Distribution Of Relaxation Times Method Research Articles

Performance optimization for proton conducting SOFCs by impregnating triple-conducting NBSCF to form composite cathode

• Triple-conducting NBSCF was evaluated as impregnating material. • H-SOFCs with NBSCF modified LSCF composite cathode achieved a relatively high performance. • Migration of protons and oxygen ions were highly enhanced. Considering the sluggish cathode reaction rate, this work applied a triple conducting coating of NdBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5+δ (NBSCF) on a state-of-the-art La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3+δ (LSCF) cathode via a common infiltrating method. The electrical measurement revealed that the polarization resistance decrease from 0.44 to 0.14Ω·cm 2 and maximum power density increase from 486 to 696mW cm -2 at 700°C. Moreover, the analysis of distribution of relaxation time method (DRT) and impedance spectrums revealed that the migration rate of proton and oxygen ion was enhanced, indicating that the impregnation of triple-conducting materials could promote oxygen adsorption and ion migration.

Comparative analysis for commercial li-ion batteries degradation using the distribution of relaxation time method based on electrochemical impedance spectroscopy

As a non-destructive method for characterizing battery dynamic behavior, electrochemical impedance spectroscopy (EIS) is becoming increasingly important to analyze electrode process kinetics, electric double layers, and diffusion in the study of battery performance. However, overlapping features of the EIS semicircle of commercial batteries over the lifetime bring ambiguities concerning their physicochemical significance spectra in the timescale distribution. In this work, we analyze the impedance of kinetic processes and corresponding time constants in three types of commercial batteries at different aging stages via the optimized distribution of relaxation time (DRT) investigation which extracts evolution details of different time scales that could not be distinguished. Firstly, informative aging tests for battery LiNi0·8Co0·1Mn0·1O2 (NCM), LixFePO4 (LFP), and LiNixCoyAlzO2 (NCA) during the whole life cycle were designed to simulate different electric vehicles working conditions, followed by periodical EIS measurements. And then, the capacity retention, as well as the deconvolution of EIS for discriminating the electrochemical mechanisms of commercial LiBs during aging were compared. Gaussian process and ridge regression are dedicated to complex superposed impedance spectra to DRT. Combining both experimental measurements and multi-peak analysis for DRT, the analysis determined the impedance distribution characteristics of batteries with different materials during cycling, which facilitates rapid identification of aging mechanisms and further life prediction. Therefore, the peak characteristic analysis method based on the DRT analysis can be employed to diagnose and predict the impact of battery performance.

Polarization resistance of the Ruddlesden-Popper nickelates Lan+1NinO3n+1 (n = 1, 2, 3): Comparative analysis using the distribution of relaxation times method

The Ruddlesden-Popper layered nickelates La2NiO4+δ (n = 1, LN21), La3Ni2O7-δ (n = 2, LN32) and La4Ni3O10-δ (n = 3, LN43) were studied as electrodes in symmetrical cells based on Ce0.8Sm0.2O2-δ (SDC) at various temperatures (600 ≤ T ≤ 800 °C) and oxygen partial pressures (−3 ≤ log(P(O2)/P0) ≤ −0.67). Electrochemical Impedance Spectroscopy (EIS) of the selected Lan+1NinO3n+1/SDC cells with similar microstructure reveals that the polarization resistance of the electrodes gradually increases with increasing n, namely by ≈0.75 Ω cm2 at 800 °C in air. The Distribution Function of Relaxation Times (DFRT) analysis of the obtained data shows that the polarization resistance of Lan+1NinO3n+1 includes four distinguishable contributions RHF (1), RHF (2), RMF and RLF. The Ri resistances at fixed temperatures and P(O2) were determined by fitting the obtained impedance spectra using the equivalent circuit consisting of electrolyte resistance and a series of R-CPE elements. Based on the analysis of the Ri = f(T, P(O2)) dependences, the RHF (1), RHF (2) resistances are assigned to the charge transfer through the interfaces and RMF, RLF are interpreted as “chemical” part of the impedance, i.e. ionic transport in the electrode and oxygen dissociation at the electrode surface, respectively. Comparison of the obtained resistances shows that RMF dominates the polarization resistance meaning that ionic transport in the electrodes is the slowest stage of the electrochemical reaction at the studied temperatures in air. The increase in the number of perovskite layers deteriorates ionic diffusion and promotes oxygen dissociation. As RMF > RLF, this leads to an increase in the overall polarization resistance from 0.75 Ω cm2 for LN21 to 2.25 Ω cm2 for LN43 at 800 °C in air.

