspensions (.Listeria innocua) was carried out on a Si-glass chip. This method was used to perform a cell viability test. This is because the metabolic products produced from viable cells modify the ionic strength of a low-conductivity medium, significantly altering its electrical characteristics. Later work also involved the detection of the presence of small numbers of bacterial cells in a Si chip 100 L. innocua cells, 200 L. monocytogenes cells, and 40 E. coli cells [93,885], In another report, impedance spectroscopy has been used for analysis of erythrocytes in a glass-polyimide chip [886]. 

Figure 48. The concentration cell experiment together with impedance spectroscopy allows one to separate ionic (

These examples and the general subjects mentioned above illustrate that ion conduction and the electrochemical properties of solids are particularly relevant in solid state ionics. Hence, the scope of this area considerably overlaps with the field of solid state electrochemistry, and the themes treated, for example, in textbooks on solid state electrochemistry [27-31] and books or journals on solid state ionics [1, 32] are very similar indeed. Regrettably, for many years solid state electrochemistry/solid state ionics on the one hand, and liquid electrochemistry on the other, developed separately. Although developments in the area of polymer electrolytes or the use of experimental techniques such as impedance spectroscopy have provided links between the two fields, researchers in both solid and liquid electrochemistry are frequently not acquainted with the research activities of the sister discipline. Similarities and differences between (inorganic) solid state electrochemistry and liquid electrochemistry are therefore emphasized in this review. In Sec. 2, for example, several aspects (non-stoichiometry, mixed ionic and electronic conduction, internal interfaces) are discussed that lead to an extraordinary complexity of electrolytes in solid state electrochemistry. 

Conventional two-electrode dc measurements on ceramics only yield conductivities that are averaged over contributions of bulk, grain boundaries and electrodes. Experimental techniques are therefore required to split the total sample resistance Rtot into its individual contributions. Four-point dc measurements using different electrodes for current supply and voltage measurement can, for example, be applied to avoid the influence of electrode resistances. In 1969 Bauerle [197] showed that impedance spectroscopy (i.e. frequency-dependent ac resistance measurements) facilitates a differentiation between bulk, grain boundary and electrode resistances in doped ZrC>2 samples. Since that time, this technique has become common in the field of solid state ionics and today it is probably the most important tool for investigating electrical transport in and electrochemical properties of ionic solids. Impedance spectroscopy is also widely used in liquid electrochemistry and reviews on this technique be found in Refs. [198 201], In this section, just some basic aspects of impedance spectroscopic studies in solid state ionics are discussed. 

Fleig reviews fundamental aspects of solid state ionics, and illustrates many similarities between the field of solid state electrochemistry and liquid electrochemistry. These include the consideration of mass and charge transport, electrochemical reactions at electrode/solid interfaces, and impedance spectroscopy. Recent advances in microelectrodes based on solid state ionics are reviewed, along with their application to measuring inhomogeneous bulk conductivities, grain boundary properties, and electrode kinetics of reactions on anion conductors. 

Guo Q, Cayetano M, Tsou Y, De-Castro ES, White RE (2003) Study of ionic conductivity profiles of the air cathode of a PEMFC by AC impedance spectroscopy. J Electrochem Soc 150(ll) A1440-9... 

Figure 4.33. Equivalent circuit of a catalyst layer [8]. (Reproduced by permission of the authors and of ECS—The Electrochemical Society, from Lefebvre MC, Martin RB, Pickup PG. Characterization of ionic conductivity within proton exchange membrane fuel cell gas diffusion electrodes by impedance spectroscopy.)...

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. 

To increase fundamental knowledge about ionic resistance, it is important to develop a methodology to experimentally isolate the contributions of the various cell components. Electrochemical impedance spectroscopy has been widely used by Pickup s research group to study the capacitance and ion conductivity of fuel cell catalyst layers [24-27] they performed impedance experiments under a nitrogen atmosphere, which simplified the impedance response of the electrode. Saab et al. [28] also presented a method to extract ohmic resistance, CL electrolyte resistance, and double-layer capacitance from impedance spectra using both the H2/02 and H2/N2 feed gases. In this section, we will focus on the work by Pickup et al. on using EIS to obtain ionic conductivity information from operational catalyst layers. 

Figure 30. Ionic space charge effects at grain boundaries in Frenkel disordered materials, (a) Theoretical profiles if u, > u .120 (b) The enhanced grain boundary conductivity can be verified by point electrode impedance spectroscopy.121 The number given are in units of nS / cm and refer to room temperature.

Impedance spectroscopy, meaning the measurement of complex resistivities with ac current methods, is an important tool to study diffusion and to correlate it with ionic transport behavior. The diffusion coefficient, D , obtained from conductivity measurements (vide infra) is related to the self-diffusion coefficient, D... 

Alternatively, an equally powerful visualization of impedance data involves Bode analysis. In this case, the magnitude of the impedance and the phase shift are plotted separately as functions of the frequency of the perturbation. This approach was developed to analyze electric circuits in terms of critical resistive and capacitive elements. A similar approach is taken in impedance spectroscopy, and impedance responses of materials are interpreted in terms of equivalent electric circuits. The individual components of the equivalent circuit are further interpreted in terms of phemonenological responses such as ionic conductivity, dielectric behavior, relaxation times, mobility, and diffusion. 

R. L. Hurt and J. R. Macdonald, "Distributed Circuit Elements in Impedance Spectroscopy A Unified Treatment of Conductive and Dielectric Systems," Solid State Ionics, 20 (1986) 111-124. 

Figure 7-8. (a) Impedance spectroscopy result for an ionically conductive epoxy composite [adapted with permission of S. Boob, 2003] (b) the equivalent circuit (inset) for the circular part of the response and (c) same as (b), but plotted on a frequency scale. 

Because of the resistance to ion flow at the electrode-electrolyte interface, normal measurement of total ionic conductivity is not possible in polymer electrolytes. In order to overcome this problem the conductivity measurements are carried out by the ac impedance spectroscopy method, which minimizes the effects of cell polarization. The measurements are often made with the electrolyte sandwiched between a pair of electrochemically inert electrodes made of platinum or stainless steel. The detailed methodology of impedance spectroscopy is reviewed thoroughly elsewhere [45-47]. 

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