Electrical Conductivity Relaxation Research Articles

Computational design and experimental realization of Zn substitution in Cr-poisoning resistant LaNi0.6Fe0.4O3-δ solid oxide fuel cell cathode for enhanced performance and durability

Chromium poisoning rapidly degrades the performance of solid oxide fuel cell (SOFC) cathode materials with superior oxygen reduction kinetics, such as La0·6Sr0·4Co0·2Fe0·8O3 and La0·6Sr0·4CoO3. Previously, LaNi0.6Fe0.4O3 (LNF) demonstrated excellent resistance to chromium poisoning; however, it has relatively low electrocatalytic activity. Here, we investigate the effect of Zn substitution on the B-site of LNF cathode and deliver La(Ni0·6Fe0.4)1-xZnxO3-δ cathode materials with enhanced electrical and electrochemical properties. The ab initio first-principles calculations showed charged oxygen-vacancy defect formation energies of Zn-doped LNF (2.07 eV for La8Ni5Fe2Zn1O24 and 1.83 eV for La8Ni4Fe3Zn1O24) are lower than those of undoped LNF (2.92 eV for La8Ni5Fe3O24). Various Zn-doped LNF materials are synthesized and tested for experimental validation of the DFT results. Electrical conductivity relaxation, temperature-programmed oxygen desorption, and electrochemical impedance spectroscopy analysis indicate enhanced oxygen reduction kinetics for LNFZ3 (La(Ni0·6Fe0.4)0.97Zn0·03O3-δ). At 700 °C, LNFZ3 has 25.4% less polarization resistance and 52.3% higher maximum power density of an anode-supported SOFC than LNF. Moreover, LNFZ3 has the lowest polarization resistance compared with various studies reporting LNF cathode material. Furthermore, LNFZ3 shows highly robust operation under accelerated chromium-poisoning conditions at 700 °C. The high performance and durability synergy makes the newly developed LNFZ3 a promising cathode material for further commercial validation studies.

Impedance spectroscopy and DFT/TD-DFT studies of diyttrium trioxide for optoelectronic fields

Yttrium (III) oxide or so-called diyttrium trioxide (Y 2 O 3 ) is an excellent candidate ceramic material for optoelectronic applications. Structural, electrical conductivity, and dielectric relaxation properties of bulk yttrium (III) oxide are studied. X-ray diffraction (XRD) results indicate that the yttrium (III) oxide compound has a crystalline cubic phase. FTIR technique is used to ascertain the chemical structure of the yttrium (III) oxide compound. Impedance spectroscopy was used to analyze frequency-dependent electrical properties as a function of temperature in the range of 303–423 K and frequency range 0.1 Hz–2 MHz. Impedance spectroscopy parameters such as dielectric constant, dielectric loss, loss factor, electric modulus, and complex impedance of the yttrium (III) oxide compound have been studied. The Nyquist plot describes the complex impedance of the yttrium (III) oxide for different temperatures. The universal Jonscher’s power law was used for the analysis of the complex electrical conductivity of the yttrium (III) oxide compound. It is found that the real ( σ ′ ) and imaginary ( σ ˊ ˊ ) parts of the complex conductivity increase with increasing frequency. The exponent frequency (s) equals unity which confirms that the predominant conduction mechanism is a nearly constant loss (NCL) mechanism. DFT/TD-DFT studies using B3LYP/LanL2DZ level of theory were used to provide comparable theoretical data and electronic energy gap of HOMO→LUMO. Spatial plot of HOMO and LUMO of yttrium (III) oxide with their energy gap.

