The impedance caused by interfacial current constriction: a case study on BaF<Subscript>2</Subscript>/YSZ bicrystals

Abstract  BaF₂/YSZ (yttria stabilized zirconia) bicrystals are investigated by means of impedance spectroscopy. The spectra show two semicircles in the complex impedance plane, and both arcs exhibit very similar temperature dependences. The high frequency semicircle can be attributed to the ideal (quasi one-dimensional) bulk resistance of BaF₂, and the low frequency arc is caused by an additional resistance in BaF₂ due to the current constriction taking place close to the contact spots at the imperfect bicrystal interface. This interpretation is supported by finite element calculations revealing that bicrystals with imperfect contacts indeed exhibit an additional semicircle in the complex impedance plane even without any ion transfer resistance between the two crystals. The low frequency arc of the BaF₂/YSZ bicrystal drastically increases upon bias voltage, but relaxes to its original value after removing the bias. This phenomenon can be associated with a strong decrease of the vacancy concentration in YSZ (close to the interfacial contact spots), which is caused by F⁻ ions being pumped into YSZ and acting as a counter dopant to Y³⁺. Graphical Abstract        

Anion transfer in ZrO2 and HfO2 bicrystals containing 12 mol % Y2O3

The oxygen-ionic conduction in the YSZ|YSZ and YSZ|YSH bicrystals, where YSZ and YSH are single-crystalline solid solutions of, respectively, ZrO₂ and HfO₂ with 12 mol % Y₂O₃ are studied using the impedance method. Bicrystals consist of two single crystals separated by a common intercrystalline boundary. The direction of the applied electrical field is chosen perpendicular to the intercrystalline boundary. The oxygen-ion transfer in the bicrystals is determined primarily by the transport processes in the bulk single crystals. This is associated with a fast exchange by oxygen vacancies between single crystals at the intercrystalline boundary.     

Oxygen Transport in the SOFC Cathode

The impedance of the La_0.75Sr_0.2MnO₃-cathode/electrolyte interface for cathodes with different porosity is measured. The impedance spectra are fitted using a developed model of the oxygen transport at this interface. After the measurements, the cathode is removed from the electrolyte. The contact area and the three-phase boundary length (TPBL) at the interface are estimated from SEM images of the electrolyte surface. The dependence of the interfacial electrical resistance on the microstructure is discussed. It is shown that the bulk diffusion of oxygen vacancies at the interface at 950°C is high enough to use the whole La_0.75Sr_0.2MnO₃/YSZ contact area F for the oxygen transport into the electrolyte for microstructures with 2F/TPBL 2 m. The impact of the surface diffusion of oxygen species on polarization resistance at operation temperatures <900°C is discussed. The polarization resistance and the morphology of composite cathodes made from La_0.75Sr_0.2MnO₃/YSZ and yttria- or scandia-stabilized zirconia powders (3YSZ, 8YSZ, 10ScSZ) are investigated by impedance spectroscopy at 800–950°C. The polarization (interfacial) resistance decreases gradually with addition of electrolyte powder in the uLSM cathode material independent of the electrolyte powder used. The interfacial resistance of the uLSM/3YSZ, uLSM/8YSZ, and uLSM/10ScSZ composite cathodes is almost the same. The interaction between uLSM and doped zirconia particles is discussed on the basis of the interfacial resistance, activation energies, and high-frequency impedance.     

An impedance study of electron transport and electron transfer in composite polypyrrole + polystyrenesulphonate films

At low potentials, the impedance of polypyrrole + polystyrenesulphonate changes from the simple transmission line response observed at high potentials to a more complex response including a high frequency semicircle in the complex plane representation. There is also a shift in the high frequency limiting real impedance as the electronic resistance R_e of the polymer becomes comparable with, and then greater than, the solution and film ionic resistances. The ionic conductivity of the polymer composite increases with decreasing potential, and becomes constant at its maximum value in the potential range where the changes in impedance behaviour occur. This greatly simplifies interpretation of the low potential impedance results, and allows unambiguous assignment of circuit elements. The charge transfer resistance R_ct, which causes the high frequency semicircles, is due to electron transfer at the polymer|electrode interface. Both R_ct and R_e decrease exponentially with increasing potential at 60 mV per decade. This observation validates the use, at low potentials, of theoretical models in which conducting polymers are treated as redox polymers despite their failure to follow the Nemst equation at higher potentials.     

