In-Situ Analysis of Reversible Deactivation and Reactivation Mechanisms of Ir Catalysts for Lifetime Extension of Proton Exchange Membrane Water Electrolysis
Authors: Lim Ahyoun , Ham Kahyun , Spanos Ioannis 
Published: December 2024

Building a hydrogen society to replace the current carbon energy systems is one of humanity's most pressing challenges. Green hydrogen production via proton exchange membrane water electrolysis (PEMWE) is thus a promising technology. However, the strong acid environment at high voltage leads to the corrosion of non-noble metal components, driving PEMWE demand towards expensive materials such as titanium and noble metal catalysts like Ir to drive the sluggish oxygen evolution reaction (OER) in PEMWE. Decreasing Ir loading in an effort to decrease costs while maintaining efficient and stable PEMWE is challenging due to Ir dissolution [1]. Reversible deactivation of iridium, along with irreversible degradation like agglomeration and dissolution, is of great interest [2], due to lifespan extension and reduced hydrogen production costs. Therefore, a comprehensive understanding of the mechanisms of OER deactivation and reactivation of Ir, which can be accomplished by operando studies, is crucial. However, only limited operando studies provide insight into the underlying mechanisms of OER de-/re-activation of Ir [3].
In this research, we study the correlation between consumption and production of μ-oxo species in Ir catalysts to explain the de-/re-activation of Ir through specially designed electrochemical protocols coupled with operando techniques such as X-ray Absorption Spectroscopy (XAS), electrochemical Quartz Crystal Microbalance (eQCM) connected with Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), and Surface Enhanced Raman Spectroscopy (SERS), including the application of the reactivation protocol in a single cell mode of operation. Surface-terminated electrophilic μ-oxo species on Ir oxide promote intermediate formation and lower the energy barrier of OER [4-8]. Operando XAS elucidates the potential-dependent formation of a thin oxide layer, including μ-oxo species, on the Ir surface while maintaining the Ir core. However, consumption of μ-oxo species and detachment of adsorbates during OER result in a reversible mass reduction of catalysts. The relationship between dissolved Ir and replenished μ-oxo species is demonstrated by ICP-OES. Although the dissolutions of Ir catalysts increase after applying certain reactivation potentials, the average OER currents are enhanced, further indicating the presence of an additional primary factor for the activity of the catalysts, apart from the amount of present catalysts. Operando SERS further clarifies the direct correlation between the generation of μ-oxo species on the catalyst surface during catalyst reactivation and the degree of the recovered catalyst activity.
Further, we implemented the reactivation protocol in a PEM single-cell and revealed the effectiveness of catalyst reactivation in PEMWE. Our findings not only contributed to an enhanced understanding of the observed phenomena on the catalyst surface during de-/re-activation but also to the development of a protocol for stable water electrolysis in a single-cell mode of operation.

Reference

Alia, S.M., et al., Activity and Durability of Iridium Nanoparticles in the Oxygen Evolution Reaction. Journal of The Electrochemical Society2016. 163(11), F3105-F3112.

Tan, X., et al., Decoupling structure-sensitive deactivation mechanisms of Ir/IrOx electrocatalysts toward oxygen evolution reaction. Journal of Catalysis 2019. 371, 57-70.

Papakonstantinou, G., et al., Electrochemical evaluation of the de-/re-activation of oxygen evolving Ir oxide. Phys Chem Chem Phys 2022. 24(23), 14579-14591.

Pfeifer, V., et al., Reactive oxygen species in iridium-based OER catalysts. Chemical Science2016. 7(11), 6791-6795.

Pfeifer, V., et al., The electronic structure of iridium oxide electrodes active in water splitting. Phys Chem Chem Phys 2016. 18(4), 2292-2296.

Pfeifer, V., et al., In situ observation of reactive oxygen species forming on oxygen-evolving iridium surfaces. Chemical Science2017. 8(3), 2143-2149.

Frevel, L.J., et al., In Situ X-ray Spectroscopy of the Electrochemical Development of Iridium Nanoparticles in Confined Electrolyte. The Journal of Physical Chemistry C 2019. 123(14), 9146-9152.

Mom, R.V., et al., Operando Structure–Activity–Stability Relationship of Iridium Oxides during the Oxygen Evolution Reaction. ACS Catalysis2022. 12(9), 5174-5184.