Abstract
In this work, impedance spectra of polyaniline film electrode recorded at some redox transition potentials have been corrected for nonstationarity by application of Stoynov's 4-D method. The procedure consisted from: a) conventional measurements of a series of impedance spectra under strictly same experimental conditions, which are accompanied by monitoring of real operating time intervals for measurement of each particular impedance spectrum of the series, b) determination of functional dependence(s) between iso-frequency impedance values and operating time, and c) calculations of instantaneous IS for the time instant of the beginning of the first IS measurement. Comparison between measured and instantaneous impedance spectra, and also between their individual impedance parameter values pointed toward significant differences in charge transfer and transport resistance values. The results suggest that erroneous (either underestimated or overestimated) charge transfer and transport resistance values will be obtained if measured impedance spectra were not corrected for nonstationarity.
Highlights
E LECTROCHEMICAL impedance spectroscopy (EIS)[1,2] is the extremely powerful technique for characterization of either different electrode materials toward their electrical/dielectrical,[3] electrocatalytic[4] and corrosive properties,[5] or different electrochemical devices such as energy conversion/storage devices[6] or electrochemical sensors.[7]
Impedance values and types of impedance/frequency responses at different polarization potentials are in general agreement to those already obtained for PANI films at various oxidation states.[9,35,36,37,38,39,40,41,42,43,44,45]
At –0.20 V vs. saturated calomel electrode (SCE), highly resistive PANI film having a closed structure of inactive PANI-LE form is, in dependence on PANI “history” before measurements,[29,30,31] indicated by either pure double-layer capacitive phase angle response (Figure 4a), or combined with a Warburglike phase angle response generated by slow charge transport within the PANI film[38,44] (Figure 4b)
Summary
E LECTROCHEMICAL impedance spectroscopy (EIS)[1,2] is the extremely powerful technique for characterization of either different electrode materials toward their electrical/dielectrical,[3] electrocatalytic[4] and corrosive properties,[5] or different electrochemical devices such as energy conversion/storage devices[6] or electrochemical sensors.[7]. Impedance is defined as the quotient between Fourier transforms of differential equations defining the sinusoidal voltage response and sinusoidal current excitation. Under conditions of linearity, causality and stationarity, the quotient would be reduced to the simple Ohm’s law by which the complex and frequency dependent resistance, i.e. impedance, becomes defined.[1,2] Violation from any of these conditions prevents all possibly measured impedance spectra to be true impedance spectra prone to accurate impedance data analysis. The most common violation is related to the condition of stationarity, because many electrochemical systems involve nonsteady-state processes that cause permanent system changes such as corrosion,[8] deposition,[9] or system poisoning.[10] Conventional impedance spectra measurements by frequency response analysis (FRA) are generally performed at a succession of different frequencies. In dependence on the frequency range, number of frequencies per decade and integration time explored, the single impedance spectrum measurement can last for hours.[11,12] A critical situation emerges when a measured system is changing between the beginning and the end of the single impedance spectrum measurement when impedance data couldn't be analysed in terms of steady-state models.[13]
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