Anion-adsorption-related frequency-dependent double layer capacitance of the platinum-group metals in the double layer region

The interfacial impedance for platinum-group metals in simple binary electrolytes, in their so-called double layer region, is generally assumed to be ideally capacitive. Using single crystalline platinum-group metals with various solutions we demonstrate that this is not necessarily the case when the anion of the solute is specifically adsorbed on the electrode. We attribute the frequency dependence of the interfacial capacitance to the relatively slow exchange of anions between the outer and inner Helmholtz planes.     

Improvement in the capacitance of a carbon electrode prepared using water-soluble polymer binder for a capacitive deionization application

It is very important to increase the wetted surface area of a carbon electrode for high capacitive deionization performance. To increase the wettability of a carbon electrode, we fabricated carbon electrodes by using water-soluble polymer binder, polyvinyl alcohol (PVA). The electrochemical properties of the PVA-bonded carbon electrode were compared with another that was prepared using hydrophobic binder, polyvinylidene fluoride (PVdF). Electrochemical methods - cyclic voltammetry, chronoamperometry, and electrical impedance spectroscopy - were used to characterize the electrochemical properties of the electrodes. As might be expected, it was confirmed that the PVA-bonded electrode was more wettable than the PVdF-bonded one, based on contact angle measurements. From the cyclic voltammetric analysis, we found that the specific capacitance was 74.4-80.3 Fg⁻¹ for the PVdF-bonded electrode and 89.6-99.8 Fg⁻¹ for the PVA-bonded electrode, depending on the potential, indicating a 13.3-30.1% increase in specific capacitance. It was observed that the ac-signal penetrated micropores of the PVA-bonded carbon electrode more deeply than the PVdF-bonded one, resulting in a higher capacitance. This was attributed to the fact that the ac-signal was able to charge more inner surface sites because micropores in the PVA-bonded electrode could be wetted due to the PVA binder.     

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.     

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.     

Measurement of particle concentration in powder coating process using capacitance computed tomography and wavelet analysis

Capacitance computed tomography techniques were used to visualize particles movement in the draft tube of a spouted fluidized bed for the coating process of drug production. A total of 512 frames images of the particle concentration distribution were obtained at 10-millisecond intervals over a coating time of 5 min using a capacitance computed tomography system. The three-dimensional capacitance CT images (time and two-dimensional space images) were decomposed to wavelet time and space levels to extract the dominant particle distribution feature using three-dimensional discrete wavelet multiresolution at different coating times. As a result, the time and space dominant particle distribution with a specific frequency level can be visualized.     

An investigation of the capacitance dispersion on the fractal carbon electrode with edge and basal orientations

Capacitance dispersion on the fractal carbon electrode with edge and basal orientations was investigated using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and a.c.-impedance spectroscopy. For this purpose, four types of as-received pyrolytic graphite electrode, as-received, mechanically polished, and as-activated glassy carbon electrodes were prepared with different surface irregularities and amounts of edge orientations. The apparent self-similar fractal dimensions of the carbon electrodes were determined from the analyses of AFM images based upon triangulation method. The amounts of edge orientations on the surface of the carbon electrodes were qualitatively estimated from the XPS analysis of surface acidic functional groups that were preferably formed on the edge planes by the heat treatment of the carbon electrodes. The values of the constant phase element exponent α determined from the apparent self-similar fractal dimensions did not accord with those α determined from the measured impedance spectra. Instead, they were closely related to the amounts of the edge orientations. From the results, it is indicated that the contribution of surface inhomogeneity is much higher than the contribution of the surface irregularity to the capacitance dispersion on the carbon electrode.     

Electrochemical multilayer deposition of polyaniline and Prussian Blue and their application in solid state electrochromic windows

In an attempt to enhance the solar modulation in electrochromic windows, the electrochemical deposition of polyaniline (PANI) and Prussian Blue (PB) has been studied more closely. Several layers of PANI and PB at different thicknesses have been electrodeposited on top of each other, or intermixed in the coating matrices. This intermixing, or multilayer deposition, of PANI and PB may lead to an overall improved coating performance. Integrating over the whole solar spectrum, a modulation as high as 49% of the total solar energy has been achieved in solid state electrochromic windows based on tungsten oxide (WO3) and multilayer PANI–PB coatings, which is comparable to earlier results with a single PANI/PB layer.     