Electrical conductivity and nuclear magnetic resonance relaxation rate of Eliashberg superconductors in the weak-coupling limit

Electrical conductivity is an important transport response in superconductors, enabling clear signatures of dynamical interactions to be observed. Of primary interest in this paper is to study characteristics of the electron-phonon interaction in weak-coupling Eliashberg theory (Eth), and to note the distinctions with Bardeen-Cooper-Schrieffer (BCS) theory. Recent analysis of weak-coupling Eth has shown that while there are modifications from the BCS results, certain dimensionless ratios are in agreement. Here we show that the conductivities in BCS theory and Eth fundamentally differ, with the latter having an imaginary gap component that damps a divergence. We focus on the dirty limit, and for both BCS theory and Eth we derive expressions for the low-frequency limit of the real conductivity. For Eth specifically, there are two limits to consider, depending on the relative size of the frequency and the imaginary part of the gap. In the case of identically zero frequency, we derive an analytical expression for the nuclear magnetic resonance relaxation rate. Our analysis of the conductivity complements the previous study of the Meissner response and provides a thorough understanding of weak-coupling Eth.

Proton conduction in tetra-n-butylammonium bromide semiclathrate hydrate

Clathrate hydrate as well as ice has been spotlighted as promising materials to investigate the properties of hydrogen-bonded networks formed by the water molecules. In the semiclathrate hydrate (SCH, a kind of clathrate hydrate) consisting of host water molecules and guest quaternary onium salts, the anion takes part in the hydrogen-bonded networks with the water molecules. In the present study, we measured the electrical conductivity and electrical relaxation time in the single-crystalline tetra-n-butylammonium bromide (TBAB) SCH by electrochemical impedance spectroscopy. The results clearly revealed that the conduction carrier was proton. The electrical relaxation time of proton conduction in TBAB SCH was similar to the reorientation time of the water molecules in TBAB SCH estimated by the results of nuclear magnetic resonance.

Structural modifications, optical response, and electrical conductivity mechanism of Bi2O3 doped in P2O5–V2O5–MoO3 nanocomposite glass systems

Structural, optical and electrical conductivity mechanism of four compositional series of xBi2O3-(1-x)(0.30P2O5–0.35V2O5–0.35MoO3) [x = 0.05, 0.15, 0.25, and 0.35] have been investigated. XRD pattern affirms the existence of nanocrystallites in the glassy network, which affects the structural, optical and electrical alterations. The structural variation leads to an increase in the density of all the glassy systems. The FTIR analysis reveals the shift of different band positions suggesting structural alterations. The electrical conductivity investigation reveals the decreasing values of ac and dc activation energy, which enhances the ac and dc conductivity. The validity of Mott's VRH model has been examined and the value of hopping energy has been estimated. The obtained DC conductivity spectra of all samples have been fitted with Greaves's model. The AC conductivity scaling model of Pan and Ghosh has been used to demonstrate the electrical conduction relaxation method. In the spectroscopic investigation, by deploying the Tauc's plot, the optical band gap energies of the studied glass compositions are estimated. The calculation of Urbach energy values allows us to estimate the disorders of glassy nanocomposites. Different spectroscopic parameters like refractive index, polarizability, reflection factor, and reflectivity have been studied.

Surface oxygen exchange kinetics of mixed conducting oxides: Dilatometric vs electrical conductivity relaxation study

The surface oxygen exchange kinetics of mixed ionic-electronic conducting (MIEC) oxides play a crucial role in a variety of applications, including solid oxide fuel/electrolysis cells (SOFCs/SOECs), permeation membranes and sensors. To date, a variety of methods, including isotope exchange, electrical conductivity, optical absorptivity and thermogravimetry relaxation, and impedance spectroscopy have been used to measure the oxygen exchange coefficient of MIEC oxides. Each of these methods have their advantages and limitations, depending on the physical and chemical properties of the investigated materials and their sample dimensions and densities. Here, we demonstrate the ability to precisely measure the surface oxygen exchange coefficient (kchem) of MIEC model material Pr0.1Ce0.9O2-δ (PCO) by use of dilatometric relaxation measurements, particularly of interest for materials that exhibit larger chemical expansion coefficients. This is achieved by use of porous bulk specimens to ensure that the contribution of oxygen exchange to the overall kinetics is dominant. We demonstrate that kchem values extracted from chemical expansion relaxation measurements on PCO are nearly identical to those derived from electrical conductivity relaxation measurements. This provides the opportunity to precisely investigate the oxygen exchange and chemical expansion kinetics of a wide range of materials used in high-temperature applications, particularly where more conventional methods are difficult or inappropriate to apply.

