Добірка наукової літератури з теми "Spatially and temporally resolved electrochemical impedance spectroscopy"

The complex operating behaviour of PEM Fuel Cells is heavily influenced by different non-linear loss processes, which limit the cell performance. Depending on the operating conditions, the different losses change in magnitude and characteristic time constants [1]. During technical operation of large-sized PEM Fuel Cells, gradients in operating conditions occur along the gas channels, resulting in an inhomogeneous current density distribution. This is caused by the locally varying electrochemical activity due to in-plane gradients in concentrations of reactants and reaction products, gas pressure and temperature. Consequently, each loss process occurs locally distributed to a varying degree. Therefore, a profound knowledge about the spatial distribution of losses is essential for optimizing the operating strategy as well as the cell and stack components. The commonly measured I/V characteristics provide only integral information about the overall performance. Thus, the data obtained represent merely the sum of all loss processes averaged over the entire cell area and do not allow detailed conclusions about the respective spatial distribution along the channel. Inevitably, in order to deconvolute the loss processes in the entire area of large sized cells, an approach is required that provides deeper insight in the processes and their interdependencies. For this purpose, an impedance-based methodology is developed that enables a spatially resolved deconvolution of loss processes. It is based on electrochemical impedance spectroscopy (EIS) and an impedance data analysis by the distribution of relaxation times (DRT) [2,3] that is applied to a segmented cell [4] with almost gradient-free segments. By systematically varying operating conditions the different loss processes are identified, quantified and separated with respect to their characteristic frequencies. In this contribution the design of the test bench and the segmented cell will be presented. Furthermore, first results regarding the electrochemical characterization and the impedance-based deconvolution of loss processes in a segmented cell will be discussed. M. Heinzmann et al., J. Power Sources 402, pp. 24-33 (2018). H. Schichlein et al., J. Appl. Electrochem. 32, pp. 875-882 (2002). E. Ivers-Tiffée et al., J. Ceram. Soc. Japan 125, pp. 193-201 (2017). T. Reshetenko et al., J. Electrochem. Soc. 163, pp. F1100-F1106 (2016)

The study of the behaviour of fuel cells by using various in-situ and ex-situ diagnostic methods is a main topic at the German Aerospace Center (DLR). The degradation of cell components of polymer electrolyte fuel cells (PEFC, DMFC) and of solid oxide fuel cells (SOFC) are of special interest. For this purpose physical and electrochemical methods are used individually as well as in combination. In addition to routinely applied electrochemical methods different methods for locally resolved current density measurements by means of segmented cell technology and integrated temperature sensors have been developed. The latest development with segmented bipolar plates based on printed circuit boards (PCB) is used both in single PEFC cells and stacks. Furthermore, a measuring system for segmented SOFC cells has been developed allowing for the spatially resolved characterisation of cells in terms of current density/voltage characteristics, impedance spectroscopy data, operating temperature and gas composition. The paper summarises the capabilities at DLR with respect to the analysis of fuel cells’ behaviour and gives examples of analytical studies to discuss the potentials and limitations of the diagnostic methodology that is applied.

