Hydrogen storage properties of Mg1−xCaxNi2−y defect solid solutions

Ternary Mg_1−xCa_xNi_2−y solid solutions were synthesized by powder sintering. The phase structures and hydrogen storage properties of the sintered samples were investigated. In a certain range of x and y values, the samples are a single C15 Laves phase with various types of defects. The reduction of Ni content leads to the formation of omission solid solution with vacancies on the sites of Ni. These vacancies increase the hydrogen storage capacity, but decrease the reversibility of hydrogen absorption and desorption.     

Nanoscale composition modulations in MgyTi1−yHx thin film alloys for hydrogen storage

A detailed structural analysis of Mg–Ti–H thin films reveals the presence of a chemically partially segregated but structurally coherent metastable phase. By combining X-Ray Diffraction and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy on Mg_yTi_1−yH_x thin films we find non-zero Chemical Short-Range Order (CSRO) parameters for all the compositions measured. Despite the positive enthalpy of mixing of Mg and Ti the degree of ordering does not increase upon loading and unloading with hydrogen. The robustness of this system and the fast and reversible kinetics of hydrogen loading and unloading are caused by the formation of nanoscale compositional modulations in the intermetallic alloy. This microstructure is responsible for the exceptional properties of Mg_yTi_1−yH_x thin films. It also shows that reversible metastable metal-hydrides offer new possibilities for hydrogen storage, beyond the limits imposed by thermodynamic equilibrium.     

Effect of Ni content on the hydrogen storage behavior of ZrCo1−xNix alloys

ZrCo_1−xNi_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared and their hydrogen storage behavior were studied. ZrCo_1−xNi_x alloys of compositions with x = 0, 0.1, 0.2 and 0.3 prepared by arc-melting method and characterized by X-ray diffraction analysis. XRD analysis showed that the alloys of composition with x = 0, 0.1, 0.2 and 0.3 forms cubic phase similar to ZrCo with traces of ZrCo₂ phase. A trace amount of an additional phase similar to ZrNi was found for the alloy with composition x = 0.3. Hydrogen desorption pressure–composition–temperature (PCT) measurements were carried out using Sievert's type volumetric apparatus and the hydrogen desorption pressure–composition isotherms (PCIs) were generated for all the alloys in the temperature range of 523–603 K. A single sloping plateau was observed for each isotherm and the plateau pressure was found to increase with increasing Ni content in ZrCo_1−xNi_x alloys at the same experimental temperature. A van't Hoff plot was constructed using plateau pressure data of each pressure–composition isotherm and the thermodynamic parameters were calculated for desorption of hydrogen in the ZrCo_1−xNi_x–H₂ systems. The enthalpy and entropy change for desorption of hydrogen were calculated. In addition, the hydrogen absorption–desorption cyclic life studies were performed on ZrCo_1−xNi_x alloys at 583 K up to 50 cycles. It was observed that with increasing Ni content the durability against disproportionation of alloys increases.     

Some physical aspects of hydrogen behaviour in the H-Storage bcc alloys Ti35VxCr65−x, Ti40VxMn50−xCr10 and TixCr97.5−xMo2.5

Hydrogen absorption/desorption has been investigated in the three series of solid solution bcc alloys Ti₃₅V_xCr_65−x (x = 18,22), Ti₄₀V_xMn_50−xCr₁₀ (x = 32,36) and Ti_xCr_97.5−xMo_2.5 (x = 43,46). It has been found that the H absorption at pressures smaller than 1 bar can only occur after elimination of the oxide films by heating the alloys to temperatures higher than 600 K. Hydrogen desorption from pre-loaded materials (n_H = H/Me ≤ 0.27) takes place on heating at much lower temperatures in the Ti₄₀V_xMn_50−xCr₁₀ and Ti₃₅V_xCr_65−x than in the Ti_xCr_97.5−xMo_2.5 alloys. The H diffusion parameters W and D_o deduced from high temperature (>450 K) absorption experiments are as follows: W = 0.318 ± 0.005 eV, D_o = (4 ± 1)×10⁻⁷ m²/s for Ti₄₀V_xMn_50−xCr₁₀; W = 0.32 ± 0.02 eV, D_o = (3 ± 2)×10⁻⁷ m²/s for Ti₃₅V_xCr_65−x; W = 0.79 ± 0.06 eV, D_o = (4 ± 2)×10⁻⁸ m²/s for Ti_xCr_97.5−xMo_2.5. The higher value of the activation energy for H diffusion in Mo containing alloys is most likely due to remarkable attractive interactions between H and Mo atoms.     

