A systematic evaluation of the role of lanthanide elements in functional complex oxides; Implications for energy conversion devices

Ji Wu, Kotaro Fujii, Masatomo Yashima, Aleksandar Tsekov Staykov, Taner Akbay, Tatsumi Ishihara, John A. Kilner

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Abstract

Lanthanide containing complex oxides, especially the ABO3 perovskite and A(n+1)BnO(3n+1) Ruddlesden-Popper series, attract much interest as promising catalytic materials in many renewable energy applications such as electro-chemical energy conversion and hydrogen production. Recent experimental and theoretical studies on some members of these materials, e.g. La2NiO4, revealed that the La-O terminated surfaces are catalytically active under operational conditions. These findings suggested that the conventional understanding of such oxides being fully ionized, and composed of catalytically inert La3+ ions needs to be revised. In this study, generalized gradient approximation and hybrid density functional theory methods were used to study and compare the electronic structures of La and Sr in related oxides. Density functional theory approaches based on both Gaussian and plane wave basis sets were employed to ensure robustness of this study. Consistent results were obtained across different ab initio methods and approaches used. Density of states plots and charge analysis results showed that La exhibits a partially occupied d-orbital and an atomic charge of +2 instead of its nominal valence number (+3) in the oxides, while Sr does not show similar characteristics. Electron density maps obtained from synchrotron X-ray diffraction experiments confirmed the simulation findings as well. The presence of the available d-orbital electron on La and associated partial covalency were postulated as being responsible for the catalytic behaviour observed in experiments. In addition, Pr and Ba electronic structures in related oxides were also calculated. A similar trend to the La and Sr charges was observed. Based on these findings, the traditional concept of atomic "ionicity" was briefly reviewed and adapted as a catalysis descriptor for possible performance evaluation.

Original languageEnglish
Pages (from-to)11819-11829
Number of pages11
JournalJournal of Materials Chemistry A
Volume6
Issue number25
DOIs
Publication statusPublished - Jan 1 2018

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Lanthanoid Series Elements
Rare earth elements
Energy conversion
Oxides
Electronic structure
Density functional theory
Hydrogen production
Synchrotrons
Perovskite
Catalysis
Carrier concentration
Experiments
Ions
X ray diffraction
Electrons

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Renewable Energy, Sustainability and the Environment
  • Materials Science(all)

Cite this

A systematic evaluation of the role of lanthanide elements in functional complex oxides; Implications for energy conversion devices. / Wu, Ji; Fujii, Kotaro; Yashima, Masatomo; Staykov, Aleksandar Tsekov; Akbay, Taner; Ishihara, Tatsumi; Kilner, John A.

In: Journal of Materials Chemistry A, Vol. 6, No. 25, 01.01.2018, p. 11819-11829.

Research output: Contribution to journalArticle

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