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Catalysts based on Fe-N_x sites have promising catalytic activity for the oxygen reduction reaction (ORR). While homogeneous Fe-N₄ macrocycle molecules and heterogeneous Fe-N-doped carbon materials have been studied extensively, systematic strategies to tune the ORR energetics and activities of Fe-N_x-based catalytic sites remain elusive. Herein, we show that carbon-supported Fe-phthalocyanine-based catalysts (FePPc/C) can be functionalized by electron-withdrawing/donating substituents to tune the electronic structures of the Fe center and the ORR catalytic activity. FePPc/C was synthesized via the polymerization of FeSO₄, pyromellitic diimide, and urea on acid-treated acetylene black. By partially replacing the bridging pyromellitic diimide with terminal phthalimide bearing different functional groups (-R), functionalized Fe-phthalocyanine-based catalysts (FePPc-R/C) were obtained with -R anchoring …