Investigation of the diffusion phenomena in lithium-ion batteries with distribution of relaxation times

The distribution of relaxation times method (DRT) has been widely used to quantify the numbers and characterize the properties of the physico-chemical processes inside the Li-ion battery (LIB). While most of the published works focused on the mid-high frequency range, the processes in the low frequency area, such as diffusion, have garnered less attention. The difficulties of applying the DRT to the diffusion processes include the more complicated mathematical treatment involved, the unknown influence of the porous electrode on the DRT spectra and the evaluation of the measured impedance with the developed theory. The galvanostatic intermittent titration technique (GITT) has been widely applied to determine the solid diffusion coefficient for the LIB. However, the GITT does not consider the geometry of the active material particles and cannot effectively separate the contribution of the solid diffusion from other processes, such as liquid diffusion. In the present work, a comprehensive theory is developed to investigate the DRT spectra of a LIB with a physics-based impedance model. Furthermore, an analytical expression is developed for the DRT spectra and analyzed in detail. The developed theory can help to determine the solid phase diffusion coefficient with a firm physico-chemical background. Based on the developed theory, the solid diffusion coefficients of the silicon graphite (SiC) and NMC811 are determined with a commercial cell. Besides, the galvanostatic intermittent titration technique (GITT) has been applied to the same cells and the measurement results are compared with that using the DRT method. The comparison indicates that GITT cannot exclude the influence of the liquid diffusion and the solid diffusion coefficients will be underestimated.

Analysis of Electrochemical Impedance by Distribution of Relaxation Time Method and Its Application

Analysis of Electrochemical Impedance by Distribution of Relaxation Time Method and Its Application

Optimization and application of the distribution of relaxation times based on characteristic frequency resolution and hyperparameters

Distribution of relaxation times (DRT) is a promising method for interpreting electrochemical impedance spectroscopy (EIS) data. Contemporary studies have made significant progress in theory, algorithm, implementation, and application of DRT. However, the DRT deconvolution is often difficult to interpret because of the pseudo-peaks, peaks deviation or peaks overlapping. In this paper, a comprehensive regularization DRT method based on characteristic frequency resolution and hyperparameters is proposed. Based on quantitative analysis of synthetic simulation data, the characteristic frequency resolution optimization is applied to avoid the peaks overlapping and improve the accuracy of DRT. Meanwhile, the hyperparameters optimization which assigns weights to each individual data point is verified to improve the robustness of DRT. Then, the optimized DRT is applied to the EIS experimental data analysis of proton exchange membrane fuel cell (PEMFC), and the polarization processes of PEMFC are identified. Moreover, a fast EIS measurement based on characteristic frequency of DRT is proposed and verified by the impedance prediction which the deviations are within 10%, but the EIS measurement time is shortened from 6 min to 5 s. Moreover, this method can provide a new direction and idea for fault diagnosis based on EIS data, especially for on-board applications.

Application of distribution of relaxation times method in polymer electrolyte membrane water electrolyzer

Distribution function of relaxation times (DRT or DFRT) is a well-established method of solving electrochemical impedance spectroscopy (EIS) data from electrochemical component, such as fuel cells and battery. It can be used to develop the most accurate equivalent circuit models without the input of any circuit architecture information, due to the complexity in equivalent circuit model (ECM). For the first time, this paper implemented DRT methods in polymer electrolyte membrane water electrolyzers (PEMECs). The regularization parameter, characteristic parameters and frequency factors of the solving process are introduced. The results show that (1) DRT method represents the original impedance spectrum of hydrogen production in the form of three characteristic peaks, which correspond to the main polarization processes in hydrogen production system, including water/gas diffusion, hydrogen evolution reaction (HER), oxygen evolution reaction (OER) reactions, and proton transfer processes. (2) The optimal regularization parameter is 0.01 for different current and temperature. (3) The frequency of water/gas diffusion, HER /OER reactions and proton transfer processes is 0.1–60 Hz, 100–1000 Hz and 2000–4000 Hz, respectively. (4) DRT methods couples with genetic algorithm can effectively identify various types of polarization impedance corresponding to different characteristic frequencies without equivalent circuit model.

