Low-frequency Inductive Loop Research Articles

Observation of negative capacitance in silicon heterojunction solar cells: role of front contact in carrier depopulation

The origin of the low-frequency inductive loop in the Nyquist plot of the Ag/indium tin oxide (ITO)/p-a-Si:H/intrinsic hydrogenated amorphous silicon (i-a-Si:H)/c-Si/i-a-Si:H/n-a-Si:H/ITO/Al heterojunction (SHJ) solar cells and their effect on the device performance are investigated by adopting impedance spectroscopy under dark and light. The negative capacitance/low-frequency inductive loop originates from the depopulation of injected charge carriers due to a transport barrier at the p-a-Si:H/ITO interface. The p-a-Si:H hole-selective SHJ device with a low-frequency inductive loop also has shown an S-shape and associated performance degradation in the light current density–voltage characteristics due to the opposing field type transport barrier present at the p-a-Si:H/ITO interface, which was overcome after vacuum annealing at ∼200 °C. However, the NiO x -based hole-selective contact Ag/ITO/NiO x /i-a-Si:H/c-Si/i-a-Si:H/n-a-Si:H/ITO/Al SHJ cells have not shown any low-frequency inductive loop or corresponding S-shape and associated performance degradation due to the optimised contact (minimum resistance) between the NiOx and ITO layers.

Investigation of nonlinear accelerated degradation mechanism in fuel cell stack under dynamic driving cycles from polarization processes

The nonlinear accelerated degradation severely limits the lifetime of fuel cells. This paper presents a comprehensive analysis of polarization processes during accelerated performance degradation. The double trap model is applied to reveal kinetic degradation, and an equivalent circuit model based on the distribution of relaxation times is proposed to identify and quantify the polarization loss and low-frequency inductive loop. Combined with monitoring dynamic voltage, the results show that the accelerated performance decline is dominated by the growth of platinum oxides rather than kinetic degradation and structural damage. The platinum oxides reduce the cathode mixed potential gradually, leading to the accelerated decay occurring earlier at low current density. Meanwhile, the high oxide coverage hinders oxygen diffusion and water removal, resulting in sudden and drastic concentration polarization in the high current density region. Furthermore, the formation and reduction of platinum oxide affect voltage stability and steady performance of loading and unloading processes due to their potential dependency. The present study helps in modeling and predicting the nonlinear accelerated ageing to improve the lifetime of fuel cells.

On the role of inductive loops at low frequencies in PEM electrolysis

Inductive loops at low frequencies have been observed in the electrochemical impedance spectra (EIS) of various electrochemical cells. Although different physicochemical models for this phenomenon have been suggested in many other applications, this topic has not been widely discussed in the field of proton exchange membrane (PEM) electrolysis.In this article, low-frequency inductive loops in PEM electrolysis cells and their impact on cell performance are analyzed. We show that this phenomenon is reproducible and occurs with different cell materials and setups. Its impact increases with increasing current density and decreasing temperature. At extreme conditions (7 A∙cm−2, 40 °C) we show that the negative polarization resistance of the inductive process can exceed the capacitive polarization processes by a factor of three, resulting in a direct current resistance less than the high frequency series resistance of the cell.

Impedance Response of Electrochemical Interfaces: Part IV─Low-Frequency Inductive Loop for a Single-Electron Reaction

Impedance Response of Electrochemical Interfaces: Part IV─Low-Frequency Inductive Loop for a Single-Electron Reaction

Low-frequency Inductive Loop and Its Origin in the Impedance Spectrum of a Graphite Anode

Low-frequency Inductive Loop and Its Origin in the Impedance Spectrum of a Graphite Anode

Low-frequency Inductive Loop and Its Origin in the Impedance Spectrum of a Graphite Anode

Graphite is a well-known anode material for commercial lithium-ion batteries, and its physical and electrochemical properties have been studied extensively. However, the origin of an inductive loop observed in the low-frequency region of the Nyquist complex plane impedance spectrum of the graphite anode has been widely debated and attributed to contrasting reasons. This paper investigates the impedance spectrum of the graphite anode at various states of charge (SoCs) using three-electrode galvanostatic Electrochemical Impedance Spectroscopy (EIS) and further explores the impedance response of the electrolyte as a function of frequency. The graphite anode EIS measurement displayed an inductive loop in the low-frequency region for almost entire SoCs, irrespective of the solid electrolyte interphase (SEI) age. To study the origin of this inductive loop in the graphite impedance spectrum, we fabricated a three-electrode pouch cell with graphite and NMC electrodes and estimated the electrolyte impedance in the frequency range from 1 MHz to 0.05 Hz. The electrolyte impedance at low frequencies exhibited inductive behavior, indicating a significant role of the electrolyte in the origin of the inductive characteristic in the low-frequency region of the graphite EIS spectrum.

