Distribution Of Relaxation Times Method Research Articles

Analysis of the Electrochemical Impedance Spectra and the Structure of the Solid Electrolyte Interphase at the Electrodeposited Metallic Lithium Using the Distribution of Relaxation Times Method

Analysis of the Electrochemical Impedance Spectra and the Structure of the Solid Electrolyte Interphase at the Electrodeposited Metallic Lithium Using the Distribution of Relaxation Times Method

Distribution of relaxation times used for analyzing the electrochemical impedance spectroscopy of polymer electrolyte membrane fuel cell

In this paper, a 25 cm2 polymer electrolyte membrane single cell is prepared by ultrasonic spraying coating method. The polarization curves and Electrochemical Impedance Spectroscopies (EIS) data of this cell are collected under different conditions, such as cell temperature, humidity, back pressure and pure O2. EIS data are analyzed by Distribution of Relaxation Time (DRT), and some results are concluded: (i) with the increase of current density, the Oxygen Reduction Reaction (ORR) time constant became lower, and the ORR reaction resistance and oxygen diffusion resistance both increases. (ii) With the increase of back pressure, the DRT peak assigned to oxygen diffusion process in cathode moves to low frequencies, the peak area decreases simultaneously. (iii) the proportion of the ORR polarization resistance gradually increases from 77 % to 84 % with the cell temperature increase from 65 °C to 80 °C. (iv) With the increase of humidity, the performance of the cell is decreased. (v) The oxygen diffusion resistance is negligible in pure oxygen working condition analyzed by DRT method.

Using distribution of relaxation times to separate the impedances in the membrane electrode assembly for high-temperature polymer electrolyte membrane fuel cells

In the membrane electrode assembly (MEA) of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), many types of resistance existed, but they have not been distinctly characterized in previous research. In this study, the distribution of relaxation times (DRT) methodology is employed, in conjunction with a home-built electrochemical electrode analysis system (EEAS), to figure out the specific resistance components in the cathode gas diffusion electrode (GDE). The impedance analysis by the DRT method was conducted using the electrochemical impedance spectrum obtained at different temperatures and current densities of the cathode only, isolated from the MEA environment. The four peaks (designated as Peak 0 to 3) were depicted at DRT plots of cathode GDE below 103 Hz. By comparing the DRT plots for different conditions and considering the EEAS structure, the four peaks were explained to be caused by distinct resistances occurring at the electrode and interface. The regions of each peak were distinguished according to their position changes with temperature and current density. Furthermore, by contrasting the DRT plots of the cathode GDE with those of MEA containing the same cathode, an additional peak (referred to as Peak 4) is identified between 103 Hz and 104 Hz, which was attributed to resistance originating from the anode. This study could provide a new approach for future research of new catalysts and electrodes utilizing the resistance analysis of MEA for HT-PEMFC.

Usefulness of the Distribution of Relaxation Time Method in Electroanalytical Systems: The Case of Voltammetric Ion-Selective Electrodes.

Despite the distribution of relaxation time (DRT) method providing clear insights about processes that go unnoticed by traditional electrochemical impedance spectroscopy (EIS) analysis, it has not yet been adopted to solve electroanalytical systems. As an illustration case, we apply the DRT method to deconvolve EIS data from solid-state voltammetric ion-selective electrodes (ISEs). The main aim is to shed light on the underlying working mechanism across the different materials and interfaces, specifically, the doping of a conducting polymer when covered with a very thin (ca. 230 nm) permselective membrane. Although frequency-dependent AC measurements in EIS allow the separation of processes that contribute to the electrical signal, interpretation of the data is challenging. DRT may overcome this inconvenience by revealing a series of peaks corresponding to the predominant electrochemical processes, without any preknowledge on those. To demonstrate our hypothesis, we examine the conducting polymer poly(3-octylthiophene) (POT) linked to a membrane with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na+TFPB-) as the cation exchanger, in which the lipophilic anionic part (TFPB-) is responsible for the POT doping when it gets electrochemically oxidized (POT+). The investigation of EIS data obtained under different conditions with the DRT method showed the occurrence of several processes. We have attributed two of these to two different conformational changes in the POT film in connection with p-type charge-transfer doping. Indeed, the kinetics is found to follow a Butler-Volmer behavior, with average charge transfers of 0.5 and 0.3 mol of electrons for each peak. Overall, we demonstrate the utility of the EIS-DRT tandem to separately study charge-transfer events that interconnect along the same (interfacial segmented) system, which cannot be reached by using classical electrochemical approaches. These kinds of insights are necessary for a more efficient design and high-level exploitation of voltammetric ISEs but also other electrochemical systems such as catalysts, batteries, and photovoltaic cells.

