Behavior of cobalt oxide electrodes during oxidative processes in alkaline medium

The behaviour of Co₃O₄ electrodes was investigated in alkaline solution during oxidative runs at different anodic potentials up to the value of oxygen evolution reaction (OER). Voltammetric results indicate the presence of several solid-state surface redox transitions (SSSRT) which lead to the formation of Co(IV) containing species which are the specific catalytic phases for OER.Electrochemical impedance measurements were done in different ranges of potential and frequency in order to better characterise the SSSRT and the capacitive response of the electrode/electrolyte interface.     

A label-free DNA sensor based on impedance spectroscopy

This paper describes a label-free detection system for DNA strands based on gold electrodes and impedance measurements. A single-stranded 18 mer oligonucleotide (ssDNA) was immobilised via a thiol linker on gold film electrodes and served as probe DNA. Residual binding places were filled with mercaptobutanol. The sensor surface clearly distinguished between complementary and non-complementary target ssDNA. Additionally, detection of single base pair mismatches was possible. The electrode was impedimetrically characterised in the presence of the redox system ferri/ferrocyanide before and after DNA hybridisation. Impedance analysis showed that the charge transfer resistance, R_ct, was increasing after DNA duplex formation, whereas the capacitive properties remain rather unaltered. The relative change of R_ct was used as sensor parameter. Concentrations in the nanomolar range have been detected by the system. The sensor was reusable because a denaturation protocol allowed effective double strand dissociation without changing the surface properties of the electrode substantially. The time for DNA detection have been reduced to about 15 min including regeneration. The sensor signal was amplified by about 20% after binding of a negatively charged molecule to the formed DNA duplex. The sensor was also capable of sensing longer target ssDNA strands as shown with 25 mer and 37 mer oligonucleotides.     

Cold Sintering Process characterization by in operando electrochemical impedance spectroscopy

The development of in situ characterization techniques is mandatory for the development of the Cold Sintering Process (CSP). Here we show the first development of in situ impedance analyzes to follow materials evolution during sintering. This characterization brings key information about grain boundary formation at the solid/liquid interface, both in terms of thermodynamic and kinetics. Through the example of ZnO sintering in the presence of acetic acid aqueous solution, known for being easily Cold Sintered, we demonstrate the role of the liquid phase, notably the acetic acid role, regarding chemical composition, structure and microstructure of the obtained densified ceramics. It turns out this is a promising in-situ characterization technique that should be applied to a wide range of materials.     

Estimation of kinetic parameters of the passive state of carbon steel in mildly alkaline solutions from electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data

The unambiguous interpretation of electrochemical impedance spectra of complex systems such as passive metals and alloys in terms of an unique kinetic model is often hampered by the large number of adjustable modeling parameters. In this paper, a combination of in situ electrochemical data and ex situ surface analytical information is employed to validate the estimates of kinetic and transport parameters of the passive state of carbon steel. For the purpose, electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data for the oxidation of carbon steel in mildly alkaline solutions are quantitatively compared with the predictions of the Mixed-Conduction Model for oxide films that represent the passive oxide as an intermediate phase between magnetite and maghemite. Estimates of the kinetic rate constants at the film interfaces, as well as the diffusion coefficients and field strength in the film are obtained and their relevance for the corrosion mechanism of carbon steel is discussed.     

On origin of resistive and capacitive contributions to impedance of memory states in Cu/TiO2/Pt RRAM devices by impedance spectroscopy

The resistive switching phenomenon is attributed to local formation and rupture of conductive filaments (CF), however some aspects of the switching mechanism still need wider consensus. The origin of resistance contribution to low resistance state (LRS) and high resistance state (HRS) is although well known, the origin of capacitive contribution to the memory state of resistive random access memory device (RRAM) is ambiguous. Here we report impedance spectroscopic studies carried on Cu/TiO₂/Pt devices to address the origin of resistive and capacitive contribution, where the effect of DC bias voltage, reset stop voltage and set compliance current on the impedance of RRAM memory states were investigated in detail. The studies revealed that the capacitive contribution was dominated by the surrounding TiO₂ bulk and contrary to existing knowledge, gap formed owing to ruptured CF has negligible role. These studies also reaffirmed the origin of the resistive contribution to the HRS and LRS state, which was dictated by the ruptured gap and connected CF, respectively. Results of this study may be significant towards improving the switching time and operating frequency performance of futuristic memory, RF switch and neuromorphic computing applications of RRAM devices.     

