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TY - THES AU - Paredes, Miquel Vega TI - Improving electrocatalysts through advanced nanostructure characterization PB - Rheinisch-Westfälische Technische Hochschule Aachen VL - Dissertation CY - Aachen M1 - RWTH-2024-11312 SP - 1 Online-Ressource : Illustrationen PY - 2024 N1 - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2025 N1 - Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2024 AB - Electrocatalysts play a pivotal role in the decarbonization of our world by enhancing the kinetics of electrochemical reactions, thereby boosting the performance of devices such as fuel cells and water electrolyzers. High-performing electrocatalysts are typically based on expensive metals like Pt or Ir, which hinders the widespread commercialization of these devices. It is therefore of great importance to study how to enhance the activity and stability of such expensive materials, which would allow for a lower loading and cost, or to find cheaper, high-performing alternatives. The stability and activity of a catalyst can be improved by modifying its structure, both on the nanometer scale (e.g., by changing the size or shape of the catalyst) and atomic scale (e.g., by introducing strain, lattice defects, or foreign elements). By studying how the structure of a catalyst affects its performance, it is possible to elucidate structure-properties relationships, which in turn enables the rational design of superior catalysts with optimized structures. Nonetheless, studying the local atomic structure in a representative manner, or its evolution during catalysis presents a formidable challenge. Recent advances in (scanning) transmission electron microscopy have made such studies possible, allowing the determination of degradation or activation mechanisms of electrocatalysts, and the systematic study of the effects of defects on catalysis.In this thesis, state-of-the-art (scanning) transmission electron microscopy-based techniques are used to develop new insights into how the structure and lattice defects of electrocatalysts influence their performance. Moreover, method development is pursued in cases where the current techniques cannot be successfully applied. Various electrocatalytic materials with applications in fuel cells and electrolyzers were studied. Firstly, the degradation mechanisms of Rh-core Pt-shell nanoparticles under electrochemical cycling -used to mimic fuel cell operation conditions- were investigated. Using identical location scanning transmission electron microscopy, it was discovered that particle detachment from the carbon support was the main degradation mechanism responsible for the loss of activity during the electrochemical tests, providing valuable insights into which strategies need to be prioritized for developing more stable catalytic systems. After having seen the potential of the identical location technique on fuel cell investigations, it was investigated how to apply it for gas-evolving reactions, such as those occurring in a water electrolyzer. It was discovered that the identical location technique could be easily and reproducibly applied to such reactions using the tweezers method, as opposed to the commonly used Teflon cap method.Next, with the help of this easy-to-implement method, a catalyst for the oxygen evolution reaction, i.e., the anodic reaction in an electrolyzer, was studied. LaNiO3 perovskite nanoparticles were selected for this part due to their low cost and high catalytic activity. The results revealed that Fe traces in the potassium hydroxide electrolyte diffuse into Ruddlesden-Popper planar faults present in the LaNiO3 structure, causing structural changes that enhance the catalytic activity of the perovskites. This work highlighted the importance of defects in the catalytic activity, motivating the study of a different class of defects, namely grain boundaries.In particular, the aim was to use grain boundaries for increasing the catalytic activity of the expensive noble metal Pt. It was discovered that the catalytic activity of grain boundary-rich Pt nanoparticle assemblies was over 35x higher than single-crystal Pt nanoparticles, thanks to their elevated number of concave grain boundary sites. Moreover, it was seen that the active sites could be stabilized by boron segregation. Lastly, having showcased the potential of grain boundaries in enhancing the catalytic activity of noble metals, the effect was explored further for Au. Using Au samples with a controlled grain boundary density as a material system, a relationship between grain boundary density, coordination number and catalytic activity was revealed. This thesis is a demonstration of how material science can contribute to improving the stability and activity of catalysts by tuning their structure and exploiting the potential of defects in catalysis. LB - PUB:(DE-HGF)11 DO - DOI:10.18154/RWTH-2024-11312 UR - https://publications.rwth-aachen.de/record/998333 ER -