polyhedra are associated, have been reported. [2f,g,5] Such vacancydriven catalysis has also been investigated in other OER/ORR catalysts, such as MnCo 2 O 4 spinel and La 1−x Sr x CoO 3−δ perovskite, [6] to propose an efficient way to improve catalytic activity.The quadruple perovskite CaCu 3 Fe 4 O 12 , in which three quarters of A-site (=A′-sites) are occupied by Cu ions, [7] allows for more active and stable catalysis for the OER than the simple perovskite CaFeO 3 (see crystal structures of simple ABO 3 and AA′ 3 B 4 O 12 perovskites in Figure 1a,b, drawn by using the VESTA-3 program [8] ). [9] The authors proposed that several features of CaCu 3 Fe 4 O 12 are probably associated with its activity and stability. These include a widespread covalent network, heavily bent FeOFe bonds to shorten distances between the neighboring adsorbates, the contribution of the A′-site Cu ions, and a possible OER mechanism on two active sites. However, in-depth studies are needed to unveil the structureactivity relationship in this system. In quadruple perovskites AMn 7 O 12 (A = Ca, La), both A′-and B-sites are solely occupied by Mn atoms. This allows investigations on pure structural features concerning catalysis (including comparison with their corresponding simple perovskite AMnO 3 ), leading to the discovery of novel structure-activity relationships. In this paper, we describe the OER/ORR catalytic activities for simple and quadruple manganese perovskites. The quadruple perovskites AMn 7 O 12 (A = Ca, La) display bifunctional catalysis for OER and ORR. On the other hand, the simple perovskites AMnO 3 (A = Ca, La) only display catalysis for the ORR. The enhancement of OER activity for AMn 7 O 12 is probably driven by the structural features of the quadruple perovskite. This finding suggests that AMn 7 O 12 perovskites are promising candidates as bifunctional catalysts.Manganese perovskite catalysts, AMnO 3 and AMn 7 O 12 (A = Ca, La), were synthesized from solid-state reactions using precursors prepared by polymerized method [10] (Supporting Information). CaMnO 3 , LaMnO 3 , and CaMn 7 O 12 were synthesized under ambient pressure, whereas LaMn 7 O 12 could be obtained by high-pressure synthesis method. All synthesized samples were almost single-phase ( Figure S1, Supporting Information). Their crystal structures, determined by the Rietveld refinement using the [11] were identical with those reported previously (Table S1, Supporting Information). [12] Based on the Rietveld refinement results, we confirmed that all the manganese perovskite samples did not contain any substantial amount of oxygen vacancies. Thus, their valence states are Ca 2+ Mn 4+ O 3 , La 3+ Mn 3+ O 3 , Ca 2+ Mn 3+ 3 (Mn 3+ 3 Mn 4+ 1 )O 12 , and La 3+ Mn 3+ 3 Mn 3+ 4 O 12 , [12b] where the Mn ions at A′-sites (squareplanar coordination) are trivalent due to the strong Jahn-Teller property of Mn 3+ (d 4 ) ions. The scanning electron microscopy

Ever proposed descriptors of catalytic activity for oxygen evolution reaction (OER) were systematically investigated. A wide variety of stoichiometric perovskite oxides ABO 3 (A = Ca, Sr, Y, La; B = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) were examined as OER catalysts. The simplest descriptor, e g electron number of transition metal ion at B-site, was not applicable for OER overpotentials (η) of the compounds tested in this study. Another descriptor, oxygen 2p band center relative to Fermi energy (ε 2p), was not necessarily adequate for the most part of perovskite oxides. Eventually, a recently proposed descriptor, charge-transfer energy (Δ), displayed a linear relationship with η the most reasonably. Since Δ values were obtained from theoretical calculations, not only by spectroscopic experiments, systematic exploration for a wide range of compounds including hypothetical ones could be allowed. This finding proposes the charge-transfer energy as the most helpful descriptor for design of perovskite oxide catalyst for OER.

The purpose of this work is to compare the two different procedures to calculate the L(2,3) x-ray absorption spectra of transition-metal compounds: (1) the semi-empirical charge transfer multiplet (CTM) approach and (2) the ab initio configuration-interaction (CI) method based on molecular orbitals. We mainly focused on the difference in the treatment of ligand field effects and the charge transfer effects in the two methods. The reduction of multiplet interactions due to the solid state effects has been found by the ab initio CI approach. We have also found that the mixing between the original and the charge transferred configurations obtained by the ab initio CI approach is smaller than that obtained by the CTM approach, since charge transfer through the covalent bonding between metal and ligand atoms has been included by taking the molecular orbitals as the basis functions.

The electronic structures of NiO, LiNiO2, and NiO2 are studied by the electron energy loss spectroscopy at Ni L(2,3), Ni M(2,3), and O K edges. The Ni L(2,3) edge spectra suggest that the formal charge of nickel is +2 in NiO, +3 with a low-spin state in LiNiO2, and +4 with a low-spin state in NiO2. This is well confirmed by first-principles calculations. The Ni M(2,3) edge spectra show similar chemical shifts to those of the Ni L(2,3) edge. Superposition of the Li K edge spectrum, however, hinders further analysis. Although the formal charge of oxygen is -2 in all the three phases, the O K edge spectra indicate a more remarkable difference in the electronic structure of the oxygen in NiO2 than that in either NiO or LiNiO2. The spectra suggest that lithium extraction from LiNiO2 reinforces the covalent bonding between the oxygen and nickel atoms and causes a notable reduction in electron density at the oxygen atoms.