From theory to practice: Unlocking the distribution of capacitive times in electrochemical impedance spectroscopy

The distribution of relaxation times (DRT) method is a non-parametric approach for analyzing electrochemical impedance spectroscopy (EIS) data. However, we must be careful when using the DRT method on electrochemical systems with blocking electrodes, such as those encountered in batteries and supercapacitors. This is because, at low frequencies, the asymptotic behavior of the DRT model cannot capture unbounded impedances. To address this issue, we explore the distribution of capacitive times (DCT), a method that, despite being developed decades ago, is still not widely used. In this work, we detail the theoretical underpinnings of the DCT, deriving DCT-specific analytical formulae based on several standard impedance models. We also draw parallels between DCT and DRT and show how these two methods differ in capturing timescales and peaks, elucidating the scenarios where DCT can serve as a viable alternative should the DRT not be applicable. Additionally, we develop a novel method featuring two deep neural networks for DCT deconvolution. We systematically tested this method using a diverse set of synthetic and actual EIS spectra to ensure its efficacy and reliability. Overall, this article seeks to expand the scope of non-parametric approaches for EIS data analysis, particularly to systems characterized by blocking electrodes.

Bayesian Approach to Estimate Distribution of Relaxation Times for Lithium-Ion Batteries

Electrochemical impedance spectroscopy (EIS) has emerged as a powerful tool for analyzing the state of lithium-ion batteries due to its ability to provide rich electrochemical information. Traditionally, equivalent circuit and physical models have been utilized for interpreting EIS data. However, these methods possess inherent drawbacks: equivalent circuit models are non-unique, while physical models can be challenging to implement. As an alternative, the distribution of relaxation times (DRT) has garnered increasing attention in recent years. The DRT method enables the analysis of EIS data without predefining a model's shape, allowing for the decomposition of internal characteristics in complex systems, such as lithium-ion batteries. Despite its advantages, DRT estimation is an ill-posed inverse problem that often requires regularization or fine-tuning based on empirical choices, which can lead to the misinterpretation of EIS data.To address these challenges, we propose a Bayesian approach that enables the estimation of DRT while considering uncertainties, even in the presence of observational errors in EIS data. Moreover, the proposed method can assist in estimating the state of health (SOH) of the battery, which provides valuable insights into the battery's overall performance, longevity, and safety. To validate the efficacy of our proposed method, we apply it to synthetic EIS data and compare its performance with existing DRT estimation methods. The results show that the proposed Bayesian approach offers a more robust and accurate estimation of DRT. We also demonstrate the practical applicability of the proposed method by applying it to real lithium-ion battery EIS experimental data, where it successfully extracts meaningful information about the battery's internal state and SOH. In summary, the proposed DRT-based Bayesian method has the potential to enable a robust diagnosis of the internal state and SOH of lithium-ion batteries through EIS measurements. By incorporating this approach, researchers and industry practitioners can gain a deeper understanding of battery performance, facilitating the development of improved diagnostic techniques and battery management strategies to enhance the performance and reliability of lithium-ion batteries. This, in turn, could contribute to the broader adoption of sustainable energy solutions, such as electric vehicles and renewable energy storage systems.This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grants funded by the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20214910100070). Figure 1

