Hydrogen Desorption Capacity Research Articles

Microstructure, hydrogen storage properties, and thermodynamic characterization of ball-milled Mg90Ni5Y5 alloy

The hydrogen storage properties of the Mg90Ni5Y5 alloy were studied, which was prepared by vacuum electromagnetic induction melting and ball milling. The microstructure and phase evolution of the ball-milled alloys during the process of hydrogen absorption and desorption were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The alloys hydrogen storage properties were measured by semi-automatic Sievert instrument. Results show that the hydrogenated sample was composed of MgH2, Mg2NiH4 and YH3 phases. After dehydrogenation, Mg2NiH4 and MgH2 phases were completely decomposed, and YH3 could only be decomposed into YH2. After 2 cycles of adsorption dehydrogenation, the alloy was completely activated, and the maximum hydrogen desorption capacity was about 5.1 wt %. According to the Arrhenius equation, the ball-milled alloy activation energy of the dehydrogenation reaction of is only 75.7 kJ/mol.

Effect of phase composition and cluster structure on de-/hydriding kinetics of as-cast and heat-treated Ti33V37Mn30-xCrx alloys

To enhance the hydrogen storage property, elements Mn in Ti33V37Mn30 alloy were partially substituted with varying concentrations of elements Cr. The Ti33V37Mn24Cr6 variant, which exhibited superior performance, was selected for further analysis. Ti33V37Mn24Cr6 samples were first heat-treated separately at 773K, 973K and 1173K for 2h, and then water-quenched. The effect of Cr substitution and heat treatment on the microstructure and de-/hydriding properties of Ti33V37Mn30-xCrx alloys were investigated. As-cast alloys were composed of BCC phases mainly, along with some C14 Laves phases and Ti-rich phases. The lattice parameter of BCC phases in Ti33V37Mn24Cr6 increased to 3.053 Å. Cr substitution changed the tetrahedral gap of the alloy. At 298K, the initial hydrogen absorption capacity and the maximum hydrogen desorption capacity of Ti33V37Mn24Cr6 were raised to 3.07 wt% and 1.81 wt%, respectively. The lattice parameter of Ti33V37Mn24Cr6 alloys heat-treated at 773K also enlarged to 3.056 Å. Elevating the heat treatment temperature to 973K and 1173K significantly raised the proportion of C14 Laves phases in the alloy. Hydrogen storage performance of heat-treated alloys was examined at 298K. The results suggested that the heat-treated alloys could be fully activated after one hydrogen ab-/desorption cycle. Ti33V37Mn24Cr6 alloy heat-treated at 773K achieved the maximum hydrogen desorption capacity of 1.83 wt%, with an accelerated hydrogen ab-/desorption rate.

Hydrogen release of NaBH4 below 60 °C with binary eutectic mixture of xylitol and erythritol additive

NaBH4 was widely regarded as a low-cost hydrogen storage material due to its high-weight hydrogen capacity of approximately 10.8% (mass) and high volumetric hydrogen capacity of around 115 g·L–1. However, it exhibits strong stability and requires temperatures above 500 °C for hydrogen release in practical applications. In this study, two polyhydric alcohols xylitol and erythritol (XE) were prepared as a binary eutectic sugar alcohol through a grinding-melting method. This binary eutectic sugar alcohol was used as a proton-hydrogen carrier to destabilize NaBH4. The 19NaBH4-16XE composite material prepared by ball milling could start releasing hydrogen at 57.5 °C, and the total hydrogen release can reach over 88.8% (4.45% (mass)) of the theoretical capacity. When the 19NaBH4-16XE composite was pressed into solid blocks, the volumetric hydrogen capacity of the block-shaped composite could reach 67.2 g·L–1. By controlling the temperature, the hydrogen desorption capacity of the NaBH4-XE composite material was controllable, which has great potential for achieving solid-state hydrogen production from NaBH4.