The mechanism of electrode process in the system silver/silver cyanide complexes

The electrode's surface inhomogeneity was taken into account studying the mechanism of process. The effect of CN⁻ adsorption and partial surface coverage was investigated by electrochemical fast Fourier transform (FFT) impedance spectroscopy (0.03-3900 Hz). The isopotential solutions (E₀=−0.350 V) as well as solutions containing constant silver cyanide complex concentration (0.05 M) and different free cyanide ion concentrations (from 0.01 to 1.0 M) were studied. The exchange current density, re-calculated for only active area, gives electrochemical reaction order (R_CN=1.8 for α=0.5, and R_CN=2 for α=0.6) which is independent from CN⁻ concentration and from the composition of complexes prevailing in the bulk of electrolyte. The value of R_CN close to 2, obtained in a series of isopotential solutions confirms that in all cases the complex ions Ag(CN)₂⁻ take direct part in the charge transfer step.     

The effect of second phase La0.67TiO2.87 on the phase structure and impedance spectroscopy of La2Ti2(1 + x)O7 piezoelectric ceramics

The Ti excess La₂Ti_{2 (1+x)} O₇ (x = 0, 0.005, 0.01, 0.02, 0.05, 0.1) piezoelectric ceramics have been prepared by sol-gel technology and solid state synthesis method. Through refinement analysis, the phase structure of the ceramics varies with Ti content. Most monoclinic phase (∼93%) and a handful of orthogonal phase (∼7%) coexist in La₂Ti_{2 (1+0)} O₇ ceramics. Pure monoclinic phase La₂Ti₂O₇ with space group P2₁ appears in La₂Ti_{2 (1+0.005)} O₇ and La₂Ti_{2 (1+0.01)} O₇ ceramics. Monoclinic phase La₂Ti₂ O₇ and a certain proportion of tetragonal phase La_0.67TiO_2.87 coexist in La₂Ti_{2 (1+0.02)} O₇, La₂Ti_{2 (1+0.05)} O₇ and Ti_{2 (1+0.1)} O₇ ceramics. With the excess of Ti content, the monoclinic phase ratio and distortion angles in a-b projection plane of the ceramics increase first and then decrease, which is consistent with the variation tendency of piezoelectric constant d₃₃. The excellent piezoelectric constant for Ti_{2 (1+0.01)} O₇ ceramics is 2.8 pC/N.Impedance analysis shows that the conductive mechanisms of all samples include both grain and grain boundary conductivity at temperature range T ≥ 500 °C. The formation of tetragonal phase La_0.67TiO_2.87 derives from Ti excess in pure monoclinic phase La₂Ti₂O₇. The existence of tetragonal phase La_0.67TiO_2.87 can obviously increase the capacitance of ceramics at x ≥ 0.05. All prepared piezoelectric La₂Ti_{2 (1+x)} O₇ ceramics have highly frequency stability and are candidates for ultrahigh temperature piezoelectric application.     

Electrochemical synthesis of poly(N-methylaniline) on an iron electrode and its corrosion performance

Synthesis of poly(N-methylaniline) (PNMA) on pure iron and Pt electrodes was carried out from aqueous 0.3 M oxalic acid solution containing 0.1 M N-methylaniline (NMA) by potentiodynamic and galvanostatic techniques. It was found that when compared to polyaniline (PAni) and its ring- and N-ethyl-substituted derivatives, PNMA can be electrosynthesized with lower upper scanning potential (upper potential limit, E_upp) of 0.8 V vs. saturated calomel electrode (SCE) on an Fe electrode. PNMA coatings were characterized by electrochemical, scanning electron microscopy (SEM) and FTIR techniques. Linear anodic potentiodynamic polarization results proved that increasing the acidity of the polymerization solution causes more effective protection against corrosion in 0.5 M H₂SO₄ medium for PNMA. Moreover, PNMA exhibited similar protective properties with PAni under the same corrosion test conditions. Tafel test results reveal that the PNMA coating appears to enhance protection for iron in 0.5 M NaCl and 0.1 M HCl solutions. According to EIS results, the PNMA coating is able to offer protection to Fe electrodes in NaCl compared to that in HCl medium over a long immersion period.     

Reactivity of CoCrMo alloy in physiological medium: Electrochemical characterization of the metal/protein interface

The influence of different concentrations of phosphate ions on the corrosive behavior of a CoCrMo alloy is investigated. The study is conducted using electrochemical techniques (polarization and potentiodynamic measurements, electrochemical impedance spectroscopy (EIS)) and X-ray photoelectron spectroscopy (XPS). It was found that phosphate ions are vastly responsible for the blocking of the active surface with the addition of calf serum having little effect. This is in contrast to titanium where the competition between the adsorption mechanisms of proteins and those relating to phosphate ions was evidenced recently. The formation and quantification of CrO₄²⁻ as a result of a dissolution of Cr₂O₃ oxide layer was evidenced using UV/vis absorption spectroscopy.     