Pressure and Impedance Based in Situ Diagnostic Methods for Water Management in Proton Exchange Membrane Fuel Cells Applications

Recent advancements in proton exchange membrane fuel cells (PEMFCs) have led to significant developments in low-carbon transport technology, enabling the generation of clean, efficient electricity with minimal noise and toxic emissions for heavy-duty applications. However, effective water management remains a critical challenge in PEMFCs due to the intricate phase changes and liquid-gas transport processes within key components, which significantly impact cell performance and longevity in industrial applications. Given the rapid dynamics of water activities and material responses within cells, in situ diagnostic techniques for water management in PEMFCs are essential for optimizing cell performance and extending the lifespan of fuel cell stack and system.Water, electrochemically produced on the cathode side, can cause potential flooding in key components including the catalyst layer (CL) and gas diffusion layers (GDLs), impeding air/oxygen supply to the cathode side. To characterize the performance loss induced by water flooding, Electrochemical Impedance Spectroscopy (EIS) is commonly employed as a fuel cell diagnostic tool. EIS records the contributions of different polarization processes to the global impedance with individual time constants during operation. However, the interpretation of complex EIS spectra can be very challenging, as polarization processes may overlap in frequency and be difficult to identify. To address this issue, Distribution of Relaxation Times (DRT) analysis is implemented as a powerful tool that allows for a clear separation and quantification of the polarization processes showing as peaks in a frequency domain [1]. Thus, we utilize EIS-based DRT analysis as an in-situ diagnostic tool to detect cathode flooding during cell operation. To validate this approach, we have conducted tests on a 300 cm2 low temperature PEMFC, varying one operation parameter at a time and evaluating its effects on cathode flooding by studying the frequency and height evolution of DRT peaks.Nevertheless, water flooding is not confined to the cathode side, as water can also permeate through the proton exchange membrane (PEM) and reach the anode side, resulting in potential fuel starvation on the cathode's counterpart and subsequent degradation of carbon-based materials. To address the sensitivity limitation of the EIS-based DRT method in detecting anode flooding, we employ an in-house developed pressure drop indicator (PDI) tool, as a non-invasive and real-time approach for diagnosing water flooding in the anode. The PDI tool assesses the pressure drop across anode channels, which is a function of the liquid water contents in bipolar plates considering the pressure reduction mainly caused by the frictional drag [2]. The effectiveness of the PDI tool is validated through cell testing at different water flooding levels by varying relative humidity and pressure conditions. This technique offers timely and sensitive detection of anode water flooding in PEMFCs, which is crucial for alarming irreversible material degradation in the electrode and facilitating appropriate follow-up actions.To conclude, we have developed and employed in situ PDI and EIS-based DRT methods within our water management framework for diagnosing water flooding in both the anode and cathode of PEMFCs in industrial applications. The combination of these techniques offers real-time and accurate assessments on water management conditions, which is crucial for optimizing overall performance and longevity of PEMFC stacks and systems.[1] Heinzmann, M., Weber, A., Ivers-Tiffée, E. (2018). Advanced impedance study of polymer electrolyte membrane single cells by means of distribution of relaxation times. Journal of Power Sources, 402, 24-33.[2] Barbir, F., Gorgun, H., & Wang, X. (2005). Relationship between pressure drop and cell resistance as a diagnostic tool for PEM fuel cells. Journal of Power Sources, 141(1), 96-101. Figure 1

Impedance Variation of All-Solid-State Battery Cell Using Li10GeP2S12; States-of-Charge Dependence Visualized by Distribution-of-Relaxation Time Analysis

