Electrochemical frequency response analyzers

An EG G PARC 273 Potentiostat/Galvanostat was used in both the electrolysis and the CV experiments, coupled with an HP 7044B X/Y recorder. A Solartron 1255 HF Frequency Response Analyzer and a Solartron 1286 Electrochemical Interface were employed for the a.c. impedance measurements, using frequencies from 0.1 to 65 kHz and a 10 mV a.c. amplitude (effective) at either the open circuit potential (OCP) or at various applied potentials. As the RE can introduce a time delay at high frequencies, observed as a phase shift owing to its resistance and capacitance characteristics, an additional Pt wire electrode was placed in the cell and was connected via a 6.8 pF capacitor to the RE lead [32-34]. 

The different behaviours of gas diffusion electrodes with different catalyst loadings were studied by Paganin et al. [4], EIS measurements of 0.5 and 1 cm2 single cells were conducted with H2/02 (air) as fuel/oxidant. In their measurements, a Solartron 1250 frequency response analyzer and a 1286 electrochemical interface were employed. The amplitude of the AC signal was 10 mV and the frequency range was typically from 10 mHz to 10 kHz. Representative EIS results are shown in Figures 6.1 and 6.2. 

The best method to measure the double-layer capacitance is to use a phase-sensitive voltmeter. This instrument is sometimes incorporated into a frequency response analyzer, designed to make electrochemical impedance spectroscopy measurements, but it can also be used independently. In Part Two we devote a full section to the operation of such instruments and the analysis of results obtained by them. Here we shall limit the discussion to the measurement of capacitance. 

Experiments were carried out under potentiostatic conditions using an 1172 Solartron Frequency Response Analyzer and 1186 Solartron Electrochemical Interface. A small (input) amplitude (10 mV) sine wave (P sin wt) was applied to the system under study. The response of the system to the applied perturbation was monitored as a sine wave current [Y sin (wt + 6 )] and a sine wave potential [X sin (tot + 0 )]. These were transformed into the complex form A + i B and A + i B, respectively. The real and imaginary part of th impedance were computed using the relation t=(A +iB)/(A +iB)... 

This operation determines the values of R and C that, in series, behave as the cell does at the measurement frequency. The impedance is measured as a function of the frequency of the ac source. The technique where the cell or electrode impedance is plotted V5. frequency is called electrochemical impedance spectroscopy (EIS). In modem practice, the impedance is usually measured with lock-in amplifiers or frequency-response analyzers, which are faster and more convenient than impedance bridges. Such approaches are introduced in Section 10.8. The job of theory is to interpret the equivalent resistance and capacitance values in terms of interfacial phenomena. The mean potential of the working electrode (the dc potential ) is simply the equilibrium potential determined by the ratio of oxidized and reduced forms of the couple. Measurements can be made at other potentials by preparing additional solutions with different concentration ratios. The faradaic impedance method, including EIS, is capable of high precision and is frequently used for the evaluation of heterogeneous charge-transfer parameters and for studies of double-layer structure. 

Instrumentation for Electrochemical Impedance Spectroscopy < 407 FREQUENCY RESPONSE ANALYZER... 

Figure 10.8.1 System for measuring the impedance of an electrochemical cell based on a frequency response analyzer (FRA).

The two codes, written in basic language for an Apple He computer, use the galvano-static mode and drive, respectively, the Solartron mod. 1286 electrochemical interface and the EG G mod. 173 potentiostat, which is equipped with a mod. 276 interface. In both cases use is made of a Solartron mod. 1250 frequency response analyzer. 

The single cell/micro-stacks were tested in a flow-through quartz tube by four-terminal method, and a schematic view of testing setup was given in Figure 1. Electrochemical properties were evaluated by a Solartron SI 1287 electrochemical interface and a Solartron SI 1260 frequency response analyzer. Under open circuit condition, impedance spectra were collected in the frequency range of... 

Potentiostat with a frequency response analyzer (FRA) or with EIS capabilities. Three-electrode electrochemical cell WE (Semiconductor material), CE, and RE (e.g., Ag/AgCl in samrated KCl). 

In addition to performance evaluations, many photoelectrochemical experiments are aimed to identify performance-Umiting steps or to determine certain materials properties. Examples of the latter are donor or acceptor densities and the flatband potential of a material, which can be determined by electrochemical impedance measurements. The challenge with these measurements is that they always yield data, but that it can be difficult - and sometimes even impossible - to translate the measured data to the desired materials parameters. Carefully performed control experiments and a good basic understanding of the measurement equipment -in particular, the potentiostat and the frequency response analyzer (FRA) - are essential for obtaining meaningful results. 

