Contribution of transition-metal d-orbital states to oxygen evolution electrocatalysis in oxides

It is identified that Fe, Cr, and Al are beneficial to enhance the oxygen evolution catalysis in LaNiO₃ although their perovskite counterparts, LaFeO₃, LaCrO₃, and LaAlO₃, are inactive. Furthermore, semiconducting LaCoO₃ is found to have more than one order higher activity than metallic LaNiO₃, in contrast to previous reports. Showing the importance of facilitating electron conduction, our work highlights the impact of the near-Fermi-level d-orbital states on the oxygen-evolution catalysis performance in perovskite oxides.

Article | Fall 2021

As many devices for electrochemical energy storage and conversion operate at room temperature, control of electrochemical redox reactions is of great importance in reducing the activation barriers and thereby boosting the overall storage and conversion efficiencies. Among different types of redox reactions, the oxygen evolution reaction (OER) is an indispensable kinetic process taking place at the anode during water splitting in electrolyzers and at the cathode during charging in metal―air rechargeable batteries. In particular, multiple transfers of electrons and protons during the OER are considered to result in a much large activation barrier compared with that of the hydrogen evolution reaction. The utilization of efficient OER electrocatalysts is thus imperative to significantly reduce the overpotential of the anodic reaction in water electrolysis for hydrogen production.

Serious constraints should be noted when the experimentally obtained activities are compared with the theoretical descriptors. First, if a catalyst is electronically insulating, a great deal of applied overpotential may be dissipated as Ohmic resistance, resulting in much lower OER current even though the catalyst may be intrinsically very active. Therefore, a direct comparison of the OER activity cannot be made when one catalyst is metallic and the other is insulating or semiconducting with a bandgap. Second, the OER activity of crystalline catalysts is considerably orientation-dependent, showing fairly different values of the OER current density on each crystalline facet. This indicates that the precise difference of the OER activity between catalysts is difficult to identify unless an identical crystallographic surface is measured. Third, most theoretical studies regarding the variation of M―O(H)* bond strength have dealt with merely one specific facet^4,23 even though the experimentally measured OER properties come from numerous random facets of polycrystalline catalysts.

To overcome these limitations, 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 utilized heteroepitaxial (001)_cubic thin films of LaNiO₃ and LaCoO₃ perovskite oxides instead of polycrystalline particles. Metallic LaNiO₃ thin films doped with a small amount of seven different trivalent metal dopants, Fe, Co, Cr, Mn, Sc, Al, and In, are thus used. Because these dopants are all 3+, neither cation vacancies nor oxygen vacancies are created during thin film fabrication. One of the significant findings in this study is that Fe, Cr, and Al, the counter La-perovskites of which (LaFeO₃, LaCrO₃, and LaAlO₃) are known to be OER inactive, make a notable contribution to the OER activity when doped in metallic LaNiO₃ films.

The DFT calculations also consistently demonstrate the appearance of a large density of the d-orbital states near the Fermi level in cases where the OER activity is enhanced. In addition, when the low conductivity limitation is eliminated in semiconducting LaCoO₃ films by introducing a metallic interlayer in the other series of experiments, the OER current density of the LaCoO₃ film is observed to be one order higher than that of the LaNiO₃ film, showing the best OER activity among LaMO₃-type perovskite oxides. In addition to clarifying the importance of the electronic conductivity of OER catalysts, this study dealing with more than 50 heteroepitaxial thin films of different compositions highlights the impact of the near-Fermi-level d-orbital states on the OER performance in perovskite oxides.

This work was published in Nature Communications (vol. 12, 824 (2021)).

Additional links for more information:

https://www.nature.com/articles/s41467-021-21055-0.pdf

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