The impedance of the Ag-CeF<Subscript>3</Subscript>/CeF<Subscript>3</Subscript> bicrystal-Ag system

The impedance spectra of CeF₃/CeF₃ bicrystal (two single crystals separated by a single intercrystalline boundary) between Ag-electrodes are studied over a 135 to 410 K temperature interval (including temperatures below room temperature). The bicrystal was prepared by thermal-diffusion welding under a pressure of 1.5 × 10⁷ Pa at 1473 K in vacuum (∼10⁻² Pa). It is shown that the intercrystalline boundary affects but insignificantly the bicrystal bulk impedance. The CeF₃/CeF₃ ionic conductivity is 3 × 10⁻⁶ S/cm at 293 K; it is mainly determined by transfer processes in the single crystal bulk.     

Impedance-based total-NOx sensor using stabilized zirconia and ZnCr2O4 sensing electrode operating at high temperature

We report here a new-type zirconia-based sensor that can detect total NO_x content at high temperatures such as 700 °C. A closed-one-end yttria-stabilized zirconia (YSZ) tube was used as a base sensor material. An oxide sensing electrode (SE) and a Pt counter electrode (CE) were formed on the outer and inner surfaces of the YSZ tube, respectively. The complex impedance of the device using a ZnCr₂O₄-sensing electrode was measured with an impedance analyzer in the frequency and the temperature ranges 0.1 Hz–100 kHz and 600–700 °C, respectively. A large semicircular arc was observed in complex impedance plots (Cole–Cole plots) in the lower frequency range examined and it seemed to correspond to the electrolyte/electrode interface. The impedance value at 1 Hz of the present device was found to vary almost linearly with the concentration of NO (or NO₂) from 50 to 400 ppm in the sample gas at 600–700 °C. Furthermore, it is noted that the sensitivity of NO is almost equal to that of NO₂. This means that the present device can detect the total NO_x at higher temperatures.     

Ac study of 10% Fe-doped La0.65Ca0.35MnO3 material by impedance spectroscopy

The ac electrical properties of La_0.65Ca_0.35Mn_0.90Fe_0.10O₃ polycrystalline sample have been investigated by complex impedance analysis over the frequency and temperature ranges of 1–100 kHz and 77–323 K, respectively. Around the metal–insulator transition temperature capacitative effect has been observed which may be due to clustering of defects induced by iron doping. The impedance plane plot shows semicircle at different temperatures and an equivalent circuit, R₁(R₂C), i.e., a resistor–capacitor, has been proposed to explain the impedance results. The correlated barrier hoping (CBH) model is found to be more appropriate for explaining the frequency and temperature dependent ac-conductivity data.     

Etude de la conductivite anionique dans quelques fluorures amorphes obtenus par hypertrempe

Fluoride ion conductivity has been investigated in amorphous materials obtained in the BaF₂-ZnF₂, PbF₂-ZnF₂ and BaF₂-MnF₂ systems using both NMR and complex impedance techniques. The activation energies appear to be about 0.53 eV with a good agreement between both methods. Ionic conductivities have been found to be larger than 10^-5ω^-1cm^-1 at 180°C, values which are among the best ones so far obtained for fluorine conducting glasses.     

Electrochemical Impedance Spectroscopic Measurements of Sexithiophene—Effect of the Electrode Nature and the Dopant (CuCl2) Content

Electrochemical impedance spectroscopic (EIS) measurements of sexithiophene (6T) were carried out according to the Pt/6T/M sandwich structure configuration, for various electrode materials (M＝GC, ITO, Ag, Cu, Al) and for different doping levels of copper chloride (CuCl2). The results demonstrate that two types of charge transport are involved in the redox process at the electrode/6T interface and inside the bulk oligomer. The complex-plane impedance plots obtained for various doping levels of CuCl2 exhibit arc shapes. The charge-transfer resistance measured from the diagrams decreases systematically with the addition of the salt, leading to an increase of the oligothiophene conductivity.     

Ionic conductivity and electrode effects on β-PbF2

The a.c. impedance of single crystal specimens of β-PbF₂ has been studied as a function of frequency and temperature in the range 10^?2 to 10⁵ Hz and 25 to 320°C. A variety of ionically blocking electrode configurations were used and equivalent circuits were derived by complex plane analysis. In addition to bulk resistive and capacitive effects, a complex frequency-dependent interface impedance was identified. The algebraic form and temperature dependence of this impedance were found and variations in its magnitude with changes in electrode material and morphology were measured.     

The anodic dissolution of silver into silver rubidium iodide — Impedance measurements

A.c. impedance measurements have been made on the Ag/Ag₄RbI₅ interphase at anodic potentials less than 50 mV vs. Ag/Ag⁺, using an automatic frequency response analyser.At overpotentials between 10 and 35 mV a high frequency semicircle and a low frequency inductive shape were seen. Spectra at these low overpotentials can be interpreted in terms of rate determining two-dimensional nucleation and growth of holes in the metal surface. The number of atoms in the critical nucleus at various overpotentials have been calculated from both steady state and impedance measurements.     

