Filtern
Dokumenttyp
- Konferenzveröffentlichung (28) (entfernen)
Schlagworte
- AEM-Electrolysis (1)
- Electrodeposition (1)
- Elektrolyseure (1)
- Erneuerbare Energien (1)
- Hydraulic compression, Carbon Nano Fibers, PEM Fuel Cells, Catalyst utilization (1)
- Kohlenstoff-Nanoröhre (1)
- Membran-Elektroden-Einheit (1)
- Ni-Mo alloy Catalyst (1)
- PEM Electrolysis, Hydrogen, Hydraulic Compression, High Pressure (1)
- PEM fuel cells; electrode preparation; carbon nanofibers; in-situ performance test (1)
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.
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.
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 this experimental work we present a novel electrolyzer system for the production of hydrogen and oxygen at high pressure levels without an additional mechanical compressor. Due to its control strategies, the operation conditions for this electrolyzer can be kept optimal for each load situation of the system. Furthermore, the novel system design allows for dynamic long-term operation as well as for easy maintainability. Therefore, the device meets the requirements for prospective power-to-gas applications, especially, in order to store excess energy from renewable sources. A laboratory scale device has been developed and high-pressure operation was validated. We also studied the long-term stability of the system by applying dynamic load cycles with load changes every 30 sec. After 80 h of operation the used membrane electrode assembly (MEA) was investigated by means of SEM, EDX and XRD analysis.
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.
For this study gas diffusion electrodes (GDE) with low platinum loading are prepared for the application as anode in polymer electrolyte membrane fuel cell (PEMFC) systems based on hydraulic compression. As catalyst support material, carbon nanofibers (CNF) are investigated because of their high specific surface area and high graphitization degree. The electrode preparation is optimized by an economic and environmental friendly pre-treatment process in oxygen plasma. For GDE manufacture an ink containing oxygen plasma activated CNFs as well as hydrophilic polymer is used. After spray coating of this CNF ink on a graphitic substrate, platinum is deposited using the pulse plating technique. Preliminary results showed a considerable improvement of CNF dispersibility as well as an increased amount and an optimized morphology of the deposited platinum. Morphology and microstructure are observed by scanning electron microscopy as well as transmission electron microscopy. Platinum loading is determined by thermogravimetric analysis to be in the range of 0.01 mg cm-2 to 0.017 mg cm-2. Furthermore, MEAs are prepared from these GDEs and testing is performed in a novel modular fuel cell test stack based on hydraulic compression. Technical information about stack design and functions is given in this work.