Loss mechanisms in PEM Fuel Cells related to charge transfer reactions or diffusive gas transport result in a strongly nonlinear performance, which is furthermore affected by operating conditions as temperature, relative humidity of the gases and stoichiometries. These dependencies have to be considered and validated in fuel cell models to ensure accuracy. Thus, the interpretation of the simulated results becomes more reliable. Direct comparison of simulated and measured current/voltage-relation only allows to evaluate deviations in resulting cell voltages but exclude internal state variables of the model such as overvoltages due to different loss mechanisms. In consequence, simulated and measured values of voltage or current can concur by unnoticed compensation of different errors in magnitude and sign. Additionally, limited numbers of measurements and comparisons increase the probability and impact of this effect. Aggravating, every single process within the cell depends on the comprehensive combination of all operating conditions and runs simultaneously with all other ones. A direct comparison of individual simulated and measured polarization curves therefore is not a suitable and sufficient validation. We address this challenge by applying measurements of electrochemical impedance spectroscopy (EIS) during systematically varied operating conditions spatially resolved along the gas flow direction [1]. The loss processes within the cell can be separated by distribution of relaxation times (DRT) and quantified by a physico-chemical meaningful transmission line model. The resulting amount and distribution of resistances caused by loss processes is compared with the corresponding simulated values of a multiphysical model for observation and monitoring during operation [2]. These insights support the investigation of deviations between simulated and measured polarization curves and therefore the validation the model. In this contribution, the resulting physical interpretation is discussed. Consequences and conclusions for the further development of the cell model are demonstrated. [1]: P. Oppek et al., „ Spatially Resolved Deconvolution of Loss Processes in PEM Fuel Cells”, 241st ECS Meeting, Vancouver [2] T. Goosmann et al. „Impedance-Based, Multi-physical DC-Performance-Model for a PEMFC Stack”, 241st ECS Meeting, Vancouver

Layered transition metal oxides like NCAs (LiNixCoyAlzO2, with x+y+z=1) and NCMs (LiNixCoyMnzO2, with x+y+z=1) are used as cathode active materials (CAMs) for high energy Li-ion batteries due to their high capacity. However, at high upper cut-off potentials, those CAMs suffer from structural instabilities, resulting in severe capacity fading and thus limiting the accessible capacity that can be obtained. Possible causes for the capacity fade at high cut-off potentials and high state-of-charge (SOC) include the (electro)chemical oxidation of the electrolyte oxidation and transition metal (TM) dissolution from the CAM surface.1 Furthermore, layered TM-oxides are known to release lattice oxygen from the near-surface region at high SOC (i.e., at ≈80% SOC when referenced to the total amount of lithium), resulting in reactive oxygen species that induce electrolyte oxidation and HF formation.2 This release of lattice oxygen results in a surface reconstruction from the pristine layered structure to a more resistive spinel- or rocksalt-like structure, thereby inducing an impedance build-up on the cathode. Diffusion of dissolved transition metals to the anode and their subsequent deposition on the anode active material particles can also have a severe effect on cell aging, as the accumulation of metal species on the graphite anode has shown to catalyze the degradation of the protective anode solid/electrolyte interphase (SEI), eventually resulting in the loss of active lithium and in an anode impedance growth. Since the dissolution of manganese is considered to have the most detrimental effect on the anode SEI compared to cobalt and nickel,3 manganese-free NCAs (e.g., LiNi0.8Co0.15Al0.05O2) might have an advantage over manganese-containing NCMs. In this study, we will examine the potential-dependent dissolution of Ni and Co in NCA/graphite cells using operando XAS, and compare it to the potential-dependent dissolution of Ni, Co, and Mn from LiNi0.6Co0.2Mn0.2O2 (NMC622) that we had determined previously by operando XAS.4 Owing to the specially designed geometry of the operando XAS cell,5 we can spectroscopically access and independently investigate the concentration and oxidation state of transition metals, both dissolved in the electrolyte and deposited within the graphite anode. This is illustrated for an NCA/graphite cell in Figure 1. We will also examine the effect of lattice oxygen release from NCA on the NCA/graphite full-cell performance by applying different techniques: We employ a three-electrode Swagelok® type T-cell with a gold wire micro reference electrode (µ-GWRE)6 to quantify the anode and the cathode impedance over the course of 100 cycles as a function of the upper cutoff voltage. In addition, on-line electrochemical mass spectrometry (OEMS)7 is applied to detect the onset SOC for the release of lattice oxygen. From these comparisons, we aim to get a detailed understanding about the influence of transition metal dissolution from NCA on capacity fade and cycle life. References: J. A. Gilbert, I. A. Shkrob, and D. P. Abraham, Journal of The Electrochemical Society, 164 (2), A389-A399 (2017). R. Jung, M. Metzger, F. Maglia, C. Stinner, and H. A. Gasteiger, Journal of The Electrochemical Society, 164 (7), A1361-A1377 (2017). S. Solchenbach, G. Hong, A. T. S. Freiberg, R. Jung, and H. A. Gasteiger, Journal of The Electrochemical Society, 165 (14), A3304-A3312 (2018). R. Jung, F. Linsenmann, R. Thomas, J. Wandt, S. Solchenbach, F. Maglia, C. Stinner, M. Tromp, and H. A. Gasteiger, Journal of The Electrochemical Society, 166 (2), A378-A389 (2019). J. Wandt, A. Freiberg, R. Thomas, Y. Gorlin, A. Siebel, R. Jung, H. A. Gasteiger, and M. Tromp, Journal of Materials Chemistry A, 4 (47), 18300-18305 (2016). S. Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind, and H. A. Gasteiger, Journal of The Electrochemical Society, 163 (10), A2265-A2272 (2016). N. Tsiouvaras, S. Meini, I. Buchberger, and H. A. Gasteiger, Journal of The Electrochemical Society, 160 (3), A471-A477 (2013). Figure 1

Achieving the high energy density targets in all-solid-state batteries (ASSBs) will require thick cathodes optimized for full utilization of active material. In composite cathodes with sulfide SSEs, the exclusion of carbon additives makes cathode design to balance ionic and electronic conductivities all the more important. Li-Ni1/3Mn1/3Co1/3O2 (NMC111) is a widely studied cathode material for its high energy density and high working voltage. The lattice parameters of this well-studied structure directly correlate to the amount of Li in the material, allowing for very accurate measurements of lithium-ion concentration gradients (lithiation gradients) throughout the cathode depth. Spatially resolved lithiation gradients in thick NMC111/ Li6PS5Cl composite cathodes were measured using operando energy dispersive X-ray diffraction (EDXRD) on sealed and pressurized ASSBs with varying relative amounts of cathode active mass (CAM) and SSE. Electrochemical impedance spectroscopy (EIS) and transmission line modeling were employed to test the ionic and electronic conductivities of composite cathodes with similar varied CAM and SSE ratios. The balance of ionic and electronic conductivities was found to influence not only the lithiation gradients and utilization of active material in the cell, but also the NMC structural change throughout the depth of the cell resulting in peak bifurcation in the NMC (003) diffraction pattern. Sulfide ASSBs suffer from a large capacity loss on the first cycle due to formation of the cathode electrolyte interface (CEI). To study this interface formation and capacity loss, in situ EIS was performed during the first charge and discharge of the full cell (Li/In| Li6PS5Cl| NMC111/Li6PS5Cl). This piecewise EIS allows us to analyze both time dependent and state-of-charge dependent interfacial processes while the battery cycles with no modifications to cell design. Acknowledgments We acknowledge financial support from the National Science Foundation under Award Number CBET-ES- 1924534. This research used resources of the Advanced Photon Source beamline 6-BM, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Over the last decade, significant progress has been achieved in the commercialization of Polymer Electrolyte Membrane (PEM) fuel cells for mobile applications. However, long-term durability and stability of such systems still needs further improvement. To overcome today’s time-consuming long-term durability tests, the development of so-called accelerated durability tests (ADT) is urgently necessary. In this work, we present our recent results on such ADT test-protocols based on research promoting a detailed understanding of the individual aging mechanisms in PEM fuel cells. Various stressors had been investigated separately to understand their impact on overall fuel cell ageing. Certain levels of cell potential, temperature and humidity could be identified as major stressors and were successively added to the test protocol. In addition to the electrochemical test results, additional focus was put on different in-situ and ex-situ characterization methods. This combination enables a detailed understanding of individual stressors and their interply. The PEM-stack used in this study is an automotive high-performance stack developed within the publically funded Auto Stack Industry (ASI) project. An extensive long-term test over 5,500 operating hours was carried out as baseline test using the New European Driving Cycle, but with additional features to mimic realistic automotive load-profiles. Based on the results of this reference protocol, four different ADT protocols of only 1,200 hours test duration were derived. All tests were performed using short stacks with automotive-size active area. In-situ measurements include spatially resolved current measurement, cyclic voltammetry and impedance spectroscopy. After each test, selected cells were additionally analyzed ex-situ using layer thickness measurement via SEM, IR-thermography and contact angle measurements. The results from ex-situ investigations underline the degradation models postulated from in-site measurements. The overall data show that detailed understanding of individual stressors is necessary to successfully develop reliable accelerated ageing tests for PEM fuel cells.

