Crystallographic hydride phase analysis and hydrogenation properties of Gd2Co7 with Ce2Ni7- and Er2Co7-type structures

We synthesized an intermetallic compound Gd₂Co₇ with Ce₂Ni₇- and the Er₂Co₇-type structures. Gd₂Co₇ with a Ce₂Ni₇-type structure was stable at 1523 K. The refined lattice parameters were a = 0.5026(1) nm and c = 2.4266(6) nm. The pressure-composition (P–C) isotherm of Ce₂Ni₇-type Gd₂Co₇ indicated that reversible hydrogen capacity reached 0.61 H/M (0.76 mass%), and two plateaus were observed in the absorption-desorption process. The crystal structures of both Gd₂Co₇H_2.5 and Gd₂Co₇H_5.5 were determined to be Ce₂Ni₇-type by X-ray diffraction.Gd₂Co₇ with Er₂Co₇-type structure is stable at temperatures below 1473 K. The refined lattice parameters were a = 0.5029(1) nm and c = 3.6403(9) nm. The determined crystal structure of Gd₂Co₇H_2.3 and Gd₂Co₇H_5.4 was Er₂Co₇-type. The P–C isotherm of Er₂Co₇-type Gd₂Co₇ was similar to that of Ce₂Ni₇-type Gd₂Co₇. In this study, the P–C isotherms of a polymorphic binary alloy with Ce₂Ni₇-type and Er₂Co₇-type structures were obtained for the first time.     

Cyclic stability and high rate discharge performance of (La,Mg)5Ni19 multiphase alloy

The cyclic stability and high rate discharge performance of (La,Mg)₅Ni₁₉ multiphase alloy were investigated in this work. The results show that the alloy is composed of Pr₅Co₁₉-type (2H), Ce₅Co₁₉-type (3R), CaCu₅-type and Ce₂Ni₇-type phase after annealing at 1123 K. The total phase abundance of Pr₅Co₁₉-type (2H) and Ce₅Co₁₉-type (3R) is 63.3%. That composition decides the good cyclic stability both in repeated hydriding/dehydriding and charging/discharging process for this alloy. Moreover, the alloy shows the higher high rate discharge capacity at room temperature and it remains 140 mA h/g at the current density of 3600 mA/g.     

Hydrogenation and structural properties of Gd2Ni7 with superlattice structure

The crystal structure and hydrogenation properties of Ce₂Ni₇-type Gd₂Ni₇ were investigated by X-ray diffraction (XRD) and the hydrogen pressure–composition (P–C) isotherm. Ce₂Ni₇-type Gd₂Ni₇ was obtained by annealing at 1523 K for 12 h and quenching in ice water. Two superlattice reflections (002 and 004) of the Ce₂Ni₇-type were clearly observed at 2θ = 7.3° and 14.6° in the XRD profile. The refined lattice parameters were a = 0.49662(9) nm and c = 2.4255(3) nm, respectively. Two plateaus were clearly observed during the absorption–desorption process in the P–C isotherm. The first and second plateaus were at 0.015 and 0.13 MPa, respectively, in the first desorption. The maximum hydrogen capacity reached was 1.13 H/M. The enthalpy and entropy were calculated as −20 kJ/mol H₂ and −80 J/mol H₂ K, respectively, from the van’t Hoff plot. After the P–C isotherm, the GdNi₅ cell expanded by 2.15%, but the Gd₂Ni₄ cell shrank by 2.83%.     

Crystal structure and hydrogen storage property of Nd2Ni7 superlattice alloy

This study investigates the crystal structure and Pressure–composition (P–C) isotherm of Nd₂Ni₇ prepared by annealing an arc-melted ingot at 1448 K for 10 h followed by ice-water quenching. The crystal structure was further refined by X-ray Rietveld analysis based on the Ce₂Ni₇-type structure. The lattice parameters were determined as a = 0.5001(1) nm and c = 2.4437(4) nm. A single plateau was observed during the first absorption–desorption cycle. In the first absorption cycle, the maximum hydrogen capacity reached 1.22 H/M (1.58 mass%) at 233 K. The absorption and desorption plateau pressures were approximately 1.0 and 0.002 MPa, respectively. In the first desorption process, 0.63 H/M of hydrogen remained in the sample. Further, a single sloping plateau was observed in the second absorption–desorption process. Heavy peak broadening was observed in the X-ray diffraction (XRD) profile after hydrogenation, with no detection of an amorphous phase.     

