Impedance spectroscopy on porous materials: A general model and application to graphite electrodes of lithium-ion batteries

Electrochemical impedance spectroscopy (EIS) was applied to porous negative graphite electrodes for lithium-ion batteries in the EC:DMC, 1 M LiPF₆ electrolyte. The effect of porosity on the electrode response time was studied and a theoretical model was developed, based on free path of the current lines between subsequent reaction sites. The effect of porosity on the electrode response is evidenced by the impedance spectra in which the high frequency capacitive semicircle is distorted. Fresh electrodes (before the formation of the solid electrolyte interphase, SEI) and cycled electrodes have different shapes of the impedance spectra indicating a change of processes at the surface. In particular, the shape of the spectrum for a fresh electrode can be related to an adsorption process. Impedance spectra of fresh electrodes were fitted using a simple model that considers porosity and the assumed electrochemical processes, giving good agreement between model and data. A correlation was found between adsorption sites and irreversible charge capacity in the first cycle.     

Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy

The most common methods used to characterize the electrochemical performance of fuel cells are to record current–voltage U(i) curves. However, separation of electrochemical and ohmic contributions to the U(i) characteristics requires additional experimental techniques. The application of electrochemical impedance spectra (EIS) is an approach to determine parameters which have proved to be indispensable for the development of fuel cell electrodes and membrane electrode assemblies (MEAs). This paper proves that it is possible to split the cell impedance into electrode impedances and electrolyte resistance by varying the operating conditions of the fuel cell (current load) and by simulation of the measured EIS with an equivalent circuit. Furthermore, integration in the current density domain of the individual impedance elements enables the calculation of the individual overpotentials in the fuel cell and the determination of the voltage loss fractions.     

Electrochemical characteristics of La0.6Sr0.4Co1−yFeyO3 (y=0.2–1.0) fiber cathodes

Electrospun La_xSr_1−xCo_1−yFe_yO₃ (LSCF) fibers with y=0.2 – 1.0 have been investigated as the cathode of intermediate solid oxide fuel cells (IT-SOFC). The electrochemical performances of LSCF (y=0.2–1.0) fibers were studied by impedance spectroscopy in symmetrical cells containing gadolinium doped ceria (CGO) electrolyte and LSCF electrode infiltrated with CGO. Impedance measurements showed that the impedance spectra have two or three semicircles, depending on the measurement temperature. The LSCF electrodes with higher cobalt content exhibit lower polarization resistance (R_p) and the La_0.6Sr_0.4Co_0.8Fe_0.2O₃ electrode displayed the lowest polarization resistance between 500 and 900 °C, classifying this composite cathode as a promising material for intermediate temperature SOFC based on CGO electrolyte.     

EIS-assisted performance analysis of non-noble metal electrocatalyst (Fe-N/C)-based PEM fuel cells in the temperature range of 23-80 °C

A carbon-supported non-noble metal catalyst, Fe-N/C, was used as the cathode catalyst to construct membrane electrolyte assemblies (MEAs) for a proton exchange membrane (PEM) fuel cell. The performance of such a fuel cell was then tested and diagnosed using electrochemical impedance spectroscopy (EIS) in the temperature range of 23-80 °C. Based on the EIS measurements, individual resistances, such as charger transfer resistance and membrane resistance, were obtained and used to simulate polarization curves (current-voltage (I-V) curves). A close agreement between the simulated I-V curves and the measured curves demonstrates consistency between the polarization and EIS data. The temperature-dependent parameters obtained via EIS, such as apparent exchange current densities and electrolyte membrane conductivities, were also used to acquire activation energies for both the oxygen reduction reaction (ORR) catalyzed by an Fe-N/C catalyst and the proton transport process across the electrolyte membrane. In addition, the maximum power densities for such a fuel cell were also analyzed.     

Fabrication and Performance of Different Structural Cathode Catalyst Layers by EHDA Deposition for DMFC

In this work, electrohydrodynamic atomization Layer‐by‐Layer deposition was used to deposit cathode catalyst layers (CLs) at different working distances of 3, 5, and 7 mm. The influence of working distance on the structural characteristics of cathode CLs was analyzed. The cyclic voltammograms of the cathode electrodes with different structures and the performance of the assembled membrane‐electrode assemblies (MEAs) were examined. It was observed that the cathode CLs presented well‐packed and porous features. The dispersity of the deposited catalyst and the thickness of cathode CL increased with higher working distance, which resulted in larger electrochemical active surface area (ESA), higher performance of the assembled MEAs and higher catalyst utilization. The ESA increased by approximately 70% when the cathode CL produced at the working distance of 7 mm compared with that at 3 mm. The peak power density of 56.1 mW cm^–2 and the peak cathode catalyst specific power of 140.3 mW mg^–1 Pt were obtained when the cathode CLs produced at the working distance of 7 mm.     

