CaMn₇O₁₂ Quadruple Perovskite Oxides Proceed by Two‐Active‐Site Reaction Mechanism for the Oxygen Evolution Reaction

Abstract

The oxygen evolution reaction (OER) is one of the essential energy conversion reactions for hydrogen production. In quadruple perovskite oxides AA’3B4O12 (A=Ca, Sr, A’=Cu, Mn and B=3d metals), the new reaction mechanism of O−O bond formation between adsorbed oxygen species at adjacent A’‐B sites has been proposed in recent studies. This idea of multiple transition metals working together to form an active site, rather than a single active site, is appealing, but has not been systematically investigated so far. This study examined catalytic OER performances of CaMn7O12 quadruple perovskite oxides with precisely controlled Mn(A′)−Mn(B) distance by doping with Sr, Cu and Al. The crystal structure and the state of cations and O were investigated through XRD with Rietveld refinement and X‐ray absorption spectroscopy. The OER catalytic activity is clearly correlated with the distance of Mn(A’)−Mn(B) while both eg state of Mn and O 2p band center unchanged. The shrinkage of the A’‐B distance affects the stability of O−O bond on A’‐B site and accelerates OER kinetics.

Effects of A‐site Cations in Quadruple Perovskite Ruthenates on Oxygen Evolution Catalysis in Acidic Aqueous Solutions

“…[ 33 ] The participation of cations at A′ site (Cu in this work) in OER catalysis is also expected from the short A′–O–B bond angle, making the OER processing preferential through the dual sites, which was also observed in other quadruple perovskite systems including CaMn 7 O 12 and CaCu 3 Ir 4 O 12 . [ 34–36 ] CaCu 3 Ru 4 O 12 was found near the top of the volcano, indicating that it had the most appropriate Cu–O bond strength. OER stabilities, that is, the current density ratio of the 100th cycle to the 1st cycle, for all samples are plotted versus the Ru–O bond length in Figure 6b (values of Ru–O bond length obtained from Rietveld refinement of the SXRD patterns were used here).…”

“…[33] The participation of cations at A′ site (Cu in this work) in OER catalysis is also expected from the short A′-O-B bond angle, making the OER processing preferential through the dual sites, which was also observed in other quadruple perovskite systems including CaMn 7 O 12 and CaCu 3 Ir 4 O 12 . [34][35][36] the Ru-O bond length in Figure 6b (values of Ru-O bond length obtained from Rietveld refinement of the SXRD patterns were used here). As the Ru-O bond length decreased, the OER stability of these quadruple perovskites increased drastically.…”

Recent Advances on Perovskite Electrocatalysts for Water Oxidation in Alkaline Medium

“…In recent years, crystal phase engineering has made great achievements in the field of the OER. ,, At the same time, the unusual phase of materials with different atomic arrangements from the thermodynamically stable phase has attracted great attention. The change of the crystal phase corresponds to the change of the arrangement of atoms in the crystal structure and exhibits different conductivity and activity units, as well as determines its electronic structure and coordination structure.…”

“…With the development of the perovskite-based OER catalytic materials, researchers have found that the OER catalytic performance of perovskite mainly relies on the catalyst surface’s adsorption strength of oxygen-containing intermediates through the adsorbate evolution mechanism (AEM). − In addition, many researchers also have illustrated that the catalytic activity of perovskite is determined by its e g orbital filling degree, transition metal 3d band center, and O 2p band center. − Based on the above theoretical guidance, several strategies of activity improvement have been successfully developed to design ideal perovskite-based OER catalysts, such as doping, morphology engineering, introducing vacancy, crystal phase engineering, surface reconstruction, and constructing hybrid-structure. − …”

Brownmillerite-type Ca2Fe0.75Co1.25O5 as a robust electrocatalyst for the oxygen evolution reaction under neutral conditions

“…However, leaching of A-site atoms and surface amorphization during the OER in a neutral solution resulted in low durability. 18 Meanwhile, PV-derived composite oxides such as Ruddlesden-Popper, 19,20 double perovskite, [21][22][23] quadruple-perovskite [24][25][26] and brownmillerite (hereaer BM)-type [27][28][29][30][31][32] oxides are also known to be active OER catalysts under alkaline conditions. We found that the OER activity of BM-type Ca 2 FeCoO 5 , which has an oxygen-deciency-ordered PV-type structure containing a layered arrangement of tetrahedral and octahedral sites, was higher than those of PV-type Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3Àd and RuO 2 .…”