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@PHDTHESIS{Paredes:998333,
author = {Paredes, Miquel Vega},
othercontributors = {Scheu, Christina and Schneider, Jochen M.},
title = {{I}mproving electrocatalysts through advanced nanostructure
characterization},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2024-11312},
pages = {1 Online-Ressource : Illustrationen},
year = {2024},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2025; Dissertation, Rheinisch-Westfälische
Technische Hochschule Aachen, 2024},
abstract = {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.},
cin = {521220 / 520000 / 080018},
ddc = {620},
cid = {$I:(DE-82)521220_20140620$ / $I:(DE-82)520000_20140620$ /
$I:(DE-82)080018_20160203$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2024-11312},
url = {https://publications.rwth-aachen.de/record/998333},
}
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