Hydrogen storage properties of flexible and porous La0.8Mg0.2Ni3.8/PVDF composite

A study on the hydrogen storage properties of flexible and porous La_0.8Mg_0.2Ni_3.8/PVDF (polyvinylidene fluoride) composite was reported. In this composite, PVDF acted as a binder to connect the alloy powders and (NH₄)₂CO₃ as a pore-forming agent to create void space. Increasing PVDF content, the hydrogen absorption kinetics of the composite gradually decreased. Increasing (NH₄)₂CO₃ from 1% to 5%, the capacity firstly increased and then decreased. 0.08–0.13 wt% increased capacity for the composite was observed at 70 °C by comparison with the intrinsic composite (La_0.8Mg_0.2Ni_3.8/1%PVDF). Varying temperature from 0 °C to 100 °C, 0.1–0.15 wt% increased capacity were obtained for the typical porous composite (La_0.8Mg_0.2Ni_3.8/1%PVDF/3%(NH₄)₂CO₃). The PVDF-assisted composite showed the flexible/solidified characteristic in hydriding/dehydriding, which maybe lowed down the oxidation of the alloy powders and preserved the void space. Finally, ∼0.1 wt% increased capacity remained after ten hydriding/dehydriding cycles.     

Microstructure and electrochemical hydrogen storage characteristics of (La0.7Mg0.3)1−xCexNi2.8Co0.5 (x = 0-0.20) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of (La_0.7Mg_0.3)_1−xCe_xNi_2.8Co_0.5 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys have been investigated. The results show that all alloys consist of (La, Mg)Ni₃ and LaNi₅ phases. The cyclic stability (S₁₀₀) of the alloy electrodes increases from 58.7% (x = 0) to 69.8% (x = 0.20) after 100 charge/discharge cycles. The high rate dischargeability (HRD) increases from 66.8% (x = 0) to 69.6% (x = 0.10), then decreases to 65.1% (x = 0.20) at the discharge current density of 1200 mA/g. Moreover, the electrochemical kinetic characteristics of the alloy electrodes are also improved by increasing Ce content.     

Comparative analysis of the hydriding kinetics of LaNi5, La0.8Nd0.2Ni5 and La0.7Ce0.3Ni5 compounds

In two-phase domains, the plateau pressure of hydride forming materials (such as intermetallic compounds or IMCs) depends markedly on the operating temperature (Van’t Hoff relationships). Therefore, for practical applications, it is necessary to select hydrogen storage materials by considering the thermal environment of the hydride tank. The thermodynamic properties (absorption and desorption pressure plateaux) of IMCs can be adjusted to some extend by chemical alloying with foreign metals and substitution on different crystallographic sites. In this paper, we report on the hydriding kinetics of substituted AB₅ compounds. Isotherms have been measured at different temperatures on La_xNd_1−xNi₅ (x ≈ 0.2) and La_xCe_1−xNi₅ (x ≈ 0.3) compounds. Pneumato-chemical impedance spectroscopy has been used to analyze the hydriding kinetics and to determine microscopic rate parameters associated with surface dissociation of molecular hydrogen, diffusion-controlled transport of atomic hydrogen to bulk regions and hydride formation. Results have been compared to those measured on LaNi₅ and the interest of using such substituted compounds for application in auxiliary power units is discussed.     

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.     

Influence of annealing treatment on the hydrogen storage properties of La2−xTixMgNi9 (x = 0.2, 0.3) alloys

La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys have been prepared by magnetic levitation melting under an Argon atmosphere, and the as-cast alloys were annealed at 800 °C, 900 °C for 10 h under vacuum. The effects of annealing on the hydrogen storage properties of the alloys were investigated systematically by XRD, PCT and electrochemical measurements. For the La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys, LaNi₅, LaMg₂Ni₉ and LaNi₃ are the main phases and a Ti₂Ni phase appears at 900 °C. The effective hydrogen storage capacity increases from 1.10, 1.10 wt.% (as-cast) to 1.22, 1.16 wt.% (annealed 800 °C) and 1.31, 1.27 wt.% (annealed 900 °C), respectively. The annealing not only improves the hydrogen absorption/desorption kinetics but also increases the maximum discharge capacity and enhances the cycling stability. The La_1.8Ti_0.2MgNi₉ alloy annealed at 900 °C exhibits good electrochemical properties, and the discharge capacities decrease from 366.1 mA h/g to 219.6 mA h/g after 177 charge-discharge cycles.     

