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- PEM fuel cell electrocatalysts, Carbon nanofibers, Oxygen plasma activation, Pulsed electroplating. (1)
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Institut
- Westfälisches Energieinstitut (106) (entfernen)
Diese Arbeit befasst sich mit der Entwicklung einer Titanträgerlage, die anschließend als Supportstruktur für den Anodenkatalysator eines PEM-Elektrolyseurs genutzt werden soll, sowie mit der Parametrisierung der hierfür genutzten Lasersinteranlage.
Dafür wird 1 mg cm-2 unterstöchiometrisches Titanoxid (TiOx) auf eine poröse Titantransportschicht gesprüht und anschließend mit einem gepulsten Laser in Argonatmosphäre gesintert, um beide Materialien zu verbinden und die Oberfläche gezielt verändern zu können. Da die benötigte Menge des unterstöchiometrischen TiOx-Pulvers nicht abschätzbar war, wurden die Arbeiten mit kommerziell verfügbarem Titandioxid-(TiOx) Pulver durchgeführt.
Im ersten Schritt wurde ein bestehender Sprühprozess optimiert. Der Fokus lag dabei auf einem gleichmäßigen Sprühergebnis und dem reproduzierbaren Erreichen einer vorgegebenen Beladung. Die Optimierung wurde auf Titanblech und auf porösen Titansubstraten durchgeführt.
Anschließend wurde das verwendetet Lasersystem, das einen luftgekühlten Neodymdotierten Yttrium-Aluminium-Granat (Nd:YAG)-Festkörperlaser mit einer Wellenlänge von 355 nm nutzt, parametrisiert. Das Erreichen der Bearbeitungsschwelle und die maximalen Spotgrößen bei verschiedenen Diodenströmen wurden untersucht. Anschließend wurde die Frequenz erhöht, dies führte zu geringeren Einzelpulsenergien und daher zu neuen Bearbeitungsschwellen bei den unterschiedlichen Diodenströmen. Die Variierung des Vorschubs führt zu einer Änderung der eingebrachten Energie pro Fläche und wirkt sich direkt auf die Sinterdauer aus. Als letztes wurde der Einfluss des Linienabstandes untersucht. Wenn der Linienabstand unterhalb des Wertes gehalten wird, ab dem eine streifenfreie Oberfläche entsteht, wirkt die Verkleinerung des Linienabstandes wie die Verringerung des Vorschubs.
Um CO2-frei, mit erneuerbaren Technologien, Energie umzuwandeln, zu speichern und Wasserstoff zu produzieren, bedarf es einer Technologie, welche zu hoher Leistungsdichte, Flexibilität und Effizienz fähig ist. Die Polymerelektrolytmembran-(PEM) Wasser-Elektrolyse besitzt das Potential alle diese Attribute in sich zu vereinen, weswegen sie eine Schlüsseltechnologie bei dem Umstieg auf erneuerbare Energien darstellt. Die aktuellen hohen Beladungen der Elektroden mit Katalysatormaterial für die Sauerstoffentwicklungsreaktion (OER) wie Iridium und die bisher noch ungenügend erforschten Alterungsmechanismen dieser über eine längere Nutzdauer, sind bisher die größten Herausforderungen für die großflächige industrielle Nutzung dieser Systeme. Mit diesem Hintergrund stellt sich diese Arbeit der Herausforderung die untersuchten PEM-Anoden mit reduzierter Beladung, hinsichtlich ihrer Stabilität und Alterungsmechanismen zu charakterisieren und den ersten Schritt in Richtung eines Degradationsprotokolls zu machen, um solche Elektroden künftig zu untersuchen. In der vorgegangenen Arbeit, aus welcher die hier untersuchten Proben stammen, wurden Elektroden hergestellt, indem auf ein Titansubstrat eine Supportschicht mit unterstöchiometrischem Titanoxid eingesintert wurde, auf welchem wiederum das Iridium galvanisch gepulst abgeschieden wurde. Die Beladung mit Katalysatormaterial war hier geringer als der der Stand der Technik, doch die katalytische Aktivität wurde durch die optimierte Elektrodenarchitektur erhöht. In dieser Arbeit wurden die Elektroden einem elektrochemischen Degradationsprotokoll unterzogen, welches sich aus Cyclovoltametrie (CV) gefolgt von Chronopotentiometrie (CP) und Chronoamperometrie, mit wechselnden Potentialen, zusammensetzt. Der dabei verwendete Elektrolyt wurde ebenfalls durch Filtration und Titration auf Zerfallsprodukte der Elektroden hin untersucht. Nach erfolgter Degradation wurden die Elektroden auf topografische Änderungen, mittels Rasterelektronenmikroskop (REM), Änderungen der Kristallinität, mittels Röntgendiffraktometrie (XRD) und Änderungen der Elementenkonzentration durch Röntgenfluoreszenzspektroskopie (XRF) hin untersucht.
