Hydrogen in materials exhibits a wide range of concentration, high mobility, quantum nature, and superior chemical reactivity. All these features of hydrogen originate from its bonding and size flexibilities; neutral hydrogen (H⁰), covalently bonded hydrogen (H^cov.), proton (H⁺), hydride ion (H⁻), and also their intermediate states. The purpose of this project (“Hydrogenomics”, i.e., hydrogen - omics (academic system)) is to develop a new materials science of hydrogen/hydrides as a guideline to “fully utilize” the diverse functionalities of hydrogen in materials. The project is focusing on the four important functionalities of hydrogen originated from its bonding and size flexibilities; i.e., “High Densification Ability (A01)”, “Interfacial Localizability (A02)”, “Fast Migration Ability (A03)” and “High Activation Ability (A04)”. Then the project merges them to induce unprecedented “higher-order hydrogen functions (synergistic effect between its individual hydrogen functions)” with the support of “Advanced Measurement and Simulation Techniques (A05)”. The current situation of the project is to be explained in this article.

Proton exchange membrane fuel cells (PEMFCs) are regarded as attractive clean energy conversion devices for residential, transportation, and portable applications. Among several hydrogen storage modes, a high-pressure tank is state-of-the-art; however, some issues such as cost, safety, and portability (or volumetric hydrogen storage capacity) hampers the widespread dissemination of PEMFCs. Recently, we have elaborated an all-polymer type rechargeable PEMFC (RCFC), where a hydrogen-storable polymer (HSP) sheet, a solid-state organic hydride, was included inside a PEMFC in the anode side. RCFC was operable without supplying hydrogen from outside. The gas impermeability of proton exchange membranes (PEMs) is crucial for the operable time of the RCFC; use of a gas impermeable in-house SPP-QP (a polyphenylene ionomer membrane) prolonged the operation time, reaching up to ca. 10.2 sec mgHSP⁻¹, which was more than two times longer than that with a commercially-available membrane (perfluorinated Nafion) cell. The RCFCs were operable repeatedly at least up to 50 cycles. The features of this RCFC system, including safety, ease of handling, and light weight, suggest applications in portable, light-weight hydrogen-based energy devices.

The conduction of hydride ions (H^–) is particularly attractive because these ions are similar in size to oxide and fluoride ions that are suitable for fast ionic conduction, while they also exhibit strong reducing properties through the standard H−/H2 redox potential (−2.3 V), which is comparable to that of Mg/Mg²⁺ (−2.4 V). Hydride ion conductors may therefore be applied in energy storage/conversion devices with high energy densities. Recently, we have developed a series of K²NiF_4-type oxyhydrides, La_2−x−ySr_x+yLiH_1−x+yO_3−y, which are equipped with anion sub-lattices that exhibit flexibility in the storage of H⁻, O²⁻, and vacancies. More specifically, the all-solid-state Ti/La_2−x−ySr_x+yLiH_1−x+yO_3−y/TiH₂ cell exhibited a redox reaction with hydrogen storage/desorption on the electrodes. The present success in the construction of this all-solid-state electrochemical cell exhibiting H⁻ diffusion confirms not only the capability of the oxyhydride system to act as a solid hydride electrolyte, but also the possibility of developing electrochemical solid devices based on H− conduction for the first time.

Electrochemical hydrogenation of non-fossil resources to produce value added chemicals has great potential to contribute to realization of sustainable material supply. In this article, we introduce electrochemical synthesis of nine amino acids from biomass-derivable α-keto acids and NH₂OH with high Faradaic efficiencies (FEs) of 60 – 99% using earth-abundant TiO₂ catalyst. We also introduce continuous electrosynthesis of alanine using newly designed flow-type electrochemical reactor equipped with TiO₂ cathode catalyst. Furthermore, we present our recent efforts on developing electrocatalytic system for utilizing oxalic acid, which can be easily produced from agro-waste, as a starting material of amino acid synthesis. Glycine and its intermediate, glyoxylic acid oxime, were electrochemically synthesized from oxalic acid and NH₂OH in one-step with FE of 28 and 28%, respectively.

Nitride-based materials such as BaCeO_3-xN_yH_z and LaN enable the activation of N₂ at nitride-vacancy sites with the help of spill-over hydrogen from transition metal surface. Fe, Co, and Ni nanopartilces combined with these nitrides function as efficient catalysts for ammonia synthesis through the lattice anion vacancy-mediated Mars-van Krevelen mechanism, while ammonia synthesis occurs over conventional catalysts through a Langmuir-Hinshelwood mechanism. As a result, these nitride-based catalysts surpass the activity of conventional Ru-based catalysts.

Hydrogen is a ubiquitous element and often controls materials properties. Since hydrogen existing in materials has small scattering cross sections for photons and electrons, it is regarded as a difficult-to-detect element. Ion and neutron beams possess unique capabilities to directly detect protons thereby allowing for advanced analysis of hydrogen-related materials. This article expounds on the experimental techniques of nuclear reaction analysis with an ion beam and neutron/ x-ray diffraction for the analysis of hydrogen in materials. We also describe studies with these experimental methods on the structural analysis of iron hydrides and palladium hydrides, the hydrogen diffusion in palladium and titanium dioxide, and absorption/desorption properties of hydrogen in palladium alloys and complex transition metal hydrides.