Perspective on high-temperature surface oxygen exchange in a porous mixed ionic-electronic conductor for solid oxide cells.

The surface exchange coefficient (k) of porous mixed ionic-electronic conductors (MIECs) determines the device-level electrochemical performance of solid oxide cells. However, a great difference is reported for k values, which are measured using presently available technologies of electrical conductivity relaxation (ECR), electrochemical impedance spectroscopy (EIS), and oxygen isotope exchange (OIE). In terms of this issue, this perspective paper estimates the possible physiochemical processes for the oxygen reduction reaction (ORR) in porous MIECs by comparing the oxygen supply/consumption fluxes through calculation. Then, the potential problems associated with ECR, EIS, and OIE for application in porous materials are discussed regarding theory, assumptions, sample requirements, and data processing. Finally, gas diffusion effects are revealed by comparing the simulated and measured ECR profiles, which show that the ORR process can be significantly delayed by gas diffusion. This perspective aims to recommend a reasonable method to characterize the true ORR kinetics of porous electrodes and quantify the effect of gas diffusion.

Method to determine the oxygen reduction reaction kinetics via porous dual-phase composites based on electrical conductivity relaxation

Mechanism–structure–performance relationship of porous single-phase La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and dual-phase La0.6Sr0.4Co0.2Fe0.8O3−δ–Sm0.2Ce0.8O1.9 (LSCF–SDC) composites.

Li2SO4 doped Li2O + B2O3 glasses: Hopping mechanism, relaxation phenomena and conductivity behavior

This oxide glass system work reports a detailed of the hopping mechanism, electrical conductivity, and dielectric relaxation phenomena in Li2O + B2O3-based glass for various Li2SO4 mol%. The conductivity, impedance, and dielectric modulus were analyzed in a temperature range from 363 K to 563 K and in a frequency range from 100 Hz to 5 MHz. The conductivity of the glass sample was verified using Cole-Davidson, Jonscher’s power law, and Cole-Cole models and is found to be consistent with each other. Conductivity at the grain and grain boundary was also determined by fitting the impedance data using constant phase elements (CPE) in an equivalent circuit model. The equivalent circuit was used to interpret the impedance semicircles obtained from the ac measurements. Interpretation of the relaxation mechanism in the glass matrix is described by employing Kohlrausch-William-Watts (KWW) model and the modified KWW model, which discloses that the non-Debye type relaxation mechanism is followed by the charge carriers. Impedance spectroscopy has been discussed to distinguish between the contribution of grain boundaries and grains in the relaxation mechanism. The doping Li2SO4 effect can be explained by observing activation energy in the investigated samples. The power law exponent ‘S’ for glass matrix with temperature depicts CBH and NSPT hopping mechanism in AC conductivity. All the above results reveal the glass’s semiconducting nature and depend on the lithium sulphate content in the glass matrix.

Tuning Surface Acidity of Mixed Conducting Electrodes: Recovery of Si-Induced Degradation of Oxygen Exchange Rate and Area Specific Resistance.

Metal oxides are an important class of functional materials, and for many applications, ranging from solid oxide fuel/electrolysis cells, oxygen permeation membranes, and oxygen storage materials to gas sensors (semiconducting and electrolytic) and catalysts, the interaction between the surface and oxygen in the gas phase is central. Ubiquitous Si-impurities are known to impede this interaction, commonly attributed to the formation of glassy blocking layers on the surface. Here, the surface oxygen exchange coefficient (kchem ) is examined for Pr0.1 Ce0.9 O2-δ (PCO), a model mixed ionic electronic conductor, via electrical conductivity relaxation measurements, and the area-specific resistance (ASR) by electrochemical impedance spectroscopy. It is demonstrated that even low silica levels, introduced by infiltration, depress kchem by a factor 4000, while the ASR increases 40-fold and weattribute this to its acidity relative to that of PCO. The ability to fully regenerate the poisoned surface by the subsequent addition of basic Ca- or Li-species is further shown. This ability to not only recover Si-poisoned surfaces by tuning the relative surface acidity of an oxide surface, but subsequently outperform the pre-poisoned response, promises to extend the operating life of materials and devices for which the catalytic oxygen/solid interface reaction is central.