Investigation and study of three different composite cathodes for proton-conducting solid oxide fuel cells

Traditional composite cathode was usually mixed with cathode materials and BaZr0.1Ce0.7Y0.2O3-δ (BZCY) to obtain thermal and chemical compatibility with proton-conducting electrolyte. Considering the poor chemical stability of BZCY in wet atmosphere, three different composite cathodes of La0.6Sr0.4Co0.2Fe0.8O3+δ (LSCF)-BZCY, LSCF-Sm0.2Ce0.8O2-δ (SCO), LSCF-La2Ce2O7+δ(LCO) were fabricated and systematically researched. Due to the high chemical stability and triple conductivity, the single cell with cathode of LSCF-LCO presented a better cell performance of 1020 mW·cm−2 at 700 °C and long-term operation durability. In addition, the analysis of distribution of relaxation time method (DRT) and impedance spectrum revealed that the migration rates of proton and oxygen ion were enhanced through adjusting electrolyte material, showing great potential for future application of new cathode materials for proton-conducting solid oxide fuel cells (H-SOFCs).

High-precision identification of polarization processes of distribution of relaxation times by polarization curve model for proton exchange membrane fuel cell

The distribution of relaxation times (DRT) is a promising impedance analysis method that is often used to decompose polarization processes with different time constants. However, uncertainty in interpreting polarization processes by DRT arises from two main sources: (1) overlap of the DRT peaks and (2) lack of peak identification methods. Thus, herein, a high-precision polarization process identification method based on polarization curve model involving electrochemical processes of proton exchange membrane fuel cell (PEMFC) was proposed by establishing the intrinsic relationship between polarization curve and electrochemical impedance spectroscopy (EIS) and comparing the quantitative relationship between the steady-state resistance obtained by the inverse slope of polarization curve model and each DRT peak. Then, an improved DRT method based on the characteristic frequency resolution optimization was suggested to avoid the overlap of the DRT peaks in case of the narrow interval time constants of the electrochemical system. Based on these two methods, each polarization resistance of PEMFC and DRT peak was quantitatively analyzed. First, it was determined that the total steady-state resistance has a resistance recovery compared to the low-frequency intercept resistance obtained by the improved DRT method, because of the ultra-low-frequency pseudo-inductive behavior arising mainly from dissolved water transport. Second, the peak with a characteristic frequency in the range of 7–54 Hz corresponds to two processes: mass transport and dissolved water transport. The peak with a characteristic frequency in the range of 3–20 Hz represents charge transfer. Interestingly, the two DRT peaks in the low-frequency region overlapped at 400 mA cm−2 because their characteristic frequencies changed in opposite directions with the current density. Moreover, the peak with a high characteristic frequency in the range of 250–300 Hz corresponds to proton transport of the catalyst layer ionomer. The methodology based on the model lays the foundation for identifying the distribution of the relaxation times for PEMFCs.

Investigating Electrode Reactions in Vanadium Redox Flow Batteries - a Distribution of Relaxation Times Analysis