Compositionally complex doping for low-V Ti-Cr-V hydrogen storage alloys

Ti-V-based BCC solid solution alloys, represented by Ti-Cr-V alloys, are considered as promising hydrogen storage materials due to their high hydrogen storage capacity (over 4 wt%) at room temperature. However, the difficult activation, low effective hydrogen desorption capacities, poor P-C-T plateau characteristics, and high cost remain significant problems for their practical applications. Herein, we present a new compositionally complex (high-entropy) doping strategy which was used to successfully fabricate a low-cost “Laves phase related BCC solid solution”, TiCrV0.7(Nb0.2Fe0.2Co0.2Ni0.2Mn0.2)0.2, that exhibits excellent activation performance and high effective hydrogen desorption capacity (Ce1atm: 2.21 wt%). The main BCC phase ensures high hydrogen storage capacity, while the minor secondary C14 phase plays a catalytic role and thus improves the hydrogen absorption kinetics. Furthermore, we confirmed that there is a synergistic effect of Nb, Fe, Co, Ni, and Mn elements in improving hydrogen storage performance. The dehydrogenation enthalpy ΔH of the heat-treated TiCrV0.7(Nb0.2Fe0.2Co0.2Ni0.2Mn0.2)0.2 is 37.5 kJ mol−1, which is significantly lower than previously reported Ti-Cr-V based systems, revealing a significant tendency towards easier dehydrogenation with high-entropy doping. This work offers a new alloying method for improving the hydrogen storage performance of hydrogen storage alloys.

Formation of eutectic structure and de/ hydriding kinetics properties: A novel Ti-Hf-V-Mn multi-principal element alloy

The novel Ti23-xV40Mn37Hfx (x = 8 and 10 at%) multi-principal element alloys (MPEAs) were prepared using vacuum non-consumable electrode arc-melting furnace. An analysis of the hydrogen sorption process unveiled a thought-provoking discovery, shedding light on the microstructure and performance of the alloys. The MPEAs exhibited a higher abundance of the C14 Laves phase in comparison to the Ti23V40Mn37 alloy. The C14 Laves phase underwent a transformation into columnar dendrites, resulting in the formation of a finely textured eutectic structure within the MPEAs. This distinctive microstructure endowed the MPEAs with superior activity and faster hydrogen absorption kinetics compared to the Ti23V40Mn37 alloy. The initial hydrogen absorption process followed a nucleation and crystal growth mechanism. In addition, the Ti13V40Mn37Hf10 alloy exhibited an impressive maximum hydrogen desorption capacity of 1.96 wt%, representing a significant 38% increase when compared to the Ti23V40Mn37 alloy. This enhanced hydrogen desorption capacity can be attributed to the presence of numerous diffusion channels for hydrogen atoms, as well as shorter diffusion distances within the MPEAs. Furthermore, the values of ΔH increased to 30.38 and 32.76 kJ/mol H2, respectively, for the MPEAs. This indicates a higher stability of hydrides within the MPEAs.

Effect of bimetallic nitride NiCoN on the hydrogen absorption and desorption properties of MgH2 and the catalytic effect of in situ formed Mg2Ni and Mg2Co phases

Magnesium hydroxide has attracted extensive attention in the past few years due to its excellent hydrogen storage capacity, good reversibility, and low cost, but its high thermodynamic stability and low rate of kinetic activity hinder its application. Therefore, in order to improve the hydrogen storage capacity of MgH2, we prepared ternary transition metal nitride NiCoN to catalyze MgH2 by hydrothermal and calcination techniques. Owing to the addition of 6 wt% NiCoN, the dehydrogenation process of MgH2 started at 164.4 °C (ball-milled MgH2 started dehydrogenation at 287.6 °C). At 285 °C, 4.95 wt% of hydrogen gas was desorption within 30 min, while 4.6 wt% of hydrogen gas was absorbed at 125 °C. And calculated by the Kissinger method, the activation energy required for the dehydrogenation reaction of MgH2-6NiCoN is reduced to 56.5 kJ/mol (ball-milled MgH2 is 127.3 kJ/mol). It is found that MgH2-6NiCoN forms Mg2Co/Mg2Ni in situ during the first dehydrogenation process, and then transforms into Mg2CoH5/Mg2NiH4 after rehydrogenation, implying a reversible phase transition between the Mg2Co/Mg2CoH5 and Mg2Ni/Mg2NiH4 phases. Due to the formation of intermediate products, it still maintains a good hydrogen desorption capacity after 40 cycles. The dehydrogenation retention rate is 94 %. The excellent behavior of MgH2 is attributed to the formation of Mg2Ni/Mg2NiH4 and Mg2Co/Mg2CoH5, which are responsible for the improved hydrogen absorption/desorption behavior of MgH2. These findings provide new insights into the hydrogen storage behavior of the catalytic MgH2 system.

