Self-ignition combustion synthesis of LaNi5 utilizing hydrogenation heat of metallic calcium

This paper describes self-ignition combustion synthesis (SICS) of LaNi₅ in a pressurized hydrogen atmosphere using metallic calcium as both the reducing agent and the heat source. In this study, the effects of hydrogen on the ignition temperature and the hydrogenation properties of the products were mainly examined. In the experiments, La₂O₃, Ni, and Ca were dry-mixed in the molar ratio of 1:10:6 and then heated up at a hydrogen pressure of 1.0 MPa until the ignition due to the hydrogenation of calcium. For the sake of comparison, the same experiments were performed in a normal argon atmosphere. The results showed that the ignition temperature was drastically lowered by hydrogen; it was only 600 K in the case of hydrogen as compared to 1100 K in the case of argon. The products also exhibited high initial activity and hydrogen storage capacity of 1.54 mass%. The proposed method offers many benefits for using cost-effective rare-earth oxide, saving productive time and energy, improving initial activity of the product and applying to any AB₅-type hydrogen storage alloy.     

Modeling and analyzing the hydriding kinetics of Mg-LaNi5 composites by Chou model

Chou model was used to analyze the influences of LaNi₅ content, preparation method, temperature and initial hydrogen pressure on the hydriding kinetics of Mg-LaNi₅ composites. Higher LaNi₅ content could improve hydriding kinetics of Mg but not change hydrogen diffusion as the rate-controlling step, which was validated by characteristic reaction time t_c. The rate-controlling step was hydrogen diffusion in the hydriding reaction of Mg-30 wt.% LaNi₅ prepared by microwave sintering (MS) and hydriding combustion synthesis (HCS), and surface penetration was the rate-controlling step of sample prepared by mechanical milling (MM). Rising temperature and initial hydrogen pressure could accelerate the absorption rate. The rate-controlling step of Mg-30 wt.% LaNi₅ remained hydrogen diffusion at temperatures ranging from 302 to 573 K, while that of Mg-50 wt.% LaNi₅ changed from surface penetration to hydrogen diffusion with increasing initial hydrogen pressure ranging from 0.2 to 1.5 MPa. Apparent activation energies of absorption for Mg-30 wt.% LaNi₅ prepared by MS and MM were respectively 25.2 and 28.0 kJ/mol H₂ calculated by Chou model. Kinetic curves fitted and predicted by Chou model using temperature and hydrogen pressure were well exhibited.     

Improved hydrogen storage properties of Mg-based nanocomposite by addition of LaNi5 nanoparticles

Mg (200 nm) and LaNi₅ (25 nm) nanoparticles were produced by the hydrogen plasma-metal reaction (HPMR) method, respectively. Mg–5 wt.% LaNi₅ nanocomposite was prepared by mixing these nanoparticles ultrasonically. During the hydrogenation/dehydrogenation cycle, Mg–LaNi₅ transformed into Mg–Mg₂Ni–LaH₃ nanocomposite. Mg particles broke into smaller particles of about 80 nm due to the formation of Mg₂Ni. The nanocomposite showed superior hydrogen sorption kinetics. It could absorb 3.5 wt.% H₂ in less than 5 min at 473 K, and the storage capacity was as high as 6.7 wt.% at 673 K. The nanocomposite could release 5.8 wt.% H₂ in less than 10 min at 623 K and 3.0 wt.% H₂ in 16 min at 573 K. The apparent activation energy for hydrogenation was calculated to be 26.3 kJ mol⁻¹. The high sorption kinetics was explained by the nanostructure, catalysis of Mg₂Ni and LaH₃ nanoparticles, and the size reduction effect of Mg₂Ni formation.     

Thermodynamic and electrochemical hydrogenation properties of LaNi5 − xInx alloys

Hydrogenation properties of LaNi_5 − xIn_x alloys (x = 0.1, 0.2 and 0.5) were examined by their direct reaction with gaseous hydrogen and by cathodic charging in 6 M KOH solution. The gas phase measurements were carried out using Sievert's type apparatus in 300–400 K temperature range and at hydrogen pressures up to 40 bars. Indium substitution for Ni in LaNi₅ significantly modifies the hydrogenation behavior, decreasing the equilibrium pressure of hydrogen and limiting the hydrogen capacity as compared to LaNi₅. The LaNi_4.9In_0.1 revealed a distinct presence of two pressure plateaus on the high temperature isotherms. Apart from the α-phase (hydrogen solid solution) and β-phase (LaNi₅H₆ hydride), formation of a new σ*-hydride phase was postulated at the hydrogen content extended over the region of H/f.u. = 1.3–1.8. Thermodynamic functions: enthalpy and entropy of the hydrogen absorption process were calculated from the H₂-pressure/composition (p–c) isotherms at several temperatures, applying the Van't Hoff's (lnp − 1/T) dependence. Electrochemical galvanostatic hydrogenation experiments at 185 mA/g charge/discharge rate revealed the greatest discharge current capacity of 319 mAh/g for LaNi_4.9In_0.1 alloy after 4–5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy.     