Study of cyclic performance of V-Ti-Cr alloys employed for hydrogen compressor

The development of a suitable hydrogen compressor plays one of the key roles to realize the fuel cell vehicle as well as for many other stationary and mobile applications of hydrogen. V-Ti-Cr BCC alloys are considered as promising candidates for effective hydrogen storage. The cyclic durability of hydrogen absorption and desorption is very important for these alloys to be realized as practical options. In connection to this, two alloys of V-Ti-Cr, (1) V₄₀Ti_21.5Cr_38.5 and (2) V₂₀Ti₃₂Cr₄₈, were selected and their cyclic hydrogen absorption-desorption performance was evaluated up to 100 cycles for temperature and pressure ranges of 20–300 °C and 5–20 MPa, respectively. It has been found that the cyclic hydrogen storage capacity continuously decreased for one composition while it was stable after 10 cycles for another composition. This performance difference of the alloys was studied in terms of their structural and microscopic properties and the results are presented in this paper.     

Structural and hydrogenation properties of SrAl2−xNix alloys

In this paper, Al was partially substituted by Ni in the Zintl phase alloy SrAl₂ and the structural and hydrogenation characteristics of the SrAl_2−xNi_x (0 ≤ x ≤ 0.4) alloys were studied by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and hydrogenation measurements. The alloy consisted of a single Zintl phase of SrAl₂ when x = 0. However, partial substitution of Al by Ni resulted in multiphase structure of the alloys. When x = 0.1, the alloy was composed of SrAl₂, Sr₅Al₉, SrAl and AlNi phases. With the increase of x, the amount of SrAl₂ and Sr₅Al₉ phases decreased, while the amount of SrAl and AlNi phases increased. Hydrogenation measurements were made at 473 K under 3 MPa hydrogen pressure. It was interesting to find that the hydriding kinetics of the alloy was improved greatly after Ni substitution, which could be attributed to the catalysis of AlNi phase.     

Dynamic measurements of hydrogen reaction with LaNi5−xSnx alloys

The study focuses on the reaction between hydrogen gas and LaNi_5−xSn_x alloys, where 0 ≤ x ≤ 0.5, in broad temperature and pressure ranges. It was performed by means of dynamic volumetric techniques using specific equipment developed at our laboratory. The substitution of Ni by Sn lowers the system equilibrium pressure and increases the hydrogen absorption reaction rate. Reaction pressures at room temperature range from 8 kPa (x = 0.5) to 250 kPa (x = 0). At 415 K the reaction pressure is within the range from 200 kPa to 4000 kPa for x = 0.5 and 0, respectively. The measured characteristic absorption time at 750 kPa for LaNi₅ is around 1 min, while it remains below 2.5 s for LaNi_4.5Sn_0.5. The maximum H concentration goes from 1.3 wt.% for LaNi₅ down to 0.95 wt.% for LaNi_4.5Sn_0.5. These results are useful to identify a metal system where the hydrogen interaction equilibrium properties can be tuned in a wide pressure range by choosing the chemical composition and the process temperature.     

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.     

Effect of La-content on the hydrogenation properties of the Ce1−xLaxNi3Cr2 (x=0.2, 0.4, 0.6, 0.8, 1) alloys

The effect of partial substitution of Ce by La in CeNi₃Cr₂ hydrogen storage alloy has been systematically investigated. All intermetallic compounds Ce_1-xLa_xNi₃Cr₂ (x = 0.2, 0.4, 0.6, 0.8, 1) synthesized by arc melting method are well characterized by the means of XRD and SEM. XRD results show that all the alloys are crystallized as a single-phase compound in the hexagonal CaCu₅ type structure. The substitution of Ce by La leads to increase the unit cell volume of the alloy. Hydrogen storage capacity has been investigated in the temperature and pressure range of 293 K ≤ T ≤ 323 K and 0.5 ≤ P ≤ 45 bar respectively using pressure-composition isotherm. The P-C isotherm curves show that the plateau pressure of the hydrogen absorption decreases and hydrogen storage capacity increases with increasing La content in the alloy. The enthalpy (?H) and entropy (?S) of dissolved hydrogen for all systems has been calculated using Van’t Hoff plot. The variation of ?H and ?S with hydrogen content has also been studied which confirm the phase boundaries.     

