Nanostructured Catalysts and Electrode Materials for PEM Fuel Cells

Doktorsavhandling, 2007

The Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a promising energy converting technology for powering automotives and electrical equipment using hydrogen gas as fuel and producing water as the only exhaust. The active carbon-supported platinum catalyst present in the electrodes limits the maximum power generation in state-of-the-art PEMFCs due to platinum’s poor activity for the oxygen reduction reaction (ORR) taking place at the cathode. Further, performance drop due to limited mass-transfer capacity of the electrode structure, as well as insufficient electrode life-time are other bottlenecks of the PEMFC technology. The aim of this thesis is to use and develop methods for preparation of active catalyst and catalyst support materials to be evaluated under lab-scale PEMFC conditions in order to find catalyst materials that improve the PEMFC performance. Nano-sized platinum particles have been synthesized using the water-in-oil microemulsion route and the phase transfer method (PTM). Platinum nanoparticles of different shape and diameter have been obtained through additions of stabilizing agents, such as alkylthiol and alkylamines, with different chain lengths. Either spherical or rod-shaped nanocrystals were obtained with the water-in-oil microemulsion whereas exclusively homogenous spherical nanoparticles were produced with the PTM. It was found that the size of the platinum nanoparticles prepared with the PTM decreased with increasing hydrocarbon chain length when stabilized with alkylamines. A non-ionic block-copolymer was employed for preparation of ordered mesoporous silica materials, which were used as templates for synthesis of ordered mesoporous carbon (OMC) materials using the nanocasting route with furfuryl alcohol as the carbon precursor. High structural long-range order and large pore volume of the OMC were obtained. Further, by using electron tomography it is shown that the water-in-oil microemulsion technique is applicable for in-situ preparation of platinum nanocrystals inside ordered mesoporous carbon. Electrode materials were prepared by deposition of platinum nanoparticles onto porous support materials of either porous carbon or titania powders. The electrochemical performance of the electrode materials was evaluated under lab-scale PEMFC conditions and compared with a commercial platinum/carbon reference material. Differences in ORR activity were observed and could be related to platinum nanoparticle size to some extent, but also to the choice and composition of the support material. The diameter of the nanoparticles increased considerably during PEMFC operation, which indicates that efforts to decrease the size of the active catalyst below 3 nm have limited benefits on the PEMFC operation. Electrode materials composed of titania-supported platinum mixed with carbon showed almost as high ORR activity as the conventional platinum/carbon materials combined with significantly improved life-time properties.