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.     

Computational study on metal hydride based three-stage hydrogen compressor

A mathematical model for predicting the performances of a three-stage metal hydride based hydrogen compressor (MHHC) is presented. The performance of the MHHC is predicted by solving the unsteady heat and mass transfer characteristics of the coupled metal hydride beds of cylindrical configuration. The governing equations for energy, momentum and mass conservations, and reaction kinetic equations are solved simultaneously using the finite volume method. Metal hydrides chosen for a three-stage MHHC are LaNi₅, MmNi_4.6Al_0.4 and Ti_0.99Zr_0.01V_0.43Fe_0.99Cr_0.05Mn_1.5. Numerical results obtained for a single-stage MHHC using MmNi_4.6Al_0.4 are in good agreement with the experimental data reported in the literature. Using three-stage compression, a maximum pressure ratio of 28 is achieved for the supply conditions of 20 °C absorption temperature and 2.5 bar supply pressure. A maximum delivery pressure of 100 bar is obtained for the operating conditions of 20 °C absorption temperature and 120 °C desorption temperature.     

Thermal modeling of LmNi4.91Sn0.15 based solid state hydrogen storage device with embedded cooling tubes

A 2-D mathematical model is developed for predicting the minimum charging/discharging time of the metal hydride based hydrogen storage device by varying the number of cooling tubes embedded in it. This study is extended to 3-D mathematical model for predicting the hydriding and dehydriding characteristics of LmNi_4.91Sn_0.15 based hydrogen storage device with 60 embedded cooling tubes (ECT) using COMSOL Multiphysics 4.3. The performance of the hydrogen storage device during hydriding/dehydriding process is presented for different supply pressure (10–35 bar), hot fluid temperature (30–60 °C) and effective thermal conductivity of hydride bed (0.2–2.5 W/(m?K)). It is observed that the rate of heat transfer and the hydriding and dehydriding rates are enhanced when the number of ECT is increased from 24 to 70. For the reactor with 60 ECT, the rate of hydrogen absorption is rapid for the supply pressure of 35 bar and hydride bed effective thermal conductivity of 2.5 W/(m?K). The numerically predicted hydrogen storage capacity (wt%) and amount of hydrogen desorbed (wt%) are compared with experimental data and found a good accord between them.     

Kinetics and electrochemical characteristics of Mg2NiH4-x wt.% MmNi3.8Co0.75Mn0.4Al0.2 (x = 5, 10, 20, 40) composites for Ni-MH battery

The structure, kinetics and electrochemical characteristics of Mg₂NiH₄-x wt.% MmNi_3.8Co_0.75Mn_0.4Al_0.2 (x = 5, 10, 20, 40) composites prepared by mechanical milling have been investigated in this paper. XRD results indicate that the as-milled Mg₂NiH₄ shows nanocrystalline or amorphous-like structure, and it does not react with MmNi_3.8Co_0.75Mn_0.4Al_0.2 during mechanical milling. As the amount of MmNi_3.8Co_0.75Mn_0.4Al_0.2 increases, the maximum discharge capacity decreases initially from 508 mAh/g (x = 5) to 440 mAh/g (x = 10) and then increases to 509 mAh/g (x = 40). Meanwhile, the capacity retention (R₁₀) increases from 12.8% (x = 5) to 23.4% (x = 40), and the corrosion potential of electrode (E_corr) increases from −0.930 V to −0.884 V (vs. Hg/HgO). Especially, the more MmNi_3.8Co_0.75Mn_0.4Al_0.2 content the composite contains, the higher high rate dischargeability (HRD) the electrode exhibits, which could be attributed to the catalytic reaction and reduction of the Mg₂NiH₄ grain size brought by MmNi_3.8Co_0.75Mn_0.4Al_0.2. The improvement in electrode kinetics has been depicted from the bulk hydrogen diffusion coefficient (D), the exchange current density (I₀) and the charge transfer resistance (R_ct) on the alloy surface.     

MmNi3.55Co0.75Mn0.4Al0.3B0.3 hydrogen storage alloys for high-power nickel/metal hydride batteries

Non-stoichiometric La-rich MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 hydrogen storage alloys using B–Ni or B–Fe alloy as additive and Ce-rich MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 one using pure B as additive have been prepared and their microstructure, thermodynamic, and electrochemical characteristics have been examined. It is found that all investigated alloys show good activation performance and high-rate dischargeability though there is a certain decrease in electrochemical capacities compared with the commercial MmNi_3.55Co_0.75Mn_0.4Al_0.3 alloy. MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 alloys using B–Ni alloy as additive or adopting Ce-rich mischmetal show excellent rate capability and can discharge capacity over 190 mAh/g even under 3000 mA/g current density, which display their promising use in the high-power type Ni/MH battery. The electrochemical performances of these MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 alloys are well correlated with their microstructure, thermodynamic, and kinetic characteristics.     

