Atomic-Scale View into the Degradation of Ir-Ru Alloys during Anodic Oxygen Evolution
Authors: Schweinar Kevin , Gault Baptiste , Greiner Mark , Raabe Dierk , Kasian Olga 
Published: May 2020

Hydrogen is likely to play a key role as central energy carrier in the world’s transition towards a more sustainable future. Electricity from renewable sources can be used to power water electrolyzers to generate carbon-neutral hydrogen [1]. ‘Green’ hydrogen can then, for example, be used for the synthesis of carbon-neutral fuels, in the transportation sector to power H2 fuel-cell fitted vehicles, as a carbon-neutral agent for the direct reduction of metals from ores, or to re-generate electricity in time of high demand. However, the technological upscaling of proton exchange membrane water electrolysis (PEM) devices is currently hindered by the low reactivity and instability of most catalyzing materials for the anodic oxygen evolution reaction (OER). The development of electrocatalysts with superior performance requires a detailed understanding of how their surfaces evolve during OER, ideally at the atomic scale [2,3]. The identification of the role of individual atoms in different chemical and structural environments in the near-surface region of the catalyst is crucial to understand the electrochemical performance and degradation processes.

Here, we provide atomic scale insights into the degradation mechanism of an Ir-Ru model alloy during electrochemical oxygen evolution. Ir-Ru alloys are promising candidates for PEM electrolysis combining the stability of Ir and high activity of Ru towards OER. The changes in electronic state of the catalysts induced by OER are studied by X-ray photoelectron spectroscopy (XPS), while an in-situ activity-stability analysis was performed online using an inductively coupled plasma mass spectrometer. Atom probe tomography (APT) was used to reveal the structure and composition of the first few atomic layers of the catalyst in three dimensions.

APT reveals strong compositional differences between intragranular and grain boundary regions in the pristine alloy, resulting from significant Ru segregation to crystalline defects during the deposition process [4,5]. Significant Ru dissolution occurs during the OER, as revealed by online dissolution measurements, resulting in a few surface layers enriched in Ir, consistent with XPS results. APT further shows that the entire near-surface region affected by the oxidation exhibits signs of Ru dissolution. In addition, oxides formed in Ru-rich regions across the catalyst´s surface, i.e. in the proximity of grain boundaries, generally exhibit a higher oxygen content, compared to oxides on top of intergranular regions. Our data suggests that the defect structure of the material governs the enrichment of Ru in grain boundaries with consecutive preferential oxidation of these regions. This preferential oxidation potentially contributes to an accelerated, localized formation of volatile RuO4 species during the OER, leading to an enhanced Ru leaching along defect regions, eventually resulting in the deterioration of the overall reactivity of the material.

Finally, using the new insights gained by this study, we discuss potential design strategy improvements of Ir-Ru alloys, to engineers an OER catalyst with a higher performance and longer durability. With this work we stress the need to investigate processes in catalytic materials with a combination of independent methods and at the near-atomic scale. Only the combination of insights enables the establishing of reliable structure-function relationships of electrocatalytic materials.

References

[1] M. Carmo, D.L. Fritz, J. Mergel, et al., Int. J. of Hydrogen Energy
37(12) (2013), pp. 4901 - 4934

[2] T. Li, O. Kasian, S. Cherevko, et al., Nat. Cat.
1 (2018), pp. 300-305.

[3] O. Kasian, S. Geiger, T. Li, et al., Energy & Environ. Sci. (2019).

[4] K. Schweinar, O. Kasian, R.L. Nicholls, et al., Microscopy and Microanalysis
25(S2) (2019), pp. 306-307

[5] K. Schweinar, R.L. Nicholls, C.R. Rajamathi, et al., submitted.