Nature Catalysis, Published online: 28 August 2026; doi:10.1038/s41929-026-01607-7 Oxygen reduction catalysts are critical for advancing the durability and efficiency of proton-exchange membrane fuel cells (PEMFCs). Now a block-copolymer micelle confinement strategy is shown to enable the decoupled synthesis of highly ordered, ultra-dense Pt-based intermetallic nanocrystals, yielding PEMFC catalysts with high activity and long projected lifetimes.