Pressure-composition Isotherm Measurements Research Articles

Stoichiometry and annealing condition on hydrogen capacity of TiCr2-x AB2 alloys

This study presents the effect of stoichiometry and annealing condition on Ti–Cr AB2-type hydrogen storage alloys. Prior to annealing the majority phase of the as-cast alloys was the C14 Laves phase, with separate Ti and Cr phases. Annealing treatment (1273 K/14 d) led to a transition from C14 to C15 Laves phase structure. Both C14 (as-cast) and C15 (annealed) cell size increased with Ti content, up to a ratio (Cr/Ti) of 1.6, due to B-site Ti substitution in the lattice up to a limit. Pressure composition isotherm (PCI) measurements demonstrated alloys containing a greater Ti content had a better maximum hydrogen storage capacity (1.5 vs. 1.03 wt%) and lower plateau pressure (9.4 vs. 15.8 bar) at 253 K. Annealing resulted in a lower storage capacity (1.05 vs. 1.49 wt%), greater plateau pressure (ca. 30 bar) and flatter plateau slope (25 % reduction in plateau slope). Reduction in hydrogen storage capacity of annealed alloys could be due to diffusion of residual Cr in the alloy into the C15 Laves phase during the annealing process, thereby changing the local composition as confirmed through X-ray diffraction (XRD).

Thermodynamic modeling of Cr and Cr–H systems up to high temperatures and high pressures

A complete literature review of the thermodynamic and phase diagram properties of pure Cr and Cr–H system has been carried out. A thermodynamic assessment using the Calphad method has been accomplished up to 50 GPa for Cr including the liquid phase and up to 18 GPa on the Cr–H system using the most relevant literature data and our own ab-initio calculations. An up-to-date phase diagram of the Cr–H system is presented. Special attention was paid to the fcc phase to clarify the structure type (NaCl, hydrogen atoms located in the octahedral site of the fcc structure) and the composition (one hydrogen per chromium atom). The robustness of our proposed Cr and Cr–H models was demonstrated by comparing our measurements of pressure-composition isotherms of a bcc Cr–Zr alloy with the prediction made by combining the thermodynamic descriptions of the three Cr–H, H–Zr and Cr–Zr binary systems.

Investigation of the phase occurrence and H sorption properties in the Y33.33Ni66.67-xAlx (0 ≤ x ≤ 33.33) system

The Y33.33Ni66.67-xAlx system has been investigated in the region 0 ≤ x ≤ 33.3. The alloys were synthesized by induction melting. Phase occurrence and structural properties were studied by X-Ray powder Diffraction (XRD). The Al solubility in each phase has been investigated by XRD and Electron Probe Micro-Analysis (EPMA). The hydrogenation properties were characterized by pressure-composition isotherm measurements and kinetic curves at 473 K. For x = 0, the binary Y33.33Ni66.67 alloy crystallizes in the cubic superstructure with F4̅3m space group and ordered Y vacancies. For 1.67 ≤ x ≤ 8.33, the alloys contain mainly Y(Ni, Al)2 and Y(Ni, Al)3 pseudobinary phases; while for 16.67 ≤ x ≤ 33.33 they are mainly formed by ternary line compounds Y3Ni6Al2, Y2Ni2Al and YNiAl. Contrary to the binary Y33.33Ni66.67, Y(Ni, Al)2 pseudo-binary compounds crystalize in C15 phase (space group Fd3¯m) with disordered Y vacancies. The solubility limit of Al in the C15 YNi2-yAly phase is y = 0.11 (i.e., x = 3.67). The Y(Ni, Al)3 phase changes from rhombohedral (PuNi3-type, R3¯m) to hexagonal (CeNi3-type, P63/mmc) structure for x increasing from 5.00 to 6.67. Upon hydrogenation, the disproportion of C15 Y(Ni, Al)2 and losses of crystallinity of YNi and Y2Ni2Al are the main reasons causing capacity decay of Y33.33Ni66.67-xAlx (0 ≤ x ≤ 33.33) alloys upon cycling.

