Naruki Endo

Site occupancy of interstitial deuterium atoms in face-centred cubic iron.

Hydrogen composition and occupation state provide basic information for understanding various properties of the metal–hydrogen system, ranging from microscopic properties such as hydrogen diffusion to macroscopic properties such as phase stability. Here the deuterization process of face-centred cubic Fe to form solid-solution face-centred cubic FeDx is investigated using in situ neutron diffraction at high temperature and pressure. In a completely deuterized specimen at 988 K and 6.3 GPa, deuterium atoms occupy octahedral and tetrahedral interstitial sites with an occupancy of 0.532(9) and 0.056(5), respectively, giving a deuterium composition x of 0.64(1). During deuterization, the metal lattice expands approximately linearly with deuterium composition at a rate of 2.21 Å3 per deuterium atom. The minor occupation of the tetrahedral site is thermally driven by the intersite movement of deuterium atoms along the ‹111› direction in the face-centred cubic metal lattice.

Li4FeH6: Iron-containing complex hydride with high gravimetric hydrogen density

Li4FeH6, which has the highest gravimetric hydrogen density of iron-containing complex hydrides reported so far, is synthesized by hydrogenation of a powder mixture of iron and LiH above 6.1 GPa at 900 °C. In situ synchrotron radiation X-ray diffraction measurements reveal that while kinetics require high temperature and thus high pressure for the synthesis, Li4FeH6 is expected to be thermodynamically stable slightly below room temperature at ambient pressure; further synthetic studies to suppress the kinetic effects may enable us to synthesize Li4FeH6 at moderate pressures. Li4FeH6 can be recovered at ambient conditions where Li4FeH6 is metastable.

Synthesis and formation process of Al2CuHx: A new class of interstitial aluminum-based alloy hydride

Aluminum-based alloy hydride Al2CuHx (x ∼ 1) is synthesized by hydrogenating Al2Cu alloy using high-temperature and high-pressure hydrogen atmosphere. Al8Cu square antiprisms in Al2Cu twist around the c axis of a tetragonal unit cell by hydrogenation. The twist enlarges the interstitial spaces for accommodating hydrogen atoms which align linearly parallel to the c axis in Al2CuHx. Thermodynamic stability of Al2CuHx results from the balance of stabilization by H 1s and Al 3sp hybridization and destabilization owing to the Fermi-level lifting upon hydrogenation. The crystal and electronic structures of Al2CuHx illustrate the formation of an interstitial hydride of aluminum-based alloy.

Formation process of perovskite-type hydride LiNiH3: In situ synchrotron radiation X-ray diffraction study

The formation process of perovskite-type hydride LiNiH3 was investigated using in situ synchrotron X-ray diffraction measurements. A mixture of LiH and Ni was hydrogenated at 873 K and 3 GPa, and the structural changes associated with hydrogenation were observed. Time-resolved diffraction profiles showed a three-step reaction: hydrogenation of Ni to NiHx, formation of LiyNi1–yH solid solution, and conversion to perovskite-type hydride LiNiH3. The solid solution, which has never been reported, plays the role of the precursor in the perovskite formation and its presence is apparently critical for synthesizing perovskite-type hydrides.

Effect of water electrolysis temperature of hydrogen production system using direct coupling photovoltaic and water electrolyzer

AbstractWe propose control methods of a photovoltaic (PV)-water electrolyzer (ELY) system that generates hydrogen by controlling the number of ELY cells. The advantage of this direct coupling between PV and ELY is that the power loss of DC/DC converter is avoided. In this study, a total of 15 ELY cells are used. In the previous researches, the electrolyzer temperature was constantly controlled with a thermostat. Actually, the electrolyzer temperature is decided by the balance of the electrolysis loss and the heat loss to the outside. Here, the method to control the number of ELY cells was investigated. Maximum Power Point Tracking efficiency of more than 96% was achieved without ELY temperature control. Furthermore we construct a numerical model taking into account of ELY temperature. Using this model, we performed a numerical simulation of 1-year. Experimental data and the simulation results shows the validity of the proposed control method.