Crystal structure and hydrogen absorption− desorption property of La5Co19

An intermetallic compound, La₅Co₁₉ is synthesized successfully for hydrogen storage, and its crystal structure is determined by X-ray diffraction. The alloy is formed by annealing the precursor at 1073 K for 10 h, and it has a Ce₅Co₁₉-type structure (space group R-3m, 3R) with a = 0.5130(1) nm and c = 4.882(1) nm. Its maximum hydrogen capacity reaches 0.92 H/M, but 0.40 H/M of hydrogen remains in the sample after the first desorption. Its reversible hydrogen capacity is 0.51 H/M. The formed hydride phases, phase I (La₅Co₁₉H₁₀) and phase II (La₅Co₁₉H₂₂) also have the Ce₅Co₁₉-type crystal structure; the hydride phases retain the same metal sublattice as that of the original alloy. Phase I is formed through anisotropic expansion of the La₅Co₁₉ lattice, while the unit cell, the MgZn₂-type and CaCu₅-type cells, of phase II is formed by the isotropic expansion of the La₅Co₁₉ lattice.     

Microstructure and electrochemical investigations of La0.76−xCexMg0.24Ni3.15Co0.245Al0.105 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys

The effects of substitution of Ce for La on the microstructure and electrochemical performance of La_0.76−xCe_xMg_0.24Ni_3.15Co_0.245Al_0.105 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses showed that the main phases of the alloys consist of (La, Mg)Ni₃ phase (PuNi₃-type rhombohedral structure), LaNi₅ phase (CaCu₅-type hexagonal structure) and (La, Mg)₂Ni₇ phase (Ce₂Ni₇-type hexagonal structure). The cell volume of the (La, Mg)Ni₃ phase, (La, Mg)₂Ni₇ phase and LaNi₅ phase decreased monotonously with increasing Ce content. Electrochemical investigations showed a decrease in the discharge capacity, while high rate dischargeability (HRD) first increased and then decreased with increasing Ce content. The Ce substitution for La slightly enhanced the cyclic stability of the alloy electrodes. The pressure–composition (P–C) isotherms showed that the plateau region was broadened with Ce content increased in the alloys, meanwhile, two plateaus appeared and pressure of the hydrogen absorption and desorption increased accordingly.     

Crystal structure of intermetallic compound Y5Co19 and its hydride phases

Y–Co intermetallic compounds and their hydrides were investigated as magnetic materials, but the hydrogenation of these alloys was not observed. We established the existence of Y₅Co₁₉ in the phase diagrams of the Y–Co system. Y₅Co₁₉, an intermetallic compound, was synthesized and its crystal structure was determined by the Rietveld refinement of X-ray diffraction (XRD) data. The structural model of Y₅Co₁₉ is a Ce₅Co₁₉-type model with lattice parameters a = 0.4994 (1) nm and c = 4.800 (1) nm. The crystal structure of the original alloy and its hydride phase is closely related to the hydrogen absorption–desorption property. The hydrogen capacity of Y₅Co₁₉ was 0.63 H/M, while two plateaus were observed in the P–C isotherm. We discovered two hydride phases: Y₅Co₁₉H_3.8 (phase I) and Y₅Co₁₉H_14.9 (phase II), and the structural model of these phases (I and II) was Ce₅Co₁₉-type, which was the same as the original alloy. The unit cell of phase I showed an expansion of 3.9% only along the c-axis from the original alloy, whereas that of phase II indicated an expansion along the a- and c-axes. Additionally, a close isotropic expansion was observed in phase II.     

Effect of Al content on the structural and electrochemical properties of A2B7 type La–Y–Ni based hydrogen storage alloy

LaY_1.9Ni_10.2−xAl_xMn_0.5 (x = 0–0.6) hydrogen storage alloys have been prepared using a vacuum induction-quenching furnace and annealed at 1148 K for 16 h. The alloys are composed of Ce₂Ni₇- and Gd₂Co₇-type phases and an extra Pr₅Co₁₉-type phase appears when x = 0.6. Aluminum tends to enter the inner AB₅ slabs of Ce₂Ni₇- and Gd₂Co₇-type phases and promotes the generation of new AB₅ slabs. The maximum discharge capacity of the alloy electrodes is stable at approximate 375 mA h/g as x increases from 0 to 0.4 and then decreases to 364.2 mA h/g (x = 0.6). The cycling capacity retention rate at the 300^th cycle is 59.4%, 62.0%, 62.7% and 58.7% for x = 0, 0.2, 0.4 and 0.6, respectively, indicating that the function of aluminum on improving the cyclic stability of the alloy electrodes is limited. The main reason is that the similar pulverization degrees of the alloys are presented during the charge/discharge cycles.     

