Electrochemical probing into the active sites of graphitic-layer encapsulated iron oxygen reduction reaction electrocatalysts.

The graphitic-layer encapsulated iron-containing nanoparticles (G@Fe) have been proposed as a potential type of active and stable non-precious metal electrocatalysts (NPMCs) for the oxygen reduction reaction (ORR). However, the contribution of the encapsulated components to the ORR activity is still unclear compared with the well-recognized surface coordinated FeN/C structure. Using the strong complexing effect of the iron component with anions, cyanide (CN) in alkaline and thiocyanate (SCN) in acidic media, the metal containing active sites are electrochemically probed. Three representative catalysts are chosen for a comparison including the as-prepared encapsulated G@Fe, commercial Fe/N/C catalyst with iron-nitrogen coordinated surface functionalities and molecular iron phthalocyanine (FePc) containing well-defined structures and compositions. It was found that all samples showed significant shifts of half-wave potentials indicating that surface Fe coordinated sites in all cases. The G@Fe catalyst showed the weakest poisoning effect (the lowest shifts of half-wave potential) compared to the Fe/N/C and FePc catalysts in both electrolytes. These results could be explained that the encapsulated iron components influence the FeN/C and/or NC surface functionality.