Itinerant Spins and Bond Lengths in Oxide Electrocatalysts for Oxygen Evolution and Reduction Reactions

Thorough analyses of structural factors in catalysis are interesting because they allow the massive prescreening of potential optimum compositions. Overall, this article shows how the orbital physics of magnetic compositions relates with spin lattice interactions and then band gaps and bond lengths together become relevant descriptors in catalytic oxygen technologies. Active electrocatalysts for the oxygen evolution reaction (OER) include magnetic oxides with metals at relatively high oxidation states, so chemisorbed molecular O-2 is not very stable. On the other hand, ideal compositions for the oxygen reduction reaction (ORR) have metals in a comparatively lower oxidation state, which can supply electrons to activate O-2 molecules toward electron-richer oxygen atoms. Spin-lattice interactions in these strongly correlated oxides relate the orbital configurations and oxidation state with distinctive metal-oxygen bond distances, indicating localized or itinerant electronic behavior and selectivity in oxygen electrochemistry. OER at low overpotentials coincides with anti-Jahn-Teller contractions in ferromagnetic (FM) metal-oxygen (M-O) bonds; however, active oxides for ORR have longer FM M-O bonds, electron-richer. In both cases of OER and ORR, dominant FM couplings moderate the binding energies of the reactants because of the stabilizing quantum spin exchange interactions associated with the open-shell orbital configurations, and correspondingly, their catalytic efficiencies improve in accordance with Sabatier's principle. The presence of FM holes in the M-O bonds also enhances spin-selective charge transport, the other crucial enthalpic contribution in electrocatalysis. These specific effects of spin-dependent potentials in heterogeneous catalysis define the explicit field of spintro-catalysis, needed to allow the inclusion of strongly correlated electrons in theoretical models, and as we show here also with the advantage of the recognizing structural descriptors ligated to spin lattice interactions.