Due to the increasing energy demand worldwide and the urgency of action because of climate change, new energy storage devices are needed to balance the fluctuations of renewable energy sources. The Vanadium Redox Flow Battery (VRFB) is a promising technology for large-scale energy storage but still needs to overcome significant lifetime and efficiency challenges, which are decreased by polarization losses during operation. It is essential to conduct experiments in a setup that closely mimics the cell's operating conditions to analyze the ongoing processes in a VRFB. Therefore, setups reflecting a half cell are well-suited. Thus, new insights into the reaction and transport processes in a VRFB can be gained. Electrochemical Impedance Spectroscopy (EIS) combined with Distribution of Relaxation Times (DRT) analysis is a well-suited method to investigate electrode reactions. This combination of methods is frequently used in research for lithium-ion batteries or in our group for fuel cell characterization (1-3). To the best of our knowledge, there is no study using EIS and DRT analysis to analyze processes in a model half cell in VRFB research published yet, and just very few studies on characterization in full cell-like setups are available (4). Here, we present a novel 3D printed flow cell for investigations under application-oriented conditions, designed to ensure steady-state conditions during the measurements. These studies focus on the characterization of the processes in the positive half cell by EIS to deepen the knowledge about the polarization losses in VRFBs.The vanadium-containing electrolyte is continuously pumped through the cell during the experiments to guarantee consistent conditions while EIS measurements are performed. The electrolyte flows through the carbon paper stack used as electrode material, undergoing an electrochemical reaction. Here, either the oxidation of vanadium(IV) to vanadium(V) or the reduction from vanadium(V) to vanadium(IV) occurs. The recorded data were analyzed with the DRT method, which allows the separation of physicochemical processes on different time scales. Parameters like the temperature, the flow rate, the electrolyte concentration, and the electrolyte species were varied independently to identify the individual processes in the positive half cell of a VRFB.This setup enables electrochemical impedance measurements of high quality, which is essential for a reliable DRT analysis. We could assign the peaks in the DRT spectrum to the electrochemical reaction, the convective transport through the electrode structure, and the diffusion processes of the vanadium species. Thus, we could identify the individual processes in the positive half cell of the VRFB and their contributions to the overall impedance. This information is vital in search of optimized operating conditions with reduced polarization losses.1. M. A. Danzer, Batteries, 5(3), 53 (2019).2. A. Weiß, S. Schindler, S. Galbiati, M. A. Danzer and R. Zeis, Electrochimica Acta, 230, 391–398 (2017).3. N. Bevilacqua, M. A. Schmid and R. Zeis, Journal of Power Sources, 471, 228469 (2020).4. J. Schneider, T. Tichter, P. Khadke, R. Zeis and C. Roth, Electrochimica Acta, 336, 135510 (2020). Figure 1

Revealing the degradation mechanism of the lanthanum nickelates based double‐layer electrodes during long‐term tests in contact with chromium‐containing steel interconnects

This paper focuses on the long-term tests for 1000 hours of different Membrane-Electrode-Interconnect Assemblies (MEIAs), consisting of a steel interconnect and an asymmetrical solid electrochemical cell based on the Ce0.8Sm0.2O2−δ electrolyte, the double-layer working electrode with the La2NiO4+δ – Ce0.8Sm0.2O2−δ composite functional layer and the LaNi0.6Fe0.4O3−δ current collector layer, and the platinum reference electrode (O2, LaNi0.6Fe0.4O3−δ|La2NiO4+δ – Ce0.8Sm0.2O2−δ|Ce0.8Sm0.2O2−δ|Pt, O2). These tests were carried out on MEIAs with two types of Cr-based stainless steels, Crofer 22 APU and 08X17T, with and without protective coatings at 850°C in air. The electrochemical performance of MEIAs was studied without polarization as well as under cathodic and anodic polarization (current density was 0.5 A cm−2) by means of electrochemical impedance spectroscopy (EIS) and the distribution of relaxation times method (DRT). It was found that the proposed protective coatings for the stainless steels inhibit chromium poisoning without polarization and under anodic polarization: for Crofer 22 APU the degradation of the polarization resistance after ~1100 hours of exposure was 0.35 ± 0.05 Ω cm2, and for 08X17T it was from 0.40 ± 0.05 Ω cm2. In the case of cathodic polarization, the degradation of the polarization resistance remained at a very high level, comparable to uncoated samples: for Crofer 22 APU and 08X17T with coatings, it was about 0.6 to 0.8 Ω cm2 after ~1100 hours. Post-test analysis of the microstructure and chemical composition of the interconnect - double-layer electrode interface was carried out using scanning electron microscopy (SEM), wavelength dispersive X-ray spectrometry (WDS), and respective image analysis of the obtained data. The influence of chromium poisoning on the degradation was considered for different stages of the electrode performance. It was demonstrated that chromium predominantly accumulates in the current collector layer in the case of anodic polarization and in the functional layer in the cases of open circuit and cathodic polarization. For the first time, a combined electrochemical-chemical degradation mechanism of the La2NiO4+δ – Ce0.8Sm0.2O2−δ functional layer was discussed. Highlights Long-term tests of Membrane-Electrode-Inteconnect Assembles (MEIAs) were done. Chromium poisoning was detected on MEIAs with the interconnects without coating. Cathodic polarization terminates the protective properties of the coatings. Chromium poisoning decreases electrode activity to oxygen exchange and diffusivity. A combined electrochemical-chemical degradation mechanism of the functional layer was revealed.