Effect of TiO2 + Nb2O5 + TiH2 Catalysts on Hydrogen Storage Properties of Magnesium Hydride

AbstractMagnesium hydride (MgH2) is a prospective material for the storage of hydrogen in solid materials. It can also be envisaged for thermal energy storage applications since it has the potential to reversibly absorb hydrogen in large quantities, theoretically up to 7.6% by weight. Also, MgH2 is inexpensive, abundant, and environmentally friendly, but it operates at relatively high temperatures, and the kinetics of the hydrogenation process is slow. Mechanical milling and the addition of catalyst can alter the activation energy and the kinetic properties of the MgH2 phase. It is known that the addition of titanium hydride (TiH2) lowers the enthalpy and enhances the absorption of hydrogen from MgH2, titanium oxide (TiO2) enhances the desorption of hydrogen and niobium oxide (Nb2O5) enhances the absorption of hydrogen. In this work, the influences of the catalysts, as mentioned above on the properties of MgH2, were studied. The samples were analyzed in terms of crystal and microstructure as well as hydrogen storage properties using a pressure-composition isotherm (PCT)measurement. It has been found that the simultaneous addition of the three catalysts enhances the properties of MgH2, lowers the activation energy and operating temperature, increases the rate of intake and release of hydrogen, and provides the largest gravimetric hydrogen storage capacity.

Crystal structure of nanocrystalline Pr5Co19 compound and its hydrogen storage properties

The present study focuses on the structure and pressure-composition (P–C) isotherm measurements of nanocrystalline Pr5Co19 hydrides. The crystal structure and hydrogenation properties of Pr5Co19 are investigated by X-ray diffraction, refined using Rietveld method, and by transmission electron microscopy (TEM). This intermetallic compound crystallizes in the rhombohedral (3R) with Ce5Co19-type structure.The pressure-composition isotherm of Pr5Co19 was measured at room temperature. A single plateau was observed during the absorption-desorption process, the reversible hydrogen capacity reached 10 H/f.u (0.4 H/M). A peak broadening was observed in the X-ray diffraction (XRD) profile after hydrogenation, with no detection of any amorphous phase.The crystal structure of the Pr5Co19Hx hydrides has not been changed, it always conserves the rhombohedral structure (space group R3¯m). The lattice parameters have increased with hydrogen content leading to an anisotropic expansion of the unit cell volume.

Thermochemical energy storage performance of zinc destabilized calcium hydride at high-temperatures.

CaH2 has 20 times the energy density of molten salts and was patented in 2010 as a potential solar thermal energy storage material. Unfortunately, its high operating temperature (>1000 °C) and corrosivity at that temperature make it challenging to use as a thermal energy storage (TES) material in concentrating solar power (CSP) plants. To overcome these practical limitations, here we propose the thermodynamic destabilization of CaH2 with Zn metal. It is a unique approach that reduces the decomposition temperature of pure CaH2 (1100 °C at 1 bar of H2 pressure) to 597 °C at 1 bar of H2 pressure. Its new decomposition temperature is closer to the required target temperature range for TES materials used in proposed third-generation high-temperature CSP plants. A three-step dehydrogenation reaction between CaH2 and Zn (1 : 3 molar ratio) was identified from mass spectrometry, temperature-programmed desorption and in situ X-ray diffraction studies. Three reaction products, CaZn13, CaZn11 and CaZn5, were confirmed from in situ X-ray diffraction studies at 190 °C, 390 °C and 590 °C, respectively. The experimental enthalpy and entropy of the second hydrogen release reaction were determined by pressure composition isotherm measurements, conducted between 565 and 614 °C, as ΔHdes = 131 ± 4 kJ mol-1 H2 and ΔSdes = 151 ± 4 J K-1 mol-1 H2. Hydrogen cycling studies of CaZn11 at 580 °C showed sufficient cycling capacity with no significant sintering occurring during heating, as confirmed by scanning electron microscopy, demonstrating its great potential as a TES material for CSP applications. Finally, a cost comparison study of known destabilized CaH2 systems was carried out to assess the commercial potential.

