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The achievement of durability targets is an important challenge for the commercialization of fuel cell electric vehicles (FCEV). In order to meet the requirements, knowledge about the most severe degradation mechanisms of fuel cel...
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The achievement of durability targets is an important challenge for the commercialization of fuel cell electric vehicles (FCEV). In order to meet the requirements, knowledge about the most severe degradation mechanisms of fuel cell stacks under automotive conditions is crucial. In the present work, degradation analysis of an automotive full size stack is performed. Herein, we focus on defects at the cathode catalyst layer and their interrelation including inhomogeneous adhesion of the microporous layer on the catalyst layer, crack formation, cathode catalyst layer thinning and wrinkling of the catalyst coated membrane. In addition, we report linear and circular Pt depositions on top of the cathode catalyst layer, which have to the best of our knowledge not been described in literature yet. For the latter, a degradation mechanism based on liquid water formation, local fuel starvation and current density distribution at the interface between microporous layer and cathode catalyst layer is postulated. Finally, a fast indication for stack degradation is suggested by correlating different degradation phenomena. This improved stack analysis approach allowed us to detect local differences in degradation on both cell and stack level.
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Fuel cell performance and durability are highly dependent on water management, wherefore wettability properties of the cell's gas diffusion layer (GDL) are important. In this work, we implement a method to determine the GDL wetted...
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Fuel cell performance and durability are highly dependent on water management, wherefore wettability properties of the cell's gas diffusion layer (GDL) are important. In this work, we implement a method to determine the GDL wetted surface area, which is based on capacitance measurements by cyclic voltammetry with a pH-neutral, aqueous electrolyte, to an automotive size fuel cell failure analysis process and demonstrate its benefit. The electrolyte penetrates large pores of the GDL, wherefore, in contrast to the most conventional methods, also inner parts of the GDL are measured. Tenside concentration of the electrolyte and penetration time, polytetrafluoroethylene treatment of the GDL, and properties of the microporous layer highly influence the capacitance values. Thus, the method is sensitive to different GDL morphologies and surface modifications. Different degradation patterns for samples either artificially chemically and mechanically aged or after real-operation (e.g., prototype vehicle) are detected by the method. For a comprehensive understanding, the obtained results are compared to ex situ degradation analysis data. A comparison to static contact angle measurements, being a state-of-the-art method to determine GDL wettability, reveals a higher sensitivity of the introduced method to detect degradation, in particular, of chemically aged and real-operation aged GDLs.
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In this study a method for the uncertainty quantification of friction induced vibrations based on the mode coupling phenomenon is shown. The main focus is the assessment of the phenomenon under consideration of uncertain input par...
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In this study a method for the uncertainty quantification of friction induced vibrations based on the mode coupling phenomenon is shown. The main focus is the assessment of the phenomenon under consideration of uncertain input parameters for the robustness evaluation. Stability assessments of the system under parameter scatter are given. It is pointed out how this is implemented within the scope of the Finite Element method. On the basis of the Euler-Bernoulli beam as a proof-of-concept model a procedure for the assessment of the system's robustness is shown. An objective function is proposed and used to evaluate a design of experiment. By means of a regression analysis an indicator for the robustness of the system is given. Numerical results are presented on the basis of the Euler-Bernoulli beam and a Finite Element brake model.
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Knowledge gained by fuel cell degradation analysis is important for meeting durability targets and thus for the commercialization of polymer electrolyte membrane fuel cells. Herein, the application of the most appropriate characte...
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Knowledge gained by fuel cell degradation analysis is important for meeting durability targets and thus for the commercialization of polymer electrolyte membrane fuel cells. Herein, the application of the most appropriate characterization method is crucial depending on which degradation mode is dominating. One major failure mode in polymer electrolyte membrane fuel cells is the formation of pinholes in the polymer membrane. In this work, a systematic study of four promising methods for pinhole detection is performed, namely detection via infrared camera, via He-gas detection as well as via pressure drop and hydrogen crossover measurements between anode and cathode. In particular, detection limits as well as influences of the material system and other relevant factors are discussed. Finally, a comparison of the different methods and a recommendation for their practical application is given to optimize detection of membrane defects in the field of failure analysis. The most successful ones are then tested for the analysis of real automotive stacks before and after operation.
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Ramping up the full-electric vehicle market share heavily relies on the extension of electrical driving range, as well as on the reduction of charging time and cost. New Li-batteries should, at the same time, offer at least the sa...
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Ramping up the full-electric vehicle market share heavily relies on the extension of electrical driving range, as well as on the reduction of charging time and cost. New Li-batteries should, at the same time, offer at least the same levels of power, lifetime and safety as the one nowadays available on the market. The achievement of these goals requires the development of new electrode materials with improved capacity, operating voltage, transport properties together with cycling and temperature stability. Several new anode materials have been proposed over the last decade. In this contribution we critically evaluate their chance to find application in the future automotive batteries. First, we discuss their properties at the material's level, subsequently, the energy density for selected candidates is calculated at the automotive battery cell level using an in-house developed software. If available, literature results concerning power capability and lifetime are also discussed with reference to the automotive targets.
