Development of a Sievert apparatus for characterization of high pressure hydrogen sorption materials

A volumetric gas absorption (Sievert) apparatus has been developed to measure hydrogen absorption and desorption at pressures up to 700 bar and temperatures between 240 K and 320 K. The apparatus is designed to reduce uncertainty for high pressure measurements while maintaining proper temperature control in the sample. Pressure-composition isotherms (PCI) and kinetics measurements of a well-studied material, LaNi₅ have been obtained for validation of the apparatus. Measurements of both absorption and desorption PCI curves as well as full absorption kinetics data have been obtained for TiCrMn to examine the performance at high pressures, as well as to examine the thermodynamic hysteresis effect in TiCrMn for applications in metal hydride system design. Due to this hysteresis, the thermodynamics of the absorption reaction differ significantly from those of the desorption reaction, which must be accounted for when considering thermal design of a metal hydride reactor and the suitability of the metal hydride for energy storage applications.     

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.     

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.     

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.     

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.     

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.     

Tests on a metal hydride based thermal energy storage system

In this paper, the performance tests on Mg + 30% MmNi₄ based thermal energy storage device is presented. Experiments were carried out at different supply pressures (10–30 bar) and absorption temperatures (120–150 °C). The effects of hydrogen supply pressure and absorption temperature on the amount of hydrogen/heat stored and thermal energy storage coefficient are presented. The maximum hydrogen storage capacity of 2.5wt% is reported at the operating conditions of 20 bar supply pressure and 150 °C absorption temperature. For a given absorption temperature of 150 °C, the thermal energy storage coefficient is found to increase from 0.5 at 10 bar to 0.74 at 30 bar supply pressure. For the given operating conditions of 20 bar supply pressure and 150 °C absorption temperature, the maximum amount of heat stored is about 0.714 MJ/kg and the corresponding thermal energy storage coefficient is 0.74.     

Study of cyclic performance of V-Ti-Cr alloys employed for hydrogen compressor

The development of a suitable hydrogen compressor plays one of the key roles to realize the fuel cell vehicle as well as for many other stationary and mobile applications of hydrogen. V-Ti-Cr BCC alloys are considered as promising candidates for effective hydrogen storage. The cyclic durability of hydrogen absorption and desorption is very important for these alloys to be realized as practical options. In connection to this, two alloys of V-Ti-Cr, (1) V₄₀Ti_21.5Cr_38.5 and (2) V₂₀Ti₃₂Cr₄₈, were selected and their cyclic hydrogen absorption-desorption performance was evaluated up to 100 cycles for temperature and pressure ranges of 20–300 °C and 5–20 MPa, respectively. It has been found that the cyclic hydrogen storage capacity continuously decreased for one composition while it was stable after 10 cycles for another composition. This performance difference of the alloys was studied in terms of their structural and microscopic properties and the results are presented in this paper.     

MgH2 intermediate scale tank tests under various experimental conditions

Magnesium hydrogenation being an exothermic reaction, the loading time of a tank is limited by heat extraction. The compaction of ball-milled MgH₂ associated with Expanded Natural Graphite was used to improve the thermal conductivity of the resulting compacts. Taking advantage of these compacts, an intermediate scale tank (1.8 kg MgH₂) cooled down by forced air circulation was designed. The absorption is initiated at 90 °C. Since the intrinsic kinetic is not the limiting factor, the hydrogen pressure does not affect the loading process. The loading time is strongly dependent on the cooling efficiency. However, beyond a given air flow rate it doesn’t decrease any more, the heat transfer being limited by the thermal conductivity of the compacted disks. During desorption, the maximum hydrogen flow (25 Nl/mn) is directly proportional to the thermal power of the heating system. The tank absorbs 1170 Nl, has a specific-energy of 270 W h/kg and a system volumetric-density of 42 gr/l.     

Development of new type Ni-Mg texture-like structure hydrogen absorber

This study demonstrated the feasibility of a novel Mg vapor deposition treatment on Ni foam to synthesize a Ni-Mg texture-like structure as a new type of hydrogen absorber. Energy dispersive spectrometry (EDS) yielded an estimative value of the weight percent ratio of Ni and Mg of 71.8 and 20.5 in as-prepared Ni-Mg texture-like structure. The microstructural changes were also characterized by X-ray diffraction (XRD) and the formed hydride tetragonal-MgH₂ was confirmed. The unique combination of large surface area of catalyst (Ni) and hydrogen acceptor (Mg) reduced the hydrogenation and dehydrogenation temperatures and performed the capability of reversible hydrogen storage capacity up to 0.72 wt.% H₂ at 25 °C. Ni-Mg texture-like structure achieved significant hydriding-dehydriding performances at lower temperature than traditional Mg-based hydrogen storage alloys. A possible hydrogen storage mechanism was also discussed where the catalytic Ni foam with large surface area was shown to be a vital factor in improving hydriding-dehydriding kinetics.