Anal. Chem. 2003, 75, 3294-3300
Articles
Real-Time Impedance Measurements during Electrochemical Experiments and Their Application to Aniline Oxidation Jung-Suk Yoo, Inja Song, Ji-Hun Lee, and Su-Moon Park*
Department of Chemistry and Center for Integrated Molecular Systems, Pohang, University of Science and Technology, Pohang 790-784, Korea
Development of an in situ technique for measuring electrochemical impedance spectra in real time during an electrochemical experiment is described. The technique is based on staircase voltammetry with relatively large step heights, in which a series of increasing potential steps are applied to an electrochemical system, and the resulting currents are sampled. The first derivatives of the currents thus obtained are then converted to ac current signals in frequency domain, and impedances are computed from them. To demonstrate the technique as a tool for studying the electrode/electrolyte interface during the electrochemical reaction, we chose an electrochemical oxidation reaction of aniline, whose reaction products have been known to continuously change the electrode surface due to the polymer film growth on its surface, and report a number of observations that would not have been obtained without such in situ experiments. A suggestion is also made on the use of staircase voltammetry for mechanistic studies on complex electrochemical reactions by simply varying the sampling time. The electrode/electrolyte interface in an electrochemical system is fully described only if complete impedance measurements have been made in a whole frequency range, for example, from 100 kHz to 1 mHz. Unfortunately, it takes relatively long, for example, somewhere between about 30 min and a few hours, depending on the frequency range and the stability of the electrochemical system, to make such measurements, because the frequency needs to be scanned for the measurements. Often the electrochemical system has been changed completely by the time the impedance measurements have been made in the full frequency range, particularly at a given bias potential. For this reason, the validity of the impedance measurements has been questioned by early investigators1 and the importance of nonstationary impedance spectroscopy, in which a set of instantaneous impedance responses are obtained by mathematically treating the impedance data obtained using a conventional frequency response * To whom correspondence should be addressed. Fax: +82-54-279-3399. E-mail:
[email protected]. (1) (a) Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1979, 102, 415. (b) Popkirov, G. S.; Schneider, R. N. Electrochim. Acta 1993, 38, 861.
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analysis (FRA) technique, has been pointed out.2 However, this technique is just the correction of the data already observed by minimizing what the investigators called the “noise” term. We have recently developed a fast impedance measurement technique for an electrochemical system3,4 in which a derivative signal was first obtained from a chronoamperometric signal at a given bias potential followed by Fourier transform of the derivative signal to a series of ac signals in the frequency domain. This is possible because the derivative of the chronoamperometric current is equivalent to a current impulse that would have been obtained if a voltage spike having a shape of the Dirac δ function would have been applied to the electrochemical system. The Dirac δ function, which has an infinitely large height with almost no base, would be obtained if all the ac waves of the same magnitudes and the same phase angles were summed.5 Since the integrated form of the Dirac δ function is just a potential step, the chronoamperometric current should be the signal that would have been obtained had the signal acquired upon application of the Dirac δ function been integrated. Thus, the current spike obtained by taking the first derivative of the chronoamperometric current can now be converted to the sum of ac current signals by Fourier transform, which allows the impedance data to be computed. The detailed theory on the technique has been fully described in our earlier report.3 The technique has a definite advantage of making impedance measurements in such a short time that it may allow simultaneous nonstationary impedance measurements to be made in situ during the other electrochemical experiments. There are, however, a number of difficulties in implementing the technique into an electrochemical experiment. One is the design of a fast rising potentiostat, for a potentiostat with a slow rise time would give a distorted step signal, which would lead to noise/errors in the highfrequency region.6 The second is a fast data acquisition system. (2) (a) Stoynov, Z. B.; Savova-Stoynov, B. S. J. Electroanal. Chem. 1985, 183, 133. (b) Savova-Stoynov, B. S.; Stoynov, Z. B. Electrochim. Acta 1992, 37, 2353. (c) Stoynov, Z. B. Electrochim. Acta 1992, 37, 2357. (d) Stoynov, Z. B. Electrochim. Acta 1993, 38, 1919. (3) Yoo, J.-S.; Park, S.-M. Anal. Chem. 2000, 72, 2035. (4) Yoo, J.-S.; Park, S.-M. Anal. Chem. 2001, 73, 4060. (5) Spanier, J., Oldham, K. B., Eds. An Atlas of Functions; Hemisphere Publishing Corporation: New York, 1987; Chapter 10. (6) Lee, J.-H.; Yoo, J.-S.; Park, S.-M. Manuscript in prepartion. 10.1021/ac0263263 CCC: $25.00
© 2003 American Chemical Society Published on Web 05/31/2003
Figure 1. Schematic diagram of the equipment for in situ impedance measurements.
In efforts to apply the technique to the real time in situ impedance measurements during the voltammetric scanning, we decided to employ the staircase voltammetric (SCV) experiments because the SCV experiments are conducted by just running a series of the chronoamperometric experiments,7-13 and there is no need to modify the experiments other than using a fast-rise potentiostat and a fast data acquisition system. The SCV experiments may sometimes be different from cyclic voltammetry (CV) experiments, which are used most frequently by electrochemists, but both may also give identical results, depending on the step size and the sampling time.9,10 Thus, the SCV experiment also imitates the CV experiments under an appropriate experimental condition. For in situ impedance measurements employing the SCV experiment, all one needs to do, therefore, is to measure a series of currents after each step is applied, take the derivatives of the signals, convert the current spike to ac current signals in the frequency domain, and compute the impedances. In this work, we report the development of real time in situ electrochemical impedance spectroscopy and apply the technique to a complex reaction, oxidation of aniline. Oxidation of aniline was chosen for the demonstration of the technique because the electrode surface undergoes continuous changes during its oxidation,14,15 which leads to the continuous change in electrical characteristics of the electrode during the experiment. (7) Barker, G. C. Advances Polarography; Longmuir, I. S., Ed.; Pergamon: New York, 1960. (8) Christie, J. H.; Lingane, P. J. J. Electroanal. Chem. 1973, 10, 176. (9) Bilewicz, R.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. 1986, 58, 2761. (10) Bilewicz, R.; Wikiel, K.; Osteryoung, R.; Osteryoung, J. Anal. Chem. 1989, 61, 965. (11) He, P. Anal. Chem. 1995, 67, 986. (12) Yu, J.-S.; Zhang, Z.-X. J. Electroanal. Chem. 1997, 427, 7. (13) Osteryoung, J. Acc. Chem. Res. 1993, 26, 77. (14) Park, S.-M. Handbook of Conductive Molecules and Polymers; Nalwa, H. S., Ed.; Wiley: Chichester, England, 1997; Vol. 3, and references therein. (15) Trivedi, D. C. In Handbook of Conductive Molecules and Polymers; Nalwa, H. S., Ed.; Wiley: Chichester, England, 1997; Vol. 2.
EXPERIMENTAL SECTION Aniline obtained from Aldrich (ACS grade) was used after vacuum distillation over zinc dust, and sulfuric acid from Samchun Chemical was used as received. Doubly distilled, deionized water was used for the preparation of solutions. A single-compartment electrochemical cell housing a gold disk working electrode with its diameter of 1.8 mm (geometric area ) 0.025 cm2), a platinum wire counter electrode, and a Ag/AgCl (in saturated KCl) reference electrode was used for the measurements. The working electrode was polished to a mirror finish successively with alumina slurries of 5.0 µm down to 0.05 µm. The electrochemistry experiments were conducted using a homebuilt fast-rise potentiostat/galvanostat whose rise time to its full voltage signal was
