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.     

Electrochemical reduction of La(Ni3.6Co0.7Al0.4Mn0.3) (LaMM) deuterides investigated by in situ neutron powder diffraction: Following the metal-hydride phase transition under technical operating conditions in a KOD electrolyte

We investigated the first charge cycle of LaNi_3.6Co_0.7Al_0.4Mn_0.3 (LaMM) during electrochemical reduction in a 6N KOD (potassium deuteroxide) electrolyte, corresponding to conditions of commercially used batteries by means of in situ neutron powder diffraction. Our measurement allowed to directly analyze the phase range of the α and β phases and the related volume change as a function of the charge transfer. The intercalation of hydrogen was followed in a home-made electrochemical cell, installed on the high intensity neutron powder diffractometer (DMC) at the Swiss continuous spallation neutron source. Compared to previous investigations following mostly in situ charging under pressure (following pressure–composition–temperature isotherms, PCT), our experimental conditions reflect closely the process as used in technical battery applications.     

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.     

Investigation on kinetics mechanism of hydrogen absorption in the La2Mg17-based composites

A new model has been successfully used to investigate the hydrogen absorption kinetics mechanism of La₂Mg₁₇-based composites. The results indicate that different preparation conditions lead to different rate-controlling steps during hydrogen absorption process. For La₂Mg₁₇–LaNi₅ composite synthesized by the method of melting, the rate-controlling step is the surface penetration of hydrogen atoms, which does not change by addition agent (LaNi₅). However, mechanical milling can change the rate-limiting steps of hydriding reaction in the La₂Mg₁₇–LaNi₅ composite from surface penetration to diffusion of hydrogen in the hydride layer. With the enhancement of milling intensity, the rate-controlling step in La_1.8Ca_0.2Mg₁₄Ni₃ alloy changes from surface penetration to diffusion. In addition, the activation energies of hydrogen absorption for La₂Mg₁₇−20 wt%LaNi₅ and La_1.8Ca_0.2Mg₁₄Ni₃ are obtained by this model.     

Effect of crystal transformation on electrochemical characteristics of La–Mg–Ni-based alloys with A2B7-type super-stacking structures

La_0.75Mg_0.25Ni_3.5 alloys with hexagonal (2H-) and rhombohedral (3R-) (La,Mg)₂Ni₇ phase were created by powder metallurgy. Partial crystal transformation of 2H- into 3R-type allotropes was realized by heat treatment and introducing LaNi₅ compound. It was found that the alloy annealed within 1073–1223 K kept (La,Mg)₂Ni₇ phase and obvious crystal transformation of 2H- into 3R-type occurred as annealing temperature reached 1223 K. Electrochemical study showed similar discharge capacity and degradation behavior for La_0.75Mg_0.25Ni_3.5 alloys with different amounts of 2H- and 3R-type allotropes while HRD was promoted by increasing 3R-type phase abundance. Introducing LaNi₅ into La_0.75Mg_0.25Ni_3.5 alloy increased 3R- to 2H-type phase ratio and led to an additional plateau in P–C isotherms. LaNi₅ introduction improved HRD, however it accelerated cycling degradation. Rietveld analysis indicated that after hydrogenation, the cell expansion of 2H- and 3R-type (La,Mg)₂Ni₇ phase was similar while the cell expansion of LaNi₅ phase was smaller than that of (La,Mg)₂Ni₇ phase. This caused discrete cell expansion between (La,Mg)₂Ni₇ and LaNi₅ phases, leading to severe pulverization and oxidation.     

Self-ignition combustion synthesis of LaNi5 at different hydrogen pressures

In this paper, we describe the self-ignition combustion synthesis (SICS) of LaNi₅ utilizing the hydrogenation heat of metallic calcium at different hydrogen pressures, and focus on the effect of hydrogen pressure on the ignition temperature and the initial activation of hydrogenation. In the experiments, La₂O₃, Ni, and Ca were dry-mixed, and then heated at 0.1, 0.5, and 1.0 MPa of hydrogen pressure until ignition due to the hydrogenation of calcium. The products were recovered after natural cooling for 2 h. The results showed that the ignition temperature lowered with hydrogen pressure. The products changed from bulk to powder with hydrogen pressure. This was probably caused by volume expansion due to hydrogenation at higher pressure. The product obtained at 1.0 MPa showed the highest hydrogen storage capacity under an initial hydrogen pressure of 0.95 MPa. The results of this research can be applied as an innovative production route for LaNi₅ without the conventional melting of La and Ni.     