Effect of proton conductivity transients on PEM fuel cell impedance: Formation of a low–frequency inductive loop

A model for PEMFC cathode impedance is developed. The model includes transient response of catalyst layer proton conductivity to cell current density variation. Analytical solution shows that transient conductivity leads to formation of a low–frequency inductive loop in the Nyquist spectrum. This loop has been observed in numerous experiments and its nature has long been discussed in literature. The results of this work provide a new mechanism of loop formation.

Low frequency inductive loop in EIS measurements of an open-cathode polymer electrolyte fuel cell stack. Impedance of water vapour diffusion in the cathode catalyst layer

• Analysis of inductive loops in EIS measurements of an open-cathode PEFC stack. • Impedance model to predict inductive loops in PEFC-EIS measurements. • Inductive loops associated to water vapour diffusion in cathode catalyst layer. • Relation between trend of inductive loops and phase angle in EIS measurements. In this study, an impedance model of a polymer electrolyte fuel cell (PEFC) is considered to simulate inductive loops at low frequencies from electrochemical impedance spectroscopy (EIS) measurements carried out in an open-cathode PEFC stack. The new impedance model proposed in this study is derived from the hypothesis that water vapour transport in the cathode catalyst layer (CCL) yields the formation of inductive loops at low frequencies. The impedance model of the PEFC considers the impedance model of the CCL reported in a previous study and considers an analytical expression for the impedance of water vapour diffusion in the CCL. The developed PEFC impedance model is validated with EIS measurements featuring inductive loops at low frequencies and carried out in the individual cells comprising an open-cathode PEFC stack at two current densities. The parameters related to the impedance of water vapour diffusion simulate the inductive loops at low frequencies and simulate a change of the size of the impedance spectrum of the PEFC. Therefore, the formation of inductive loops at low frequencies can be attributed to not only water vapour transport, but also ionomer swelling/shrinking (hydration/dehydration) in the CCL. The impedance model can provide an insight into the relation between the trend of the inductive loops at low frequency and the change in the phase angle in the EIS measurements which can indicate the hydrated state in the membrane-electrode assembly of the individual cells of the open-cathode PEFC stack.

Electrochemical Characterization of Low-Temperature Direct Ethanol Fuel Cells using Direct and Alternate Current Methods

Here, we report for the first time the results of systematic characterization of a low-temperature polymer electrolyte membrane direct ethanol fuel cell using DC and AC electrochemical methods. Model catalysts (carbon supported Pt nanoparticles) painted on carbon paper are used as anode and cathode. Influence of physical parameters, such as cell temperature, current density, and ethanol concentration, and anode fuel flow rate on overall cell impedance is studied. Analysis of the obtained impedance spectra in connection with DC measurements allows us to comment on cell properties and to separate different contributions to the overall cell polarization. Our results suggest that the cell impedance is dominated by anode faradaic impedance, with a small or negligible contribution from cathode faradaic impedance. The anode impedance depends strongly on current density and cell temperature, but is not significantly influenced by ethanol concentration. Presence of anode mass-transfer impedance, even when ethanol was fed to the cell in high excess, is confirmed. Based on the results, we conclude that changes in ethanol electro-oxidation mechanism might manifest themselves on the impedance spectra in the low-frequency inductive loop. Nonetheless, further studies involving equivalent circuit modelling are needed to determine the exact influence of the cell parameters on the anode kinetics.Graphical

Effect of Pulse Current Mode on Microstructure, Composition and Corrosion Performance of the Coatings Produced by Plasma Electrolytic Oxidation on AZ31 Mg Alloy

Plasma electrolytic oxidation (PEO) coatings were grown on AZ31 Mg alloy in a silicate-based electrolyte containing KF using unipolar and bipolar (usual and soft-sparking) waveforms. The coatings were dual-layered consisting of MgO, MgF2 and Mg2SiO4 phases. Surface morphology of the coatings was a net-like (scaffold) containing a micro-pores network, micro-cracks and granules of oxide compounds. Deep pores were observed in the coating produced by unipolar and usual bipolar waveforms. The soft-sparking eliminated the deep pores and produced the lowest porosity in the coatings. It was found that the corrosion performance of the coatings evaluated using EIS in 3.5 wt. % NaCl solution is mostly determined by the inner layer resistance, because of its higher compactness. After 4 days of immersion, the inner layer resistances were almost the same for all coatings. However, the coatings produced by unipolar and usual bipolar waveforms showed sharp decays in inner layer resistances after 1 week and even the barrier effect of outer layer was lost for the unipolar-produced coating after 3 weeks. The low-frequency inductive loops appeared after a 3-week immersion for all coatings indicated that the substrate was under local corrosion attack. However, both coatings produced by soft-sparking waveforms provided the highest corrosion performance.