Measurements of Carbon Diffusivity and Surface Transfer Coefficient by Electrical Conductivity Relaxation during Carburization: Experimental Design by Theoretical Analysis

The diffusion coefficient (D) and surface transfer coefficient (β) of carbon are important parameters governing the kinetics of carburization, and some other heat treatment processes accompanied by redistribution of carbon in steel. Here, we propose to use an electrical conductivity relaxation (ECR) method for the in situ measurement of D and β of carbon. The feasibility of the method is discussed by the theoretical modeling of carburization for an infinitely long rectangular sample. The synthetic ECR data for the carburization is simulated by tracking the relaxation of electrical conductivity upon a sharp or a gradual change of carbon potential. Then, by Fourier transform, the synthetic ECR data is transformed to an impedance spectroscopy, which is used for estimation of D and β by fitting with a one-dimensional equivalent circuit model. The effects of the width-to-thickness ratio of the sample and the duration of carbon potential buildup on the accuracy of the estimated D and β are studied. The feasibility of the ECR method is verified, and rational guidance for experimental design is proposed.

High performance La2NiO4+δ impregnated La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes for solid oxide fuel cells

La2NiO4+δ (LNO) impregnated La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes were prepared and the effects of the impregnation on the electrical conductivity and oxygen surface exchange property were studied to reveal the enhancement mechanism of oxygen reduction reaction (ORR) catalytic activities. Results of electrochemical impedance spectroscopy (EIS) with distribution of relaxation times (DRT) deconvoluting and electrical conductivity relaxation (ECR) indicate that LNO impregnation into LSCF cathodes enhances the oxygen diffusion process and oxygen surface exchange property. The optimal ORR catalytic activity is achieved after LNO impregnation for two times on the LSCF scaffold (LNO2-LSCF). LNO2-LSCF cathodes also have outstanding performance for their compromise thermal expansion coefficients (TEC) and conductivities.

(Pr0.9La0.1)1.9(Ni0.7Cu0.3)0.9Mn0.1O4+δ nanofiber cathode with Pr site cation defect prepared by electrospinning and its application in reversible fuel cells

In this contribution, the Pr-site cation-deficient (Pr0.9La0.1)1.9(Ni0.7Cu0.3)0.9Mn0.1O4+δ nanofiber electrode material is successfully prepared by electrospinning and microwave sintering, and the effect of defects on the crystal structure and electrochemical performance are systematically studied. The resulted cations deficiency within material can form a certain amount of oxygen vacancies, which improves its hydration performance and accelerates the surface exchange coefficient of oxygen. Based on the thermogravimetric analysis (TGA) and electrical conductivity relaxation (ECR) technology, it is concluded that the enhanced proton conductivity of the material benefits from the oxygen vacancies caused by internal crystal defects. The doping of Mn element weakens the binding energy of the Ni–O bond, which avails the release of lattice oxygen. In addition, the distribution of relaxation time (DRT) analysis demonstrates that the main electrode reaction is the dissociation reaction process of the adsorbed oxygen. As a result, the polarization resistance (Rp) of the symmetrical cells at 700 °C is only 0.101 Ω cm2. In the single-cell fuel mode, the power density reaches 894 mW cm−2, and the excited current density in the electrolysis mode is 1563 mA cm−2 at 700 °C.

