Article pubs.acs.org/JPCC
Localized Impedance Measurements for Electrochemical Surface Science Aliaksandr S. Bandarenka,*,† Artjom Maljusch,‡ Volodymyr Kuznetsov,‡ Kathrin Eckhard,‡ and Wolfgang Schuhmann*,†,‡ †
Center for Electrochemical SciencesCES, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany Analytische ChemieElektroanalytik & Sensorik, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany
‡
ABSTRACT: An approach for in-depth characterization of complex electrode/electrolyte interfaces based on localized impedance measurements is described in detail. The local ac probing of the interface is performed at different frequencies by means of scanning electrochemical microscopy (SECM) using ultramicroelectrodes (SECM tips) which enables visualization of dependences of the localized impedance spectra as a function of spatial coordinates. Subsequent fitting of these spectra to physical models visualize the local distribution of parameters describing the electrochemical interface, such as the electric double layer capacitance and the charge transfer resistance. Three model examples are analyzed dealing with typical situations, when the measurements are either affected or not by specific adsorption of anions at the SECM-tips. It is demonstrated that the approach holds promise for electrochemical surface science, particularly for better understanding of corrosion processes taking place at metal surfaces in aggressive, particularly aqueous electrolytes.
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INTRODUCTION Techniques that are based on impedance measurements are among the most powerful and informative in physical chemistry and electrochemistry.1−8 The popularity of these approaches originates from their ability to help in efficient elucidation of physical models of the interface, as well as in distinguishing and comprehensive characterization of processes taking place at the electrode surface simultaneously. However, classical electrochemical impedance spectroscopy (EIS) deals with acquisition and analysis of spectra that contain only averaged information over the whole surface area. From this point of view, common EIS can be considered as a “global” analytical technique. While the latter fact is of less importance for quasi-uniform, welldefined or single crystal surfaces, it can clearly be a problem in the case of nonuniform electrodes or specific localization of electrochemical reactions at different sites of the surface. One typical example of those processes is corrosion of metal surfaces: in order to gain insight into mechanisms of formation and progression of corrosion sites, localized measurements across the electrode surface are mandatory. A noticeable progress in electrochemical surface science and physical electrochemistry9−15 has recently contributed to elaboration of new approaches for localized impedance measurements capable to address the methodological issues related to nonuniform electrode surfaces. Among them, one can arbitrarily distinguish vibrating probe EIS techniques,16−18 microcapillary-based impedance methods,19 bielectrode probe techniques,20,21 and EIS-approaches based on the application of scanning electrochemical microscopy (SECM).22−24 © 2014 American Chemical Society
In this work, we primarily focus on localized impedance measurements in the alternating current mode of SECM (ACSECM), as this is one of few approaches that has a big potential to provide comprehensive information about the properties of the electrochemical interface itself as a function of spatial coordinates, particularly, for corrosion investigations. A unique feature of this method is that the samples are not connected to a potentiostat making characterization of corrosion processes at the sample surface possible under “real-world” conditions. In particular, we consider model systems where dissolved chloride or sulfate ions can largely affect the measurements due to, e.g., their specific adsorption at the probing tip. New opportunities for visualization of spatial distributions of the key parameters describing the electrode/electrolyte interfaces are also discussed.
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EXPERIMENTAL SECTION Measurement Scheme and Data Analysis. Acquisition of localized impedance spectra was performed according to procedures described elsewhere in detail22,23 using a setup shown schematically in Figure 1. Briefly, the spectra were recorded using either a Bio-Logic VSP-300, Gamry Reference 600 potentiostat with a FRA module (Gamry Instruments, Inc.) or a PAR 273A potentiostat (Ametek, Oak Ridge, U.S.A.) equipped with an EG&G 7280 digital lock-in amplifier. The Received: December 20, 2013 Revised: April 4, 2014 Published: April 7, 2014 8952
dx.doi.org/10.1021/jp412505p | J. Phys. Chem. C 2014, 118, 8952−8959
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Figure 1. (A) Schematic representation of the setup for the localized scanning impedance microscopy measurements. (B) Photographic image of a ultramicroelectrode (SECM tip, 10 μm Pt) used for AC-SECM experiments. (C) Schematic representation of the measurement protocol.