Phase evolution, hydrogen storage thermodynamics and kinetics of ternary Mg90Ce5Sm5 alloy

Greatly stable thermodynamics and sluggish kinetics impede the practical application of Mg-based hydrogen storage alloys. The modifications of composition and structure are important strategies in turning these hydrogen storage properties. In this study, Mg-based Mg90Ce5Sm5 ternary alloy fabricated by vacuum induction melting was investigated to explore the performance and the reaction mechanism as hydrogen storage material by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and pressure-composition isotherms (PCI) measurements. The results indicate that the Mg-based Mg90Ce5Sm5 ternary alloy consists of two solid solution phases, including the major phases (Ce, Sm)5Mg41 and the minor phases (Ce, Sm)Mg12. After hydrogen absorption, these phases transform into the MgH2 and (Ce, Sm)H2.73 phase, while after hydrogen desorption, the MgH2 transforms into the Mg phase, but the (Ce, Sm)H2.73 phases are not changed. The alloy has a reversible hydrogen capacity of about 5.5 wt% H2 and exhibits well isothermal hydrogen absorption kinetics. Activation energy of 106 kJ/mol was obtained from the hydrogen desorption data between 573 and 633 K, which also exhibits the enhanced kinetics compared with the pure MgH2 sample, as a result of bimetallic synergy catalysis function of (Ce, Sm)H2.73 phases. The rate of hydrogen desorption is controlled by the release and recombination of H2 from the Mg surface. Furthermore, the changes of enthalpy and entropy of hydrogen absorption/desorption were calculated to be −80.0 kJ/mol H2, –137.5 J/K/mol H2 and 81.2 kJ/mol H2, 139.2 J/K/mol H2, respectively.

Effect of annealing temperature on the microstructure of Pr2Co7 alloy and its hydrogen absorption-desorption kinetics

In this work, we studied the effect of annealing temperature on the kinetics of hydrogen absorption/desorption in nanocrystalline Pr2Co7 alloy. The pressure-composition (P–C) isotherm measurements revealed that the plateau pressure of Pr2Co7 alloy after annealing becomes comparatively flat. The maximum hydrogen absorption content of as-milled alloy and the annealed one reached 1.01 and 1.2 H/M (H/M: hydrogen per metal), respectively. The derived formation enthalpy ΔH and Gibbs free energy ΔG of the hydride in the annealed alloy are higher than those of the as-milled compound, indicating that the hydrides stability increases after annealing. The mechanism of hydrogen absorption-desorption process in the as-milled and annealed alloys was further investigated according to the Johnson-Mehl-Avrami formula. The hydrogenation of the as-milled and annealed Pr2Co7 alloys was found to be controlled by the three-dimensional-interface process. The activation energies Ea were determined and indicate that annealing is a very beneficial way to improve hydrogen absorption-desorption kinetics of the nanocrystalline Pr2Co7 alloy. The latter has a very fast absorption kinetic at 298 K. Throughout the study, it has been shown that Pr2Co7 alloy exhibits excellent hydrogenation properties which open a new perspective to be a new candidate for important uses in the field of hydrogen storage such as negative electrodes for nickel metal hydride (Ni-MH) batteries, mobile and short range vehicular applications.

Microstructure and hydrogen storage properties of V48Fe12Ti15-xCr25Alx (x=0, 1) alloys

The microstructure and hydrogen storage characteristics of V48Fe12Ti15-xCr25Alx (x = 0, 1) alloys prepared by vacuum arc melting were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and pressure–composition isotherm measurements. It was confirmed that all of the alloys comprise a BCC phase, a Ti-rich phase, and a TiFe phase. Al as a substitute for part of the Ti content caused an increase of lattice parameters of the BCC phase and of the equilibrium pressures of hydrogen desorption, but decrease of the hydrogen storage capacities. The kinetic mechanism of the hydrogenation and dehydrogenation of the alloys was investigated by the classical Johnson–Mehl–Avrami equation. The reaction enthalpies (ΔH) for the dehydrogenation of alloys without and with Al were calculated by the Van't Hoff equation based on the PCI measurement data, which are 30.12 ± 0.14 kJ/mol and 28.02 ± 0.46 kJ/mol, respectively. The thermal stability of the metal hydride was measured by differential scanning calorimetry. The hydrogen desorption activation energies were calculated using the Kissinger method as 79.41 kJ/mol and 83.56 kJ/mol for x = 0 and 1, respectively. The results suggest that the substitution of titanium with aluminum improves the thermodynamic properties of hydrogen storage and reduces the kinetic performance of hydrogen desorption.

