LONG TERM OPERATION OF AFC ELECTRODES WITH CO2 CONTAINING GASES

Kurzfassung

Low temperature fuel cells represents an important componets in pollution free energy con¬version for mobile application. Alkaline cells are an interesting alternative for polymer elec¬trolyte membrane fuel cells, because they do not need expensive membranes and catalysts. It is commonly accepted, that carbon dioxide intolerance is the most pronounced disadvantage of air breathing alkaline fuel cells. Thus, alkaline technology is commonly not considered for mobile fuel cell application. In this study the long term influence of carbon dioxide containing reaction gases on electrodes manufactured by a rolling process were investigated. To investigate the mechanism of carbon dioxide poisoning, gas diffusion electrodes (GDEs) designed for alkaline fuel cells have been operated in a half cell configuration up to several thousand hours with a constant load of l50 mA/cm2. The reactants were contaminated with 5% CO2. The experiments have been performed with both electrodes, anode and cathode. Control measurements were performed by operating electrodes with pure reactants. During these long term experiments the V-i curves were recorded daily. Recently also the chemical surface composition of the gas diffusion electrodes has been investigated by x-ray induced photoelectron spectroscopy (XPS) after long term operation. In addition to the characterization of the electrode the carbonate concentration in the electrolyte was determined after different operation times. The electrochemical performance of the electrodes, anodes and cathodes, decreases during the operation time in both operation modes, operated with pure gases as well as with CO2 containing gases. The decrease in both modes are comparable; this means the CO2 neither enhances the degradation process nor induces a new, detrimental degradation process. The XPS characterizations of used cathodes operated with pure and with CO2 containing oxygen yielded no significant difference. On the surface of the anodes opperated with CO2 containing hydrogen a slightly higher carbon concentration was observed than on surfaces of anodes operated with pure hydrogen. The carbonate concentration in the electrolyte increases during fuel cell operation with CO2 containing gases. The long term behavior of the investigated AFC electrodes supplied with CO2 containing gases gives no evidence that CO2 affects significantly the degradation process.