This report gives a brief overview to the state of the art of PEM fuel cell technology and a description of a newly developed fuel cell stack concept. One main research activity at the Westphalian Energy Institute of the Westphalian University of Applied Sciences is the development of PEM fuel cells, for which a range of different materials have been investigated for fuel cell pole plate construction. Whereas graphite is a material which has suitable properties concerning conductivity as well as manufacturing e.g. for milling, stainless steel foils are suitable for economical hydroforming processes. However, with steel coating is necessary to increase corrosion resistance as well as electrical conductivity. A new fuel cell stack design is currently under development using separated single fuel cells with hydraulic cell compression. The advantages of this stack concept are modularity, effective heat exchanging and constant, uniform cell compression which are further described in this work.
Since the 1980’s, against the backdrop of global warming and the decline of conventional energy resources, low emission and renewable energy systems have gotten into the focus of politics as well as research and development. In order to decrease the emission of greenhouse gases Germany intents to generate 80% of its electrical energy from renewable and low emission sources by 2050. For low emission electricity generation hydrogen operated fuel cells are a potential solution. However, although fuel cell technology has been well known since the 19th century cost effective materials are needed to achieve a breakthrough in the market.
Proton Exchange Membrane Fuel Cells with Carbon Nanotubes as Electrode Material
At the Westphalian Energy Institute of the Wesphalian University of Applied Sciences one main focus is on the research of proton exchange membrane fuel cells (PEMFC). PEMFC membrane electrode assemblies (MEA) consist of a polymer membrane with electrolytic properties covered on both sides by a catalyst layer (CL) as well as a porous and electrical conductive gas diffusion layer (GDL).
For PEMFC carbon nanotubes (CNT) have ideal properties as electrode material concerning electrical conductivity, oxidation resistance and media transport. CNTs are suitable for the use as catalyst support material within the CL due to their large surface in comparison to conventional carbon supports. Furthermore, oxygen plasma treated CNTs show electrochemical activity referred to hydrogen adsorption and desorption, which has been shown by cyclic voltammetry in 0.5 M sulfuric acid solution. According to the PEMFCs anode a GDL coated with oxygen plasma activated CNTs has promising properties to significantly reduce catalyst content (e.g. platinum) of the anodic CL.
Um die Wasserstofftechnik in Zukunft wirtschaftlich und damit kommerziell am Markt verfügbar werden zu lassen, sind heute noch immer große Forschungs- und Entwicklungsanstrengungen notwendig. Dabei erfordert die Entwicklung von optimierten Komponenten wie beispielsweise der Membran-Elektroden-Einheit (MEA – engl. Membrane Electrode Assembly) für Brennstoffzellen sowie Elektrolyseure reproduzierbare und homogene Prüfbedingungen. Für diesen Zweck ist ein Prüfsystem auf Basis eines von der Westfälischen Hochschule (WHS) patentierten modularen Stackkonzepts mit hydraulischer Verpressung entworfen und realisiert worden. Mit dem hier vorgestellten System ist es möglich, auf Einzelzellenbasis mehrere Proben zum gleichen Zeitpunkt unter identischen Umgebungsbedingungen auf ihre Charakteristik hin zu untersuchen.
