LONG TERM INVESTIGATIONS OF SILVER CATHODES FOR ALKALINE FUEL CELLS

Kurzfassung

Alkaline fuel cells (AFC) are an interesting alternative to polymer electrolyte fuel cells (PEFC). In AFC no expensive platinum metal is necessary; silver can be used for the oxygen reduction reaction (cathode catalyst). To enhance its catalytic activity, silver is used in high surface area forms such as porous gas diffusion electrodes (GDE). The gas diffusion electrodes (cathodes) of AFC were prepared by a reactive powder-polymer mixing and rolling (RMR) technology. They consist of a mixture of a silver catalysts and polytetrafluorethylene (PTFE) as organic binder rolled onto a metal web. Different silver catalysts, e.g. silver deposited onto PTFE, silver oxide or silver catalyst formed from Raney silver, can be used. In gas diffusion electrodes hydrophobic and hydrophilic pore systems are required. In AFC the hydrophilic system allows the penetration of the electrolyte into the electrode and the transport of the ions to or from the reaction zone; the hydrophobic pore system is required for the transport of the oxygen to the reaction zone. In addition, to give the mechanical stability, the PTFE in the electrodes forms a hydrophobic pore system, whereby the spin webs of PTFE fibers in and on the electrodes are formed during preparation. For technical use of AFC the long-term stability of AFC components is important, especially that of the gas diffusion electrodes. In order to analyse in detail the long term behaviour - the degradation (ageing) process of silver electrodes during oxygen reduction, electrochemical impedance spectroscopy (EIS) has been performed [1]. The silver GDE has been operated galvanostatically at -100 mA/cm2 in 30 wt % KOH at 70°C with pure oxygen (1 bar) over 1400 h and impedance spectra were recorded at the open circuit potential (OCP), at the beginning daily and after 2 weeks in 48 and 72 h intervals respectively. The evaluation of the measured impedance spectra was performed with the simulation software program. The spectra can be simulated by an equivalent circuit which consists of a series combination of uncompensated electrolyte resistance (Rel), charge transfer resistance (Rct,C), double layer capacity (Cdl) and infinite diffusion impedance (Warburg impedance, W). From the charge (time) dependency of the impedance parameters, the ageing effects can be related to the decrease of the double layer capacity, probably a decrease of the electrochemically active surface and the increase of the Warburg impedance, suggesting that diffusion of reacting gas is hindered. In addition, the fuel cell cathodes were physically characterized by x-ray photoelectron spectroscopy (XPS). By successive measurement of XP spectra and ion etching the surface depth profiles were recorded. Due to electrochemical operation the chemical composition is changed. The PTFE in the electrode is partially decomposed, which can be seen in the depth profile of the electrochemically stressed electrode by the enhanced decrease of the fluorine concentration. Similar, the electrochemical induced decomposition of PTFE is also observed for other low temperature fuel cell electrodes like AFC or PEFC anodes. The second difference in the depth profiles is that the recorded silver concentration is increased. This is induced by the decomposition of the PTFE and the resulting lower concentration of the elements related to PTFE. The decomposition of the PTFE changes the hydrophobic/hydrophilic behavior and in this way it affects directly the three phase zone between catalyst, electrolyte and gas phase relevant for the reaction. Typically, the decomposition of PTFE reduces the hydrophobic property and so the electrode will be more flooded by the liquid electrolyte. Consequently the gas transport in the electrode is more hindered. In addition, the thickness of the three phase zone will be reduced.