Mo doped Ruddlesden-Popper Pr1.2Sr0.8NiO4+δ oxide as a novel cathode for solid oxide fuel cells

A new type of Ruddlesden-Popper Pr1.2Sr0.8Ni1−xMoxO4+δ (PSNMO) cathode with x = 0, 0.025, 0.050, 0.075 was prepared by the sol-gel method. The conductivity of Pr1.2Sr0.8NiO4+δ cathodes reaches the max values and then decreases the continuous increase of Mo amount in all the Mo doped cathodes. Results of electrochemical impedance spectroscopy show that the Pr1.2Sr0.8Ni0.950Mo0.050O4+δ has the lowest polarization resistance of 0.110 Ω cm2 at 750 °C among PSNMO cathodes. And results of electrical conductivity relaxation and X-ray photoelectron spectroscopy indicate that Mo-doping improves the oxygen surface exchange properties of PSNMO cathodes, which can be mainly ascribed to co-interaction of oxygen vacancy and interstitial oxygen in PSNO cathodes after Mo-doping.

Tuning oxygen vacancy of the Ba0·95La0·05FeO3−δ perovskite toward enhanced cathode activity for protonic ceramic fuel cells

Innovation of highly active cathode is of great significance to the development of protonic ceramic fuel cells (PCFCs). Herein, tailoring oxygen vacancies in Zn-doped Ba0·95La0·05FeO3−δ (BLFZ) perovskite is proved to be beneficial for promoting the formation of proton defects. Hydration ability of the triple conducting BLFZ perovskites is confirmed by electrical conductivity relaxation (ECR). The results demonstrate that BLFZ exhibits a proton surface exchange coefficient of 1.34 × 10−3 cm s−1 at 600 °C, which greatly extends active sites from the electrolyte/cathode interface to the entire electrode. Mechanism and process elementary steps of the oxygen reduction reaction (ORR) of BLFZ-BaCe0.7Zr0·1Y0.1Yb0.1O3−δ (BCZYYb) are detailedly studied. It is found that the rate-determining step of ORR is surface dissociative adsorption of oxygen on BLFZ-BCZYYb cathode. A maximum power density of 673 mW cm−2 at 700 °C is achieved and BLFZ-BCZYYb based single-cell shows no obvious degradation at 600 °C for 200 h. The good performance is ascribed to the rapid proton diffusion of BLFZ-BCZYYb composite electrode by regulating the oxygen vacancies.

Effect of Ag2S on electrical conductivity and dielectric relaxation in Ag2O-MoO3-P2O5 ionic glassy systems

The influence of Ag2S incorporation on the electrical and dielectric properties of the host Ag2O-MoO3-P2O5 glassy matrix has been systematically studied in the present communication. By applying the well-known Archimedes principle, the density of the samples has been determined. The ionic property for all the as-prepared glassy systems has been explored methodically. The nearly identical obtained values of the crossover frequency and the activation energy for DC and AC conductivity suggest that the same mechanism is responsible for electrical conduction. For the purpose of inspecting the frequency and temperature dependent AC conductivity, the Almond-West formalism model has been used. The observed values of dielectric constant and dielectric loss are found to increase with the temperature rise and drop with rising frequency. The coinciding scaled complex electric modulus spectra suggest a non-Debye type dynamical relaxation mechanism, which also indicates that the relaxation mechanism is temperature independent but composition dependent.

On the effect of electrode material in electrical conductivity relaxation - An alternative interpretation of the two-fold relaxation behavior in La5.4WO11.1-δ

Oxygen and proton transport properties of La5.4WO11.1-δ are investigated using electrical conductivity relaxation. Chemical diffusion coefficients and surface exchange coefficients of both hydrogen and oxygen presumed to govern conductivity relaxation are extracted from fitting the two-fold conductivity relaxation observed after hydration/dehydration steps at fixed oxygen partial pressure to the model equations. Surprisingly, the kinetic parameters obtained from fitting are found to depend on the magnitude of the current used in the measurements. A two-fold relaxation behavior with characteristics depending on the magnitude of the current is also observed after oxidation/reduction steps under dry conditions. Furthermore, at fixed temperature, oxygen and water partial pressures, the conductivity is found to exhibit relaxation behavior after a current step, while the apparent steady-state conductivity is found to depend on the applied current. The observations are attributed to the use of gold electrodes in the experiments and associated interfacial capacitances. It is obvious from this study that the use of partially ion-blocking electrodes in conductivity and conductivity relaxation measurements on mixed ionic-electronic conducting oxides with prevailing ionic conduction may lead to erroneous results and interpretations.

