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 systematically modulate the ordering degree of PtNi nanowires from 4.5% to 65.8% to dynamically track this structural evolution. A key finding that refines the conventional understanding of strain evolution is the emergence of characteristic biaxial strain during the disorder‐to‐order transition, which we quantitatively distinguish from the universally assumed isotropic compression model. Specifically, this transition induces in‐plane lattice expansion coupled with out‐of‐plane lattice contraction. As the 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. Density functional theory calculations confirm this anisotropic strain reduces the energy barrier of the rate‐determining *O protonation step from 0.51 to 0.39 eV. The optimized catalyst exhibits a mass activity of 1.30 A mg Pt −1 with 86.0% retention after 60 000 cycles, and a peak power density of 1014.5 mW cm −2 in membrane electrode assemblies. This work identifies tunable ordering‐induced biaxial strain as a critical structural parameter for designing high‐performance Pt‐based ORR catalysts.