Electrochemical impedance spectroscopy (EIS) will support understanding of electrochemical phenomena in emerging energy devices, including all-solid-state Li ion batteries (ASSLiBs), only when the results are analyzed in a consistent way without arbitrariness. This study1 aimed to establish such a robust analysis for ASSLiB cells using a fast lithium ion conductor Li10 +x Ge1+x P2 −x S12 (LGPS)2, by applying distribution-of-relaxation time (DRT) analysis3. The DRT method distinctively visualized impedance changes depending on states of charge (SOCs). Pellet-type cells were prepared with an In–Li anode, LGPS separator, cathode composite comprising LGPS and LiNbO3-coated LiCoO2 powders4. Figure 1a shows the discharge curve that was recorded for the cell at the current density of 13.2 μA cm−2 (1/40 C-rate) and with the cut-off voltage of 1.9–3.6 V vs. Li+/In–Li (2.55–4.25 V vs. Li+/Li). The EIS measurements were performed during discharge process at various SOCs from 100% to 20%. Nyquist diagrams obtained from the EIS data are shown in Figure 1b, where two broad semicircles are observed at low and high frequency sides. This result indicates at least two electrochemical processes exist in the cell. Both semicircles become large in size upon decreasing SOCs from 100 % down to 20%, indicating that the cell has SOC-dependent electrochemical processes. Figure 1c shows DRT transformation of the EIS data, which helps to separate impedance components in detail. Here horizonal and vertical axes respectively represent the measurement frequency and polarization contribution at each frequency. The center frequency of each distinct peak corresponds to time constant [τ =1/(2πf)] for the respective electrochemical process, and thus the number of peaks matches that of electrochemical processes in the cell. Meanwhile, the peak area represents polarization contribution of each process. Consistently in the whole SOC range, the above DRT analysis indicates the semicircle at high frequency side is divided into two impedance contributions and that at low frequency side cannot be separated any more. This result is consistent with the DRT spectra from additionally prepared symmetric cells; in total three DRT peaks were observed with two peaks from interphase and interface between In–Li/LGPS, and one from LiNbO3-coated LiCoO2/LGPS interface. Accordingly, DRT diagram in Figure 1c was interpreted as indicated by arrows for each peak. This study demonstrated that DRT analysis supports a consistent interpretation of EIS data from ASSLiB cells at various SOCs and with different electrode configurations, and thus implied that the method will also help subsequent equivalent circuit model analysis for reproducible parametrization of electrochemical processes.1. S. Hori et al., “Understanding the impedance spectra of all-solid-state lithium battery cells with sulfide superionic conductors,” J. Power Sources 556, 232450 (2023).2. N. Kamaya et al., “A lithium superionic conductor,” Nat. Mater. 10, 682–686 (2011).3. S. Dierickx, A. Weber, E. Ivers-Tiffée, “How the distribution of relaxation times enhances complex equivalent circuit models for fuel cells,” Electrochim. Acta 355, 136764 (2020).4. N. Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries,” Electrochem. Commun. 9, 1486-1490 (2007). Figure 1

Mechanistic Study on the Corrosion of (La,Sr)(Co,Fe)O3-δ Cathodes Induced by CO2.

Solid Oxide Fuel Cell (SOFC) cathodes operating in ambient atmospheric conditions inevitably encounter CO2 contamination, leading to sustained performance deterioration. In this investigation, we examined the impact of CO2 on three variants of (La,Sr)(Co,Fe)O3-δ cathodes and employed the distribution of relaxation times method to distinguish distinct electrochemical processes based on impedance spectra analysis. We meticulously analyzed and discussed the corrosion resistance of these (La,Sr)(Co,Fe)O3-δ cathodes under high CO2 concentrations, relying on the experimental data. Electrochemical impedance spectroscopy results revealed that La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF-6428), La0.4Sr0.6Co0.2Fe0.8O3-δ (LSCF-4628), and La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (LSCFN-46271) cathodes exhibited persistent degradation when exposed to CO2 at temperatures of 650 °C or 800 °C during the durability-testing period. An increase in electrode polarization resistance was observed upon CO2 introduction to the electrode, but electrode performance recovered upon returning to a pure air environment. Furthermore, X-ray diffraction and scanning electron microscopy analyses confirmed that CO2 did not cause permanent damage to the (La,Sr)(Co,Fe)O3-δ cathodes. These findings indicate that the (La,Sr)(Co,Fe)O3-δ cathodes exhibit excellent resistance to CO2-induced corrosion.

