Electrochemical impedance spectroscopy of fully hydrated Nafion membranes at high and low hydrogen partial pressures

The proton transport mechanism in fully hydrated Nafion 117 membranes was examined via electrochemical impedance spectroscopy (EIS) and steady-state current–potential measurements both in a symmetric H₂, Pt|Nafion|Pt, H₂ cell and in a H₂, Pt|Nafion|Pt, air PEM fuel cell with hydrogen partial pressure values, PH₂, varied between 0.5 kPa and 100 kPa. In agreement with recent studies it is found that for low PH₂ values the steady-state current–potential curves exhibit bistability and regions of positive slope. In these regions the Nyquist plots are found to exhibit negative real part impedance with a large imaginary component, while the Bode plots show a pronounced negative phase shift. These observations are consistent with the mechanism involving two parallel routes of proton conduction in fully hydrated Nafion membranes, one due to proton migration in the aqueous phase, the other due to proton transfer, probably involving tunneling, between adjacent sulfonate groups in narrow pores. The former mechanism dominates at high PH₂ values and the latter dominates in the low PH₂ region where the real part of the impedance is negative.     

Electrochemical impedance spectroscopy study of waterborne coatings film formation

In order to establish electrochemical impedance spectroscopy (EIS) as a viable quantitative method for characterization of latex film formation, three waterborne acrylate and styrene–acrylate polymer dispersions were periodically analyzed during a course of 2 weeks. Impedance spectra were fitted on the base of equivalent circuit consisting of a capacitor in parallel with a Warburg element representing film capacitance and the extent of ion diffusion through the film. Calculated EIS parameter values showed a decrease in Warburg diffusion over time, which is a result of particle coalescence and in agreement with the established theory of latex film formation. Atomic force microscopy (AFM) of the samples showed a smoothing of the surface and blurring of interparticle boundaries which confirmed that EIS can be used to study film formation of latex.     

Electrochemical impedance spectroscopy of the alkaline manganese dioxide electrode

Two-probe electrochemical impedance spectroscopy measurements were carried out on the electrolytic manganese dioxide electrode in concentrated KOH electrolytes under a variety of experimental conditions. These included varying the electrode thickness and compaction pressure, electrolyte content and concentration, degree of manganese dioxide reduction and the presence of TiO₂ (anatase) as an additive. The overall electrode impedance was found to decrease when thin electrodes, prepared under high compaction pressures, with an excess of electrolyte, were used. The impedance of the EMD/electrolyte interface was also minimized when 5.0 M KOH was used as the electrolyte. This correlates with a maximum in electrolyte conductivity. The electrode impedance also increased as the degree of EMD reduction was increased, as was expected. Under these experimental conditions the electrode impedance increased in the presence of TiO₂ (anatase), which has negative implications for its commercial use. This conclusion was reached despite the differences in experimental conditions between this work and in commercial applications. An equivalent circuit was also derived and used as an aid in interpreting the impedance data.     

Electrochemical impedance spectroscopy evidence of dimethyl-silicon-oil enhancing O2 transport in a porous electrode

Faster oxygen transport is critical to guarantee reliable power output of polymer electrolyte membrane fuel cells (PEMFCs). In order to enhance oxygen transfer in a porous electrode especially in the case of water flooding, water-proof oil (dimethyl-silicon-oil (DMS)) was introduced into the conventional Pt/C electrode. Owing to the capability of electrochemical impedance spectroscopy (EIS) in discriminating individual contribution of ohmic, kinetic, and mass transport from all PEMFC processes, EIS was carried out to evaluate the effect of the DMS on the oxygen reduction reaction (ORR). The equivalent circuits corresponding to the EIS spectra were employed. The parameters in the equivalent circuits were obtained by curve fitting to the EIS spectra with the aid of the frequency response analysis software (FRA) attached in the electrochemical station Autolab PGSYAT302. The EIS analysis has shown that the introduction of DMS reduces the oxygen diffusion resistance as well as the charge transfer resistance in the flooded state. The single cell tests show that even in the case of normal operating condition the accumulated water with PEMFC operation also worsens the oxygen transfer in the conventional Pt/C gas diffusion electrode (GDE) with more and more water produced at the cathode. GDE containing DMS, which is defined as a flooding tolerant electrode (FTE), is fortunately quite good at alleviating water flooding. Success of the FTE in alleviating water flooding is ascribed to (1) its high oxygen transfer flux due to the higher solubility of oxygen in DMS than in water as long as parts of pores are occupied beforehand by DMS rather than by water, and (2) enhanced hydrophobic property of the FTE with DMS adsorption on the walls of the pores, which makes more hydrophobic pores be open to oxygen transport.     

