Search

The difference in electronic structure between the β-phase (space group: Pa- 3) and the γ-phase ( P- 43 n ) of Li 7 VN 4 is studied. Li 7 VN 4 crystallizes in the γ-phase with all the VN 4 tetrahedral clusters isolated or the β-phase with all the VN 4 formed into the isolated pairs of two neighbor VN 4 clusters jointed at their corners, depending on the distribution of V atoms in a zinc blende GaN-like Li 7 VN 4 framework. γ- and β-Li 7 VN 4 have similar optical band gaps (1.13 eV and 1.10 eV, respectively). β-Li 7 VN 4 shows a metallic electron configuration due to the partially occupied V-3d related valence band as a result of the V-3d orbital spreading into the neighbor VN 4 clusters, while γ-Li 7 VN 4 possesses a semiconducting configuration with the band gap between the completely occupied N-2p valence band and the V-3d related conduction band. γ-Li 7 VN 4 shows a much higher electric resistivity (∼10 8 Ω cm) than that of β-Li 7 VN 4 (∼10 4 Ω cm), which reflect the difference in the electronic structure between γ- and β-Li 7 VN 4 .

The band gap and disordered structure in Li 8 GeN 4 (lattice constant: 9.622 A) are studied by optical absorption, photoacoustic spectroscopy, X-ray diffraction, and Raman scattering spectroscopy. Li 8 GeN 4 is a semiconductor with the band gap of 2.61 eV, suggesting the transition from the N-2p valence band to the conduction band mainly consisting of Ge-4s and/or -4p orbitals. A broad Raman peak is observed at 530 cm −1 , indicating the homogenously random distribution of Li and Ge atoms. A possible distribution of Li and Ge is proposed. Li 8 GeN 4 crystallizes in a superstructure of eight face-centered N sublattices. In each sublattice, one Li and one Ge atoms are randomly and diagonally occupied at two tetrahedral sites next to N, while six Li atoms reside at the other six tetrahedral sites next to N. The possible locations of one remaining Li atom are also discussed.

Fabrication and charge/discharge behaviors of all-solid-state individual Li ion secondary battery with a Li free anode and a polymer electrolyte are studied. The battery with a 300 × 300 μm2 area, consisting of multiple layers (Poly Si anode/a poly methacrylate acetate (PMMA)/LiMn2O4), is prepared by combining a sol–gel spin-coating method and Si very-large-scale integration technologies. The PMMA solid electrolyte containing LiClO4 for the battery is embedded into an individual trench (300 × 300 μm2 in area and 2.0 μm in depth) of a double insulating layer (Si3N4/SiO2), which is directly stacked on a poly Si layer, leading to a Li free anode. The individual battery demonstrates a specific capacity of 3.70 nAh/cm2 obtained for the charge of 3 nA and discharge of 10 pA. A Rutherford backscattering study of Cl in the PMMA electrolyte suggests that 2.3 × 1021 cm−3 of Li is included in the electrolyte.

Zr7Ni10 has three hydrogen occlusion phases, α, β and γ, and the following unusual features are known for the phase transitions in the Zr7Ni10–H2 system: (1) The intermediate hydride phase (β) appears only during dehydrogenation but not during hydrogenation, and (2) The continuous hydrogen solid solution phase (α) exhibits a much higher hydrogen solubility during hydrogenation than during dehydrogenation. In order to clarify the mechanism about the difference in the hydrogen solubility of the α phase, the relation between the pressure-composition isotherms and corresponding structural change has been examined by a conventional volumetric method and X-ray diffraction. Through the examination, we discovered that the crystal structure of the α phase, which undergoes hydrogenation followed by dehydrogenation, is different from that of its pure metal phase, where the crystal structure of the dehydrogenated α phase changes from an orthorhombic structure to a tetragonal structure. The conditions causing the structural change were then examined, and it has been found that the α phase maintains its original orthorhombic structure as long as it is hydrogenated so as not to absorb enough hydrogen to change it to the hydride with a higher hydrogen content (γ). The phenomenon can be understood as one of the hydrogen-assisted phase transitions such as hydrogen-induced amorphization (HIA) in the sense that the phase transition requires hydrogenation under special conditions.