contact with 1 mM Na2SO4 were used. Before the experiments, these samples were polished with diamond aqueous slurries (3, 1, 0.5 μm; Leco Corporation) and cleaned in an ultrasonic bath with acetone, isopropanol, and water. The “line scan” experiments were performed in the electrolyte solution containing specifically adsorbing Cl− anions over a flat “dielectric−metal−dielectric” structure. The latter consisted of poly(methyl methacrylate) (PMMA) plastic with Al-alloy (AA 3003) inserts. The surface of the sample was grounded with subsequently finer emery paper (400, 800, 1500, 2000, and 2500 grid). Prior to each experiment the surface was polished using diamond-based water-free polishing suspensions (3 μm, 1 and 0.25 μm, Struers), cleaned by ultrasonication in isopropanol, and dried in an argon stream. To perform area scans, a “metal-protective film” model sample was used. For this, low-carbon steel was coated with Sn and covered with a 9−15 μm layer of epoxy phenolic varnish. A circular defect of approximately 220 μm in diameter had been introduced approximately in the middle of the sample in order to expose the underlying metal to the electrolyte. The defect was produced by pressing a thin sharp needle perpendicular to the sample surface. It can be assumed that a conically shaped defect was created while both Sn and steel were exposed. Glass insulated disc Pt-microelectrodes (Figure 1B) of either 10 or 25 μm were used for the localized AC-SECM experiments with the corresponding x and y steps (Figure 1C) during the scans.
data were collected at each point during the line and area scans in the frequency range either (i) between 100 kHz and 200 Hz or (ii) from 8 kHz to 270 Hz, using probing signal amplitudes of 30 mV or 100 mV, respectively. Calibration of localized measurements were performed in the frequency range between 100 kHz and 30 Hz. Lower probing frequencies were avoided to maintain a reasonable time for data acquisition during the measurements. The quality of the localized impedance spectra was evaluated using the “linear”25 and “logarithmic”26 Kramers−Kronig check procedures. The quality of the fitting was controlled by the root-mean-square deviations and estimated individual parameter errors to ensure the validity of the model and the correctness of the fitting, as described in detail elsewhere.27,28 It should be mentioned that the relatively high amplitude of the probing signal used in this work was the maximal allowed by the Kramers−Kronig check procedures, to maximize the signal-to-noise ratio. The large experimental impedance data sets generated as the result of the line and area scans were analyzed with a homemade “EIS Data Analysis 1.0” software according to the procedure described previously23,29 using a hybrid fitting algorithm described elsewhere.29 The latter provides high stability of the sequential spectra fitting, allowing semiautomatic data analysis. Chemicals, Samples and Electrodes. One mM KClO4 (Riedel-de-Haen, Seelze, Germany), 1 mM Na2SO4 (SigmaAldrich), and 10 mM KCl (Merck) aqueous solutions were used as working electrolytes. A Pt wire and an Ag/AgCl electrode were used as the counter electrode (CE) and the reference electrode (RE), respectively. Three model samples were utilized in this work. In order to perform “calibration” experiments, brass (37% Zn, 63% Cu, Goodfellow) samples in
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RESULTS AND DISCUSSION Modeling, Analysis and Interpretation of Localized Impedance Spectra. Figure 2 schematically shows a generalized physical model (equivalent electric circuit, EEC) 8953
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interfacial processes. Obviously, under these conditions many Faradaic (particularly corrosion) processes at the sample surface (if any) normally do not experience diffusion limitations. For the simplicity, considering that the contribution of the sample response into the localized spectra is significant only at relatively high frequencies, the localized Faradaic impedance, Z(F.,sample), can be represented by a charge transfer resistance, R(F.,sample), only. Further assumptions are related to the response of the overall sample interface. At relatively high probing frequencies used in this work and taking into account a large ratio between the overall sample surface and the area covered by the glass shield of the tip, Z(int.,tot.) and Z(F.,tot) in Figure 2 can be neglected due to their small relative contribution to the measured response. In the remaining model, it is normally not possible to separate the local charge transfer resistance and the electrolyte resistance R(electrolyte) connected in parallel giving together an effective resistance, R(eff.,sample)−1 = R(F.,sample)−1 + R(electrolyte)−1. Therefore, the simplified EEC shown in Figure 3A represents the model describing the ac response in the simplest situation; excluding any effects of possible Faradaic processes at the tip.
