0. Concept
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National Institute for Materials Science (NIMS)
Hydrogen is the most abundant element in the universe. Owing to its small size, it can be readily incorporated into materials, where interactions with electronic orbitals alter its properties and sometimes create highly active functional sites.
Hydrogen-ion materials containing such activated hydrogen form open systems in equilibrium with the hydrogen-rich environment of the Earth, incorporating hydrogen to maintain constant chemical potentials of hydrogen and electrons. Once incorporated, hydrogen interacts with the host lattice and transforms into new chemical species known as hydrogen defects. The structural and functional diversity of these defects—depending on the host material—is a central feature of hydrogen-ion materials research.
A remarkable property of hydrogen in materials is its fast diffusion.
In certain solids, hydrogen diffuses as rapidly as, or even faster than, in water, enabling hydrogen-ion systems to reach thermal equilibrium efficiently.
In hydrogen research, the development of novel analytical methods is indispensable. Our group is dedicated to developing instrumentation using low-energy ion beams with high sensitivity to hydrogen on the outermost surfaces of solids. We are also tackling the ambitious challenge of "Functional Material Design from the Surface."
By activating and manipulating environmentally sourced hydrogen through host material design and external fields, and applying these concepts to CO₂ reduction electrocatalysts, we aim to contribute to a sustainable, circular society.
1. hydrogen defects
In general, the term defect carries negative connotations such as imperfection or lack. In materials science, however, defects refer to atoms introduced into materials from the external environment (interstitial atoms) or vacancies formed by the removal of atoms originally present in the lattice.
Hydrogen defects, through their electronic interactions with host materials, can exhibit diffusivity and chemical reactivity far exceeding those of molecular hydrogen, thereby playing a decisive role in determining material functionalities. In this sense, hydrogen defects constitute the very core of hydrogen-ion materials research.
Throughout the history of materials science, numerous examples demonstrate that hydrogen defects have governed material functions. Protons introduced by hydration of positively charged oxygen vacancies (VO•• + H2O → OO× + 2Hi• ) serve as ionic charge carriers in intermediate-temperature fuel cells based on perovskite oxides and act as acidic active sites in zeolites for petroleum refining. Similarly, interstitial hydrogen in amorphous silicon passivates dangling bonds that form gap states (DBSi× + 2Hi× → (Si-H)Si× ), thereby enabling the development of low-cost solar cell technologies.
In our group, we have achieved:
1．Electron doping and superconductivity induced by hydride substitution in oxides (VO× + Hi× → HO• + e′).
2．Insulating behavior and fast hydride-ion conduction realized by oxygen substitution in metal hydrides (2HH× + Oi× → OH• + VH′).
3．Elucidation of degradation mechanisms in oxide multilayer ceramic capacitors (MLCCs) caused by the introduction of interstitial hydrogen (OO× + Hi× → (OH)O• + e′).
By designing the functionality of hydrogen defects based on the electronic structure and bonding states of host materials, we continue to explore new materials and novel functionalities.
Nano-SIMS mapping of 16OD− distribution in a fractured BaTiO₃-based MLCC cross-section (Ref.4).
Electronic phase diagram of the iron-based superconductor. The horizontal axis represents the electron doping level (proportional to the hydride substitution amount), and the vertical axis indicates the transition temperatures (for SC: Superconductivity, for AFM: Antiferromagnetism, for Orth: Orthorhombic phase) (Ref.3).
Related Publications
1. Yamasaki, T.; Iimura, S.; Kim, J.; Hosono, H., Extremely Shallow Valence Band in Lanthanum Trihydride. Journal of the American Chemical Society 2022, 145 (1), 560–566.
2. Iimura, S.; Okanishi, H.; Matsuishi, S.; Hiraka, H.; Honda, T.; Ikeda, K.; Hansen, T. C.; Otomo, T.; Hosono, H., Large-Moment Antiferromagnetic Order in Overdoped High-Tc Superconductor 154SmFeAsO1-xDx. Proceedings of the National Academy of Sciences 2017, 114 (22), E4354–E4359.
3. Iimura, S.; Matuishi, S.; Sato, H.; Hanna, T.; Muraba, Y.; Kim, S. W.; Kim, J. E.; Takata, M.; Hosono, H., Two-Dome Structure in Electron-Doped Iron Arsenide Superconductors. Nature Communications 2012, 3, 943.
