1,721,011 research outputs found
Electrolyte distribution in partially flooded porous electrodes of electrochemical energy conversion devices
No full textThis thesis investigates the electrolyte distribution in porous electrodes of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) and vanadium redox flow batteries (VRFBs). Overall, the electrolyte distribution directly impacts performance and lifetime of these systems and hence system efficiency. The analysis of both systems allows to highlight the influence of various parameters on the electrolyte distribution in porous electrodes and helps to develop an approach for optimization.In dieser Dissertation wurde die Electrolytverteilung in porösen Electroden von Hochtemperatur-Polymerelektrolytmembran Brennstoffzellen und Vanadium Redox Flow Batterien untersucht. Die Electrolytverteilung hat einen direkten Einfluss auf die Leistungsfähigkeit und Lebensdauer dieser beiden System und damit auch auf die Effizienz. Die Analyse fokussiert sich auf den Einfluss verschiedener Parameter auf die Elektrolytverteilung in porösen Elektroden und hilft, einen Optimierungsansatz zu entwickeln
Untersuchung der Reaktionen und Prozesse in Vanadium-Redox-Flow-Batterien – von der Grundlagenforschung zur Anwendung
No full textRenewable energy sources such as wind, water, or sunlight are good alternatives to fossil fuels, which are the largest source of carbon dioxide emissions worldwide and contribute significantly to climate change. However, wind and solar power are intermittent. Therefore, large-scale energy storage devices must be installed to balance the intermittency and combat greenhouse gas emissions and climate change. Vanadium redox flow batteries (VRFBs) are a commercially available technology that can provide the required flexibility and scalability to store large amounts of energy for long periods of time, thereby balancing the intermittency of renewable energy sources. This thesis focuses on bridging the gap between the fundamental characterization of the processes and reactions in VRFBs and the application of this knowledge to the investigation of electrodes and their treatments and the development of novel materials.
First, a novel approach to study the processes in the half-cells of a VRFB was presented. Electrochemical impedance spectroscopy combined with the distribution of relaxation times analysis allows to study the chemical and physical processes in the positive and negative half-cells, namely the electrochemical reaction, the transport through the porous structure of the electrode and the ion transport. The frequency ranges and impedances associated with these processes are different for each half cell. Therefore, the electrode materials for the positive and negative half-cells must be individually optimized to achieve excellent overall efficiency of VRFBs.
This approach was combined with several other characterization techniques to study the electrode materials in detail. Three key properties were identified that are required for excellent VRFB performance: the 3D electrode structure, the wettability, and the electrochemical performance of the electrode material. Using this multimodal characterization approach, a wide range of potential electrodes were investigated, including different thermal treatments, modifications, aging treatments, and novel materials. Significant differences in overall performance were found and the knowledge was used to further develop the conditions for treatments, modifications, or fabrication parameters of novel materials.
In addition, the unwanted side reaction hydrogen formation, which significantly reduces the efficiency of a VRFB, was studied in detail using synchrotron X-ray imaging with a focus on electrode materials and applied potential. Based on the results, the material optimization process should also include a step to evaluate hydrogen bubble generation in the electrode to prevent degradation in the VRFB.
In conclusion, VRFBs are already commercialized and several companies worldwide offer products for long-term energy storage. However, VRFBs still need to overcome challenges related to long-term stability, efficiency, and cost in order to compete in the energy storage market
Der Einfluss der Adsorption auf die Sauerstoffreduktionsreaktion : perfluoralkylierte Phosphorsäurederivate als Modellverbindungen zur Untersuchung der Anionenadsorption an Platin
No full textIn this work, the influence of anion-adsorption on the oxygen reduction reaction (ORR) at platinum electrocatalysts is evaluated. By chemical modification of the electrolyte composition, polarization-losses for the ORR shall be reduced in order to enhance power-densities in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs).
This was studied systematically for the first time by substitution of hydroxyl groups in phosphoric acid by perfluorinated alkyl chains. Perfluoro-alkylated phosphonic- and phosphinic acids proved to be both electro-chemically and thermally stable enough to serve as potential HT-PEMFC electrolytes. Compared with H3PO4 (PA), these acids exhibit substantially weaker adsorption tendencies on the surface of a platinum-based electrocatalyst. Consequently, overpotentials for the ORR on a flat platinum surface are reduced by 75%. The number of hydroxyl groups within the acid molecules and the negative charge density of their corresponding anions were identified as the main descriptor for their adsorption behavior, whereas the length of the perfluorinated side chain did not affect the ORR kinetics negatively. The activation barrier for the ORR on a flat platinum surface was reduced by 40% by adding bis(pentafluoroethyl)-phosphinic acid (BPPA) to the host electrolyte instead of PA. Under the same experimental conditions, the charge transfer resistance for the ORR decreased by more than three orders of magnitude.
