Mechanism and equivalent circuits in electrochemical impedance spectroscopy

The utility and limitations of using equivalent circuits to analyse electrochemical impedance spectroscopy (EIS) data for electrochemical reaction mechanisms are reviewed. The difficulty of assigning physical meaning to elements is discussed. The utility of equivalent circuits as measurement models is emphasized, and ways to use them to find mechanistic information are discussed. The rules about which mechanisms can show inductive behavior are of interest, since this is a visually obvious feature. We review our previous rules for mechanisms that can show inductive behavior, and show that inductive behavior is more common for mechanisms with two adsorbed species. We discuss two variations of a simple cycle mechanism (A → B → C → A) in more detail.The interpretation of the charge-transfer resistance and the polarization resistance has some subtleties. Transfer coefficients extracted from Tafel plots of steady-state current–potential measurments are often used to determine where in the mechanism the rate-determining step is. We show that transfer coefficients from Tafel plots of the charge-transfer resistance do not have the same mechanistic significance. The polarization resistance is simply related to the slope of the polarization curve. We discuss the validity of this relationship and its utility in interpreting spectra.     

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.     

A comparative study on electrochemical co-deposition and capacitance of composite films of conducting polymers and carbon nanotubes

Composite films of carbon nanotubes (CNTs) with polyaniline (PANI), polypyrrole (PPY) or poly[3,4-ethylenedioxythiophene] (PEDOT) were prepared via electrochemical co-deposition from solutions containing acid treated CNTs and the corresponding monomer. In the cases of PPY and PEDOT, CNTs served as the charge carriers during electro-deposition, and also acted as both the backbone of a three-dimensional micro- and nano-porous structure and the effective charge-balancing dopant within the polymer. All the composites showed improved mechanical integrity, higher electronic and ionic conductivity (even when the polymer was reduced), and exhibited larger electrode specific capacitance than the polymer alone. Under similar conditions, the capacitance was enhanced significantly in as-prepared PPY-CNT and PEDOT-CNT films. However, the fresh PANI-CNT film was electrochemically similar to PANI, but PPY-CNT and PEDOT-CNT differed noticeably from the respective polymers alone. In continuous potential cycling tests, unlike the pure polymer and other composite films, PANI-CNT performed much better in retaining the capacitance of the as-prepared film, and the possible cause is analysed.     

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.     

Cyclability study of silicon–carbon composite anodes for lithium-ion batteries using electrochemical impedance spectroscopy

The effects of carbonization process and carbon nanofiber/nanotube additives on the cycling stability of silicon–carbon composite anodes were investigated by monitoring the impedance evolution during charge/discharge cycles with electrochemical impedance spectroscopy (EIS). Three types of Si–C anodes were investigated: the first type consisted of Si nanoparticles incorporated into a network of carbon nanofibers (CNFs) and multi-walled carbon nanotubes (MWNTs), with annealed polymer binder. The second type of Si–C anodes was prepared by further heat treatment of the first Si–C anodes to carbonize the polymer binder. The third Si–C anode was as same as the second one except no CNFs and MWNTs being added. Impedance analysis revealed that the carbonization process stabilized the Si–C anode structure and decreased the charge transfer resistance, thus improving the cycling stability. On the other hand, although the MWNTs/CNFs additives could enhance the electronic conductivity of the Si–C anodes, the induced inhomogeneous structure decreased the integrity of the electrode, resulting in a poor long term cycling stability.     

A new transfer function in electrochemistry: Dynamic coupling between Raman spectroscopy and electrochemical impedance spectroscopy

A new type of transfer function is described in this work as a tool to give support to modeling in electrochemical kinetics. It is based on the response analysis with frequency to a sine wave potential modulation applied to an electrode, of a Raman band, the intensity of which is proportional to the response of surface concentration of an adsorbed species.A set-up, consisting of a Raman spectrometer with a CCD detector from which the Raman intensity was defined and extracted to be converted into a tension sent to the input of a multichannel FRA, was implemented. The procedure for extracting the Raman band intensity is explained and was validated by using, as a substitute to the Raman band, a LED light.The method was applied to the study of the polyaniline doping process in a pH range around 3. Five input channels of the FRA were devoted to Raman bands characterizing leucoemeraldine, emeraldine base and/or salt. For pH lower than 3, the concentrations of all moieties are in phase with the charge, while above 3, emeraldine base shows a significant phase lag. This behavior is consistent with a model in progress involving two electrochemical steps and a chemical one.     

