This title appears in the Scientific Report : 2014 

Eigenschaften des Phosphorsäure-Polybenzimidazol-Systems in Hochtemperatur-Polymerelektrolyt-Brennstoffzellen
Majerus, Anne (Corresponding Author)
Elektrochemische Verfahrenstechnik; IEK-3
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 2014
141 S.
RWTH Aachen, Diss., 2014
Dissertation / PhD Thesis
Fuel Cells
Schriften des Forschungszentrums Jülich Reihe Energie & Umwelt / Energy & Environment 210
Please use the identifier: in citations.
The principle of a fuel cell is the direct conversion from chemical to electrical energy. Depending on the application, different fuel cell types are used. As there is no extensive hydrogeninfrastructure in Germany, research is currently focused on fuel cell types that can be operatedwith reformed hydrocarbons. This includes the high temperature polymer electrolyte fuel cell. In high temperature polymer electrolyte fuel cells, phosphoric acid doped polymer membranesare used as electrolyte. Compared to conventional sulfonated polymer electrolytes, thesedo not need to be continually hydrated and can thus be operated at higher temperatures. Thephosphoric acid interacts with the polymer and influences the properties of the membrane. However, the composition of the phophoric acid is dependent on the temperature and the watervapour pressure. The aim of this work was the examination of the influence of the interactionbetween acid and polymer as well as the composition of the acid on the properties of theacid-base-system. To this end, different physico-chemical analysing methods were used on three different samplesystems: i) the pure acid, ii) the acid interacting with the membrane, and iii) the acid interactingwith the membrane in the fuel cell under operating conditions. The composition of thepure acid as well as the acid in the membrane at different ambient conditions was examinedthermogravimetrically. Studies of the conductivity using impedance spectroscopy provided aninvestigation of the influence of the ambient conditions as well as the acid composition onthe proton conductivity of the doped membrane ex situ as well as under operating conditions. The interaction between acid and membrane was examined by vibrational spectroscopy. Anew characterisation method that allowed the recording of in situ Raman spectra of theelectrolyte under operating conditions was developed, thus enabling the examination of thebinding characteristics in the electrolyte under dynamic operating conditions.The application of these methods on the polymer-acid-system led to the following findings: $\bullet$ Two different types of phosphoric acid molecules can be distinguished in the membrane,bound and „unbound“ molecules. They influence the properties of the electrolyte indifferent ways. $\bullet$ The interaction between acid and polymer influences the properties of the acid in themembrane. As the acid is bound by hydrogen bonds to the polymer, these bonds mustbe broken before the condensation of the orthophosphoric acid can start. Furthermore,the dehydration of the acid is sterically hindered by the polymer structure. $\bullet$ The dehydration of the phosphoric acid has an influence on the proton conductivityof the electrolyte. However, the water production at the cathode and the crossover ofhydrogen and air prevent a complete dehydration of the phosphoric acid in the fuel cell.The deydration process of phosphoric acid is reversible.