Optimizing the Copper Content in GdBa0.5Sr0.5Co2-XCuxO5+δ Double Perovskites as the High Performance Oxygen Electrodes for Reversible Solid Oxide Cells

Sustainable development and better protection of environment are among the top priority issues for human society. Seeking for cleaner and less greenhouse-emissive energy resources or solutions has been a dominating trend around the world, since daily functioning of the current society is still dependent on fossil energy sources, such as oil, coal, and also natural gas. Tremendous efforts have been made to develop alternative energy production technologies, among them, fuel cells have been found as very promising ones. Especially, reversible Solid Oxide Cells (rSOCs) have become famous for their all-solid structure, which consist of the porous fuel electrode, dense solid oxide electrolyte, porous oxygen electrode, and interconnects. Reversible SOCs can effectively work either in the fuel cell (e.g. H2 + 0.5O2 → H2O and heat) or electrolyzer modes, making them useful for grid balancing and utilization of surplus electrical energy. It is worth noting that the oxygen electrode has a decisive role regarding performance and long-term operation of the whole cell. Until now most of the widely-studied and also practically-implemented oxygen electrode materials are cobalt-based perovskite-type oxides or related compounds having the relatively high concentration of Co. This can be regarded as one of drawbacks, due to the toxicity, carcinogenicity, high costs, and poorly available resources of cobalt, despite impressive electrocatalytic activity and excellent electrochemical properties of such materials. Accordingly, numerous attempts have been carried out, which aim at replacing Co with other transition metal elements, such as Ni, Fe, Mn, and Cu, at least to some extent. For example, copper-doping into Co-based perovskites enabled to limit the excessive thermal expansion, as well as positively influenced redox stability.The A-site ordered double perovskites, REBaCo2O5+δ (RE: rare earth elements and Y), feature the layered structure with alternating configuration of REOδ and BaO layers along the c-axis, which generates fast diffusion channels for migration of oxygen anions, as well as the materials exhibit excellent electrical conductivity. In this work, an attempt has been made to further optimize the compounds, with the target compositions proposed as GdBa0.5Sr0.5Co2-xCuxO5+δ (0 ≤ x ≤ 2). Intermediate-size Gd3+ facilitates layered structure formation, while introducing Sr at Ba sites is beneficial to electrical conductivity, due to alleviated lattice distortion. Comprehensive studies have been directed to optimize the copper content in the selected oxides. After synthesis by a self-combustion sol-gel method, the samples were systematically characterized, including identification of the phase composition and crystal structure, thermal stability and chemical reactivity with solid electrolytes, thermal expansion behavior, oxygen content, total electrical conductivity and electrochemical properties. The adopted characterization methods include the X-ray diffraction at room and high temperatures with Rietveld refinements, iodometric titration and thermogravimetric tests, dilatometric measurements, total electrical conductivity studies. Also, laboratory-scale symmetrical and full cells have been manufactured and characterized concerning morphology of the electrodes, electrode-electrolyte interface, electrode polarization resistance with distribution of relaxation times method, as well as power output. High copper content (x > 1.15) was found as hindering factor for synthesis of pure samples. Bearing in mind a need to reduce cobalt content, GdBa0.5Sr0.5Co2-xCuxO5+δ compositions with x = 1, 1.05, 1.1, and 1.15 were selected for further evaluation. The oxides could be synthesized as practically phase-pure, revealing the desired P/4mmm tetragonal symmetry. All of them maintain stable lattice structure up to 1000 °C in air. They are chemically compatible with La0.8Sr0.2Ga0.8Mg0.2O3-δ and Ce0.9Gd0.1O2-δ electrolytes up to at least 900 °C. A moderate thermal expansion coefficients were measured, which decrease with the increasing Cu-doping content, from 15.8·10-6 K-1 (x = 1) to 14.5·10-6 K-1 (x = 1.15) in the temperature range of 300-900 °C. The equilibrated oxygen content in the considered samples ranges from 5.69 (x = 1) to 5.49 (x = 1.15). All oxides exhibit a sufficiently high total conductivity, above 10 S cm-1 in the working temperature range of 600-900 °C. The recorded electrochemical impedance spectra indicate that the lowest polarization resistance (0.017 Ω cm2 at 850 °C) can be obtained for the electrode with GdBa0.5Sr0.5Co0.9Cu1.1O5+δ composition (on La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte), which is lower than for the higher cobalt-content GdBa0.5Sr0.5CoCuO5+δ, 0.025 Ω cm2 at the same temperature. The developed GdBa0.5Sr0.5Co0.9Cu1.1O5+δ material used as the oxygen electrode in the La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte-supported full cell enabled to achieve power density output exceeding 1.2 W cm-2 at 850 °C, and 0.57 A cm-2 current density at 1.5 V in electrolysis mode at 700 °C. All the evidence prove that Cu-doping is an effective approach, since it allows for limiting the Co content and obtaining the desirable performance at the same time.

