Brønsted-acidic Ionic Liquids as conducting electrolytes in polymer membrane fuel cells for operating temperatures around 120 °C - Synthesis and electrochemical characterisation.

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The polymer electrolyte fuel cell (PEFC) using a membrane made of sulfonated fluoropolymers (PFSA) has set the standards for mobile applications in recent decades and has now reached a mature level of performance and durability. The PFSA membranes in PEFCs require a sufficient water swelling and a humidification of the feed gases hydrogen and air. This restricts the atmospheric operation to temperatures <80°C, as a dehydration of the membrane will decrease its conductivity significantly. A PEFC operated at 120°C allows a more effective cooling at high electrical performance and a better heat management. These increased operating temperatures, however, require a new electrolyte material whose proton conduction mechanism is not based on the presence of the amphoteric water. A non-aqueous proton conducting electrolyte would also make an active water management obsolete. All this would allow to construct complete fuel cell systems with an easier design and thus enable a much more efficient system integration of PEFCs, especially in the field of electro mobility. A medium temperature PEFC would meet the requirements in the transport and traffic sector (cars, trucks, trains) much better than a conventional PEFC. To enable operating temperatures between 100–140°C new membrane materials respectively (non-aqueous) proton-conducting electrolytes with a sufficient proton conductivity also at low water partial pressures are required. In this project, proton-conducting ionic liquids (PILs) with strong Brønsted-acidic sulfoalkylphosphonium cations will be prepared and characterized. Strongly acidic and thus strongly hygroscopic PILs based on sulfonic acids are a promising technological approach. The focus is on the synthesis of new structures with varying polar and hydrophilic/hydrophobic properties and the effect on proton transport (vehicle or cooperative mechanism) and on the kinetics of the electrode redox reactions in such non-aqueous electrolytes. The structure of the new PILs will be optimized in successive steps by considering the measured physical and electrochemical properties. There are only few investigations of the oxygen reduction reaction (ORR) in non-aqueous electrolytes on a platinum surface. Thus, another aim is also to determine the critical kinetic parameters and to get insights in the mechanism. The optimized PIL will be immobilized in a polymer matrix to obtain free standing mechanically stable electrolyte membranes. In order to enable the development of more efficient future cell concepts, it is necessary to proceed with basic investigations on electrocatalysts but also on new customised electrolyte materials.