Synthesis and characterization of a novel Zn-Ni and Zn-Ni/Si3N4 composite coating by pulse electrodeposition

To explore better or utterly new performance of Zn–Ni alloy for more challenging applications, a cauliflower-like Zn–Ni and a plate-like Zn–Ni/Si3N4 nanocomposite coating was successfully synthesized by pulse electrodeposition. Their morphology, microstructure, surface and electrochemical properties were studied by various methods. The effects of Si3N4 nanoparticles on microstructure and properties of Zn–Ni matrix were evaluated. The results show that the Zn–Ni alloy with cauliflower-like structure contains about 7–8 wt.% Ni including nanocrystal phase. The plate-like Zn–Ni/Si3N4 nanocomposite coating comprises 4–7 wt.% Ni and 1.5–3 wt.% Si3N4 nanoparticles. Si3N4 nanoparticles were uniformly distributed in Zn-Ni matrix. The inclusion of Si3N4 nanoparticles changed the cauliflower-like Zn–Ni to plate-like Zn–Ni/Si3N4 covered by a nanocrystal layer. The Si3N4 nanoparticles could fill crevices and micron holes, enhancing the effect of physical barrier. The Zn–Ni alloy is rougher than Zn–Ni/Si3N4 composite coating. The coating consists of metallic zinc, nickel and an amount of oxidized/hydroxide zinc species. The Nyquist plot is characterized by two capacitive loops and one low-frequency inductive loop, indicating a zinc dissolution process involving adsorption/desorption of the intermediate. The incorporation of Si3N4 nanoparticles in Zn–Ni coating promotes grain refinement, decreases roughness and improves the performance of the coating.

The corrosion characteristic of Mg-3Nd alloy without and with Al addition

The microstructure and corrosion behaviors of Mg-3Nd alloy without and with Al addition were characterized in the present work. The experimental results show that the microstructure of Mg-3Nd alloy is significantly refined by Al addition. There are two kinds of precipitated phases in the alloy with Al addition, which are Al11Nd3 phase and Al2Nd phase. With increasing the Al addition, the Ecorr increases and the Icorr decreases in Mg-3Nd alloy. The average immersion corrosion rate of Mg-3Nd alloy without Al addition is lower than that of Mg-3Nd alloy with 1% and 2% Al, but higher than that of Mg-3Nd alloy with 3% Al. When the Al content was 3%, the corrosion potential is −1.482 V, corrosion current density is 6.93 μA/cm2 and the corrosion rate is 0.69 mg cm−2·day−1. The Mg-3Nd alloy without Al addition have a high frequency and a low frequency capacitive loop. The Mg-3Nd alloy with 1% and 2% Al has only a high frequency capacitive loop. When the Al addition is 3%, there are one high frequency capacitive loop and one low frequency inductive loop. The corrosion mechanism is different for Mg-3Nd alloy with increasing the Al addition.

