Tuning the Catalytic Activity of Fe-Phthalocyanine-Based Catalysts for the Oxygen Reduction Reaction by Ligand Functionalization

Catalysts based on Fe-Nx 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-Nx-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 on the edge of the polymer. Notably, the Fe2+/3+ redox potential of Fe sites from FePPc-R/C was shifted by 0.35 V via different functional groups, where increasing redox potential was correlated with greater Hammett constants (i.e., stronger electron-withdrawing) of functional groups. Moreover, the specific and mass ORR activity of FePPc-R/C in 0.1 M HClO4 could be increased by up to 20 times with increasing electron-withdrawing functional groups, where a linear relationship was observed between the Fe2+/3+ redox potential and ORR activity, with dicarboxylate-functionalized FePPc-(COOH)2/C showing the highest activity. This versatile method can be used to further design M-N4-based catalysts for ORR and beyond.