Effect of cycling on hydrogen storage properties of Ti2CrV alloy

In the present work, we have studied the hydrogen absorption–desorption properties of the Ti₂CrV alloy, and effect of cycling on the hydrogen storage capacity. The material has been characterized for the structure, morphology, pressure composition isotherms, hydrogen storage capacity, hydrogen absorption kinetics and the desorption profile at different temperatures in detail. The Ti₂CrV crystallizes in body centered cubic (bcc) structure like TiCrV. The pressure composition isotherm of the alloy has been measured at room temperature and at 373K. The Ti₂CrV alloy shows maximum hydrogen storage capacity of 4.37 wt.% at room temperature. The cyclic hydrogen absorption capacity of Ti₂CrV alloy has been investigated at room temperature upto 10th cycle. The hydrogen storage capacity decreased progressively with cycling initially, but the alloy can maintain steady cyclic hydrogen absorption capacity 3.5 wt.% after 5th cycle. To get insight about the desorption behavior of the hydride in-situ desorption has been done at different temperatures and the amount of hydrogen desorbed has been calculated. The TG (Thermo gravimetric) and DTA analysis has been done on uncycled hydride shows that the surface poisoned sample gives a desorption onset temperature of 675K. The DSC measurement of uncycle and multi-cycled saturated hydrides shows that the hydrogen desorption temperature decreasing with cycling.     

Crystal structure and hydrogen storage properties of AB-type TiZrNbCrFeNi high-entropy alloy

The crystal structure and hydrogen storage properties of a novel equiatomic TiZrNbCrFeNi high-entropy alloy (HEA) were studied. The alloy, which had an AB-type configuration (A: elements forming hydride, B: elements with low chemical affinity with hydrogen), was selected with the aid of thermodynamic calculations employed by the CALPHAD method. The arc-melted AB-type TiZrNbCrFeNi alloy showed the presence of two C14 Laves phases in different fractions but with slight differences in unit cell parameters. Hydrogen storage properties investigated through pressure-composition-temperature absorption and desorption isotherms at different temperatures revealed that the alloy could absorb 1.5 wt% of hydrogen at room temperature without applying any activation procedure, but full desorption was not obtained. At 473 K, the alloy was able to reversibly absorb and fully desorb 1.1 wt% of hydrogen. After full hydrogenation at 473 K, the initial metallic C14 Laves phases were converted into their respective Laves phase hydrides. Under cycling, the fractions of two C14 Laves phases changed while one of the phases was more active to accommodate the hydrogen atoms. After dehydrogenation at 473 K, the alloy presented a single C14 Laves phase. The microstructural analysis, before and after cycling, showed a very well homogeneous microstructure and good distribution of elements.     

Study on hydrogen storage properties of LaNi3.8Al1.2−xMnx alloys

Effects of the Mn substitution on microstructures and hydrogen absorption/desorption properties of LaNi_3.8Al_1.2−xMn_x (x = 0.2, 0.4, 0.6) hydrogen storage alloys were investigated. The pressure-composition (PC) isotherms and absorption kinetics were measured in a temperature range of 433 K ≤ T ≤ 473 K by the volumetric method. XRD analyses showed that with the increase of the Mn content in the LaNi_3.8Al_1.2−xMn_x alloys, the lattice parameter a was decreased, c increased and the unit cell volume V reduced. It was found that the absorption/desorption plateau pressure was increased and the hydrogen storage capacity was enhanced with the increase of Mn content. The absorption/desorption plateau pressure of the alloys was linearly changed with the Mn content x and the lattice parameter a, while the hydrogen storage capacity was linearly increased with the increase of c/a ratio. It was also found that the slope factor S_f was closely correlated with the lattice strain of the alloys.     

