Epitaxial growth of hexagonal Pd on Co₃O₄/NC heterostructures for high-performance ORR electrocatalysis

Multicomponent hybrid systems with engineered interfaces are promising candidates for enhancing electrocatalytic performance. Palladium, with an electronic structure similar to that of platinum but with a significantly lower cost, has emerged as a viable alternative to Pt-based catalysts in which the oxygenated intermediate coverage during the oxygen reduction reaction (ORR) limits the long-term performance of conventional Pt/C catalysts. This work introduces a strategically engineered multicomponent heterointerface comprising interstitially modified palladium (Pd-O_x) nanoparticles with a rare hexagonal crystal structure, spinel Co₃O₄, and nitrogen-doped porous carbon (NC). The formation of hexagonal palladium under ambient conditions using NaBH₄, rather than high-pressure or high-temperature routes, is exceptionally rare and represents a significant advancement in non-conventional phase stabilization. This unusual morphology, stabilized by the lattice strain and surface oxygen adsorption from Co₃O₄, modulates the d-band center of Pd, optimizing the adsorption energies of the oxygenated intermediates and accelerating ORR kinetics. Furthermore, the tight NC filling of the porous Co₃O₄ matrix imparts high conductivity and structural robustness, even after calcination in air, which is itself a novel achievement. The synergistic interaction across these engineered interfaces enhances active site exposure, electronic conductivity, and stability, collectively driving the outstanding electrocatalytic ORR performance (onset potential (E_onset) ≈ 0.99 V_RHE, half-wave potential (E_1/2) ≈ 0.89 V_RHE, mass activity ≈ 840 mA mg_Pd^-1, electrochemical surface area (ECSA) ≈ 13.6 m² g_Pd^-1, and enhanced durability). This heterostructure thus serves as a powerful blueprint for the future catalyst designs, where lattice-strained phases and interfacial effects are deliberately exploited.