  The synthesis of new 'Pt-free' electrocatalysts (ECs) for the oxygen
reduction reaction (ORR) is reported. The ECs are characterized by a
hierarchical 'core-shell' morphology; the 'core' is made of graphene, that is
covered by a cratered, microporous carbon nitride (CN) 'shell'. The latter
supports nanoparticles of M1 and Sn metals (M1 = Co; Ni) in 'coordination
nests'. These latter are holes in the CN matrix, whose walls consist of N- and
C-ligands. Two groups of ECs are studied: (i) 'pristine' ECs; and (ii)
'activated' ECs, that are obtained from the 'pristine' ECs by means of a
suitable activation process (A) aimed at improving the performance in the ORR.
Here is clarified the interplay existent between: (i) the chemical composition,
morphology, structure and A; and (ii) the ORR performance and mechanism as a
function of the pH of the environment. The resulting insights improve the
fundamental understanding of this family of ECs and open the door to the
devising of new preparations of 'Pt-free' ECs for the ORR, which: (i) are
stabilized by a CN matrix and; (ii) exhibit an improved performance.
