Hydrogen storage in rapidly solidified and crystallized Mg–Ni-(Y,La)–Pd alloys

Amorphous-crystalline composite ribbons of quaternary Mg–Ni–(Y,La)–Pd alloys are produced via rapidly solidification and used as precursors for creating nanocrystalline hydrogen storage materials. The resulting materials demonstrate relatively high hydrogen capacity of around 4.5 mass% H and excellent absorption/desorption kinetics at 573 K. Additionally, the alloys demonstrate reversible hydrogen storage at 473 K. A composition of Mg₈₅Ni₁₀Y_2.5Pd_2.5 fully absorbs and desorbs 4.6 mass% H in 90 min. The cyclability of the quaternary alloys demonstrates good stability, with little loss in maximum capacity through 8–10 cycles. This has been attributed to the improved stability of the nanocrystalline structure attained via the Y and La additions. Thermodynamically, the enthalpy of the hydrogen absorption reaction is reduced by 5 kJ/mol in the quaternary alloys, compared to Mg-MgH₂; while the entropy of reaction is also reduced.     

Hydrogen storage measurements in novel Mg-based nanostructured alloys produced via rapid solidification and devitrification

Nanostructured materials for hydrogen storage with a composition of Mg₈₅Ni_15−xM_x (M = Y or La, x = 0 or 5) are formed by devitrification of amorphous and amorphous-nanocrystalline precursors produced by melt-spinning. All three compositions exhibit a maximum storage capacity of about 5 mass % H at 573 K. When ball-milled for 30 min in hexanes, the binary alloy can be activated (first-cycle hydrogen absorption) at 473 K. DSC experiments indicate that desorption in this sample begins at 525 K, compared to 560 K when the material is activated at 573 K; which indicates an improvement in the hydride reaction thermodynamics due to capillarity effects. Additions of Y and La improve the degradation in storage capacity observed during cycling of the binary alloy by slowing microstructural coarsening. Alloying with La also shows a decrease of about 8 kJ/mol and 5 kJ/mol in the enthalpy of reaction for MgH₂ and Mg₂NiH₄ formation, respectively, compared to the binary alloy; resulting in some desorption of H₂ at 473 K. The improved thermodynamics are discussed in terms of destabilization of the hydrides relative to new equilibrium phases introduced by alloying additions. The proposed hydriding reaction for the La-containing material is in agreement with previously reported experimental results.     

Stress measurement of MlNi4.5Cr0·45Mn0.05 alloy during hydrogen absorption-desorption process in a cylindrical reactor

Hydrogen stored in a solid state form of metal hydrides offers a safe and efficient storage technique for hydrogen application. In a closed metal hydride tank, stresses may occur on the tank wall due to hydride expansion during hydrogen absorption process. In the present investigation, a novel testing system for stress evolution of MlNi_4.5Cr_0.45Mn_0.05 alloy in a closed cylindrical reactor during hydrogen absorption-desorption process was built. The results show that considerable swelling stress is developed on the inner reactor wall during activation process though a high free space of 45% is presented. Increasing hydrogen charging pressure and alloy loading fraction increase the as-generated swelling stress. The metal hydride particle expansion caused by hydrogen absorption is the intrinsic factor for swelling stress evolution. The presence of particle agglomerate in a closed tank in which its expansion is constrained is responsible for the observed swelling stress accumulation.     

Experimental investigations and a simple balance model of a metal hydride reactor

The metal hydride reactor filled with 5 kg of the AB₅-type (LaFe_0.5Mn_0.3Ni_4.8) alloy was investigated with respect to the hydrogen discharge rates classified using C-rate value, which is discharge of the maximum hydrogen capacity 750 st L within 1 h. The reactor cannot be fully discharged with a constant flow rate, for each temperature of hot water and flow rate there exists a moment of crisis at which the hydrogen flow drops under the constant value. The nominal capacity of the reactor reaches 80% of maximum capacity if sufficient heat transfer is provided. The simple balance model of a metal hydride reactor is developed based on the assumption of uniform temperature and pressure inside a metal hydride bed. The model permits to predict behavior of the metal hydride reactor in different operation regimes, quantitative agreement is obtained for low C-rates (less than 4) and sub-critical modes.     

