PREPARATION OF GAS DIFFUSION ELECTRODES WITH SILVER CATALYSTS FOR ALKALINE FUEL CELLS

Kurzfassung

Alkaline fuel cells have the opportunity of becoming important for mobile energy systems as, in contrast to other low temperature fuel cells, the alkaline type requires neither expensive noble metal catalysts nor an expensive polymer electrolyte [1]. In alkaline fuel cells (AFC) silver can be used as cathode catalyst in gas diffusion electrodes. The metal catalyst was mixed with polytetraflourethylene (PTFE) as organic binder in a knife mill and rolled onto a metal web in a calandar to prepare the gas diffusion electrodes. The electrodes are manufactured as an endless tape with a thickness of approx. 0.4 mm and widths up to 20 cm. As catalyst Raney-silver, carbon supported silver or silver deposited as a closed film on PTFE are used. The silver catalyst is frequently used in an oxidized form. In the preparation process of the gas diffusion electrodes the silver catalyst can be used in this oxidized form. Before using the gas diffusion silver cathodes in the alkaline fuel cell the catalyst will be reduced to the metallic form. The silver oxide in the gas diffusion electrodes is reduced galvanostatically at approx. 7 mA/cm2 with measurement of the potential. The reduction process is finished if the electrode potential is in the region of the hydrogen generation potential. In the chrono-potential curves the oxide states of silver yield a plateau; from the integration of the charging current the silver loading in the electrodes can be calculated. During the reduction process the dark electrodes turn bright. The electrodes are characterized electrochemically in half cells and physical by x-ray photoelectron spectroscopy (XPS). With V-i-curves the electrochemical performance is investigated. The XP spectra give information about the chemical composition of the electrodes. By successive measurements of XP spectra and ion etching of the surface depth profiles are recorded.