Physical-chemical processes at Mo-doped ceria-yttria stabilized zirconia (YSZ) anodes in solid oxide fuel cells using syngas derived by the distribution of relaxation times method

The use of environment-friendly carbonaceous fuels (such as, biomass and synthetic fuels) to produce electricity using solid oxide fuel cells (SOFCs) has emerged as a promising alternative to the use of high-purity hydrogen, which remains as a difficult to produce and store gas. Much effort has been put in the development of SOFC anodes highly active and stable for the use of carbonaceous fuels, such as ceria-based materials, but only recently research has focused on the comprehensive analysis of the interfacial physical-chemical processes affecting their performance.This work presents an in-depth analysis of the interfacial processes occurring at a Mo-doped ceria-yttria stabilized zirconia (YSZ) cermet anode in a SOFC using humidified syngas as fuel. To this end, electrochemical impedance spectroscopy (EIS) data is generated and analysed by the distribution of relaxation times (DRT) method. In addition, gas chromatography analysis of the internal reforming products generated under open circuit voltage (OCV) conditions and post-mortem analysis of the electrode materials after polarization are discussed in support of the DRT results.It is observed that the presence of carbon monoxide (CO) in fuel, as compared to pure hydrogen, influences the anodic charge transfer processes not only due to promotion of the water-gas-shift reaction at the anode two-phase (fuel | electrode) and three-phase (fuel | electrode | electrolyte) interfaces, but also because of its direct electrooxidation at the anode three-phase interfaces. The electrooxidation of carbon monoxide presents a slightly higher activation energy than the electrooxidation of hydrogen (ca. 1.86 and 1.21 eV, respectively), which explains the slower kinetics of SOFCs when using syngas as fuel. Additionally, other oxidation processes of CO at the anode interfaces (such as, the Boudouard reaction) promote the formation of carbon deposits lacking ordered structure, which are more reactive than graphitic structures formed at Ni-YSZ anodes but still can negatively influence the electrochemical kinetics.

High performance composite Pr4Ni3O—Ce0.75Gd0.1Pr0.15O solid oxide cell air electrode

A composite electrode composed of Pr4Ni3O—Ce0.75Gd0.1Pr0.15O (50 wt.%–50 wt.%) was thoroughly investigated in terms of the electrochemical performance as a function of microstructure. The electrochemical performance was characterized by electrochemical impedance spectroscopy and the microstructures, characterized by focused ion beam-scanning electron microscopy and 3D reconstructions, were modified by changing the particle size of Pr4Ni3O and the electrode thickness. The distribution of relaxation time method was applied to help resolve electrochemical processes occurring in the electrodes. It was found that an appropriate increase in electrode thickness and an appropriate decrease in particle size enhanced the oxygen reduction reaction (ORR) kinetics. The lowest area specific resistance obtained in this study at 670 ∘C under pO 2 of 0.21 atm was 0.055 Ω cm2. Finally, a comparison to the Adler-Lane-Steele (ALS) model was made and the main active site for the ORR was concluded to be triple phase boundaries. A fuel cell made of the composite material as the cathode was fabricated and tested. The peak power density was 1 W cm−2 at 800 ∘C, which demonstrates that this composite material is promising for solid oxide fuel cell cathodes.

