Spatially resolved studies in direct methanol fuel cells.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2012)
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|Item Type:||Ph.D. Thesis|
|Title:||Spatially resolved studies in direct methanol fuel cells|
The thesis mainly focuses on the spatially resolved characterization of a direct methanol fuel cell. Initially spatially resolved analyses were carried out on an end of life (5000 hrs operated) stack membrane electrode assembly (MEA) using various techniques, like X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive X-ray (EDX) mapping and X-ray absorption spectroscopy (XAS). The fate of the Ru in the direct methanol fuel cell (DMFC) with ageing is carefully analyzed in these studies. It was found that the large oxidized ruthenium fraction in the anode catalyst plays a significant role in particle growth and ruthenium dissolution. Ru was also found in the form of precipitates in the Nafion membrane preferentially at the methanol outlet regions. Ex-situ studies were preceded by in-situ spatially resolved XAS studies. For these, in-situ cells for spatially resolved DMFC studies are developed and optimized. The relative OH and CO coverages on both the anode and cathode were followed using the XANES technique at different regions of a DMFC during operation at several current levels in dependence on the oxygen flow. For the first time, a very strong “cross-talk” between the anode and cathode is seen with the anode dictating at high O2 flow rate the OH coverage on the cathode. The fuel starvation studies on the single DMFC cell revealed a non-uniform degradation pattern with a high degradation at the methanol inlet and low degradation at methanol outlet.
Finally, shape-selected Pt nanoparticles were synthesized using different surfactants like tetradecyltrimethylammonium bromide (TTAB) and polyvinylpyrrolidone (PVP) and tested fuel cell performance. These shape-selected Pt nanoparticles were characterized by TEM and their electrocatalytical activity tested by cyclic voltammetry. High potential cycling of the shape-selected particles revealed a preferential degradation of Pt (100) facets over Pt (110). The TEM analysis of the cycled samples showed predominantly shape-selected particles with very few spherical particles. Finally, supported shape-selected particles showed excellent fuel performance even with low Pt loading. Tuning of the shape of Pt nanoparticles is expected to increase the Pt utilization, i.e. Pt loading can be reduced in the MEA. Further higher durability is expected for the shape-selected particles than the commercial catalyst. Thus by tuning the shape of the Pt nanoparticles, cost reduction and increased durability can be achieved.
|Place of Publication:||Darmstadt|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 540 Chemie
600 Technik, Medizin, angewandte Wissenschaften > 660 Technische Chemie
|Divisions:||Fachbereich Material- und Geowissenschaften|
|Date Deposited:||25 Jul 2012 13:50|
|Last Modified:||07 Dec 2012 12:05|
|Referees:||Roth, Prof Christina and Riedel, Prof Ralf and Ensinger, Prof Wolfgang and Claus, Prof Peter|
|Refereed:||9 May 2012|