Effect of Cr substitution by Ni on the cycling stability of Mg2Ni alloy using EXAFS

This paper describes the hydrogen storage properties of Mg₂Ni_0.9Cr_0.1 alloy and aims to elucidate the effect of doping Cr on the hydrogen sorption/desorption kinetics upon cycling. Mg₂Ni_0.9Cr_0.1 alloy shows stable absorption capacity, and its absorption/desorption rates further improve after cycling. The calculated activation energy for dehydrogenation was 53 kJ/mol at the 3rd cycle, and decreased to 36 kJ/mol at the 20th cycle. XRD combined with SEM exhibits that Cr dopant substitutes for Mg or Ni after ball milling and the lattice structure remains stable over 20 cycles. EXAFS was used to investigate the local coordination of Ni and Cr atoms in the ball-milled and cycled samples. For the ball-milled sample, the strong Cr–Ni bonds weaken the Cr–Mg bonds, thereby destabilizing all Cr-doped phases. After 20 cycles, the stable Ni₁–Mg₁ bonds may be dominant and control the structural stability of Mg₂Ni phases.     

Cyclic properties and ammonia by-product emission of Li/Mg-N-H hydrogen storage material

A magnesium amide-based hydrogen storage material, 3 Mg(NH₂)₂ + 8LiH, was subjected to cycling tests of dehydrogenation and hydrogenation, in which the cyclic trend in the hydrogen storage capacity as well as the amount of the ammonia by-product contained in the desorbed hydrogen gas were recorded. After 300 cycles at 473 K, the initial hydrogen capacity of 4.2 mass% dropped to 3.6 mass%, corresponding to the decay rate of 0.0004 per cycle. The average ammonia concentration through the 300 cycles was determined to be 0.05 ± 0.01 mol%(NH₃/H₂) which is entirely responsible for the hydrogen capacity decay because the ammonia emission leads to the loss of elemental nitrogen from the system. When the dehydrogenation temperature was raised to 573 K, the hydrogen capacity decay became more significant and the ammonia concentration increased to 0.27 ± 0.06 mol%(NH₃/H₂). The reaction kinetics also severely deteriorated during cycling at the higher temperature.     

Structure, morphology and hydrogen storage properties of a Ti0.97Zr0.019V0.439Fe0.097Cr0.045Al0.026Mn1.5 alloy

The Ti_0.97Zr_0.019V_0.439Fe_0.097Cr_0.045Al_0.026Mn_1.5 alloy is a hexagonal C14 Laves phase material that reversibly stores hydrogen under ambient temperatures. Structural changes are studied by XRD and SEM with regard to hydrogenation and dehydrogenation cycling at 25, 40 and 60 °C. The average particle size is reduced after hydrogenation and dehydrogenation cycling through decrepitation. The maximum hydrogen capacity at 25 °C is 1.71 ± 0.01 wt. % under 78 bar H₂, however the hydrogen sorption capacity decreases and the plateau pressure increases at higher temperatures. The enthalpy (ΔH) and entropy (ΔS) of hydrogen absorption and desorption have been calculated from a van’t Hoff plot as −21.7 ± 0.1 kJ/mol H₂ and −99.8 ± 0.2 J/mol H₂/K for absorption and 25.4 ± 0.1 kJ/mol H₂ and 108.5 ± 0.2 J/mol H₂/K for desorption, indicating the presence of a significant hysteresis effect.     

Stability of LaNi5−xSnx cycled in hydrogen

In this work we study the effect of cycling in hydrogen with a purity grade 4.5 of six alloys belonging to the LaNi_5−xSn_x family for 0 ≤ x ≤ 0.5. Measurements consist in the alternate repetition of absorption reactions at a temperature of 316 K and an initial pressure of 800 kPa, each followed by a desorption reaction at the same temperature and a maximum backpressure of 2 kPa. All samples present good stability, preserving at least 98% of their initial capacity after 100 cycles. Samples with composition LaNi₅ and LaNi_4.55Sn_0.45 were subjected to 1000 cycles, after which we observe a higher stability from the Sn-containing alloy (96% of the initial capacity preserved versus 92% for LaNi₅). Absorption characteristic times do not suffer important changes in either case. Desorption is gradually retarded when Sn content is higher than 0.4 at.     