Review on hydrogen absorbing materials—structure,microstructure, and thermodynamic properties

Hydrogen is a promising renewable fuel for transportation and domestic applications. Many systems have been investigated in order to improve the maximum hydrogen storage capacity (reversible), high kinetics, moderate equilibrium pressure and/or decomposition temperature and better cyclability. In this paper, a review of studies related to stability of Zr-based Laves phase system as well as in situ neutron diffraction investigation, the kinetics of TiFe, surface treatment of LaNi₅ system, mechanically alloyed Mg-based hydrides and graphite nanofibres are reported.     

Measurement and analysis of reaction kinetics of La – based hydride pairs suitable for metal hydride – based cooling systems

The reaction kinetics of metal hydride pairs consisting of La_0.9Ce_0.1Ni₅, La_0.8Ce_0.2Ni₅, LaNi_4.7Al_0.3 and LaNi_4.6Al_0.4 were measured at different temperatures to determine their suitability for metal hydride – based cooling systems (MHCSs). The effect of operating conditions and compositional changes on driving potential and reaction rates during cooling and regeneration processes was studied. The reaction rates were increased with Ce content and decreased with Al content. The cooling and regeneration time of MHCS, for working temperature range of 140 °C (heat source), 25 °C (heat sink) and 10 °C (cooling), were measured for each pair. The estimated cycle time took the following trend (La_0.8Ce_0.2Ni₅ – LaNi_4.7Al_0.3) < (La_0.9Ce_0.1Ni₅ – LaNi_4.7Al_0.3) < (La_0.8Ce_0.2Ni₅ – LaNi_4.6Al_0.4) < (La_0.9Ce_0.1Ni₅ – LaNi_4.6Al_0.4). Two reaction kinetics models namely Jander diffusion model (JDM) and Park – Lee model (PLM) were studied and employed for reaction kinetics analyses. The activation energies (E) of these hydrides were calculated using the Arrhenius plot. Estimated values of activation energies from these models were compared by substituting in the hydriding expression and accurate values of activation energies established for these hydrides.     

Combined effects of hydrogen back-pressure and NbF5 addition on the dehydrogenation and rehydrogenation kinetics of the LiBH4–MgH2 composite system

It is well known that the dehydrogenation pathway of the LiBH₄–MgH₂ composite system is highly reliant on whether decomposition is performed under vacuum or a hydrogen back-pressure. In this work, the effects of hydrogen back-pressure and NbF₅ addition on the dehydrogenation kinetics of the LiBH₄–MgH₂ system are studied under either vacuum or hydrogen back-pressure, as well as the subsequent rehydrogenation and cycling. For the pristine sample, faster desorption kinetics was obtained under vacuum, but the performance is compromised by slow absorption kinetics. In contrast, hydrogen back-pressure remarkably promotes the absorption kinetics and increases the reversible hydrogen storage capacity, but with the penalty of much slower desorption kinetics. These drawbacks were overcome after doping with NbF₅, with which the dehydrogenation and rehydrogenation kinetics was significantly improved. In particular, the enhanced kinetics was observed to persist well, even after 9 cycles, in the case of the NbF₅ doped sample under hydrogen back-pressure, as well as the suppression of forming Li₂B₁₂H₁₂. Furthermore, the mechanism that is behind these effects of NbF₅ additive on the reversible dehydrogenation reaction of the LiBH₄–MgH₂ system is discussed.     

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.     