Electro-catalytic activities for the ORR at Pt3Co/C- and Pt/C-electrocatalysts were found to be significantly higher in perfluorinated electrolytes than in PA. In the pres-ence of BPPA, an enhancement of 85% was achieved. After accelerated aging proce-dures, the electro-chemical active surface area (ECA) decreased significantly.
However, electro-catalytic activities for the ORR were still three orders of magnitude higher compared with PA under the same experimental conditions. The ORR-selectivity at Pt3Co/C- and Pt/C was measured to be dependent on the nature of the adsorbed species. The rate of peroxide production was correlated with the steric demand of the anionic species, with larger anions yielding more hydrogen peroxide.
A HT-PEMFC single cell setup was successfully operated with mixtures of PA and 20% perfluoroalkyl-substituted phosphonic acids. Compared to operation with PA as an electrolyte, the performance was enhanced significantly
Characterization and modification of carbon electrodes for vanadium redox flow batteries
No full textThis thesis aims to provide an extended insight into the carbon felt electrodes of vanadium redox flow batteries and suitable characterization techniques. The carbon felt electrodes are a crucial component of the battery since they facilitate the redox reactions of the electroactive species in the electrolyte. They provide a large surface area, due to their porous structure, and a sufficient number of active sites, which is affected by the chemical composition and pretreatment procedures. Additionally, each characterization technique itself unveils various aspects of the carbon felt and only the combination of several methods leads to a comprehensive insight. Moreover, the utilization of sophisticated methods, such as differential electrochemical mass spectrometry and synchrotron X-ray radiography and tomography, enabled an unprecedented view inside the vanadium redox flow battery.
The first objective of this thesis was to develop an accelerated aging procedure to establish a systematic benchmark for carbon felt electrodes. The procedures include thermal treatment, which is widely utilized to increase the wettability and electrochemical performance of the carbon felt, as well as a chemical and an electrochemical aging to distinguish the influence of the acidic electrolyte and a long term application in a full-cell of a vanadium redox flow battery, respectively. In the first publication (A1), the procedures were applied on three different carbon felt electrodes, which subsequently were characterized by a wide range of state-of-the-art methods to outline the effects of the accelerated aging, and to single out the differences in the carbon felt types. In a complementing study (A2), a pore network model was applied to investigate the transport properties of carbon felt electrodes for the first time in the context of vanadium redox flow batteries. The comprehensive characterization introduced in the first publication was also applied to carbon-carbon composite materials synthesized by a novel method. This provided extensive insight into the properties and performance of the composite materials and led to the publication reprinted in A3. Beyond that, the artificial aging procedures were also adopted in the context of the investigation on the carbon-based gas diffusion electrodes of fuel cell systems, as shown in publication A4. Here, the stability of a novel hydrophobic coating applied on electrospun gas diffusion electrodes was studied via previously introduced degradation protocols and characterization methods.
Based on the findings of Part A, Part B of this thesis applied the differential electrochemical mass spectrometry via a modification that enabled the mounting of a carbon felt electrode for the first time. This method provided an unprecedented insight into the influence of the electrolyte on the side reactions taking place on a carbon felt electrode by the comparison of vanadium-containing electrolyte and the solely sulfuric acid-based electrolyte (B1). Additional experiments in B2 focused on how the above-mentioned accelerated aging procedures affect the side reactions and reveal how the stability of the carbon felt electrodes and their affinity to side reactions changes throughout the operational life of a vanadium redox flow battery.
The third pillar of this thesis (Part C) comprises the comprehensive characterization of carbon felt electrodes via synchrotron X-ray radiography and tomography. In publication C1, a detailed study is presented that provides an experimental approach to the pore network model applied in Part A of this thesis. It investigated the influence of the electrode compression ratio and the vanadium species on the saturation – and thus utilization – of the carbon felt electrode. Here, a small sample size allowed a high resolution of the obtained images and an accurate measurement of the pressure drop during electrolyte injection experiments. This approach was previously applied to investigate fuel-cell materials and successfully adopted in the context of the vanadium redox flow battery for the first time. Based on these results, a consequential step was the development and design of a novel vanadium redox flow full-cell that enables a comprehensive investigation via synchrotron X-ray radiography and tomography. In the final publication (C2), a set of viable experiments were presented that investigate the electrolyte flow through the porous electrode in an operating vanadium redox flow battery
Materials and characterization techniques for vanadium redox flow batteries
No full textThe ever-increasing demand for energy and the negative impact of traditional, fossil fuel-based energy production on climate change drive the investment in alternative renewable energy sources to reduce greenhouse gas emissions. However, this change in power generation sources requires load-balancing technologies to store energy and buffer the grid during fluctuations. Redox flow batteries are one type of electrochemical storage technology with a flexible and easily scalable design useful for small-scale energy storage in private homes and large-scale energy storage in solar or wind farms. However, their development is still at an early stage compared to other types of batteries.