Crystal structures of MgyTi100−y thin film alloys in the as-deposited and hydrogenated state

In situ X-ray diffraction was used to identify the crystal structures of as-deposited and hydrogenated Mg_yTi_100-y thin film alloys containing 70, 80 and 90 at.% Mg. The preferred crystallographic orientation of the films in both the as-prepared and hydrogenated state made it difficult to unambiguously identify the crystal structure up to now. In this work, identification of the unit cells was achieved by in situ recording diffraction patterns at various tilt angles. The results reveal a hexagonal closed packed structure for all alloys in the as-deposited state. Hydrogenating the layers under 10⁵ Pa H₂ transforms the unit cell into face-centered cubic for the Mg₇₀Ti₃₀ and Mg₈₀Ti₂₀ compounds, whereas the unit cell of hydrogenated Mg₉₀Ti₁₀ has a body-centered tetragonal symmetry. The (de)hydrogenation kinetics changes along with the crystal structure of the metal hydrides from rapid for fcc-structured hydrides to sluggish for hydrides with a bct symmetry and emphasizes the influence of the crystal structure on the hydrogen transport kinetics.     

Structural characteristics and hydrogen storage properties of Ca3.0−xMgxNi9 (x = 0.5, 1.0, 1.5 and 2.0) alloys

The effect of Mg content on the structural characteristics and hydrogen storage properties of the Ca_3.0−xMg_xNi₉ (x = 0.5, 1.0, 1.5 and 2.0) alloys was investigated. The lattice parameters and unit cell volume of the PuNi₃-type (Ca, Mg)Ni₃ main phase decreased with increasing Mg content. The 6c site of PuNi₃-type structure was occupied by both Ca and Mg atoms. Moreover, the occupation factor of Ca on the 6c site decreased with the increase of Mg content. The hydrogen absorption capacity of the alloys decreased due to higher Mg content. However, the thermodynamic properties of hydrogen absorption and desorption were improved and the plateau pressures were increased. When x = 1.5–2.0, the Ca_3.0−xMg_xNi₉ alloys had favorable enthalpy (ΔH) and entropy (ΔS) of hydride formation.     

Microstructure and electrochemical hydrogen storage characteristics of La0.67Mg0.33−xCaxNi2.75Co0.25 (x = 0-0.15) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of La_0.67Mg_0.33−xCa_xNi_2.75Co_0.25 (x = 0, 0.05, 0.10 and 0.15) alloys are investigated. The results show that all alloys mainly consist of (La, Mg)Ni₃ and LaNi₅ phases, besides a small amount of (La, Mg)₂Ni₇ phase. The cycle stability (S₈₀) after 80 charge/discharge cycles of all alloy electrodes first increases from 60.1% (x = 0) to 64.2% (x = 0.05), then decreases to 45.9% (x = 0.15). The high rate dischargeability of all alloy electrodes first increases from 52.6% (x = 0) to 61.4% (x = 0.10), then decreases to 57.2% (x = 0.15). Moreover, the charge-transfer resistance (R_ct) first decreases from 168.2 mΩ (x = 0) to 125.7 mΩ (x = 0.10), then increases to 136.6 mΩ (x = 0.15). All the results indicate that the substitution of Mg with a certain amount of Ca can improve the overall electrochemical characteristics.     

Effect of niobium on the microstructure,hydrogen embrittlement,and hydrogen permeability of NbxHf(1−x)/2Ni(1−x)/2 ternary alloys

The influence of niobium (Nb) on the microstructure, hydrogen embrittlement, and hydrogen permeability of the Nb_xHf_(1−x)/2Ni_(1−x)/2 ternary alloys has been studied in particular. The results show that the quantity of the primary (Nb, Hf) phase decreases with the increase of Nb content from 14 mol% to 16 mol%, and then increases with the increase of the Nb content from 16 mol% to 40 mol%. The Nb₁₄Hf₄₃Ni₄₃ alloy is brittle at all temperatures from 523 K to 673 K, as it possesses the largest amount of the B_f-HfNi compound; however, the Nb₄₀Hf₃₀Ni₃₀ alloy has high resistance to hydrogen embrittlement at all temperatures from 523 K to 673 K, as it possesses the largest amount of the primary (Nb, Hf) phase. The hydrogen permeability (Φ) increases with the increase in Nb content, as the quantity of the primary (Nb, Hf) phase in the Nb_xHf_(1−x)/2Ni_(1−x)/2 ternary alloys also increases with the increase in Nb content.     