Cycling stability of RNi5 (R = La,La+Ce) hydrides during the operation of metal hydride hydrogen compressor

This work presents results of the experimental studies (XRD, SEM, PCT) of hydride forming intermetallides used in the first (LaNi₅) and the second (La_0.5Ce_0.5Ni₅) stages of industrial-scale metal hydride hydrogen compressor providing H₂ compression from 3.5 to 150 atm with the productivity about 10 Nm³/h. During the operation, both materials underwent 18,180 hydrogenation/dehydrogenation (h/d) cycles which included H₂ absorption at the pressure of 3.5 atm (LaNi₅) and 35–38 atm (La_0.5Ce_0.5Ni₅) at T = 15–20 °C followed by H₂ desorption at the pressure of 35–38 atm (LaNi₅) and 150 atm (La_0.5Ce_0.5Ni₅) at T = 150–160 °C. It was found that the observed ～30% drop of the productivity of the compressor by the end of its operation is associated with a degradation of the first stage hydride material (LaNi₅) under conditions specified above. The cycling resulted in the appearance of Ni and LaH_2+x phases in addition to the parent intermetallide. In turn, the cycled LaNi₅ exhibited more than 20% lower hydrogen storage capacity than the alloy at the beginning of the cycling; the cycling was also found to result in a noticeable sloping of initially flat plateau. Conversely, the degradation effects in La_0.5Ce_0.5Ni₅ were found to be much less pronounced, in spite of the higher operating H₂ pressures. The observed effect was associated with the decrease of thermodynamic driving force (TDF) of AB₅ disproportionation in H₂ when substituting La with Ce.     

Design and optimization of single particle electrodes for the kinetic analysis of hydrogen evolution and absorption on hydrogen storage alloys

In the literature, there is a large discrepancy between reported values of electrochemical, kinetic and transport parameters of hydrogen storage alloys. These discrepancies arise, because in most cases, electrodes are prepared with the powdered alloy supported within a porous matrix, constituted by carbon and additive binders such as PTFE. The main drawback, of this preparation technique, for the identification of kinetic parameters, is the uncertainty in the specific active area value, where the hydrogen evolution and absorption processes take place. To overcome the disadvantages described, a new type of electrode, was designed, using a single particle of AB₅ and AB₂ hydride forming alloys. The data obtained from electrochemical impedance measurements were adjusted in terms of a physicochemical model that takes into account the processes of hydrogen evolution and absorption coupled to hydrogen diffusion. From the study it can be concluded that the differences in the behavior of the AB₅ and AB₂ alloys, presenting the first best performance during the activation and operation at high discharge currents, are mainly due to higher values of the exchange current density and the diffusion coefficient of H for the AB₅ alloy.     

Thermodynamical properties of La–Ni–T (T = Mg, Bi and Sb) hydrogen storage systems

The hydrogen absorption properties of LaNi_4.8T_0.2 (T = Mg, Bi and Sb) alloys are reported. The effects of the substitution of Ni in the LaNi₅ compound with Mg, Bi and Sb are investigated. The ability of alloys to absorb hydrogen is characterized by the pressure–composition (p–c) isotherms. The p–c isotherms allow the determining thermodynamic parameters enthalpy (ΔH^des) and entropy (ΔS^des) of the dehydrogenation processes. The calculated ΔH^des and ΔS^des data helps to explain the decrease of hydrogen equilibrium pressure in alloys doped with Al, Mg and Bi and its increase in the Sb-doped LaNi₅ compound. Generally, partial substitution of Ni in LaNi₅ compound with Mg, Bi and Sb cause insignificant changes of hydrogen storage capacity compared to the hydrogen content in the initial LaNi₅H₆ hydride phase. However, it is worth to stress that, in the case of LaNi_4.8Bi_0.2, a small increase of H/f.u. up to 6.8 is observed. The obtained results in these investigations indicate that the LaNi_4.8T_0.2 (T = Al, Mg and Bi) alloys can be very attractive materials dedicated for negative electrodes in Ni/MH batteries.     

The effect of particle size on hydrolysis properties of Mg3La hydrides

The effect of particle size on hydrolysis properties of hydrogenated Mg₃La was investigated through fixing the grain size of in-situ formed LaH₃ and MgH₂ from hydrogenated Mg₃La. The hydrolysis rate and hydrogen yield were affected by its particle size. The hydrogenated Mg₃La with smaller particle size of [<12] μm had a higher hydrolysis yield of 863 ml g⁻¹ (7.70 wt.%) hydrogen. The surface area and defect of samples were increased through reducing the particle size and thus promoted the complete hydrolysis. Reducing the particle size is an effective and simple method to improve hydrolysis properties of magnesium hydrides-based materials.     