Deterioration behavior of an electrode made of V 4 TiNi 0.65 Co 0.05 Nb 0.047 Cr 0.058 was studied by means of impedance spectroscopy, scanning electron microscopy (SEM) and impedance spectroscopy. The reaction resistance related to the lowest frequency semi-circle increased considerably and dischargeability became worse with cycling. The double layer capacitance for the same semi-circle became smaller after 50 cycles. The SEM observation of the cross-section of the cycled electrodes indicated that voids were formed around alloy particles embedded in a matrix of Cu powder, and crack formation and dissolution of the TiNi second phase proceeded with cycling. These phenomena indicate that dissolution of the second phase caused loss of reaction sites and TiNi networks as a current collector.

An evaluation of the substitution of a third element for the constituent elements (Ca, Ni) in the compound was conducted in order to obtain information about stabilization of the CaNi compound which does not exist as a stable phase. Prior to the experimental study of the substitution, the candidates for the third element were investigated using Miedema’s semi-empirical model, from the standpoint of formation enthalpy, and as a result, Pd was chosen as the element to stabilize the compound. The samples with the nominal compositions of CaNi1−xPdx (0.1≤x≤1) were prepared by sintering and high frequency heating, and the constituent phases and hydrogenation properties were examined. New ternary phases such as Ca1(Ni, Pd)1 were not obtained under the present conditions; however, some pieces of new information such as the substitution of Ni for Pd in CaPd2 and reversible hydrogenation of CaPd at 373 K were obtained through the studies.

X-ray diffraction patterns of NaAlH4 doped with up to 10 mol.% of either titanium or zirconium do not contain Bragg peaks for the bulk metals or their aluminum alloys. Instead the hydride lattice parameters a and c undergo significant contraction upon 2 mol.% doping and then expand as the doping level increases from 2 to 5 mol.%. These results are explained by a model that entails substitution of sodium cations by variable valence transition metal cations and the creation of Na+ vacancies in the bulk hydride lattice.

The hydrogenation properties of CaNi 3 were investigated at temperatures ranging from 298 to 773 K by differential thermal analysis (DTA). This compound exhibited four exothermic peaks at 373, 498, 543 and 653 K followed by an endothermic peak at 748 K when the compound was heated to 773 K under a hydrogen atmosphere of 3 MPa, and its final products were CaH 2 and Ni. However, the formation of a new ternary hydride was observed in the X-ray diffraction (XRD) profiles below the endothermic reaction temperature at which the final products were formed. The Rietveld refinement of the obtained XRD profiles and a volumetric measurement of the hydrogen content based on the Sieverts’ method indicated that this new phase was CaNiH 3 with a cubic CsCl type structure for the metal constituents. The experimental data obtained by XRD and DTA were discussed from the viewpoint of the disproportionation of CaNi 3 in the hydrogen atmosphere.

A volumetric apparatus for gas phase hydrogen/helium adsorption and desorption measurement aimed at carbon materials was constructed. The performance of the apparatus was assessed using activated carbons and vapor grown carbon nanofibers, and was proved to be applicable for these materials of low apparent densities with sufficient accuracy. Materials used in this study did not show a significant storage capacity of hydrogen. The obtained result, however, will provide reference data for future study to develop the carbon material for hydrogen storage.

We report the evaluation of a novel air–metal hydride rechargeable battery. A copper plated MmNi3.5Co0.7Al0.7Mn0.1 alloy was used as the active material in the metal hydride electrode. The metal hydride electrode was charged and discharged via a gas diffusion electrode based on a La0.6Ca0.4CoO3 perovskite (supported on carbon) as the catalyst for both oxygen reduction (discharge) and oxygen evolution (charge). In a laboratory cell, measurements on a 2.5×2.5 cm metal hydride electrode sandwiched between two air-electrodes of the same size were performed in a 6 M KOH electrolyte at 25°C with oxygen or air supply. A very long cycle life of 250 cycles at 1C charge and discharge (1 h charge and 1 h discharge) was achieved. After 250 cycles, the capacity of the metal hydride electrode had decreased by about 10%, from 268 mAh g−1 to 242 mAh g−1. After more than 220 cycles, the discharge voltage of the cell was 0.64 V for oxygen and 0.55 V for air. At 10 mA cm−2 (approx. 2 h charge and 2 h discharge) the cell voltage was 1.57 V for charge and 0.77 V for discharge (using oxygen), measured after more than 100 cycles. Peak power was 51.0 mW cm−2 for oxygen and 34.8 mW cm−2 for air. These values were measured at currents of 150 mA cm−2 for oxygen and at 100 mA cm−2 for air.