4. Saito, Y.; Oguni, T.; Nakamura, T.; Nada, K.; Sano, H.; Hashiguchi, M.; Sakaguchi, I., Hydrogen infiltration into BaTiO3-based dielectrics for multi-layer ceramic capacitors under highly accelerated temperature and humidity stress test, Japanese Journal of Applied Physics, 2021, 60, SFFC02.
2. hydrogen diffusion
Remarkably, hydrogen in certain solids is known to diffuse at room temperature as fast as, or even faster than, hydrogen in water.
In fuel cells, an electric circuit is established by rapid proton transport through electrolytes sandwiched between two electrodes.
Likewise, in various hydrogenation catalysts, fast hydrogen diffusion on catalyst surfaces and supports plays an essential role in catalytic performance.
In our group, we have achieved:
The world’s first demonstration of room-temperature hydride-ion conduction in oxygen-substituted lanthanum hydrides,
A novel resistive switching memory device based on hydride-ion-conducting thin films.
Currently, we employ molecular dynamics simulations based on machine-learning interatomic potentials to efficiently search for new fast hydrogen-ion conductors and to elucidate the underlying diffusion mechanisms.
Arrhenius plots of proton and hydride ion conductivities. The lanthanum oxyhydride exhibits ion conductivity at room temperature comparable to that of conventional solid proton electrolytes (Ref.5).
Related Publications
5. Fukui, K.; Iimura, S.; Iskandarov, A.; Tada, T.; Hosono, H., Room-Temperature Fast H⁻ Conduction in Oxygen-Substituted Lanthanum Hydride. Journal of the American Chemical Society 2022, 144 (4), 1523–1527.
6. Yamasaki, T.; Takaoka, R.; Iimura, S.; Kim, J.; Hiramatsu, H.; Hosono, H., Characteristic Resistive Switching of Rare-Earth Oxyhydrides by Hydride Ion Insertion and Extraction. ACS Applied Materials & Interfaces 2022, 14 (17), 19766–19773.
7. Fukui, K.; Iimura, S.; Tada, T.; Fujitsu, S.; Sasase, M.; Tamatsukuri, H.; Honda, T.; Ikeda, K.; Otomo, T.; Hosono, H., Characteristic Fast H⁻ Ion Conduction in Oxygen-Substituted Lanthanum Hydride. Nature communications 2019, 10 (1), 2578.
3. hydrogen catalysis
We consider that intentionally introducing diverse hydrogen defects into metals can impart excellent hydrogenation catalytic activity.
By analyzing electronic orbitals in metals—often obscured by free electrons—we design and synthesize metallic materials containing desired hydrogen defects and evaluate their performance as electrode catalysts for electrochemical CO₂ reduction.
Because chemical reactions occur at solid surfaces, catalyst development fundamentally requires precise control over surface structure and composition.
However, solid surfaces often possess structures and compositions distinct from those of the bulk, and even today, such surface structures have been identified for only a limited number of materials.
Moreover, hydrogen—owing to its single electron and the fact that nearly all materials contain trace amounts of hydrogen—is among the most challenging elements to analyze. Consequently, structural analysis of surface hydrogen is regarded as one of the most difficult measurements in materials science.
To address this challenge, we focus on low-energy ion beam analysis techniques with high sensitivity to hydrogen and surfaces, including Elastic Recoil Detection Analysis (ERDA), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and Low-Energy Ion Scattering (LEIS). By independently developing original instrumentation, we investigate the structure of surface hydrogen and elucidate surface reaction mechanisms.
The relationship between the energy and TOF of scattered ions and recoiled particles. Hydrogen on rutile TTiO₂ surface was detected by combining energy and mass analysis. Reprinted from Ref8, with permission from Elsevier (Ref.8).
Related Publications
8. Suzuki, T. T.; Sakaguchi, I.; Iimura, S.; Nishimura, T.; Tsuchiya, B., Combined Energy and TOF Analyses of Low-Energy Elastic Recoil Detection for Analyzing the Topmost Surface of Atomic Deuterium-Irradiated Rutile TiO₂ (001) Crystals. Surface Science 2026, 769, 122967.
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