  This work reports the synthesis, the physicochemical characterization and the
electrochemical studies of new electrocatalysts (ECs) for the oxygen reduction
reaction (ORR) that: (i) are based on a hierarchical graphene-based support;
and (ii) do not comprise platinum. The active sites of the ECs consist of Fe
and Sn species stabilized in 'coordination nests' of a carbon nitride (CN)
matrix. The latter exhibits a rough, microporous morphology and acts as a
'shell' covering a graphene 'core'. This paper: (i) discusses the role played
by Fe as the 'active metal' in this family of ECs; and (ii) examines in detail
how the physicochemical properties and, correspondingly, the electrochemical
performance are affected by a suitable activation procedure A meant to boost
the ORR kinetics. The results lead to an improved fundamental understanding on
the features of the active sites, including the impact of both A and the pH of
the environment in their performance and ORR mechanism. These insights clarify
the most desirable features to be included in high-performing ECs belonging to
this family, paving the way to the synthesis of next-generation, efficient ECs
for the ORR that do not comprise platinum.