Promoted electrochemical performance of one-step sintered intermediate temperature solid oxide fuel cells using nanoscale electrodes

Solid oxide fuel cells (SOFCs) require simple cell fabrication procedure and reduced sintering temperature to improve system economics and durability. In this work, we propose a one-step low temperature (950 °C vs. conventional ≥1300 °C) sintering technology to prepare doped ceria electrolyte-based SOFCs by adding 3 mol% of Li2O sintering aid. Super low temperature sintering grants the direct application of nanoporous Ni-based cermet anode and hierarchical SrNb0.1Fe0.9O3 cathode with improved catalytic activities, which improves SOFC efficiency. The ionic conductivity of the electrolyte and catalytic activities of electrodes are systematically investigated by the electrochemical impedance spectroscopy technique and analyzed by the distribution of relaxation time method. These electrolyte-supported SOFCs exhibit an OCV value of 1.01 V at 500 °C and a peak power density of 291 mW cm−2 at 700 °C as well as sufficient stability for 50 h. Our results demonstrate a reliable and affordable method for manufacturing high-performance SOFCs.

Electrochemical performance enhancement of electrospun nanofiber cathode through impregnation method

Intermediate-temperature proton-conducting solid oxide fuel cells often require impregnation techniques to improve the sluggish cathode reaction rate. Nevertheless, the limited impregnation efficiency poses a significant challenge. To tackle this issue, an innovative approach involving the synthesis of La0.6Sr0.4Co0.2Fe0.8O3+δ (LSCF) nanofibers (LSCF-NF) through electrospinning and their use as an impregnation substrate was employed. Through meticulous optimization of the skeleton microstructure, Bi0.75Ba0.25FeO3-δ (BBF) was successfully impregnated in the LSCF substrate, leading to a remarkable enhancement of cell performance. Specifically, the cell performance improved from 680 mWcm−2 to an impressive 1350 mW cm−2 at 700 °C, underscoring the efficacy of this novel technique. The density functional theory (DFT) simulation confirms that BBF is beneficial for proton migration. The distribution of relaxation time method (DRT) analysis coupled with electrochemical impedance spectroscopy has revealed a noteworthy increase in oxygen adsorption and reduction rates following the implementation of impregnation combined with microstructure optimization. These results establish impregnation combined with microstructure optimization as a highly effective strategy for boosting the performance of fuel cells, paving the way for further advancements in this exciting field.

Impact of strontium non-stoichiometry of SrxTi0.3Fe0.7O3-δ on structural, electrical, and electrochemical properties for potential oxygen electrode of intermediate temperature solid oxide cells

This work presents the results of a comprehensive study on the impact of the A-site non-stoichiometry of SrxTi0.3Fe0.7O3-δ (x = 0.90, 0.95, 1.00, 1.05) ceramics on their physicochemical properties. The materials were fabricated by the conventional solid-state reaction method and their structure was determined by X-ray diffractometry, X-ray photoelectron spectroscopy and electron microscopy. Their sintering and thermal expansion properties were then evaluated. The electrical properties of the materials were determined by electrical conductivity and electrical relaxation measurements (on bulk materials) and by electrochemical impedance spectroscopy (EIS) studies of symmetrical, CGO-electrolyte based, porous electrodes. Finally, fuel cell tests with the non-stoichiometric electrodes were evaluated. To elucidate the electrochemical reaction pathways for oxygen reduction/evolution reaction, EIS measurements were carried out in different pO2 and were analysed via the distribution of relaxation times method. The results showed a dependence of materials’ properties on the A-site non-stoichiometry, which can be used to fine-tune their properties, e.g. increase the surface exchange rate or decrease the thermal expansion coefficient.

