Novel catalyst materials for the cathode side of MEAs suitable for transportation applications
Novel low temperature fuel cell (FC) cathode catalyst and support systems will be designed and synthesized. The focus will be on highly active catalyst materials for polymer electrolyte membrane fuel cells (PEMFC) for transportation applications.
These materials will be fully characterized, benchmarked and validated with a multi-scale bottom up approach in order to significantly reduce the amount of precious metal catalyst loadings (< 0.15 g/kW) and to vastly improve fuel cell efficiency and durability. Thereby, materials compatible and stable under automotive fuel cell environment and conditions will be investigated in order to reach a FC lifetime of 5000h. These targets are highly relevant to the call topic requesting ambitious, highly novel concepts for next generation European membrane electrode assemblies (MEAs) for transportation applications.
Numerical simulations will be used in order to identify which alloy compositions to strive for in the experimental work. These alloys will be synthesized both in the form of well defined model compounds as well as in the form of nanoparticles. Different modified support materials will be studied. For the NPs, there will be two stages of preparation, the small scale preparation to create well defined NPs for preliminary assessment of their performance and stability, and, subsequently, up-scaling for MEA production. Supported NP catalysts and model catalysts will be tested using electrochemical methods and Surface Science approaches. After up-scaling MEAs based on improved cathode catalysts and improved supports will be assembled using advanced Nafion- based and high temperature membrane based electrolytes. These will be tested for performance and durability using procedures established in automotive industry and previous EU projects.
Final Report Summary - CATHCAT (Novel catalyst materials for the cathode side of MEAs suitable for transportation applications)
The aim of the CathCat project was to investigate Pt- and Pd-rare earth alloys as catalysts for low temperature fuel cells, and advanced support materials to enhance both activity and durability of actual catalyst layers. The project targeted the entire process chain from theoretical materials screening and validation, materials synthesis, preparation and characterization of model compounds and actual nanosized catalyst powders to MEA fabrication and single cell testing.
A comprehensive understanding of the detailed physical mechanisms that lead to the enhanced catalytic activity of Pt-rare earth alloys for the oxygen reduction reaction was achieved by close collaboration between theoretical work and experimental studies on model alloys. It was shown that the formation of different compressive strains in the Pt skin layers induced by the underlying alloy lattice parameters determine the ranking in catalytic activity, but that there is a limit to the maximum possible strain, so that the maximum of the activity volcano cannot be reached with these alloys. Also the dependence of the catalytic activity of nanoparticles on the size was studied and understood, and the performance degradation of the alloys could be explained. Different approaches for the preparation of larger amounts of practical catalyst materials were pursued, including vacuum-based methods, i.e. sputtering, chemical reduction in solution-phase and dry methods, and electrochemical methods. Sputtering allowed to prepare thin catalyst films with correct composition and good catalytic properties. The electrochemical methods permitted to deposit the rare earth metals, but did not yet succeed in making catalyst material. The dry method resulted in the preparation of Pt-Y nanoparticles that showed a better performance than an excellent commercial Pt benchmark catalyst, but did not yet permit to control the particle size. The efforts to prepare these catalysts in a form and amount suitable for MEA testing was much more difficult and time-consuming as originally anticipated. With respect to Pd-based catalysts, the influence of particle size on the electrocatalytic properties and stability as well as the nature of the support were studied using electrochemically deposited Pd on HOPG and nitrogen-doped HOPG. Pd-Y alloys and Pd-Ce alloys were studied. In the latter case, through combined theoretical and experimental efforts, an understanding of the lowered catalytic activity of the alloy as compared to pure Pd was understood. Aside from the studies regarding Pt and Pd rare earth catalysts, several other binary catalysts like PdCu catalysts were prepared and tested.
Studies on nitrogen-doped HOPG did not unequivocally demonstrate a beneficial influence for the catalytic activity of Pd catalysts. However, a large set of different N- and S-doped mesoporous carbon materials were prepared and demonstrated to be beneficial for ORR activity of supported Pd and Pt. Similarly, advanced oxide supports demonstrated improved performance in ORR experiments. Different deposition methods were tested on these materials and oxide-carbon composites, and photochemical deposition proved the best option for simple Pt catalysts.
MEA single cell testing was carried out on Pt-based benchmark MEAs and several MEAs with catalyst materials and advanced support materials from the project. However, the only actual alloy samples tested were not present in optimized catalyst layers, as only one set of PtY/carbon powder had been prepared for MEA testing. Nevertheless, they performed better than pure Pt present in the same configuration, but still worse than the benchmark MEAs. None of the MEAs tested so far surpassed the performance of the benchmark MEAs so that further efforts regarding catalyst preparation and optimization of the catalyst layer structure and loading are required in future work.