Abstract A new method for acquiring temporal and spatially resolved electrochemical impedance spectroscopy data on a segmented proton exchange membrane fuel cell is developed. The cell used in this study consists of a segmented anode with 20 galvanically insulated segments. By measuring all segments simultaneously, restricting the frequency range, imposing all frequencies within one measuring task, and decoupling the acquisition and post-processing steps, we can reduce the acquisition time to 1 second. The results yield the spatial and temporal evolution of the TLM’s parameters, e.g., membrane resistance Rmem. Our results show that the temporal evolution of local Rmem strongly differs from the temporal evolution of the global Rmem. Moreover, we can see strong differences in the temporal evolution of local Rmem. The results further indicate that re-humidification after a decrease of stoichiometry appears almost instantly, while after an increase of stoichiometry it takes several minutes until the cell is dried out and equilibrated again. Furthermore, we can show, that the ratio of the 1 kHz resistance R1kHz to Rmem locally changes over time, making it an unsuitable substitute for Rmem. We therefore suggest R1kHz not to be taken as an indicator or substitute for Rmem.

La pile à combustible à membrane échangeuse de protons (PEMFC) est une technologie prometteuse, de plus en plus supérieure au moteur à combustion interne, en tant que groupe motopropulseur électrique pour l'application automobile. Au cours des dernières années, la déconvolution des processus internes a été étudiée de manière intensive, ce qui a permis d'améliorer les performances et la durée de vie tout en réduisant les coûts. Pourtant, de nombreux mécanismes sous-jacents ne sont pas encore totalement compris, car les méthodes expérimentales sont en partie limitées. La distribution d'eau dans une PEMFC joue un rôle central dans sa performance et sa réponse transitoire. L'étude expérimentale de son influence est difficile et coûteuse. En outre, les méthodes expérimentales qui peuvent combiner des données résolues spatialement et temporellement pour étudier les impacts au niveau de la cellule ou du stack industriel sont rares. Ce travail se concentre sur le développement de méthodes qui peuvent fournir un aperçu de l'influence de la distribution spatiale et temporelle de l'eau sur la performance des PEMFCs. Pour celà, une PEMFC hautement instrumentée et segmentée avec des canaux parallèles droits et son système de contrôle adapté sont conçus et optimisés. La flexibilité du système de contrôle « fait maison » permet de réaliser une mesure locale de spectroscopie d'impédance électrochimique (Lo EIS) le long du canal. De plus, un travail de paramétrage est réalisé : les résistances sont mesurées in situ en fonction de l'humidité et de la température. Cet étalonnage préliminaire permet d'estimer avec une resolution spatiale une humidité équivalente, sa distribution le long du canal, et son effet sur les performances en régime permanent. Afin d'étudier non seulement les conditions statiques, mais aussi les conditions transitoires, la méthode Lo-EIS a été développée pour devenir la méthode EIS rapide et locale (RaLo-EIS). La méthode RaLo EIS acquiert des spectres dans une gamme réduite de fréquences (10 kHz→20 Hz) pour les 20 segments de la cellule en 1 seconde. De telles données d'impédance résolues spatialement et temporellement constituent une nouveauté pour les PEMFC et offrent de nouvelles perspectives pour la compréhension du comportement transitoire des PEMFC. Avec la méthode RaLo, des expériences transitoires EIS sont menées, montrant l'inhomogénéité de la réponse d'une PEMFC. Cela révèle également que de nombreuses informations sont perdues, si uniquement les données moyennes globales sont acquises.Cependant, l'utilisation de résistances issues de l'ajustement des spectres pour remonter à l'humidité équivalente est assez compliqué. Dans un but de simplification, le même travail est réalisé à partir d'impédances à fréquence fixe. Pour prouver l'indépendance de l'humidité estimée par rapport à la fréquence selectionnée, une comparaison robuste de l'humidité relative résultante basée sur des fréquences multiples est effectuée. La comparaison montre que l'humidité équivalente résultante est presque indépendante de la fréquence choisie. Pour accélérer encore la vitesse d'acquisition de RaLo EIS, une version optimisée en termes de vitesse, le rapid and local fixed frequency (RaLoff) EIS, est développée pour acquérir des données à haute résolution temporelle. Cette méthode est ensuite utilisée pour étudier et caractériser la réponse transitoire de la PEMFC à des changements soudains de ses conditions de fonctionnement. Un modèle qualitatif est produit : il explique le comportement résultant en termes de performance et de distribution d'humidité. Une méthode de caractérisation des réponses transitoires est créée
