Browsing by Author "Panchenko, Alexander"

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    Polymer electrolyte membrane degradation and oxygen reduction in fuel cells : an EPR and DFT investigation
    (2004) Panchenko, Alexander; Roduner, Emil (Prof. Dr.)
    The present work deals with the investigation and characterisation of different processes which take place in a working PEMFC. The SQUID technique was used to investigate magnetic properties of the electrode material. The EPR was the main approach to investigate the radical formation in a working fuel cell. The second part of the thesis comprises the results and discussion of the quantum chemical calculations of the O2 and its reduction intermediates adsorption on low index Pt surfaces. The work aims at an understanding of the pathways of oxidative degradation of membranes, and it wants to provide guidance in the choice of favorable fuel cell operating conditions and in the preparation of alternative membranes with improved durability. For this a miniature fuel cell which can operate in a resonator of an X-band EPR spectrometer was constructed. The concentration of free radicals produced in a fuel cell is extremely low and their lifetime is relatively short, so that it is not possible with conventional methods to observe them directly. We therefore employed the spin trapping technique, using the spin trap molecules POBN, DMPO, DBNBS, and DEPMPO. Radical formation was studied separately at the anode and cathode side of the in situ EPR fuel cell. At the anode side of the cell formal addition of hydrogen atoms to the spin trap molecules was observed. No traces of membrane degradation were detected at the anode side of the fuel cell for any membrane used. At the cathode side we were able to demonstrate the ·OH radical formation during the oxygen reduction by introducing the DMPO spin trap water solution into the cell equipped with the Nafion-115 membrane. The formed ·OH radicals manifested their destructive nature when F-free membranes were used instead of a very stable Nafion membrane. They attacked the membrane and formed different organic radicals on the membrane surface. The formation of radicals was confirmed by the addition of a spin trap water solution at the cathode side. The spin trap molecules react with the radicals under formation of the stable spin trap adducts. In the theoretical part of the manuscript we investigated the energetics of the oxygen reduction reaction intermediates on Pt surfaces in the conditions relevant for the fuel cells. The presence of applied electric fields and coadsorption of water were considered. The adsorption properties of the oxygen molecule and intermediates of the ORR (Oxygen Reduction Reaction) on the (111), (100), and (110) platinum surfaces were calculated in the DFT-GGA framework (PW91-GGA/PAW) using periodic boundary conditions and a slab model of the Pt surface. The electric field dependence of the adsorbate properties was studied using a cluster model of the adsorption system. The B3LYP functional and a 6-311G** basis set for the O and H atoms and a LANL2DZ basis set for the Pt atoms were employed in this case. For all adsorbed species the applied electric field is predicted to have a strong impact and to cause considerable changes in the bond lengths, charge transfer characteristics and vibrational frequencies. The presence of coadsorbed water on the catalyst surfaces was modeled by the coadsorption of two water molecules together with the O2 and ·OH species. The presence of water leads to the formation of hydrogen bonds and strengthens the adsorption significantly.