Electrochemical studies of self-assembled monolayers using impedance spectroscopy

Self-assembled monolayers (SAMs) using various organic molecules were constructed on thin thermal silicon dioxide on silicon structures. These structures have potential bio-applications. The monolayers are part of a capacitor sensor based on the electrolyte-insulator-silicon (EIS) structure. The main goal of this work is to investigate the ability of SAMs for acting as an intermediate layer between aqueous solutions and silicon based biosensors. This work presents monolayers based on trimethoxy silanes with various functional groups such as: (a) (3-aminopropyl)-trimethoxysilane, (b) octadecyltrimethoxysilane and (c) trimethoxy(3,3,3-trifluoropropyl)silane. The biosensor is a differential sensor with a built in reference based on the structure of electrolyte-insulator-silicon or in ion-sensitive-field-effect-transistors (ISFETs). While the methyl and fluorine groups can act as the reference part, which is passive to the bio-molecules detection, the capacitor containing the amine groups has the sensing capabilities to the biological molecules. The amine (NH₂) tail groups can be active for those attachments via glutaraldehyde.The film's study and characterization was made by spectroscopic ellipsometry, contact angle method (CA) and FT-IR spectroscopy. The electrical methods such as capacitance-voltage (C-V) and electrochemical impedance spectroscopy (IS) were used to investigate the whole electrolyte/SAM/SiO₂/Si structure. We will show the detection capabilities of these 2D structures, its electrical equivalent model through a simulation fitting and will discuss the application of those structures to bio-sensitive metal/oxide/silicon (MOS) devices.     

Photovoltaic performance and impedance spectroscopy of ZnS-Cu-Go nanocomposites

In this work, novel, non-toxic and cost effective ZnS-Cu-GO nanocomposite is synthesized via wet chemical route to study its photovoltaic properties. Three samples including ZnS,ZnS-Cu and ZnS-Cu-GO were prepared and deposited as sensitizers on ZnO coated FTO substrates to assemble PV devices.The samples were characterized using UV–Vis NIR spectroscopy, Atomic force microscopy (AFM).Electrochemical impedance spectroscopy (EIS) and AM 1.5 Sun Simulator. It was observed that ZnS-Cu-GO exhibitedsuperiorchargetransport, remarkably high open circuit voltage(0.8 V) and Fill factor (0.806).The current density significantly enhanced and maximum solar cell efficiency was observed for ZnS-Cu-GO based PV device. A pronounced red shift of 360 nm in the absorption spectra was observed in the ZnS-Cu-GO due to fine dispersion of GO sheets.The AFM analysis showed that incorporation of GO and Cu maximized grain density and trench like grain boundaries in ZnS-Cu-GO which facilitated charge transport mechanism.A detailed electrochemical impedance study to probe charge dynamics in the prepared PV devices is presented herein.     

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.     

Mechanistic study of the reduction of copper oxides in alkaline solutions by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy was used to study the mechanism by which copper oxides are reduced in alkaline solutions. For the reductions of CuO and Cu₂O, a capacitive loop and also an inductive loop under certain conditions were observed in the complex plane. The electrochemical impedance for CuO reduction was not greatly dependent on the solution alkalinity and the kind of alkali hydroxide. However, the electrochemical impedance for Cu₂O reduction was considerably affected by the kind and concentration of alkali hydroxide. The diameter of the capacitive loop, i.e., the charge-transfer resistance (R_ct), was increased with increase in solution alkalinity. It should also be noted that R_ct was increased in the order of KOH < NaOH < LiOH. These dependences were consistent with the good separation between the reduction potentials of CuO and Cu₂O in chronopotentiometric and voltammetric measurements with strongly alkaline electrolytes containing Li⁺. The inductive loop observed for the Cu₂O reduction at higher concentrations of KOH (>6 M) and LiOH (>0.2 M) suggested the existence of an intermediate species (probably CuOH). The specific inhibitory effect of Li⁺ ions on the reduction of Cu₂O might be explained by a possible stabilization of CuOH by Li⁺ ions.     

Model parameterisation of nonlinear devices using impedance spectroscopy

Spatially resolved simulation models are valuable tools to predict the behaviour (electrical, thermal, ageing) of energy storage systems. However, parameterisation of the electrical impedance-based model is often difficult because local measurements are hardly possible. For homogeneous conditions, e.g. homogeneous current, state of charge and temperature distribution, the measured overall impedance of a cell or electrode can be divided equally over the volume elements of the spatially resolved model. Homogeneous conditions are though very rare, so all larger devices show at least a current distribution because of non-zero ohmic resistances in current collectors, active masses and electrolyte.In this paper, a method is presented and validated for calculating the local impedance of nonlinear inhomogeneous devices from the overall impedance of the device. The method consists of iteration between impedance parameter identification and calculation of current distribution.     

