Size-Controlled Synthesis of IrO2 nanoparticles at High Temperatures for the Oxygen Evolution Reaction
Authors: Malinovic Marko , Paciok Paul , Koh Ezra Shanli , Geuß Moritz , Choi Jisik , Pfeifer Philipp , Hofmann Jan Philipp , Göhl Daniel , Heggen Marc , Cherevko Serhiy , Ledendecker Marc 
Published: September 2023

Polymer electrolyte membrane (PEM) electrolysis is considered to play a vital role in the sustainable energy transition. The efficient generation of hydrogen is largely influenced by the slow rate of the anodic oxygen evolution reaction (OER). Iridium oxide represents one of the most promising catalysts for the electrochemical oxidation of water in an acidic environment. Under harsh operating conditions at the anode, iridium oxide is found to be among the most dissolution-resistant catalysts while offering acceptable OER activity. However, iridium’s limited availability dictates high costs centralizing the research in direction of reducing noble metal content while maintaining favorable electrochemical properties.[1] Designing nanostructured catalyst with an increased surface-to-volume ratio improves the application-oriented mass-specific activity.[2] Hydrous iridium oxide is known for superior OER activity, but for a successful application, drastic dissolution of the catalyst must be addressed by stabilization. This can be achieved by heat treatment to temperatures ≥400ºC with the formation of crystalline order. However, managing to avoid agglomeration of nanoparticles at high temperatures is not trivial, thus, temperature studies on electrochemical stability and activity on similar particle sizes are missing.[3]. Here, we demonstrate how nanoparticles below 10 nm can be obtained at high preparation temperatures up to 800 °C with unprecedented control over particle size and morphology. A detailed understanding of the structural evolution during heating was obtained by in-situ scanning transmission electron microscopy (in-situ STEM) with locally resolved nanoparticles, high spatial resolution, and chemical specificity. Additionally, changes in surface properties at different temperatures were tracked ex-situ by X-ray photoelectron spectroscopy (XPS), the crystal structure was investigated by X-ray diffraction analysis (XRD), size and morphology were characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The OER activities of synthesized iridium oxide nanoparticles were measured in half-cell measurements at forced convection. The stability was carefully studied by operando flow cell measurements that were coupled to online inductively coupled plasma mass spectrometry.[4] The iridium oxide catalyst calcined at the lowest temperature resulted in outstanding mass-specific activity outperforming the reference iridium oxide catalyst by a factor of 40. By gradual increase in calcination temperatures up to 800 °C, we observe improvement in the durability of the synthesized catalysts, being comparable to the reference catalyst, yet still with notable improvement in catalytic activity. This is the first report to synthesize iridium oxide nanoparticles at high temperatures with preserved size and morphology not exceeding 10 nm and allows for the determination of activity and durability of similarly sized nanostructures.[5]

Literature:
[1] M. Bernt et al. Chemie Ingenieur Technik 2020, 92, 31-39.
[2] T. Reier, et al. ACS Catalysis 2012, 2, 1765-1772.
[3] Y. Lee et al. The Journal of Physical Chemistry Letters 2012, 3, 399-404.
[4] S. Geiger et al. Nature Catalysis 2018, 1, 508-515.
[5] M.Malinovic et al. Advanced Energy Materials 2022, Manuscript submitted for publication.