Nano-Pd/CeO2 catalysts for hydrogen storage by reversible benzene hydrogenation/dehydrogenation reactions

Nano-CeO₂ supports, which have different structure from different preparation methods, were used to prepare nano-Pd/CeO₂ catalysts. The hydrogen storage capacity of prepared nano-Pd/CeO₂ catalysts were studied via vapor phase benzene hydrogenation and cyclohexane dehydrogenation reactions. Results show that the prepared Pd/CeO₂ catalysts exhibit excellent benzene hydrogenation and cyclohexane dehydrogenation performances. The catalytic performance of the Pd/CeO₂ catalysts is related to the dispersion of metallic Pd, hydrogen adsorption-desorption ability and structure of Pd/CeO₂ catalysts and so on. And those properties are also directly affected by the morphology and mesoporous structure of the prepared nano-Pd/CeO₂ catalysts that can be regulated by CeO₂ support preparation methods. The synergistic effect between metal Pd, CeO₂ support and their structures can effectively promote benzene hydrogenation and cyclohexane dehydrogenation, thus promoting hydrogen storage capacity. The prepared Pd/CeO₂-HT catalyst, which has high specific surface area, developed pore structure and highly dispersed metal Pd species, exhibits superior catalytic performances. And, the Pd/CeO₂-HT catalyst exhibits superior catalytic hydrogen storage performances. The benzene conversion over it at 200 °C reaches 99.5%. Whereas the cyclohexane conversion at 450 °C is 65.3%, and the H₂ production capacity is 73.77 g/h.     

200 NL H2 hydrogen storage tank using MgH2–TiH2–C nanocomposite as H storage material

MgH₂-based hydrogen storage materials are promising candidates for solid-state hydrogen storage allowing efficient thermal management in energy systems integrating metal hydride hydrogen store with a solid oxide fuel cell (SOFC) providing dissipated heat at temperatures between 400 and 600 °C. Recently, we have shown that graphite-modified composite of TiH₂ and MgH₂ prepared by high-energy reactive ball milling in hydrogen (HRBM), demonstrates a high reversible gravimetric H storage capacity exceeding 5 wt % H, fast hydrogenation/dehydrogenation kinetics and excellent cycle stability. In present study, 0.9 MgH₂ + 0.1 TiH₂ +5 wt %C nanocomposite with a maximum hydrogen storage capacity of 6.3 wt% H was prepared by HRBM preceded by a short homogenizing pre-milling in inert gas. 300 g of the composite was loaded into a storage tank accommodating an air-heated stainless steel metal hydride (MH) container equipped with transversal internal (copper) and external (aluminium) fins. Tests of the tank were carried out in a temperature range from 150 °C (H₂ absorption) to 370 °C (H₂ desorption) and showed its ability to deliver up to 185 NL H₂ corresponding to a reversible H storage capacity of the MH material of appr. 5 wt% H. No significant deterioration of the reversible H storage capacity was observed during 20 heating/cooling H₂ discharge/charge cycles. It was found that H₂ desorption performance can be tailored by selecting appropriate thermal management conditions and an optimal operational regime has been proposed.     

Theoretical prediction of structure,electronic and optical properties of VH2 hydrogen storage material

The vanadium hydrides have better hydrogen storage capacity in comparison to the other metal hydrides. Although the structure of VH₂ hydride has been reported, the structural stability, electronic and optical properties of VH₂ hydride are unclear. To solve these problems, we apply the first-principles method to study the structural stability, electronic and optical properties of VH₂ hydrides. Similar to the metal dihydrides, four possible VH₂ hydrides such as the cubic (Fm-3m), tetragonal (I4/mmm), tetragonal (P4₂/mnm) and orthorhombic (Pnma) are designed. The result shows that the cubic VH₂ hydride is a thermodynamic and dynamical stability. In particular, the tetragonal (I4/mmm) and the orthorhombic (Pnma) VH₂ hydrides are firstly predicted. It is found that these VH₂ hydrides show metallic behavior. The electronic interaction of V (d-state)-H (s-state) is beneficial to improve the hydrogen storage in VH₂ hydride. In addition, the formation of V–H bond can improve the structural stability of VH₂ hydride. Based on the analysis of optical properties, it is found that all VH₂ hydrides show the ultraviolet response. Compared to the tetragonal and orthorhombic VH₂ hydrides, the cubic VH₂ hydride has better storage optical properties. Therefore, we believe that the VH₂ hydride is a promising hydrogen storage material.     