Enhanced mechanisms of oxygen reduction on Pr0.4Sr0.6Co0.2Fe0.8O3-δ impregnated La1−xSrxCo1−yFeyO3-δ cathodes for solid oxide fuel cells

Surface modification is an effective approach for the improvement of the electrochemical performance of cathodes. Here effects of perovskite-structured Pr0.4Sr0.6Co0.2Fe0.8O3-δ (PSCF) modification are investigated on the electrochemical performance of La1−xSrxCo1−yFeyO3-δ (LSCF) cathodes with experimental and theoretical methods. The formation energies of oxygen vacancy are reduced with PSCF modification and the adsorption energies of oxygen adsorption on PSCF modified LSCF surface are lower than those on the LSCF surface. Also the dissociation barriers of oxygen molecules on the LSCF surface are greatly decreased, indicating that the rate of oxygen reduction reaction is greatly improved and the LSCF surface is activated with PSCF modification. Results of electrochemical impedance spectra tests show that polarization resistance (Rp) of LSCF cathodes decreases with the appropriate amount of PSCF impregnation and reaches the lowest Rp (0.055 Ω cm2) with about 10 wt% impregnation at 750 °C. Results of electrical conductivity relaxation indicate that the performance improvement of LSCF cathodes mainly results from the increase of oxygen surface exchange property (5.4 ×10−4 and 1.1 ×10−3 cm s−1), which is consistent with the calculation results.

Electrical conductivity and relaxation phenomena in Li2O·B2O3 based glass and glass-ceramic: A comprehensive and comparative analysis

This work reports a detailed investigation on the electrical conductivity and dielectric relaxation phenomena in Li2O·B2O3 based glass and glass-ceramic materials. Measurements reveal that the electrical conductivity of glass-ceramic is lower than glass. This owes to occurrence of grain boundaries which impede the mobility of charge carriers in glass-ceramic as confirmed by FESEM micrographs. The frequency-dependent behavior of electrical conductivity, within the measured temperature range, is analysed using Jonscher's power law. It is found that the conductivity rises with an increase in temperature and the range of conductivity reveals the semiconducting behavior of the material. The ac conductivity mechanism of system is observed to follow the overlapping large polaron tunneling model. The conduction and relaxation mechanism studies are performed using electric modulus and impedance spectroscopy within the frequency and temperature ranges varied from 103 Hz to 107 Hz and 593 K–773 K, respectively. Analysis of the relaxation mechanism in both samples is performed employing Kohlrausch-William-Watts (KWW) model, which reveals that the charge carriers are following non-Debye type relaxation. To distinguish between the contribution of grain boundaries and grains in the relaxation mechanism, a combined analysis of complex electric module and impedance spectroscopy has been discussed.

Rational design of Sr2Fe1.5Mo0.4Y0.1O6-δ oxygen electrode with triple conduction for hydrogen production in protonic ceramic electrolysis cell

Protonic ceramic electrolysis cells (PCECs) are emerging as potential technology to achieve efficient electrochemical hydrogen production and purification due to their high conversion efficiency and low cost. However, their maturity on a large-scale hydrogen production is notoriously hindered by the poor activity and stability of oxygen electrodes. In this work, a Sr2Fe1.5Mo0.4Y0.1O6-δ (SFMY) oxygen electrode was designed, which shows a stable phase structure, enhanced hydration capacity, and special triple conduction (H+/O2−/e−). Electrical conductivity relaxation measurement demonstrated that SFMY showed faster H2O surface exchange process and improved bulk ion mobility. The as-fabricated PCEC using SFMY as the oxygen electrode can achieve a current density as high as 1570, 1235, 827, and 516 mA·cm−2 at 750 °C, 700 °C, 650 °C, and 600 °C, respectively, at a voltage of 1.3 V while displaying excellent operation stability at 700 °C during the test period of about 240 h. These results suggest that SFMY can be a potential oxygen electrode for PCECs.