Time Constants Of Processes Research Articles

Investigating the Impedance of an Iron/Iron Redox Flow Battery at Different State of Charge Conditions – a Distribution of Relaxation Times Analysis

New energy storage technologies are needed to offset the erratic power supply of renewable energy sources due to the rising global energy demand and the urgency of climate change action. The Iron/Iron redox flow battery is a viable contender for large-scale energy storage, even if there are still issues with operational parameters and efficiency. Due to the positive electrode's moderate pH and low voltage, which preserves minimum toxicity and aggressivity, iron is a very affordable active material, making it feasible to develop inexpensive energy storage systems and batteries. The Electrochemical impedance spectroscopy technique was utilized to understand the degradation of the iron/iron redox flow battery during charging and discharging operation modes. However, discriminating between processes occurring at similar time scales is difficult since they overlap in the generally used Nyquist or Bode representation. Distribution of relaxation times (DRT) analysis tackles this issue by converting the EIS data with respect to the time constants of the individual processes. Individual processes such as electrochemical reactions inside the iron/iron redox flow cell, transport processes through porous structures, and iron ion transport were investigated.The impedance measurements were carried out to evaluate the ohmic resistance during the charging and discharging phases of the iron/iron redox flow battery with varied states of charge ranging from 0-100% while charging and 100-0% while discharging. In this series, a DC offset load of 100 mA in the charging and discharging directions was used to test the resistance differential associated with the battery's state of charge. In the charging phase of the battery of three DC offset load parameters, the iron ions were transported faster in the process while the state of charge of a battery increased at lower frequencies. The impedance increased when the state of charge decreased at a higher frequency range, involving the iron ions transported through the negative half-cell in an iron/iron redox flow battery. The overall ohmic resistance in the charging phase of the battery decreased up to 6 % until the battery was fully charged, but in discharging phase, the overall ohmic resistance increased to 2 %. This indicated that the DC offset load in the charging and discharging direction were dependent on the battery's state of charge. Figure 1

Investigation of the degradation of proton exchange membrane water electrolysis cells using electrochemical impedance spectroscopy with distribution of relaxation times analysis

Reduction of catalyst loading on Proton Exchange Membrane (PEM) Electrolysis Cells (EC) is needed because of scarcity and high price of the catalyst. Monitoring the degradation of these cells is important to be able to allocate the different processes taking place. Electrochemical Impedance Spectroscopy (EIS) might be suitable for this purpose, but in its usual implementation, the system and the contribution of its components to the total resistance must be already known to build a suitable equivalent circuit model (ECM). Furthermore, the overlap of different processes in the Nyquist and Bode plots hinders the identification of single components of the system. The Distribution of Relaxation Times (DRT) analysis converts the EIS data into a distribution of time constants of individual processes. The contribution of each component to the full cell resistance can be identified by varying different operation parameters. In this work, a PEMEC with low iridium loading was analyzed by DRT and the faradaic processes were identified. A long-term test was carried out and the degradation of the cell was investigated by DRT analysis, to determine the components and processes which limit the cell performance.

Photophysical Properties of Homo- and Hetero-Aggregate Assemblies Made of N-Annulated Perylene Derivatives.

We report the absorption spectra and photophysical properties of homo and hetero-aggregate assemblies of a strongly emissive N-annulated perylene dye (P) and of a dyad made of P and a methyl viologen derivative (P-MV), in ethanol-water solutions. In homo-aggregate assemblies of P, the π-π* fluorescence of the isolated chromophore is replaced by excimer emission at lower energy, with a lifetime of 900 ps, due to excimer formation from the initially prepared excitons. In homo-aggregate assemblies of P-MV, photoinduced charge separation, with formation of P+ -MV- species, occurs in 3 ps with a charge recombination of 20 ps. In hetero-aggregate P/P-MV systems, the light energy absorbed by the P components delocalizes over various P subunits, and when a P-MV unit is reached, charge separation occurs; however, excimer emission is present for P/P-MV ratio larger than 3 : 1, indicating that delocalized excitons within the hetero-aggregate systems extend over a limited number of P chromophores.