Advanced reactor concept for complex hydrides: Hydrogen absorption from room temperature

Complex hydride materials (CxH) are potential candidates for hydrogen storage in automotive applications due to their high hydrogen storage capacities. However, the reaction rates of these materials are rather low at temperatures below 100 °C implying negative effects on absorption performance e.g. at a fuelling station. In this paper simulated and experimental results of a new reactor concept that can improve the dynamic reactor performance are presented. This concept is based on the combination of a metal hydride (MeH) and a CxH in one reactor, separated by a gas permeable layer. The storage capacity of available MeH materials is just ∼1 wt.%, however, they show very high reaction rates even at room temperature. Thus, the idea of this concept is to combine both: the high storage capacity of the CxH material and the high reaction rate of the MeH material. The two reference materials for this study are 2LiNH₂–1.1MgH₂–0.1LiBH₄–3 wt.%ZrCoH₃ (Li–Mg–N–H) and LaNi_4.3Al_0.4Mn_0.3 (MeH). In the first part, 2D simulation results are presented showing the development of a reaction front from the core to the annulus of the tubular reactor caused by the fast exothermal absorption reaction of the MeH material. In the second part, experimental results of a 50 g lab-scale reactor and simulated scenarios are presented and used for model validation. In the present scenario it has been possible to reduce the time to initiate the absorption reaction from room temperature by approximately 500 s.     

Preparation of the composite LaNi3.7Al1.3/Ni–S–Co alloy film and its HER activity in alkaline medium

The composite LaNi_3.7Al_1.3/Ni–S–Co alloy film was prepared by molten salt electrolysis and aquatic electrodeposition orderly. With Na₃AlF₆–La₂O₃–Al₂O₃ (91:8:1) system as molten salt electrolyte, the LaNi_3.7Al_1.3 alloy film was obtained by galvanostatic electrolysis at 100 mA cm⁻². The results showed that the La³⁺ and Al³⁺ ions could be co-reduced on the nickel cathode to form LaNi_3.7Al_1.3 film, i.e. La³⁺ + 1.3Al³⁺ + 6.9e + 3.7Ni = LaNi_3.7Al_1.3 at c.a. −0.5 V, which is much lower than that of the theoretical decomposition potential of lanthanum and aluminum. With high HER activity, the composite LaNi_3.7Al_1.3/Ni–S–Co film (η₁₅₀ = 65 mV, 353 K) could absorb large amount of H atoms, which would be oxidized and therefore effectively avoid the dissolution of the Ni–S–Co film under the state of open-circuit and consequently prolong the lifetime of the cathode.     

Effect of Mg content on structure,hydrogen storage properties and thermal stability of melt-spun Mgx(LaNi3)100−x alloys

Amorphous Mg_x(LaNi₃)_100−x (x = 40, 50, 60, 70) alloys with ribbon shape (5 mm wide, 0.2 mm thick) have been prepared by rapid solidification, using a melt-spinning technique. Their microstructure, hydrogen storage properties and thermal stability were studied by means of XRD, SEM, PCTPro2000 and DSC analysis, respectively. The results indicated that when Mg_x(LaNi₃)_100−x alloys have been hydrogenated at 573 K under 2 MPa hydrogen pressure, LaH₃ phase is formed in the case of x (x = 40, 50, 60, 70), Mg₂NiH₄ phase formed in the case of x (x = 40, 50, 60, 70), Mg₂NiH_0.3 phase formed in the case of x (x = 40, 50), and MgH₂ phase formed in the case of x = 70. Experimental data of hydrogen desorption kinetics, tested at 523 K, 573 K and 623 K, are in good agreement with Avrami–Erofeev equation. The maximum hydrogen absorption capacity is 2.71 wt.% for Mg₇₀(LaNi₃)₃₀ and 2.35 wt.% for Mg₇₀(LaNi₃)₃₀, the increase of hydrogen desorption capacity is in the order of x = 70 > x = 60 > x = 50 > x = 40. Based on DSC analysis, the activation energies for dehydrogenation of these samples are calculated to be 122 ± 2 kJ/mol (x = 40) > 101 ± 3 kJ/mol (x = 50) > 84 ± 5 kJ/mol (x = 60) > 64 ± 3 kJ/mol (x = 70), which are in agreement with the results of hydrogen desorption kinetics.     