Investigation of the distribution of relaxation times of a porous electrode using a physics-based impedance model

Recently, the analysis of the distribution of relaxation times (DRT) has drawn wide attention in battery engineering due to its nondestructive nature and the ability to separate the electrochemical processes with different time constants. Since proposed, the DRT is only interpreted using the equivalent circuit model (ECM), and an interpretation based on physicochemical model is still missing. To characterize the cell parameters and extract the valuable information more effectively, a new theory is developed based on the deep understanding of the physics-based impedance model and the DRT method. Particularly, a closed-form analytical expression for the DRT spectrum is derived, therefore the DRT spectrum can be explained and interpreted with a clear physical meaning. In addition, it is found that the relaxation time dispersion of EIS may have multiple origins: the porous structure of the electrode, the regularization technique used to calculate the DRT and the constant phase element type behavior of the active material particles. Based on the theory developed in this work, an optimized data analysis framework is created to better interpret the DRT spectrum. Finally, the developed DRT analysis framework is applied to the impedance data of a commercial LIB to demonstrate its advantages and effectiveness.

Electrochemical analysis of BaZr0.8Y0.2O3-δ-Gd0.2Ce0.8O2-δ composite electrolytes by distribution of relaxation time method

BaZr0.8Y0.2O3-δ (BZY)-Gd0.20Ce0.80O2-δ (GDC) composite electrolytes have been fabricated by mechanical mixing the BZY and GDC powders with different weight ratio (30 wt%, 50 wt % and 70 wt %). The ceramic powders were synthesized by a sol-gel method, and the crystalline structure of the ceramic powders and composite electrolytes was examined by X-ray diffraction (XRD). Good chemical compatibility between BZY and GDC has been shown based on the XRD analysis. The morphology and element distribution of the BZY and composite electrolyte were observed by the scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDX). The conductivity measurement was conducted in both oxidizing and reducing atmospheres by the means of electrochemical impedance spectroscopy (EIS). The equivalent circuit and distribution of relaxation times (DRT) method were employed to analyze the obtained EIS data. For the composite electrolytes, two different types of grain boundaries can be identified in the DRT diagram derived from the impedance data measured in reducing atmosphere at lower temperature (< 450 °C). The influence of the GDC content on the resistance between the two grain boundaries was investigated and the result can be attributed to the different average grain size of the samples. The total conductivity of the composite electrolytes in each atmosphere was also analyzed and discussed, and the composition of 30 wt%BZY-70%GDC (BG37) shows the highest conductivity of 0.0269 Scm−1 at 700 °C in reducing atmosphere. Single cell based on BG37 electrolyte was fabricated and tested, maximum power density of 0.442 Wcm−2 at 700 °C with the OCV of 0.89V were achieved.

Extended Distribution of Relaxation Time Analysis for Electrochemical Impedance Spectroscopy

The distribution of relaxation time (DRT) is increasingly investigated as a novel analytical method for electrochemical impedance spectroscopy. However, this method has not yet been generalized, as it cannot be applied to a spectrum influenced by an isolated capacitator and a resistor connected in parallel with an inductor. To generalize the DRT analysis, we propose a novel principal relation and actual calculation methods using an iterative elastic net regularization algorithm. The elastic net regularization was implemented using the Levenberg–Marquardt method. The proposed algorithm aids in analyzing the DRT for spectra that cannot be solved using conventional methods. We compared the DRT results in different regularization methods obtained from the proposed program with those obtained from other open-source programs. Additionally, the realistic problems of the DRT analysis are discussed considering the calculated results and their theoretical basis. Different DRT curves can be obtained depending on the particular program and regularization methods, even when the spectrum is the same. Moreover, overconfidence in the electrochemical DRT method can be avoided with a clear understanding of DRT basics.