Properties of La0.2Y0.8Ni5−xMnx alloys for high-pressure hydrogen compressor

Hydrogen absorption/desorption properties of La_0.2Y_0.8Ni_5−xMn_x (x = 0.2, 0.3, 0.4) alloys for high-pressure hydrogen compression application were investigated systematically. The Pressure–Composition isotherms and absorption kinetics were measured at 293, 303 and 313 K by the volumetric method. XRD analyses showed that all the investigated alloys presented CaCu₅ type hexagonal structure and the unit cell volume increased in both a and c lattice axes with Mn substitution. Hydrogen absorption/desorption measurements revealed that Mn could lower the plateau pressure effectively, improve the hydrogen storage capacity and absorption kinetics but slightly increase the slope of the pressure plateau and hysteresis. The study results suggest that La_0.2Y_0.8Ni_4.8Mn_0.2 is suitable for the high-pressure stage compression of the hydrogen compressor and the other two alloys, La_0.2Y_0.8Ni_4.7Mn_0.3 and La_0.2Y_0.8Ni_4.6Mn_0.4, for the preliminary stage.     

Development of large MH tank system for renewable energy storage

Metal Hydrides (MH) can absorb large quantities of hydrogen at room temperature and ordinary pressure. Because MH can store hydrogen at a pressure less than 0.1 MPa safely and compactly, it is looked to as a method of storing hydrogen produced by electricity derived from renewable energy sources. To study this method of storing renewable energy, we made a MH tank system which could store hydrogen in the range of 1000 Nm³. A Mm-NiMnCo alloy was used for this MH tank system. MH becomes pulverized with absorbing and desorbing hydrogen, and this causes the problem of MH tank transformation owing to the partial distribution of the pulverized MH powders. Our MH material, named “Hydrage?,” was made using a technique to compose the MH powders with polymer materials without decreasing the hydrogen absorption and desorption rate. With this technique, the MH powders were immobilized, and strain on the MH tank was reduced. Furthermore, this technique enabled uniform dispersion of the MH powders, and high-density filling in MH tank was achieved relative to that attainable in a conventional MH tank. An MH tank system with a capacity of 1000 Nm³ is 1,800 mm in width, 3,150 mm in length, and 2,145 mm in height. The system for renewable energy storage consists of 9 tanks. About 7.2 tons of MH were used in this system. This system could work at temperatures from 25 to 35° C, and its maximum hydrogen absorption and desorption rate is 70 Nm³/h with a medium flow rate of 30 NL/min. This type of MH tank system, which can store a large amount of hydrogen safely and compactly, has the potential to become popular with various applications in the future.     

Influence of electrostatic field on the interaction of AB5-type alloy LaNi4.4Al0.3Fe0.3 with hydrogen

The effect of the electrostatic field on hydrogen absorption is experimentally studied for the case of AB₅-type intermetallic compound LaNi_4.4Al_0.3Fe_0.3 with low equilibrium pressure. Experimental facility contained control and measurement system for PCT-isotherms and a non-conductive polymer vessel immersed in a bath of a thermostat with transformer oil. The test sample with 100 g of the activated alloy powder was used. Electrostatic field was created between a copper tube, which simultaneously served as a hydrogen inlet, connected to a high voltage source and a grounded nickel plate rolled in the form of a cylinder around the outer wall of the vessel. The electrodes were arranged coaxially, the maximum voltage on the internal electrode was 15 kV. The high voltage source also allowed changing the polarity on the internal electrode.It was found that the electrostatic field had no effect on the already established equilibrium in the hydrogen-alloy system at a voltage at the electrode up to 15 kV, regardless of the polarity. However, the process of hydrogen absorption is noticeably slowed down when a voltage of up to 15 kV with negative polarity is applied to the internal electrode, and the effect increases with increasing voltage. At a voltage of 15 kV and the positive polarity of the internal electrode, there was no noticeable effect on the hydrogen absorption process.     

Hydrogen desorption from a hydride container under different heat exchange conditions

The desorption behavior of a hydrogen storage prototype loaded with AB₅H₆ hydride, whose equilibrium pressure makes it suitable for both feeding a PEM fuel cell and being charged directly from a low pressure water electrolyzer without need of additional compression, was studied. The nominal 70 L hydrogen storage capacity of the container (T = 20 °C, P = 101.3 kPa) suffices for ca. 2.5 h operation of a 50 W hydrogen/oxygen fuel cell stack. The hydride container is provided with aluminum extended surfaces to enhance heat exchange with the surrounding medium. These surfaces consist of internal disk-shaped metal foils and external axial fins. The characterization of the storage prototype at different hydrogen discharge flow rates was made by monitoring the internal pressure and the temperatures of the external wall and at the center inside the container.     

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.