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_x/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_x/C and/or N_xC surface functionality.