Akihiko Takamatsu

Bifunctional electrocatalysts for oxygen evolution/reduction reaction (OER/ORR) are desirable for the development of energy conversion technologies. It is discovered that the manganese quadruple perovskites CaMn7O12 and LaMn7O12 show bifunctional catalysis in the OER/ORR. A possible origin of the high OER activity is the unique surface structure through corner-shared planar MnO4 and octahedral MnO6 units to promote direct OO bond formations. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Ever-proposed descriptors of catalytic activity for the oxygen evolution reaction (OER) were systematically investigated. A wide variety of stoichiometric perovskite oxides ABO3 (A = Ca, Sr, Y, La; B = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) were examined as OER catalysts. The simplest descriptor, eg, electron number of transition-metal ions at the 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 (ε2p), 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 most reasonably. Because 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 the design of perovskite oxide catalysts for the 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′3B4O12-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, CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), are all superior to those of simple perovskite counterparts CaBO3. The order of activity of B-site transition metal ions for CaBO3 (Fe4+ > Co4+ ≫ Ti4+, V4+, Cr4+, Mn4+) is retained in CaCu3B4O12, indicating that B-site ions play a primary role whereas A′-site Cu ions secondarily contribute to OER activity for CaCu3B4O12. 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.

> 0.2 valence unit) correspond to the difference in total energy of 50-200 meV per formula unit.

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., CaCu3Fe4O12 and LaMn7O12) displayed catalytic activity that was higher than that of simple perovskites (e.g., LaMnO3), but the reason has not yet been unveiled. We have conducted first-principles calculations of the several surface energies of LaMn7O12 and adsorption energies of OER intermediates on LaMn7O12 using slab models to clarify the reaction mechanism. The Mn-rich surfaces, i.e., the (001) with BO2 termination and (220) surfaces, are found to be more stable for LaMn7O12. It is found that all intermediates are preferentially adsorbed on the A′–B-bridge site on the LaMn7O12 (220) surface, although only the B-top site was a stable adsorption site on the (001) surface of LaMn7O12 and LaMnO3. The difference between theoretical overpotentials on the (220) surface of LaMn7O12 and the (001) surface with BO2 termination of LaMnO3 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.