Pressure artifacts at isothermal operation of a metal hydride tank

The paper presents the results of experimental investigations on hydrogen absorption and desorption in the metal hydride reactor RS-1 containing 81 kg of La0.5Nd0.5Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound at 60 °C isothermal conditions. During the reactor-scale measurements of pressure-composition isotherms we observe pressure artifacts; pressures for absorption are higher and for desorption are lower than equilibrium pressures obtained after cooling down and reheating of the reactor. This thermal relaxation procedure removes the pressure artifacts. The observed effect is similar to the large aliquot effect, and mostly affects the desorption isotherm. The highest measured pressure difference is 0.7 bar (11%).

Experimental investigation and thermodynamic assessment of the yttrium-hydrogen binary system

A coupled experimental investigation and thermodynamic study of the yttrium-hydrogen (Y-H) binary system were carried out to provide more comprehensive and quantitative insights into the key thermodynamic properties of this system. Y-H system in the full range of H/Y = 0–3.0 was investigated by accurate pressure composition isotherm (PCI) measurement to provide credible phase equilibria information and thermodynamic data. The phase boundaries obtained were in agreement with previous experimental data but with improved accuracy. With the guide of the crystal structures, all the solid phases were modelled using the three sublattice model. The Y-H phase diagram and thermodynamic parameters were calculated and assessed with the CALPHAD technique. The obtained results are in very good agreement with our experimental data and the published data reported in literature. The obtained thermodynamic database of Y-H system can be used to predict the hydrogenation behavior and decomposition temperatures of hydrides.

Effect of Mg substitution on crystalline structure and hydrogenation of Gd4MgNi19

The effect of Mg substitution on Gd4MgNi19 was investigated using X-ray diffraction (XRD) and pressure-composition (P-C) isotherm measurements. Gd4MgNi19 consists of 56% Gd2Co7-type, 27% Ce5Co19-type, and 17% CaCu5-type structures, which are retained in the hydride phase. Peak broadening was observed in the XRD profile of the hydride phase, while the degree of peak broadening decreased in the hydrogen-desorbed alloy after the P-C isotherm measurement. The reversible hydrogen capacity reached 1.0 H/M. There was a wide plateau region between 0.10 H/M and 0.80 H/M during the absorption-desorption process.Gd5Ni19 consists of 89% Sm5Co19-type and 11% CaCu5-type structures. After the P-C isotherm measurement, the hydrogen-desorbed Gd5Ni19 alloy showed severe −peak broadening in the XRD profile. The reversible hydrogen capacity was 0.80 H/M, and there was a sloping plateau in the P-C isotherm.Mg substitution decreases the severe peak broadening and lattice strain after the P-C isotherm measurement, and enhances the hydrogenation property.

The enhanced dehydrogenation performances of 17MgH2-12Al composite with additive TiH2

Abstract In this work, we introduced TiH 2 into the 17MgH 2 -12Al composite by ball milling for improving the dehydrogenation performance of the reactive system. According to the measurements of non-isothermal dehydrogenation, the onset temperature of hydrogen release from 17MgH 2 -12Al-TiH 2 was reduced by 50 °C from that of 17MgH 2 -12Al. The investigations by Kissinger's method showed that the activation energy of hydrogen desorption of 17MgH 2 -12Al-TiH 2 (107 kJ mol −1 ) is much lower than 17MgH 2 -12Al (168 kJ mol −1 ), leading to enhanced dehydrogenation kinetics due to the TiH 2 additive. X-ray diffraction (XRD) analysis suggests that TiH 2 catalyzed the reaction between MgH 2 with Al of the system during the dehydrogenation below 280 °C. At temperatures between 280 °C to 400 °C, however, TiH 2 will thoroughly react with produced Mg 17 Al 12 and liberate hydrogen. The overall reaction of the system may be represented by 17 MgH 2 + 12 Al + TiH 2 → 3/4 Mg 17 Al 12 + Al 3 Ti + 17/4 Mg + 18 H 2 . Its theoretical dehydrogenation capacity (4.38 wt% H 2 ) agrees well with that achieved by temperature programmed dehydrogenation (TPD) measurements, demonstrating the occurrence of such reaction(s). The dehydrogenation enthalpy of 17MgH 2 -12Al-TiH 2 (71.7 kJ mol −1 H 2 ) proceeding at lower hydrogen desorption plateau was significantly lowered from that of 17MgH 2 -12Al (81.9 kJ mol −1 H 2 ) according to the measurements of pressure-composition isotherms (PCI) and van't Hoff plots. It means that TiH 2 was involved in the end reaction proceeding in the 17MgH 2 -12Al system. We believe that TiH 2 improves the dehydrogenation kinetics of 17MgH 2 -12Al initially and tailors its thermodynamics finally.