The capacitance of the diffuse layer of electric double layer of electrodes in molten salts

Similarly to aqueous electrolytes, the electric double layer of electrodes in molten salts is assumed to be composed of compact and diffuse layers. The charge density of the compact layer, formed as a monolayer of specifically adsorbed anions (primary ionic shell), is calculated as the difference between the charge of the primary ionic shell and the charge removed by the exchange current density. The centre of the specifically adsorbed anions create the inner Helmholtz plane (iHp). The counterions to the specifically adsorbed anions in the primary ionic shell, take place in the numerous neighbouring holes, introduced into the molten salt structure by the melting process, and being subjected to thermal motion, create the diffuse layer. The electrostatically adsorbed metal cations form the outer Helmholtz plane (oHp) with the value of the inner potential equal φ₂. Using the Boltzman and Poissone equations, the equation for the capacitance of the diffuse layer of the metallic electrode in molten salt is derived and tested on some literature experimental results.     

Thermal dynamics of side-chain copolymethacrylates as studied by the dielectric spectroscopy and relaxation of second-harmonic generation

A series of copolymethacrylates with different contents of tolane-based mesogenic groups have been synthesized. The mesogenic group content was characterized with ¹H-n.m.r.. The phase behaviours were determined using a differential scanning calorimeter and optical polarizing microscopy. A smectic A phase was obtained when the mesogenic group content was increased up to 80 mol.%. Dielectric relaxation results indicated that the amplitude of the -relaxation was suppressed significantly due to the presence of the liquid crystalline phase. The reduction of the molecular motion is beneficial to the enhancement of the temporal stability of the effective second-harmonic coefficient for the polymer with a higher mesogenic group content. Moreover, the second harmonic coefficient is enhanced as the mesogenic group content increases. The self-alignment nature of the liquid crystal phase is favourable for alignment of the NLO-active mesogenic group under an applied electric field and preserving such alignment after removal of the electric field. The relationship between thermal dynamic behaviour and second-order nonlinear optical properties is also discussed.     

Electrochemical and structural characterizations of an experimental track-etched membrane in KCl solutions

Characteristic electrochemical transport parameters for an experimental poly(ethylene)terephtalate (PET) track-etched membrane with well-defined structure and low porosity (Θ=0.13%) were determined with the membrane in contact with KCl solutions at different concentrations. Membrane potential, Δφ_m, measurements were performed to investigate the effective fixed charge concentration, X_f, and transport number of the ions, t_i, in the membrane using two different procedures: keeping the concentration ratio constant, or keeping one concentration constant and changing the other one. Results show the membrane presents a weak cation-exchanger character, since the following values were obtained: X_f=−(2.5±0.2)×10⁻² M, t_K =(0.56±0.06), t_Cl⁻=(0.44±0.05); taking into account these values, concentration dependence of membrane potential was predicted. Membrane electrical resistance, R_m, was obtained from Impedance Spectroscopy (IS) measurements using equivalent circuits as models, and the membrane porosity Θ=(0.11±0.02)% was also obtained from resistance values, which agrees very well with the value determined from geometrical parameters. From R_m, Δφ_m and Θ values, the diffusion coefficient of the ions in the membrane pores can be calculated, and the following average values were obtained: D_K⁺=(1.9±0.4)×10⁻⁹ m²/s and D_Cl⁻=(0.8±0.2)×10⁻⁹ m²/s, but for an average concentration higher than 0.06 M, their values do not differ practically from solution in agreement with the small negative charge previously indicated.     

Investigation of alkylamine self-assembled films on iron electrodes by SEM, FT-IR, EIS and molecular simulations

Alkylamine were used to form self-assembled films for the inhibition of the corrosion of iron in 0.5 M H₂SO₄ solution. The films were characterized by electrochemical impedance spectroscopy (EIS) and surface analysis techniques including SEM and FT-IR. EIS results indicate that the inhibition ability of these alkylamine self-assembled films depends on the immersion time and the alky chain length of the adsorbate. Both longer immersion time of the electrode in the solution and longer alky chain will result in stronger inhibition ability of the films. However, when the immersion time was increased over some critical point, the inhibition ability was almost invariable. Also when the chain length reached a certain degree such as 14 carbon atoms, the inhibition ability decreased on the contrary. The ability of the corrosion inhibition of the tetradecylamine and dodecylthiol mixed films improved remarkably comparing with the tetradecylamine or dodecylthiol single films. In addition, molecular simulation was used to discuss the adsorption mechanism and good agreement with electrochemical results was obtained.     

