Commercially Available Impedance Measurement Systems

Electrochemical impedance tests usually investigate the interface between an electrode material and a solution (for example corrosion tests may investigate different coated metals in a salt solution, while battery/fuel cell tests may investigate different electrode materials in an electrolyte). Electrochemical impedance tests provide complementary information to that obtained from dc electrochemical techniques such as cyclic voltammetry, pulse voltammetry, ohmic drop analysis, and chronoamperometry. 

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The simplest experimental arrangement for measuring the impedance of an electrochemical cell involves the use of an a.c.bridge. This type of experimental procedure is extremely laborious, since a balance point must be obtained at each frequency ([2], p. 178). A considerable improvement can be made by measuring, directly, the in-phase and quadrature components of the potential and the current, using phase-sensitive detectors (lock-in amplifiers). The computer-aided electrochemical impedance measurement systems are now commercially available. 

Electrochemical impedance. spectro.scopy, AC probe.s. EIS, although around since the 1960s, has primarily been a laboratory technique. Commercially available probes and monitoring systems that measure EIS are becoming more widely used, especially in plants that have on-staff corrosion experts to interpret the data or to train plant personnel to do so. 

There are a wide variety of measurement systems available that will determine capacitance and loss. These include capacitance bridges (manual and auto-balance), impedance analysers, network analysers etc. It is important for the user to consider the frequency range over which the pyroelectric devices are to be used, as this will largely determine the selection of the instrument to use. As most pyroelectric detectors are used in the range 0.1 to 100 Hz, the instrument should ideally permit measurement over this frequency range. There are very few commercial capacitance and impedance analysers that will work below 20 Hz, and many low cost units... 

By simultaneously exciting a broad range of frequencies, Fourier-based impedance analyzers have inherently shorter measurement times [9], [10], but most are optimized for the range below 1 MHz. Other issues are the relatively large dimensions of most commercially available systems as well as their relatively high cost. 

Figure 10.26 shows an example of cell attachment and motion as measured with the Electric Cell-substrate Impedance Sensing instrument, which is a commercially available version of this system (Applied BioPhysics Inc., Troy, New York, USA). 

Some commercially available electrochemical impedance test systems have the ability to measure not only the overall impedance of a complete electrochemical cell, but also the impedance contribution of various component parts of the cell allowing, for example, direct comparison of different anode or cathode materials in a battery. Multielectrode measurement techniques may also be used to investigate individual cells in a battery or fuel cell stack (Figures 3.2.6 and 3.2.7). 

The previous edition of this book became a standard textbook on impedance spectroscopy. This second extended edition updates the book to include the results of the last two decades of research and adds new areas where impedance spectroscopy has gained importance. Most notably, it includes completely new sections on batteries, supercapacitors, fuel cells, and photochromic materials. A new section on commercially available measurements systems reflects the reahty of impedance spectroscopy as a mainstream research tool. 

Coaxial line reflectometry has to be employed at frequencies from 1 MHz to -1 GHz. In contrast to the low-frequency techniques already described, here the sample capacitor is used as the termination of a low-loss precision coaxial line. The complex reflection factor is measured with a microwave reflectometer at the analyzer end of the fine, depending on the sample impedance. For this purpose, the incoming and reflected waves are separated with two directional couplers and their amplitudes are measured. The BDS concept 70 is a commercially available system (Novocontrol Instruments) that uses the coaxial line reflectometry method. This system covers the frequency range between IMHz and 3 GHz with loss factor tan 5 resolution of less than 3 x 10 . The... 

FRAs provide a very convenient, high-precision, wide-bandwidth method of measuring impedances in electrochemical systems. Commercial instruments are available which provide up to 4 digits of precision in the real and imaginary components, in frequency ranges covering 10 to lO Hz. These are direct-measuring devices and therefore are not susceptible to limitations on imposed potentiostat control. 

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