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

Abstract

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 O2 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 O2 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.

Spin pinning effect to reconstructed oxyhydroxide layer on ferromagnetic oxides for enhanced water oxidation

“…The reduction of the electronic repulsions, dominant over Coulomb attractions, enhance the stabilization of electrons in the orbitals, also associated with extended spin mobility in dominant FM orderings. In relation with typical concepts in catalysis, QSEI make the stable Co 3− x Fe x O 4 /Co(Fe)O x H y catalytic interfaces to be more noble like, optimizing the spin-polarized kinetics 12 .…”

“…On the other hand, on the basis of current knowledge of OER, the OER kinetics is spin dependent 10 – 12 . In OER, the reactants including OH − and H 2 O are singlet while the product oxygen has a triplet ground state with parallel spin alignment (↑0 = 0↑).…”

“…Note that for spin-dependent catalysis, spin selection, spin-dependent electron mobility, and spin potentials in activation barriers could be optimized as quantum spin exchange interactions (QSEI) introduce a significant reduction of the electronic repulsions in the active d-orbitals of catalysts 15 . The maximum kinetic rates occur at catalytic interfaces with dominant ferromagnetic (FM) electronic delocalization, , where is the activation enthalpy in a general spin-selective step and can reduce the barriers because of QSEI 10 , 12 .…”

Spin-polarized oxygen evolution reaction under magnetic field

“… 20 , 21 It is reasonable to consider that the active sites with suitable thermodynamic paths for OER should allow a way to facilitate the spin alignment in the product. As suggested by recent theoretical studies by J. Gracia, 22 – 24 the spin-polarized electrons in catalysts promote the generation of parallel spin aligned oxygen by quantum spin-exchange interactions (QSEI), which further promote the OER kinetics. Therefore, facilitating the spin polarization should be an effective strategy for improving OER efficiency.…”

Spin-Polarized Oxygen Evolution Reaction Under Magnetic Field

“…20,21 It is reasonable to consider that the active sites with suitable thermodynamic paths for OER should allow a way to facilitate the spin alignment in the product. As suggested by recent theoretical studies by J. Gracia, [22][23][24] the spin-polarized electrons in catalysts promote the generation of parallel spin aligned oxygen by quantum spin-exchange interactions (QSEI), which further promote the OER kinetics.…”