An electrochemical impedance spectroscopy study of fuel cell electrodes

Electrochemical impedance spectroscopy (EIS) was used to study the capacitance and ion transport properties of fuel cell catalyst layers. It was found that limiting capacitance correlates with active area. The capacitance per gram of catalyst was calculated and is proposed as a measure of catalyst utilization. Results obtained with catalyst layers immobilized on glassy carbon electrodes agree very well with results obtained with gas diffusion electrodes. EIS was also used to study ion conductivity and active area in fuel cell electrodes that contain the electroactive probe Os(bpy)₃²⁺. Together, these results validate the hypothesis that the non-ideal behavior of fuel cell electrodes is due to variations of conductivity across the layer, rather than variations in capacitance.     

Study of electrical properties of polymeric materials using electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) has been used to determine the water absorption characteristics of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyaromatic amide (Aramid), and amine–quinone polyurethane (AQ) free-standing polymer films. A method for calculating percent moisture uptake is presented based on capacitance measurements via EIS, which requires initial or “dry” capacitance values, C₀. Three different methods for obtaining C₀ values were described and employed, but the most reliable values were obtained via EIS measurement using Hg contacts. C₀ values obtained by this method resulted in calculated water uptake values nearly identical to those obtained by measured weight gain. Lower capacitance values were observed for films immersed in distilled water compared to 0.1M NaCl, which indicates that water uptake is greater in NaCl than in pure water. The four polymers examined in this study can be listed in order of decreasing resistance to moisture absorption as AQ > PET > PEN > Aramid. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 553–560, 1998     

Evaluation of sub-micrometer parylene C films as an insulation layer using electrochemical impedance spectroscopy

We investigated the insulation performance of sub-micrometer parylene C films over time using electrochemical impedance spectroscopy (EIS). For this, interdigitated electrodes were fabricated and completely encapsulated with parylene C in thicknesses of 50, 100, 200, and 500 nm. The EIS was measured in phosphate buffered saline (PBS) solution under an accelerated aging condition at 90 °C over 45 days. To analyze the EIS data, the equivalent circuit models of coating at different stages of coating degradation were used and the lumped circuit parameters of the best fitted equivalent circuit model were extracted by curve fitting. The analysis of impedance using the equivalent circuit model and the FTIR measurements suggest that sub-micrometer parylene C coatings exhibited delamination resulting from water diffusion from the top surface as soon as being immersed in PBS solution, although the degree of delamination varied depending on the film thickness. The penetration of water through sub-micrometers thick parylene C films can occur as quickly as the film is in contact with solution, unlike for thicker coatings in several micrometers where water diffusion would be saturated before water reaches the bottom surface of the coating.     

Determination of water uptake and diffusion of Cl ion in epoxy primer on aluminum alloys in NaCl solution by electrochemical impedance spectroscopy

The electrochemical impedance spectroscopy (EIS) of epoxy-coated aluminum alloy LY12 has been investigated during exposure to 3.5% NaCl solution. Using the continuous simulation of EIS by expanded general electrical model, the time-dependent impedance model of the alloy/coating/solution system was deduced. The results shown that the composite electrode displayed a barrier behavior before water and oxygen penetrated to alloy base. After water and oxygen reached the base, the impedance associated with corrosion of alloy base changed with the immersion time as following: (i) active corrosion period at the beginning (double-layer capacitance, C_dl, in parallel to the charge transfer resistance of electrochemical corrosion R_ct), (ii) impeding of the diffusion of corrosion production at the intermediate period as a result of the presence of coated film (a constant phase element Z_diff was additionally in series with R_ct), and (iii) appearance of the characteristic impedance related to Cl⁻ ion-participating reaction with alloy base at the later stage. From the linear part of ln C_c–t^0.5 curve in the early immersion stage, the apparent diffusion coefficient of water was obtained. The diffusion coefficient of water and Cl⁻ ion through the coating was also calculated by the required time for diffusion of permeation species through the coating to the metal interface obtained from the simulation of EIS data by which the occurrence of characteristic impedance element(s) corresponding to special species arrival can be determined.     