Identifying Non-Stationarity in Non-Linear Harmonics of EIS Measurements of Li Batteries

Non-linear Harmonic Analysis is a technique used to investigate the distortions in Electrochemical Impedance Spectroscopy (EIS) measurements. The distortions are in both non-linear and non-stationary nature that appear as high order harmonics in the frequency domain. Differentiating the origin of the harmonics is important in understanding their nature and relating them to electrochemical phenomena. Here, we will show how the harmonics are in direct relation to the initial non-stationary response (initial transient) of the excited system [1]. We will show by both experimental measurements and simulations of dummy cell circuits, representing the electrochemical interface, that the initial transient exhibits high order harmonics that show the highest value at the characteristic frequency of the process [1]. We also extend the analysis to include more complex systems of primary Lithium batteries [2]. It is observed that the same non-stationary initial transient behavior is persistent in the time constants of the electrochemical processes. It is concluded that the capacitive components of the electrochemical processes are responsible for such non-stationary behavior in electrochemical systems [2].[1] G. Katırcı, M. A. Zabara, and B. Ülgüt, “Methods—Unexpected Effects in Galvanostatic EIS of Randles’ Cells: Initial Transients and Harmonics Generated,” J Electrochem Soc, vol. 169, no. 3, p. 030527, Mar. 2022, doi: 10.1149/1945-7111/ac5ad7.[2] M. A. Zabara, G. Katırcı, and B. Ülgüt, “Non-linear harmonics in EIS of batteries with lithium anodes: Proper controls and analysis,” Electrochim Acta, vol. 429, Oct. 2022, doi: 10.1016/j.electacta.2022.140969.

Impact of xenon doping in the scintillation light in a large liquid-argon TPC

The use of xenon-doped liquid argon (Xe-doped LAr) is a promising alternative for large-scale liquid argon Time Projection Chambers (LAr-TPC), since it mitigates the light suppression due to impurities and it also improves the photon-detection efficiency and uniformity with the distance. This study analyses the impact of using Xe-doped LAr in ProtoDUNE Dual-Phase, a 750 ton Dual-Phase LAr-TPC placed at CERN. ProtoDUNE Dual-Phase completed a Xe-doping data-taking campaign in summer 2020 by re-filling the detector with 230 tons of Xe-doped LAr contaminated with nitrogen, and performing dedicated nitrogen injections. The effects of the presence of Xe at 5.8 ppm in the scintillation light production and propagation are analysed in this paper, showing an increase of the collected photons, but a suppression of the light signal amplitude. A 60% increase of the light attenuation length is measured. The impact on the scintillation time profile is also studied. A model to fit the time profile is proposed and the time constants of the physics processes are obtained.

Quantifying Percolated Triple Phase Boundary Density and Its Effects on Anodic Polarization in Ni-Infiltrated Ni/YSZ SOFC Anodes

Increasing the density of percolated triple phase boundaries (TPBs) by infiltrating nanoscale electrocatalysts can improve the performance of solid oxide fuel cell (SOFC) anodes. However, the complex microstructure of these infiltrated nanocatalysts creates challenges in quantifying their role in anode performance improvements. In this research, scanning electron microscopy of fractured cross-sections of a Ni-nanocatalyst infiltrated anodic symmetric cell along with three-dimensional (3-D) reconstruction of the same anode have been used to quantify the changes in percolated TPB densities due to infiltration. This change in percolated TPB density has been compared to the improvement in anode activation polarization resistance measured by electrochemical impedance spectroscopy (EIS). It was found that increased TPB densities only partially accounted for the measured performance improvement. Distribution of relaxation times (DRT) analyses showed that a reduction in the time constants of the catalytic processes in the anode also play a role, suggesting that the added nanoscale percolated TPB boundaries are more electrochemically active as compared to the cermet TPB boundaries.

