Thin-Film Electrocatalysts for Polymer Electrolyte Fuel Cells - Activity, Durability and Proton Conduction

Licentiate thesis, 2007

The polymer electrolyte membrane fuel cell (PEMFC) is a potential technology for future energy conversion in sustainable energy systems. By using hydrogen from renewable energy sources and oxygen from the air, the fuel cell produces electricity with water as the only exhaust. However, in order for fuel cells to be compatible with current techniques, several technological improvements are needed. The major issues are related to the platinum used to catalyze the oxygen reduction reaction on the cathode electrode, the proton conducting membrane and the durability of the fuel cell. PEMFC electrodes typically consist of nanometer sized platinum electrocatalyst particles mixed with an ionomer and supported on carbon particles. The structural complexity of the electrode renders fundamental studies of its function difficult. Such studies are needed in order to increase the understanding and performance of the PEMFC electrode. For this purpose, model systems with well defined nanostructures should be used so as to isolate different governing mechanisms. In this thesis, a structurally well controlled thin-film model system has been used to investigate PEMFC electrodes. The model electrodes were fabricated using thermal evaporation of the catalyst material directly on proton conducting membranes or gas diffusion layers. This procedure allows for short fabrication times, control over the amount of material deposited and a wide range of materials can be used. The samples were electrochemically evaluated in a conventional fuel cell under realistic operating conditions. Evaluation of several different thin-film model systems demonstrated that the methodology can be used to qualitative compare and rank catalyst material in the PEMFC environment. Clear changes in the catalytic and structural properties of platinum were observed when the metal was mixed in bi-layers with iridium and titanium oxide. TiO2 was found to operate as a proton conducting electrolyte in the PEMFC. In addition, adding TiO2 between platinum and the carbon gas diffusion layer indicated an increased durability of the PEMFC electrode.