DFT studies of the OER and ORR on a FeMo-MIL-88b model system

Abstract

The development of efficient electrocatalysts for oxygen evolution and oxygen reduction requires a detailed understanding of the relationship between the local electronic structure of transition-metal centers and their catalytic activity. In this work, we investigate a multinuclear Fe/Mo molecular system designed as a model for a metal–organic framework (MIL-88b) containing Fe3O cores connected through a (MoO4)2− linker as it can be used for Li-air batteries. Density functional theory calculations are employed to elucidate the oxidation states, magnetic interactions, and catalytic properties of the individual metal centers. The neutral Fe3O units exhibit mixed-valence character, with two Fe(III) and one Fe(II) centers. Broken-symmetry DFT calculations reveal an antiferromagnetically coupled spin arrangement within each Fe3O core, resulting in a frustrated spin system (duu). Upon incorporation of the (MoO4)2− linker, the two Fe centers coordinated to the (MoO4)2− group retain Fe(III) character, whereas the non-bridged Fe center remains Fe(II). The two possible relative orientations of two magnetic Fe3O units are found to be essentially degenerate, indicating weak magnetic coupling between the units mediated by the (MoO4)2− linker. The catalytic properties of the structurally and electronically distinct Fe sites are subsequently investigated for the oxygen evolution and oxygen reduction reactions. Reaction free-energy diagrams are constructed using DFT electronic energies together with zero-point energy, entropic, and solvation contributions. The OER and ORR pathways are evaluated through the formation and transformation of oxygenated intermediates, including *OH, *O, *OOH, and *O₂. Particular emphasis is placed on the influence of the local spin configuration and coordination environment on the stability of the reaction intermediates and the potential-determining steps. Comparison of the different active Fe sites reveals differences in reaction energetics and predicted overpotentials, providing insight into the origin of catalytic activity in the multinuclear system.

Keywords: Density Functional Theory; Metal Organic Framework; Electrocatalysis