Characterization by electrochemical impedance spectroscopy of magnetite nanoparticles supported on carbon paste electrode

Magnetite nanoparticles were supported on carbon paste electrode and characterized by low scan rate voltammetry and electrochemical impedance spectroscopy (EIS) to obtain mechanistic information related to its oxidation and reduction in acid media.The voltammograms showed only one reduction and one oxidation peak for the supported magnetite, which were attributed to formation of ferrous ion and ferric oxide, respectively. Both peaks are fairly wide, indicating complex mechanisms.Using EIS, a mechanism showing up to three time constants, capacitive all of them, was evidenced, both in anodic and cathodic domain. These were attributed to charge transfer at the highest frequencies, adsorption of generated species at intermediate frequencies, and proton adsorption at low frequencies. Discussion about the nature of the adsorbed species and the concerned mechanism for each domain is developed.     

Electrochemical impedance spectroscopy as a tool to measure cathodic disbondment on coated steel surfaces: Capabilities and limitations

The disbondment of protective organic coatings under excessive cathodic protection potentials is a widely reported coating failure mechanism. Traditional methods of evaluating cathodic disbondment are based on ex situ visual inspection of coated metal surfaces after being exposed to standard cathodic disbondment testing conditions for a long period of time. Although electrochemical impedance spectroscopy (EIS) has been employed as an effective means of evaluating various anti-corrosion properties of organic coatings; its application for assessing the cathodic disbondment resistance of coatings has not been sufficiently exploited. This paper reports an experimental study aimed at developing EIS into a tool for in situ measurement and monitoring of cathodic disbondment of coatings. A clear correlation between EIS parameters and the disbonded coating areas has been confirmed upon short term exposure of epoxy-coated steel electrodes to cathodic disbondment conditions; however the degree of this correlation was found to decrease with the extension of exposure duration. This observation suggests that EIS loses its sensitivity with the propagation of coating disbondment, and that in order to achieve quantitative determination of the coating cathodic disbondment localized EIS measurements are required to measure the parameters related to local disbonded areas.     

Electrochemical behaviour of PANi/polyurethane antifouling coating in salt water studied by electrochemical impedance spectroscopy

Electrochemical behaviour of polyaniline–polyurethane (PANi–PU) antifouling coating in 3.5 wt% NaCl is studied by electrochemical impedance spectroscopy (EIS). A thick coating (∼1 mm) of 10, 15 and 20% PANi in marine grade PU, is cast over corrosion resistant aluminium alloy 2024 and its impedance characteristics are measured by EIS and compared with neat PU. On addition of 10% PANi, the impedance of the coating drastically comes down from 10⁹ to 10⁷ Ω. 20% is the maximum processable amount of PANi for the selected PU system. The coatings are exposed to 3.5 wt% NaCl and its impedance characteristics are monitored as a function of time. Changes in the impedance characteristics of the systems were found to occur as a function of the exposure time in all cases, though their evolution with time showed marked differences with PANi content. Water sorption and break down frequency are derived from the experimental results and analysed.     

Electrochemical impedance spectroscopy of carboxylic-acid terminal alkanethiol self assembled monolayers on GaAs substrates

Adsorbate-induced charge depolarization can influence the organization of self assembled monolayers (SAMs) on semiconductor surfaces, especially as a function of the SAM functional group, SAM length and substrate dopant level and type. Based on systematic differences in the frequency response of the electrochemical impedance and phase data for carboxylic acid (COOH) terminal alkanethiol monolayers of varying alkane chain length assembled on GaAs substrates of different dopant level and type, we assessed the relative monolayer quality through fits to an appropriate equivalent circuit analog to compare the proportion of defects and SAM-induced semiconductor depolarization. At the open circuit potential in the NaCl-phosphate buffer, while SAMs on p+ GaAs were of marginally better quality than those on p GaAs, SAMs on n+ GaAs exhibited a far superior quality than those formed on n GaAs. COOH-terminal SAMs of longer chain lengths formed higher quality monolayers at all the substrate doping levels. COOH-terminal SAM modified n and n+ GaAs surfaces were passivated and stable over a wider voltage range than SAM modified p and p+ GaAs surfaces, from cyclic voltammetry. The poorer quality of COOH-terminal SAMs formed on GaAs substrates at the lower doping levels is attributed to the disorder as a result of the enhanced degree of charge depolarization at these surfaces, as substantiated by systematic variations in the space charge capacitance upon SAM modification that suggest a negative surface dipole.     

