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.

Transition metal oxides have been extensively investigated as novel catalysts for oxygen evolution reaction (OER). Partial elemental substitutions are effective ways to increase catalytic performance and such electronic interactions between multiple elements are known as synergistic effects. However, serious issues such as random atomic arrangement and ambiguous roles of constituent elements humper theoretical investigations for rational materials design. Herein, we describe systematic study on OER activity of AA′ 3 B 4 O 12 -type quadruple perovskite oxides, in which multiple transition metal ions are located at distinct crystallographic sites. Electrochemical measurements demonstrate that OER catalytic activities of quadruple perovskite oxide series, CaCu 3 B 4 O 12 (B = Ti, V, Cr, Mn, Fe, and Co), are all superior to those of simple perovskite counterparts CaBO 3 . The order of activity of B-site transition metal ions for CaBO 3 (Fe 4+ > Co 4+ ≫ Ti 4+ , V 4+ , Cr 4+ , Mn 4+ ) is retained in CaCu 3 B 4 O 12 , indicating that B-site ions play a primary role whereas A′-site Cu ions secondarily contribute to OER activity for CaCu 3 B 4 O 12 . Charge-transfer energies, energy differences between oxygen 2p band center and unoccupied 3d band center of B-site transition metal obtained from first-principles electronic-state calculations, illustrate that OER overpotentials of quadruple perovskite oxides are lower than simple perovskite oxides by ∼150 mV. These findings propose a simple avenue to realize enhanced OER activity for multiple transition-metal ions.

Perovskite-type oxides composed of earth-abundant elements have been extensively studied as possible candidates for oxygen evolution reaction (OER) catalysts. In our recent study, quadruple perovskite oxides (e.g., Ca-Cu 3 Fe 4 O 12 and LaMn 7 O 12 ) displayed catalytic activity that was higher than that of simple perovskites (e.g., LaMnO 3 ), but the reason has not yet been unveiled. We have conducted firstprinciples calculations of the several surface energies of LaMn 7 O 12 and adsorption energies of OER intermediates on LaMn 7 O 12 using slab models to clarify the reaction mechanism. The Mn-rich surfaces, i.e., the (001) with BO 2 termination and (220) surfaces, are found to be more stable for LaMn 7 O 12 . It is found that all intermediates are preferentially adsorbed on the A′−B-bridge site on the LaMn 7 O 12 (220) surface, although only the B-top site was a stable adsorption site on the (001) surface of LaMn 7 O 12 and LaMnO 3 . The difference between theoretical overpotentials on the (220) surface of LaMn 7 O 12 and the (001) surface with BO 2 termination of LaMnO 3 is in good agreement with the experimental overpotential for OER. We propose a new design principle in which OER is enhanced via adsorption on the A′−B-bridge site consisting of two adjacent Mn sites {coordination-unsaturated pyramid [coordination number (CN) = 5] and coordination-saturated pseudosquare (CN = 4)}, where the adsorbates are strongly bound to the former and weakly bound to the latter.

Estimation of structure stability is an essential issue in materials design and synthesis. Global instability index (GII) based on bond-valence method is applied as a simple indication, while density functional theory calculation is adopted for accurate evaluation of formation energy. We compare the GII and total energy of typical ABO3-type perovskite oxides and rationalize their relationship, proposing that the criteria for empirically unstable structures (GII > 0.2 valence unit) correspond to the difference in total energy of 50–200 meV per formula unit.