Enhanced hydrogen desorption/absorption properties of magnesium hydride with CeF3@Gn

In order to improve the hydrogen storage performance of MgH₂, graphene and CeF₃ co-catalyzed MgH₂ (hereafter denoted as MgH₂+CeF₃@Gn) were prepared by wet method ball milling and hydriding, which is a simple and time-saving method. The effect of CeF₃@Gn on the hydrogen storage behavior of MgH₂ was investigated. The experimental results showed that co-addition of CeF₃@Gn greatly decreased the hydrogen desorption/absorption temperature of MgH₂, and remarkably improved the dehydriding/hydriding kinetics of MgH₂. The onset hydrogen desorption temperature of Mg + CeF₃@Gn is 232 °C，which is 86 °C lower than that of as-milled undoped MgH₂, and its hydrogen desorption capacity reaches 6.77 wt%, which is 99% of its theoretical capacity (6.84 wt%). At 300 °C and 200 °C the maximum hydrogen desorption rates are 79.5 and 118 times faster than that of the as-milled undoped MgH₂. Even at low temperature of 150 °C, the dedydrided sample (Mg + CeF₃@Gn) also showed excellent hydrogen absorption kinetics, it can absorb 5.71 wt% hydrogen within 50 s, and its maximum hydrogen absorption rate reached 15.0 wt% H₂/min, which is 1765 times faster than that of the undoped Mg. Moreover, no eminent degradation of hydrogen storage capacity occurred after 15 hydrogen desorption/absorption cycles. Mg + CeF₃@Gn showed excellent hydrogen de/absorption kinetics because of the MgF₂ and CeH_2-3 that are formed in situ, and the synergic catalytic effect of these by-products and unique structure of Gn.     

Effect of boron additive on the cycle life of low-Co AB5-type electrode consisting of alloy prepared by cast and rapid quenching

In order to modify the cycle stability of low-Co AB₅-type alloy, a trace of boron was added in MmNi_3.8Co_0.4Mn_0.6Al_0.2 hydrogen storage alloy. The low-Co AB₅-type alloys MmNi_3.8Co_0.4Mn_0.6Al_0.2B_x(x=0, 0.1, 0.2, 0.3, 0.4) were prepared by cast and rapid quenching. The cycle lives and microstructures of the as-cast and quenched alloys were measured and analyzed. The effects of boron additive on the microstructures and cycle lives of as-cast and quenched alloys were investigated comprehensively. The obtained results showed that the addition of boron could dramatically enhance the cycle lives of the as-cast and quenched alloys. When boron content x increases from 0 to 0.4, the cycle lives of the as-cast alloys were increased from 118 to 183 cycles, and for as-quenched alloys with quenching rate of 38 m/s from 310 to 566 cycles.     

Experimental studies on industrial scale metal hydride based hydrogen storage system with embedded cooling tubes

The present article reports the activation and testing of large scale metal hydride based hydrogen storage system (MHHSS) for industrial application. The metal hydride reactor is fabricated using SS316 material with 99 embedded cooling tube and filled with 40 kg of LaNi_4.7Al_0.3. The activation was carried out by successive absorption and desorption processes. In the third absorption cycle, MHHSS had absorbed 552.356 g of hydrogen to reach a maximum storage capacity of 1.4 wt% at 40 bar pressure and 30 °C temperature. The testing of MHHSS was carried out by varying H₂ supply pressure, absorption and desorption temperatures and heat transfer fluid (HTF) flow rate. It was observed that the supply pressure has significant effect on absorption rate, and the optimum supply pressure was observed in the range of 10–15 bar. Similarly, during the desorption cycle, optimum desorption temperature was found in the range of 80–90 °C. The optimum flow velocity for HTF was observed in the range of 20–30 lpm.     