First insights into the borohydride oxidation reaction mechanism on gold by electrochemical impedance spectroscopy

Direct borohydride fuel cells (DBFC) exhibit some potential regarding the powering of small portable electronic devices, thanks to their high energy density as well as the facile and safe storage of borohydride salts. However, DBFC are hindered because (i) the borohydride oxidation reaction (BOR) is complex, (ii) its mechanism imperfectly determined yet and (iii) no practical electrocatalyst exhibits both fast BOR kinetics and high faradaic efficiency. In this context, we characterized the BOR mechanism for polycrystalline bulk gold (a classical model BOR electrocatalyst) in the rotating disk electrode (RDE) setup. Modeling cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data, we propose a simplified reaction pathway, the theoretical behavior of which agrees with the experimental findings. This pathway includes at least a first irreversible electrochemical step (E) for BH₄⁻ oxidation, which competes with the electrochemical adsorption reaction (EAR) of OH⁻ anions at high potentials.     

In situ FTIR spectroscopic studies of electrooxidation of ethanol on Pd electrode in alkaline media

Electrooxidation of ethanol on a polycrystalline Pd disk electrode in alkaline media was studied by in situ Fourier transform infrared (FTIR) reflection spectroscopy. The emphasis was put on the quantitative determination of intermediates and products involved in the oxidation. It has revealed that most of ethanol was incompletely oxidized to acetate. The selectivity for ethanol oxidation to CO₂ (existing as CO₃²⁻ in alkaline media) was determined as low as 2.5% in the potential region where Pd electrode exhibited considerable electrocatalytic activity (−0.60 to 0.0 V vs. SCE). Nevertheless, the ability of Pd for breaking C-C bond in ethanol is still slightly better than that of Pt under the same conditions. Besides, a very weak band of adsorbed intermediate, bridge-bonded CO (CO_B) was identified on the Pd electrode for the first time, suggesting that CO₂ and CO₃²⁻ species may also be generated through CO pathway (i.e., indirect pathway).     

Influence of the migration of chloride ions on the electrochemical impedance spectroscopy of mortar paste

During the last decade, electrochemical impedance spectroscopy (EIS) started to be effectively exploited to investigate the electrical properties of hydrated cement pastes.This paper is devoted to the study of the high- and medium-frequency regions of the impedance spectrum and focused on determining the relationship between chloride migration and impedance spectroscopy in accelerated diffusion tests carried out on samples of mortar.After introducing an experimental protocol based on a four-electrode arrangement, we present the results of a parametric study which is related to the thickness and water-cement ratio (W/C) of the samples.Firstly, all our measurements show that the presence of chloride ions modifies the impedance response of mortar and reveals a small loop as of modification of the composition of the upstream solution. This loop is probably due to the development of interfacial phenomena between material and the solution.Secondly, as the migration of chloride process progresses, an increase followed by a decrease of the bulk electrical resistance is observed, whereas the second loop, due to the presence of chlorides, remains constant.An equivalent electrical circuit is then proposed to fit the different experimental data.     

Effect of nickel on copper anode passivation in a copper sulfate solution by electrochemical impedance spectroscopy

Electrochemical Impedance Spectroscopy (EIS) was applied on Cu-Ni samples passivated in a specially designed flat electrochemical cell which was heated at 60 °C. The electrolyte consisted of 160 g l⁻¹H₂SO₄, 40 g l⁻¹ Cu²⁺ and 0, 10, 20, 30 or 40 g l⁻¹ Ni²⁺ and the copper anodes contained nickel ranging from 0 w% to 10 w%. The oxygen content of the anodes and the electrolyte was also measured. An AC excitation signal of 10 mV and of 1mHz 100 MHz frequency was applied at the open circuit potential as well as at a passivation potential, the latter having been determined previously. The results indicate that nickel ion additions to the electrolyte increased the resistance of the electrolyte and altered the porosity, thickness and constituents of the passivation layer formed. The equivalent circuit models generated from the data acquired during the EIS experiments and the values for the electrical components were in the predicted range. The results are supported by supplementary XRD and SEM findings.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.     

Distribution of relaxation time in solution-processed polycrystalline CZTS thin films: Study of impedance spectroscopy

Here we report the complex impedance spectroscopic analysis of polycrystalline CZTS thin films synthesized by sol-gel spin coating technique without any post deposition sulphurization. The films are characterized by microstructural, compositional, optical and electrical studies to confirm the formation of kesterite phase of CZTS comprises of well distributed compact grains with the optical band gap 1.44?eV. Room temperature electrical characterizations of the CZTS thin films by four-probe and Hall effect technique revealed the p-type conductivity of the films with resistivity ~ 1.45?×?10^?2 Ω?cm, mobility ~ 3.7?×?10³ cm² V^?1 s^?1 and carrier concentration ~ 1.82?×?10¹⁷ cm^?3. The distribution of relaxation time (DRT) function with improved frequency resolution is reconstructed from the impedance spectra of CZTS film recorded in the frequency range 50?Hz to 5?MHz at room temperature to identify the number of electrical processes in the polycrystalline film. The Nyquist plot is fitted into electrical model consist of three parallel combinations of resistor (R) and capacitor (C) in series as three major peaks in DRT function indicates the presence of different relaxation processes with major contributions from core grains along with smaller contributions from grain boundary and interfaces. The room temperature frequency dependence of dielectric constant, loss tangent and ac conductivity is also studied for the CZTS films.     