Mid-temperature solid oxide fuel cells with thin film ZrO<Subscript>2</Subscript>: Y<Subscript>2</Subscript>O<Subscript>3</Subscript> electrolyte

Data on the mid-temperature solid-oxide fuel cells (SOFC) with thin-film ZrO₂-Y₂O₃ (YSZ) electrolyte are shown. Such a fuel cell comprises a carrying Ni-YSZ anode, a YSZ electrolyte 3–5 μm thick formed by vacuum ion-plasma methods, and a LaSrMnO₃ cathode. It is shown that the use of a combined method of YSZ electrolyte deposition, which involves the magnetron deposition of a 0.5–1.5-μm thick sublayer and its pulse electron-beam processing allows a dense nanostructured electrolyte film to be formed and the SOFC working temperature to be lowered down as the result of a decrease in both the solid electrolyte Ohmic resistance and the Faradaic resistance to charge transfer. SOFC are studied by the methods of voltammentry and impedance spectroscopy. The maximum power density of the SOFC under study is 250 and 600 mW/cm⁻² at temperatures of 650 and 800°C, respectively.     

LSM-YSZ nano-composite cathode with YSZ interlayer for solid oxide fuel cells

Low temperature prepared(La_(0.8)Sr_(0.2))_(0.9)MnO_3-δ-Y_(0.15)Zr_(0.85)O_(1.93)(LSM-YSZ) nano-composite cathode has high three-phase boundary(TPB) density and shows higher oxygen reduction reaction(ORR) activity than traditional LSM-YSZ cathode at reduced temperatures. But the weak connection between cathode and electrolyte due to low sintering temperature restrains the performance of LSM-YSZ nano-composite cathode. A YSZ interlayer, consisted of nanoparticles smaller than 10 nm, is introduced by spinning coating hydrolyzed YSZ sol solution on electrolyte and sintering at 800 °C. The thickness of the interlayer is about 150 nm. The YSZ interlayer intimately adheres to the electrolyte and shows obvious agglomeration with LSM-YSZ nano-composite cathode. The power densities of the cell with interlayer are 0.83, 0.46 and 0.21 W/cm~2 under 0.7 V at 800, 700 and 600 °C, respectively, which are 36%, 48% and 50% improved than that of original cell. The interlayer introduction slightly increases the ohmic resistance but significantly decreases the polarization resistance. The depressed high frequency arcs of impedance spectra suggest that the oxygen incorporation kinetics are enhanced at the boundary of YSZ interlayer and LSM-YSZ nanocomposite cathode, contributing to improved electrochemical performance of the cell with interlayer.     

Self diffusion and conductivity in Nafion membranes in contact with NaCl+CaCl2 solutions

The ion-exchange isotherm of sodium and calcium ions is drawn for the Nafion^R 117 membrane in contact with [NaCl] + 2[CaCl₂] = 0.1 M. The self-diffusion coefficients of sodium and calcium ions, obtained from steady-state self-diffusion ionic fluxes by a radiotracer method are independent of the ionic composition of the membrane phase. They are compared to those obtained from high frequency electrical resistance by means of the ac impedance technique. The values of electric conductivity of the membrane can be derived from a simple additivity law.     

AC impedance and state-of-charge analysis of a sealed lithium-ion rechargeable battery

A 7.2 V, 1.25 Ah sealed lithium-ion rechargeable battery has been studied for estimating its state-of-charge (SOC) by AC impedance. The dispersion of impedance data over the frequency range between 100 kHz and 25 mHz comprises an inductive part and two capacitive parts. As the inductive behaviour of the battery is attributed to the porous nature of the electrodes, only the capacitive components have been examined. The data obtained at several SOC values of the battery have been analyzed by a non-linear least-squares fitting procedure. The presence of two depressed semicircles in the capacitive region of the Nyquist plots necessitated the use of an electrical equivalent circuit containing constant phase elements instead of capacitances. The impedance parameters corresponding to the low-frequency semicircle have been found useful for predicting the SOC of the battery, mainly because the magnitude of these parameters and their variations are more significant than those of the high-frequency semicircle. The frequency maximum (f _max) of the semicircle, the resistive component (Z′) corresponding to f _max, the phase angle (φ) in the 5.0 Hz–0.1 Hz frequency range, the equivalent series resistance (R _s) and the equivalent series capacitance (C _s) have been identified as suitable parameters for predicting the SOC values of the lithium-ion battery. Received: 23 September 1998 / Accepted: 22 February 1999     