Experimental study and thermodynamic assessment of the dysprosium-hydrogen binary system

The dysprosium-hydrogen (Dy-H) binary system in the full range of H/Dy = 0 to 3.0 is investigated by accurate pressure composition isotherm (PCI) measurement, including a PCI measurement of the β-DyH2/γ-DyH3 transition for the first time. The Dy/β-DyH2 biphasic region shows well defined plateaus in the range of H/Dy = 0.41 to 1.65 at 650 °C and gradually shrinks to 0.54 to 1.23 at 900 °C, with transition enthalpy of 213.1 ± 2.8 kJ/mol H2. The β-DyH2/γ-DyH3 biphasic region also shows well defined plateaus from 250 °C to 325 °C, but with much narrower composition region of H/Dy from 2.3 to 2.9. The transition enthalpy is 79.3 ± 2.2 kJ/mol H2. The phase diagram and thermodynamic parameters are calculated with the CALPHAD method using the measured PCI data. The obtained results are in very good agreement with our experimental data and the published data reported in literature. The results provide more comprehensive and quantitative insights into the key thermodynamic properties of the Dy-H system.

Hydrogen sorption behavior of CaAl1.5Li0.5

The hydrogen sorption properties of an alloy with nominal composition CaAl1.5Li0.5 have been investigated in a pursuit for hydrogen-absorbing Li-containing intermetallics. X-ray analysis of the original alloy indicated a coexistence of three closely related Laves phases. The maximum hydrogen capacity, recorded at about 6 MPa and 300 °C, was approximately 2.5 H atoms per formula unit (f.u.). Pressure-composition (p-c) isotherm measurements were taken in the temperature range between 350 and 450 °C up to pressures of 133 kPa. Thermodynamic parameters are derived for two plateau regions in the p-c isotherms. Analysis of these parameters and supporting evidence from X-ray patterns of hydrogenated and dehydrogenated samples suggest: (a) an initial irreversible disproportionation of the original alloy and (b) subsequent reversible hydrogenations, featuring reversible disproportionations of CaAl2 and LiAl intermetallic compounds. Attempts to form additional Li-containing intermetallics, namely CaAlLi, TiMn2−xLix (x = 0.2, 0.3, 0.4, 0.6) and TiAl2−xLix (x = 0.3, 0.5), and to hydrogenate them, are reported in brief.

Study on the thermodynamics of the gadolinium-hydrogen binary system (H/Gd = 0.0–2.0) and implications to metallic gadolinium purification

The thermodynamics of the gadolinium-hydrogen (Gd–H) binary system (H/Gd = 0.0–2.0) between 650 and 900 °C is studied by pressure composition isotherm measurement. Significant H dissolution in Gd is observed, up to H/Gd = 0.34 at 650 °C and 0.55 at 900 °C. The metal-rich phase boundary of nonstoichiometric gadolinium dihydride was found to occur with nominal composition of GdH1.80 at 650 °C and GdH1.53 at 900 °C. The results are in fairly good agreement with previous experimental work but with improved accuracy. The binary phase diagram is obtained using the CALPHAD method. The thermodynamic study here assists the understanding on the efficient deoxygenation effect by the H in Gd for Gd purification.

Evolution of the phase structure and hydrogen storage thermodynamics and kinetics of Mg88Y12 binary alloy

