A substantial amount of interest has been focused on AB O 3 -type perovskite oxides over the past decade as oxygen electrocatalysts. Despite many studies on various compositions, the correlation between the structure of the oxygen octahedra and electrocatalytic property has been overlooked, and there accordingly have been a very limited number of attempts regarding control of atomistic structure. Utilizing epitaxial Ln NiO 3 ( Ln = La, Pr, Nd) thin films, here we demonstrate that simple electrochemical exchange of Fe in the surface region with several-unit-cell thickness is notably effective to boost the catalytic activity for the oxygen evolution reaction by different orders of magnitude. Furthermore, we directly establish that strong distortion of oxygen octahedra at the angstrom scale is readily induced during the Fe exchange, and that this structural perturbation permits easier charge transfer. The findings suggest that structural alteration can be an efficient approach to achieve exceptional electrocatalysis in crystalline oxides.
Atomic-scale direct probing of active sites and subsequent elucidation of the structure-activity relationship are important issues involving oxide-based electrocatalysts to achieve better electrochemical conversion efficiency. By generating Ruddlesden-Popper (RP) two-dimensional homologous faults via simple control of the cation nonstoichiometry in LaNiO thin films, we demonstrate that strong tetragonal distortion of [NiO] octahedra is induced by more than 20% elongation of Ni-O bonds in the faults. In addition to direct visualization of the elongation by scanning transmission electron microscopy, we identify that the distorted [NiO] octahedra in the faults show considerably higher electrocatalytic activities than other surface sites during the electrochemical oxygen evolution reaction. This unequivocal evidence of the octahedral distortion and its impact on electrocatalysis in LaNiO suggests that the formation of RP-type faults can provide an efficient way to control the octahedral geometry and thereby remarkably enhance the oxygen catalytic performance of perovskite oxides.
redox reactions in fuel cells and metaloxygen batteries as well as water-splitting devices. [1][2][3][4] Including the initial notable works in the 1980s, [5][6][7] many experimental and theoretical studies on the ABO 3 -type perovskite catalysts have dealt with the number of d-electrons of B-site cations, the bond strength of BOH, the position of the O 2p band center, the effect of oxygen vacancies, and the role of lattice oxygen redox in efforts to account for the crucial contributions that correlate with the high catalytic activity during the oxygen evolution reaction (OER). [6][7][8][9][10][11][12] By extensive comparison of more than ten perovskite oxides, recent studies also suggested activity descriptors on the basis of the number of e g -orbital electrons of the 3d transition metals [13] and the charge transfer energy between metal and oxygen electrons [14] to understand the OER activity differences among the perovskite family. Although various structural and electronic factors should be taken into account to precisely depict the catalytic behavior of perovskites [15][16][17] and other transitionmetal oxides, [18][19][20][21][22][23] it appears to be accepted in general that relative difference of energy level between the 3d orbitals of B-site cations and the 2p orbitals of oxygen anions play an important role in significant promotion of charge transfer between [BO 6 ] at the surface and adsorbates, resulting in much higher OER efficiency at a lower overpotential. [9,[11][12][13][14][24][25][26][27][28] In stark contrast to the [BO 6 ] octahedra in the bulk, most [BO 6 ] units at grain boundaries (GBs) in polycrystals have severely distorted geometric configurations of B and O. [29,30] Note that the atomic displacement from this distortion at GBs is usually much greater than the displacement that occurs at the crystal surface for relaxation [31] and/or is induced by the coherent strain via using the epitaxial thin-film growth. [26,32] As a result, it can be reasonably anticipated that the electronic states of transition-metal 3d and oxygen 2p orbitals at GBs considerably differ from those in the bulk. In this regard, a combination of selective analysis of the OER characteristics at GBs and observation of the grain-boundary atomic structure can provide direct and insightful information that has not been offered in previous studies to elucidate the correlation between A grain boundary forms as an internal interface when two crystalline grains with mutually different crystallographic orientations are in direct contact with each other. As a result, atomic arrangement at grain boundaries differs from that of the bulk, showing serious displacements deviating from the original symmetric positions. As these symmetry-broken configurations are difficult to achieve in the bulk crystals, grain boundaries are considered distinctive platforms that can exhibit new physical properties. By using both sintered polycrystals with various grain sizes and thin films on bicrystal substrates, it is directly verified that surfac...
As chemical reactions and charge-transfer simultaneously occur on the catalyst surface during electrocatalysis, numerous studies have been carried out to attain an in-depth understanding on the correlation among the surface structure and composition, the electrical transport, and the overall catalytic activity. Compared with other catalysis reactions, a relatively larger activation barrier for oxygen evolution/reduction reactions (OER/ORR), where multiple electron transfers are involved, is noted. Many works over the past decade thus have been focused on the atomic-scale control of the surface structure and the precise identification of surface composition change in catalyst materials to achieve better conversion efficiency. In particular, recent advances in various analytical tools have enabled noteworthy findings of unexpected catalytic features at atomic resolution, providing significant insights toward reducing the activation barriers and subsequently improving the catalytic performance. In addition to summarizing important surface issues, including lattice defects, related to the OER and ORR in this Review, we present the current status and discuss future perspectives of oxide- and alloy-based catalysts in terms of atomic-scale observation and manipulation.
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