Study on the structure and hydrogen absorption-desorption characteristics of as-cast and annealed La0.78Mg0.22Ni3.48Co0.22Cu0.12 alloys

La_0.78Mg_0.22Ni_3.48Co_0.22Cu_0.12 alloy is one kind of nonstoichiometric AB_3.5 type hydrogen storage alloy with low cost and high capacity. In this paper, the effect of annealing treatment on the structure and hydrogen absorption-desorption characteristics of the alloy is discussed. The annealing temperature is determined by using thermogravimetry-differential scanning calorimetry (TG-DSC) tests. The structure of as-cast and annealed alloys is examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD results show that the crystal cell volume decreases after annealing treatment. SEM tests reveal that the structure of the annealed alloys is more regular than that of the as-cast alloy. Influences of annealing treatment on maximum and reversible hydrogen storage capacity, hysteresis factor (H_f) between hydrogen absorption and desorption, sloping factor (S_f), enthalpy changes (ΔH) and entropy changes (ΔS) of hydrogen absorption are discussed in detail.     

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.     

Development of sample holder for in situ neutron measurement of hydrogen absorbing alloy

We developed a sample holder for in situ measurement of hydrogen absorbing alloy. In order to prevent the hydrogen absorption by vanadium, copper is coated with 2 μm thickness on inner surface of the vanadium holder. The effect of copper coating and the performance of the holder were evaluated by neutron diffraction and PDF profiles. The lattice parameters a and c of La₂Ni₇ with Ce₂Ni₇-type structure were refined as 0.505921(4) and 2.468608(4) nm by Rietveld analysis. The Cu-Cu correlation peak around r = 0.255 nm was not observed in the PDF profile. Thus the holder is useful for in situ measurement of hydrogen absorbing alloy. The diffraction and PDF profiles of La₂Ni₇D_x (0 < x < 10.5) were collected using a deuterium pressure of 3.7 MPa, and the changes of crystal and local structures were clearly observed.     

High temperature performance of La0.6Ce0.4Ni3.45Co0.75Mn0.7Al0.1 hydrogen storage alloy for nickel/metal hydride batteries

La_0.6Ce_0.4Ni_3.45Co_0.75Mn_0.7Al_0.1 hydrogen storage alloy has been prepared and its electrochemical characteristics and gas hydrogen absorption/desorption properties have been investigated at different temperatures. X-ray diffraction results indicated that the alloy consists of a single phase with CaCu₅-type structure. It is found that the investigated alloy shows good cycle performance and high-rate discharge ability, which display its promising use in the high-power type Ni-MH battery. The exchange current density and the diffusion coefficient of hydrogen in the bulky electrode increase with increasing temperature, indicating that increasing temperature is beneficial to charge-transfer reaction and hydrogen diffusion. However, the maximum discharge capacity, the charge retention and the cycling stability degrade with the increase of the temperature.     

Phase formation of Ce5Co19-type super-stacking structure and its effect on electrochemical and hydrogen storage properties of La0.60M0.20Mg0.20Ni3.80 (M = La,Pr, Nd,Gd) compounds

In order to investigate the formation mechanism of Ce₅Co₁₉-type super-stacking structure phase, La_0.60M_0.20Mg_0.20Ni_3.80 (M = La, Pr, Nd, Gd) compounds are synthesized by powder sintering method. Rietveld refinements of X-ray diffraction patterns find that La_0.80Mg_0.20Ni_3.80 compound has a single Pr₅Co₁₉-type structure. The Ce₅Co₁₉-type phase appears and increases with the decrease of atomic radius of M, until the La_0.60Gd_0.20Mg_0.20Ni_3.80 compound shows a Ce₅Co₁₉-type single phase structure. The cycling stability and high rate dischargeability (HRD) of the alloy electrodes both improve with the increase of Ce₅Co₁₉-type phase. The capacity retention of La_0.60Gd_0.20Mg_0.20Ni_3.80 compound at the 100th cycle is high to 93.6% and the HRD reaches 66.9% at a discharge current density of 1500 mA g^?1. Moreover after 50 charge/discharge cycles, the Ce₅Co₁₉-type particle retains an intact crystal structure while severe amorphization occurs to Pr₅Co₁₉-type particle as shown in graphical abstract. The cohesive energy obtained from the First-principle calculations is analyzed combined with the experimental results. It is found that the La_0.60Gd_0.20Mg_0.20Ni_3.80 compound with Ce₅Co₁₉-type single phase structure has the highest cohesive energy indicating a more stable structure. This work provides new insights into the superior composition-structure design of LaMgNi system hydrogen storage alloys that may improve the cycling stability.     