We studied reproducibility of discharge capacity, dischargeability, activation behavior, activity and porosity for charge–discharge reaction of a pellet-type electrode consisting of one part by weight of Ti 0.5 Zr 0.5 Ni 1.3 V 0.7 Mn 0.1 Cr 0.1 and three parts of the copper powder. We examined electrodes immediately after preparation and electrodes stored for 1 month under the following conditions: vacuum, dry air and humid air. Electrolyte impregnation in vacuo was found to be very effective in improving the reproducibility of electrochemical measurements, reducing measuring effects due to the storage conditions of the electrodes.

Ternary Ti4Ni2X (X=O, N, C) compounds and a binary Ti2Ni compound were compared based on their hydrogen pressure–composition–temperature (PCT) relations. It was demonstrated that the ternary compounds desorbed most of the absorbed hydrogen under moderate conditions such as room temperature and atmospheric pressure. The hydrogen desorption pressures of the Ti4Ni2X compounds were more than two orders of magnitude higher than the desorption pressure of the binary compound. In spite of the significant increase in the hydrogen desorption pressure, the slope of the PCT curve of each ternary compound was not large compared with that of the Ti2Ni one. These four Ti2Ni-based compounds and their corresponding hydrogen occlusion ones were discussed in terms of their relative thermal stability.

Electrical resistance of the Mg 2 Ni and Mg 2 NiH 4 powders was measured under compressing up to 6 gigapascals (GPa), using the four-point probe method at 20°C. Effects induced by the high-pressure treatment and heating at 400°C with or without hydrogen source LiAlH 4 were also studied. The structure and hydrogenation properties of the samples recovered after quenching from high pressure and high temperature were investigated employing the techniques of X-ray diffraction and thermal analyses such as differential scanning calorimetry, thermogravimetry and differential thermal analysis. Decreased absorption/desorption temperatures were observed for the treated samples owing to the changes of microstructure and kinetic factor.

Phase relations in the Zr7Ni10-H system were examined with a conventional Sieverts’-type apparatus. The hydrogen pressure–composition–temperature (PCT) relations obtained under the conditions such as a temperature of 410 K and a pressure range of 0.01–3 MPa demonstrated the existence of two hydride phases (β, γ) in the system and indicated the following unprecedented hydrogenation behavior of the Zr7Ni10 compound: (1) the β-hydride phase, which has lower hydrogen content than the other (γ), appears only in hydrogen desorption but not in hydrogen absorption; (2) the hydrogen capacity of the continuous solid solution phase (α) is 0.56H/M (H/M; the atomic ratio of hydrogen to metal) in hydrogen absorption and only 0.40H/M in hydrogen desorption; (3) The Zr7Ni10 compound absorbs hydrogen accompanied by the phase transition from α to γ (α→γ transition) and desorbs hydrogen accompanied by the γ→β and β→α transitions, however, the reverse of these three transitions, i.e. γ→α, β→γ and α→β transitions are not observed in the PCT relations under the conditions mentioned above. From the fact about the irreversibility of the phase transitions, the Zr7Ni10 compound can reversibly absorb and desorb hydrogen by the irreversible phase transition.

The effects of substitution of La by Ce, Nd and Pr in the LaNi5-based alloys were systematically studied, with the use of factorial analysis. The changes of equilibrium pressure, hydrogen capacity and hysteresis were analyzed in relation to the structure changes of the hexagonal unit cell of (La,Ce,Nd,Pr)Ni5. Both individual effects of each alloy component, and their interaction in the alloy were studied. It has been shown that variations in the basal plane parameter (a) can be used as an indication for the plateau pressure changes resulting from substitution, giving better fits than variations in the unit cell volume. Structural anisotropy is discussed as a possible tool for the prediction of the decrepitation resistance of the alloy. Methods for optimization of the electrode alloys (including reduction of the cobalt content) are proposed.

