Mit dem DLR-Trockensprühverfahren wurden PEFC-Modellelektroden hergestellt, die unterschiedliche Katalysatortypen (ungeträgertes und auf Kohle geträgertes Platin) und unterschiedliche Elektrolytmengen enthielten. In Abhängigkeit vom Katalysatortyp wurde eine erforderliche minimale Festelektrolytmenge in der Elektrode festgestellt. Durch den zusätzlichen Festelektrolyt in der Elektrode wird die Protonenleitung der Elektrodenschicht nicht signifikant erhöht. Ex-situ Untersuchungen ergaben, dass flüssiges Wasser die Protonenleitung der Katalysatoren bei hohen Stromdichten durch die Dissoziationserhöhung aufgrund lokaler elektrischer Felder auf Werte erhöht, die der Leitfähigkeit des Festelektrolyten entsprechen. Beim geträgerten Katalysator führt Wasser in der Elektrode zu einer deutlich erhöhten Elektronenleitung. Dabei ergibt sich die Elektronenleitung aus einer Paralleschaltung des Wasser- und Katalysatorwiderstandes. Es wird gefolgert, dass bei Einsatztemperaturen über 120 Grad C und Betriebsdrücken unter 2 bar aufgrund der fehlenden Protonenleitung über flüssiges Wasser dem Katalysator Festelektrolyt über der berechneten Minimalmenge zugemischt werden muss.
The high cost for a Polymer electrolyte Fuel Cell (PEFC) System is still a barrier for commercial breakthrough, which cannot be compensated by the advantages of being pollution free, or nearly noiseless. The most effective way of saving costs is to reduce expensive materials, because the material costs only for the Membrane Electrode Assemblies (MEAs) is more than 70% of the total costs of a PEFC Stack. Within the MEA a main part of the costs is due to the catalyst. It is one of the main goals to decrease the catalyst loading by simultaneously increasing the performance or keeping it at least constant. Because in most electrodes only 20-50% of the catalyst in the electrodes is used, enlarging the electrochemical active area is one of the key problems of the PEFC. For being electrochemical active, the catalyst must be reachable for the gases, he must have a good ionic conductivity to the membrane and he must be attached to the Gas Diffusion Layer (GDL) by electron conductivity. In literature often an inferior ionic contact of the catalyst to the membrane is responsible for the low catalyst utilization. In the first part of the work, model electrodes with different kinds of catalysts and different amounts of electrolyte in the electrodes were investigated to explore the interrelationship between platinum and electrolyte content. Three different catalysts, unsupported Pt- black, 60 wt.% Pt carbon-supported and 20 wt.% Pt carbon-supported with an addition of Nafion? powder of 0%, 20%, 40%, 60 wt.%, and 80 wt.% were used. The electrodes were prepared by spraying the electrode material with the DLR dry spray technique directly onto the membrane and then rolling them while hot. Because material solutions were not used, the structure of the electrodes are determinable and predictable. Numerous different in- and ex-situ characterization methods like impedance spectroscopy, U-i characteristic, cylic-voltammetry, proton conductivity measurements, halfcell measurements and REM and EDX investigations were applied. As a result of the investigations, optimum electrolyte content for a specific catalyst was obtained by using a percolation model. In the second part of the work, the influence of water in the electrode was investigated for the ionic and the electron conductivity. It turned out that the ionic conductivity increased with the amount of water for the supported and unsupported catalyst, whereas the electron conductivity, particularly for the supported catalyst, decreased dramatically by increasing the water content. With the help of electron conductivity models for composite materials, where water is the matrix and the catalyst the filler material, electron conductivity can be described as a parallel circuit of the single conductivities. With this model, the difference in performance between the supported and unsupported catalyst can be explained. With the results of the work, ideal electrode compositions and structures for different catalysts and applications can be predicted and so the catalyst loading can be decreased. This together with a cost effective MEA production technique is an important step towards fuel cell commercialization.