This report presents the interplay between the synthesis parameters, physicochemical properties, and electrochemical performance of core-shell low-Pt electrocatalysts (ECs) for the oxygen reduction reaction (ORR) based on PtxNi active sites stabilized on a carbon nitride shell. The impact of the pyrolysis temperature (Tf), of the support core (H) composition and of an electrochemical dealloying-activation step on the EC morphology and on the accessibility and stability of the active sites are studied in detail. Three supports are employed based on carbon nanoparticles and/or graphene platelets. The ORR performance of activated ECs measured by cyclic voltammetry with the thin-film rotating ring-disk electrode approach is strongly affected by Tf and H. The best performing ECs are tested in single proton exchange membrane fuel cells under operating conditions. The simultaneous presence of graphene and carbon in H improves the dispersion of active sites, resulting in a vastly improved mass activity and durability in comparison with a benchmark state-of-the-art Pt/C EC. Display omitted •Hierarchical core-shell Pt–Ni ORR electrocatalysts (ECs) are obtained.•Interplay between synthesis, properties, and performance of ECs is discussed.•Hierarchical Pt–Ni ECs exhibit mass activities surpassing DOE targets.•Hierarchical Pt–Ni ECs are more durable than Pt/C reference ECs.