Atomic Probing Surface Restructuring of Epitaxial IrO2 Thin Films Towards Extreme Oxygen Evolution Reaction Conditions

Electrochemical water splitting offers efficient energy conversion potential, but it faces hurdles in the form of slow oxygen evolution reaction (OER) kinetics and harsh acidic conditions. Acidic OER electrocatalysis directly impacts making the proton exchange membrane viable on an industrial scale. IrO2 is widely recognized as a benchmark electrocatalyst for OER which dictates the overall performace on electrochemical water splitting, in particular in regard with its superior catalytic stability under long term operations. Investigating the interaction between IrO2 and aqueous solutions is crucial for understanding the intriguing properties and active sites of this catalyst in operation. Recently, many groups have used epitaxial single crystal films as model surfaces to address these fundamental questions. These developments enable for the first time the ability to assess the activities on different facets, which has significantly improved the understanding of the OER and degradation mechanism. However, there is no deep understanding of IrO2 atomic level surface structure during electrocatalytic reactions due to the extreme challenge of synthesizing epitaxial IrO2 thin film. Very recently, our collaborators have demonstrated the successful growth of atomically smooth epitaxial IrO2 films on TiO2 single crytal substrates by the unique hybrid oxide MBE growth method, which allows us to utilize in-situ surface scattering techniques, including crystal truncation rod (CTR) analysis and X-ray reflectivity (XRR), to examine epitaxial IrO2 thin films with various crystallographic orientations and their relationship with OER performance. By employing coherent Bragg rod analysis during in-situ electrochemical reactions, the interfacial structure, specifically the presence of oxygenated adsorbed species, can be resolved as a function of potential. Our study endeavors to uncover the surface chemistry of IrO2, shedding light on its activity, stability, and degradation mechanisms under extreme electrochemical conditions.<br/><br/>In the talk, we will present our investigation of epitaxial IrO2 films with three distinct crystallographic orientations (e.g., 001, 101, and 110 orientations) using in-situ CTR and XRR techniques, which could comprehend the influence of orientation or facet dependence on surface structuring when exposed to controlled OER conditions. Additionally, we will demonstrate surface modifications induced by different electrolytes (KOH, H2SO4, and HClO4) that lead to diverse catalytic performance and stability. Preliminary synchrotron in-situ grazing incident X-ray absorption spectroscopy (GIXAS) measurements have revealed surface structure modifications, which will be further elucidated using CTR/XRR analysis. By employing energy-modulated measurements at the element-specific Ir L-edge, taking advantage of the resonant anomalous effect, we can gather detailed information on the concentration of Ir cations at each monolayer, providing direct experimental evidence on surface composition. The resulting quantitative structural and compositional information from CTR measurements will be correlated with electrochemical properties, enabling profound mechanistic insights into the activity and stability of IrO2 toward extreme OER conditions. Our experimental results offer a comprehensive understanding of the surface properties and electrochemical behavior of IrO2, contributing to the rational design and development of efficient catalysts for the oxygen evolution reaction.