Synergic effect investigation of carbon monoxide and other compositions on the high temperature proton exchange membrane fuel cell

The high temperature proton exchange membrane fuel cell integrated with the methanol steam reformer is a promising technology because it avoids the challenges of high pressure hydrogen storage and refueling. The high temperature fuel cell can tolerate the CO content in the methanol reformate with acceptable performance degradation. However, the synergic effects of CO and other impurities in the methanol reformate on the fuel cell performance need further investigation. This study thoroughly evaluates performances of the high temperature proton exchange membrane fuel cell fed in by mixture of H2, CO and other impurities such as CH3OH, H2O and CO2. The transient voltage, polarization curve, and electrical impedance spectroscopy are in-situ measured. The distribution of relaxation times method is used to characterize different time constants in the impedance spectroscopy. Meanwhile, the equivalent circuit model is developed to represent various reaction processes. The results show that the presence of CH3OH with CO in the fuel gas benefits the fuel cell performance at low current densities but aggravates the CO poisoning at high current densities. The addition of H2O with CO decreases both the Ohmic and anodic charge transfer resistances. The combination of CO2 and CO could enhance the effect of CO poisoning. Moreover, the multi-component methanol reformate test results suggest that the coexistence of multiple impurities in the methanol reformate have positive effect on the fuel cell performance.

Identification of gas diffusion phenomena on highly active Ni–ceramic anodes using the DRT technique

Nickel–ceramic composites are conventional and basic materials for anodes of solid oxide fuel cells. Usually, nickel–cermets have insufficient electrochemical activity due to the fact that the hydrogen oxidation reaction is limited by the triple phase boundary (TPB) length. The electrochemical activity of nickel–cermets can be improved by increasing the TPB length or through the wet impregnation with ceria. A distinctive feature of the electrochemical impedance spectra of highly active nickel–ceramic anodes impregnated with ceria is the response from gas diffusion phenomena in the low frequency range. Under certain conditions, the resistance of gas diffusion phenomena cannot be determined by conventional approaches to the analysis of impedance spectra. A tremendous assistance in the analysis of impedance spectra over the past few years has become the method of distribution of relaxation times (DRT). This paper presents the results of studies of highly active nickel–ceramic electrodes made from NiO and Zr0.84Sc0.16O1.92 powders of various dispersion and impregnated with ceria. Using the DRT method was found that in the case of electrode with low porosity the hydrogen oxidation rate is limited by the rates of two parallel gas diffusion processes relaxing in almost the same frequency range.

Oxygen vacancy concentration, electrochemical oxygen reduction, and performance stability of a LaNiO3-δ porous electrode

The oxygen vacancy concentration (δ), electrochemical reaction, and durability of LaNiO3-δ, a perovskite-type oxide, were determined as functions of temperature (773–1073 K) and oxygen partial pressure, p(O2). We established the δ value of LaNiO3-δ using high-temperature gravimetry and used the delocalized electron model to analyze by defect equilibrium. LaNiO3-δ has a higher oxygen partial molar enthalpy (△hO0) at δ ≈ 0 than other Ni-based oxides, which indicates that oxygen vacancies are easily formed. The electrochemical properties and durability of LaNiO3-δ were examined to investigate the oxygen reduction reaction and its long-term operational durability. Impedance spectra were analyzed using the distribution of relaxation times method, which separated the electrode reaction into three components: gas conversion/diffusion, reaction at the surface, and reaction at the electrode/electrolyte interface. Further electrochemical experiments demonstrated that the LaNiO3-δ porous electrode performs stably at 1073 K and − 0.2 A.cm−2 for 200 h.

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.