Effects of NH4+ doping on the hydrogen storage properties of metal hydrides

Doping can modify the properties of metal hydrogen storage materials significantly. Currently, the metal doping is a frequent strategy, while the non-metal cation doping has not been examined extensively so far. In this study, the effects of NH₄⁺ doping on the hydrogen storage properties of different metal hydrides, including TiH₂, Ti_0·25V_0·25Nb_0·25Zr_0·25H₂, Ti_0·5V_0·5H₂ and VH₂, are investigated by first-principles calculations. It is found that the NH₄⁺ presents a good affinity for metal hydrides and the NH₄⁺ incorporation leads to charge redistribution and formation of dihydrogen bond. Furthermore, the NH₄⁺ doping in metal hydrides is favorable for enhancing the hydrogen storage capacity and decreasing the thermal stability simultaneously. The possible reason for the NH₄⁺ doping induced destabilization in metal hydrides is the relatively weak interaction between NH₄⁺ and hydrogen atoms.     

Modification of NaAlH4 properties using catalysts for solid-state hydrogen storage: A review

Energy is an essential requirement in our daily lives. Currently, most of our energy demands are fulfilled by fossil fuels. After 20 years, non-renewable fossil fuels are estimated to plummet rapidly. The world will face energy shortage and will seek for a new environmental method of energy generation for transportation, economy and application. Hydrogen is a fascinating energy carrier that is considered as ‘hydrogen economy’ for the future. The key challenge in developing the hydrogen economy is the context of hydrogen storage. Storing hydrogen via the solid-state method has received special attention and consideration because of its safety and larger storage capacity. A light complex hydride, NaAlH₄, is considered as an attractive material for solid-state hydrogen storage owing to its high hydrogen capacity, bulk in availability and low cost. Sluggish sorption kinetics and poor reversibility have driven research into various catalysts to enhance its hydrogen storage properties. This review article examines the development of different catalysts and their effects on the hydrogen storage properties of NaAlH₄. The addition of catalyst offers synergistic catalytic effect on the dehydrogenation performance of NaAlH₄. Doping NaAlH₄ with catalyst promote promising results such as lower decomposition temperature, improved kinetics and reduced activation energy. Superior performance on the dehydrogenation performance of NaAlH₄ doping with the catalyst may be due to the nanosized catalyst particle and in situ formed active species that may serve as nucleation sites at the surface of the NaAlH₄ matrix and benefiting the kinetics properties of NaAlH₄.     

Improved reversible hydrogen storage of LiAlH4 by nano-sized TiH2

Transition metal halides are mostly used as dopants to improve the hydrogen storage properties of LiAlH₄, but they will cause hydrogen capacity loss because of their relatively high molecular weights and reactions with LiAlH₄. To overcome these drawbacks, active nano-sized TiH₂ (TiH₂^nano) prepared by reactive ball milling is used to dope LiAlH₄. It shows superior catalytic effect on the dehydrogenation of LiAlH₄ compared to commercial TiH₂. TiH₂^nano-doped LiAlH₄ starts to release hydrogen at 75 °C, which is 80 °C lower than the onset dehydrogenation temperature of commercial LiAlH₄. About 6.3 wt.% H₂ can be released isothermally at 100 °C (800 min) or at 120 °C (150 min). The apparent activation energies of the first two dehydrogenation reactions of LiAlH₄ are reduced by about 20 and 24 kJ mol⁻¹, respectively. Meanwhile, the regeneration of LiAlH₄ is realized through extracting the solvent from LiAlH₄·4THF, which is obtained by ball milling the dehydrogenated products of TiH₂^nano-doped LiAlH₄ in the presence of THF and 5 MPa H₂. This suggests that TiH₂ is also an effective catalyst for the formation of LiAlH₄·4THF.     

