title: The role of the central Fe atom in the N4-macrocyclic structure for the enhancement of oxygen reduction reaction in a heteroatom nitrogen-carbon nanosphere.
abstract: In the present work, we demonstrated the significance of a central transition metal, Fe, in a N4-macrocycle for the enhancement of ORR activity and other electrochemical properties. The catalysts were synthesized by a solution plasma process. Fe-phthalocyanine/benzene and phthalocyanine/benzene were chosen as the precursors of Fe-phthalocyanine based mesoporous carbon (FP-MCS) and phthalocyanine based mesoporous carbon (P-MCS) catalysts, respectively. The existence of Fe-N4 and N4 macrocyclic structures was confirmed by X-ray photoelectron spectroscopy. From the chemical bonding structure, FP-MCS demonstrated that the Me-N peaks increased as the amount of iron-phthalocyanine introduced in the experiment increased. The dominant active site was shifted from pyridinic nitrogen to Me-N when iron-phthalocyanine was present. The analysis of Tof-SIMS indicated that the relative intensity of FeN4Cy(+) ions was approximately 50% of the total amount of ionized species of ∑FeNxCy(+). Both XPS and Tof-SIMS results confirmed that the Fe-N4 site was the most favourable structure in the matrix. From CV measurements, the cathodic peak current corresponding to ORR activity slightly shifted from -0.19 V to -0.17 V when the active site changed from N4 to Fe-N4 macrocyclic structure. The current density increased more than 30% in the presence of iron. Based on the calculation of Koutecky-Levich plots, the electron transfer numbers for ORR reaction in P- and FP-MCSs were 3.25 and 3.98, respectively. These results clearly demonstrated that the presence of a Fe central ion in the N4-macrocyclic structure significantly enhanced the ORR and charge transfer number in ORR activities.
authors.affiliation: Department of Materials, Physics and Energy Engineering, Nagoya University, 464-8603, Furo-cho, Chikusa-ku, Nagoya, Japan. dw@rd.numse.nagoya-u.ac.jp.
authors.forename: Dae-wook
authors.initials: DW
authors.lastname: Kim
authors.forename: Oi Lun
authors.initials: OL
authors.lastname: Li
authors.forename: Nagahiro
authors.initials: N
authors.lastname: Saito