This thesis and its associated publications cover various aspects impacting the performance of vanadium redox flow batteries (VRFBs), such as the use of different electrode materials, modifications, or treatments, the (side) reactions occurring in VRFBs, and the electrolyte transport and distribution in VRFB electrodes. For this, traditional characterization techniques are combined with and related to innovative approaches, many of which are new to the field of VRFB research
Entwicklung und Verifikation von beschleunigten Lebensdauertests für PEM-Brennstoffzellen basierend auf realen Alterungsmechanismen
No full textDie Technologie der Polymer-Elektrolyt-Membran-Brennstoffzellen (PEMFC) erlebt derzeit einen bedeutenden Aufschwung. Trotz großer Entwicklungsfortschritte sind verlässliche Prognosen hinsichtlich der erwarteten Lebensdauer von Brennstoffzellenstacks und -systemen in mobilen Anwendungen nach wie vor eine Herausforderung. Grund dafür ist der Zeitaufwand, der mit belastbaren Lebensdaueruntersuchungen einhergeht, und nicht immer mit den schnellen Entwicklungszyklen der Industrie vereinbar ist. Beschleunigte Alterungstests werden im Forschungsumfeld bereits seit vielen Jahren entwickelt und eingesetzt. Die im Englischen als Accelerated Stress Tests (ASTs) bezeichneten Protokolle sind besonders effektiv bei der Untersuchung von Materialien, einzelnen Komponenten oder spezifischen Degradationsmechanismen. Da ASTs jedoch typischerweise an kleinen Einzelzellen unter sehr extremen Betriebsbedingungen durchgeführt werden, liefern sie keine verlässlichen Vorhersagen für die in realen Anwendungen erwarteten Lebensdauern. Die vorliegende Arbeit versucht die bestehende Lücke zwischen etablierten ASTs im Forschungskontext und weniger standardisierten Lebensdauertests im industriellen Entwicklungsumfeld zu schließen. Dafür wurde eine systematische Methode zur Entwicklung beschleunigter Lebensdauertests (Accelerated Durability Tests, ADTs) erarbeitet, die auf realistischen Betriebsbedingungen basiert, und speziell für den automobilen Einsatz von PEMFC geeignet ist. Ein zentrales Element der Forschung war die Definition und schrittweise Verschärfung spezifischer Stressoren, wie Lastprofile, Betriebstemperaturen oder Feuchtezyklen. Es wurden mehrere ADTs auf Kurzstapeln im Automobilformat durchgeführt, um eine detaillierte Untersuchung der Auswirkungen dieser Stressoren auf die Alterung der einzelnen Komponenten der Membran-Elektroden-Einheit (MEA) zu ermöglichen. Die Ergebnisse zeigen, dass durch die gewählten Stressoren eine beschleunigte Degradation mit einem Faktor von drei bis sieben im Vergleich zum durchgeführten Referenz-Langzeittest erreicht werden konnte. Zur wissenschaftlichen Bewertung der ADT-Protokolle wurde ein sogenannter Alterungs- Fingerabdruck unter Verwendung von diversen in-situ und ex-situ Charakterisierungsmethoden ermittelt. Damit können die ADTs mit dem Referenztest verglichen werden und die Auswirkungen auf die Kernkomponenten der PEMFC: Membran, Elektroden, Gasdiffusionslage (GDL) und Bipolarplatte (BPP), analysiert werden. Durch den systematischen Ansatz lassen sich klare Zusammenhänge zwischen den gewählten Stressoren und den beobachteten Degradationserscheinungen ermitteln. Die Arbeit zeigt einerseits, dass die beschleunigte Alterung von PEMFC-Stacks unter realistischen Betriebsbedingungen möglich ist, und andererseits, dass die derzeit angestrebten Lebensdauerziele im automobilen Einsatz mit state-of-the-art MEAs erreicht werden können. Die erarbeitete Methodik bietet darüber hinaus einen universellen Ansatz, der auf unterschiedliche Anwendungen und Materialien angepasst werden kann, und stellt damit einen wichtigen Beitrag zur Weiterentwicklung der Brennstoffzellentechnologie dar.The technology of polymer electrolyte membrane fuel cells (PEMFC) is currently experiencing significant scale-up. Despite substantial progress in development, reliable predictions regarding the expected lifetimes of fuel cell stacks and systems in mobile applications remain challenging. This challenge is primarily due to the time-intensive nature of durability testing, which is not always compatible with the rapid development cycles in the industry. Accelerated Stress Tests (ASTs) have been developed and employed in fundamental research for many years. These protocols are particularly effective in examining materials, individual components, and specific degradation mechanisms. However, since ASTs are typically conducted on small-scale single cells under very harsh operating conditions, they do not yield reliable predictions for the lifetimes expected in real-world applications. This work addresses the gap between well-established ASTs in research settings and the less standardized durability testing approaches