Electrochemical behavior of high-pressure synthetic boron doped diamond powder electrodes

Boron doped diamond (BDD) was synthesized under high pressure and high temperature using B-doped graphite intercalation compositions (GICs) as carbon sources. The electrochemical characteristics of high-pressure synthetic BDD powder electrodes were investigated by measuring the cyclic voltammetry curves and AC impedance spectrum. For the [Fe(CN)₆]^3−/4− redox couple, the electrode reaction process is reversible or quasi-reversible at the scan rates of 0.01-1.0 V/s. At the low scan rate the linear relation between peak current and square root of scan rate indicates that the electrode process was a diffusion-controlled mass transport process. The electrochemical behavior is similar to a planar electrode. With the increasing of the scan rate the electrode process is controlled by the mass transport plus kinetic process. AC impedance spectra exhibit the porous structure characteristic of BDD powder electrode.     

Pore‐forming technology and performance of MEA for direct methanol fuel cells

BACKGROUND: The commercialization of DMFCs is seriously restricted by its relatively low power density. Lots of work has been concentrated on catalysts with high activity, the optimization of flow path design, development of new kinds of proton exchange membrane and modification of Nafion membrane. Meanwhile, very few reports have involved the structure optimization of the membrane electrode assembly (MEA). To improve the performance of direct methanol fuel cells (DMFCs), the catalyst layer (CL) structures of anode and cathode were optimized by utilizing ammonium carbonate as pore forming agent. RESULTS: The polarization curves showed that in catalyst slurry the optimal content of ammonium carbonate was 50 wt%, and the DMFC performance was enhanced from 75.65 mW cm^?2 to 167.42 mW cm^?2 at 55 °C and 0.2 MPa O₂. Electrochemical impedance spectroscopy and electrochemical active surface area (EASA) testing revealed that the improved performance of optimized MEAs could be mainly attributed to the increasing EASA and the enhanced mass transfer rate of CLs. But poor methanol crossover limited the performance enhancement of MEAs with porous anodes. CONCLUSION: With regard to improving cell performance, this pore‐forming technology is better applied to the cathode catalyst layer to improve its structure rather than the anode catalyst layer. © 2012 Society of Chemical Industry     

Characteristics and performance of membrane electrode assemblies with operating conditions in polymer electrolyte membrane fuel cell

The degradation behavior of a membrane-electrode assembly (MEA) was investigated in accelerated degradation tests under constant voltage (0.8 V and 0.7 V) and load cycling (from open circuit voltage to 0.35 V) conditions. Changes in the structural and electrochemical characteristics of MEA after the durability tests give information as to the degradation mechanism of MEAs. The results of cyclic voltammogram and postmortem analysis by X-ray diffraction and high resolution-transmission electron microscopy indicate that the cathode catalyst layers of the MEAs showed no extreme degradation under constant voltage mode, whereas MEAs under repetition of load cycling mode showed very severe degradation after 280 h. However, the single cell performance of the MEA under repetition of load cycling mode was higher than under constant voltage mode. In addition, although the Pt band in the membrane of the MEA under repetition of load cycling mode was observed by field emission scanning electron microscopy, it did not affect the ohmic resistance.     

A study on the effect of compositing silver oxide nanoparticles by carbon on the ‎electrochemical behavior and electronic properties of zinc‐silver oxide batteries ‎