Evaluation of electrochemical hydrogenation and corrosion behavior of LaNi5-based materials using galvanostatic charge/discharge measurements

A detailed thermodynamic analysis of galvanostatic −i/+i characteristics of epoxy resin bonded, LaNi₅ powder based electrodes enables not only alloy hydrogenation ability but also oxidation susceptibility of their constituents to be determined. It is shown that electrode cycling is prone to oxidation of nickel during discharging process, however, the NiO/Ni equilibrium is reversible and nickel oxides undergo reduction during charging. Similar behavior exhibits bismuth as Ni partial substitute. The way of determining alloy constituent oxidation/reduction times as well their corrosion rate in 6 M KOH is presented. The effects of substitution of 20 at% of nickel by Bi or Zn on hydrogen capacity, corrosion behavior and surface development are discussed. It is shown that multiphase LaNi₄Bi alloy possesses much worse whereas monophase LaNi₄Zn alloy possesses clearly better hydrogen absorption properties as compared with LaNi₅ reference. The plots of charge curves allow to distinguish processes of atomic hydrogen absorption and evolution of molecular H₂ for the tested alloys.     

Hydrogen storage by Mg-based nanocomposites

Hydrogen storage materials research is entered to a new and exciting period with the advance of the nanocrystalline alloys, which show substantially enhanced absorption/desorption kinetics, even at room temperatures. In this work, hydrogen storage capacities and the electrochemical discharge capacities of the Mg₂(Ni, Cu)-, LaNi₅-, ZrV₂-type nanocrystalline alloys and Mg₂Ni/LaNi₅-, Mg₂Ni/ZrV₂-type nanocomposites have been measured. The electronic properties of the Mg₂Ni_1-xCu_x, LaNi₅ and ZrV₂ alloys were calculated. The nanocomposite structure reduced hydriding temperature and enhanced hydrogen storage capacity of Mg-based materials. The nanocomposites (Mg,Mn)₂Ni (50 wt%)-La(Ni,Mn,Al,Co)₅ (50 wt%) and (Mg,Mn)₂Ni (75 wt%)-(Zr,Ti)(V,Cr,Ni)_2.4 (25 wt%) materials releases 1.65 wt% and 1.38 wt% hydrogen at 25 °C, respectively. The strong modifications of the electronic structure of the nanocrystalline alloys could significantly influence hydrogenation properties of Mg-based nanocomposities.     

Exergy analysis of self-ignition combustion synthesis for producing rare-earth-based hydrogen storage alloy

A new production system for rare-earth-based hydrogen storage alloys is proposed. We applied self-ignition combustion synthesis (SICS) utilizing hydrogenation heat of metallic calcium. The required primary energy and total exergy loss (EXL) for the production of 1 kg of LaNi₅ alloy with the proposed and conventional systems were evaluated. The results revealed that the production of raw materials accounted for more than 90% of the total EXL in both systems. Specifically, the use of calcium had decisive effects on the total EXL of the system for producing LaNi₅ alloy. The proposed system reduced the total EXL by 14.6 MJ/kg-LaNi₅ as compared with the conventional system. The SICS was remarkably exergy-saving because the heating temperature was decreased by utilizing the hydrogenation heat of calcium and the product absorbed hydrogen without an activation treatment.     

The effect of Ni and Al addition on hydrogen generation of Mg3La hydrides via hydrolysis

Hydrogen generation by hydrolysis of hydrogenated Mg₃LaNi_0.1, and Mg₃LaNi_0.1Al_0.1 in pure water at room temperature has been investigated and compared with that of Mg₃La. It has been found that hydrolysis reaction rate of those hydrides, which are obtained by induction melting and then hydrogenated, is fast when they are immersed in pure water. The highest hydrogen yield is 1024 ml/g for hydrogenated Mg₃LaNi_0.1. The kinetics of hydrogen production reaction for hydrogenated Mg₃LaNi_0.1, and Mg₃LaNi_0.1Al_0.1 hydrides at 298 K was also measured. The effect for addition of Ni and Al was also discussed in this paper.     