Improved hydrogen storage properties of low-cost Ti–Cr–V alloys by minor alloying of Mn

V-based BCC solid-solution alloys have been considered as promising hydrogen storage materials due to their high hydrogen storage capacity; however, the low effective hydrogen desorption capacity and high cost of pure V have limited their practical application in fuel cell vehicles or devices. Herein, a low-V TiCr1.2(V–Fe0.203)0.6 hydrogen storage alloy with a hydrogen storage capacity of 3.35 wt% has been developed by using a low-cost FeV80 as raw material. To further increase the effective hydrogen desorption capacity, cheap Mn is first used to partially substitute for Cr in FeV80-based alloy. The Mn substitution alloy achieves an effective hydrogen desorption capacity of 2.07 wt% cutting-off at 0.1 MPa with good activation performance. It is ascribed that the minor alloying of Mn increases the dehydrogenation plateau pressure and reduces the hysteresis and the enthalpy change value. The dehydrogenation enthalpy decreases from 34.84 kJ/mol to 31.51 kJ/mol. The synergistic effect of Mn and Fe increases the abundance of the C14 Laves phase, which plays a catalytic role in hydrogen absorption and desorption. The cost of TiCr1.1Mn0.1(V–Fe0.203)0.6 alloy is much lower than that of using pure V, and even lower than some AB2 alloys based on cost estimation. The results of this study would provide a reference for the application of low-cost Ti–Cr-(FeV80) alloys for hydrogen storage.

Microstructure Characteristics and Hydrogen Storage Kinetics of Mg77+xNi20-xLa3 (x = 0, 5, 10, 15) Alloys.

Mg77+xNi20-xLa3 (x = 0, 5, 10, 15) alloys were successfully prepared by the vacuum induction melting method. The structural characterizations of the alloys were performed by using X-ray diffraction and scanning electron microscope. The effects of nickel content on the microstructure and hydrogen storage kinetic of the as-cast alloys were investigated. The results showed that the alloys are composed of a primary phase of Mg2Ni, lamella eutectic composites of Mg + Mg2Ni, and some amount of LaMg12 and La2Mg17. Nickel addition significantly improved the hydrogen-absorption kinetic performance of the alloy. At 683 K, Mg77Ni20La3 alloy and Mg82Ni15La3 alloy underwent hydrogen absorption and desorption reactions for 2 h, respectively, and their hydrogen absorption and desorption capacities were 4.16 wt.% and 4.1 wt.%, and 4.92 wt.% and 4.69 wt.%, respectively. Using the Kissinger equation, it was calculated that the activation energy values of Mg77Ni20La3, Mg82Ni15La3, Mg87Ni10La3 and Mg92Ni5La3 alloys were in the range of 68.5~75.2 kJ/mol, much lower than 150~160 kJ/mol of MgH2.

Regulating the microstructure and hydrogen storage properties of Ti23V40Mn37 + 10 wt% ZrNi alloys via ultrasonic treatment

To improve the activation performance and kinetic property of the Ti23V40Mn37 + 10 wt% ZrNi alloy, ultrasonic treatment was applied during its solidification. Besides, the effect of microstructure evolution on the hydrogen storage properties was investigated. Both as-cast and ultrasonically-treated (UST) Ti23V40Mn37 + 10 wt% ZrNi alloys were composed of BCC, C14 Laves and β-Zr phases. BCC phases appeared to be dendritic-shaped, of which the lattice parameter increased and the dendritic trunk was refined from 13.5 μm to 5.0 μm by ultrasonic processing. Hydrogen ab-/desorption rate accelerated and the maximum hydrogen desorption capacity was improved to 1.52 wt% at 303 K, owing to the increasing number of grain boundaries formed. The apparent dehydriding activation energy of the UST alloy decreased to 32.530 kJ/mol, while its dehydriding enthalpy (ΔH) enlarged to 40.22 kJ/mol H2 due to the lowered stability of the hydride.

Synthesis of α-AlH3 by organic liquid reduction method and its hydrogen desorption performance

Aluminum hydride (AlH3) is considered to be a promising hydrogen storage material due to its moderate dehydrogenation temperature and high hydrogen desorption capacity. However, AlH3 has multiple crystal forms, and it is difficult to prepare a single crystalline, which restricted the development of this material. In this study, a new organic liquid reduction method was used to prepare pure α-AlH3. The synthesis process includes two steps: primary reduction reaction in ether and subsequent desolvation reaction in toluene. The effects of reaction conditions on product composition were discussed in detail. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis were used to characterize the crystal structure and morphology of reaction products. Research results show that the final product is composed of α-AlH3 and γ-AlH3. The composition proportion of α-AlH3/γ-AlH3 can be adjusted by controlling reaction conditions. Through comparison and analysis, an optimal synthetic condition has been gained to obtain pure α-AlH3. The dehydrogenation performance of the obtained pure α-AlH3 were studied through thermal analysis methods. It is found that the α-AlH3 has a hydrogen desorption capacity of about 9.6 wt% below 200 °C. The dehydrogenation activation energy was also evaluated by using Kissinger and Friedman methods. This study provides a more simple and efficient method for synthesising pure α-AlH3, and also offers some reference for the future research of AlH3.