Structural,hydrogen storage,and electrochemical performance of LaMgNi4 alloy and theoretical investigation of its hydrides

Structural, hydrogen storage, and electrochemical properties of LaMgNi₄ alloy were investigated in this study to determine whether it can be used as an active material of the negative electrode in nickel–metal hydride (Ni/MH) batteries. X-ray diffraction study showed that amorphization occurs at the first dehydrogenation cycle and was recovered crystallization after 873 K annealing.Maximum hydrogen storage capacity reached 1.4 wt% in the first hydrogenation under 373 K. The reannealed alloy showed improved reversible hydrogen storage capacity at ～0.9 wt% due to more LaNi₅ phase composition. Electrodes prepared from the investigated alloy showed maximum discharge capacities of ～340 mAh/g at 10 mA/g. The LaMgNi₄ alloy electrode exhibited satisfactory cycling stability remaining 47% of its initial capacity after 250 cycles. The negative cohesive energy indicated the exothermic process and stable compound structures of the LaMgNi₄ alloy and its hydrides via Density functional theory calculations.     

Cycling performance and hydriding kinetics of LaNi5 and LaNi4.73Sn0.27 alloys in the presence of CO

We analyzed the sorption cycling behavior of LaNi₅ and LaNi_4.73Sn_0.27 alloys in H₂ containing 10 and 100 ppm of CO. The effect of temperature was studied for the Sn-containing alloy. When cycling in the presence of CO, we found the reaction was strongly retarded due to surface contamination but no loss of capacity was observed when samples were given enough time for both absorption and desorption. The retardation was stronger at lower temperatures and higher CO concentration. The results also indicate that a fraction of the adsorbed CO is released during the desorption process. For the Sn-containing alloy, a stationary state is met after about 10 cycles, with no further degradation occurring past this point. The retarding factor at 40 °C and 100 ppm in this condition, with respect to the kinetics in pure hydrogen, is of about 600.     

Improvement in the hydrogen storage properties of Mg by mechanical grinding with Ni, Fe and V under H2 atmosphere

Mg-5wt%Ni-2.5wt%Fe-2.5wt%V (named Mg-5Ni-2.5Fe-2.5V) powder was prepared by reactive mechanical grinding using a planetary ball mill. The activation process, the changes in phase and microstructure with hydriding-dehydriding cycling, and the variations in the hydriding and dehydriding rates with temperature were investigated. The rate-controlling step for the dehydriding reaction of Mg-5Ni-2.5Fe-2.5V was analyzed by using a spherical moving boundary model. As the temperature increased from 473 K through 623 K, the initial hydrogen absorption rate under 12 bar H₂ decreased, while the hydrogen desorption rate under 1.0 bar H₂ increased.     

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 high entropy alloy. It was found that the V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.     

Reduction and unusual recovery in the reversible hydrogen storage capacity of V1−xTix during hydrogen cycling

The effect of the vanadium content on the cyclic stability of V–Ti binary alloys was investigated. V_1−xTi_x, x = 0.2 and 0.5 samples were hydrogenated and dehydrogenated at 410 K and 553 K respectively, for more than 100 times. During hydrogen cycling, reduction in the reversible hydrogen storage capacity was clearly observed from both samples. No prominent V-effect was found. In fact, the reduction rates of two samples were similar; both samples showed a ∼25% reduction in the reversible hydrogen storage capacity after 100 cycles. In addition, the shape of the pressure–composition-isotherm (PCT) curves was significantly altered over the testing cycle period; the absorption and desorption plateaus got markedly inclined and the hysteresis became evidently smaller. We found that even after the hydrogen storage capacity of V_1−xTi_x was significantly reduced, at low enough temperature V_1−xTi_x was able to absorb hydrogen as much as it did at the first cycle. Furthermore, the reversible hydrogen storage capacity of V_0.8Ti_0.2 at 410 K was recovered to a certain degree after hydrogenating the sample at low temperatures.     

Hydrogen storage behaviour of Cr- and Mn-doped Mg2Ni alloys fabricated via high-energy ball milling

This paper describes the efficient preparation of an Mg₂Ni alloy for hydrogen storage via high-energy ball milling mechanical alloying for 2 h. The degree of alloy amorphisation increases with increasing ball-milling time. Ball milling for 4 h affords partially amorphous alloys exhibiting the best hydrogen storage performance. Partial substitution of Ni with Cr and Mn improves the hydrogen absorption/desorption thermodynamics, kinetics and cycling performance of the alloy. Specifically, partial Mn substitution improves the cycling performance and reduces the activation energy of the hydrogen desorption reaction, effectively improving the hydrogen desorption kinetic performance. Mg₂Ni_0.8Mn_0.2 shows the best cycling and hydrogen absorption/desorption kinetic performances. Partial Cr substitution reduces the entropy and enthalpy changes of the hydrogen absorption/desorption reaction and effectively reduces the temperature of the initial hydrogen absorption/desorption reaction. In particular, Mg₂Ni_0.9Cr_0.1 shows the best thermodynamic performance.     