Microstructural characteristics and hydrogen storage properties of the Mg–Ni–TiS2 nanocomposite prepared by a solution-based method

Magnesium hydride is considered as a promising solid-state hydrogen storage material due to its high hydrogen capacity. How to improve hydrogen desorption kinetics of MgH₂ is one of key issues for its practical applications. In this study, we synthesize a Mg–Ni–TiS₂ composite through a solution-based synthetic strategy. In the as-prepared composite, the co-precipitated Mg and Ni nanoparticles are highly dispersed on TiS₂ nanosheets. As a result, the activation energy for hydrogen desorption decreases to 79.4 kJ mol⁻¹. Meanwhile, the capacity retention rate is kept at the level of 98% and only slight kinetic deterioration is caused after fifty hydrogenation-dehydrogenation cycles. Further investigation indicates that the superior hydrogen desorption kinetics is attributed to the synergistically catalytic effect of the in situ formed Mg₂NiH₄ and TiH₂, and the remained TiS₂. The excellent cycle stability is related not only to the inhibition effect of the secondary phases on powder agglomeration and crystallite growth of Mg and MgH₂ but also to the prevention effect of MgS and TiS₂ on redistribution of catalytic Mg₂NiH₄ and TiH₂ nanoparticles during cycling. This work introduces a feasible approach to develop Mg-based hydrogen storage materials.     

Hydrogen storage properties of the Mg–Ti–H system prepared by high-energy–high-pressure reactive milling

Magnesium-based alloys are among the promising materials for hydrogen storage and fuel cell applications due to their high hydrogen content. In the present work, we investigated the hydrogen release/uptake properties of the Mg–Ti–H system. Samples were prepared from the mixtures of MgH₂ and TiH₂ in molar ratios of 7:1 and 4:1 using a high-energy-high-pressure (HEHP) mechanical ball-milling method under 13.8 MPa hydrogen pressure. Thermogravimetric analysis (TGA) showed that a relatively large amount of hydrogen (5.91 and 4.82 wt.%, respectively, for the above two samples) was released between 126 and 313 °C while temperature was increased at a heating rate of 5 °C min⁻¹ under an argon flow. The onset dehydrogenation temperature of these mixtures, which is 126 °C, is much lower than that of MgH₂ alone, which is 381 °C. The activation energy of dehydrogenation was 71 kJ mol⁻¹, which is much smaller than that of as-received MgH₂ (153 kJ mol⁻¹) or as-milled MgH₂ (96 kJ mol⁻¹). Furthermore, the hydrogen capacity and the dehydrogenation temperature remained largely unchanged over five dehydrogenation and rehydrogenation cycles.     

Progress of graphene and loaded transition metals on Mg-based hydrogen storage alloys

Mg-based hydrogen storage alloys have become a research hotspot in recent years owing to their high hydrogen storage capacity, good reversibility of hydrogen absorption/desorption, low cost, and abundant resources. However, its high thermodynamic stability and slow kinetics limit its application, so the modification of Mg-based hydrogen storage alloys has become the development direction of Mg-based alloys. Transition metals can be used as catalysts for the dehydrogenation of hydrogen storage alloys due to their excellent structural, electrical, and magnetic properties. Graphene, because of its unique sp² hybrid structure, excellent chemical stability, and a specific surface area of up to 2600 m²/g, can be used as a support for transition metal catalysts. In this paper, the internal mechanism of graphene as a catalyst for the catalysis of Mg-based hydrogen storage alloys was analyzed, and the hydrogen storage properties of graphene-catalyzed Mg-based hydrogen storage alloys were reviewed. The effects of graphene-supported different catalysts (transition metal, transition metal oxides, and transition metal compounds) on the hydrogen storage properties of Mg-based hydrogen storage alloys were also reviewed. The results showed that graphene played the roles of catalysis, co-catalysis, and inhibition of grain aggregation and growth in Mg-based hydrogen storage materials.     