Effect of substituting Mn for Ni on the hydrogen storage and electrochemical properties of ReNi2.6−xMnxCo0.9 alloys

ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys were prepared by induction melting. The effects of partially substituting Mn for Ni on the phase structure and electrochemical properties of the alloys were investigated systematically. In the alloys, (La, Ce)₂Ni₇ phase with a Ce₂Ni₇-type structure, (Pr, Ce)Co₃ phase with a PuNi₃-type structure, and (La, Pr)Ni₅ phase with a CaCu₅-type structure were the main phases. The (La,Pr)Ni phase appeared when x increased to 0.45, and the (La, Pr)Ni₅ phase disappeared with further increasing x (x > 0.45). The hydrogen-storage capacity of the ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys initially increased and reached a maximum when Mn content was x = 0.45, and then decreased with further increasing Mn content. The ReNi_2.6−xMn_xCo_0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloy exhibited a hydrogen-storage capacity of 0.81, 0.98, 1.04, 0.83 and 0.53 wt.%, respectively. Electrochemical studies showed that the maximum discharge capacity of the alloy electrodes initially increased from 205 mAh/g (x = 0.0) to 352 mAh/g (x = 0.45) and then decreased to 307 mAh/g (x = 90). The hydrogen absorption rate first increased and then decreased with addition of Mn element. The ReNi_2.15Mn_0.45Co_0.9 alloy showed faster hydrogen absorption kinetics than that of the other alloys. The presence of Mn element slowed hydrogen desorption kinetics.     

The morphology and electrochemical properties of La1-xMgxNi3.4Al0.1 (x = 0.1–0.4) hydrogen storage alloys

In view of the importance of Mg in R-Mg-Ni-type alloys (R generally represents rare earth elements), the effect of Mg on the morphology and electrochemical performance of the as-cast La_1-xMg_xNi_3.4Al_0.1 (x = 0.1, 0.2, 0.3 and 0.4) hydrogen storage alloys were studied in this work. The samples possess multiphase structures, including Gd₂Co₇-, Ce₂Ni₇-, Pr₅Co₁₉-and CaCu₅-type, PuNi₃-and MgCu₄Sn-type phases. It is found that reasonably increasing Mg contents can promote the formation of Gd₂Co₇-and Ce₂Ni₇-type phase as well as Mg contents have important effects on phase morphology. Furthermore, fine-dispersed LaNi₅ structure in (La, Mg)₂Ni₇ matrix is beneficial to facilitate the hydrogen diffusion and exert the electrochemical properties for the alloys.The EIS results indicate that the charge transfer resistance decreases nonlinearly with increase of (La, Mg)₂Ni₇ phase content and presents approximately linear relationship with LaNi₅ phase content. When x equals to 0.2, the alloy displays more optimal comprehensive electrochemical properties, i.e. the discharge capacity reaches 357.4 mA/g, the high rate dischargeability at 1200 mA/g 60.1% and the cycling performance 74.5%.     

Characteristics of A2B7-type LaYNi-based hydrogen storage alloys modified by partially substituting Ni with Mn

LaY₂Ni_10.5?xMn_x (x = 0.0, 0.5, 1.0, 2.0) alloys are prepared by a vacuum induction-quenching process followed by annealing. The structure, as well as the hydriding/dehydriding and charging/discharging characteristics, of the alloys are investigated via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), pressure-composition isotherms (PCI), and electrochemical measurement. The alloys have multiphase structures mainly composed of Gd₂Co₇-type (3R) and Ce₂Ni₇-type (2H) phases. Partial substitution of Ni by Mn clearly increases the hydrogen storage capacity of the alloys. The x = 0.5 alloy exhibits a maximum hydrogen storage capacity of 1.40 wt % and a discharge capacity of 392.9 mAh g^?1, which are approximately 1.5 and 1.9 times greater than those of the x = 0.0 alloy, respectively. The high-rate dischargeability (HRD) of the x = 0.5 alloy is higher than that of the other alloys because of its large hydrogen diffusion coefficient D, which is a controlling factor in the electrochemical kinetic performance of alloy electrodes at high discharge current densities. Although the cyclic stability of the x = 0.5 alloy is not as high as that of the other alloys, its capacity retention ratio is as high as 56.3% after the 400th cycle. The thermodynamic characteristics of the x = 0.5 alloy satisfy the requirements of the hydride electrode of metal hydride–nickel (MH–Ni) batteries.     