Proton exchange membrane fuel cells (PEMFCs) represent a promising technology as an electric powertrain in automotive application that becomes increasingly superior to the internal combustion engine. Over the past years, the deconvolution of internal processes has been studied intensively, leading to an improvement of performances and lifetime while reducing the costs. Yet, many underlying mechanisms are still not fully understood, as the experimental methods are partly limited. Water distribution in a PEMFC plays a pivotal role in its performance and transient response. The experimental study of its influence is challenging and expensive. In addition, there is a lack of experimental methods that can combine spatially and temporally resolved data at a scale that allows investigation of impacts at the industrial cell or stack level.This work focuses on developing methods that can provide insight into spatially and temporally resolved influences of water distribution on the performance of PEMFCs and their investigation. Therefore, a highly instrumented and segmented PEMFC with straight parallel channels in combination with a self-developed control system is installed. The flexibility of the self-developed control system allows to establish a local electrochemical impedance spectroscopy (Lo EIS) measurement along the channel. In addition, a parametrization workflow is established, that links humidity sensitive resistances to temperature and applied relative humidity. These resistances, spatially resolved by Lo EIS, can then be transferred into a equivalent local humidity to study the humidity distribution along the channel and its effect on steady-state performances.In order to investigate not only steady-state but also transient conditions, Lo-EIS is further developed into the rapid and local EIS method (RaLo-EIS). The RaLo EIS acquires spectra within a reduced range of frequencies (10 kHz→20 Hz) for all 20 segments of the cell within 1 second. Such spatially and temporally resolved impedance data are a novelty for PEMFC and offer new insights into the transient behavior of PEMFCs. With RaLo EIS transient experiments are conducted, showing the inhomogeneity of the response of a PEMFC. It further reveals how much information is lost, if only globally averaged data is acquired.Since the use of humidity sensitive resistors that need to be fitted is challenging for the parameterized calculation of local humidity, fixed frequency impedances are used for this purpose. To prove the independence of fixed frequency a robust comparison of the resulting relative humidity based on multiple frequencies is conducted. The comparison shows that the resulting equivalent humidity is almost independent of the chosen frequency.To further speed up the acquisition rate of RaLo EIS, a speed optimized version, the rapid and local fixed frequency (RaLoff) EIS is developed to acquire temporally high resolved data. This method is then used to investigate and characterize the transient response of PEMFC to sudden changes in its conditions. A qualitative model is developed, that explains the resulting behavior in terms of performance and humidity distribution. A characterization method for transient responses is created, that can be used to reduce the complexity of such processes, making it possible to compare different materials in terms of their transient behavior.In the end a new approach for a transmission line model (TLM) is developed, as the established ones from literature are incapable of properly fitting experimental data for some cases. This new TLM is based on not yet validated assumptions but provides excellent matches with experimental data