Figure 2. A generalized equivalent electric circuit used to model the localized impedance spectra. The ac-response typically contains the contributions of the probing tip associated with the impedance of its electrochemical interface, Z(int.,tip) and possible electrochemical (Faradaic) processes, Z(F.,tip). Additionally, the response contains the information about the sample itself through the contribution of the local interfacial Z(int.,sample) and local Faradaic Z(F.,sample) impedances. R(electrolyte) represents the contribution of the electrolyte resistance. Finally, Z(int.,tot.) and Z(F.,tot.) denote the overall interfacial and Faradaic impedances of the sample.
describing the impedance response that can be obtained as a result of the localized measurements. If the tip (playing the role of the working electrode, WE) is in close proximity to the sample surface, then the measured ac-response would consist of several contributions. Obviously, it will contain the contributions originating from the impedance of the tip/electrolyte interface and some possible electrochemical processes taking place at the tip, including adsorption, formation of surface oxides or side reactions. Additionally, as it is illustrated in Figure 2, the impedance spectra will contain the information about the local properties of the sample through the responses of the sample/electrolyte interface and Faradaic processes at the sample surface domains which are covered by the glass sheath of the tip. Finally, the overall interfacial and Faradaic impedances of the sample as well as the electrolyte resistance (which changes during the scans) should be taken into account. The EEC shown in Figure 2 reveals the situation that if the tip is close enough to the sample surface, the current can flow either through the electrolyte or the sample and corresponding interfaces. Depending on the properties of the sample, tip, and electrolyte composition, the EEC shown in Figure 2 can be simplified. Consider an aqueous electrolyte containing some amount of anions and cations like Na+ and ClO4− that normally do not directly participate in Faradaic processes both at the tip and the sample, including specific adsorption with interfacial charge transfer. In this case, Z(F.,tip) → ∞, and the contribution of the tip is represented by the interfacial impedance, Z(int.,tip), only. The latter can be represented in a general case by a constant phase element (CPE), so that Z(int.,tip) = Z(CPE) = CDL′−1(jω)−n (where C′DL is proportional to the double layer capacitance related to the tip, j is the imaginary unit, ω is the angular frequency, and the exponent n is a parameter which practically varies between 1 and 0.75, if the CPE is used to model the double layer response). As mentioned above, the sample is not connected to any potentiostat so that its potential depends only on the electrode and electrolyte composition, as well as on the relevant
Figure 3. Simplified equivalent electric circuits describing the localized ac-response expected in electrolytes (A) without and (B) with chloride anions capable to specifically adsorb at the surface of the Pt tip (see text for details).
Consider another situation when the working electrolyte contains components that can significantly change the simple model shown in Figure 3A. One example is the introduction of SO42− or Cl− anions into the electrolyte, as widely utilized in corrosion test protocols. These anions can be specifically adsorbed on the surface of both sample and the tip. As the SO42− and Cl− adsorption on Pt surface of the tip is reversible (the fractional chloride surface coverage oscillates under acprobing), the Faradaic response of the tip can be described by the classical Dolin-Ershler adsorption model,30−33 resulting in the following equation for Z(F.,tip) = RADS + (jωCADS)−1, where RADS and CADS are the charge transfer resistance and the capacitance associated with the adsorption, respectively. In turn, the EEC will be transformed to the one shown in Figure 3B. Calibration Experiments. First of all, it is important to ensure that the measurement and analysis of the localized impedance spectra are able to provide reasonably accurate quantitative information about the local properties of the electrified interface. For that, special calibration experiments have been performed using polished brass samples in contact with 1 mM Na2SO4 electrolytes. The main idea of those 8954
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electrolyte interface in the presence of an electrochemical reaction involving reversible adsorption of intermediates. This EEC is often used in corrosion research to assess the properties of the corroding samples under potentiostatic conditions (see, e.g. ref 34). We are mostly interested in the quantification of two parameters of this model: the “global” charge transfer resistance, Rct,G, and the double layer capacitance, CDL,G. Namely, these two parameters largely contribute to the response at higher probing frequencies and, therefore, can be correlated with those obtained from the localized impedance data (at lower frequencies the contribution of the tip will dominate). In order to accurately extract the Rct,G, CPE was used to model the “global” response of the double layer. Excellent fitting was obtained (Figure 5A) if the EEC shown in Figure 4 was used. The average value of the charge transfer resistance obtained for three independent measurements at freshly polished brass surface was 175 ± 5 Ω cm2 (the uncertainty takes those obtained from the measurements and estimated from the fitting as independent ones). However, to estimate the true double layer capacitance values, the same spectra were fitted to the model shown in Figure 4 with ZDL,G = (jωCDL,G)−1, i.e., with the ideal capacitance representing the double layer. As can be seen from Figure 5B, the fitting results with the ideal capacitance are noticeably worse; however, this is the necessary compromise to obtain physically the clearer parameter of the double layer capacitance for the calibration purposes. The average CDL,G was estimated to be 9.9 ± 1.3 μF cm−2. In the following, three independent localized experiments were performed using a freshly polished brass surface. Typical “local” impedance spectrum is shown in Figure 5C together with the results of the fitting to the model shown in Figure 3B.