Combining the Distribution of Relaxation Times from EIS and Time-Domain Data for Parameterizing Equivalent Circuit Models of Lithium-Ion Batteries

Equivalent circuit models (ECMs) are a widely used modeling approach for lithium-ion batteries in engineering applications. The RC elements, which display the dynamic loss processes of the cell, are usually parameterized by fitting the ECM to experimental data in either the time-domain or the frequency-domain. However, both types of data have limitations with regard to the observable time constants of electrochemical processes. This work proposes a method to combine time-domain and frequency-domain measurement data for parameterization of RC elements by exploiting the full potential of the distribution of relaxation times (DRT). Instead of using only partial information from the DRT to supplement a conventional fitting algorithm, we determine the parameters of an arbitrary number of RC elements directly from the DRT. The difficulties of automated deconvolution of the DRT, including regularization and the choice of an optimal regularization factor, is tackled by using the L-curve criterion for optimized calculation of the DRT via Tikhonov regularization. Three different approaches to merge time- and frequency-domain data are presented, including a novel approach where the DRT is simultaneously calculated from electrochemical impedance spectoscropy (EIS) and pulse relaxation measurements. The parameterized model for a commercial 18650 NCA cell was validated during a validation cycle consisting of constant current and real-world automotive cycling and yields a relative improvement of over 40% compared to a conventional EIS-fitting algorithm.

Determination of time constants of diffusion and electrochemical processes in Polymer Electrolyte Membrane Fuel Cells

This work presents an experimental analysis of the time constants associated to diffusion and electrochemical processes in a 50 cm2 Polymer Electrolyte Membrane (PEM) fuel cell. The experimental techniques and results include polarization curves and Electrochemical Impedance Spectroscopy (EIS) analysis of the fuel cell, where the time constants are determined from the analysis of the Distribution of Relaxation Times (DRT). EIS results are also used to determine the cell ohmic resistance, where High Frequency Resistance (HFR) values are calculated from the Nyquist plots. A wide range of operating conditions of the fuel cell are analysed, including back pressure (0.5 bar–1 bar), cell temperature (55 °C, 65 °C, 75 °C), reactant gases relative humidity (30%, 60%, 90%), cathode stoichiometry (λc 2.5–3.5), and oxygen concentration (air and pure oxygen). The effect of the operating conditions on the time constants are discussed, and Damköhler number is introduced and discussed.

Comparative analysis of isothermal decay of the surface potential of fluoroethylenepropylene electrets and of the sensitivity of electret microphones at elevated temperature

It is well known that fluorocarbon electrets, although thermally very stable, suffer a surface potential decay if exposed to high temperatures. This potential decay is of considerable interest in applications, for example, in the so-called prepolarized microphones. As a result, these devices suffer a loss in sensitivity approximately proportional to the decay of the electret surface potential. Since the potential and the related sensitivity losses are very slow at room temperature, a common approach in the literature is to perform accelerated isothermal depolarization experiments at elevated temperatures, and extrapolate the results to lower temperatures by assuming an Arrhenius-type behavior. In this paper, we investigate experimentally the potential decay of differently pre-annealed fluoroethylenepropylene electrets of different thicknesses, as well as the drop of sensitivity of commercially available measurement microphones from several manufacturers by the exposure to an ambient temperature of 95 °C for up to three years. Until now, no other reports compare electret and microphone decays over such a long period. The experimental data presented here could not be fitted with only one exponential decay function over the whole time-span investigated. However, assuming two or more discharge processes results in a good agreement between measurement and model functions. The time constants of these decay processes are specified in the text.

Deconvolution of electrochemical impedance data for the monitoring of electrode degradation in VRFB

Understanding degradation phenomena occurring during the operation of vanadium redox-flow batteries (VRFB) requires a measurement technique which allows for differentiating the overall performance losses into individual performance losses of the cell components. For this purpose, electrochemical impedance spectroscopy (EIS) is a valuable and well established tool. However, the discrimination of processes taking place at similar time scales is challenging since they overlap in the commonly used Nyquist or Bode representation. Distribution of relaxation times (DRT) analysis tackles this issue by deconvoluting EIS data with respect to the time constants of the individual processes. It hence circumvents the necessity of finding a suitable equivalent circuit model and thus allows for data evaluation without any a-priori knowledge of the system under study. For the first time, we herein present the application of DRT transform to EIS data of a VRFB. By varying experimental conditions and employing full cell as well as double half cell operational modes, we are able to identify the faradaic process of the negative half cell. This enables us to visualize the negative half cell’s contribution to the overall impedance of a VRFB even in a full cell EIS measurement. By an accelerated degradation experiment we finally demonstrate the great potential of DRT analysis for future application in the monitoring of electrode degradation in VRFB.