Alloy samples with a single phase structure of Ti 4 Cu 2 O were prepared and their hydrogenation characteristics were investigated. Pressure–composition isotherms of hydrogen absorption and desorption at 273, 303 and 313 K for the oxometallic alloys were measured over the range of hydrogen pressure from 0.004 to 2 MPa. The Ti 4 Cu 2 O alloys absorbed hydrogen up to 0.47 in the atomic ratio of hydrogen to metal at 273 K and under 2 MPa and reversibly desorbed at 313 K more than half of hydrogen absorbed. No distinct pressure plateau was observed in the isotherms. For all the samples after hydrogenation at temperatures and pressures below 523 K and 3 MPa, no phase other than Ti 4 Cu 2 O was observed in the X-ray diffraction profiles. The thermodynamic properties for the hydrogenation of the alloys were discussed.

Influences of carbon on microstructure and electrode properties were investigated for battery alloy V 4 TiNi 0.65 Co 0.05 Nb 0.047 Ta 0.047 . With increasing carbon concentration, the cycle-life of charge/discharge became longer. One of main factors that affected the cycle-life was the expansion and reduction of the (Ti,V)-based solid solution phase. Cracks yielded in TiNi phase more easily than in other phases. V-based battery alloys were more hardly pulverized than AB 5 and AB 2 intermetallic compound type materials.

LaNi5 powders were mechanically ground under argon atmosphere. After 2 h of milling, a limit particle size of 1.5 μm was obtained. This powder was easily activated under hydrogen atmosphere and the time for a 90% maximum hydrogen uptake was found to be close to that of the unmilled powder. The first hydrogenation cycle of the milled powder shows a strong slope indicating the presence of several hydrides. The hydrogenated ground powder was stabilized with a carbon monoxide surface treatment that markedly slows down the desorption kinetics. From the ball milled and hydrogenated powder, the formation of the intermediate phase LaNi5Hx with x≈3 is observed, contrarily to what is found with hydrogenated but unground powders, such a different behavior was questioned in terms of electrochemical conditions, as milled or unmilled LaNi5 powders are intensively used as the negative electrode of reversible metal hydrogen batteries. The unmilled but activated LaNi5 electrode exhibits the lowest discharge capacity but the best cycle life time (loss of only 5% of the discharge capacity after 50 cycles). The powder milled for only 1 h before activation does not present any intermediate hydride and shows a better discharge capacity. The best performance is obtained for the powder containing the intermediate phase with a discharge capacity of 307 mA h g−1. Nevertheless, the loss in capacity after 50 cycles is large (24%).

The influence of the carbon impurity on the microstructure, hydrogen storage, and electrochemical performance of alloy V4TiNi0.65Co0.05Nb0.047Ta0.047 were investigated by varying the weight percent of added carbon in the range of 0–1%. The hydrogen storage ability and discharge capacity were significantly decreased in the presence of a smaller content carbon. This is ascribed to the solution of carbon into the V–Ti-based main phase. A new TiC phase was clearly observed when the added carbon content was over 0.6 wt%.

Three different cast LmNi 3.6 Co 0.7 Al 0.3 Mn 0.4 alloys (Lm=Lanthanum rich mischmetal) were used for the investigation. One alloy was rapidly cooled, a second was rapidly cooled and annealed and the third one was cast in a conventional way. Our main interest are the kinetic properties of these alloys, measured by means of Electrochemical Impedance Spectroscopy (EIS) and electrochemical charge discharge experiments. The rapidly cooled alloy sample showed better fast discharge behavior and smaller reaction resistance compared to annealed samples. In combination with X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) a possible explanation on the improved kinetics of the rapidly cooled electrode can be given: The smaller grain size and the high amount of grain boundary in the alloy leads to a faster activation and therefore to an enhanced alloy surface. This results in a smaller reaction resistance and an improved fast discharge behavior.