experiments was to correlate the parameters obtained from the “global” and “local” impedance measurements. It should be however noted that the measurements and modeling in the case of corroding samples using “global” EIS represent a certain challenge. The first complication arises because of the properties of the sample surface changes with time significantly under potentiostatic conditions, even at quasi open circuit potential and utilizing low probing amplitudes. Therefore, acquisition of reliable data at lower frequencies is more difficult due to nonstationarity effects. The second issue rises from the fact that the double layer response can be in many cases not ideally capacitive. Due to numerous reasons, the measured double layer capacitance depends on the applied probing frequency. Therefore, the constant phase element is often used to take into account this effect. However, this complicates determination of the true double layer capacitance. Figure 4 shows the equivalent electric circuit that describes the “global” high frequency response of the electrode/
Figure 4. Equivalent electric circuit describing the “global” impedance response of the brass electrode/electrolyte interface. RS,G is the electrolyte resistance, ZDL,G is the impedance of the electric double layer, Rct,G is the charge transfer resistance, Ca,G and Ra,G are the capacitance and resistance associated with reversible adsorption of intermediates, respectively.
Figure 5. (A,B) Typical experimental “global” impedance spectrum (open circles, corrected for the electrolyte resistance and normalized for the geometric electrode surface area) characterizing the brass samples in contact with 1 mM Na2SO4 electrolyte with fitting (solid lines) to the equivalent circuit shown in Figure 4 with the impedance of the double layer represented by (A) the constant phase element and (B) the ideal capacitance. (C) Representative “local” impedance spectrum (taken at ca. 2 μm above the surface) for the same sample with fitting (solid lines) to the model shown in Figure 3B. 8955
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Figure 6. Typical localized impedance spectra of the model “dielectric−metal−dielectric” sample recorded over the dielectric (open squares) and metal (open circles) surfaces. Solid lines represent the fitting of the data to the equivalent circuit shown in Figure 3B.
The latter confirms the general validity of the EEC. The following average normalized parameters related to the localized sample/electrolyte interface were obtained, taking into account the geometric area under the glass tip: Reff.,sample = 222 ± 77 Ω cm2 and CDL,sample = 5.8 ± 2.2 μF cm−2. The higher uncertainties compared to the “global” case can be explained by certain microheterogeneity of the sample and that Reff.,sample contains the contribution from the electrolyte resistance, which can in many cases (taking into account e.g. the cell geometry, the shape of RE or electrolyte conductivity) depend on the position of the tip with respect to the position of RE. One can notice that the relevant parameter values obtained from the localized experiments are reasonably close to those obtained from the “global” measurements within the errors of the experiments and taking into account above-mentioned difficulties in the analysis of the “global” spectra. The results of the calibration experiments, therefore, confirm the earlier hypothesis22,23 that the localized impedance data characterize the sample/electrolyte interface covered by the glass tip, i.e., the diameter of the glass tip plays the decisive role in relatively diluted electrolytes, not the diameter of the Pt electrode itself. However, it should be noted that this quite counterintuitive behavior of the measurement system will likely depend on many other parameters, such as the distance between the tip and the sample or the electrolyte composition. Therefore, dedicated calibration experiments would be necessary in each particular case. Nevertheless, the localized measurements are able to provide reasonably accurate quantitative information about at least two important local parameters: the double layer capacitance and the charge transfer resistance. The latter can be used, e.g., to estimate the corrosion currents in the case of corroding samples. However, it should be noted that the acceptable fitting using the ideal capacitance to represent the localized double layer capacitance in the EEC shown in Figure 3B does not necessarily mean the absence of the CPE-behavior of the localized sample/electrolyte interface. The use of CPE (Figure 3B) to approximate the behavior of the tip also likely takes into account possible imperfections of the behavior of the interface between the sample and the electrolyte. Line and Area Scan Experiments. Figure 6 shows typical examples of the localized impedance spectra obtained using the model “dielectric−metal−dielectric” sample over the nonconducting part as well as over the metallic surface in 10 mM KCl electrolyte. It should be, however, noted that the Alalloy surface (further denoted as the metal surface) is likely covered with a nonuniform oxide layer under these conditions. As can be seen from Figure 6, the shape of the spectra is drastically different depending on the location of the micro-
scope tip. However, the equivalent circuit shown in Figure 3B fits perfectly (solid lines in Figure 6) all the spectra in the data set with relatively low root-mean-square deviations (