Electrochemical impedance spectroscopy of poly[carbazole-co-N-p-tolylsulfonyl pyrrole] on carbon fiber microelectrodes,equivalent circuits for modelling

Polycarbazole (PCz) and copolymerization of carbazole (Cz) and N-p-tolylsulfonyl pyrrole (pTsp), P(Cz-co-pTsp), thin films have been cyclovoltammetrically coated onto carbon fiber electrodes as an active functionalized microelectrode in sodium perchlorate (NaClO₄)/acetonitrile (ACN) medium. The resulting thin films of homopolymer and copolymer were characterised by using Fourier transform infrared reflectance spectroscopy (ATR-FTIR), energy dispersive X-ray (EDX) point analysis, scanning electron microscopy (SEM) and atomic force microscopy (AFM). An electrical impedance study on the prepared electrodes is reported in the present paper under different feed ratios of [pTsp]₀/[Cz]₀ during electrochemical impedance spectroscopic (EIS) measurements. Specific capacitance (C_sp) were calculated, P(Cz-co-pTsp) in feed ratio of [pTsp]₀/[Cz]₀ = 200 has preserved more capacitive behavior especially at lower frequency (C_sp = ∼156 mF g⁻¹) than polycarbazole (C_sp = ∼2.1 mF g⁻¹. The electrochemical impedance data fitted to three different equivalent models were used to find out numerical values of the proposed components.     

Electrochemical impedance studies of corrosion protected surfaces covered by epoxy polyamide coating systems

Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of different types of commercial quality epoxy polyamide dry coatings on mild steel, with thickness between 150 and 250 μm, which were previously weather accelerated in a wet chamber. The following data were established on the basis of impedance curves and corresponding equivalent circuits: the coating capacity that relates to the coating dimension, the pore resistance that represents conductive paths through the pores, and Warburg coefficients, which are the measure of ion diffusion through the coating. EIS data were compared with a criterion in the European standard, and samples were classified on the basis of their quality, also receiving a final ranking by summing-up all the individual rankings. Suggestions have been made, namely that the use of anticorrosive active pigments is obsolete, when the thickness of the coatings exceeds 200 μm.     

Electrochemical impedance spectroscopy of pure copper exposed in bentonite under oxic conditions

The corrosion rate of pure copper has been followed by thin electrical resistance (ER) sensors placed in a test package containing an oxic bentonite/saline groundwater environment at room temperature for a period of more than three years. Potential measurements have verified oxic conditions in the test package. A series of electrochemical impedance spectroscopy (EIS) measurements has also been performed on the ER sensors and results from one of the sensors are presented.The impedance spectra for copper in the oxic bentonite/saline groundwater environment changed considerably with time of exposure. The impedance increased over the whole frequency range. Three methods were applied to estimate the corrosion rate from the EIS spectra.The EIS measurements have revealed decreasing but measurable corrosion rates all through the exposure period. The former reveals a desired behaviour of copper in the environment. After three-year exposure the estimated corrosion rate from the EIS measurements fell in the range 0.4–0.7 μm/year, which is somewhat lower than the value of 1.0 μm/year, obtained from the ER measurements. Gravimetric data from six years exposure in the Äspö Hard Rock Laboratory showed an average corrosion rate of 0.5 μm/year. The corrosion resistance of copper is improved by the presence of bentonite.     

Electrochemical impedance spectroscopy response of water uptake in organic coatings by finite element methods

The corrosion of metals is a long standing topic of research which is of immense economic importance. One of the principal means of preventing surface corrosion of metals is thin polymer coatings, i.e. paint. These coatings often fail because of the passage of water or other active ionic species into and through the polymer coating. Since both the capacitance and the resistance of a polymer coating change as it absorbs water, electrochemical impedance spectroscopy (EIS) gives a general idea of how much water a polymer has been absorbed. Theories, such as the Brasher-Kingsbury approximation, are effective medium theories based on the assumption that the absorbed water is randomly distributed in spherical inclusions. The water in a coating may be distributed as spherical inclusions, as discrete channels, or as some combination that transports water from the coating surface until the water reaches the metal substrate and corrosion can begin.The resistance and the capacitance of a coating depend on both the amount of water (volume fraction) and the shape of the water inclusions. EIS gives only a general idea of how much water has been absorbed by a coating but does not provide the distribution or shape of the water inclusions. EIS circuit response is often modeled with the equivalent circuit elements describing the material properties for water inclusions that are implicitly assumed to be randomly distributed spherical inclusions. Numerical calculations using the finite element analysis (FEA) are reported here to solve Maxwell's equations for various shapes and sizes of water inclusions within the polymer. Calculations here have been based on the electrical properties of a polyvinyl fluoride film, as an exemplar, with water inclusions of different shapes and concentrations (water volume fraction). The Brasher-Kingsbury approximation gives the correct outcome only for a random distribution of spherical inclusions, as expected. Other shapes and distributions can vary from the Brasher-Kingsbury prediction of water volume fraction by more than 50% of the actual gravimetric water volume fraction. Results are presented here for spherical and cylindrical randomly distributed water inclusions. Understanding the sensitivity to different distributions and numbers of inclusions is an objective planned for future research.     