Direct determination of mode-projected electron-phonon coupling in the time domain.

Ultrafast spectroscopies have become an important tool for elucidating the microscopic description and dynamical properties of quantum materials. In particular, by tracking the dynamics of nonthermal electrons, a material's dominant scattering processes can be revealed. Here, we present a method for extracting the electron-phonon coupling strength in the time domain, using time- and angle-resolved photoemission spectroscopy (TR-ARPES). This method is demonstrated in graphite, where we investigate the dynamics of photoinjected electrons at the [Formula: see text] point, detecting quantized energy-loss processes that correspond to the emission of strongly coupled optical phonons. We show that the observed characteristic time scale for spectral weight transfer mediated by phonon-scattering processes allows for the direct quantitative extraction of electron-phonon matrix elements for specific modes.

A timely look into electron-phonon coupling

Solid-State Physics The coupling between electrons and phonons—lattice vibrations in solids—is responsible for macroscopic quantum phenomena such as superconductivity. Yet, experimentally measuring this coupling as a function of momentum and for a particular phonon mode is tricky. Na et al. used time- and angle-resolved photoemission spectroscopy to excite electrons in graphite and monitor their decay, which was accompanied by the release of phonons. The time constants of these decay processes provided direct information on electron-phonon couplings in this system. Science , this issue p. [1231][1] [1]: /lookup/doi/10.1126/science.aaw1662

What Is the Rate Limiting Mechanism in Solid-State Lithium Cells at Different Pulse Operating Conditions?

Solid-state lithium battery could reduce the safety concerns due to thermal runaway in conventional lithium-ion batteries. As lithium has a higher gravimetric and volumetric capacity compared to graphite, the use of lithium metal as the negative electrode in the solid-state lithium batteries could also improve the cell gravimetric and volumetric energy density. In practical use, batteries are often charged and discharged with a dynamic current profile of different C-rates and durations. Understanding the different bottlenecks limiting the cell performance under realistic operational conditions such as pulse current profiles could accelerate the process of cell design for solid-state lithium batteries. This study focuses on the electrochemical modelling of the solid-state lithium cells based on Li|LiPON|LiCoO2 cell configuration, in which most of the model parameters are estimated from experimental measurements. By calculating the time constants of the electrochemical processes, which are derived from the distribution relaxation time analysis, we show that ionic conductance and charge-transfer kinetics in solid-state lithium batteries happen at the millisecond timescale. The in-situ pulse and impedance measurements show that solid diffusion in the positive electrode is limiting the cell performance as the state-of-charge decreases. Figure 1(a) shows the comparison of model prediction (yellow solid curve) to the experimental pulse measurement (black solid curve) for the current density of 0.69 A/m2. The experimentally validated model is then used to compute and analyse the contributions of different overpotentials for a range of other pulse current densities. Figure 1(b) illustrates the breakdown of different maximum overpotentials as a function of the current densities derived from Figure 1(a). At low current densities, solid diffusion overpotential in the positive electrode is the dominant overpotential, but at high current densities, the ohmic drop in the solid electrolyte becomes the rate-limiting mechanism. In Figure 1(c), the maximum overpotentials at different state-of-charge are compared. While the magnitude of different overpotentials at the state-of-charge more than 30% is similar (less than ten mV), the model prediction shows that the solid diffusion overpotential becomes the performance bottleneck as the state-of-charge decreases to less than 30%. The contributions of different overpotentials are also analysed as a function of pulse durations. Figure 1(d) shows that the ohmic drop is limiting the cell performance for a short pulse duration. However, as the pulse duration increases, solid diffusion overpotential becomes more dominant than the ohmic drop. Hence, our simulation results show that the solid diffusion in the positive electrode is the rate-limiting mechanism as the state-of-charge decreases and pulse duration increases, but the ohmic drop across the solid electrolyte becomes the dominant overpotential for a short pulse time constant and with increasing current density. Based on these rate-limiting mechanisms at different operating conditions, we propose possible cell designs for future solid-state lithium batteries. Our simulations using different cell design parameters show that the solid diffusion overpotential would increase by more than 100% if the electrode thickness is doubled. This result implies that if the phase separation behaviour in the positive electrode is not addressed in the future cell design, increasing the thickness to achieve a higher capacity will only lead to more voltage losses. We also show that the electrolyte thickness should be as thin as possible to minimise the ohmic drop in the solid electrolyte. If the bulk LiCoO2 is used as the positive electrode, further improvement in the ionic conductivity beyond 10-4 S cm-1 is shown to yield marginal reduction in the ohmic drop for both low and high current densities studied in this model. Figure 1