Hydrogen absorption properties of Zr(V1−xFex)2 intermetallic compounds

The Zr(V_1−xFe_x)₂ (x = 0.02, 0.05, 0.10, 0.15, 0.25) alloys were prepared by the arc-melt method and annealed at 1273 K for 168 h in an argon atmosphere. Phase structure investigations of the as-cast and annealed Zr(V_1−xFe_x)₂ alloys indicate the annealing treatment can eliminate the minority phases originating from the non-equilibrium solidification of as-cast alloys. The ZrV₂-type phase becomes the dominant one in each annealed alloy. The substitution of Fe in V sites leads to the contraction of their lattice. For annealed Zr(V_1−xFe_x)₂ alloys, the P–t and PCT curves obtained between 673 K and 823 K give the evidence that the absorption process is controlled by a rate-controlling hydrogen diffusion. With the increase of iron, the equilibrium pressure and the plateau slope increase while the hydrogenation capacity and the absolute value of enthalpy and entropy decrease accordingly. The stability of metal hydride reduces gradually as the Fe content varies from x = 0.02 to 0.25 which promotes the hydrogen release and favors the practical applications of the Zr(V_1−xFe_x)₂ alloys.     

Effect of Al and Mo substitution on the structural and hydrogen storage properties of CaNi5

A simple mechanical milling and annealing process has been used to synthesize CaNi₅-based hydrogen storage alloys. Heat treatment at 800 °C under vacuum results in the formation of a crystalline CaNi₅ phase. Secondary phases, including Ca₂Ni₇ and Mo–Ni, are formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in the unit cell volume of the CaNi₅ phase. The hydrogen storage capacity of all substituted alloys is reduced and the plateau pressures are lower than those of pure CaNi₅. Fairly flat plateau regions are retained for all compositions except the CaNi_4.8Mo_0.2 composition where a Ca₂Ni₇ phase is dominant. The incorporation of Mo also causes slow sorption kinetics for the CaNi_4.9Mo_0.1 alloy. CaNi_4.9Al_0.1 maintains its initial hydrogen absorption capacity for 20 cycles performed at 85 °C but the other substituted alloys lose their capacity rapidly, especially the CaNi_4.8Mo_0.2 composition.     

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.     

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.     

A low-cost BCC alloy prepared from a FeV80 alloy with a high hydrogen storage capacity

A V₃₀Ti₃₂Cr₃₂Fe₆ alloy prepared from a FeV80 master alloy is reported. It has a high hydrogen absorption/desorption capacity, good activation performance and kinetics. Heat-treatment at 1673 K is an effective way to increase the capacity and flatten the plateau due to the homogenization of the compositions in the alloy and the disappearance of Laves phase after heat-treatment. The heat-treated alloy can absorb 3.76 wt.%H at 298 K. It desorbs 2.35 wt.%H at 298 K and 2.56 wt.%H at 373 K. The development of this alloy could be of great significance to the application of V-based BCC hydrogen storage alloys.     

Effect of rare earth (RE) elements on V-based hydrogen storage alloys

In this work, the effect of RE additives on the properties of _{V55Ti22.5Cr16.1Fe6.4}${\mathrm{V}}_{55}{\mathrm{Ti}}_{22.5}{\mathrm{Cr}}_{16.1}{\mathrm{Fe}}_{6.4}$ alloy (RE=La$\mathrm{RE}=\mathrm{La}$, Pr, Ce and Nd, separately) was discussed. It was demonstrated that RE additives improve the activation property rather than kinetics during cycling, absorption capacity and the plateau pressure. Two phases, including BCC main phase and Ce second one, were found in Ce-containing alloy. It is inferred that RE element offers a route for hydrogen to enter the alloys more easily, which leads to the improvement of activation property of the alloys.     

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.     

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.     

Microstructure and electrochemical behavior of Cr-added V2.1TiNi0.4Zr0.06Cr0.152 hydrogen storage electrode alloy

The microstructure and electrochemical behavior of V_2.1TiNi_0.4Zr_0.06Cr_0.152 hydrogen storage electrode alloy have been investigated in comparison with V_2.1TiNi_0.4Zr_0.06 alloy. The results show that V_2.1TiNi_0.4Zr_0.06Cr_0.152 alloy consists of a V-based solid solution main phase and a C14-type Laves secondary phase in the form of three-dimensional network, being similar to V_2.1TiNi_0.4Zr_0.06 alloy, the secondary phase precipitates along the grain boundaries of the main phase. As compared with V_2.1TiNi_0.4Zr_0.06 alloy, the unit cell volume of each phase in the V_2.1TiNi_0.4Zr_0.06Cr_0.152 alloy contracts. It is found that adding Cr restricts the dissolution of vanadium and titanium into the KOH electrolyte, and improves the corrosion resistance of the alloy, thus the cycling stability after 30 cycles increases from 22.34% (V_2.1TiNi_0.4Zr_0.06) to 77.96% (V_2.1TiNi_0.4Zr_0.06Cr_0.152). Furthermore, V_2.1TiNi_0.4Zr_0.06Cr_0.152 alloy has a better high-rate dischargeability and higher exchange current density compared with V_2.1TiNi_0.4Zr_0.06 alloy, but its maximum discharge capacity decreases.     

