Identification of the Intrinsic Ordering Effect of PtCo Intermetallic Catalysts on Fuel Cell Durability

The dissolution of 3d transition metals is a central bottleneck limiting the practical use of Pt‐alloy cathode catalysts in fuel cells. Although ordered intermetallic compounds (IMCs) have been explored for nearly 4 decades as a route to mitigate this instability, the durability benefit of structural ordering itself remains debated and has rarely been reliably validated under practical membrane electrode assembly (MEA) conditions. Herein, we design a phase‐diagram‐guided sequential annealing protocol that yields an ordered/disordered PtCo catalyst pair with closely matched particle size and composition, enabling the durability effect of structural ordering to be rigorously isolated. In practical MEAs, structural ordering does not translate into a significant H 2 ‐air performance gain but markedly slows catalyst degradation. For example, after 90 000 cycles of accelerated durability test, the ordered catalyst loses only 28 mV at 1.5 A cm −2 , less than half that of its disordered counterpart (65 mV). Correlative in situ MEA diagnostics, electron microscopy, and online inductively coupled plasma mass spectrometry trace this enhanced durability to suppressed Co outward diffusion and dissolution. Machine‐learning‐accelerated theoretical calculations further rationalize this stabilization by revealing a higher thermodynamic penalty for Co surface segregation and a larger kinetic barrier for Co diffusion toward neighboring vacancies in the ordered IMC.