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Development of low-cost and high-performance oxygen evolution reaction catalysts is key to implementing polymer electrolyte membrane water electrolysers for hydrogen production. Iridium-based oxides are the state-of-the-art acidic oxygen evolution reaction catalysts but still suffer from inadequate activity and stability, and iridium’s scarcity motivates the discovery of catalysts with lower iridium loadings. Here we report a mass-selected iridium–tantalum oxide catalyst prepared by a magnetron-based cluster source with considerably reduced noble-metal loadings beyond a commercial IrO₂ catalyst. A sensitive electrochemistry/mass-spectrometry instrument coupled with isotope labelling was employed to investigate the oxygen production rate under dynamic operating conditions to account for the occurrence of side reactions and quantify the number of surface active sites. Iridium–tantalum oxide nanoparticles …