Characterization of polymers in PEFC-electrodes with EDX and XPS

In polymer electrolyte fuel cells (PEFC) preferably Nafion(R) (Du Pont) is used as electrolyte material. The electrodes contain polytetrafluoroethylen e (PTFE) as organic binder and the electrolyte Nafion(R) as additive to enl arge the three phase boundary zone. Since Nafion(R) is chemically related c losely to PTFE, using surface analytical methods the two polymers can only be distinguished by the sulfonic acid groups in Nafion(R). XPS measurements give information about the chemical composition of the electrode surface. By electron microscopy and EDX analysis it is possible to image the surface structure and to determine the local elements distribution, but hereby the distinction of the different polymers is a problem. In EDX spectra the sul fur peak induced from the sulfonic acid in the Nafion(R) is overlayed by a platinum peak from the catalyst component in the electrodes. Consequently, the distribution of the pure Nafion(R) cannot be determined by EDX. To make Nafion(R) distinguishable from PTFE and platinum the Nafion(R) in the elec trode has to be marked, e.g. by exchange of the conducting protons in Nafio n(R) by alkali ions like sodium. After having marked Nafion(R) its spatial distribution in fuel cell electrodes can be determined by EDX mapping of fl uorine and sodium.