In this study, the effect of compositing silver oxide nanoparticles by carbon on the electrochemical behavior and electronic properties of zinc‐silver oxide batteries have been investigated. For this purpose, firstly four silver oxide electrodes containing 5, 10, 15, and 20 wt% carbon powder were produced by powder metallurgy method. For the next step, all four silver oxide electrodes were sintered at 500°C for 10 minutes. Afterward and in order to investigate the microstructure, phase and elemental analysis of the electrodes were carried out using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X‐ray Diffraction (XRD), and Energy Dispersive Spectroscopy (EDS), respectively. Moreover, in order to investigate the effect of compositing silver oxide nanoparticles by carbon on the electrochemical behavior and electronic properties of zinc‐silver oxide, electrochemical tests (potentiodynamic polarization and electrochemical impedance spectroscopy) and electric discharge test in 1.4 wt%KOH electrolyte were carried out respectively. The microstructural observations revealed that increasing carbon content in the silver oxide electrodes results in increasing the apparent porosities in these electrodes. Investigating the phase and elemental analysis results showed that by increasing the content of carbon in the silver oxide electrode, the amount of Ag₂O and AgO phases in this electrode reduces and also the extent of pure silver formation increases. Investigations on the results of electrochemical tests showed that increasing carbon content results in the reduction of corrosion resistance in silver oxide electrodes. Moreover, the results of electric discharge test revealed that the silver oxide electrode containing 10wt% carbon yields the highest energy efficiency in the zinc‐silver oxide batteries.     

Investigation of the Ethanol Electro‐Oxidation in Alkaline Membrane Electrode Assembly by Differential Electrochemical Mass Spectrometry

The fuel cell differential electrochemical mass spectrometry (FC‐DEMS) measurements were performed for studying the ethanol oxidation reaction (EOR), using alkaline membrane electrode assemblies (MEAs) made up of nanoparticle Pt catalyst and alkaline polymeric membranes. The obtained results indicate that in an alkaline medium, ethanol undergoes significantly more complete electro‐oxidation to CO₂ than in an acidic MEA using the same Pt anode. The CO₂ current efficiency (CCE) can be compared for acidic and alkaline MEA with similar electrochemical active area on the anode side. The CCE estimated, in case of alkaline MEA with Pt anode, is around 55% at 0.8 V/RHE, 60 °C and 0.1 M ethanol. In comparison, under similar conditions, acidic MEAs using the same anode catalyst show only 2% CCE. This might indicate that the C–C bond scission rates are much higher in alkaline media. However, the mechanism of ethanol oxidation in alkaline media is not exactly known. CO₂ produced in electrochemical reaction forms soluble carbonates in the presence of aqueous alkaline electrolyte. This makes it difficult to study ethanol oxidation in alkaline media using FTIR or model DEMS systems. The alkaline polymer electrolyte membranes as used in this study for making alkaline MEAs provide an important opportunity to observe CO₂ produced during EOR using FC‐DEMS system.     

Multi-variable optimization of PEMFC cathodes using an agglomerate model

A comprehensive numerical framework for cathode electrode design is presented and applied to predict the catalyst layer and the gas diffusion layer parameters that lead to an optimal electrode performance at different operating conditions. The design and optimization framework couples an agglomerate cathode catalyst layer model to a numerical gradient-based optimization algorithm. The set of optimal parameters is obtained by solving a multi-variable optimization problem. The parameters are the catalyst layer platinum loading, platinum to carbon ratio, amount of electrolyte in the agglomerate and the gas diffusion layer porosity. The results show that the optimal catalyst layer composition and gas diffusion layer porosity depend on operating conditions. At low current densities, performance is mainly improved by increasing platinum loading to values above 1 mg cm⁻², moderate values of electrolyte volume fraction, 0.5, and low porosity, 0.1. At higher current densities, performance is improved by reducing the platinum loading to values below 0.35 mg cm⁻² and increasing both electrolyte volume fraction, 0.55, and porosity 0.32. The underlying improvements due to the optimized compositions are analyzed in terms of the spatial distribution of the various overpotentials, and the effect of the agglomerate structure parameters (radius and electrolyte film) are investigated. The paper closes with a discussion of the optimized composition obtained in this study in the context of available experimental data. The analysis suggests that reducing the solid phase volume fraction inside the catalyst layer might lead to improved electrode performance.     

固体电解质膜对PEMFC电极动力学的影响

研究了不同种类的固体电解质膜对质子交换膜燃料电池性能的影响,采用相同组成的气体扩散电极与制作工艺,分别用Nafion 111、Nafion 112、Nafion 1135、Nafion 115、Dow 800固体电解质膜制成MEA,并组装成单电池,用极化曲线法与交流阻抗法研究了单电池的极化行为与电气特性,并用zsimwin软件模拟了电气特性参数。结果表明,随着固体电解质膜变薄,电池内阻变小,但电池开路电压却反而降低,固体电解质膜的厚度较大(如Nafion 115)或较小(如Nafion 111)都会使电极双电层微分电容变小,使电极电化学反应动力学变差,离子交换容量大的电解质膜电导率大,电极电荷传递阻力较小,反应阻抗小。     