An electrochemical–thermal model of lithium-ion battery and state of health estimation

The performance of lithium-ion batteries deteriorates with the passage of time, estimating State-of-Health of batteries correctly is important. In this paper, an electrochemical-thermal model including the electric double layer model is developed. In the frequency domain, sensitivity analysis of the electrochemical processes during battery aging are carried out based on electrochemical impedance spectroscopy and distribution of relaxation time method. In the time domain, single parameter sensitivity analyses are performed. Methods of obtaining different types of parameters in the model are studied, and the electrochemical parameter values of the battery under different cycles are obtained. Finally, some key parameters with higher correlation with State-of-Health are selected to achieve State-of-Health estimation of the battery.

Impedance Evaluation By Distribution of Relaxation Times Applied to a Lead-Acid Storage Battery

Introduction – electrical impedance and its evaluation. Electrical Impedance Spectroscopy (EIS) is used to identify the physical characteristics of a system or, in the absence of adequate equivalent circuits; it is at least used to compare overall impedance response as a change of a particular parameter. A novel approach to acquired impedance data treatment is to process them mathematically and obtain what is known as distribution of relaxation times, abbreviated as DRT. Here the method is evaluated to explore its suitability for evaluation of state of charge of a lead-acid battery.The typical impedance evaluation is based on fitting to equivalent circuits. However, oftentimes both finding the equivalent circuit and the physical component assignment are tedious, if not impossible. Recently a new approach to data reduction has been treatment via distribution of relaxation times (DRT) [1, 2]. In principle, this approach has been outlined already in 1936 [3]. Its present use was made possible by available computation power of current PCs [4, 5]. While this is not a substitute to equivalent circuits and subsequent physical data interpretation, it is possibly a method, when used appropriately and in automated way, that can provide more information that simple comparison of the Bode or Nyquist graphs.Here we treated impedance data from lead-acid battery studies by the DRT approach. Of interest are charged and discharged states of the battery as well as aging of the battery. During the operation of a lead-acid battery, its operating properties gradually deteriorate. Experimental part A lead-acid commercial maintenance free cell Hawker Cyclon 2 V/2.5 Ah (EnerSys) of spiral construction with AGM separator was used. The discharge step was carried out at current 0.4 A until the voltage dropped to 1.6 V. After that charging commenced using current of 0.4 A, with upper voltage limit 2.4 V, which took about 7 hours. During the experiment impedance of the cell was measured every approximately 150 s. The frequency range was 6 kHz to 351 mHz, 6 steps per decade, amplitude 5 mV. DRT – resolving model data Application of the DRT method makes it possible, without any preliminary considerations, to estimate how many meaningful time constants can be expected in the evaluated system. It is possible to illustrate, for example, that an impedance response generated by five RC circuits separated by 1/2 order of magnitude in a time constant, which in a Nyquist plot looks like a depressed semicircle, can be resolved into five peaks in DRT.State of charge of a lead-acid battery can be investigated by impedance measurements [6-8]. A large series of the impedance data obtained in our laboratory previously from gradual discharge of a lead-acid battery was treated by the DRT routine and the response is shown in the attached figure (Impedance data treated by the DRT method for a lead-acid battery. Front to back a fully charged to a discharged state.). Remarkable is the growing-in feature, with also changing its time-constant over four orders of magnitude can be associated with the state of charge. Conclusion The mathematical method of applying the distribution of relaxation times numerical method to evaluation of large sets of data appears to be a promising approach to data reduction and subsequent studies of devices and material properties with changing impedance properties. Acknowledgement This work was supported with the help of the grant for specific research at the Brno University of Technology (FEKT-S-20-6206).

Quality‐Indicator‐Based Preprocessing for the Distribution of Relaxation Times Method

AbstractThe deconvolution of impedance data to their respective distribution of relaxation times (DRT) is accompanied by an increase in the resolution of the frequency domain and is, thus, advantageous. Yet, most of the techniques used for computing DRT spectra from experimental data are valid only for impedance spectra that solely show polarization processes. Therefore, most experimental electrochemical impedance spectra must be corrected from perturbing inductive, capacitive, or diffusive contributions. This preprocessing is time consuming, and its quality has an enormous impact on the obtained DRT spectra. It also prohibits using the DRT method within an automated impedance evaluation. In this work, we propose a software‐aided approach for deriving optimal preprocessing. For this purpose, we first introduce a quality indicator that reflects the quality of a certain preprocessing. Based on this quality indicator, we furthermore introduce a new batch fitting approach for deriving the optimal equivalent electrical circuit within the preprocessing of experimental impedance spectra. Besides, the batch fitting displays the optimal frequency range for modeling the unwanted impedance fractions. Based on this information, the considered impedance data can finally be optimally corrected from any non‐polarization processes. The new methods and their application are tested for simulated data and experimental impedance data of a lithium‐ion cell.

