Nanoconfinement Mitigates the Size-Ordering-Loading Trade-Off to Enable High-Loading Sub-3 nm Intermetallic PtCu for Oxygen Reduction.

Pt-based intermetallic nanoparticles (NPs) are among the most effective electrocatalysts for the oxygen reduction reaction (ORR), yet simultaneously achieving high metal loading, ultrasmall particle size, and atomic ordering remains a longstanding challenge because the high-temperature annealing required for intermetallic ordering inevitably induces particle coarsening. Here, we demonstrate that mesopore-induced nanoconfinement mitigates this size-ordering-loading trade-off, enabling the synthesis of highly dense (∼40 wt.%), sub-3 nm intermetallic PtCu nanoparticles within ordered mesoporous carbon (CMK-3). In contrast, microporous carbon supports with comparable surface areas produce much larger nanoparticles (∼8.3 nm), highlighting the critical role of mesoporous confinement rather than surface area alone. Molecular dynamics simulations reveal that geometric confinement suppresses PtCu atomic mobility while interactions between the mesopore walls and PtCu nanoparticles promote chemical ordering during annealing. The intermetallic PtCu/CMK-3 catalyst exhibits substantially enhanced ORR activity and durability compared with disordered PtCu alloys and carbon black-supported intermetallic PtCu. Furthermore, sulfur-doped CMK-3 strengthens metal-support interactions, leading to improved nanoparticle dispersion and further enhanced ORR performance. These findings establish mesopore-engineered nanoconfinement as a general strategy for decoupling particle size, atomic ordering, and metal loading, providing a versatile platform for designing high-performance intermetallic nanomaterials for electrocatalysis and beyond.