Functional group effects on the electrocatalytic activity of iron phthalocyanine for the ORR, OER, and HER in an aqueous medium: insights from density functional theory

Abstract

This study used density functional theory (DFT) to analyze the ORR, OER, and HER activities of iron-doped phthalocyanine (FePc) functionalized with fluorine (–F), amine (–NH₂), and carboxyl (–COOH) groups such as FePc, FePc(F)₄, FePc(F)₈, FePc(NH₂)₄, FePc(NH₂)₈, FePc(COOH)₄, and FePc(COOH)₈ in a solvent medium (water). Structural analyses of the E_HOMO–LUMO gaps, XAS spectra, and free energy profiles showed that FePc(COOH)₈, FePc(NH₂)_4, and FePc(NH₂)₈ were highly favorable for the ORR and HER. FePc and its derivatives exhibit substantial Fe(3d)–N(2p)/C(2p) hybridization near the Fermi level, enhancing the TDOS and providing a more metallic or semi-metallic character than Pc, thereby improving charge transfer for catalytic reactions. The Fe²⁺ centers on all surfaces have half-filled d⁶ configurations, which improves their ORR and HER catalytic activity. Carboxyl functionalized structure FePc(COOH)₈ has the lowest overpotential for the ORR at 1.16 V compared to the other considered surfaces. In contrast, amine functionalized FePc(NH₂)₄ and FePc(NH₂)₈ structures exhibited modest HER overpotentials of −0.01 and −0.08 V, respectively, indicating greater activity towards the HER in the solvent phase. Overall, the FePc(COOH)₈ structure demonstrates excellent bifunctional (ORR/HER) electrocatalytic properties.