Mechanistic understanding of thermodynamic metastability of core-shell catalysts in the polymer electrolyte fuel cell catalyst layer durability

Core-shell nanoparticles in cathode catalyst layers of Polymer Electrolyte Fuel Cells (PEFCs) are a promising class of electrocatalysts that have received considerable attention owing to their high mass activity compared to their single metal counterparts. The nature of its bimetallic design offers a potential range of tunable parameters to improve its stability mediated by the long-range interactions arising from the strain effect. In this work, we reveal the emergence of thermodynamic metastability in the surface energetics at a critical design limit of large core diameter and shell thickness when the elastic strain energy of the electrocatalyst exceeds the energy to form dislocations. The progression of degradation events induces a transition in the metastability front in the catalyst layer which can be attributed to the heterogeneous nature of particle aging. We performed a screening of key candidate core materials and found that negative misfit scenarios such as Nickel (Ni), Copper (Cu), Cobalt (Co), and Palladium (Pd) impart durability against the primary mode of electrochemical dissolution. The positively mismatched structures however exhibit a disparate trend where the strain ramp at high lattice misfits is suppressed mechanistically by exerting a pseudo-compressive force field. Consequently, the critical onset of the nanocatalysts to reach a limiting diameter is governed by the property mismatch between the core and the shell, in terms of the bulk moduli and molar volume. The contrariety in the metastability-durability characteristics stemming from a variation in the initial thickness of the protective Pt skin is further delineated.