Effects of MEA fabrication method on durability of polymer electrolyte membrane fuel cells

To study the effects of fabrication methods on the durability of polymer electrolyte membrane fuel cells (PEMFCs), membrane-electrode assemblies (MEAs) were fabricated using a conventional method, a catalyst-coated membrane (CCM) method, and a CCM-hot pressed method. Single cells assembled with the prepared MEAs were operated galvanostatically at 600 mA cm⁻² for 1000 h for the conventional MEA and the CCM MEA and for 500 h for the CCM-hot pressed MEA. During operation, i-V curves, impedance spectra, and cyclic voltammograms were measured roughly every 100 h. Before and after long-term operation, the physical and chemical characteristics of the MEAs were analyzed using mercury porosimetry, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and Fourier transformation infrared spectroscopy (FTIR). Under the operating conditions, the CCM MEA exhibited the lowest degradation rate as well as the highest initial performance.     

Impedance studies of copper foil and graphite-coated copper foil electrodes in lithium-ion battery electrolyte

The electrochemical impedance spectroscopy of battery grade copper foil and graphite-coated copper foil electrodes in 1 M LiPF₆ in a ternary organic carbonate electrolyte has been obtained. Detailed studies showed that both electrodes gave similar impedance spectra of two successive semicircular arcs. When overpotential was increased for both electrodes, the high frequency semicircles remained the same on each electrode, but the second semicircle increased for both electrodes. Based on the impedance spectra results of the electrodes, it appeared that the high frequency response represented a surface oxidation layer. At low frequency further oxidation occurs at both electrodes, but is kinetically controlled for bare copper, while the graphite-coated copper undergoes diffusional blocking through the porous carbon layer. An equivalent circuit of the impedance spectrum was then proposed.     

Impedance study of the ion-to-electron transduction process for carbon cloth as solid-contact material in potentiometric ion sensors

Carbon cloth was studied as solid-contact material in potentiometric ion sensors by using electrochemical impedance spectroscopy and potentiometry. The ion-to-electron transduction process was studied by electrochemical impedance spectroscopy by using a two-electrode symmetrical cell where a liquid electrolyte was sandwiched between two solid electrodes, including bare glassy carbon (GC), GC/carbon cloth and GC/poly(3,4-ethylenedioxythiophene). Impedance data for different electrode/electrolyte combinations were evaluated and compared. Solid-contact K⁺-selective electrodes were fabricated by coating the carbon cloth with a conventional plasticized PVC-based K⁺-selective membrane via drop casting. These K⁺-sensors showed proper analytical performance and acceptable long-term potential stability (potential drift ≈ 1 mV/day). Solid contact reference electrodes were fabricated in an analogous manner by coating the carbon cloth with a plasticized PVC membrane containing a moderately lipophilic salt. The results indicate that carbon cloth can be used as a solid-contact material in potentiometric ion sensors and pseudo-reference electrodes.     

The impedance of fast charge transfer reactions on boron doped diamond electrodes

Reversible charge transfer on boron doped diamond (BDD) electrodes was studied using cyclic voltammetry and electrochemical impedance spectroscopy. Polycrystalline diamond films of 5 μm thickness with 200 and 3000 ppm boron content were prepared by chemical vapour deposition on niobium substrate. The samples were mounted in a Teflon holder and used as rotating disk electrodes (RDE) with rotation frequencies between 0 and 4000 rpm. The electrochemical measurements were carried out in aqueous electrolyte solutions of 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] and 0.1 M KCl + 5 mM [Ru(NH₃)₆]Cl₂/[Ru(NH₃)₆]Cl₃. The electrochemical redox behaviour of the BDD electrodes was found to differ significantly from that of an active Pt electrode. The deviations are indicated by a large peak potential difference and a shift of the peak potentials in cyclic voltammograms with increasing sweep rate. At rotating electrodes lower limiting current densities are found and the impedance diagrams exhibit an additional capacitive impedance element at high frequencies. The results are described quantitatively by an impedance model which is based on partial blocking of the diamond surface.     

