FeCo-N-C oxygen reduction electrocatalysts: Activity of the different compounds produced during the synthesis via pyrolysis

•Synthesized electrocatalysts arranged in an order of increasing structural complexity.•Carbon-encapsulated metal nanoparticles presented the same onset potential of carbon.•N-doped carbon-encapsulated iron-cobalt nanoparticles have intermediate ORR activity.•M-N-C species having decisive role on the electrocatalyst activity for the ORR.•Activity of encapsulated nanoparticles dependent on the presence of M-N-C on the shell.