Electrochemical impedance spectroscopy study of a surface confined redox reaction: The reduction of azobenzene on mercury in the absence of diffusion

The kinetics of azobenzene reduction on mercury electrodes in the absence of diffussional mass transport is studied by electrochemical impedance spectroscopy (EIS) in acetic acid/acetate buffered solutions at different pH values. Cyclic voltammetry experiments confirm the absence of diffusion effects and provide the values of the surface equilibrium potential. The analysis of the impedance frequency spectrums at every potential within the faradaic region conforms well the model and provides the global rate constant of the process, k_f. The potential dependence of k_f suggests the existence of an EE mechanism, with two electron transfers controlling the overall rate. The kinetic parameters of every step are obtained and their pH dependences clarify the role played by the protonation steps.     

Review study of electrochemical impedance spectroscopy and equivalent electrical circuits of conducting polymers on carbon surfaces

Electrochemical impedance spectroscopy (EIS) is an experimental method for characterizing electrochemical systems. This method measures the impedance of the concerned electrochemical system over a range of frequencies, and therefore the frequency response of the system is determined, including the energy storage and dissipation properties. The aim of this article is to review articles focusing on electrochemical impedance spectroscopic studies and equivalent electrical circuits of conducting polymers, such as polypyrrole, polycarbazole, polyaniline, polythiophene and their derivatives, on carbon surfaces. First, the conducting polymers are introduced. Second, the electrochemical impedance spectroscopic method is explained. Third, the results of EIS applications using equivalent electrical circuits for conducting polymers taken from the literature are reviewed.     

Electrochemical impedance spectroscopy of lead(II) ion-selective solid-state membranes

Silver sulphide/lead sulphide membranes were studied using electrochemical impedance spectroscopy. The influence of the electrolyte concentration and the membrane thickness were evaluated. The complex impedance plots have shown two capacitive loops: one at high frequency range, related with the charge transfer resistance at the interface membrane-Ag and a second one at low frequency range, associated with the diffusion process through the membrane. A simple model was used to take into account the experimental results: the changes of the potential with the time and the electrolyte concentration; the changes of the charge transfer resistance and the diffusion resistance with the electrolyte concentration. An empirical equation was used to calculate the diffusion coefficient of Ag⁺ inside the membrane.     

Electrochemical impedance spectroscopic investigation of adhesion formation mechanism at an epoxy/aluminum interface

A theoretical method for characterizing the structure of a coating/metal interface by electrochemical impedance spectroscopy using water molecules as the probe was established. The properties of coating/metal interfaces for a series of epoxy resins with different water affinities were studied using this method. It was found that as the water affinity of the coating decreased, it became much more difficult for water molecules to reach the coating/metal interface. This suggests that, during the adhesion formation, a more hydrophobic layer is formed along the epoxy/metal interface.     

Investigation of electrode composition of polymer fuel cells by electrochemical impedance spectroscopy

In order to optimize the electrode composition and performance of Polymer Fuel Cells and to reduce the production cost of membrane electrode assemblies (MEAs), different MEAs using different catalyst powders, carbon supported and unsupported catalysts with different proton conducting electrolyte powder (Nafion) content were produced by using a dry powder spraying technique developed at German Aerospace Research Center (DLR, Deutsches Zentrum fuer Luft- und Raumfahrt). The electrochemical characterization was performed by recording current-voltage curves and electrochemical impedance spectra (EIS) in the galvanostatic mode of operation at 500 mA cm⁻². The evaluation of the measured impedance spectra with an adequate equivalent circuit shows that the cathode of the fuel cell is very sensitive to the electrode composition whereas the contribution of the anode is very small and invariant to the electrode composition. Furthermore, it could be shown for the first time using electrolyte powder in the electrodes that the charge transfer of the cathode decreasing monotonically with increasing electrolyte content in the cathode. These findings suggest that with increasing electrolyte content in the electrodes, in particular in the cathode, the utilization degree of the catalyst increasing linearly with increasing electrolyte content in the electrode.     