Study of the storage performance of a Li-ion cell at elevated temperature

A series of Li-ion cells with a LiCoO₂ cathode, artificial graphite anode and a LiPF₆-based nonaqueous electrolyte were stored at 55 °C in a series of state of charge (SoC) from 0 to 100%. After storage, all the cells except the one stored in 0% SoC exhibited capacity fade and cycling performance decline, which were aggravated by increasing storage SoC. Furthermore, storage at higher SoC increased the safety risk of the cells. The cells stored at SoC higher than 50% could not pass the 3 C/5 V overcharge test, while such a test was easy to pass for the fresh cells and those stored at 0% SoC. The above results show that the fully discharged state is a favorable storage condition to maintain good storage performance of Li-ion cells. In addition, to clarify the aging mechanisms of the cells, XRD (X-ray diffraction), SEM (scanning electron microscopy) and EIS (electrochemical impedance spectra) measurements were carried out. The results indicate that the performance fading of the stored cells is not due to the bulk structure change of the electrode materials, but instead due to the microstructure variation of the cathode, including the decrease in the crystallite dimension, the change of the micro-stress, and the precipitation of the surface films over the electrodes. According to EIS analysis, the increase of the cathode impedance may be the main contributor to the overall degradation of the Li-ion cells after storage.     

Effects of electrolytic composition on the electric double-layer capacitance at smooth-surface carbon electrodes in organic media

As a fundamental research on the optimization of electrolyte composition in practical electrochemical capacitor device, double-layer capacitance at Glassy Carbon (GC) and Boron-doped Diamond (BDD), as typical smooth-surface carbon electrodes, has been studied as a function of the electrolyte composition in organic media. Specific capacitance (differential capacitance: F cm⁻²) determined by an AC impedance method, in which no contribution of mass-transport effects is included, corresponded well to integrated capacitance evaluated by conventional cyclic voltammetry. The specific capacitance at the GC electrode varied with polarized potential and showed clear PZC (potential of zero charge), while the potential dependence of the capacitance at BDD was very small. The effects of the solvent and the electrolytic salt on the capacitance behavior were common for both electrodes. That is, the sizes of the solvent molecule and the electrolytic ion (cation) strongly affected the capacitance at these smooth-surface carbon electrodes.     

Charge storage mechanism of sonochemically prepared MnO2 as supercapacitor electrode: Effects of physisorbed water and proton conduction

The charge storage mechanism of nanostructured hydrated manganese dioxide, as a supercapacitor electrode, was investigated with respect to the role of amount of hydrates on the electrolyte cations diffusion. The MnO₂ materials (γ- and layered types), prepared by a novel ultrasonic aided procedure. Thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy were employed to characterize the water content of the samples. The samples then were heat-treated in air for 2 h at 70, 100 and 150 °C to prepare different γ- and L-series of electrodes with various amounts of hydrates. To determine the role of the protonic conduction on the charge storage mechanism, the electrochemical properties of the electrodes were investigated in two different electrolyte pH values of 3.3 and 6.Compared to γ25, the higher specific capacitance of L25, especially in more acidic electrolytes, is attributed to the higher amount of physically adsorbed water molecules and their contribution in diffusion process. Furthermore, it is clearly demonstrated that the total electrochemical performance of the systems under consideration is also influenced by the crystalline structure of the prepared electrodes, especially when the size of the tunnels limits the intercalation of cations.Analyzing the results of cyclic voltammetry and electrochemical impedance spectroscopy for both series of the electrodes, revealed that, increasing the heat-treatment temperature makes the charge-transfer resistance increase and the Warburg impedance decrease. This effect can be attributed to the more amount of surface physisorbed water lost by the higher heat-treatment temperatures.     

Model‐Based Ex Situ Diagnostics of Water Fluxes in Catalyst Layers of Polymer Electrolyte Fuel Cells

The ability to predict the electrochemical performance of the cathode catalyst layer in a polymer electrolyte fuel cell hinges on a precise knowledge of water distribution and fluxes. Water transport mechanisms that must be accounted for include vapor diffusion, liquid water permeation and vaporization exchange. In order to facilitate experimental efforts to this effect, we propose an ex situ model of water fluxes in catalyst layers. The model formulation is similar to transmission line models that are widely used in the analysis of electrochemical impedance spectra of porous composite electrodes. Focusing in this article on steady state and isothermal conditions, we rationalize the response function between defined environmental conditions, i.e. gas pressures, partial vapor pressures and temperature, which are defined at the boundaries of the catalyst layer, and the net water flux. This response function provides diagnostic capabilities to isolate and extract water transport parameters of catalyst layers from measurements of water fluxes through membrane electrode assemblies or half cell systems. An important asset of the model is the ability to analyze catalyst layer transport properties under partial saturation.