New Frontiers in Electrocatalyst Characterization – Three Dimensional Atomic-Scale Insights By Atom Probe Tomography
Authors: Schweinar Kevin , Kim Se-Ho , Lim Joohyun , Scheu Christina , Raabe Dierk , Kasian Olga , Gault Baptiste 
Published: May 2020

Developing electrocatalysts with enhanced properties requires a fundamental understanding of the processes that occur during operation as well as side reactions during the synthesis. Many of those processes, e.g. solute segregation to defects [1], the hydroxylation of near-surface regions [2], or the preferential oxidation/reduction of certain regions, are often governed by the local microstructure. Such phenomena commonly result in significant chemical heterogeneities within the material at the nanoscale, affecting the potential distribution across the surface during operation. This way, localized heterogeneities can control the local binding energetics for reactants, intermediates and products, and determine the overall reactivity and stability of the material.
Atom probe tomography (APT) offers the opportunity to measure the composition of localized chemical fluctuations at the near-atomic scale and in three dimensions. Targeted APT specimen preparation of regions-of-interest, e.g. at specific grains or grain boundaries, enables the correlation of local structure and local composition. Recent method developments have enabled the measurement of free surfaces with APT [2,3], offering a new perspective into the first few atomic layers of electrocatalytic materials. APT thereby provides the chance to elucidate chemical processes within the near-surface region which underpin the electrochemical performance of the material.
In this contribution, we showcase how APT can be instrumental to the precise characterization of various electrocatalytic materials. In a first example, we demonstrate how APT revealed the structural features in hydrous Ir oxides that are responsible for an accelerated degradation of the catalyst during the oxygen evolution reaction, due to release of molecular oxygen from the lattice [4]. Secondly, we present how APT can be combined with scanning photoemission electron microscopy to obtain spatially correlated chemical state information of the surface investigated with APT [1,3]. And finally, we show how APT enables the investigation of trace-element distribution in nanostructured materials, such as nanoparticles [5] or nanorods [6]. For example, APT can shed light on the incorporation of B, Na and N during the hydrothermal synthesis and partial reduction of hollow TiO2 nanorods, that are commonly used as a photocatalyst or catalyst support. Trace amounts of such light elements often remain undetected with other methods but may have a dramatic effect on the functionality of the material.

References

[1] K. Schweinar, R.L. Nicholls, C.R. Rajamathi, et al., submitted.
[2] T. Li, O. Kasian, S. Cherevko, et al., Nat. Cat.
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[3] K. Schweinar, O. Kasian, R.L. Nicholls, et al., Microscopy and Microanalysis
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[4] Kasian, S. Geiger, T. Li, et al., Energy & Environ.
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[5] S. Kim, P.W. Kang, O.O. Park, et al., Ultramicroscopy
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[6] J. Lim, S. Kim, R.A. Armengol, et al., submitted.