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.     

Metallic Ni nanocatalyst in situ formed from LaNi5H5 toward efficient CO2 methanation

LaNi₅ alloy can be utilized to directly store and release hydrogen in mild condition, thus it is considered as a long-term safe and stable solid-state hydrogen storage material. In this work, LaNi₅H₅ was used as the solid-state hydrogen source in the CO₂ methanation reaction. Impressively, the carbon dioxide conversion can be achieved to nearly 100% under 3 MPa mixed gas at 200 °C. The microstructure and composition analysis results reveal that the high catalytic activity may originate from the promoted elementary steps over in situ formed metallic Ni nanoparticles during the CO₂ methanation process. More importantly, as the lowered reaction temperature prevented the agglomeration of Ni nanoparticles, this catalyst exhibited durable stability with 99% conversion rate of CO₂ retained after 400 h cycling test.     

Hydrogen absorption–desorption mechanisms for the ball-milled Li3N–MgH2 (1:1) mixture

The Li–Mg–N–H system is a very promising hydrogen storage material due to its high capacity, reversibility and moderate operating conditions. In this work, the LiMgN/2LiH was directly synthesized by ball-milling the mixture of Li₃N–MgH₂ at 1:1 molar ratio by a reaction of Li₃N + MgH₂ → LiMgN + 2LiH. The hydrogenation/dehydrogenation properties of the as-prepared LiMgN/2LiH were investigated by a Sieverts'-type apparatus. The mixture of LiMgN/2LiH started to absorb hydrogen at 130 °C, and 2.2 wt%, 3.2 wt% hydrogen were absorbed under a pressure of 5 MPa and 10 MPa, respectively. Powder X-ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) spectrometer measurements were used to identify the phase characterizations of the products during the hydrogen absorption–desorption process. The reaction mechanism during the hydrogenation/dehydrogenation process for the Li₃N–MgH₂ system is discussed.     

Hydrogen absorption characteristics of mechanically alloyed Ti–Zr–Ni and Ti–V–Ni powders

A first investigation into the production of amorphous and nanostructured Ti-based alloys with nominal compositions Ti_41.5Zr_41.5Ni₁₇, Ti₆₁Zr₂₂Ni₁₇, Ti_41.5V_41.5Ni₁₇ and Ti₆₁V₂₂Ni₁₇ by mechanical alloying (MA) technique is presented. This technique was adopted to produce alloys' powders with high fresh surface area that were active for hydrogen storage. Hydrogen absorption characteristics and structure changes in the alloys after hydrogenation were investigated. Gas phase hydrogenation of the Ti–Zr–Ni alloys, at 573 K and an initial hydrogen pressure of 2 MPa, exhibited good hydriding properties and started at a maximal rate without induction period with a hydrogenation capacity up to 1.2 wt%. However, hydriding of Ti–V–Ni alloys at the same conditions exhibited slower rates. The Ti₆₁V₂₂Ni₁₇ composition showed high hydrogen absorption capacity of 1.8 wt% and exceeded 4 wt% at 345 K. In addition, the Ti–V–Ni alloys showed structure stability after hydrogenation and retained the amorphous structure.     

Improved hydrogen storage performance of the LiNH2–MgH2–LiBH4 system by addition of ZrCo hydride

Significant improvements in the hydrogen absorption/desorption properties of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ composite have been achieved by adding 3wt% ZrCo hydride. The composite can absorb 5.3wt% hydrogen under 7.0 MPa hydrogen pressure in 10 min and desorb 3.75wt% hydrogen under 0.1 MPa H₂ pressure in 60 min at 150 °C, compared with 2.75wt% and 1.67wt% hydrogen under the same hydrogenation/dehydrogenation conditions without the ZrCo hydride addition, respectively. TPD measurements showed that the dehydrogenation temperature of the ZrCo hydride-doped sample was decreased about 10 °C compared to that of the pristine sample. It is concluded that both the homogeneous distribution of ZrCo particles in the matrix observed by SEM and EDS and the destabilized N–H bonds detected by IR spectrum are the main reasons for the improvement of H-cycling kinetics of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ system.     