Mg88Y12 binary alloy was fabricated by vacuum induction melting technique. The phase compositions and microstructures of the alloy were analyzed by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Then the alloy powders were subjected to isothermal hydrogen absorption/desorption characterizations, differential scanning calorimetry (DSC) analysis and pressure-composition isotherms (PCI) measurements. XRD and SEM results showed that the alloy was composed of primary Mg24Y5 intermetallic and fine dendritic eutectic Mg–Mg24Y5 mixture. First hydrogenation process resulted in an irreversible two-step disproportion reaction: Mg24Y5 + H2 → Mg + YH2 → MgH2 + YH2 + YH3, and 6.479 wt.% hydrogen was absorbed at 380 °C. For further absorption/desorption cycles, the alloy had a reversible hydrogen capacity of about 5.6 wt.% due to the fact that the YH2 has a very high thermal stability and can not desorb hydrogen at the present experimental temperature. The hydrogen absorption and desorption rates were gradually improved by increasing hydriding/dehydriding cycle times because the alloy particles significantly cracked and pulverized during the first few cycles. The activated alloy exhibited well isothermal hydrogen absorption kinetics, but the dehydrogenation occurred at relatively high temperature. DSC measurements indicated that the desorption process of the fully hydrogenated alloy began at about 320 °C and peaked at 367 °C when the heating rate was 3 °C/min. The activation energy was determined as 122 and 116 kJ/mol according to the isothermal dehydrogenation kinetics at different temperatures and DSC curves at different heating rates. Moreover, the enthalpy and entropy changes for hydrogen absorption/desorption were calculated to be −79.8 kJ/mol H2, –139 J/K/mol H2 and 81.2 kJ/mol H2, 141 J/K/mol H2, respectively.

A Study of the Consecutive Absorption/Desorption Cycles of ZrCo–H2System

Consecutive absorption/desorption cycles of the ZrCo–H2 system were studied to simulate the real International Thermonuclear Experimental Reactor (ITER) hydrogen getter system. A ZrCo getter was used in this paper instead of the depleted uranium (DU) getter material, which has been recently considered as the hydrogen getter in ITER. In a cyclic pressure-composition isotherm (PCI) measurement, the high-pressure Sievert apparatus seems impractical to describe the equilibrium state of the ZrCo–H2 system in detail, especially for the desorption stage. This high-pressure PCI apparatus, however, shows cause and effect well, from the previous getter state to the following state in presenting hydriding/dehydriding performance. In case of the ZrCo–H2 system or in case of the DU–H2 system having multiple getter bed battery, a similar affection by previous getter status might be related and a similar aspect could be shown to consider further ITER design; for example, a need for control logic from PCI measurements using a high-pressure Sievert apparatus.

Complex transition metal hydrides incorporating ionic hydrogen: thermal decomposition pathway of Na2Mg2FeH8 and Na2Mg2RuH8.

Complex transition metal hydrides have potential technological application as hydrogen storage materials, smart windows and sensors. Recent exploration of these materials has revealed that the incorporation of anionic hydrogen into these systems expands the potential number of viable complexes, while varying the countercation allows for optimisation of their thermodynamic stability. In this study, the optimised synthesis of Na2Mg2TH8 (T = Fe, Ru) has been achieved and their thermal decomposition properties studied by ex situ Powder X-ray Diffraction, Gas Chromatography and Pressure-Composition Isotherm measurements. The temperature and pathway of decomposition of these isostructural compounds differs considerably, with Na2Mg2FeH8 proceeding via NaMgH3 in a three-step process, while Na2Mg2RuH8 decomposes via Mg2RuH4 in a two-step process. The first desorption maxima of Na2Mg2FeH8 occurs at ca. 400 °C, while Na2Mg2RuH8 has its first maxima at 420 °C. The enthalpy and entropy of desorption for Na2Mg2TH8 (T = Fe, Ru) has been established by PCI measurements, with the ΔHdes for Na2Mg2FeH8 being 94.5 kJ mol(-1) H2 and 125 kJ mol(-1) H2 for Na2Mg2RuH8.

Shape-dependent hydrogen-storage properties in Pd nanocrystals: which does hydrogen prefer, octahedron (111) or cube (100)?

Pd octahedrons and cubes enclosed by {111} and {100} facets, respectively, have been synthesized for investigation of the shape effect on hydrogen-absorption properties. Hydrogen-storage properties were investigated using in situ powder X-ray diffraction, in situ solid-state (2)H NMR and hydrogen pressure-composition isotherm measurements. With these measurements, it was found that the exposed facets do not affect hydrogen-storage capacity; however, they significantly affect the absorption speed, with octahedral nanocrystals showing the faster response. The heat of adsorption of hydrogen and the hydrogen diffusion pathway were suggested to be dominant factors for hydrogen-absorption speed. Furthermore, in situ solid-state (2)H NMR detected for the first time the state of (2)H in a solid-solution (Pd + H) phase of Pd nanocrystals at rt.