Why ball lightning disappears suddenly traceless

We show that the mysterious and intriguing property of Ball Lightning to disappear suddenly traceless in any point of its trajectory can be explained in a natural way in a frame of the optical model of Ball Lightning (BL) BL consists of the white light and air only. BL is a ball light or a bubble of light. Unlike a soap bubble, a shell of the light bubble consists of the compressed air where an intense white light is circulating in all possible directions. Due to the electrostriction pressure, the light compresses the air. In turn, the refractive index of the compressed air is greater than that of surrounding space. The compressed air is a spherical lightguide that confines the light from radiation in free space. A reason of a sudden disappearance of BL is the same as for a soap bubble. This is appearance of instability after a certain time. For a soap bubble, this is the thinning and breaking of the soap film. For a light bubble this is a decrease of the light intensity because of light radiation, a corresponding decrease of the refractive index of the compressed air and its ability to confine circulating light. Since time constants of processes accompanied the disappearance is connected with the light speed, the velocity of these processes is great and the light bubble disappear abruptly. The light radiates in free space, the air expands and no trace of BL left.

Lack of tone in mouse small mesenteric arteries leads to outward remodeling, which can be prevented by prolonged agonist-induced vasoconstriction.

Inward remodeling of resistance vessels is an independent risk factor for cardiovascular events. Thus far, the remodeling process remains incompletely elucidated, but the activation level of the vascular smooth muscle cell appears to play a central role. Accordingly, previous data have suggested that an antagonistic and supposedly beneficial response, outward remodeling, may follow prolonged vasodilatation. The present study aimed to determine whether 1) outward remodeling follows 3 days of vessel culture without tone, 2) a similar response can be elicited in a much shorter 4-h timeframe, and, finally, 3) whether a 4-h response can be prevented or reversed by the presence of vasoconstrictors in the medium. Cannulated mouse small mesenteric arteries were organocultured for 3 days in the absence of tone, leading to outward remodeling that continued throughout the culture period. In more acute experiments in which cannulated small mesenteric arteries were maintained in physiological saline without tone for 4 h, we detected a similar outward remodeling that proceeded at a rate several times faster. In the 4-h experimental setting, continuous vasoconstriction to ~50% tone by abluminal application of UTP or norepinephrine + neuropeptide Y prevented outward remodeling but did not cause inward remodeling. Computational modeling was used to simulate and interpret these findings and to derive time constants of the remodeling processes. It is suggested that depriving resistance arteries of activation will lead to eutrophic outward remodeling, which can be prevented by vascular smooth muscle cell activation induced by prolonged vasoconstrictor exposure. NEW & NOTEWORTHY We have established an effective 4-h method for studying outward remodeling in pressurized mouse resistance vessels ex vivo and have determined conditions that block the remodeling response. This allows for investigating the subtle but clinically highly relevant phenomenon of outward remodeling while avoiding both laborious 3-day organoid culture of cannulated vessels and in vivo experiments lasting several weeks.