On the Fletcher's two-terminal equivalent network of a three-terminal electrochemical cell

The Fletcher's proposition [S. Fletcher, Electrochemistry Communications 3 (2001) 692-696] to represent a three-terminal electrochemical cell by its two-terminal electrical equivalent for the purpose of the analysis of its electrical responses helps significantly to elucidate peculiarities of the electrochemical impedance (EI) such as inductive or capacitive artifacts. The Fletcher's two-terminal equivalent cell of 3rd order appears however to be redundant as including more circuit elements then necessary to represent the 2nd order impedance of the electrochemical cell. As the alternative to this we recommend two equivalent circuits of 2nd order, both simpler then the original Fletcher's circuit and both better candidates to play the role of the canonical electrical model. Transformation from three-terminal to two-terminal circuit done here with Mathematica^® program appeared relatively simple and it was also possible to relax Fletcher's restraint of representing the working electrode by single resistance. Instead, we used two-terminal electrical models with the working electrode represented by series resistance, double layer capacitance and charge transfer resistance. EI spectra of such extended configuration also present capacitive and inductive artifacts referred to by Fletcher in his model. These artifacts do depend on the impedance of the working electrode.     

Chemical reactivity of thermo-hardenable steel weld joints investigated by electrochemical impedance spectroscopy

The chemical reactivity of oxide-free weld joints made of thermo-hardened carbon steel in different electrolytes was investigated by chronopotentiometry, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The objective was to identify the role of different electrolyte constituents on the electrochemical behaviour of the different materials constituting the weld joint, namely the weld material, the heat affected zone (HAZ) and the base carbon steel. Hardness measurements by Vickers and nano-indentation techniques indicated that the weld material is harder than the heat affected zone and the base carbon steel due to a Widmanstätten ferrite-type structure of the weld. Electrochemical measurements were performed on polished cross-sections on these weld joints in four electrolytes containing different additives. The weld joints are active in all tested electrolytes and the composition of the electrolytes dictates the dissolution even though the main chemical reactivity mechanism remains unaffected. A balanced presence of oxidative agent, inhibitor and HF in the electrolyte is necessary to obtain a homogeneous chemical attack on weld joint and Si-rich inclusion removal in weld material, while avoiding excessive attack roughening and/or pitting of the carbon steel.     

Electrochemical deposition of platinum nanoparticles on different carbon supports and conducting polymers

Electrodeposition of Pt nanoparticles under potentiostatic conditions was performed on several types of carbon electrode supports: commercial macroporous carbon (a three-dimensional electrode), glassy carbon and graphite. Conducting polymers (poly-aniline and poly-o-aminophenol) were also used. The platinum nanoparticles were obtained by different Potential Step Deposition (PSD) methods in 5 mM H₂PtCl₆ + 0.5 M H₂SO₄ aqueous solutions. The effect of the final potential, time and number of steps on the quantity, distribution and size of the platinum nanoparticles was analysed. The mechanism of the electrochemical deposition of platinum was studied through the application of theoretical modelling. The progressive nucleation mechanism provided the closest agreement with the results obtained. In addition, the chemical state and morphology of the electrodeposited materials were determined by means of SEM, TEM and XPS. The results show that the carbon material structure has a strong influence on the Pt particle structure and this, in turn, affects the catalytic activity.     

Local electrochemical impedance measurements on inclusion-containing stainless steels using microcapillary-based techniques

MnS inclusions are good precursor sites for pitting corrosion of stainless steel. The objective of this paper was to quantify the passive properties of resulfurized stainless steel after immersion in chloride media. This was done by combining microcapillary techniques with electrochemical impedance spectroscopy and numerical analysis (specific equivalent circuit). It was shown that sulfur species produced in the electrolyte during the dissolution of inclusions react with the native passive film to CrS and FeSO₄. Local electrochemical impedance spectroscopy measurements provided data describing the behaviour of the affected matrix at the microscale. For example, the value of the charge transfer and migration of point defects resistance decreases from 51,700 Ω cm², in sites free of any metallurgical heterogeneity down to 12,200 Ω cm², in sites containing a high density of inclusions. It was also shown that the integrity of the microcapillary can be altered by the presence of high quantity of sulfur in the electrolyte. Local impedance data allowed the detection of such problems.     

电化学阻抗谱在电沉积研究中的应用(一)

介绍了电化学阻抗谱在各种金属及合金的电沉积研究中的应用。文章分3期连载。第一部分介绍了电化学阻抗谱的基础知识,包括复数、复阻抗的概念,以及在各种常见条件下电解池的等效电路图。     

Identification of inductive behavior for polyaniline via electrochemical impedance spectroscopy

Polyaniline (PANI) film electrodeposited in HCl medium using cyclic voltammetry (CV) with an upper potential limit of 0.90 V, exhibited an inductive behavior. PANI films deposited with different conditions were subjected to various applied potentials and the impedance characteristics were recorded through electrochemical impedance spectroscopy (EIS). The impedance results clearly reveal the existence of inductive behavior to PANI. Inductive behavior was observed for PANI films deposited with conditions which favor benzoquinone/hydroquinone (BQ/HQ) formation and further evidenced by X-ray photoelectron spectroscopy (XPS). A comparative analysis of the EIS and XPS results of PANI films prepared under similar conditions with the upper potential limits of 0.75 and 0.90 V, respectively, clearly documented that the presence of BQ/HQ, the degradation product of PANI, formed during the electrochemical polymerization at the upper potential limits causes inductive behavior to PANI.     

