Kurzfassung: Proton exchange membrane water electrolysis (PEMWE) offers a promising route for the production of green hydrogen from renewable energy sources and could be the master key to unlocking a future sustainable energy system (1). One of the main barriers delaying the wide-spread adoption of PEMWE technologies is the slow kinetics of the oxygen evolution reaction (OER) occurring at the anode and the need for high-cost, low-abundance precious metal electrocatalysts. Iridium-based oxides are still considered the only feasible option for practical applications due to their high activity and considerable corrosion stability under the harsh electrochemical reaction conditions. To improve the overall efficiency of PEMWEs, future electrocatalyst development strategies must concomitantly address performance metrics in terms of activity, stability and material cost. Towards enhancing iridium utilisation, efforts are aimed at increasing the electrochemical surface area of the catalyst per mass of iridium, thereby increasing the number of available electrocatalytic surface sites. In this regard, amorphous iridium oxide (IrO x ) nanoparticles have been shown to achieve a high intrinsic activity (2). However, this comes at the expense of catalyst stability (3) and a loss of intrinsic electronic conductivity (4) associated with the lower degree of crystallinity. By maximising the dispersion of Ir-based nanoparticles, the use of high surface area support materials have also been shown to improve OER performance (5, 6). However, to address long-term stability concerns for Ir-based OER catalysts, highly crystalline rutile iridium dioxide (IrO 2 ) materials may still offer the best prospects. The drawback of using this approach is that the formation of crystalline IrO 2 nanoparticles often involves high temperature thermal oxidative treatment causing particle growth and loss of surface area, ultimately leading to decreased OER activities (7). Therefore, novel synthesis methods that retain a high degree of crystallinity without a loss of surface area for IrO 2 nanoparticles are required. In this talk, we discuss two synthesis strategies geared towards the preparation of highly crystalline IrO 2 nanoparticles with high OER performance. Firstly, we present a novel wet-chemistry synthesis method that avoids the use of reducing agents and eliminates the need for high temperature thermal oxidative treatment. The resultant nano-sized IrO 2 nanoparticles were found to have excellent Ir mass-specific OER activity and durability attributed to the small nanoparticle size and high degree of crystallinity. Secondly, we present a novel metalorganic chemical deposition process as a simple, one-step preparation method for highly crystalline IrO 2 nanoparticles supported on Sb-doped tin oxide (ATO) (8). The superior OER performance was attributed to the epitaxial anchoring of well dispersed, crystalline IrO 2 nanoparticles onto the ATO support. We further discuss the versatility of the method to the application of other conductive oxide support materials such as indium tin oxide and F-doped tin oxide, with the ability of tuning the chemical state of the Ir-based nanoparticles by changing the reaction conditions, i.e., temperature and gas environment as well as the nature of the support. Finally, using a series detailed physico-chemical characterisation techniques to elucidate the nature the iridium phase, composition, morphology and structure, we relate these properties to the electrochemical activity and stability of the prepared materials for the OER. 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