Deuteration properties of CaNi5−xCux system

Intermetallic compounds with nominal formula CaNi_5−xCu_x (x = 0, 1, 2.5) have been prepared in order to investigate their hydrogenation properties. The samples were obtained by arc-melting and were deuterated in a Sieverts reactor. For x = 0 and 1, we have found that the fast kinetics and the different shape of the curve (non sigmoidal) in the second absorption process indicate an improvement of the hydrogen absorption due to the activation of the alloys. The deuterium desorption spectra are similar for x = 0 and 1 whereas for x = 2.5 the desorption ranges a broader temperature interval (∼100-350 °C) indicating a certain degree of chemical inhomogeneity or amorphization intrinsic to the parent sample or induced by the deuterium absorption. The formed deuterides were passivated in the presence of air in order to carry out a neutron diffraction study, allowing us to determine the deuterium positions in the samples. While in CaNi₄CuD_y the deuterium is randomly distributed over seven different positions, in CaNi₅D_y the deuterium only occupies five of them. This wider distribution in CaNi₄CuD_y can explain its higher stability, and therefore, its higher desorption temperature for deuterium.     

Effect of non-stoichiometry on hydrogen storage properties of La(Ni3.8Al1.0Mn0.2)x alloys

The La(Ni_3.8Al_1.0Mn_0.2)_x (x = 0.94, 0.96, 0.98, 1.0) hydrogen storage alloys have been investigated to examine the effect of non-stoichiometry on the crystal structure, activation performance, hydrogen absorption/desorption properties and cycle life. It was found that for the stoichiometric compound, only single phase with CaCu₅ type structure exists. However, for B-poor compounds of AB₅ alloys, there is a principal CaCu₅ type phase with a small amount of second phase and the amount of second phase increased with decreasing x when x ≥ 0.96 and reached a maximum when x = 0.96. The activation becomes harder with decreasing x until x = 0.96 and easier when x decreased to 0.94. The plateau pressure increased and the hydrogen uptake capacity decreased with decreasing x when x ≥ 0.96, and then decreased and increased, respectively, when x further decreased to 0.94. Both the change in the lattice strain which could be estimated by FWHM (full width at half maximum) and the degree of slope factor S_f in the alloys show the same trend with the change of x, exhibiting a maximum at x = 0.96. The ΔH decreased with decreasing x when x ≥ 0.96 and then increased when x = 0.94 and it was found that the larger the cell volume, the larger the absolute value of the enthalpy. The pulverization resistance of the alloys was greatly improved by the non-stoichiometric. The kinetics of the alloys was very fast and almost not influenced by the change of non-stoichiometric x. After 300 absorption/desorption cycles, the hydrogen uptake capacity of the stoichiometric and non-stoichiometric alloys almost kept the same, but the particle size decreased greatly.     

Electrocatalytic activity of perovskite La1−xSrxMnO3 towards hydrogen peroxide reduction in alkaline medium

Perovskite-type series of compounds La_1−xSr_xMnO₃ are synthesized by a sol-gel method using Chitosan as the gelling agent. Their catalytic activity for hydrogen peroxide electroreduction in 3.0 mol dm⁻³ KOH at room temperature is evaluated by means of cyclic voltammetry and chronoamperometry. Effects of annealing temperature and the ratio of La to Sr of La_1−xSr_xMnO₃ on their catalytic performance are investigated. Among this series of compounds, La_0.4Sr_0.6MnO₃ calcined at 650 °C exhibits the highest activity, which is comparable with Co₃O₄. An aluminum-hydrogen peroxide semi-fuel cell using La_0.4Sr_0.6MnO₃ as cathode catalyst achieves a peak power density of 170 mW cm⁻² at 170 mA cm⁻² and 1.0 V running on 0.6 mol dm⁻³ H₂O₂.     

Hydrogen storage properties of the pseudo binary laves phase (Sc1−xZrx)(Co1−yNiy)2 system

The (Sc_1−xZr_x)(Co_1−yNi_y)₂-H_z system has been studied using both experimental techniques and ab initio calculations. The material was synthesised through high temperature synthesis and characterised using powder XRD. Hydrogen absorption and desorption was studied in-situ using synchrotron radiation. Maximal storage capacity increased when Co replaced Ni and substitution of Sc for Zr increased the equilibrium pressure. Density functional based calculations reproduce the experimental trends in terms of cell parameters both for the non-hydrogenated systems as well as for the hydrogenated systems, and helped to quantitatively understand the observed hydrogen uptake properties.     

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.     

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.