Synchrotron EXAFS studies of Ti-doped Mg2Ni alloy on the cycling behavior

In this study, Mg_1.9NiTi_0.1 alloy was synthesized by mechanical alloying and its cyclic hydrogen storage behaviors were investigated. It was found that titanium substituting magnesium in Mg_1.9NiTi_0.1 alloy notably improves the absorption/desorption kinetics. Upon cycling, the kinetic rates of absorption/desorption further increase, whereas the hydrogen storage capacity decreases. To identify the micro-structural evolution of Mg_1.9NiTi_0.1 alloy during cycling, we used X-ray diffraction, scanning electron microscope, and extended X-ray absorption fine structure. After ball milling, the decrease of Mg–Ni atomic interaction lowers the stability of Ti-doped phases and has positive effect on the absorption/desorption behaviors. After 20 cycles, the decrease in the cycling capacity may be attributed to the increasing MgNi₂ content. Further studies revealed that some amounts of Mg₂Ni transform into MgNi₂, which result in great decrease in effective hydrogen storage capacity.     

Study on preparation and properties evaluation of Mg/Ni/Ti hydrogen storage material

The improvement of the capacity and rate of hydrogen absorption/desorption at the same time had been focus hydrogen storage field. A new method was proposed to prepare a new type sintered body with a special layer structure due to the crystal structure of the sintered body distinguish with a powder and alloy. Therefore, the sintered body not only effectively overcame the inconvenience from the powder on storage aspect, but also avoided negative factors of small surface area. The sintered body had a foam structure whose surface area was large, which provided a strong chemical channel for the physical absorption and chemical diffusion by generating hydrogen absorption phase Mg₂Ni and catalytic phase NiTi, so that great improved the property of hydrogen absorption/desorption. The capacity of hydrogen absorption/desorption reach to 3% within 3 min and the highest capacity reach to 5.26% within 16 min; hydrogen desorption process was completed within 30 min and the released capacity was about 2%.     

Effect of partial niobium and iron substitution on short-term cycle durability of hydrogen storage Ti–Cr–V alloys

The effect of partial niobium and iron substitution on the short-term (up to 10 cycles) cycle durability of hydrogen absorption and desorption was evaluated for Ti–Cr–V alloys. Partial iron substitution improved the durability of Ti₁₆Cr₃₄V₅₀ alloy, but reduced its hydrogen storage capacity. In contrast, partial niobium substitution improves its durability while not affecting its hydrogen storage capacity. Similar experimental results were obtained for Ti₂₅Cr₅₀V₂₅ alloy. The effective hydrogen storage capacity decreased to 84.9 and 94.2% of its initial value after 10 cycles of hydrogen absorption and desorption for Ti₂₅Cr₅₀V₂₅ and Ti₂₅Cr₄₅V₂₅Nb₅ alloys, respectively. This reduction in the effective hydrogen storage capacity of Ti₂₅Cr₅₀V₂₅ alloy during hydrogen absorption and desorption is attributed to reduced hydrogen storage capacity during absorption and a greater residual hydrogen during desorption.     

Improved hydrogen storage behaviours of nanocrystalline and amorphous Mg2Ni-type alloy by Mn substitution for Ni

The nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by melt spinning technique. The structures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system. The results show that the as-spun (x = 0) alloy holds a typical nanocrystalline structure, whereas the as-spun (x = 0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg₂Ni-type alloy. The hydrogen absorption capacity of the alloys first increases then decreases with rising Mn content, but the hydrogen desorption capacity of the alloys grows with increasing Mn content. Furthermore, the substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving both the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn from 0 to 0.4, the discharge capacity of as-spun (30 m/s) alloy grows from 116.7 to 311.5 mAh/g, and its capacity retaining rate at 20th charging and discharging cycle rises from 36.7 to 78.7%.     