IEEE-488 systems of considerable complexity have been developed for electrochemical data acquisition and experimental control. One such system, shown in Eigure 3.1.22, uses a microcomputer to monitor temperature and dc signals wim an IEEE-488 multiplexer and multimeter, to control and IEEE-488 potentiostat, and to output and input data for an IEEE-488 frequency response analyzer, m order to measure impedances in a sodium/sodium-polysulfide cell at elevated temperatures (McKubre and Sierra-Alcazar [1985]). 

Electrochemical impedance measurement systems used for the analysis of the ac properties of electrochemical cells typically consist of a potentiostat (sometimes called an electrochemical interface) together with a frequency response analyzer (FRA) or a spectrum analyzer, or even a combination of the two. The potentiostat provides buffered connections to the cell under investigation together with circuitry for applying a controlled voltage or current stimulus and for the measurement of the dc properties of the cell. The FRA is connected through the potentiostat to the cell and therefore the bandwidth of the potentiostat is a very important consideration for accurate high frequency analysis. 

One important capability of the potentiostat is to maintain the required dc conditions on the cell while the frequency response analyzer is performing the impedance (ac) analysis. The actual dc conditions required depend on the application. For tests on fuel cells it may be necessary to set up a particular dc steady state current to investigate the impedance of the cell under load conditions. In other cases it may be necessary to run the impedance test at the open circuit potential of the electrochemical cell, in which case the open circuit voltage is measured (Figure 3.2.1)... 

The experimental cell is controlled by a potentiostat/galvanostat, which is also coupled with a frequency response analyzer for EIS measurements. The potentiostat (connected to a computer) measures the WE potential ( ) with respect to the RE, and the current (/) through the CE. The resistor (> 1 Gf2) is internal to the potentiostat and prevents current flow in the RE. The electrochemical cell shown in Figure 3.4(a) can also be used with rotating disc electrodes (RDEs), with the addition of an RDE rotor/controUer. RDE-based experiments do not necessarily mimic the hydrodynamic conditions of CMP, because the fluid velocity prohle at the surface of an RDE (Bard, 2001) is different from that expected for a CMP pad (Thakurta et al., 2002). Nevertheless, certain details of the CMP-related reaction kinetics and the effects of convective mass transfer on such reactions can be examined using RDEs. 

Electrochemical Impedance Spectroscopy (EIS) Applications to Sensors and Diagnostics, Fig. 6 Example for impedance measurement setup with a frequency response analyzer and a 4-electrode cell... 

Impedance measurement system usually integrates an AC measurement unit such as frequency response analyzer (FRA), a potentiostat or galvanostat of suitably high bandwidth, and the electrochemical cell composed of 2, 3, or 4 electrodes in contact with an investigated sample (Fig. 6). The analyzed electrochemical interface is located between the sample and the working electrode (WE). A counter electrode (CE) is used to supply a current through the cell. Where there is a need to control the potential difference across the interface, one or two reference electrodes... 

The impedance of an RE can be measured with any potentiostat with built-in electrochemical impedance spectroscopy (EIS) capabilities or with the addition of a separate frequency response analyzer (ERA). One method is described below. 

Generally, there are two categories in impedance measurement electrical and electrochemical impedance. Electrical impedance (El) is a two-electrode measurement performed using a lock-in amplifier and function generator or frequency response analyzer (FRA) with a pair of electrodes. 

All electrochemical measurements (PP, LPR and EIS) were accomplished with Autolab frequency response analyzer (FRA) and general purpose electrochemical system (GPES) for... 

Determination of the Internal Resistance of the Cells AC bulk resistances were measured from impedance plots for each membrane using a Solatron 1186 Electrochemical Interface and Solatron 1250 Frequency Response Analyzer (FRA) controlled by a Hewlett-Packard 85B computer (75). 

Figure 7-2. Schematical diagram of electrochemical impedance measurements (Upper part) set-up with the electrochemical cell, the potentiostat, and the frequency response analyzer (FRA) (Lower part) potential perturbation A (f) and the current response A/(/) superimposed to the steady state point (E, 1 ) of the polarization curve.

The AC impedance technique coupled to the complex plane method of analysis is a powerful tool to determine a variety of electrochemical parameters. To make the measurements, instrumentation is somewhat more complex than with other techniques. It requires a Wheatstone bridge arrangement with series capacitance and resistance in the comparison arm, a tuned amplifier/detector, and an oscillator with an isolation transformer. A Wagner ground is required to maintain bridge sensitivity, and a suitably large inductance should be incorporated in the electrode polarization circuit to prevent interference from the low impedance of this ancillary circuitry. Sophisticated measurement instruments or frequency response analyzers with frequency sweep and computer interface are currently available such as the Solartron frequency response analyzers. Data obtained can be analyzed or fitted into proper equivalent circuit using appropriate software. 

A Solartron 1287 electrochemical interface was employed for all of the galvanostatic e2q)eriments. For the electrochemical impedance measurements, the Solartron 1287 electrochemical interface was coupled with a Solartron 1455A frequency response analyzer. 

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