Effects of electrolytes and temperature on high-rate discharge behavior of MmNi5-based hydrogen storage alloys

A series of experiments have been performed to investigate the effects of electrolyte composition and temperature on the high-rate discharge behaviors of MmNi₅-based AB₅ hydrogen storage alloy electrodes. Two types of AB₅ electrodes have been used using different alloys: Ce-rich alloy V (La_0.26 Ce_0.44Pr_0.1Nd_0.2Ni_3.55Co_0.72Mn_0.43Al_0.3) and La-rich alloy N (La_0.58Ce_0.25Pr_0.06 Nd_0.11Ni_3.66Co_0.74Mn_0.41Al_0.18). Electrolytes EN were obtained by adding a saturated amount of Al₂(SO₄) ₃ to the original electrolyte EO (6 M KOH + 1 wt% LiOH). The electrolyte EN has previously been shown to be very effective to stop the self-discharge of the AB₅ electrodes, better charge/discharge cycle life have been observed. The electrochemical properties of the electrodes were measured by two methods: step mode high-rate discharge and continuous mode high-rate discharge. The results indicate that at 298 K and 333 K, high-rate discharge capacity of Ni–MH battery was mostly affected by the chemical composition of the electrolyte, then the type of alloy. Better dischargeabilities in high-rate discharge capacity have been observed in electrolyte EO than in electrolyte EN. The Ce-rich alloy V has a higher high-rate discharge capacity than La-rich alloy N. High-rate discharge capacity decreases in the following order: VEO > NEO > VEN > NEN (VEO denotes the combination of alloy V and electrolyte EO used in the test battery, similarly equivalent representations for NEN, VEO and VEN).     

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.     

An air–metal hydride battery using MmNi3.6Mn0.4Al0.3Co0.7 in the anode and a perovskite in the cathode

Hydrogen storage alloy MmNi_3.6Mn_0.4Al_0.3Co_0.7 (MH) was tested as anode material in a metal hydride–air cell. The cathode was a non-noble metal air electrode based on a mixture of perovskite and pyrolyzed macrocycles on carbon. Polarization and discharge capacities of the electrodes were measured and compared at 22 °C and 40 °C using air or oxygen at the cathode. Discharge capacity reaching 330 mAh/g MH with pure oxygen at 40 °C and 305 mAh/g MH with air at 22 °C were obtained. Power densities and/or energy densities were found to significantly depend on the increase of the electrode kinetics on both the ORR (oxygen reduction reaction) and HOR (hydrogen oxidation reaction). However, for air electrode, an increase of oxygen concentration by using pure oxygen gas plays a more important role than an 18 °C temperature increase.     

First principles study to identify the reversible reaction step of a multinary hydrogen storage “Li–Mg–B–N–H” system

A density functional theory study with the generalized gradient approximation (GGA) and projected augmented wave (PAW) method is performed for the hydrogen storage properties of the complex multinary storage Li–Mg–B–N–H system. Using ab initio methods, stability of the structures at finite temperatures is confirmed via. phonon spectrum calculations. Thermodynamic properties such as heat of reaction, and Gibbs energy for each reactant and product in the reaction steps in different temperature zones are calculated. It is found that reversibility occurs in the temperature range of 160–225 °C with approximately 4.38 wt % hydrogen storage capacity. The enthalpy of reversible re-/de-hydrogenation is found to be 55.17 kJ/mol H₂, which is supported by experimental data. The total hydrogen storage capacity of this material is calculated to be 8.76 wt% from the desorption behavior observed at different temperatures up to 350 °C. These theoretically established reactions are validated with the suggested mechanism from experimental observations for the dehydrogenation reaction of this Li–Mg–B–N–H multinary system. These efforts are expected to contribute toward identification of suitable hydrogen storage materials.     

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.     

Effects of the preparative parameters of hydriding combustion synthesis on the properties of Mg–Ni–C as hydrogen storage material

Magnesium may be the most promising solid-state hydrogen storage material owing to its high storage capacity (7.6 wt%) and highest volumetric density (2 times of liquid H₂). On the other hand, suffers from its sluggish absorption/desorption characteristics. In the present study, the simple/cost-effective hydriding combustion synthesis (HCS) was used to prepare highly-active Mg-based-samples. The preparative parameters of HCS were varied, and its effects on the micro-structural and hydrogen storage properties were determined. The results and its analysis showed that the simple HCS process possesses a multifaceted dependence on a range of experimental factors and affect the final product. The estimated dependence enabled us to explain the combined effect of individual experimental factors on the prepared samples. The Mg–Ni–C sample prepared at 610 °C with 6%wt-nano-Ni and 4 wt%-multi-walled-CNTs as reactants, resulted in sample with a surface area as high as 19.01 m²/g and a desorption capacity of 5.77 wt%, highlighting the promising characteristics of HCS to prepare highly-active Mg-based-materials.     