Preparation and electrochemical properties of composite LaNix/Ni–S–Co alloy film

The composite LaNi_x/Ni–S–Co film with considerable stability and high HER activity (η₁₅₀ = 70 mV, 353 K) was obtained by molten salt electrolysis combined with aquatic electrodeposition. LaNi_x film was prepared by galvanostatic electrolysis at 100 mA cm⁻² under 1273 K. The results showed that the La³⁺ ions could be reduced on the nickel cathode and the LaNi_x film could form, i.e. La³⁺ + 3e + xNi = LaNi_x (x = 5 or 3) at ca. −0.6 V, which is much lower than that of the decomposition potential of lanthanum, due to the strong depolarization effect of nickel. Furthermore, compared with the traditional amorphous Ni–S film, the composite LaNi_x/Ni–S–Co film could absorb large amount of H atoms, which would be oxidized and avoid the dissolution of the Ni–S–Co film under the state of open-circuit effectively and increase the HER activity.     

Advanced reactor concept for complex hydrides: Hydrogen desorption at fuel cell relevant boundary conditions

Hydrogen storage in complex hydrides can be a storage option in automotive applications due to the high theoretical hydrogen storage capacities. As hydrogen is bonded by a chemical reaction to the solid state storage material, it is just released when heat is provided. In an automotive application for complex hydrides, this heat source can possibly be provided by the waste heat of a high temperature PEM fuel cell. However, for the application of existing complex hydride materials the temperature level of the fuel cell at 180 °C is still quite low leading to low desorption rates. A new reactor concept based on the addition of a metal hydride (MeH) to a complex hydride (CxH) reactor, separated by a gas permeable separation layer, can improve the desorption performance. In this paper, the effects of this reactor concept on desorption performance are studied using the two reference materials 2LiNH₂–1.1MgH₂–0.1LiBH₄–3 wt.%ZrCoH₃ and LaNi_4.3Al_0.4Mn_0.3. First, a model is developed and 2D simulations are performed using a driving scenario. Then, the model is validated by experimental data that has been obtained using a ∼600 g lab scale reactor. Concluding, there exist two main advantages of the combination reactor concept: The desorption time at a pressure above the fuel cell supply pressure is extended by a factor of 1.2, and 94 instead of 84% of the max. mass of hydrogen stored in the material can be desorbed at technically relevant boundary conditions.     

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.     

Comprehensive hydrogen storage properties and catalytic mechanism studies of 2LiBH4–MgH2 system with NbF5 in various addition amounts

In the present work, the role of NbF₅ addition amount in affecting the comprehensive hydrogen storage properties (dehydrogenation, rehydrogenation, cycling performance, hydrogen capacity) of 2LiBH₄–MgH₂ system as well as the catalytic mechanism of NbF₅ have been systematically studied. It is found that increasing the addition amount of NbF₅ to the 2LiBH₄–MgH₂ system not only results in dehydrogenation temperature reduction and hydriding–dehydriding kinetics enhancement but also leads to the de/rehydrogenation capacity loss. Compared with other samples, 2LiBH₄–MgH₂ doping with NbF₅ in weight ratios of 40:4 exhibits superior comprehensive hydrogen storage properties, which can stably release ∼8.31 wt.% hydrogen within 2.5 h under 4 bar H₂ and absorb ∼8.79 wt.% hydrogen within 10 min under 65 bar H₂ at 400 °C even up to 20 cycling. As far as we know, this is the first time that excellent reversibility as high as 20 cycles without obvious degradation tendency in both of hydrogen capacity and reaction rate has been achieved in the 2LiBH₄–MgH₂ system. The further experimental study reveals that the highly catalytic effects of NbF₅ on the 2LiBH₄–MgH₂ system are derived from the reaction between NbF₅ and LiBH₄, which provides a fundamental insight into the catalytic mechanism of NbF₅.     

