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

During the synthesis of earth-abundant oxygen reduction reaction (ORR) electrocatalysts formed by metal, nitrogen and carbon via pyrolysis, a variety of side products arise in addition to the intended structure of nitrogen-coordinated metal inserted on a carbon matrix (M-N-C). In this study, some of these side products, such as nitrogen-doped carbon and carbon-encapsulated metal nanoparticles were selectively synthesized, and their electrocatalytic activities for the ORR were measured in acid and alkaline electrolytes. The ORR polarization curves showed that carbon-encapsulated iron-cobalt nanoparticles do not present an increased activity in relation to pure carbon. When the surfaces of these encapsulated nanoparticles were doped with nitrogen using thermal treatment in N2 or in NH3, the resulting ORR curves presented a slight shift of the onset potentials to higher values in both electrolytes. Remarkably, when a nitrogen-rich molecule (imidazole) was used as the nitrogen precursor, the highest ORR activity was achieved in both media. Only for those materials thermally treated in N2 or in NH3 atmosphere or synthesized in the presence of imidazole, the EXAFS characterization revealed the occurrence of iron-nitrogen and cobalt-nitrogen bonds, this being an indicative of the formation of M-N-C species. The presence of imidazole may lead to Fe and Co complexation during the first step of the synthesis, permitting better dispersion of the metallic atoms on the carbon powder, favoring the formation of M-N-C to a higher extent. Therefore, the results disclosed herein pointed out that carbon-encapsulated metal nanoparticles or even nitrogen-doped carbon-encapsulated metal nanoparticle cannot be responsible for the high half-wave potentials encountered in polarization curves for the ORR in M-N-C materials synthesized via pyrolysis. Only M-N-C species, even being present at low amounts, can result in high ORR activity whether in acid or alkaline media.