Nanostructured Catalysts and Electrode Materials for PEM Fuel Cells

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

Nanostructured Catalysts and Electrode Materials for PEM Fuel Cells

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

The Sonogashira reaction catalyzed by palladium leached from ordered mesoporous carbon

The Sonogashira carbon–carbon coupling reaction between iodobenzene and phenylacetylene was performed using palladium nanoparticles formed directly inside the pores of ordered mesoporous carbon as catalyst. The nanoparticles were generated by two methods, a phase-transfer method and a microemulsion procedure. The coupling reaction was performed using different bases and varying amounts of palladium and the yields and reaction profiles were determined by 1H NMR. Recycling experiments were also performed and the recyclability was found to be very dependent on the choice of base. From the so-called Maitlis’ filtration test it was shown that soluble palladium species were present in solution and these were found to be active in catalyzing the coupling reaction. The degree of leaching depended on the type of base used. The results hint at the Sonogashira reaction being an example of homogeneous rather than heterogeneous catalysis, as has recently been suggested by other authors as well

The Sonogashira reaction catalyzed by palladium leached from ordered mesoporous carbon

The Sonogashira carbon–carbon coupling reaction between iodobenzene and phenylacetylene was performed using palladium nanoparticles formed directly inside the pores of ordered mesoporous carbon as catalyst. The nanoparticles were generated by two methods, a phase-transfer method and a microemulsion procedure. The coupling reaction was performed using different bases and varying amounts of palladium and the yields and reaction profiles were determined by 1H NMR. Recycling experiments were also performed and the recyclability was found to be very dependent on the choice of base. From the so-called Maitlis’ filtration test it was shown that soluble palladium species were present in solution and these were found to be active in catalyzing the coupling reaction. The degree of leaching depended on the type of base used. The results hint at the Sonogashira reaction being an example of homogeneous rather than heterogeneous catalysis, as has recently been suggested by other authors as well

On the influence of Pt particle size on the PEMFC cathode performance

Colloidal suspensions of almost spherical and crystalline Pt nanoparticles between 1.6 and 2.6 nm in diameter and with narrow size distribution were synthesized using the phase transfer method (PTM) with alkylamines, C n NH 2 , as stabilizing agents. Batches of such homogenous Pt-C n NH 2 (n = 8, 12) nanocrystals were deposited onto Vulcan XC-72 carbon powder, and the activity for the oxygen reduction reaction (ORR) of this series of Pt/C materials was evaluated under PEMFC conditions. The aim was to elucidate whether this type of stabilized Pt nanoparticles were as active for the ORR as a corresponding commercial Pt/C material, and if any difference in mass activity could be observed between catalysts with different Pt particle size. In the PEMFC experiments, i.e. voltammetry in oxygen and nitrogen, it was found that, after an initial electrode activation, the ORR activity of the catalysts prepared from the alkylamine-stabilized Pt nanoparticles deposited on carbon was as high as that of the employed commercial reference catalyst. In fact, all samples in the Pt/C series showed high and very similar ORR activity normalized to Pt-loading, without significant dependence on the initial Pt particle size. However, pre- and post-electrochemical characterization of the Pt/C material series with TEM showed that structural changes of the Pt nanoparticles occurred during electrochemical evaluation. In all samples studied the mean Pt particle size increased during the electrochemical evaluation resulting in decreased differences between the samples explaining the observed similar ORR performance of the different materials. These results emphasize the necessity of post-operation characterization of fuel cell catalysts when discussing electrocatalytic activity. In addition, employing complex preparation efforts for lowering the Pt particle size below 3 nm may have limited practical value unless the particles are stabilized from electrochemical sintering. \ua9 2007 Elsevier Ltd. All rights reserved