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Remote laser welding of high strength aluminum alloys is still a field of extensive research due to the hot cracking phenomena. Recent research activities have been focused on center-line hot cracks in welds that are located close...
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Remote laser welding of high strength aluminum alloys is still a field of extensive research due to the hot cracking phenomena. Recent research activities have been focused on center-line hot cracks in welds that are located close to the edge of the material. To avoid this type of crack from occurring, a fillet weld joint design could be utilized. However, within the fillet welds there are still transverse hot cracks present on a microscopic scale. This paper presents a thermomechanical analysis of the formation of transverse hot cracks in EN AW-6082 alloy fillet welds. In order to calculate the local deformation near the mush zone, a welding simulation based on the finite element method was created. The simulation was able to depict different settings for the welding speed, laser power, beam position, protruding length of the lower sheet, and the sheet thickness. For this purpose, an automated method for heat source calibration was developed based on image processing of polish of cross sections of different weld seams. As a result, it was possible to investigate the influencing factors on the formation of local strain and strain rate during the hot crack sensitive temperature range. It was found that the welding speed and laser power significantly increased the strain rate, but had no effect on the strain. In addition, it was determined that the position of the laser beam caused a major difference in the formation of strain and strain rate if weld seams changed from partial penetration to full penetration. Full penetration welds had lower strain and strain rate. The protruding length of the lower sheet and the sheet thickness had a minor impact on strain and strain rate. By linking the computerized results of strain and strain rate with data obtained from experiments on the hot crack susceptibility, a hot cracking criteria based on strain rate was found. (C) 2016 Laser Institute of America.
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Objective: The feasibility of measuring drivers' automation trust via gaze behavior during highly automated driving was assessed with eye tracking and validated with self-reported automation trust in a driving simulator study.
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To make a Lithium Ion Battery (LIB) reliably rechargeable over many cycles, its graphite-based negative electrode requires the solid electrolyte interphase (SEI) as a protection layer. The SEI is formed through chemical and partic...
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To make a Lithium Ion Battery (LIB) reliably rechargeable over many cycles, its graphite-based negative electrode requires the solid electrolyte interphase (SEI) as a protection layer. The SEI is formed through chemical and particularly electrochemical side reactions of electrolyte components in the first charging cycle(s) after manufacturing of a LIB. The SEI ideally serves two purposes: (i) act as a sieve permeable to Li ions but not to other electrolyte components and (ii) passivate the electrode against further electrolyte decomposition. Core element of conventional SEI formation is a lengthy, low-current galvanostatic charging step, which due to its time consumption contributes heavily to cell manufacturing costs. Here, we report on some non-conventional SEI formation protocols for composite carbon electrodes, inspired by recent experimental findings at smooth model electrodes. Acknowledging that the SEI forms in two main steps, taking place in a high-potential and a low-potential region, respectively, we demonstrate that less time spent in the high-potential region not only makes the process faster but even yields SEIs with superior kinetic properties. We tentatively explain this via basic rules of thin film growth and the role of grain boundaries for ion transport. We also report on the positive influence of multi-frequency potential modulations applied between high-potential and low-potential formation. Given that any new cell chemistry in principle requires its own tailor-made formation process, technologic success of future LIB cells will benefit from a systematic, well-understood toolbox of formation protocols. This paper is meant as a first step, highlighting potentially low-hanging fruits, but also flagging the demand for further systematic studies on model systems and on commercially manufactured cells. (C) 2018 Elsevier Ltd. All rights reserved.
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In order to gain a better understanding of the crashworthiness of lithium-ion cells, a test-setup for dynamic impact and crush tests has been designed. An experimental study was carried out using prismatic automotive cells compris...
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In order to gain a better understanding of the crashworthiness of lithium-ion cells, a test-setup for dynamic impact and crush tests has been designed. An experimental study was carried out using prismatic automotive cells comprising increasing levels of manufacturing quality and specific energy made of lithium nickel manganese cobalt oxide as the cathode active material. Different loading scenarios and more than four orders of magnitude of deformation rates were applied to specimens at fully charged state. The presented work describes the test program, the experimental setup and an objective evaluation method, which finally allows for a detailed summary of the observed mechanical behavior. A distinct strain-rate dependence of hardening, failure parameters, and compressibility of the cells is found. No significant dependence on cell type and state of charge could be observed. The results constitute essential new insights into the material behavior of EV battery cells during a crash event.
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We compare the stability of alkyl carbonate electrolyte on NMC111, -622, and -811, LNMO, and conductive carbon electrodes. We prove that CO, and CO evolution onset potentials depend on the electrode material and increase in the or...
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We compare the stability of alkyl carbonate electrolyte on NMC111, -622, and -811, LNMO, and conductive carbon electrodes. We prove that CO, and CO evolution onset potentials depend on the electrode material and increase in the order NMC811 < NMC111 approximate to NMC622 < conductive carbon approximate to LNMO, which we rationalize by two fundamentally different oxidation mechanisms, the chemical and the electrochemical electrolyte oxidation. Additionally, in contrast to the widespread understanding that transition metals in cathode active materials catalyze the electrolyte oxidation, we will prove that such a catalytic effect on the electrochemical electrolyte oxidation does not exist.
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