Multiple timescales in the photoswitching kinetics of crystalline thin films of azobenzene-trimers

Functional materials that exhibit photoinduced structural phase transitions are highly interesting for applications in optomechanics and mechanochemistry. It is, however, still not fully understood how photochemical reactions, which are often accompanied by molecular motion, proceed in confined and crystalline environments. Here we show that thin films of azobenzene trimers exhibit high structural order and determine the crystallographic unit cell. We demonstrate that thin film can be switched partially reversibly between a crystalline and an amorphous phase. The time constant of the photoinduced amorphisation as measured with real-time x-ray diffraction (220 s) lies between the two time constants (120 s and 2870 s) of the ensemble photoisomerisation processes that are measured via optical spectroscopy. Our observation of a photoinduced shrinking of the crystalline domains indicates a cascading process, in which photoisomerisation starts at the surface of the thin film and propagates deeper into the crystalline layer by introducing disorder and generating free volume. This finding is important for the rapidly evolving research field of photoresponsive thin films and smart crystalline materials in general.

Connecting Time and Frequency in the RC Circuit

Charging and discharging processes of a capacitor through a resistor, as well as the concept of impedance in alternating current circuits, are topics covered in introductory physics courses. The experimental study of the charge and discharge of a capacitor through a resistor is a well-established lab exercise that is used to introduce concepts such as exponential increase or decrease and time constant. Determining the time constant of the RC circuit has important practical applications because, for example, it can be used to measure unknown values of resistance or capacitance. The transient experiment can be done by using a voltmeter and stopwatch, signal generator and oscilloscope, or even low-cost data acquisition systems such as Arduino. An equivalent topic when studying alternating current circuits arises from the characterization of the impedance of the series or parallel combination of the capacitor and the resistor as a function of frequency. Determining the time constant of the RC circuit by means of impedance measurements for different frequencies is a known experimental technique that can be done using not only LCR meters but also basic instrumentation in the physics lab such as a signal generator, frequency counter, and multimeter. However, lab exercises dealing with RC circuits in alternating current usually focus on their use as filters, and the potential applications in the field of the electrical characterization of material systems are ignored. In this work, we describe a simple exercise showing how the time constant of the RC circuit can easily be determined in the introductory physics lab by means of impedance measurements as a function of frequency. This exercise allows students to learn experimental techniques that find application to characterize the time constants of the charge transport processes in material systems. Moreover, comparison of the time constants obtained from transient and frequency analysis allows us to relate the time and frequency domains, which plays a central role in the advanced analysis of electric circuits, once the concept of Laplace transform has been introduced in order to simplify the problem of dealing with differential equations in the time domain by converting them into algebraic equations within the frequency domain.

Significant reductions in human visual gamma frequency by the gaba reuptake inhibitor tiagabine revealed by robust peak frequency estimation.

The frequency of visual gamma oscillations is determined by both the neuronal excitation–inhibition balance and the time constants of GABAergic processes. The gamma peak frequency has been linked to sensory processing, cognitive function, cortical structure, and may have a genetic contribution. To disentangle the intricate relationship among these factors, accurate and reliable estimates of peak frequency are required. Here, a bootstrapping approach that provides estimates of peak frequency reliability, thereby increasing the robustness of the inferences made on this parameter was developed. The method using both simulated data and real data from two previous pharmacological MEG studies of visual gamma with alcohol and tiagabine was validated. In particular, the study by Muthukumaraswamy et al. [2013a] (Neuropsychopharmacology 38(6):1105–1112), in which GABAergic enhancement by tiagabine had previously demonstrated a null effect on visual gamma oscillations, contrasting with strong evidence from both animal models and very recent human studies was re‐evaluated. After improved peak frequency estimation and additional exclusion of unreliably measured data, it was found that the GABA reuptake inhibitor tiagabine did produce, as predicted, a marked decrease in visual gamma oscillation frequency. This result demonstrates the potential impact of objective approaches to data quality control, and provides additional translational evidence for the mechanisms of GABAergic transmission generating gamma oscillations in humans. Hum Brain Mapp 37:3882–3896, 2016. © 2016 Wiley Periodicals, Inc.