Effect of dehydrogenation depth on cyclic hydrogen desorption properties of V40Ti25.5Cr26.5Fe8 alloy

The cyclic stability of hydrogen absorption and desorption is an important indicator of hydrogen storage alloy. The decay mechanism was roundly investigated by controlling the depth of dehydrogenation (DOD) of the V40Ti25.5Cr26.5Fe8 alloy during the process of hydrogen ab/desorption cycles. The V40Ti25.5Cr26.5Fe8 alloy exhibits the hydrogen desorption capacity of 2.36 wt% at 323 K in the first cycle. After 30 cycles, the decay rate of the hydrogen desorption capacity is only 4.37 % at 40 % DOD, but rises to 8.74 % at 100 % DOD. The hydrogen desorption capacity (C), plateau pressure (P) and micro-strains (S) all show an “acceleration followed by slowing” exponential variation trend with the increase of cycle number (x). Additionally, any two of C, P and S show linear correlation. It is considered that the micro-strains in the crystal lattice is the main factor for the decay of the hydrogen desorption capacity and the plateau pressure. On the premise of ensuring the valid hydrogen storage capacity to meet the requirements of the application, properly reducing DOD per cycle can effectively reduce the micro-strains accumulation and cell volume expansion, thus improving the cyclic stability of the alloy. Moreover, the application of this alloy in a fuel cell bicycle was investigated, and the total range and mileage per unit weight of the alloy are up to 100.8 km and 15.7 km/kg, respectively. Compared with the lead-acid battery, lithium-ion battery and LaNi5-based hydrogen fuel cell, the V-Ti-Cr-Fe-based fuel cell shows significant advantages.

Carbon composite support improving catalytic effect of NbC nanoparticles on the low-temperature hydrogen storage performance of MgH2

Ultrafine carbon-based transition metal compounds have been widely investigated as efficient catalysts for enhancing the hydrogen storage performance of magnesium hydride. In this work, the carbon thermal shock method is applied to synthesize the ultrafine carbon-encapsulated NbC nanoparticles with an average grain size of 17.3 nm. The MgH2–10 wt% NbC/C composites show excellent low-temperature hydrogen storage performance with the onset dehydrogenation temperature of 196.1 °C, which is 92.2 °C and 98 °C lower than that of MgH2–10 wt% NbC and undoped MgH2, respectively. Specifically, MgH2–10 wt% NbC/C can absorb 6.71 wt% H2 at 100 °C within 30 min around and retain almost 100% reversible hydrogen desorption capacity after 10 cycles. For the catalytic mechanism, the electron transfer process between multi-valence Nb cations of in-situ formed NbHx and Mg, H atoms can greatly improve the cyclic de/rehydrogenation kinetics of MgH2-NbC/C. Besides, the enhancement of dehydrogenation kinetics can also be ascribed to MgH2 particle refinement by NbC nanoparticles, and destabilization of the Mg-H bond caused by carbon substrate. This investigation not only proves that carbon-encapsulated NbC nanoparticles can greatly enhance the hydrogen storage performance of MgH2 but provides an idea of preparing carbon-based transition metal carbides as effective catalysts for magnesium-based hydrogen storage materials.

In situ formation of TiB2 and TiH2 catalyzed Li/Mg based dual-cation borohydride with a low onset dehydrogenation temperature below 100 °C