The membrane electrode assemblies (MEA) for polymer electrolyte membrane fuel cells (PEMFC) developed at the Westphalian Energy Institute are based on oxygen plasma activated carbon nanotubes (CNT) doped with platinum particles. For electrode preparation an ink is used containing the activated CNTs as well as hydrophobic and hydrophilic material in solved form. After this ink is sprayed onto a graphitic substrate platinum particles are deposited by pulse plating method, where the plasma activation enhances CNT dispersibility as well as platinum deposition. This materials mixture is structured in nanoscale with the aim to increase the catalyst particles’ specific surface. For low reactance at operation, homogeneous compression of the MEA’s layers is necessary within a PEMFC. A novel stack architecture for electrochemical cells, especially PEMFC as well as PEM electrolysers, has been developed in order to achieve ideal cell operation conditions. Single cells of such a stack are inserted into flexible slots that are surrounded by a hydraulic medium which is pressurised during operation in order to achieve an even compression and cooling of the stack’s cells. With this stack design it has been possible to construct a test facility for simultaneous characterisation of several MEA samples. As compression and temperature conditions of every single sample are the same, the effects of e.g. different electrode configurations can be investigated with the novel test system.
To further increase platinum utilisation in PEM fuel cells CNFs are investigated as catalyst support material due to the CNF’s high specific surface area. Furthermore, CNFs provide suitable properties concerning corrosion resistance as well as electrical conductivity in contrast to conventional carbon supports.
This work presents the results of an electrode preparation procedure based on O2 plasma activated CNFs. The plasma treatment leads to CNF dispersibility in alcohol/water for a spray coating process. Furthermore, O2 plasma activation enhances metal deposition on the CNF’s surface. Pulse plating procedure as well as wet chemical metal synthesis have been used for particle deposition. For pulse plating a potentiostat/galvanostat type MMates 510 AC from Materials Mates, Italy has been used. Electrode morphology has been determined in SEM type XL 30 ESEM from Philips, The Netherlands.
Membrane electrode assemblies (MEA) developed at the Westphalian Energy Institute for polymer electrolyte membrane fuel cells (PEMFC) are high tech systems containing various materials structured in nanoscale, at which electrochemical reactions occur on catalyst nano particle surfaces. For low reactance homogeneous compression of the MEA’s layers is necessary. A novel stack architecture for electrochemical cells, especially PEMFC as well as PEM electrolysers, has been developed according to achieve ideal cell operation conditions. Single cells of such a stack are inserted into flexible slots that are surrounded by hydraulic media. While operation the hydraulic media is pressurised which leads to an even compression and cooling of the stack’s cells. With this stack design it has been possible to construct a test facility for simultaneous characterisation of several MEA samples. As compression and temperature conditions of every single sample are equal, with the novel test system the effect of e.g. different electrode configurations can be investigated. Furthermore, the modular stack design leads to the development of hybrid energy applications combining fuel cells, electrolysers, batteries as well as metal hydride tanks in one system.
In this study, a novel design concept for PEMFC (polymer electrolytemembrane fuel cell) stacks is presented with singlecells inserted in pockets surrounded by a hydraulic medium. Thehydraulic pressure introduces necessary compression forces to themembrane electrode assembly of each cell within a stack. Moreover, homogeneous cell cooling is achieved by this medium. First,prototypes presented in this work indicate that, upscaling of cells for the novelstack design is possible without significantperformancelosses. Due to its modularity and scalability, this stackdesign meets the requirements for large PEMFC units.