Hydrogen storage properties of Ti0.72Zr0.28Mn1.6V0.4 alloy prepared by mechanical alloying and copper boat induction melting

In the present study, two process techniques, mechanical alloying and innovative vacuum copper boat induction melting, were used to produce Ti_0.72Zr_0.28Mn_1.6V_0.4 alloy for hydrogen storage applications. The hydrogen absorption and desorption properties of the alloy were studied. The material structure and phases were characterized by XRD and SEM. The hydrogen absorption and desorption properties of the alloy were measured by an automatically controlled Sieverts apparatus. The results showed that the samples consisted of two main phases, C14 Lave phase and V-base solid solution phase. The maximum capacity of abs/desorption was achieved at mediate temperature (150 °C). The hydrogen capacity of the induction melted samples in various temperatures was higher than that for the samples produced by mechanical alloying method. The maximum absorption capacity of the induction melted and mechanically alloyed samples were 2 and 1.2 wt%, respectively. The maximum desorption capacity of the induction melted and mechanically alloyed samples were 0.45 and 0.1 wt%, respectively.     

Nanoscale microstructures and hydrogenation properties of an as-cast vanadium-based medium-entropy alloy

Vanadium-based hydrogen storage alloys have been widely investigated; however, alloys in the cast state are typically coarse-grained. In this study, an as-cast V₄₅Fe₁₅Ti₂₀Cr₂₀ medium-entropy alloy was prepared by arc melting, and microstructural analysis revealed that the alloy was composed of nanocrystals. The initial pretreatment temperature of the alloy was approximately 100 K lower than that of the as-cast coarse-grained alloy. At room temperature, the time required for the alloy to reach 90% saturation was only 140 s, indicating excellent hydrogen absorption kinetics. The alloy is fully activated after two hydrogen absorption/desorption cycles. The phase transformation of the alloy in the early hydrogenation stage was investigated using X-ray diffraction, and the results showed that the BCC phase was completely transformed into the BCT phase when hydrogen uptake was performed for 6 s. Furthermore, the apparent activation energy of dehydrogenation in the present alloy calculated using the Kissinger method was 69.8 ± 0.8 kJ/mol. The pressure-composition-isotherms tests showed that the hydrogen absorption capacity of the alloy at 295 K was 2.12 wt%. The hydrogenation/dehydrogenation enthalpy change of the alloy was calculated by the Van't Hoff equation, which was 30.90 ± 1.47 and 33.95 ± 0.41 kJ/mol, respectively. The present work demonstrates that nanostructured vanadium-based hydrogen storage alloys can be fabricated using traditional casting techniques. Our study also enriches the understanding of the microstructures of medium-entropy alloys, which may provide positive guidance for the design of novel vanadium-based hydrogen storage alloys.     

Enhanced hydrogen sorption properties of Ni and Co-catalyzed MgH2

MgH₂ is one of the most promising hydrogen storage materials due to its high capacity and low cost. In an effort to develop MgH₂ with a low dehydriding temperature and fast sorption kinetics, doping MgH₂ with NiCl₂ and CoCl₂ has been investigated in this paper. Both the dehydrogenation temperature and the absorption/desorption kinetics have been improved by adding either NiCl₂ or CoCl₂, and a significant enhancement was obtained in the case of the NiCl₂ doped sample. For example, a hydrogen absorption capacity of 5.17 wt% was reached at 300 °C in 60 s for the MgH₂/NiCl₂ sample. In contrast, the ball-milled MgH₂ just absorbed 3.51 wt% hydrogen at 300 °C in 400 s. An activation energy of 102.6 kJ/mol for the MgH₂/NiCl₂ sample has been obtained from the desorption data, 18.7 kJ/mol and 55.9 kJ/mol smaller than those of the MgH₂/CoCl₂, which also exhibits an enhanced kinetics, and of the pure MgH₂ sample, respectively. In addition, the enhanced kinetics was observed to persist even after 9 cycles in the case of the NiCl₂ doped MgH₂ sample. Further kinetic investigation indicated that the hydrogen desorption from the milled MgH₂ is controlled by a slow, random nucleation and growth process, which is transformed into two-dimensional growth after NiCl₂ or CoCl₂ doping, suggesting that the additives reduced the barrier and lowered the driving forces for nucleation.