Etching of n-type silicon in (HF+oxidant) solutions: in situ characterisation of surface chemistry

The etching of n-type silicon in hydrofluoric acid solutions containing active oxidising agents has been studied. The effect of different parameters on the etch rate of silicon in these solutions has been examined. Based on electrochemical experiments coupled to in situ real-time measurements by infrared spectroscopy, the evolution of surface chemistry during etching of Si has been investigated. It has been found that dissolution of silicon may take place with and without oxide formation on the electrode surface, depending on the composition of etching solution. The transition between conditions giving rise to porous silicon generation and those leading to surface polishing was analysed, when the concentration of oxidising species is increased. In the polishing regime, oxide thickness was estimated from the intensity of the SiO absorption band.     

电化学阻抗谱在缓蚀剂研究中的应用进展

概括了电化学交流阻抗谱的基本原理,介绍了电化学阻抗谱在缓蚀剂研究中的应用进展,并对电化学阻抗谱在腐蚀科学领域其他方面的应用进行了展望。     

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.     

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.     

Instantaneous electrochemical impedance spectroscopy of electrode reactions

Continuous STFT transforms of a sinusoidal perturbation signal and current have been found for the first order electrode reaction. Electrode impedance has been determined in the joint time-frequency domain. Measurements of the Cd(II) reduction reaction have been performed on a dropping mercury electrode as a function of time. A possibility of instantaneous impedance spectra generation has been presented. Time characteristics of charge transfer resistance, Warburg coefficient and double layer capacitance has been described.     

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.     

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.     

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 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.     

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.     

Indirect identification of Hads relaxation on different metals by electrochemical impedance spectroscopy

For the hydrogen evolution reaction, the coverage rate of the electrode surface by adsorbed hydrogen is generally difficult to evidence especially in presence of bubbles. In the present paper, a parallel competing reaction was incorporated to the interface by the addition of ferricyanide to a NaOH solution. This reaction was supposed to take place on the free electrode surface and allowed the relaxation of adsorbed hydrogen to be identified. Models taking into account hydrogen evolution, hydrogen absorption, and ferricyanide reduction were proposed. Three metals were tested: platinum, iron, and palladium, which absorb hydrogen in very different quantities. In these conditions, low frequency loops related to the H_ads surface coverage appeared in the electrochemical impedance diagrams, whose properties depend on the tested metal. Very good agreement was found between the experimental data and model predictions.     

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.     

Stainless steel electrode characterizations by electrochemical impedance spectroscopy for dye-sensitized solar cells

Electrochemical impedance spectroscopy (EIS) was used to understand the electrochemical mechanisms which appear in dye-sensitized solar cells (DSSCs). This qualitative and quantitative technique permits identification of the phenomena proceeding within the different elements composing the cell and at their interfaces.In this study, the classical conducting glass substrate was replaced by a protected stainless steel (304 type) substrate as the counter-electrode (cathode) in dye-sensitized solar cells. Platinum was deposited at the substrate surface to optimize the charge transfer resistance of the electrode.After a few days of immersion in the electrolytic solution, stainless steel substrates coated with low thickness of Pt show pitting corrosion due to iodine. Defects in the Pt layer such as discontinuity of the film and micro-cracks may explain the corrosion of the stainless steel substrate. However the Pt layer degradation is retarded for thicker films. On the other hand, polished substrates show a better behaviour probably due to the elimination of the defects on the stainless steel surface.Electrolytic solution was optimized. For this, components such as 1-butyl-3-methylimidazolium iodide (BMII), guanidine thiocyanate (GT) and 4-tert-butylpyridine (TBP) were added. No corrosion phenomena on stainless steel 304 appeared within 3 days when TBP was added. This means that TBP acts as a corrosion inhibitor.A schematic equivalent circuit is also proposed.     

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%.     

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.     

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.     

Simulation of transient electrochemical impedance spectroscopy due to water uptake or oxide growth

Coatings are often employed to prevent, or at least delay, corrosion of a metallic substrate. The topcoat is often also intended to be a barrier to environmental moisture and any entrained, corrosive ions reaching the substrate. The protective ability of a coating system is very well monitored by electrochemical impedance spectroscopy since it can measure changes as the protective coatings deteriorate, as they absorb water and as they permit the substrate to corrode. Infiltration of water and ions into the coating can also be measured by EIS to deduce approximate water uptake. EIS analysis in coating systems is often predicated on the system being stable over the timeframe of the spectral measurement, which if taken from 100 kHz to 0.01 Hz, or even 0.001 Hz, may take as long as an hour, or more. EIS measurement is most often accomplished by the use of an aqueous electrolyte solution as the electrical contact at the top of the coating, with the metallic substrate used as the other electrical contact. The ingress of aqueous solution is likely to result in time-dependent dielectric or chemical changes to the coating/substrate system. This work shows how these changes, during the spectral measurement period, produce a Bode plot that has a slope of magnitude slightly less than 1. This capacitive response to non-steady state water uptake is commonly characterized by a constant phase element (CPE) in an equivalent circuit model for the material, and equations developed here can be used to deduce the diffusion coefficient based on the CPE.     

Study of passivation of Al and Al-Sn alloys in borate buffer solutions using electrochemical impedance spectroscopy

Properties of thin oxide films on Al and Al-Sn alloys (with Sn content of 0.02, 0.09, 0.20 and 0.40 wt.%) formed either naturally or anodically in borate buffer solutions were investigated by means of electrochemical impedance spectroscopy. Equivalent circuits have been proposed that completely illustrate the Al(Al-Sn alloy)/oxide film/electrolyte systems examined, and properties of oxide films were determined. The stability (thickness and resistance) of oxide films has been found to increase with increased Sn content in the alloy, with increased passivation potential, and with longer time of anodising. The increase in temperature of anodising significantly reduces impedance in systems observed.     

Evaluation of anti-corrosive pigments by pigment extract studies, atmospheric exposure and electrochemical impedance spectroscopy

The inhibition efficiencies of zinc chromate, barium metaborate, calcium silicate, amino carboxylate, calcium barium phosphosilicate, aluminum triphosphate and a modified zinc phosphate on the corrosion of steel and zinc were determined by polarization experiments on pigment extracts. Zinc phosphate and zinc chromate were the best and were studied further to determine the effect of pH and chloride concentration on their inhibition of steel. Zinc chromate is adversely affected by high concentration of chloride ions, which effect seems to be less pronounced on zinc. A low pH, although increasing the solubility of zinc phosphate, does not increase its efficiency. The pigments were also incorporated into an epoxy-poly(amide) binder, applied to cold-rolled steel and galvanized steel, exposed at a marine exposure station and the degradation monitored by electrochemical impedance spectroscopy. There was a general correlation between the results of pigment extract studies and atmospheric exposure except in the case of phosphate pigments on cold rolled steel.