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.     

Recent progress of nanotechnology in enhancing hydrogen storage performance of magnesium-based materials: A review

Hydrogen has attracted wide attention in the field of new energy, triggering a comprehensive study of hydrogen production, storage and application. This paper mainly studies the hydrogen storage capacity of magnesium-based materials with nanostructure. The reversible hydrogen capacity of Mg-based hydrogen storage materials can reach 7.6 wt%, but due to its poor kinetic and thermodynamic properties, its hydrogen storage performance is not as good as other hydrogen storage materials. In order to reduce the desorption temperature of materials, many studies have been carried out. Alloying, nanostructure and adding catalyst are feasible methods to improve the properties of Mg-based hydrogen storage alloys. By adding catalyst and alloy with other transition elements, the dehydrogenation temperature of magnesium-based materials has been reduced to less than 200 °C. The hydrogen storage of magnesium-based alloys has been practically applied.     

Magnesium-based alloys for solid-state hydrogen storage applications: A review

Magnesium hydrides (MgH₂) have attracted extensive attention as solid-state H₂ storage, owing to their low cost, abundance, excellent reversibility, and high H₂ storage capacity. This review comprehensively explores the synthesis and performance of Mg-based alloys. Several factors affecting their hydrogen storage performance were also reviewed. The metals addition led to destabilization of Mg–H bonds to boost the dehydrogenation process. A unique morphology could provide more available active sites for the dissociation/recombination of H₂ and allow the activation energy reduction. Also, an appropriate support prevent the agglomeration of Mg particles, hence, sustains their nanosize particles. Moreover, the perspective of avenues for future research presented in this review is expected to act as a guide for the development of novel Mg-based H₂ storage systems. New morphological shape of catalysts, more unexplored and highly potential waste materials, and numerous synthesis procedures should be explored to further enhance the H₂ storage of Mg-based alloys.     

Catalytic effect of Al2TiO5 on the dehydrogenation properties of LiAlH4

Lithium alanate (LiAlH₄) is considered as a promising material for storing hydrogen (H₂) in solid-state form for onboard applications due to its advantage of high gravimetric H₂ capacity. LiAlH₄ could release H₂ ～7.9 wt.% when heated up to ～250 °C. Nevertheless, the high desorption temperature, sluggish desorption kinetics, and irreversibility hamper the application of LiAlH₄ for solid-state H₂ storage materials. Therefore, in this study, we have used aluminum titanate (Al₂TiO₅) as an additive to diminish the desorption temperature and enhance the desorption kinetics of LiAlH₄. The addition of a small amount of Al₂TiO₅ (5 wt.%) into LiAlH₄ significantly decreased the decomposition temperature and enhanced the desorption kinetics, in which Al₂TiO₅-doped LiAlH₄ started to release H₂ at ～90 °C and was able to desorb H₂ as much as ～3.5 wt.% at 90 °C within 1 h. Without the catalyst, pure LiAlH₄ starts to release H₂ at ～145 °C and only desorbs H₂ as low as 0.3 wt.% at 90 °C within 1 h. The activation energies for H₂ release in the two-step desorption process of LiAlH₄ were reduced after catalysis with Al₂TiO₅. The activation energies of as-milled LiAlH₄ were 80 kJ/mol and 91 kJ/mol, respectively, as calculated by the Arrhenius plot. The activation energies were lowered to 68 kJ/mol and 79 kJ/mol after milling with Al₂TiO₅. The scanning electron microscopy images revealed that the LiAlH₄ particles became smaller and less agglomerated when Al₂TiO₅ was added. It is believed that the in-situ formation of active species during the desorption process and reduction in particles size play a vital role in improving the dehydrogenation properties of the Al₂TiO₅-doped LiAlH₄ system.     

Synthesis of solid-state NaBH4/Co-based catalyst composite for hydrogen storage through a high-energy ball-milling process

Solid-state composites of NaBH₄ and Co-based catalyst have been fabricated for hydrogen generation via a novel mechanochemical technique, i.e. the high-energy ball milling, in which the gravimetric storage capacity of hydrogen has reached 6.7 wt%, meeting the 2010 target of at least 0.06 kg H₂/kg set by the U.S. Department of Energy (DOE). The active catalysts used in the hydrolysis reaction of sodium borohydride for hydrogen generation are mainly cobalt oxides. Controlled addition of water, namely water used as a limiting agent, to the solid composites of NaBH₄ and Co-based catalyst greatly improves the H₂ storage capacity and resolved the issues of low gravimetric H₂ storage in conventional aqueous system of sodium borohydride. Factors influencing the performance of hydrogen production such as amounts of H₂O added, catalyst loadings and durations of ball-milling processes are investigated. Moreover the hydrolyzed products of NaBH₄ and spent catalysts are analyzed as well.     

Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4

Study on the catalytic roles of MgFe₂O₄ on the dehydrogenation performance of LiAlH₄ was carried out for the first time. Notable improvement on the dehydrogenation of LiAlH₄–MgFe₂O₄ compound was observed. The initial decomposition temperatures for the catalyzed LiAlH₄ were decreased to 95 °C and 145 °C for the first and second stage reactions, which were 48 °C and 28 °C lower than the milled LiAlH₄. As for the desorption kinetics performance, the MgFe₂O₄ doped-LiAlH₄ sample was able to desorb faster with a value of 3.5 wt% of hydrogen in 30 min (90 °C) while the undoped LiAlH₄ was only able to desorb 0.1 wt% of hydrogen. The activation energy determined from the Kissinger analysis for the first two desorption reactions were 73 kJ/mol and 97 kJ/mol; which were 31 and 17 kJ/mol lower as compared to the milled LiAlH₄. The X-ray diffraction result suggested that the MgFe₂O₄ had promoted significant improvements by reducing the LiAlH₄ decomposition temperature and faster desorption kinetics through the formation of active species of Fe, LiFeO₂ and MgO that were formed during the heating process.     

