Analysis of the Magnetic Entropy in Oxygen Reduction Reactions Catalysed by Manganite Perovskites

Abstract

Manganese oxides with a half‐metallic ground state are particularly active for oxygen reduction reactions (ORR). La0.67Sr0.33MnO3 (LSMO) perovskite is the archetypal example for compositions with a Curie temperature (TC) above room temperature and with a high intrinsic activity for the partial reduction of triplet‐state O2. The ferromagnetic (FM) character of the superexchange interactions in LSMO facilitates both charge and spin transport below 370 K. Other than the enhanced electronic conductivity, the reduced spin entropy seems to be relevant in oxygen catalysis because the magnetic ordering extends to the surface. The sign of the exchange interactions determines the adsorption of the triplet oxygen molecule with its spin antiparallel to the FM catalysts. Based on the transition‐state theory, we report that on LSMO, the hindrance resulting from the magnetic entropy for the initial reduction of O2 by two antiparallel electrons to diamagnetic intermediates (such H2O2) is minimum. On the other hand, the additional reduction of H2O2 to H2O, diamagnetic steps, prefers paramagnetic catalysts with higher magnetic entropy such as La0.4Sr0.6MnO3 to avoid spin accumulation.

Ferromagnetic ligand holes in cobalt perovskite electrocatalysts as an essential factor for high activity towards oxygen evolution

“…Eq. 3 correlates the overall activation energy of FM catalysts with the specific influence of , added via a Heisenberg type ℎ exchange term 14,22 .…”

“…Spin selection in polarized density of states facilitates the oxidation/reduction of triplet state O 2 . 21,22 Nature has evolved excellent magnetic catalysts from abundant 3d-metals, e.g. during photosynthesis 23 , via engineering QSEI.…”

On the Role of Ferromagnetic Interactions in Highly Active Mo‐Based Catalysts for Ammonia Synthesis

“…Transition metal oxides with preferential FM spin delocalization are excellent catalysts for oxygen electrochemistry ,,. The explanation lies on the fact that augmented QEI in spin‐polarized conductors assists the electron transfer and spin selection ,.…”

Spin-polarized oxygen evolution reaction under magnetic field

“… 10 – 12 Some exceptions have been found not well fitted with the e g theory, which is partially resulted by the diverse and complicated magnetism in TMOs family. 13 – 16 Besides, the produced O 2 is in triplet ground state, where the frontier π* orbitals are occupied by two electrons with parallel alignment. In contrast, the ground spin state of reactant OH - /H 2 O is singlet with all paired electrons.…”