Structural Effects of FeN4 Active Sites Surrounded by Fourteen-Membered Ring Ligands on Oxygen Reduction Reaction Activity and Durability

FeN4 active sites have been studied as non-Pt group metal (non-PGM) catalysts for the oxygen reduction reaction (ORR). The authors recently developed Fe(II) 1,14:7,8-ditethenotetrapyrido-[2,1,6-de:2′,1′6′-gh:2″,1″,6″-na][1,3,5,8,10,12] hexaazacyclotetradecine (Fe-14MR) as an FeN4 complex incorporating a 14-membered ring. This complex exhibited higher ORR activity and greater durability than Fe phthalocyanine, a conventional FeN4 complex having a 16-membered ring. In the present study, the ORR activity and durability of this Fe-14MR complex supported on C (Fe-14MR/C) were enhanced through modification of the active site structure by heating at 600 °C and removing protons on amine groups in the Fe-14MR. Density functional theory calculations indicated that the Fe-14MR/C structure generated by heating resulted in oxygen species absorption energies close to optimal values for the ORR. The improved durability of the heat-treated Fe-14MR/C was attributed to reduced distortion of the FeN4 sites. The results suggest that designing Fe-14MR structures can be a promising approach to developing non-PGM catalysts.