Structural Effects of FeN<sub>4</sub> Active Sites Surrounded by Fourteen-Membered Ring Ligands on Oxygen Reduction Reaction Activity and Durability
Zhiqing Feng, Soutaro Honda, Junya Ohyama, Yasushi Iwata, Keisuke Awaya, Hiroshi Yoshida, Masato Machida, Kotaro Higashi, Tomoya Uruga, Naomi Kawamura, Ryota Goto, Takeo Ichihara, Ryoichi Kojima, Makoto Moriya, Hideo Notsu, Shinsuke Nagata, Mami Miyoshi, Teruaki Hayakawa, Yuta Nabae
Abstract
High Resolution Image Download MS PowerPoint Slide FeN 4 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 FeN 4 complex incorporating a 14-membered ring. This complex exhibited higher ORR activity and greater durability than Fe phthalocyanine, a conventional FeN 4 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 FeN 4 sites. The results suggest that designing Fe-14MR structures can be a promising approach to developing non-PGM catalysts.