An Identical-Location STEM Study of the Degradation of Oer Electrocatalysts for PEM Electrolyzers
Authors: Yu Haoran , Cullen David A. , Meyer Harry M , Kariuki Nancy N. , Myers Deborah J. , Padgett Elliot , Alia Shaun M 
Published: July 2021

The slow kinetics of the oxygen evolution reaction (OER) and high cost of OER electrocatalysts are among the main barriers for proton exchange membrane water electrolysis (PEMWE) technology [1]. Improving the design of active and low-loaded OER catalysts requires a fundamental understanding of catalyst degradation mechanisms.

Current state-of-the-art OER catalysts for PEM electrolyzers are iridium-based, such as pure iridium oxide or mixed oxides of iridium and ruthenium. The dissolution of iridium catalyst in acidic environment during OER has been reported extensively in both liquid electrolyte [2] and membrane electrode assemblies (MEAs) [3]. Current understanding of the iridium catalyst degradation reveals a trade-off between stability and activity. Hydrous iridium oxide (IrOx) shows higher activity, but less stability compared to rutile iridium oxide (IrO2) [4, 5]. Mechanistic studies reveal Ir (IV) oxide being the stable oxide and Ir (III) oxide being the intermediate species during OER and dissolving rapidly [4, 5]. Furthermore, density functional theory (DFT) and experimental work show that oxygen binding energy is affected by surface facets and sub-surface structures for iridium-based catalysts [6]. Surface oxide skins can exhibit unique structural features, including different coordination, compression or expansion related to lattice mismatch, and weaker or stronger binding relative to the pure metal or metal oxide, which govern the OER kinetics at end-of-life (EOL). [6]

Obtaining an electrocatalyst structure that provides both high activity and high stability, particularly at high current densities, remains a challenge. Stabilizing the weak iridium intermediate caused by oxygen vacancies will be of great importance [5] and requires a fundamental understanding of iridium surface oxides to address this challenge. It is unclear how the surface oxides are developed as degradation progresses and what roles of other accompanied phenomena, such as surface reconstruction and segregation [6], are playing during degradation. Therefore, the formation and structural change of iridium surface oxide and subsequent impact on dissolution rates during accelerated stress test (AST) is the focus of this work.

Identical-location scanning transmission electron microscopy (IL-STEM) is a powerful approach for tracking the evolving morphology and chemistry of a catalyst particle during degradation [7, 8]. In this work, commercially available iridium oxide catalyst will be studied using IL-STEM. Aberration-corrected STEM coupled with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) will be used to study the catalyst surface structure and composition at the atomic scale. The effect of different upper potential limits and potential scanning profiles will also be investigated. The dissolution rates of iridium under these different AST protocols will be determined using time-resolved inductively coupled plasma-mass spectrometry (ICP-MS). Changes in the composition and structure of iridium surface oxide before and after ASTs will be analyzed with EDS and EELS and corroborated with results from X-ray photoelectron spectroscopy (XPS) and, further down the road, operando X-ray absorption spectroscopy (XAS). The outcome of this work can shed light on the strategies to maintain high activity for precious metal catalysts.[9]

References

[1] Buttler, A., Spliethoff, H., Renewable and Sustainable Energy Reviews, 2018, 82, 2440-2454.

[2] Cherevko S., Geiger S., Kasian O., et al., J. Electroanal. Chem. 2016, 774, 102–110.

[3] Yu, H., Bonville, L., Jankovic, J., et al. Appl. Catal. B: Environ., 2020, 260, 118194.

[4] Pfeifer V., Jones T.E., Velasco Velez J.J., et al., Phys. Chem. Chem. Phys. 2016, 18, 2292–2296.

[5] Geiger, S., Kasian, O., Ledendecker, M. et al. Nat. Catal. 2018, 1, 508–515.

[6] Alia, S.M., Ha, M-A., Anderson, G.C., et al. J. Electrochem. Soc. 2019, 166, F1243-F1252.

[7] Hartl, K., Hanzlik, M, Arenz, M., Energy Environ. Sci., 2011, 4, 234-238.

[8] Rasouli, S., Myers, D., Kariuki N., et al., Nano Lett. 2019, 19, 46−53.

[9] This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the H2NEW Consortium. Electron microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).