Challenge for electrochemical impedance spectroscopy in the dynamic world
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Challenge for electrochemical impedance spectroscopy in the dynamic world G.A. Ragoisha 1 Received: 26 May 2020 / Revised: 26 May 2020 / Accepted: 27 May 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Electrochemical impedance spectroscopy is a powerful method which provides separation and analysis of individual constituents of the electrochemical response originated from different objects (processes and structures) in an electrochemical system. The impedance spectrometer, after having been a separate instrument which was used by specially trained people, transformed into computer programs supplied with modern potentiostats and thus became available to any researcher who uses electrochemical methods. Complexity of theory of impedance spectroscopy has become hidden behind simple user interfaces and powerful software for impedance spectra analysis. This is of great help for efficient use of impedance spectroscopy but also a catch for those users who misunderstand its concepts. In order to minimize risk of obtaining artifacts instead of meaningful data, several restrictions are applied as prerequisite on impedance spectrum acquisition and the strongest restriction contains in the requirement of stationarity of the object under investigation. The two kinds of restrictions related to the stationarity prerequisite in electrochemical impedance spectroscopy have to be taken into account. First, very many objects which require frequency response analysis are in fact non-stationary objects. Electrochemical reactions of dissolved substances often strictly comply with the prerequisite, but stationarity is less characteristic to reactions of solid reagents. Second, even in the perfect stationary state, the frequency response not necessarily contains the anticipated information which motivated the use of impedance spectroscopy. The characteristic examples of the latter case are the electrochemically irreversible surfacelimited reactions which lack back current at the potential of voltammetric peak of the forward reaction. Attaining the stationary state at the peak potential would result in completion
* G.A. Ragoisha [email protected] 1
Research Institute for Physical Chemical Problems, Belarusian State University, 220006 Minsk, Belarus
of the forward reaction, so neither forward nor back reaction contributes to the impedance spectrum. Though impedance spectrum in such a stationary state may be obtained accurately, parameters derived from impedance spectrum turn to be unrelated to the completed reaction. The most lacking in informative frequency response characterization are various electroactive materials, i.e., materials in which capacitive charging is strongly interrelated with Faradaic current. Especially important among the latter are supercapacitors with capacitances originated from “pseudocapacitance”, i.e., capacitance other than doublelayer capacitance. A typical example of pseudocapacitance, identical to physical capacitance of a capacitor in frequency response analysis, is the a
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