Tuning the Catalytic Activity of Fe-Phthalocyanine-Based Catalysts for the Oxygen Reduction Reaction by Ligand Functionalization
Shuai Yuan, Jiayu Peng, Yirui Zhang, Daniel J. Zheng, Sujay Bagi, Tao Wang, Yuriy Román‐Leshkov, Yang Shao‐Horn
Abstract
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.