In state of the art polymer electrolyte membrane fuel cells (PEMFC) rare and expensive platinum group metals (PGM) are used as catalyst material. Reduction of PGM in PEMFC electrodes is strongly required to reach cost targets for this technology. An optimal catalyst utilisation is achieved in the case of nano-structured particles supported on carbon material with a large specific surface area. In this study, graphitic material in form of carbon nanofibres (CNFs) is decorated with platinum (Pt) particles serving as catalyst material for PEMFC electrodes with low Pt loading. For electrode preparation CNFs have been previously activated by means of radio frequency induced oxygen plasma. This kind of treatment results in formation of functional groups on the CNF’s surface which directly influences the characteristics of subsequent Pt particle deposition. Different plasma parameters (plasma power, gas flow or exposure time) have to be set in order to achieve formation of oxygen containing functional groups (hydroxylic, carboxylic or carbonylic) on the CNF’s surface. In the frame of this experimental work, electrodes are investigated in respect of optimal morphology, microstructure as well as electrochemical properties. Therefore, samples were characterised by means of scanning electron microscopy combined with energy dispersive X-ray analysis, transmission electron microscopy, thermogravimetry, X-ray diffraction, X-ray fluorescence as well as polarisation measurements.
Platinum is one of the most effective electro catalysts for PEMFCs (proton exchange membrane fuel cells), but because of its prohibitive price, the use of this metal in industrial purposes is limited. As a consequence, during last years, several materials have been investigated, in order to obtain an efficient catalyst for both ORR (oxygen reduction reaction) and HOR (hydrogen oxidation reaction), which can replace the expensive platinum but preserving the same properties: high electrical conductivity, structural stability and good corrosion resistance. Moreover, one of the most important parameters for catalyst materials is the electrochemical surface area (real surface area), which has a strong influence on the reaction rate and also on the current density.
CNFs (carbon nanofibers) are considered to be a promising catalyst support material due to their unique characteristics, excellent mechanical, electrical and structural properties, high surface area and nevertheless, good interaction with platinum particles.
The possibility of preparing CNFs decorated with platinum by electrochemical methods was tested, using a hexachloroplatinic solution bath. The experiments were carried out with the aid of a Potentiostat/Galvanostat MMate 510, in a three – electrode cell.
The aim of the present work was to determine the electrochemical surface area of the CNFs – Pt catalysts, using an electrochemical method. The obtained results correlate very well with the particles size and distribution of platinum, analyzed by SEM (scanning electron microscopy) respectively with the quantity of deposited platinum determined by TG (thermo gravimetrical analyses). Cyclic voltammetry is a suitable method for estimation of the real surface area for catalyst particles.
In polymer electrolyte membrane fuel cells (PEMFC) noble metal nano particles are deposited on graphitic supports serving as electrocatalysts for devices with high power density. In this study anodes are analysed with low platinum loading of about 0.1 mg cm-2. These electrodes are prepared by carbon nano fibres (CNF) decorated with platinum nano particles. For electrode manufacturing two sorts of fibres, which are produced in an industrial scale, are used with different graphitisation degree and surface area. CNF layers are applied on commercially available graphitic substrate by spray coating which leads to a porous structure with high surface area. Subsequently, platinum deposition is achieved by pulsed electroplating for an improved platinum utilisation in PEMFC electrodes. Spray coating and platinum deposition are assisted by a previous oxygen plasma activation process. Prepared anode material is characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), X-ray fluorescence spectroscopy (XRF) and thermogravimetry (TGA). Electrochemical analyses (cyclic voltammetry and corrosion test) are carried out in 0.5 M sulphuric acid. The effect of graphitisation degree of carbon nano fibres on the performance of prepared electrodes is investigated in-situ in a PEM fuel cell test bench.
Im Rahmen eines gemeinsamen Forschungsprojekts mit dem Titel „Energieautarke Bohrlochsensorik mittels Brennstoffzellen – GeoFuelCells“ wurde vom Geothermie-Zentrum Bochum und dem Westfälischen Energieinstitut, unterstützt aus dem Förderprogramm Ziel 2 (2007-2013 EFRE) des Landes NRW, ein brennstoffzellenbasiertes Energieversorgungssystem für Bohrloch-Anwendungen entwickelt.