A new ternary magnesium-based alloy CaMg 2 Ni 9 has been synthesized, and its crystal structure was determined by Guinier–Hagg X-ray and neutron powder diffraction. The compound has a hexagonal structure and is isostructural with LaMg 2 Ni 9 type, with Ca at the La atom site. The hydrogen absorption/desorption properties have been determined by thermal analysis and pressure–composition isotherms. The electrochemical properties of the alloy were also examined.

The effects of KOH aqueous solution saturated with Co(OH)2 on LaNi4.7Al0.3 electrodes were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The electrochemical activity of an LaNi4.7Al0.3 electrode was considerably improved by a treatment with Co(OH)2-KOH solution below the potential of −0.8 V (vs. Hg/HgO) after several cycles in pure KOH electrolyte. These electrodes exhibited higher metallic-nickel and lower lanthanum concentrations in the subsurface region of the alloy than those without any treatment. This metallic-nickel enrichment was concluded to be caused by oxidation of metallic lanthanum with water catalyzed by Co(OH)42− ions followed by a dissolution of lanthanum hydroxide. In contrast, the electrode treated with the Co(OH)2-KOH solution without regulation of potential before cycling in KOH solution exhibited lower electrochemical activity and metallic-nickel concentration than that without any treatment. In this case, Co(OH)42− ions were considered to act as an oxidant to the alloy.

The hydriding behavior of pseudobinary CaAl 2− x M x (M = B, Si, 0 ⩽ x ⩽ 1) alloys with the C15 Laves phase structure is examined in the search for new metal hydrides for hydrogen storage. The initial activation for hydriding proceeded at an H 2 pressure of 3 MPa and at temperatures between 300 and 400°C without changing their phase structure or decomposing thermally. The CaAl 1.8 B 0.2 alloy absorbed the largest amount of hydrogen among these alloys, forming CaAl 1.8 B 0.2 H 0.6 at an H 2 pressure of 1 MPa at 60°C. For alloys with compositions around CaAl 1.8 M 0.2 , pressure plateaus were observed in the pressure-composition isotherms. Lattice contractions were also observed in these alloys.

Microstructures, pressure-composition isotherms, and charge-discharge characteristics of TiV 2 Ni x (0 ≦ x ≦ 0.75) alloys were investigated. The alloys for x ≧ 0.25 composed of a vanadium rich (βTi, V) main phase and a TiNi based b.c.c. secondary phase. The secondary phase formed a three dimensional network, enhancing electrode kinetics of the alloys. The maximum discharge capacity was 420 A h kg −1 for x = 0.56.

Crystal structures and pressure-composition isotherms were investigated for the vanadium rich ternary TiVNi solid solution. The alloy containing more than 85at.% vanadium absorbed hydrogen of 1.6 in H/M ratio that amounted to 3.2 mass% H. The pressure-composition isotherm for Ti 22 V 66 Ni 12 in the hydrogen pressure range of 10 0 to 10 −8 MPa was obtained by the electrochemical method. Two pressure plateaus were found at hydrogen pressures of 10 −2 MPa and 10 −6 MPa. The upper plateau would be ascribed to the reaction from dihydride to monohydride and the lower plateau would be ascribed to the reaction from monohydride to solid solution of the alloy and hydrogen.

Electrochemical impedance spectroscopy (EIS) was applied to metal hydride electrodes. Cole-Cole plots for the electrodes consisted of two obvious semicircles and a slope related to Warburg impedance. The semicircle in the high-frequency region was mainly related to the resistance and capacitance between the current collector and the pellet of alloy powder. The semicircle in the low-frequency region, which exhibited appreciable dependence on hydrogen content, was attributed to electrode reactions on the alloy particles and double-layer capacitance on the alloy particles. Resistance and capacitance between alloy particles in the electrodes also need to be taken into account. Deterioration processes of metal hydride electrodes using a mischmetal-based alloy, MmNi 3.5 Co 0.7 Al 0.8 , were also studied employing EIS. Deterioration of a metal hydride electrode using copper-coated alloy powder was dominated by a decrease in reactivity of the alloy surface. In contrast, an increase in the contact resistances and a decrease in the amount of electrochemically utilizable alloy particles were significant in the deterioration of electrodes using uncoated alloy powder. Deterioration of the electrodes was avoided to some extent by elevating the hot-press temperature during electrode preparation.