Performance degradation study of a direct methanol fuel cell by electrochemical impedance spectroscopy

A stability test of a direct methanol fuel cell (DMFC) was carried out by keeping at a constant current density of 150 mA cm⁻² for 435 h. After the stability test, maximum power density decreased from 68 mW cm⁻² of the fresh membrane-electrode-assembly (MEA) to 34 mW cm⁻² (50%). Quantitative analysis on the performance decay was carried out by electrochemical impedance spectroscopy (EIS). EIS measurement of the anode electrode showed that the increase in the anode reaction resistance was 0.003 Ω cm². From the EIS measurement results of the single cell, it was found that the increase in the total reaction resistance and IR resistance were 0.02 and 0.05 Ω cm², respectively. Summarizing the EIS measurement results, contribution of each component on the performance degradation was determined as follows: IR resistance (71%) > cathode reaction resistance (24%) > anode reaction resistance (5%). Transmission electron microscopy (TEM) results showed that the average particle size of the Pt catalysts increased by 30% after the stability test, while that of the PtRu catalysts increased by 10%.     

Influence of electrochemical potential on the tribocorrosion behaviour of high carbon CoCrMo biomedical alloy in simulated body fluids by electrochemical impedance spectroscopy

The corrosion and tribocorrosion behaviour of a high carbon CoCrMo alloy sliding against alumina in simulated body fluids under potentiostatic conditions was investigated. The electrochemical behaviour of the sample in two electrolytes at different potentials (−1 V_Ag/AgCl, −0.5 V_Ag/AgCl, +0.05 V_Ag/AgCl, +0.5 V_Ag/AgCl and +0.75 V_Ag/AgCl) was studied by means of electrochemical impedance spectroscopy (EIS). The effects of solution chemistry and applied potential on the wear volume and anodic current were determined. Result shows that wear of CoCrMo alloy is negligible under cathodic and in the cathodic-anodic transition and considerably increases in the passive domain. Third body behaviour depends on surface chemistry which also varied depending on solution chemistry and electrochemically applied potential thus, modifies the tribocorrosion rate of CoCrMo alloy.     

Interaction of BSA protein with copper evaluated by electrochemical impedance spectroscopy and quartz crystal microbalance

The interaction of bovine serum albumin (BSA) protein with copper in phosphate buffer solution has been studied by a combination of electrochemical impedance spectroscopy (EIS) close to the open circuit potential, with simultaneous monitoring by the electrochemical quartz crystal microbalance (EQCM), in order to throw light on BSA adsorption. Copper films were electroplated onto gold quartz crystals and mounted in the EQCM. Experiments were conducted in the presence and absence of dissolved oxygen and of BSA and the results show the influence of O₂ on the protein/metal interaction and also show specific interactions between BSA and copper. The good reproducibility obtained in these experiments suggests future application to other systems and which should lead to a better understanding of the use of such types of protein as corrosion inhibitors.     

An electrochemical impedance spectroscopy study of copper in a bentonite/saline groundwater environment

Pure copper samples have been exposed for 6 years in a bentonite test parcel in the Äspö Hard Rock Laboratory, which offers a realistic environment for the conditions that will prevail in a deep repository for high-level radioactive waste disposal in Sweden. After retrieval of the test parcel, a bentonite test package containing the samples was placed in a container and sealed with a thick layer of paraffin, and later on new copper and platinum samples were installed. An electrochemical impedance spectroscopy study has been performed on the pre-exposed and new copper samples for different durations in the retrieved test package.The impedance spectra for copper in the bentonite/saline groundwater environment change considerably with time of exposure. The change is more pronounced at higher than at lower frequencies. It is presumed that the corrosion resistance of pure copper in this environment mainly depends on a thin protective cuprite film that forms readily, and on a thick porous outer corrosion product layer (mainly cuprite) that develops slowly and partly intermixes with the bentonite. The impedance data revealed that both the inner and the outer corrosion product layer were involved in the impairment and the recovery of the corrosion resistance when the copper interface was disturbed during retrieval of the bentonite test package.