within industrial development. A systematic methodology for developing so-called Accelerated Durability Tests (ADTs) was established, based on realworld operating conditions and specifically tailored for automotive applications of PEMFCs. A key focus of this research was the definition and progressive intensification of specific stressors, such as load profiles, operating temperatures, and humidity cycles. ADTs were carried out on short stacks with an automotive-relevant active area, enabling a detailed investigation into the effects of these stressors on the degradation of individual components of the membrane electrode assembly (MEA). The results demonstrate that the selected stressors induced accelerated degradation by a factor of three to seven compared to a conducted reference long-term test. A so-called aging fingerprint was determined using various in-situ and ex-situ characterization techniques to compare the ADTs with the reference test, and to assess the impacts on the core components of the PEMFC: membrane, electrodes, gas diffusion layer (GDL), and bipolar plate (BPP). This systematic approach allows clear correlations between the imposed stressors and the observed degradation phenomena. The findings of this study indicate that accelerated aging of PEMFC stacks under realistic operating conditions is feasible, and that the current automotive lifetime targets can be achieved with state-of-the-art MEAs. Moreover, the developed methodology provides a versatile framework that can be adapted to various applications and materials, thereby contributing significantly to the advancement of fuel cell technology
Feldeffekttransistoren auf der Basis von Schicht-Einkristallen
No full textWith parylene as a gate-dielectric material, I was able to successfully produceFETs based on a variety of organic as well as transition metal dichalcogenidesemiconductors. The results of this work are briely summarized below.By employing single crystals, the device performance, including thecharge carrier mobility, the field effect threshold, and the subthresholdslope, have been significantly improved. The FET characteristics areno longer limited by the disorder common for thin films. To limit the concentration of impurities, it is preferable to grow crystalsby physical vapor transport since crystals grown from solution tend toincorporate the solvents into the intra-molecular position in the weaklybonded Van der Waals network of molecules. However, organic materials especially are known for undergoing severaldisproportionation reactions during the sublimation process. The productsof these reactions may become embedded into the host crystal. Tolimit the formation of impurities and therefore improve the electroniccharge transport in the crystal the sublimation temperature and thepresence of oxygen during the growth process should be reduced.If the structure of a molecules is conjugated, impurities themselves canshow field effect activity. In general, an important criterion for thechoice of potential organic semiconductor material seems to be to pickmolecules that consist of an alternating sequence of single and doublebonds which allow charge transport though the molecules. So far, of all the organic semiconductor materials, rubrene exhibits the best device performance. A mobility anisotropy and an increase of mobility with cooling indicating intrinsic charge transport was observed only for rubrene single crystal FETs. Modifying tetracene molecules allows changing the herring bone packing, thus affecting the electronic transport properties of the system. The pi- stacking structure was obtained when two hydrogen atoms of tetracene were substituted by chlorine, and a mobility exceeding that of tetracene was observed along the stack direction. Overall, the pi-orbital overlap in the crystal plays a crucial role for the device performance of the semiconductor material. The perylene-TCNQ charge-transfer salt presents a different approach,where the combination of two different molecules in a crystal produces a partial charge transfer from one molecule type to the other. Here, the molecules alternate in stacks forming a quasi-one-dimensional semiconductor material. This arrangement leads to a small band gap system where n-type field effect activity is observed. Layered transition metal dichalcogenides are interesting alternative to organic semiconductors. Similar to organic materials their surface is Van der Waals determined. Therefore, an intrinsic low density of trap states at semiconductor/dielectic interface is observed. The WSe2-based devices with their high carrier mobilities and ambipolar operation are especially promising.publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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