35 research outputs found

    Application of Electrochemical Impedance Spectroscopy on Different Battery Circuits

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    For the operation of a battery pack, the cell state estimation plays a central role. For that, enough information about the current charge condition (SoC, state of charge) and the health status (SoH, state of health) of the individual cells or cell strings must be available. One way to draw out conclusions about the state of charge and health provides the electrochemical impedance spectroscopy (EIS) [1]. The test cells are thereby stimulated with an alternating current signal, and the resulting voltage signal is detected. These results in cell impedances, which are addicted to the signal frequencies and the respective cell states. This poster shows an experimental platform which uses the EIS to detect asymmetries in SoC and/or SoH on circuited cells. For that, the behavior of the amplitudes and frequencies of the signals should be analyzed, because for the calculation of the precise impedance, these factors are crucial. Thereby the required alternating current and voltage signals are acquired and analyzed separately for each single cell. As cell type lithium iron-phosphate round cells of the size 18650 are used. The investigations are made on a series circuit (Fig.1) made up of three cells and on a parallel circuit made up of two strings, each having two cells in series. It shows that both a series and a parallel connection within the working range the experimental platform impedances of individual cells can be determined. For these cases, differences in state of charge and state of health can be highlighted and assigned to the respective cells

    The importance of nerve microenvironment for schwannoma development

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    Schwannomas are predominantly benign nerve sheath neoplasms caused by Nf2 gene inactivation. Presently, treatment options are mainly limited to surgical tumor resection due to the lack of effective pharmacological drugs. Although the mechanistic understanding of Nf2 gene function has advanced, it has so far been primarily restricted to Schwann cell-intrinsic events. Extracellular cues determining Schwann cell behavior with regard to schwannoma development remain unknown. Here we show pro-tumourigenic microenvironmental effects on Schwann cells where an altered axonal microenvironment in cooperation with injury signals contribute to a persistent regenerative Schwann cell response promoting schwannoma development. Specifically in genetically engineered mice following crush injuries on sciatic nerves, we found macroscopic nerve swellings in mice with homozygous nf2 gene deletion in Schwann cells and in animals with heterozygous nf2 knockout in both Schwann cells and axons. However, patient-mimicking schwannomas could only be provoked in animals with combined heterozygous nf2 knockout in Schwann cells and axons. We identified a severe re-myelination defect and sustained macrophage presence in the tumor tissue as major abnormalities. Strikingly, treatment of tumor-developing mice after nerve crush injury with medium-dose aspirin significantly decreased schwannoma progression in this disease model. Our results suggest a multifactorial concept for schwannoma formation-emphasizing axonal factors and mechanical nerve irritation as predilection site for schwannoma development. Furthermore, we provide evidence supporting the potential efficacy of anti-inflammatory drugs in the treatment of schwannomas

    FTIR spectroscopic imaging of aged fuel cell components

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    Regarding fuel cells as an environmen-tally friendly energy converter one of the major unsolved problems beside the costs is an insufficient lifetime. In poly-mer electrolyte fuel cells humidity management is one of the key point for their performance. The water management is mainly determined by the hydrophobic/hydrophilic properties of the gas diffusion layer (GDL) including the micro porous layer. Thus the degradation of GDL and micro porous layers MPL therein are the key point for the understanding of reduced fuel cell lifetimes. In the past the partially decomposition of the polymers in the GDL and the MPL was observed by X-ray photoelec-tron spectroscopy measurements on fresh and aged GDL and MPL. The decomposition of the polymers due to fuel cell operation changes the hydrophobic/hydrophilic properties of the GDL, MPL and catalyst layer and conse-quently also the water balance in the cell.This poster demonstrates the applicability of Fourier transform infrared microscopy (FTIR) and Raman microscopy to investigate membrane-electrode-assemblies (MEA) and their components, e.g. the gas diffusion layer (GDL) and micro porous layer (MPL). For the study of the catalyst layer other methods that provide direct information on elemental material composition are more suitable. The presented data comprise fresh MEAs only that can be used as reference samples to study specific degradation mechanis

    Influence of realistic load profiles on Lithium-Ion Cell Degradation

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    The degradation of batteries is dependent of the use in their particular application. The loss of capacity because of cycling correlates in some degree to utilization in consumer electronic devices. But the utilization in battery electric vehicles (BEV) or stationary applications like frequency regulation for grid stability imply a different strain for the used battery cells. For example a BEV (with recuperation) in urban traffic accelerates and decelerates periodically. This represents a special stress influence due to the alternating load between discharge and charge. In this study the effects of BEV utilization on lifetime performance of commercial Lithium-Ion cells were investigated. Therefor single cells were stressed in a test bench with different realistic load profiles according to standardized driving cycles. The used data were obtained from a Modelica simulation of the city car Hotzenblitz which is a limited-lot production vehicle. The actual vehicle is equipped with a 12kW electric traction drive and reaches a maximum speed of 112km/h. For the simulation an increased traction motor of 30 kW was assumed, so the applied driving cycles could be fulfilled. For the tests the Common Artemis Driving Cycle (CADC) Urban were compared with the CADC Road to face the SOH loss due to frequent acceleration and deceleration in urban traffic to high kilometric performance in overland traffic. The New European Driving Cycle (NEDC) was used as reference