Nanoconfinement effects on hydrogen storage properties of MgH2 and LiBH4

To find a solution to efficiently exploit renewable energy sources is a key step to achieve complete independence from fossil fuel energy sources. Hydrogen is considered by many as a suitable energy vector for efficiently exploiting intermittent and unevenly distributed renewable energy sources. However, although the production of hydrogen from renewable energy sources is technically feasible, the storage of large quantities of hydrogen is challenging. Comparing to conventional compressed and cryogenic hydrogen storage, the solid-state storage of hydrogen shows many advantages in terms of safety and volumetric energy density. Among the materials available to store hydrogen, metal hydrides and complex metal hydrides have been extensively investigated due to their appealing hydrogen storage properties. Among several potentials candidates, magnesium hydride (MgH₂) and lithium borohydride (LiBH₄) have been widely recognized as promising solid-state hydrogen storage materials. However, before considering these hydrides ready for real-scale applications, the issue of their high thermodynamic stability and of their poor hydrogenation/dehydrogenation kinetics must be solved. An approach to modify the hydrogen storage properties of these hydrides is nanoconfinement. This review summarizes and discusses recent findings on the use of porous scaffolds as nanostructured tools for improving the thermodynamics and kinetics of MgH₂ and LiBH₄.     

Kinetics of dehydrogenation of the Mg-Ti-H hydrogen storage system

Among the proposed hydrogen storage systems, magnesium alloys have proved to be promising since they are rechargeable with high hydrogen capacities (theoretically up to 7.6 wt.%), reversibility and low costs. Small particle size, which can be achieved by milling, and small amounts of transition-metal compounds as catalysts result in increased hydrogen release/uptake kinetics. In this work, we developed the rate expression for the dehydrogenation of milled 7MgH₂/TiH₂, 10MgH₂/TiH₂, and MgH₂ samples. The complete rate expressions, together with the values of activation energy and other rate parameters, were determined for the three milled samples by analyzing data obtained from non-isothermal thermogravimetric analysis (TGA). The MgH₂ doped with TiH₂ by high-energy milling displayed substantially reduced apparent activation energy of 107-118 kJ/mol and significantly faster kinetics, compared with 226 kJ/mol for similarly milled MgH₂ without TiH₂ doping.     

Excellent catalytic effect of LaNi5 on hydrogen storage properties for aluminium hydride at mild temperature

The catalytic effect of rare-earth hydrogen storage alloy is investigated for dehydrogenation of alane, which shows a significantly reduced onset dehydrogenation temperature (86 °C) with a high-purity hydrogen storage capacity of 8.6 wt% and an improved dehydrogenation kinetics property (6.3 wt% of dehydrogenation at 100 °C within 60 min). The related mechanism is that the catalytic sites on the surface of the hydrogen storage alloy and the hydrogen storage sites of the entire bulk phase of the hydrogen storage reduce the dehydrogenation temperature of AlH₃ and improve the dehydrogenation kinetic performance of AlH₃. This facile and effective method significantly improves the dehydrogenation of AlH₃ and provides a promising strategy for metal hydride modification.     

Mg/MgH2 hydrogen storage system destabilized by recyclable AlH3–NaBH4 composite

While borohydrides, such as NaBH₄, were often used as supplements to improve hydrogen storage properties of Mg/MgH₂ systems, they have long suffered from high decomposition temperature and irreversible dehydrogenation process. Here, we report that NaBH₄ can reversibly serve as a hydrogen storage host and reactant for Mg/MgH₂ systems under mild reaction conditions with the help of Al/AlH₃. 90 wt%MgH₂–5 wt.%AlH₃–5 wt.%NaBH₄ (M-5AB) has been successfully synthesized using the conventional mechanical alloying technique. The dehydrogenation activation energy and enthalpy are 20% and 9% reduced than those of pure Mg/MgH₂. After 10 hydrogen absorption and desorption cycles, the hydrogen storage capacity of M-5AB can reach 6.35 wt%. The X-ray diffraction (XRD) and the transmission electron microscope (TEM) measurements revealed that the interface of additives and Mg/MgH₂ decompose to Mg₁₇Al₁₂, MgAlB₄ and NaH phases. The Mg₁₇Al₁₂ and MgAlB₄ phases reduces the barrier of free energies of hydrogenated and dehydrogenated states, helping NaBH₄ to recover after rehydrogenation. These discoveries indicate that Al species can boost the decomposition and reformation of NaBH₄, providing a wider degree of freedom for the material design of Mg-based hydrogen storage materials.     