Composition dependent microstructure evolution,activation and de-/hydrogenation properties of Mg–Ni–La alloys

In this work, in order to elucidate the effect of different alloying elements on the microstructure, activation and the de-/hydrogenation kinetics performance, the Mg–20La, Mg–20Ni and Mg–10Ni–10La (wt.%) alloys have been prepared by near equilibrium solidification combined with high-energy ball milling treatment to realize the internal optimization as well as particle refinement. The results show that the microstructures of the prepared alloys are significantly refined by forming different types and sizes of intermetallic compounds. Meanwhile, the effects of LaH₃ and Mg₂Ni within the activated samples on de-/hydrogenation kinetics are also discussed. It is observed that the alloy containing LaH₃ preserves stable hydrogenation behavior between 573 and 623 K, while the hydrogenation properties of the alloy containing Mg₂Ni is susceptible to temperature. A preferable hydrogenation performance is observed in Mg–10Ni–10La alloy, which can absorb as high as 5.86 wt% hydrogen within 15 min at 623 K and 3.0 MPa hydrogen pressure. Moreover, the desorption kinetics and the desorption activation energies are evaluated to illustrate the mechanism based on improved dehydrogenation performance. The addition of proper alloying elements Ni and La in combination with reasonable processing is an effective strategy to improve the de-/hydrogenation performance of Mg-based alloys.     

Measurement and modelling of kinetics of hydrogen sorption by LaNi5 and two related pseudobinary compounds

The hydriding/dehydriding rates and the pressure–composition isotherms were measured for LaNi5, LaNi_4.85Al_0.15 and LaNi_4.75Fe_0.25 under quasi-isothermal and variable pressure conditions. Isothermal conditions were obtained by reducing the thermal time constant of the experimental device. Empirical rate equations to describe the sorption reaction kinetics were derived. These rates are expressed as a function of temporal composition, saturated composition, temperature, applied pressure and essentially the initial operating conditions which were not considered in most of all the previous studies related to the reaction kinetics of metal hydrides. Besides, the rate equations presented in this work can be integrated easily in the numerical models that predict dynamic flow and heat and mass transfer within realistic metal–hydrogen devices. This paper also discusses the effects of Fe and Al as substituents for Ni on P–C isotherms and reaction rates of LaNi₅ alloy.     

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.     

Role of atomic hydrogen supply on the onset of CO2 methanation over La–Ni based hydrogen storage alloys studied by in-situ approach

Mechanochemical CO₂ methanation reactions using LaNi₅ and LaNi_4.6Al_0.4 hydrogen storage alloy powders were investigated by the in-situ monitoring of the gas pressure change during ball-milling. Methane generation begins when the H₂ partial pressure drops due to the H-uptake by the powder. Phase transition occurred in the sample after milling for 15 min and 224 min, with separate metallic Ni, La-oxide and La-hydroxide phases observed. Methane generation continued even after this phase separation. Our results imply that the formation of La-hydroxide at the surface and sub-surface contributed to methane generation during ball-milling. A comparison of LaNi₅ and LaNi_4.6Al_0.4 suggests the amount of hydrogen stored in the hydrogen storage powder dominates the timing of the onset of the methane generation.     

The effect of transition metals on hydrogen migration and catalysis in cast Mg-Ni alloys

The inexpensive fabrication technique of casting is applied to develop new Mg-Ni based hydrogen storage alloys with improved hydrogen sorption properties. A nanostructured eutectic Mg-Mg₂Ni is formed upon solidification which introduces a large area of interfaces along which hydrogen diffusion can occur with high diffusivity. After a few cycles of hydrogenation and dehydrogenation, an ultrafine porous structure formed in the eutectic Mg-Mg₂Ni and some cracks developed along the interface between the eutectic and the α-Mg matrix. This indicates that hydrogen atoms introduced into the alloys preferentially migrate along the interfaces in the nanostructured eutectic which enables effective short-range diffusion of hydrogen. Furthermore, transition metals (TMs) such as Nb, Ti and V in the range 240-560 ppm are added directly to molten Mg-10 wt% Ni alloys and are found to form intermetallic compounds with Ni during solidification. The alloys can store 5.6-6.3 wt% hydrogen at 350 °C and 2 MPa. TM-rich intermetallics distributed homogeneously in the cast alloys appear to play a key role in accelerating the nucleation of Mg from MgH₂ upon dehydrogenation. This leads to a significant improvement in the hydrogen desorption kinetics.     

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.