    Development of self-supporting MPLs for investigations of water transport in PEM fuel cells

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    The performance of a polymer electrolyte membrane (PEM) fuel cell has a strong dependence of its water management. The membrane needs humidity to have sufficient ion conductivity. But at high humidity, especially at high current densities, flooding of the electrodes can occur and consequently the available active area begins to decrease. The primary purpose of a micro porous layer (MPL) on a gas diffusion layer (GDL) is the effective wicking of liquid water from the catalyst layer into the diffusion media as well as reducing electrical contact resistance with the adjacent layers. In synchrotron radiography studies the importance of liquid water pathways through the porous structure for the water management is proven. These pathways can be formed by natural cracks in the MPL and the texture of the carbon fibre substrate or by artificial pore paths through the GDL. With artificial paths in a carbon fibre GDL produced by laser perforation an overall performance gain has been obtained. To get additional information about the function of the MPL as an interconnection between the reaction layer and the macro porous carbon fibre substrate a self-supporting MPL was developed. This allows the manufacturing and the following treatments of the MPL independent from the GDL substrate. This MPL consists of a thin nonwoven of synthetics coated on one side with a mixture of carbon and PTFE produced by the dry spraying technology. It is possible to perforate this layer alone and press it with the non-coated side on a commercial GDL without MPL (Sigracet® GDL25BA from SGL). Thus it was feasible to perform experiments for investigation of the influence of artificial pores in the MPL on the water management. As a consequence, the liquid water transport of non-perforated GDL/MPLs is compared to the perforation of both layers as well as to the exclusive perforation of MPL and the GDL, by means of in-situ synchrotron imaging. Further measurements, in particular Ucell(i)-curves up to limiting current densities and electrochemical impedance spectra were done in a 5 cm² fuel cell setup, to obtain a correlation of the global intrinsic properties of the MPL, like through-plane permeability, electrical conductivity or hydrophobicity, with fuel cell performance

    Diagnostics of PEM fuel cells

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    The analysis of processes in low temperature fuel cells (PEFC, DMFC) is a main topic at the Ger-man Aerospace Center (DLR). The degradation of cell components, especially electrodes, and changes during start-up processes are of foremost interest. For this purpose physical and elec-trochemical methods are used individually as well as in combination in order to study the cell behaviour in terms of performance and durability. In addition to routinely applied electrochemi-cal methods such as I-V characteristics, impedance spectroscopy and chronopotentiometry and chronoamperometry, 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 cells and different fuel cell stacks. In addition, the current density distribution measurements have a high potential for the application of controlling fuel cell systems and early detection of malfunctions and failures in fuel cell stacks. The use of ex-situ methods before and after operation is needed to complete the interpretation of in-situ electrochemical data and to identify degradation mechanisms. Physical characterization methods such as SEM/EDX, XPS, XRD, TPDRO, porosimetry etc. are applied to investigate electrodes, gas diffusion layers and bi-polar plates. Recently, a test facility for the investigation of time-resolved water sorption of membrane materials in combination with electrochemical impedance spectroscopy and also a test rig which enables electrochemical measurements at low pressure has been set up. The elec-trochemical characterization at low pressure can simulate fuel cell operation at high altitudes as it is relevant for aircraft application (0.2 bar (abs.) corresponds to approximately 12.000 m). The main objectives with regard to the use of the analytical tools mentioned are the understand-ing of the fuel cell’s behaviour and its processes and the support of the development of fuel cell components. The investigation of degradation processes allows identifying critical operating conditions and, in combination with the in-situ diagnostics, to develop intelligent control strate-gies. The presentation summarizes the capabilities at DLR with respect to the analysis of fuel cell’s behaviour and gives examples of analytical studies to discuss the potentials and limitations of the diagnostic methodology that is applied

    Investigation of GDL degradation behaviour by half cell tests

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    The objective of this work is to investigate the wettability of gas diffusion layers (GDL) by electrochemical tests and to analyse the change of hydrophobicity due to degradation effects. The common GDL consists of conductive, hydrophilic (coal) and non conductive, hydrophobic (PTFE) components. The change of the ratio of conductive to non conductive area, which can be measured by wetting with liquid electrolyte in an electrochemical measurement setup induces a change of the electrochemical double layer capacitance (Fig. 1). Therefore measurements were performed with five different GDLs to determine the double layer capacitances. Four GDL samples were pre-treated by exposure at defined time in an autoclave (Fig. 2) at 130°C in 1 N sulphuric acid to enforce a degradation effect. One untreated GDL sample was used as reference
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