Oxygen Evolution at Manganite Perovskite Ruddlesden-Popper Type Particles: Trends of Activity on Structure, Valence and Covalence

Abstract

An improved understanding of the correlation between the electronic properties of Mn-O bonds, activity and stability of electro-catalysts for the oxygen evolution reaction (OER) is of great importance for an improved catalyst design. Here, an in-depth study of the relation between lattice structure, electronic properties and catalyst performance of the perovskite Ca1−xPrxMnO3 and the first-order RP-system Ca2−xPrxMnO4 at doping levels of x = 0, 0.25 and 0.5 is presented. Lattice structure is determined by X-ray powder diffraction and Rietveld refinement. X-ray absorption spectroscopy of Mn-L and O-K edges gives access to Mn valence and covalency of the Mn-O bond. Oxygen evolution activity and stability is measured by rotating ring disc electrode studies. We demonstrate that the highest activity and stability coincidences for systems with a Mn-valence state of +3.7, though also requiring that the covalency of the Mn-O bond has a relative minimum. This observation points to an oxygen evolution mechanism with high redox activity of Mn. Covalency should be large enough for facile electron transfer from adsorbed oxygen species to the MnO6 network; however, it should not be hampered by oxidation of the lattice oxygen, which might cause a crossover to material degradation. Since valence and covalency changes are not entirely independent, the introduction of the energy position of the eg↑ pre-edge peak in the O-K spectra as a new descriptor for oxygen evolution is suggested, leading to a volcano-like representation of the OER activity.

Mechanistic Parameters of Electrocatalytic Water Oxidation on LiMn2O4 in Comparison to Natural Photosynthesis

“…For calculation of the Mn valence from IPFY spectra, the centroid of the Mn L 3 edges was determined using the tool "peak analyzer" in . Again, the calculated valence did not differf rom that obtained by the previous calibration curve [101] for the TEY spectra.…”

Perovskite Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

“…For the back reaction of oxygen evolution, greater hybridization also led to higher activity in the LaBO3 series at single eg occupancy [51,201]. However, Abrishami et al [202] report in this Special Issue that lower hybridization at an eg occupancy of 0.3 corresponds to higher activity and stability for first-order Ruddelsden-Popper phases that are related to the perovskite structure (see Section 2). This suggests that the details of the correlation No single descriptor can predict the optimal material for a given reaction, which was shown statistically for the reverse reaction of oxygen evolution [197].…”

Structure–Activity Relationship in Manganese Perovskite Oxide Nanocrystals from Molten Salts for Efficient Oxygen Reduction Reaction Electrocatalysis

“…Ruddlesden-Popper (RP) manganites A n+1 Mn n O 3n+1 (or (AO)(AMnO 3 ) n n = 1, 2…) that withstand numerous cationic substitutions, mixed Mn valences and lattice distortions. 3,[6][7][8][9][10][11] RP manganites contain layers of n AMnO 3 perovskite units alternating with AO rock-salt layers, which provide additional anionic conductivity. 9,12,13 This feature should increase electrocatalytic activity.…”

“…3,[6][7][8][9][10][11] RP manganites contain layers of n AMnO 3 perovskite units alternating with AO rock-salt layers, which provide additional anionic conductivity. 9,12,13 This feature should increase electrocatalytic activity. 14 Strategies explored to enhance the ORR electrocatalytic activity of manganese-based oxides focus on increasing the surface area of the materials in order to provide more active sites per mass unit compared to the large crystals typically obtained with solid-state synthesis.…”