Chemical complex borohydride is a promising hydrogen storage material due to its large gravimetric and volumetric hydrogen capacities. However, the high dehydrogenation temperature and sluggish kinetics still place strong restrictions on its practical application in the hydrogen storage field. In this work, a synergetic approach of partial cation substitution and catalysis is developed to enhance the hydrogen storage properties of LiBH4. The Li/Mg based dual-cation borohydride (LiMg2(BH4)5, LMBH) was successfully synthesized by wet chemical ball milling of LiBH4 and MgCl2. The optimal (LMBH (4.5:1) sample, LiBH4 and MgCl2 in molar ratios of 4.5:1, possesses a maximum hydrogen desorption capacity (11.27 wt%) and the outstanding initial decomposition temperature (∼250 °C). Importantly, the LMBH (4.5:1) doped with TiF3 shows a remarkable onset dehydrogenation temperature as low as 97.2 °C, which is about 190 °C lower than that of pristine LiBH4. The LMBH (4.5:1) doped with TiF3 system releases 7.98 wt% H2 within 170 min below 350 °C. And the dehydrogenation product of doped composite can reversibly absorb ∼4.72 wt% H2 at a relatively moderate temperature of 280 °C, which is substantially lower than the reversible hydrogen absorption temperature of previous modified borohydride systems. Based on the structural characteristic analyses, the TiF3 reacts with LMBH (4.5:1) to in-situ form actual catalytic components of TiB2 and TiH2 as the actual catalysts for LMBH (4.5:1), resulting in the improved hydrogen re/dehydrogenation properties. The synergetic modification of Li/Mg dual-cation substitution and TiB2/TiH2 catalysis may lead to the development of light-metal borohydrides with outstanding hydrogen storage properties.

Carbon coated La2Mg17 nanocrystalline for enhanced hydrogen storage performances

• Carbon coated nanoscale La-Mg alloy are constructed through a facile strategy; • The hydrogen ab/desorption kinetics were improved; • The dehydrogenation active energy was reduced significantly. Herein, carbon layer coated La-Mg intermetallic nanocrystallines were constructed via a facile process for an enhanced hydrogen storage performance. The hydrogen desorption capacity of carbon-covered sample after ten cycles was 3.5 wt.%, which was four times as high as that for the original. The coated sample presented the higher hydrogen storage capacity, lower dehydrogenation temperature and activity energy. Otherwise, the compositions and microstructures of the various samples were also investigated.

High pressure solidification of alloying substitution and promotion of hydrogen storage properties in AB2-type Y-Zr-Ti-Fe based alloys

Metastable phase has great potential in searching hydrogen storage alloys. In this work, high pressure solidification has been successfully used to realize the supersaturation of Ti in YFe2 based C15 Laves phase to obtain a metastable Y0.7Zr0.24Ti0.06Fe2 alloy. The segregation of Ti, Y, Zr and related multi-phase formation were substantially suppressed and the obtained HPS alloy contained a primary C15 phase(C15–1) and a secondary C15 phase(C15–2) with 77.1% and 22.9% of the total content, respectively. Meanwhile, the hydrogen absorption and desorption capacities increased substantially. This study proves that the combination of high-pressure solidification and alloying is a very effective method for exploring new hydrogen storage materials with metastable structure and high performance.

Impacts of Ce dopants on the hydrogen storage performance of Ti-Cr-V alloys

Vanadium-based alloys are considered to be one of the most promising hydrogen storage materials due to their high hydrogen storage capacity under ambient conditions. However, their complex activation at high temperature and poor stability pose serious challenges for large-scale applications. In this work, a series of TiCr3V16Cex (x = 0, 0.1, 0.2, 0.4, 1) hydrogen storage alloys were developed with different Ce contents using arc melting. The hydrogen storage and desorption performance, activation mechanism, and hydrogen absorption mechanism of the prepared alloys were investigated. Physical characterization confirms that the alloy is body-centered cubic (BCC) with Ce dopants, which exist in the form of oxides. The pressure-composition-temperature (PCT) test showed that the hydrogen storage plateau pressure of the Ce-doped alloy is increased compared to the Ce-free counterparts, while the hydrogen storage capacity decreased slightly with increasing Ce content. In addition, the influence of Ce doping on the alloy kinetics and thermodynamics is also discussed. The results showed that the TiCr3V16Cex (x = 0.2, 0.4, 1) alloys could absorb and release hydrogen at room temperature without activation. As an optimum, the TiCr3V16Ce0.2 alloy shows a hydrogen absorption rate of up to 3.69 wt%, and an effective hydrogen desorption capacity of 2.29 wt% at 25 °C. After hydrogen absorption and desorption cycles, the alloy almost maintains its original capacity. The Ce-doped BCC alloy developed in this work provides a new route to achieve high hydrogen storage performance under mild conditions.