(Invited) Effects of PrOx and Gd-Doped CeOx Infiltration: Enhancement of Solid Oxide Cell Performance and Stability

PrOx-infiltrated oxygen electrodes can yield relatively low polarization resistances even at reduced operating temperature. Similarly, Gd-doped Ceria (GDC) infiltration into Ni-YSZ fuel electrodes has been shown to reduce polarization resistance while also helping to maintain stable electrolysis operation. Here we review the literature on these electrodes and discuss recent results on electrochemical impedance spectroscopy (EIS) studies of PrOx-infiltrated La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and SrTi0.3Fe0.55Co0.15O3 (STFC) oxygen electrodes and GDC-infiltrated Ni-YSZ. Physically-based equivalent circuit models are developed using the distribution of relaxation times method and used to fit the symmetric cell EIS data, comparing PrOx- and GDC-infiltrated electrodes with un-infiltrated electrodes. The results suggest that PrOx improves oxygen dissociative adsorption/desorption, oxygen surface exchange, and transport of oxygen in the perovskites; while GDC reduces the reaction resistance associated with three-phase boundaries and improves oxygen transport in Ni-YSZ. Full cells employing the PrOx-infiltrated STFC and Ni-YSZ:GDC electrodes achieve high performance (e.g. fuel cell power density nearly 3 W/cm2 at 750 oC and > 1 W/cm2 at 600 oC; and electrolysis current density at 1.3 V of nearly 4 A/cm2 at 750 oC), while exhibiting promising stability. Combining the equivalent circuit models from the symmetric cell data for each electrode provides a detailed explanation and fit of the full cell EIS results.

Supercapacitive properties of MnO2 and underlying kinetics by distribution of relaxation time method

Charging and discharging processes in supercapacitors and secondary batteries are critically dependent on elementary rate determining sub-processes (ERDSPs) with different relaxation time (τ) at different frequencies. In this work, distribution of relaxation time (DRT) method is employed to investigate the ERDSPs and relevant kinetics of electrodeposited MnO2 electrodes. Two ERDSPs are identified: ion diffusion at low frequencies (100-0.1Hz), and the mechanism of ERDSP at high frequencies (105-102 Hz) is found to be charge transfer across the MnO2/electrolyte interface. The rate capability is found to reduce with the increasing electrode thickness (l), which is attributed to the increasing relaxation time of diffusion (τd), proportional to l2. Oxygen vacancies (VO••) can lead to an increased specific capacitance. Meanwhile, VO•• with positive effective charge may also lead to repulsion to cations, and thus increase the activation energy (Ea) of charge transfer (Ea-ct) and diffusion (Ea-diff) processes. Enhancing cycling stability with reducing Mn3+ and VO•• concentration may result from reducing Mn element loss due to disproportionation reaction of Mn3+, and alleviating polarization and increasing reversibility due to reducing Ea-ct and Ea-diff.

Impregnation of Pt|YSZ Oxygen Electrodes with Microquantities of Praseodymium Oxide

The method of electrochemical impedance is used to study the polarization conductivity of porous platinum electrodes in the Pt|YSZ|Pt cells over the temperature range from 500 to 700°C in air. Electrodes are impregnated with microquantities of praseodymium oxide PrOx (from 10–4 to 0.1 mg/cm2) by the impregnating them with praseodymium nitrate solution, with further thermal treatment: slow heating (50°С/h) up to 850°C. This processing resulted in the formation of the praseodymium oxide nanofilm on the electrolyte surface. The introducing of PrOx, even in an amount of 10–4 mg/cm2, is found to significantly increase the electrode polarization conductivity. Analysis of the electrochemical impedance spectra was performed by DRT (distribution of relaxation times) method, which revealed a change of the rate-determining stage of the electrode process in the concentration range of praseodymium oxide from 1 × 10–3 to 3.3 × 10–3 mg/cm2. The studied electrodes are discussed in terms of the island model of dense-oxide-PrOx-electrodes in which platinum plays the role of the current collector.