Rational Grain Boundary Segregation Enables Long‐Term Thermally Stable, Catalytically Active, and CO‐Tolerant Nanograined Metals

Abstract Grain boundaries (GBs) impart nanocrystalline metals with unique functional properties but are often plagued by poor thermal stability. While solute segregation is commonly used to stabilize GBs, stability alone is insufficient—effective strategies must also preserve, or ideally enhance, functional activity, a critical aspect largely overlooked. Here, it is shown that GB segregation is not universally beneficial: the same segregant can either stabilize or destabilize GBs depending on its local concentration and spatial distribution. Using boron‐segregated platinum as an example, a concentration‐dependent duality is uncovered: at sub‐equilibrium levels, boron strengthens GB cohesion and preserves structure during prolonged annealing; when oversaturated, boron clusters near GBs, generating repulsive interactions that trigger decohesion. In extreme cases, segregation can render GBs less stable than those without segregants. To address this, a three‐step framework is established for rational GB design: 1) theoretical selection of segregants with strong GB affinity and minimal impact on catalytic energetics, 2) thermodynamic determination of segregation limits, and 3) kinetic trapping during nanoparticle attachment for targeted GB incorporation. Guided by this approach, thermally stable, GB‐rich platinum nanograins that retain high catalytic activity and exhibit enhanced carbon monoxide tolerance are fabricated. This work provides a blueprint for engineering robust, high‐performance nanocrystalline materials.