Principles determining the activity of magnetic oxides for electron transfer reactions

Abstract

Electrons in covalent oxides are fermions interacting through overlapping atomic orbitals, and quantum exchange interactions incorporate influential spin-dependent potentials in their electrocatalytic properties. The Goodenough-Kanamori rules explain the magnetic coupling between metals connected via ligands, known as super- (or double-) exchange interactions, which regulate their charge transport properties. To describe the electrocatalytic activity of magnetic metal oxides, we must extend their spin-dependent mechanisms of electron tunnelling to catalytic interfaces, because the exchange coupling between orbitals, in the catalysts and with the chemisorbed reactants, influences the kinetics of electron transfer reactions. The principles for developing magnetic coupling rules in electrocatalysis must guarantee spin passages, which are optimum for intrinsically degenerate configurations of the frontier orbitals oriented in the direction of the bonds at both sides of the Fermi level. A continuous energy landscape between the reactants and the catalyst minimizes the overpotentials during coherent redox electron tunnelling. Consequently, in this paper we derive the guidelines of the ferromagnetic (FM) exchange interactions, an extension of the Goodenough-Kanamori rules, to electrocatalytic interfaces, which anticipates minimum Gibbs energy of activation. We focus on the electronic coordinates, targeting reaction conditions where the electrons are the main energy carriers to trigger the steps; nonetheless they are inter-related with the atomic movements. We will use the oxygen evolution and reduction reactions as examples where quantum exchange interactions, a landmark of solid-state magnetism, and the chemistry of the triplet state O2 molecule, are crucial for optimum kinetics.One sentence summary: Delocalizing spin potentials facilitate the coherent propagation of electrons at covalent magnetic interfaces; this is a physical principle that links ferromagnetic exchange interactions, antibonding orbitals and optimum viable electrocatalysis: spintro-catalysis.