Effects of Zr doping on activation capability and hydrogen storage performances of TiFe-based alloy

Activation difficulty is the key problem limiting the application of TiFe-based hydrogen storage alloys. The addition of transition group elements helps to improve the activation properties of TiFe-based hydrogen storage alloy. In our previous work, the Ti1.08Y0.02Fe0.8Mn0.2 alloy exhibits extremely high hydrogen storage capacity (1.84 wt%) at room temperature with excellent kinetic properties, but it still needs an incubation period of about 1500s. In this study, the composition of Ti1.08Y0.02Fe0.8Mn0.2Zrx (x = 0, 0.02, 0.04, 0.06, 0.08) alloys was prepared by electromagnetic induction melting. The quantitative analysis of elements by energy dispersive spectrometer shows that in the second phase region containing Zr, the content of Ti element is significantly higher than that of Fe. Meanwhile, the first-principle calculation on Zr-doped TiFe system indicates that Zr is more attractive to substitute Ti than Fe. Therefore, the doping of Zr partially replaces the Ti. The solubility of Zr in TiFe is limited, when x ≤ 0.04, the alloy consists of pure TiFe phase. When x > 0.4, the excess Zr forms precipitates, which reduces the reversible hydrogen absorption and desorption capacity of the TiFe alloy. The addition of Zr significantly shortens the activation time and reduces the plateau pressure of TiFe alloys. The Ti1.08Y0.02Fe0.8Mn0.2Zr0.04 alloy can be directly activated without the incubation period and its absolute values of enthalpy change (ΔH) and entropy change (ΔS) are minima (ΔH for 23.2 kJ/mol and ΔS for 83.1 J/mol/K).

Improved hydrogen storage properties of Li-Mg-N-H system by lithium vanadium oxides

The Mg(NH2)2-2LiH system is considered as one of the most potential hydrogen storage materials due to its high hydrogen storage capacity (5.6 wt%) and excellent reversible hydrogen adsorption and desorption performance. However, its poor kinetic characteristics of hydrogen absorption and desorption limit its practical application. In this work, Li3VO4 @LiVO2 was synthesized by a hydrothermal method, and was then introduced into Mg(NH2)2-2LiH system to enhance the hydrogen absorption and desorption kinetic characteristics. The results show that the hydrogen storage properties were greatly improved with addition of 10 wt% Li3VO4 @LiVO2 addition. As for the hydrogen absorption and desorption kinetics, the average hydrogen absorption rate of 10 wt% Li3VO4 @LiVO2 modified Li-Mg-N-H sample within 20 min at 200 ℃ increased by 95.2% compared with that of the pristine Li-Mg-N-H sample, and the average hydrogen desorption rate of 10 wt% Li3VO4 @LiVO2 modified sample within 30 min at 180 ℃ increased by 42% compared with that of the pristine sample. The Ea of hydrogen desorption decreased from 117.86 kJ/mol to 99.43 kJ/mol, which also indicates that Li3VO4 @LiVO2 catalyst effectively improved the kinetic performance of hydrogen desorption and desorption of Li-Mg-N-H. Moreover, the hydrogen desorption capacity and hydrogen absorption capacity of 10 wt% Li3VO4 @LiVO2 modified Li-Mg-N-H increased by 18.4% and 13.8% compared with that of the pristine sample. The catalytic mechanism of Li3VO4 @LiVO2was discussed based on XRD, XPS and FTIR analysis results.

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy. It was found that the V0.3Ti0.3Cr0.25Mn0.1Nb0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.

Synergistic effect of TiH2 and air exposure on enhancing hydrogen storage performance of Mg2NiH4

Air stability of magnesium-based hydrogen storage materials is one of the key challenges for its large-scale application, as the hydrides could be oxidized easily during air exposure. In this work, TiH2 and graphite were added into magnesium nickel hydride (Mg2NiH4) by short-time (0.5 h) mechanical milling to clarify the effect of air exposure on hydrogen storage behavior of the composites. Among the various Mg2NiH4–additive composites, the Mg2NiH4–TiH2 composite showed the highest hydrogen desorption and absorption capacity retention of 76.9 % and 75.6 % after air exposure for 30 days, respectively. It is worth noting that both the hydrogen absorption and desorption kinetics of the composites were enhanced after air exposure, especially for the Mg2NiH4 co-added with TiH2 and graphite composite. The excellent air stability and enhanced hydrogen reaction kinetics of Mg2NiH4–TiH2 composite should be attributed to the ternary-synergistic effect, including the protective effect of passivation layer, catalytic and sacrificial effect of TiH2, and catalysis of in-situ formed Ni-based products. They prevented further oxidation of the active matrix and provided more active sites for hydrogen absorption and desorption reactions. Our findings might guide the design and development of reversible metal hydrides with excellent reaction activity and air stability.