Ordering-Driven Biaxial Strain Engineering in PtNi Intermetallic Nanowires for Oxygen Reduction Catalysis

Precise regulation of lattice strain in platinum (Pt)-based intermetallic catalysts is essential for optimizing oxygen reduction reaction (ORR) performance, yet strain evolution during atomic ordering is often simplified as isotropic compression, masking its structural complexity. Herein, we address this critical gap by systematically modulating the ordering degree of PtNi nanowires by hydrogen-mediated annealing, obtaining continuously tunable ordering from 4.5% to 65.8% without structural coarsening. A key finding that breaks with long-standing paradigms is the emergence of characteristic biaxial strain during the disorder-to-order transition, distinct from the universally assumed isotropic compression. Specifically, this transition induces in-plane lattice expansion coupled with out-of-plane lattice contraction; as ordering degree increases, this biaxial strain progressively relaxes the compressive stress on the catalytically active (111) surface, rather than amplifying it as traditional models predict. This anomalous strain evolution modulates the electronic structure of Pt, optimizing the Pt d-band center, resolving the core issue of disordered PtNi alloys with excessive compression, and weakening oxygen intermediate binding. The highly ordered PtNi nanowires deliver a mass activity of 1.30 A mg _Pt ^-1 and maintain nearly unchanged activity after 30,000 cycles for ORR, while achieving a peak power density of 1014.5 mW cm ^-2 in the membrane electrode assembly. This work identifies tunable ordering-induced biaxial strain as a critical structural parameter for designing high-performance Pt-based ORR catalysts.