Electrochemical impedance of microelectrodes

The influence of the radius (10 <a < 1500 μm) of a Pt disk electrode on the impedance frequency spectrum of the [Fe(CN)₆]^3-/4- reversible system is studied in the frequency range 5 × 10^-3 to 10³ Hz. The impedance is calculated by applying the Fourier transform to potential and current pulses of various lengths obtained when imposing a step potential. For electrodes witha < 100 μm, the spectrum in the complex plane has the form characteristic of microelectrodes, while for electrodes of higher radii, it corresponds to a finite Warburg impedance. The main impedance parameters, such as the charge transfer resistance, the diffusion resistance, and the frequency in the maximum of the imaginary constituent are determined. The latter decreases with an increase ina by the lawf* ∼ 1/a² at lowa and is independent ofa on electrodes witha > 100 μm, which agrees with the impedance theory for microelectrodes     

Properties of Ag/AgCl electrodes fabricated with IC-compatible technologies

The purpose of this work is to fabricate and characterize Ag/AgCl electrodes made on a silicon chip at the wafer level with integrated circuit-compatible fabrication techniques. Such electrodes are useful as reference electrodes in several kinds of chemical sensors. Two types of electrode were investigated. The first type uses an evaporated AgCl layer that is patterned with lift-off photolithography. The second type is formed by exposing a selected part of the silver substrate to a KCrO₃Cl solution. Both types of electrode give the thermodynamically expected potential response to variations of Cl⁻ ion concentration. The potential generated by the KCrO₃Cl-formed electrodes was more stable, however. Auger electron spectroscopy depth profiles indicate that immersion in a KCrO₃Cl solution produces a thin layer of AgCl on top of a layer of AgO. The low electronic resistance of AgO then reduces the measured series resistance of the KCrO₃Cl-formed electrodes. Impedance plane plots and the impedance as a function of frequency were measured for both types of electrode, and the impedance of the evaporated AgCl electrodes was indeed considerably higher. The impedance measurements could be successfully modelled by assuming a Randles equivalent circuit for the AgCl/electrolyte interface. For the KCrO₃Cl-formed electrodes, the impedance was modified by the porosity these electrodes manifested.     

Performance studies of electrolyte-supported solid oxide fuel cell with Ni–YSZ and Ni–TiO2–YSZ as anodes

Redox cycling of Ni-based anode induces cell degradation which limits the cell's lifetime during solid oxide fuel cell operation. In the present study, the redox testing of electrolyte-supported cells has been investigated with TiO₂-added NiO–YSZ anode matrix. Button cells were fabricated by die-pressing YSZ powder as electrolyte, and onto which NiO–YSZ or NiO–TiO₂–YSZ anode and LSM–YSZ composite cathode were painted. The electrochemical performance and stability have been evaluated by measuring current–voltage characteristics followed by impedance spectroscopy after each redox cycling. Anode matrices before and after cell operation have been characterized by X-ray diffraction (XRD), elemental dispersive X-ray (EDX), and scanning electron microscopy (SEM). During cell operation the peak power density decreases from 111 mW cm^?2 (239 mA cm^?2) to 84 mW cm^?2 (188 mA cm^?2) between 23 and 128 h with five redox cycles for cell having NiO–YSZ (40:60) anode. But for cell with NiO–TiO₂–YSZ (30:10:60), the anode peak power density was constant and stable around 85 mW cm^?2 (194 mA cm^?2) throughout the cell run of 130 h and five redox cycles. No loss in the open circuit voltage was observed. SEM and XRD studies of NiO–TiO₂–YSZ (30:10:60) anodes revealed formation of ZrTiO₄, which may be responsible for inhibition of Ni coarsening leading to stable cell performance.     

Microstructure and luminescence of transparent glass ceramic containing Er:BaF2 nano-crystals

Transparent BaF₂-SiO₂ glass ceramics doped with different content of Er³⁺ were prepared by sol-gel method. The microstructural evolution of the samples was studied with X-ray diffraction (XRD), transmission electron microscope (TEM), absorption and infrared spectra (IR). BaF₂ nano-crystals with 2-15 nm in size, depending on the crystallization temperature, distributed homogeneously among the amorphous silica matrix. The BaF₂ lattice parameters decreased with the increasing of Er³⁺ doping, indicating the incorporation of Er³⁺ into nano-crystals, which was further confirmed by energy dispersive X-ray spectroscopy (EDS) and absorption spectra analysis. The upconversion emissions of Er³⁺ emerged under the excitation at 980 nm for glass ceramic heat-treated at 800 °C.