Hydrogen storage property improvement of La–Y–Mg–Ni alloy by ball milling with TiF3

Ball milling the powders of Mg-based alloys with transition metal compounds is effective for improving their hydrogen storage performances. In this experiment, the alloys of La_1.7Y_0.3Mg₁₆Ni + x wt.% TiF₃ (x = 0–10) were prepared through mechanical milling technology. XRD, SEM, HRTEM and granulometry were used to measure the composition and microstructure of alloys. The isothermal hydrogen storage property was measured by a Sievert apparatus. The results reveal that the TiF₃ additive in ball-milled samples transforms into MgF₂ and TiH₂ after the first hydrogen absorption. Adding TiF₃ enhances the crystallinity and reduces the average particle and crystallite sizes of alloys, which is beneficial to accelerating hydriding and dehydriding kinetics. Adding 7 wt% TiF₃ into alloy decreases the dehydrogenation activation energy from 72.2 to 64.0 kJ/mol and improves the hydrogen absorption rate at low temperatures, absorbing 3.50 wt% H in 0.5 min at 323 K.     

Catalytic mechanisms of TiH2 thin layer on dehydrogenation behavior of fluorite-type MgH2: A first principles study

DFT calculations were carried out to investigate hydrogen release and diffusion behaviors. Results demonstrated that MgH₂/TiH₂ interface is thermodynamically stable with negative adhesion energy of −1.33 J/m² with respect to the individual MgH₂ and TiH₂ slabs. The formation of MgH₂/TiH₂ interface alters the interstice structure and space of the interstitial sites where H atoms located and then significantly lowers the dehydrogenation energy of hydrogen releasing from both the MgH₂ and TiH₂ slabs nearby the interface comparing the bulk MgH₂ and TiH_2. The smallest dehydrogenation energy of 0.06 eV/H could be reached when H releases from MgH₂ side. The study also illustrates that the existence of the MgH₂/TiH₂ interface promotes the diffusion of hydrogen vacancy. The lowest diffusion barrier of hydrogen vacancy in the MgH₂ slab (from the sublayer to the frontier layer to the interface) is estimated as 0.21 eV. Based on the present study, one can deduce that the dehydrogenation of the MgH₂/TiH₂ system will start by H releasing from MgH₂ slab, which generates H vacancies near the interface, then the interior H of MgH₂ migrates to the H vacancies (diffuse of H vacancies in the opposite direction) and releases. The TiH₂ acts as a catalyst promoting the generation and diffusion of H vacancies in MgH₂. Therefore synthesizing of MgH₂/TiH₂/MgH₂ sandwich structure could be an effective approach to promote the dehydrogenation process of MgH₂, and an ideal structure owning geometric hydrogen capacity of 6.45 wt%.     

Enhanced hydrogen storage properties of LiBH4-MgH2 composite by the catalytic effect of MoCl3

Though LiBH₄-MgH₂ system exhibits an excellent hydrogen storage property, it still presents high decomposition temperature over 350 °C and sluggish hydrogen absorption/desorption kinetics. In order to improve the hydrogen storage properties, the influence of MoCl₃ as an additive on the hydrogenation and dehydrogenation properties of LiBH₄-MgH₂ system is investigated. The reversible hydrogen storage performance is significantly improved, which leads to a capacity of about 7 wt.% hydrogen at 300 °C. XRD analysis reveals that the metallic Mo is formed by the reaction between LiBH₄ and MoCl₃, which is highly dispersed in the sample and results in improved dehydrogenation and hydrogenation performance of LiBH₄-MgH₂ system. From Kissinger plot, the activation energy for hydrogen desorption of LiBH₄-MgH₂ system with additive MoCl₃ is estimated to be ∼43 kJ mol⁻¹ H₂, 10 kJ mol⁻¹ lower than that for the pure LiBH₄-MgH₂ system indicating that the kinetics of LiBH₄-MgH₂ composite is significantly improved by the introduction of Mo.     

Enhanced hydrogen desorption kinetics and cycle durability of amorphous TiMgVNi3-doped MgH2

MgH₂ is considered as a promising hydrogen storage material for on-board applications. In order to improve hydrogen storage properties of MgH₂, the amorphous TiMgVNi₃-doped MgH₂ is prepared by ball milling under hydrogen atmosphere. It is found that the catalytic (Ti,V)H₂ and Mg₂NiH₄ nanoparticles are in situ formed after activation. As a result, the amorphous TiMgVNi₃-doped MgH₂ exhibits enhanced dehydrogenation kinetics (the activation energy for hydrogen desorption is 94.4 kJ mol^?1 H₂) and superior cycle durability (the capacity retention rate is up to 92% after 50 cycles). These results demonstrate that the in situ formation of highly dispersed catalytic nanoparticles from an amorphous phase is an effective pathway to enhance hydrogen storage properties of MgH₂.     