Electrochemical impedance spectroscopy of electrodes modified with thin films of MgMnCO3 layered double hydroxides

The electrochemical impedance spectra of MgMnCO₃ LDH films oxidized at different dc potentials were recorded. The results were fitted to a Randles type cell by replacing the Warburg impedance with a mass transfer resistance in parallel with a constant phase element. The films charge transfer resistances decreased dramatically at the onset of manganese oxidation. In thin films, R_ct decreased from 10⁴ Ω for an un-oxidized film to a minimum of 40 Ω in a film oxidized at 0.32 V, before increasing back to 10⁴ Ω in a film oxidized at 0.5 V. Iodometry measurements show these changes correspond to increases in the manganese average valence in the films from 3.09+ prior to oxidation, to 3.80+ at 0.32 V and 3.95+ at 0.5 V. In thicker films, however, a much higher dc potential, 1.0 V, was required to return R_ct to 10⁴ Ω. There was also less change in the manganese average valence in the thicker films. Oxidation at 1.0 V only increased the manganese valence to 3.33+. For the partially oxidized films, the Nyquist plots consisted of depressed semicircles at high frequency, followed by linear regions at lower frequency where the impedance was controlled by mass transport. The effective diffusion coefficient estimated from the low frequency impedance was 1 × 10⁻⁹ cm² s⁻¹, consistent with proton diffusion in solid electrodes. The impedance spectrum of a partially oxidized film reduced at −0.2 V was similar to that of the un-oxidized film.     

Electropolymerization of chlorinated phenols on a Pt electrode in alkaline solution Part III: A Fourier transformed infrared spectroscopy study

FTIR spectroscopy was employed to investigate high molecular weight substances formed on a platinum electrode surface during the electrochemical oxidation of phenol and its chlorinated derivatives. Potentiodynamic (potential range from –0.80 V to 0.85 V vs SHE; scan rate 200 mV s^–1) and potentiostatic (at 0.78 V vs SHE) electropolymerization was used in alkaline solutions (1 M NaOH) containing 0.1 M of phenol, monochlorophenols, dichlorophenols, trichlorophenols and pentachlorophenol. The IR spectra of the corresponding monomers were recorded for the comparison. The FTIR spectroscopy studies revealed that the polymers formed under potentiodynamic and potentiostatic conditions are of aromatic nature (–C=C– stretching vibrations at 1450–1600 cm^–1), they have ether-linkages (=C—O—C= stretching vibrations at 1100–1300 cm^–1) and quinone groups (–C=O stretching vibrations at 1630–1800 cm^–1 and –C—H out-of-plane bending at 760 cm^–1). The intensities of the hydroxyl group bands in most of the polymers are rather weak compared to those in the corresponding monomers. Vibrations at 2850–2960 cm^–1, which are present in most of the IR spectra of polymers formed under cyclic voltammetry conditions, correspond to the stretching vibrations of the sp³ hybridized C—H bond and suggest that the cleavage of the benzene ring occurs to some extent during electrooxidation–electropolymerization of phenol and its chlorinated derivatives when reaching the potential of oxygen evolution (0.85 V vs SHE).     

Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: Role of Zn content on corrosion protection mechanism

Effect of zinc content in the epoxy zinc rich coating on the mechanism of corrosion protection was investigated using electrochemical impedance spectroscopy (EIS). The zinc content in the coating (on dry film) was varied from 40% to 90% in steps of 10%. Open circuit potential (OCP) measurements and salt spray tests were also carried out to generate supporting evidences and to deepen the understanding in the area of zinc rich coatings. The healing or bridging ability of these coatings was studied by making a scribe on the coating and monitoring the evolution of OCP. EIS was also utilized to screen the amount of zinc particles required to provide efficient galvanic protection and to study the effect of purity of Zn on corrosion protection performance. Both EIS and OCP measurements indicate that coatings containing 40% Zn (on dry film) provides very good barrier protection coming mainly from polymer whereas the one containing >80% Zn provides excellent galvanic protection to the steel substrate. When the zinc loading is between 60% and 70%, coatings neither show barrier protection nor galvanic protection.