Mechanical Basis of Nafion’s High-Frequency Inductive Impedance

Recent literature has highlighted how the coupling of electrochemical and mechanical phenomena can affect the way batteries and fuel cells perform [1, 2]. Local volume changes and stresses due to water transport play critical roles in the operation of polymer exchange membranes. A non-isobaric version of Newman’s concentrated-solution theory [3] was applied to study hydrated ionomers, in which mechanical effects intricately entangle the transport of material, momentum, and energy. Electrochemical impedance spectroscopy (EIS) provides a useful tool to study the most pronounced effects arising from this electrochemical/mechanical coupling in Nafion. Nafion is relatively compressible, so changes in pressure affect the concentration of sulfonic acid; thus pressure gradients affect the voltage drop. Also, pressure diffusion can drive proton flux, directly coupling current with mechanical state. When AC signals are driven at low frequencies, these piezoelectric effects are minimal and the Warburg signature of water diffusion dominates the impedance. At higher frequencies, however, current oscillates too fast for the water concentration to respond. Pressure effects dominate the impedance. Strikingly, in this high-frequency regime the membrane can resonate, leading to divergent pressure impedances at a fundamental frequency and its harmonics. This resonance can be related to the speed of sound in the membrane. Above this speed, which is much higher than the characteristic diffusion speed, the signal is carried across the ionomer by elastic (mechanical) waves. Inductive loops on the Nyquist plot, and corresponding peaks in the Bode plots, occur as a consequence of this electrochemical-mechanical coupling above the critical resonant frequency. Various mechanical characteristics can be associated with resistance (viscosity), capacitance (elastic modulus), and inductance (mass). A parametric study reveals how transport and mechanical properties affect the magnitudes and shapes of these impedance signatures. The model rationalizes the inductive responses seen experimentally in several recent high-quality Nyquist plots [4]. [1] A. Kusoglu and A.Z. Weber, Electrochemical/Mechanical Coupling in Ion-Conducting Soft Matter, J. Phys. Chem. Lett., 2015, 6(22), pp 4547-4552 [2] G. Bucci, Formulation of the Coupled Electrochemical-Mechanical Boundary-Value Problem, with Applications to Transport of Multiple Charged Species, Acta Mater., 2016, 104, pp 33-51 [3] P. Goyal and C.W. Monroe, New Foundations of Newman’s Theory for Solid Electrolytes: Thermodynamics and Transient Balances, J. Electrochem. Soc., 2017, 164(11), pp E3647-E3660 [4] B.P. Setzler and T.F. Fuller, A Physics-Based Impedance Model of Proton Exchange Membrane Fuel Cells Exhibiting Low-Frequency Inductive Loops, J. Electrochem. Soc., 2015, 162(6), pp F519-F530

Experimental diagnostics and modeling of inductive phenomena at low frequencies in impedance spectra of proton exchange membrane fuel cells

Representation of fuel cell processes by equivalent circuit models, involving resistance and capacitance elements representing activation losses on both anode and cathode in series with resistance representing ohmic losses, cannot capture and explain the inductive loop that may show up at low frequencies in Nyquist diagram representation of the electrochemical impedance spectra. In an attempt to explain the cause of the low-frequency inductive loop and correlate it with the processes within the fuel cell electrodes, a novel equivalent circuit model of a Proton Exchange Membrane (PEM) fuel cell has been proposed and experimentally verified here in detail. The model takes into account both the anode and the cathode, and has an additional resonant loop on each side, comprising of a resistance, capacitance and inductance in parallel representing the processes within the catalyst layer. Using these additional circuit elements, more accurate and better fits to experimental impedance data in the wide frequency range at different current densities, cell temperatures, humidity of gases, air flow stoichiometries and backpressures were obtained.

Influence of Vanadium on the Corrosion Behavior of High Manganese Steel in 0.5 M H2SO4 Solution

The effect of vanadium on the corrosion behavior of high manganese steel in 0.5 M H?SO? solution has been investigated using scanning electron microscopy, potentiodynamic polarization and impedance spectroscopy. The results have shown that the addition of vanadium to high manganese steel decreases the corrosion current density from 2.1 mA cm¯² to 1.29 mA cm¯². Impedance diagrams show the existence of a high frequency capacitive loop and a low frequency inductive loop. The addition of vanadium increases charge transfer resistance from 5.18 Ω cm² to 12.45 Ω cm².

Understanding the electrochemical behaviour of LSM-based SOFC cathodes. Part II - Mechanistic modelling and physically-based interpretation

This study presents a physically-based model for the impedance simulation of the oxygen reduction reaction in porous strontium-doped lanthanum manganite (LSM) cathodes. The model describes the surface mechanism only, taking into account the co-limited adsorption/diffusion of oxygen and the charge-transfer reaction at the three-phase boundary (TPB). After calibration with experimental impedance spectra, the model is used to identify the transition of kinetic regime from the surface to the bulk path mechanism, which occurs at cathodic dc bias of ca. 0.2V within 700–800°C. The transition is highlighted by a significant decrease in impedance and the appearance of a low-frequency inductive loop. The model consistently reproduces the impedance spectra before the transition of kinetic regime with a single set of parameters, allowing for the deconvolution of two features, one associated with the co-limited adsorption/diffusion process (ca. 5Hz) and another minor contribution due to the charge-transfer at the TPB (ca. 35Hz). The model and its parameters, which quantitatively agree with the literature, can be used as a basis to optimize the microstructural and surface properties of technical LSM-based cathodes, showing that the TPB length is not the main parameter to be maximized.