Characterising the Charge Storage Mechanisms in Electrochemical Capacitors Using a Combination of Electrochemical Impedance Spectroscopy (EIS) and Step Potential Electrochemical Spectroscopy (SPECS)

Electrochemical capacitors are energy storage devices which have been demonstrated to exhibit both high specific capacitance and high specific power. Electrochemical capacitors are a promising energy storage technology due to their unique combination of specific energy and power output in addition to being relatively inexpensive and environmentally friendly. The performance of electrochemical capacitors is largely influenced by the electrode material properties. The specific properties of the electrode will determine both the nature and magnitude of the charge storage processes occurring within the electrode, such as double layer capacitance (non-faradaic) and redox reactions (faradaic; pseudo-capacitance). For example, activated carbons store charge almost exclusively via double layer capacitance, whereas metal oxides, such as ruthenium oxide and manganese oxide, will store charge via a combination of double layer and pseudo-capacitance. In metal oxides, pseudo-capacitance arises due to the reduction and oxidation of the material via proton insertion (and de-insertion), with ruthenium oxide and manganese dioxide exhibiting different charge storage characteristics. Understanding how the charge storage mechanism is influenced by material (and electrolyte) properties is vital for designing electrodes with optimised performance characteristics. However, this requires an understanding of how each charge storage mechanism contributes to capacitive performance. In terms of evaluating electrode performance, conventional electrochemical methods, such as cyclic voltammetry and constant current charge-discharge, cannot differentiate the capacitance contributions from charge storage processes involved. Characterising the different charge storage contributions from double-layer charge storage (non-faradaic) and pseudo-capacitive redox processes (faradaic) is a vital step in relating electrode performance to its material properties. In this work, both electrochemical impedance spectroscopy (EIS) and step potential electrochemical spectroscopy (SPECS) have been applied to electrochemical capacitor electrodes as a performance analysis method to determine the charge storage contributions from different processes. EIS has a number of advantages over other techniques, namely, the different time constants of electrochemical processes can be utilized to separate the different mechanisms occurring at the electrode. The SPECS experiment uses the same principle as EIS, i.e. separating the electrochemical mechanisms based on their different time constants. The SPECS procedure involves applying a small (±25 mV) potential step to the working electrode followed by a long equilibration time (300 s). This process is repeated over and entire charge-discharge cycle. By scanning at such a slow rate, the electrode has time to equilibrate at each potential, and the maximum charge storage capabilities of the electrode can be accessed. Each of the different charge storage processes occurring at the electrode has a unique time- dependent current response, and hence each potential step profile can be fitted to a model describing each of these processes. From this, values for series resistance (RS), double layer capacitance (CDL), diffusion limited capacitance (CD) and residual capacitance (CR) can be extracted. When the potential is stepped over an entire capacitor cycling range, contributions from each process can be determined at each point in the cycle. Additionally, by varying the equilibration time over which the current response is analysed, the scan rate can be effectively increased therefore the electrode behaviour can be analysed over a range of scan rates. This allows the development of a Ragone diagram for the different charge storage processes, indicating how specific charge storage mechanisms contribute to the power and energy characteristics of different electrode materials. This technique has been applied to a range of commonly used electrochemical capacitor systems including activated carbon, manganese dioxide and ruthenium oxides.

Pulse-fitting – A novel method for the evaluation of pulse measurements, demonstrated for the low frequency behavior of lithium-ion cells

The impedance of a commercial lithium ion cell (2 Ah) is evaluated over a wide frequency range of 10–5 Hz < f < 102 Hz using a new approach. The so-called pulse-fitting method (PFM) derives the cell impedance from time domain measurements, which are transferred into frequency data by a using a newly developed evaluation algorithm in combination with the distribution of relaxation times (DRT). The time constants of the individual polarization processes taking place during charging/discharging were analyzed at 20 different state-of-charge levels at 23 °C in a wide frequency range spanning from 10–5 Hz < f < 102 Hz. The pulse-fitting method seems to be especially useful for monitoring the change of individual polarization losses in the course of time, as self-discharge effects can be sorted out.