Observation of oxide electrode catalysts with synchrotron radiation leads to understanding of surface structure changes during electrolysis

A research group led by Professor Yusuke Wakabayashi of the Graduate School of Science at Tohoku University, in collaboration with Professor Kohei Miyazaki of the Graduate School of Engineering at Kobe University, has revealed through interface structure analysis using synchrotron radiation, how the atomic arrangement at the interface between the electrode and electrolyte solution changes over time during water electrolysis. With information about surface structure, clear guidelines can be established for catalyst development, and significant progress is expected. The results were published in ACS Applied Materials & Interfaces.

It is difficult to directly observe the atomic arrangement in electrode catalysts where water electrolysis occurs. Notably, actual catalysts improve in efficiency or degrade during use, but if the causes can be understood from the perspective of atomic arrangement, it becomes possible to suppress degradation and create structures that maintain high activity. The research group fabricated a thin film of La_0.6Sr_0.4CoO₃, known to have relatively high activity, on a strontium titanate (SrTiO₃) substrate. They measured the surface structure under the following conditions using diffraction experiments with synchrotron X-rays combined with Bayesian estimation: ① in vacuum, ② immediately after placing a catalyst in an electrochemical environment where the catalyst is immersed in potassium hydroxide aqueous solution with controlled thin film potential, ③ with the potential controlled to a level just before strong chemical reactions such as water electrolysis occur, and ④ after changes had stabilized from a conventional measurement perspective. The measurements utilized the X-ray diffraction equipment at the KEK synchrotron radiation facility, Photon Factory.

When the sample was placed in an electrochemical environment capable of causing water electrolysis, and the thin film's potential was controlled to achieve conditions immediately preceding the onset of strong chemical reactions, within the range where no bubbles formed. From this it was found that not only did the structure change with potential, but the surface structure gradually changed over time. Since the time evolution ended after approximately 1.5 days, examination of the surface structure at that stage revealed that a different surface structure had formed. This structure is similar to the structure proposed for the surface of highly efficient cobalt-iron oxide electrode catalysts. Surface structure measurements at solid-liquid interfaces for oxide catalysts are extremely rare, and this measurement represents the world's first for oxides with perovskite structure.

The edge-sharing structure formed affects the electron orbitals near the surface and changes the catalytic activity. Additionally, the process by which oxygen contained on the oxide electrode side and oxygen on the electrolyte solution side exchange is thought to be included in the intermediate stages of water electrolysis, and the steric hindrance during this process also changes. Through these changes, the newly formed surface structure is expected to exhibit different catalytic activity from the structure immediately after film formation. In fact, the degree of chemical reaction measured during this measurement was greater after the structure changed than before.

Traditionally, in catalyst research, structural information has been something researchers "want but cannot obtain." In particular, unintended structural changes have been recognized as resulting in high-performance materials for unknown reasons, which has been a major obstacle to new material development. This research creates a path to overcome this challenge.

Journal Information
Publication: ACS Applied Materials & Interfaces
Title: Surface Structure Modulation of La_0.6Sr_0.4CoO₃ Films on SrTiO₃ (001) Substrate under Electrochemical Conditions
DOI: 10.1021/acsami.5c11807