XAS and XPS Analysis on Surface Reconstruction By Chemical Treatment of Ba0.5Sr0.5Co0.8Fe0.2O3
Authors: Ikeda Yusaku , Shibata Daisuke , Irizawa Akinori , Zhong Chengchao , Okazaki Ken-ichi , Shimoda Keiji , Orikasa Yuki 
Published: December 2024

Alkaline water electrolysis is expected to play a vital role in hydrogen production systems using renewable electricity. However, the intermittent nature of renewable electricity can often cause electrolysis cells to shut down. Such interruptions result in reverse currents that accelerate catalyst degradation. Extensive studies of catalytic activity have elucidated that the oxygen evolution reaction (OER) is contingent on certain descriptors [1]. Recently, the focus has been placed on the surface structure of the catalysts, as the actual OER occurs on the catalyst surface [2]. Previous research has demonstrated that the surfaces of numerous oxide materials with high OER activity undergo surface reconstruction under OER, resulting in the formation of metal oxyhydroxide phases [3]. This surface reconstruction has been reported after OER experiments, although it is challenging to analyze large numbers of samples in this manner. In order to efficiently study and utilize the state of the catalyst surface for material design, a method is needed to induce the surface reconstruction through chemical treatments. Consequently, the present study examines the chemical state of the surface following chemical treatment of perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) by stirring in an alkaline solution, employing X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS).
BSCF was synthesized using the sol-gel process. The as-prepared samples were subjected to stirring under four different conditions: in 1 M or 7 M KOH solutions with either oxygen or nitrogen bubbling for a duration of 21 hours. The structural and chemical properties were characterized using X-ray diffraction (XRD), XAS, and XPS. XAS measurements were conducted at the BL-2 and BL-11 beamlines of the Ritsumeikan University SR Center. Both total electron yield and partial fluorescence yield modes were employed.
XRD analysis did not reveal any significant peak shifts or the formation of a second phase, indicating that the bulk structure of the samples remained unchanged. Comparative studies of the O K-edge XAS for both as-prepared and chemically treated BSCF samples in 7 M KOH with oxygen bubbling were conducted. The peak at approximately 530 eV is attributed to the transition from O 1s to the 3d orbitals of transition metals combined with O 2p. In the as-prepared sample, this peak exhibited low intensity. However, in the chemically treated sample, sharp peaks were observed at approximately 529 eV and 532 eV. These findings indicate that the surface of the perovskite-type oxide underwent a transformation to an oxyhydroxide of Co or Fe, as evidenced by the presence of peaks characteristic of transition metal oxyhydroxides. Despite these surface changes, the results from Co and Fe L-edge XAS showed no peak shifts, indicating that the valence states of Co and Fe remained unchanged. XPS analysis revealed a decrease in the surface concentrations of Ba and Sr.
This work is based on results obtained from the project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. (JPNP 20003).
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