Assembly of Hollow Carbon Nanospheres on Graphene Nanosheets and Creation of Iron-Nitrogen-Doped Porous Carbon for Oxygen Reduction

Triblock copolymer micelles coated with melamine-formaldehyde resin were self-assembled into closely packed two-dimensional (2D) arrangements on the surface of graphene oxide sheets. Carbonizing these structures created a 2D architecture composed of reduced graphene oxide (rGO) sandwiched between two monolayers of sub-40 nm diameter hollow nitrogen-doped carbon nanospheres (N-HCNS). Electrochemical tests showed that these hybrid structures had better performance for oxygen reduction compared to physically mixed rGO and N-HCNS that were not chemically bonded together. Further impregnation of the sandwich structures with iron (Fe) species followed by carbonization yielded Fe_1.6-N-HCNS/rGO-900 with a high specific surface area (968.3 m² g^-1), a high nitrogen doping (6.5 at%), and uniformly distributed Fe dopant (1.6 wt %). X-ray absorption fine structure analyses showed that most of the Fe in the nitrogen-doped carbon framework is composed of single Fe atoms each coordinated to four N atoms. The best Fe_1.6-N-HCNS/rGO-900 catalyst performed better in electrocatalytic oxygen reduction than 20 wt % Pt/C catalyst in alkaline medium, with a more positive half-wave potential of 0.872 V and the same limiting current density. Bottom-up soft-patterning of regular carbon arrays on free-standing 2D surfaces should enable conductive carbon supports that boost the performance of electrocatalytic active sites.