Catalyst Activation and Degradation in Hydrous Iridium Oxides

The development of efficient catalysts for the Oxygen Evolution Reaction (OER) is crucial for advancing Proton Exchange Membrane (PEM) water electrolysis, with iridium-based OER catalysts showing promise despite the challenges related to their dissolution. Collaborative research by the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH and the Fritz-Haber-Institut has provided insights into the mechanisms of OER performance and iridium dissolution for amorphous hydrous iridium oxides, advancing the understanding of this critical process.

Water electrolysis is a cornerstone of global sustainable and renewable energy systems, facilitating the production of hydrogen fuel. This clean and versatile energy carrier can be utilized in various applications, such as chemical CO₂ conversion, and electricity generation. Utilizing renewable energy sources such as solar and wind to power the electrolysis process may help reducing carbon emissions and promoting the transition to a low-carbon economy.

The development of efficient and stable anode materials for the Oxygen Evolution Reaction (OER) is essential for advancing Proton Exchange Membrane (PEM) water electrolysis technology. OER is a key electrochemical reaction that generates oxygen gas (O₂) from water (H₂O) or hydroxide ions (OH⁻) during water splitting. This seemingly simple reaction is crucial in energy conversion technologies like water electrolysis, as it is hard to efficiently realize and is a concurrent process to the desired hydrogen production. Iridium (Ir)-based materials, particularly amorphous hydrous iridium oxide (am-hydr-IrOx), are at the forefront of this research due to their high activity. However, their application is limited by the high dissolution rates of the precious iridium.

A collaborative effort led by scientists from the Department of Interface Design at the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH and the Theory Department at the Fritz-Haber-Institut der Max-Planck-Gesellschaft provided now fundamental insights into the intertwined mechanisms of OER and Ir dissolution in amorphous, hydrous iridium oxides (am-hydr-IrOx). Traditionally, the understanding of these processes has been limited by reliance on crystalline iridium oxide models. In this joint effort, Hydrous Iridium Oxide Thin Films (HIROFs) was explored as a model system, which revealed a unique iridium suboxide species associated with high OER activity. In situ X-ray photoelectron and X-ray absorption spectroscopy at BESSY II and ALBA synchrotrons and Density Functional Theory (DFT) was employed to investigate the local electronic and geometric structures of these materials under operating conditions, leading to the introduction of a novel surface H-terminated nanosheet model. This model better represents the short-range structure of am-hydr-IrOx, revealing elongated Ir-O bond lengths compared to traditional crystalline models.

Moreover, Ir dissolution was identified as a spontaneous, thermodynamically driven process, already occurring at potentials lower than OER activation, while the prevalent mechanistic picture assumes degradation to be driven by rare events during OER. This discovery required the development of a new mechanistic framework to describe Ir dissolution through the formation of Ir defects. The study also offered insights into the relationship between activity and stability of am-hydr-IrOx by systematically analyzing the DFT-calculated OER activity across different Ir and O chemical environments.

Overall, the current research results challenge conventional perceptions of iridium dissolution and OER mechanisms, offering an alternative dual-mechanistic framework. By examining a highly active and porous catalyst with a singular hydroxylated Ir suboxide species, the study develops a nanosheet atomistic model that surpasses conventional crystal-based models. This research not only challenges traditional understanding but also offers a new atomistic perspective on the delicate relationship between OER activity and durability of precious metal oxide catalysts. The findings are expected to be broadly applicable, potentially guiding the development of more efficient and stable anode materials for advancing PEM.