Anisotropic Strain Enhances Oxygen Reduction Catalysis in Pt-Based High-Entropy Intermetallics

The oxygen reduction reaction (ORR) remains a major bottleneck for the efficiency of proton exchange membrane fuel cells. While alloying Pt with 3d transition metals can reduce Pt loading and improve catalytic activity of ORR catalysts, Pt-based solid solutions suffer from reduced activity and stability at Pt fractions below 75-80 at. %. Here, we investigate the role of atomic ordering in multimetallic Pt catalysts by comparing high-entropy solid solutions (HESOSO) and high-entropy intermetallics (HEI) with identical nominal composition, Pt5MnFeCoNiCu. Gas diffusion electrode (GDE) setup measurements at industrially relevant current densities (>1 A cm-2 geo) show that catalysts containing the intermetallic phase outperform solid solution counterparts in both specific and mass activity despite their larger particle sizes. Density functional theory calculations attribute this improvement to anisotropic strain arising from the tetragonal intermetallic lattice, which generates distinct adsorption environments with near-optimal *OH binding energies. These results identify strain anisotropy in ordered multimetallic intermetallics as a promising catalyst design principle for independently tuning adsorption energetics and Pt loading. On-line inductively coupled plasma mass spectrometry (ICP-MS) measurements reveal reduced 3d-metal dissolution in the HEI sample compared to HESOSO sample and binary PtNi reference. Operando X-ray total scattering and pair distribution function analysis further demonstrate that both crystallographic structure and local atomic arrangement are retained during accelerated stress testing, indicating that nanoparticle structural collapse is not the dominant degradation pathway. Instead, the observed activity losses are primarily associated with catalyst-layer degradation and mass transport limitations. Overall, the results identify atomic ordering and strain anisotropy as key parameters for designing low-Pt ORR catalysts with enhanced activity and structural stability under operating conditions.