The corrosion resistance of coated steel dowels determined by impedance spectroscopy

The corrosion resistance of coated smooth steel dowels in simulated pore solution with and without 3.5% sodium chloride solution is reported. The dowels are coated with a double-layer structure composed of a 180 μm of Nickel-Chromium-Boron (referred to as NiCrB) that is overlaid by a 20 μm inorganic layer made of silicon powder, silica fume, blast furnace slag, and combinations thereof. The composition of the synthetic pore solution is based on the composition of pore fluids expressed from Type I portland cement that is characterized by pH of about 13.4 and an ionic strength of 0.4.Alternating current impedance spectroscopy measurements conducted in the range of 10 mHz to 10 kHz were found to be instrumental in detecting any change in the resistance of the coatings. It was found that the NiCrB sub-layer provides a good corrosion resistance that outperforms that of the inorganic coating. Of the five inorganic compositions tested, the combination of slag and silica fume has the best performance. The protection against corrosion is attributed mostly to particle packing rather than densification of the matrix due to pozzolanic reaction.     

Effects of electrolyte in dye-sensitized solar cells and evaluation by impedance spectroscopy

Effects of the electrolyte of DSCs on impedance spectra were evaluated by changing concentration of redox couple, viscosity, and additives to electrolyte. The relation with current-voltage characteristics (I-V characteristics) was investigated. In many cases, the impedance component attributed to charge transfer at TiO₂|electrolyte interface demonstrated strong relation with the I-V characteristics. The recombination of electrons in TiO₂ with I₃⁻ in electrolyte was a key factor in determining performance of DSCs. To evaluate the effect of I₃⁻, diffusion-limiting current in the electrolyte for various viscosities was evaluated by cyclic voltammetry. When the short circuit current (SCC) was almost equal to the diffusion-limiting current, strong influence of the diffusion coefficient on the impedance spectra was observed: impedance arcs were enlarged as the diffusion coefficient was decreased. On the other hand, when the diffusion-limiting current was larger than the SCC, photo-excitation and electron injection processes became dominating factors in the DSCs performance. The SCC was regulated by the charge recombination process at TiO₂|electrolyte interface, and thus the impedance component ω₃ was related to the performance in such condition.     

Characterization of active pigments in damage of organic coatings on steel by means of electrochemical impedance spectroscopy

Active anticorrosive pigments are solid additives for primers which can give further protection for areas with coating damage in addition to their barrier effect. These pigments are expected to prevent corrosion of metal substrate in coating damage by build-up of permanently passive conditions at the metal surface (electrochemical protection) and/or by build-up of solid compounds which plug the coating damage (chemical protection). Electrochemical Impedance Spectroscopy (EIS) was applied to characterize the corrosion protection behaviour of alkyd primers containing different pigments. Impedance spectra were recorded in the frequency range 50 mHz f 50 kHz at the open-circuit potential as a function of the type of pigment and the exposure time in different corrosive media. In general, two different parts can be distinguished in the impedance diagrams. The higher frequency part is related to the insulating properties of the primer and the lower frequency part can be attributed to electrochemical processes taking place within the coating defects. The parameters derived from EIS results show that the low frequency data can be used for characterization of the protective properties of anticorrosive pigments in the presence of defects in organic coatings.     

Electrochemical impedance spectroscopy investigation of spinel type cobalt oxide thin film electrodes in alkaline medium

Spinel type Co₃O₄ thin films, for the oxygen evolution reaction (OER) in 1 M KOH, have been prepared, on stainless steel supports, using the thermal decomposition method at 400 °C. The electrochemical behaviour of the oxide film/1 M KOH interface was investigated by cyclic voltammetry and impedance techniques. The impedance measurements were carried out at different positive potentials, from the open circuit potential to a potential in the OER region and the electrical equivalent circuit, L (R₁Q₁) (R₂Q₂) (R₃Q₃) was used to fit the experimental results. At each potential, a good correlation between experimental and simulated data is found, thereby validating the proposed equivalent circuit model. The roughness factor value determined in the potential region where the charge transfer reaction is negligible is similar to that obtained by cyclic voltammetry, with a value of 70 ± 2.