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.     

Electrochemical growth of Co nanowires in ultra-high aspect ratio InP membranes: FFT-impedance spectroscopy of the growth process and magnetic properties

The electrochemical growth of Co nanowires in ultra-high aspect ratio InP membranes has been investigated by fast Fourier transform-impedance spectroscopy (FFT-IS) in the frequency range from 75 Hz to 18.5 kHz. The impedance data could be fitted very well using an electric circuit equivalent model with a series resistance connected in series to a simple resistor-capacitor (RC) element and a Maxwell element. Based on the impedance data, the Co deposition in ultra-high aspect ratio InP membranes can be divided into two different Co deposition processes. The corresponding share of each process on the overall Co deposition can be determined directly from the transfer resistances of the two processes. The impedance data clearly show the beneficial impact of boric acid on the Co deposition and also indicate a diffusion limitation of boric acid in ultra-high aspect ratio InP membranes. The grown Co nanowires are polycrystalline with a very small grain size. They show a narrow hysteresis loop with a preferential orientation of the easy magnetization direction along the long nanowire axis due to the arising shape anisotropy of the Co nanowires.     

Electrochemical behavior of nickel in nitric acid and its corrosion inhibition using some thiosemicarbazone derivatives

The adsorption and corrosion inhibition behavior of three selected thiosemicarbazone derivatives, namely 3-pyridinecarboxaldehyde thiosemicarbazone (META), isonicotinaldehyde thiosemicarbazone (PARA) and 2-pyridinecarboxaldehyde thiosemicarbazone (ORTHO) at the nickel surface were studied electrochemically by Tafel and impedance methods and computationally by carrying out Monte Carlo searches of configurational space on nickel/thiosemicarbazone derivative system. Electrochemical measurements showed that the inhibition efficiency of these compounds increased with increase in their concentration. The recorded inhibition efficiencies of the three tested thiosemicarbazone increase in the order: META > PARA > ORTHO. Polarization studies showed that these compounds act as mixed-type inhibitors for nickel corrosion in 1.0 M HNO₃ solutions. Results obtained from Tafel and impedance methods are in good agreement. Thiosemicarbazone derivatives have been simulated as adsorbate on Ni (1 1 1) substrate and the adsorption energy, binding energy and the low energy adsorption sites have been identified on nickel surface.     

A theoretical approach of impedance spectroscopy during the passivation of steel in alkaline media

A theoretical impedance function is deduced for a proposed mechanism of passive film formation of steel in contact with alkaline aqueous media involving two reaction intermediates: mixed oxide with similar stoichiometry to magnetite and Fe(III)-oxides. The reduction reaction of dissolved oxygen is considered as the only cathodic reaction compensating the anodic current induced by the formation of iron oxides at open circuit potential. The iron dissolution takes place through a chemical dissolution of ferric oxide. A two-layered passive film with 3D structure is considered. A satisfactory agreement between the digital simulations on the basis of the theoretical impedance function and experimental spectra validates the proposed model.     

Effect of Nafion ionomer and catalyst in cathode layers for the direct formic acid fuel cell with complex capacitance analysis on the ionic resistance

Using platinum (Pt) black and carbon-supported Pt (Pt/C) as cathode catalysts, membrane-electrode assemblies (MEAs) were fabricated with various Nafion ionomer content, and their direct formic acid fuel cell (DFAFC) performances were investigated. In MEAs incorporating Pt black catalysts, the current density at 0.6 V was highest at ionomer/catalyst volume ratio of 1.0, which was consistent with the electrochemical active area (EAS) variation measured by cyclic voltammetry. However, the current density measured at 0.3 V, the cell performance increased with Nafion ionomer content, especially at low ionomer loading, indicating that proton transport rate played an important role. The variation in ionic resistance (R_ion) of cathode layers with Nafion ionomer content was experimentally confirmed by using the complex capacitance analysis of impedance data implemented with nitrogen (cathodes)/hydrogen (anodes) atmosphere. For Pt/C, the layer thickness and EAS of cathode were larger than those of MEA cathode using Pt black; and the current densities at 0.6 V were lower than those of Pt black, suggesting that smaller fraction of EAS was utilized.