An investigation on electrochemical and gaseous hydrogen storage performances of as-cast La1−xPrxMgNi3.6Co0.4 (x = 0–0.4) alloys

The as-cast RE–Mg–Ni-based AB₂-type La_1−xPr_xMgNi_3.6Co_0.4 (x = 0–0.4) alloys were prepared by vacuum induction furnace with a high purity helium gas as the protective atmosphere. The phase composition and microstructure of the as-cast alloys were characterized by XRD, SEM equipped with EDS. The results indicate that the as-cast alloys consist of two phases of LaMgNi₄ and LaNi₅. The measurements of the electrochemical properties show that the discharge capacity of the alloys slightly decreases with Pr content rising. As the Pr content grows from 0 to 0.4, the maximum discharge capacity decreases from 347.0 to 310.4 mAh/g. However, the cycle stability and the high-rate dischargeability of the alloy obviously augment with the Pr content increasing. Furthermore, the measurements of the electrochemical hydrogen storage kinetics reveal that the limiting current density (I_L) first increases then decreases whereas the exchange current density I₀ of the alloys first decreases then increases with the rising amount of Pr substitution, which indicates that the electrochemical dynamic of the alloy electrode are jointly dominated by the charge-transfer resistance and diffusion ability of hydrogen atoms. The measuring of the gaseous hydrogen storage reveals two pressure plateaus appear on each pressure–concentration–isotherm (P–C–T) curve of the as-cast alloys, which correspond to the LaMgNi₄ and LaNi₅ phases. Furthermore, we note that the pressure plateau of the P–C–T curve visibly rises with Pr content increasing.     

Enhanced reversible hydrogen storage performance of NbCl5 doped 2LiH–MgB2 composite

2LiBH₄ + MgH₂ system is considered as an attractive candidate for reversible hydrogen storage with high capacity and favorable thermodynamics. However, its reaction kinetics has to be further improved for the practical application. In this work, we investigated the effect of NbCl₅ additive on the de/hydrogenation kinetics and microstructure refinement in 2LiH–MgB₂ composite systematically. The hydrogenation and dehydrogenation kinetics of 2LiH–MgB₂ composite can be significantly enhanced with the increase of NbCl₅ content. The 3 mol% NbCl₅ doped 2LiH–MgB₂ composite exhibits the superior reversible hydrogen storage performance, which requires 50 min to uptake 9.0 wt% H₂ at 350 °C and release 8.5 wt% H₂ at 400 °C, respectively. In contrast, the undoped 2LiH–MgB₂ sample uptakes 6.2 wt% H₂ and releases 3.1 wt% H₂ under identical measurement conditions. Moreover, the 3 mol% NbCl₅ doped 2LiH–MgB₂ composite can release more than 9.0 wt% H₂ within 300 min at 400 °C without obvious degradation of capacity over the first 10 cycles. Microstructure analyses clearly indicate that NbCl₅ additive first reacts with LiH to form Nb and LiCl during ball-milling process, and then NbH is formed after the first hydrogenation and stabilized upon further de/hydrogenation cycling. The well-distributed NbH active species play an important role in the improvement of de/hydrogenation kinetics for Li–Mg–B–H system through facilitating hydrogen diffusion rapidly as well as prevent the particles from further growth in the subsequent hydrogenation and dehydrogenation processes.     

Synthesis and hydrogen storage characteristics of Mg–B–H compounds by a gas–solid reaction

Magnesium borohydride [Mg(BH₄)₂] is an attractive complex hydride for hydrogen storage. In this study, attempts to synthesize Mg(BH₄)₂ were carried out by a solid–gas reaction through MgH₂ and B₂H₆ in the absence of a liquid medium. The source of B₂H₆ was obtained by heating a mixture of NaBH₄ and ZnCl₂. The profile of pressure versus temperature indicated that the absorption kinetics of B₂H₆ by MgH₂ were slow. Structural analysis confirmed the formation of Mg–B–H compounds. The reaction products presented two-step hydrogen release during heating. A small amount of hydrogen could be released from the as-synthesized Mg–B–H compounds at a low temperature of 215 °C. The slow reaction kinetics were significantly affected by the surface conditions of the MgH₂ powders.