Transient Relaxations of Ionic Conductance during Growth of Porous Anodic Alumina Films: Electrochemical Impedance Spectroscopy and Current Step Experiments

Transient electrochemical relaxations during growth of anodic oxide films can provide insight into mechanisms of transport processes. Few such studies have focused on porous anodic oxides, which are of significant interest as functional materials. Conductance relaxations during anodizing of aluminum in acidic porous anodic oxide-forming solutions are investigated, using high voltage impedance spectroscopy and constant current reanodizing experiments. Both experiments follow phenomenology demonstrated in prior studies of anodic barrier films. Low-frequency inductive loops in impedance spectra exhibit time constants with inverse proportionality to current density and characteristic resistances nearly equal to those of high-frequency capacitive features. Potential decays during reanodizing depend on charge passed instead of time. Both sets of experiments are analyzed with a model including conductance relaxations due to electric field-dependent generation and removal of bulk charge-carrying defects. The model is found to be quantitatively consistent with nearly all aspects of experimental behavior; the inverse current density dependence of the inductive time constant and the charge parameterization of potential decays both arise because of exponential electric field dependence of ionic conduction and defect generation/removal rates. Oxide volume changes accompanying defect relaxations may help explain mechanical stress generation during anodizing, which is thought to drive pore formation.

Origin of low frequency inductive impedance loops of O2 reduction reaction of solid oxide fuel cells

Inductive impedance loop at low frequencies on electrochemical impedance spectroscopy curves is a common phenomenon in electrochemical reactions such as fuel cells. Here, we study the occurrence of low frequency inductive impedance loops and their evolution for the oxygen reduction reaction on Gd0.2Ce0.8O1.9 infiltrated La0.8Sr0.2MnO3 (GDC-LSM), GDC infiltrated Pt (GDC-Pt) and mixed ionic and electronic conducting (MIEC) La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes under solid oxide fuel cells (SOFCs) operation conditions. The incorporation of GDC nanoparticles (NPs) substantially enhances the electrochemical activity of LSM and Pt electrodes with the concomitant occurrence of an inductive impedance loop at low frequencies. However, the low frequency impedance loop disappears for the GDC-LSM with GDC NPs larger than 41nm or after polarization at 200mAcm−2 for more than 60min. In the case of LSCF electrode the low frequency inductive loop transfers to capacitive arc after polarization for 30min. The occurrence of low frequency inductive loops is closely related to the electrode electrocatalytic activity and microstructure and is primarily determined by the ability of the electrode materials to supply atomic oxygen for the reaction at the interface. A mechanism of competitive atomic oxygen supply and dissociative oxygen adsorption and diffusion is proposed for the occurrence of the inductive loops at low frequencies for the O2 reduction reaction of SOFCs.

Corrosion Behavior of Galvanized Q235 Steel for Grounding in Acid Soils

Electrochemical behavior of galvanized Q235 steel for grounding in acidified soils with different pH value was investigated utilizing electrochemical impedance spectrum (EIS) and poten- tiodynamic polarization tests. Results showed that the corrosion behavior of galvanized Q235 steel in acid soils presents an adsorption of intermediate corrosion products in the early stage, and a low frequency inductive loop then induced in Nyquist plot. With the formation of corrosion products on the surface of the steel, the inductive loop disappeared, the low frequency capacitive arc enlarged gradually. The corrosion process is mainly controlled by the cathodic process. The corrosion rate of the galvanized Q235 steel decreased with the increasing pH of the acidified solution, it also in- creased and then decreased with the increasing time.

A Kinetic Model for the Anodic Dissolution of Ti in HF in the Active and Passive Regions

We investigated the anodic dissolution of polycrystalline titanium rotating disc electrode in 0.1 M hydrofluoric acid in active and passive regions using potentiodynamic polarization and electrochemical impedance spectroscopy. In the active region, complex plane plots of the impedance spectra exhibited three capacitive loops indicting the presence of at least two adsorbed intermediates. In the passive region, they exhibited a negative resistance and a low frequency inductive loop. The high frequency loops exhibit constant phase element behavior indicating that a heterogeneous 2D film with partial surface coverage is present in the active and passive regions. The impedance data was fit to a reaction model and a four step mechanism with two adsorbed intermediate species is proposed to explain the observed trends in the active and passive regions. The change in the surface coverage of the adsorbed intermediate species, with the overpotential is estimated. This model describes the dissolution of Ti via two parallel paths, viz. a chemical step and an electrochemical step. The onset of passivation ensues when the rate of TiO2 film formation is more than that of its dissolution. Transpassive dissolution becomes dominant when the electrochemical dissolution from the bare surface becomes negligible.