Study on the gaseous and electrochemical hydrogen storage properties of as-milled CeMgNi-based alloys

For gaining further insight into the involvement of the gaseous and electrochemical hydrogen storage properties of CeMg₁₂-type alloys, partial substitution and ball milling were both used to synthesize the nanocrystalline and amorphous CeMg₁₁Ni + x wt.% Ni (x = 100, 200) samples. This research aims at elucidating the functional roles of Ni content and milling time on samples' structure and hydrogen storage performance. X-Ray diffraction and high-resolution transmission electron microscope were used to reveal the micro constructions of alloys. To determine the gaseous hydrogen storage property, Sievert's apparatus and a thermal gravity analysis bonded with a H₂ probe were adopted. The dehydrogenation activation energy was computed in the Kissinger method. The electrochemical performances of the as-milled samples were measured through a constant current system. Further researches showed that the electrochemical performance of as-milled samples had been dramatically improved by increasing Ni content. With milling duration lengthens, the gaseous hydrogen absorption capacity, gaseous hydriding rate and high rate discharge capability of samples reached the maximal values, but electrochemical discharge capacity and gaseous dehydriding rate always increased. The dehydrogenation activation energy decrease resulted by improving Ni percent and milling duration was deemed as the cause of the excellent gaseous kinetics of samples.     

In-situ synthesis of amorphous Mg(BH4)2 and chloride composite modified by NbF5 for superior reversible hydrogen storage properties

The solvent-free amorphous Mg(BH₄)₂ composite was in-situ synthesized by ball milling LiBH₄ and MgCl₂. It is found that the onset dehydrogenation temperature of the as-synthesized composite is 126.9 °C, which is roughly 156 °C lower than that of pristine Mg(BH₄)₂. The activation energy of the amorphous Mg(BH₄)₂ and pristine Mg(BH₄)₂ for the first dehydrogenation step was calculated as 120.01 kJ/mol and 487.99 kJ/mol, respectively. Hence the kinetics improvement is certified by the lower E_a value of the dehydrogenation process. When adding NbF₅ into the composite, the catalyzed composite exhibits better hydrogen storage properties compared to pristine and amorphous Mg(BH₄)₂. The catalyzed composite starts to release hydrogen at proximately 120 °C with a total capacity of 10.04 wt%. The reversibility of the catalyzed composite is also improved. The capacity of the catalyzed composite at the second cycle is 5.5 wt%. For the third and fourth cycles the catalyzed composite can still liberate 4 wt% H₂. Besides, the onset hydrogen desorption temperature during four cycles are extremely lower than those of pristine and amorphous Mg(BH₄)₂. The peaks of the intermediate MgB₁₂H₁₂ is detected by FTIR as the regenerated hydrogenation product in the catalyzed composite. It can be speculated from the detailed analysis that there are mainly three reasons for the improved properties. Firstly, the additive NbF₅ is favorable to enhance the hydrogen storage properties by modifying the dehydrogenation path and producing MgF₂ and NbB₂ as new products. Secondly, the in-situ formation of amorphous Mg(BH₄)₂ is likely to improve the dehydrogenation properties of the samples due to its different reactivity comparing to crystal ones. Finally, LiCl can serve as buffer in the composite and thus improve the dehydrogenation properties.     

The enhanced de/re‐hydrogenation performance of MgH2 with TiH2 additive

Catalytic effects of TiH₂ on hydrogenation/dehydrogenation kinetics of MgH₂ were investigated in this study. The TG analysis showed that the addition of the x wt% TiH₂ exhibited lower onset temperature of 160°C which is 100°C and 190°C lower than as‐milled and as‐received MgH₂. The dehydrogenation and hydrogenation kinetics were significantly improved compared with the pure MgH₂. The activation energy for the hydrogen desorption of MgH₂ was reduced from ?137.13 to ?77.58 kJ/mol by the addition of TiH₂. XRD and XPS results showed that the phase of TiH₂ remained same during the dehydrogenation without any intermediate formation confirming its role as catalyst.