Kanagawa University discovers a proton-functional material with high thermal stability characteristics — Potential applications in fuel cells and catalysts

A research group led by Professor Teruki Motohashi (Faculty of Chemistry and Biochemistry, Kanagawa University), together with Professor Yusuke Nambu from the Institute for Integrated Radiation and Nuclear Science, Kyoto University, Professor Kunihisa Sugimoto and Dr. Zi Lang Goo (at the time of the research) from Faculty of Science and Engineering, Kindai University, Professor Katsuro Hayashi and Associate Professor Miki Inada from Graduate School of Engineering, Kyushu University, Dr. Koji Kimoto from Center for Basic Research on Materials, National Institute for Materials Science (NIMS), and Dr. Maxim Avdeev from Australian Nuclear Science and Technology Organization (ANSTO), discovered a strontium-gallium oxy-hydroxide with exceptional thermal stability.

This newly discovered compound was successfully obtained using a uniquely developed synthesis technique. Advanced analyses using electron microscopy, X-ray diffraction, neutron diffraction, and infrared spectroscopy revealed detailed crystal structure of the compound and the existence of hydrogen bonds contributing to its high thermal stability. This outcome has the potential to accelerate the exploration of oxy-hydroxides to create innovative proton-functional materials that can be used in fuel cells and solid-acid catalysts. This achievement was published in Inorganic Chemistry.

Metal oxy-hydroxides, which contain hydroxide ions (OH⁻) derived from water molecules in their crystal structures, are a class of compounds incorporating a large quantity of proton (H⁺) sources. As proton-mediated functional materials such as solid-acid catalysts and proton conductors are often vital, there is a strong demand for new synthesis techniques of novel oxy-hydroxides.

Using a uniquely developed method, termed "vapor hydroxidation," the research group discovered a novel oxy-hydroxide Sr₂Ga₃O₆ (OH). This synthesis method directly hydroxylates metal oxide precursors under high-temperature, highly concentrated water vapor in a custom-designed reactor. Typical reaction conditions are 500 ℃ or higher and 80 vol % water vapor, which differ greatly from conventional synthesis conditions. This water vapor concentration is 30 times higher than that of room-temperature saturated water vapor (corresponding to 3000% humidity), enabling the discovery of new materials that only exist in high-temperature, highly concentrated water vapor atmospheres. Generally, oxy-hydroxides decompose into oxides releasing water molecules at about 300 ℃; therefore, attempts to synthesize oxy-hydroxides at high temperatures above 500 ℃ were considered impractical and unprecedented—an important point.

Comprehensive investigations using electron microscopy, X-ray diffraction, and neutron diffraction revealed that strontium (Sr) and gallium (Ga) are arranged in a hexagonal structure, and the incorporated OH⁻ is localized in a narrow space sandwiched between two strontium atoms. Furthermore, the compound was found to exhibit exceptional thermal stability and can retain OH⁻ ions in its crystal structure up to approximately 850 ℃. Detailed infrared spectroscopy measurements suggested that multiple hydrogen bonds formed in the crystal structure contribute to the thermal stability of this compound.

The "vapor hydroxidation" technique is extremely effective for discovering novel oxy-hydroxides. Owing to high crystallinity of the products with good reproducibility, this enables precise characterization of chemical composition and crystal structure. In fact, the research group has succeeded in synthesizing several types of novel oxy-hydroxides. Vital progress is expected in the development of oxy-hydroxides as potential proton-functional materials and the creation of innovative proton functionalities such as solid-acid catalysts and proton conductors that can be used even in high-temperature environments.

Motohashi commented: Our "vapor hydroxidation" is a uniquely developed synthesis technique for the exploration of novel oxy-hydroxides under high-temperature, highly concentrated water vapor conditions. Although steam-cooking ovens are commercially available, this approach—analogous to 'water vapor-heated cooking of ceramics'—has enabled the discovery of numerous new materials.

Journal Information
Publication: Inorganic Chemistry Vol. 64, Issue 36, 18294-18303.
Title: A Sr-Ga Oxy-Hydroxide with High Thermal Stability: Unraveling Its Characteristic Hydrogen-Bond Network
DOI: 10.1021/acs.inorgchem.5c02586