Elettra | Materials Science Beamline Unraveling the secret of Ir-Ru catalysts for water electrolysis: Why are they so active and yet so durable? The growing demand for clean energy and energy independence calls for a rapid expansion of solar and wind power – alongside better storage solutions to handle their fluctuating output. Powerto-gas conversion, such as water electrolysis, offers a possible solution by turning excess electricity into hydrogen. This hydrogen can then be used in industry, power generation, or fuel cell vehicles. Among electrolyzers, Proton Exchange Membrane Water Electrolyzers (PEM-WEs) stand out for their efficiency, responsiveness, and compact design. However, their reliance on iridium-based catalysts poses a critical challenge: global iridium supply is extremely limited, and current PEM-WE designs use far too much of it. Reducing Ir loading by at least a factor of 10 is essential for large-scale deployment. Several research directions are being pursued to overcome this challenge, one of which involves alloying iridium with more abundant elements to retain its original performance but with lower price. Perhaps the most explored bimetallic system is Ir-Ru which should in theory combine the superior activity of ruthenium with stability of iridium. Various alloys of different ratios, prepared by numerous methods have been investigated in the past – generally achieving better activity but compromised stability in comparison to pure Ir. Our approach to the Ir-Ru catalyst was to prepare it unorthodoxly by magnetron sputtering – a method well known in industry but still underutilized in catalysis. This technique allowed us to precisely control the composition and identify the most promising ratio, Ir-Ru 25:75, which exhibited not only high activity but also unexpected stability considering its low Ir content. However, a thorough understanding of the interplay between iridium and ruthenium that could explain this intriguing behavior was missing. It was not until we combined the powerful methods of operando electrochemical Wide Angle X-ray Scattering (WAXS) at ESRF (Grenoble) with Synchrotron Radiation Photoelectron Spectroscopy (SRPES) at Elettra (Trieste) that we uncovered the true nature of the magnetron-sputtered Ir-Ru catalyst during its operation at the anode of a PEM-WE. The first goal was to determine which element is actually catalytically active for the reaction in Ir-Ru catalysts. The superior performance of Ir-Ru 25:75 could in principle be due to increased Ir activity, stabilized Ru, or a synergistic effect of both. However, conventional X-ray Photoelectron Spectroscopy (XPS), with sampling depth of around 5.3 nm, detected both Ir and Ru, making it difficult to resolve the surface composition and answer our question. To probe the very surface, we used SRPES with fine-tuned energy to get information from the topmost 1.3 nm and 2.3 nm of our activated Ir-Ru catalysts (Fig. 1 a). Combined with complementary Scanning Transmission Electron Microscopy (STEM) and Energy-Dispersive X-ray spectroscopy (EDX) results, we concluded that upon activation the Ir-Ru 25:75 catalyst dynamically forms Ir-Ru@IrOx coreshell structure. This structure is unique because it is formed during operation without intentional engineering. The remaining question was why Ir-Ru 25:75 outperforms pure Ir even though the surface is dominated by IrOx not by the more active Ru. This suggested that the surface IrOx shell possesses enhanced properties influenced by the underlying Ir-Ru core. We therefore studied the strain evolution in Ir-Ru catalysts through operando WAXS by utilizing a newly developed synchrotron-compatible PEM-WE single cell. The data confirmed alloying between Ir and Ru in the magnetron-sputtered films, following Vegard’s law. Notably, the strain in Ir-Ru 25:75 remained stable throughout the operando electrochemical procedure, while in different compositions, such as 50:50, it quickly disappeared. This led us to hypothesize that Ir-Ru 25:75 forms a compact and stable coreshell structure that resists degradation unlike for example Ir-Ru 50:50, which suffers from extensive Ru leaching, resulting in the loss of surface strain and catalytic performance (Fig. 1 b). Finally, we investigated the chemical state of Ir in the IrOx shell. Both XPS and SRPES showed that Ir in Ir-Ru 25:75 had a lower oxidation state than in the Ir 100 reference. This has been associated with higher activity and stability, which is caused by less dissolution of the higher valence Ir species as well as kinetic inhibition of the unstable hydrous iridium oxide growth into the subsurface regions. To sum up our key findings, the magnetron sputtered Ir-Ru thin film with the atomic ratio of 25:75 outperforms the conventional catalysts with just a fraction of their Ir loading, ~ 0.1 mgIr cm⁻², i.e. an order of magnitude reduction. Shortly after the introduction of the electric potential, the catalyst undergoes dynamic transformation to Ir-Ru@IrOx core-shell structure, which induces compressive strain on the Ir surface. This structural strain influences the electronic structure of surface iridium, allowing it to remain in active and stable oxidation states while preventing its further dissolution. However, the practical implication of our research goes beyond this conclusion as it opens up the question whether such coreshell behavior cannot be achieved with other elements with structure similar to ruthenium which could improve catalysts in other areas as well. Elettra Highlights 2024–2025 84
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