New Insights into the Structure - Activity - Durability - Relationships of Pt-Co Nanoparticle Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells

In order to further commercialize polymer electrolyte membrane fuel cells (PEMFCs), it is essential to reduce their costs. The high costs are mainly caused by the utilization of expensive Pt-based noble metal catalysts to overcome the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. Thereby, the understanding of the structure - activity - durability relationships is of enormous importance for the future development of active and especially durable Pt-based ORR catalysts. This dissertation provides new insights into the structure - activity - durability relationships and the degradation mechanisms of Pt-Co nanoparticle (NP) catalysts for ORR. At first, two PtCo alloy NP catalysts were prepared at different synthesis temperatures and holding times. A higher temperature and longer holding time leads to the formation of larger NPs, which show a lower electrochemically active surface area (ECSA). However, higher temperature and longer holding time leads to improved alloy formation, resulting in an enhanced ORR performance. This study shows that controlling the initial structure and chemical homogeneity of PtCo alloy NPs allows tuning the ORR performance of the subsequently formed core-shell catalyst materials by electrochemical activation. In another study, the analysis of the fine-structure of PtCoOx NPs using scanning transmission electron microscopy in combination with electron energy loss spectroscopy (STEM-EELS) reveals that oxidized Co species in the core of PtCo core-shell catalysts can serve as a stable alternative to metallic cobalt. Knowledge and control of the detailed atomic distribution inside the core-shell NPs, especially in the sub-surface layers, is crucial to tune the ORR activity and durability of these core-shell catalyst materials. Finally, in-situ small-angle and wide-angle X-ray scattering (SAXS and WAXS) investigations provide new and detailed insights into the degradation processes of acid-treated PtCo NPs, so-called skeleton-PtCo catalysts. Under the corrosive start-up/shut-down (SUSD) conditions, small and Pt-rich NPs grow significantly faster than larger and more Co-rich NPs due to Ostwald ripening and particle coalescence. However, the larger and Co-rich NPs lose massive amounts of cobalt immediately at the beginning of the accelerated stress test (AST). This Co depletion results in a thickening of the Pt-enriched shell and ultimately in a reduction of the catalytic ORR performance. An improved understanding of the highly complex degradation mechanisms of Pt-based NPs especially under the harsh SUSD conditions is essential for the further development of highly ORR active catalyst materials with enhanced durability.