Atomic-scale engineering to boost oxygen evolution electrocatalysis in perovskite oxides

By utilizing epitaxial thin-film materials, it is directly demonstrated that simple exchange of Fe in the surface region is highly effective in boosting the catalytic activity for the oxygen evolution reaction. Furthermore, strong distortion of oxygen octahedra at the angstrom scale is identified to be readily induced during the Fe exchange, and this structural perturbation permits easier charge transfer.

Article | Spring 2020

Oxygen evolution and reduction are the major electrochemical reactions in electrolysers, metal-air batteries, fuel cells, and water-splitting devices.

Facilitating these reactions has been a central issue for effective fuel generation and higher energy conversion/storage efficiency. It is generally accepted that the activation barrier of the oxygen evolution and reduction reactions is fundamentally large and that multiple intermediate steps of electron transfer are necessarily involved during the reactions. Adequate electrocatalysts are thus essential for promoting the reactions at lower overpotentials. In particular, since both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in metal-air batteries and water-splitting devices take place at room temperature, enhancing the oxygen electrocatalysis has a crucial impact not only on the efficiency of such devices, but on their longevity and ultimate performance.

Prof. Sung-Yoon Chung’s group in the Department of Materials Science and Engineering at the Korea Advanced Institute of Science and Technology (KAIST) has successfully demonstrated that the structural perturbation of metal-oxygen octahedra via simple electrochemical Fe exchange at the surface of perovskite oxides is particularly efficient at enhancing OER activity by an order of magnitude (see Figure 1). To conduct atomic-column resolved observation of oxygen displacement, integrated differential phase contrast (iDPC) scanning transmission electron microscopy (STEM), a recently developed phase-contrast imaging technique, was used in addition to well-known annular bright-field (ABF) STEM (see Figure 2). Electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS) were also carried out to verify the chemical exchange of the Fe for the Ni site. Combined with ab initio density functional theory (DFT) calculations, the experimental findings in the present study provide significant evidence that a high density of states of O 2p and transition-metal 3d orbitals near the Fermi level can be achieved by the strong distortion of the oxygen octahedra, boosting the OER activity in the perovskite nickelates.

Many previous studies on the OER catalysis of oxides have attempted to relate the observed variation in OER activity with the electronic structure of the bulk and the structural change of the overall polycrystals, even though there is substantial crystallographic anisotropy in the catalytic activity which strongly depends on both electronic and atomic structures near the surface. To move past these previous limitations, this study employed a combination of (001) epitaxial thin films, atomic-scale structure and composition analyses, and DFT calculations using more realistic supercells from direct observation. The present work was thus able to reliably demonstrate a solid correlation among the structural perturbation, the electronic structure, and the resultant OER property in nickelate perovskites. In addition to providing evidence that the Fe exchange has a rather general impact on the exceptional increase in OER activity in nickelate perovskites, the findings in this work show that symmetry-breaking configurational control of atoms on the surface can offer an important platform toward exceptional oxygen electrocatalysis in perovskite oxides.

This work was published in Nature Communications (vol. 10, 2713, (2019)).

Additional links for more information:
https://www.nature.com/articles/s41467-019-10838-1
https://sites.google.com/site/atomicscaledefects/home