Hydrogen storage and cyclic properties of V60Ti(21.4+x)Cr(6.6−x)Fe12 (0 ≤ x ≤ 3) alloys

Hydrogen storage and cyclic properties of V₆₀Ti_(21.4+x)Cr_(6.6−x)Fe₁₂ (0 ≤ x ≤ 3) alloys were investigated systematically. All alloys were composed of single BCC phase and exhibited good activation performance. V₆₀Ti_22.4Cr_5.6Fe₁₂ showed the highest desorption capacity of 2.12 wt% with the plateau pressure of 0.061 MPa. In the absorption–desorption cycle tests, both the hydrogen desorption capacity and the micro-strain of V₆₀Ti_22.4Cr_5.6Fe₁₂ alloy showed exponential relationship with the increase of cycle numbers, which indicated that the micro-strain induced and thereafter accumulated during the absorption–desorption cycles might lead to the decrease of the desorption capacity.     

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.     

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.     

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.     

Synthesis of catalytically active rock salt structured MgxNb1−xO nanoparticles for MgH2 system

This communication deals with the ex-situ synthesis of rock salt type Mg_xNb_1−xO whose structural characteristics are closely related with MgO. XRD examination of 30 h ball milled MgH₂ + Nb₂O₅ confirms the formation of a rock salt product Mg_xNb_1−xO_, which is comparable to the recently reported active catalyst Mg_xNb_1−xO formed in-situ in MgH₂ milled with 8 mol.% Nb₂O₅. It is shown that MgH₂ catalyzed with the pre-made 2 wt.% Mg_xNb_1−xO desorbs hydrogen at least 50 °C lower than the in-situ 2 wt.% Nb₂O₅ catalyzed MgH₂ with improved reversible absorption. This result highlights that the proposed pathway mechanism on the basis of Nb₂O₅ catalyst may need further verification and that the addition of the Mg_xNb_1−xO catalyst in a pre-reduced state can offer distinct performance advantages over its in-situ preparation.     

Perovskite-type Sr(Mn1−xNix)O3 materials and their chemical-looping oxygen transfer properties

This paper contains the results of research on chemical-looping combustion (CLC). CLC is one of the most promising combustion technologies and has the main advantage of producing a concentrated CO₂ stream, which is obtained after water condensation and without any energy penalty for CO₂ separation. The objective of this work was to study the chemical-looping reaction performance of novel perovskite-type oxygen carriers. The Sr(Mn_1−xNi_x)O₃ family was tested for its suitability as an oxygen carrier in hydrogen (syngas component) combustion for power generation. Sr(Mn_1−xNi_x)O₃ perovskite-type oxides with x = 0, 0.2, 0.5, 0.8, and 1.0 were prepared. Thermogravimetric measurements were performed to investigate the oxidation/reduction of the obtained materials. Reactivity tests were performed under isothermal conditions during multiple redox cycles using a thermogravimetric analyzer (TGA). For the reduction reaction, 3% H₂ in Ar was used, and air was used for the oxidation cycle. The effect of reaction temperature (600–800 °C) and the number of reducing/oxidizing cycles (up to 5 cycles) on the performance of the oxygen-carrier samples developed in this study were evaluated. The stability, oxygen transport capacity, and reaction rates were analyzed on the basis of thermogravimetric TG results. The Sr(Mn_1−xNi_x)O₃ oxides showed stable chemical-looping performance with rapid changes in their oxygen content (2–3 min) while maintaining their chemical properties. The cyclic redox reaction revealed that Sr(Mn_1−xNi_x)O₃ exhibits excellent structural stability and provides a continuous oxygen supply during redox reactions. Good oxygen capacity was maintained during the cycling hydrogen combustion tests. These new perovskite-type materials were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements and by surface area (BET), particle size distribution (PSD) and melting behavior analyses. The Sr(Mn_1−xNi_x)O₃ oxides exhibited high melting temperatures and small surface areas. The promising results obtained from chemical-looping combustion experiments indicate that the Sr(Mn_1−xNi_x)O₃ oxides are potentially useful oxygen carriers for chemical-looping combustion processes where hydrogen is one of the fuel components.