Effects of Mg substitution on crystal structure and hydrogenation properties of Pr1−xMgxNi3

The effects of substitution of Pr by Mg in PrNi₃ with a PuNi₃-type structure were investigated using pressure–composition (P–C) isotherm measurements and X-ray diffraction. The unit cell of Pr_0.68Mg_0.32Ni_3.04 contracted anisotropically in comparison to that of PrNi₃. The maximum hydrogen capacity of PrNi₃ reached 1.25 H/M in the first absorption. A plateau region was observed between 0.82 H/M and 1.04 H/M in the first absorption cycle. However, 0.85 H/M of hydrogen remained in the sample after the first full desorption. Pr_0.68Mg_0.32Ni_3.04 showed reversible hydrogenation properties. The maximum hydrogen capacity was 1.22 H/M. The plateau region of Pr_0.68Mg_0.32Ni_3.04 was between 0.08 H/M and 0.87 H/M, which was wider than that of PrNi₃. Pr_0.68Mg_0.32Ni_3.04 retained the PuNi₃-type structure after hydrogenation, whereas the crystal structure of PrNi₃ changed from that of PuNi₃-type to an unknown structure. The structural change in PrNi₃ during hydrogenation was evidently different from that in Pr_0.68Mg_0.32Ni_3.04.     

Microstructures and hydrogen absorption–desorption behavior of an A2B7-based La-Mg-Ni alloy

The microstructural changes during hydrogen absorption–desorption cycles of an A₂B₇-based La-Mg-Ni alloy with a nominal composition of La_1.5Mg_0.5Ni_7.0 were systematically investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The ternary La-Mg-Ni alloy was mostly composed of 2H-A₂B₇ phase with minor inclusions of 3R-A₅B₁₉, 2H-A₅B₁₉ and 3R-AB₃ phases existing as parts of intergrowth structures with the major A₂B₇ phase. Most parts of the major 2H-A₂B₇ phase containing Mg exhibited an excellent crystal structure retention after the hydrogen absorption–desorption cycles at 80 °C. Two types of defected bands were found to develop after the first hydrogen absorption–desorption cycle. The first ones are amorphous bands developed inside the minor 3R-AB₃ phase, while the second ones develop as heterogeneously strained regions inside the major 2H-A₂B₇ phase. Both the defected bands are considered to be responsible for the irreversible hydrogen capacity of the A₂B₇-based La_1.5Mg_0.5Ni_7.0 alloy during the hydrogen absorption–desorption cycles at 80 °C.     

Solid solubility of Mg in Ca2Ni7 and hydrogen storage properties of (Ca2−xMgx)Ni7 alloys

The phase relations and hydrogen storage properties of the (Ca_2−xMg_x)Ni₇ alloys were investigated. It was found that the maximum solid solubility of Mg in the (Ca,Mg)₂Ni₇ phase is about x = 0.5 in the present study. The ‘inter-block-layer’ type stacking faults exist in the (Ca,Mg)₂Ni₇ phase when Mg content is very low. However, the density of stacking faults decreases and the lattice parameters reduce as Mg content increases to its maximum solid solubility. Thus the (Ca_1.5Mg_0.5)Ni₇ alloy has a good reversibility of hydrogen absorption–desorption.     

Hydrogen storage and electrochemical properties of annealed low-Co AB5 type intermetallic compounds

The structure, hydrogen storage and electrochemical properties of annealed low-Co AB₅-type intermetallic compounds have been investigated. La-alloy, Nd-alloy and Cr-alloy are used to represent La_0.8Ce_0.2Ni₄Co_0.4Mn_0.3Al_0.3, La_0.6Ce_0.2Nd_0.2Ni₄Co_0.4Mn_0.3Al_0.3 and La_0.6Ce_0.2Nd_0.2Ni_3.8Co_0.4Mn_0.3Al_0.3Cr_0.2, respectively. The XRD results indicated that annealed samples are all single-phase alloys with CaCu₅ type structure. The maximum of both hydrogen content and discharge capacity is obtained for La-alloy 1.23 wt%H₂ and 321.1 mA h/g, respectively. All the investigated alloys are quiet stable with ΔH of hydrogen desorption about 36–38 kJ/mol H₂. Cycle life of alloy electrode has been improved by partial substitution of La for Nd and Ni for Cr. The highest capacity retention of 92.2% after 100 charge/discharge cycles at 1C has been observed for Nd-alloy. The hydrogen diffusion coefficient measured by PITT is higher at the start of charging process and dramatically reduces by 2–3 order of magnitude with saturation of β-hydride. The highest value 6.9 × 10^?13 cm²/s is observed for La alloy at 100% SOC. Partial substitution La for Nd and Cr for Ni in low-Co AB₅ metal hydride alloys slightly reduces maximum discharge capacity, HRD performance and hydrogen diffusion kinetics. Low-Co alloys show good overall electrochemical properties compared to high-Co alloys and might be perspective materials for various electrochemical applications.