Long-term operation of AFC electrodes with CO₂ containing gases

Kurzfassung

Low temperature fuel cells represent an important component in pollution-free energy conversion for mobile application. Alkaline fuel cells are an interesting alternative for polymer electrolyte membrane fuel cells (PEFC), 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 mechanisms of carbon dioxide poisoning, gas diffusion electrodes (GDEs) which were designed for alkaline fuel cells had been operated in a half cell configuration up to several thousand of hours with a constant load of 150 mA/cm2. The reactants were contaminated with 5% CO2. The experiments were 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 compositions of the gas diffusion electrodes were 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 decreases in both modes are comparable; this means the CO2 neither enhances the degradation process nor induces a new, detrimental degradation process. The XPS characterization of used cathodes operated with pure and with CO2 containing oxygen yielded no significant difference. A slightly higher carbon concentration was observed on the surface of the anodes which was operated with CO2 containing hydrogen than on surfaces of anodes which were 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 alkaline fuel cell (AFC) electrodes supplied with CO2 containing gases gives no evidence that CO2 affects significantly the degradation process.