A comparative study for synthesis methods of nano-structured (9Ni–2Mg–Y) alloy catalysts and effect of the produced alloy on hydrogen desorption properties of MgH2

9Ni–2Mg–Y alloy powders were prepared by arc melting, induction melting, mechanical alloying, solid state reaction and subsequent ball milling processes. The results showed that melting processes are not suitable for preparation of 9Ni–2Mg–Y alloy due to high losses of Mg and Y. Therefore, 9Ni–2Mg–Y alloy powder was prepared by three methods including: 1) mechanical alloying, 2) mechanical alloying + solid state reaction + ball milling, and 3) mixing + solid state reaction + ball milling. The prepared 9Ni–2Mg–Y alloy powders were compared for their catalytic effects on hydrogen desorption of MgH₂. It is found that 9Ni–2Mg–Y alloy powder prepared by mechanical alloying + solid state reaction + ball milling method has a smaller particle size (1–5 μm) and higher surface area (1.7 m² g⁻¹) than that of other methods. H₂ desorption tests revealed that addition of 9Ni–2Mg–Y alloy prepared by mechanical alloying + solid state reaction + ball milling to MgH₂ decreases the hydrogen desorption temperature of MgH₂ from 425 to 210 °C and improves the hydrogen desorption capacity from 0 to 3.5 wt.% at 350 °C during 8 min.     

Destabilization of the LiNH2–LiH hydrogen storage system by aluminum incorporation

The lithium amide–lithium hydride system (LiNH₂–LiH) is one of the most attractive light-weight materials for hydrogen storage. In an effort to improve its hydrogen sorption kinetics, the effect of 1 mol% AlCl₃ addition to LiNH₂–LiH system was systematically investigated by differential scanning calorimetry, X-ray diffraction, Fourier transform infrared analysis and hydrogen volumetric measurements. It is shown that Al³⁺ is incorporated into the LiNH₂ structure by partial substitution of Li⁺ forming a new amide in the Li–Al–N–H system, which is reversible under hydriding/dehydriding cycles. This new substituted amide displays improved hydrogen storage properties with respect to LiNH₂–LiH. In fact, a stable hydrogen storage capacity of about 4.5–5.0 wt% is observed under cycling and is completely desorbed in 30 min at 275 °C for the Li–Al–N–H system. Moreover, the concurrent incorporation of Al³⁺ and the presence of LiH are effective for mitigating the ammonia release. The results reveal a common reaction pathway for LiNH₂–LiH and LiNH₂–LiH plus 1 mol% AlCl₃ systems, but the thermodynamic properties are changed by the inclusion of Al³⁺ in the LiNH₂ structure. These findings have important implications for tailoring the properties of the Li–N–H system.     

Hydrogen-storage properties of Mg–23.5Ni–(0 and 5)Cu prepared by melt spinning and crystallization heat treatment

Mg–23.5 wt% Ni and Mg–23.5 wt% Ni–5 wt% Cu alloys for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The alloys were ground by a planetary ball mill for 2 h in order to obtain a fine powder. The activated Mg–23.5Ni and Mg–23.5Ni–5Cu alloys absorbed 4.34 and 4.84 wt% H, respectively, at 573 K under 12 bar H₂ for 60 min. The activated Mg–23.5Ni and Mg–23.5Ni–5Cu alloys desorbed 4.27 and 4.81 wt% H, respectively, at 573 K under 1.0 bar H₂ for 30 min. The hydriding rates of the alloys are quite high, even at 473 K, while the dehydriding rates of the samples at 473 K are nearly zero.     

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.     

Ternary compounds in the magnesium–titanium hydrogen storage system

Mg–Ti–H samples were mechano-chemically synthesized by ball milling in argon atmosphere or under elevated hydrogen pressure. The detailed reaction mechanism during hydrogen release and uptake during continuous cycling was investigated by in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) experiments. The thermal behaviour of the samples and hydrogen desorption properties were examined by simultaneous thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and mass spectrometry (MS) measurements. A ternary Ti–Mg–H compound with a fcc lattice form during mechano-chemical sample preparation in hydrogen atmosphere using metal powders, but not using metal hydrides as reactants. The amount of β-MgH₂ increases during the first hydrogen absorption cycle at 300 °C at the expense of the high-pressure polymorph, γ-MgH₂ and the amount of β-MgH₂ remain constant during the following hydrogenations. This study reveals that the ternary compound tends to absorb increasing amounts of magnesium in the dehydrogenated state during cycling. A strong coupling between the amounts of magnesium in the ternary Ti–Mg–H phase and the formation of magnesium and magnesium hydride during hydrogen release and uptake at 300 °C is observed. The composition and the amount of the Ti–Mg–H phase appear to be similar in the hydrogenated state. Fast absorption–desorption kinetics at 300 °C and lower onset temperatures for hydrogen release is observed for all investigated samples (lowest onset temperature of desorption T_on = 217 °C).