A review on recent advances in hollow spheres for hydrogen storage

Recent advancements in synthesizing materials potential for hydrogen storage have greatly forced the hydrogen storage technology ahead in recent years. Hollow spheres, with unparalleled characteristics like low density and high specific surface area, have emerged as one of the most promising alternatives for hydrogen storage applications. In the present review, the main synthesis approaches of hollow spheres including spray drying, Kirkendall, template-free and, template-assisted methods are surveyed and concisely described. In addition, different types of hollow spheres such as hollow carbons, hollow glasses and other less-common types like Boron nitrides and metal hollow spheres have been tackled with special focus on adsorption/desorption capacities as well as the kinetic of hydrogen storage/release. In addition to the recent progresses, some perspective and outlook on the advancement of hollow spheres and challenges in terms of synthesis methods and hydrogen storage performance were presented.     

Recent advances in carbon nitride-based nanomaterials for hydrogen production and storage

There are different types of materials comprising of carbon and nitrogen elements. Typical materials are the cyanogen family, beta carbon nitride, graphitic carbon nitride, azafullerenes, and heterofullerenes, N-containing heterocycles. Except cyanogen (C₂N₂) is a gas, most others are solid. Among these solids, the graphitic carbon nitride, with the general chemical formula of C₃N₄, is widely studied in heterogeneous catalysis and energy storage. Such applications exploit the resiliency of the material in different environments due to its labile protons and Lewis acid functionalities, as well as its layered structure. The structure of graphitic C₃N₄ allows it to store a significant amount of hydrogen. Furthermore, it offers the space for dopants, which are used purposely for tuning the band gap and the electronic properties of C₃N₄ to make it suitable for water splitting using sun light, or many other applications in waste water treatment under radiation. We think that the material is important and it is not being exploited at its highest capability, especially in hydrogen production via water splitting technique. This review aims to summarize recent outcomes using the carbon nitride material in hydrogen production, and a brief about hydrogen storage. We also highlight future research directions which might worth being persuaded.     

Enhancing hydrogen storage performance of MgH2 through the addition of BaMnO3 synthesized via solid-state method

Currently, magnesium hydride (MgH₂) as a solid-state hydrogen storage material has become the subject of major research owing to its good reversibility, large hydrogen storage capacity (7.6 wt%) and affordability. However, MgH₂ has a high decomposition temperature (>400 °C) and slow desorption and absorption kinetics. In this work, BaMnO₃ was synthesized using the solid-state method and was used as an additive to overcome the drawbacks of MgH₂. Interestingly, after adding 10 wt% of BaMnO₃, the initial desorption temperature of MgH₂ decreased to 282 °C, which was 138 °C lower than that of pure MgH₂ and 61 °C lower than that of milled MgH₂. For absorption kinetics, at 250 °C in 2 min, 10 wt% of BaMnO₃-doped MgH₂ absorbed 5.22 wt% of H₂ compared to milled MgH₂ (3.48 wt%). Conversely, the desorption kinetics also demonstrated that 10 wt% of BaMnO₃-doped MgH₂ samples desorbed 5.36 wt% of H₂ at 300 °C within 1 h whereas milled MgH₂ only released less than 0.32 wt% of H₂. The activation energy was lowered by 45 kJ/mol compared to that of MgH₂ after the addition of 10 wt% of BaMnO₃. Further analyzed by using XRD revealed that the formation of Mg_0·9Mn_0·1O, Mn₃O₄ and Ba or Ba-containing enhanced the performance of MgH₂.     

An overview of reactive hydride composite (RHC) for solid-state hydrogen storage materials

Hydrogen storage using the metal hydrides and complex hydrides is the most convenient method because it is safe, enables high hydrogen capacity and requires optimum operating condition. Metal hydrides and complex hydrides offer high gravimetric capacity that allows storage of large amounts of hydrogen. However, the high operating temperature and low reversibility hindered the practical implementation of the metal hydrides and complex hydrides. An approach of combining two or more hydrides, which are called reactive hydride composites (RHCs), was introduced to improve the performance of the metal hydrides and complex hydrides. The RHC system approach has significantly enhanced the hydrogen storage performance of the metal hydrides and complex hydrides by modifying the thermodynamics of the composite system through the metathesis reaction that occurred between the hydrides, hence enhancing the kinetic and reversibility performance of the composite system. In this paper, the overview of the RHC system was presented in detail. The challenges and perspectives of the RHC system are also discussed. This is the first review report on the RHC system for solid-state hydrogen storage.     

Novel fabrication of solid-state NaBH4/Ru-based catalyst composites for hydrogen evolution using a high-energy ball-milling process

Solid-state NaBH₄/Ru-based catalyst composites have been fabricated for hydrogen generation through a high-energy ball-milling process, providing uniform dispersion of resin-supported Ru³⁺ catalysts among pulverized NaBH₄ (SBH) particles, so as to increase the contacts of SBH with active catalytic sites. Consequently, the gravimetric hydrogen storage capacity as high as 7.3 wt% could be achieved by utilizing water as a limiting reagent to overcome the issue of deactivated catalysts whose active sites are often blocked by precipitates caused by limited NaBO₂ solubility occurring in conventional aqueous SBH systems for hydrogen productions. Products of hydrolyzed SBH that greatly influence the gravimetric H₂ storage capacity are found to be most likely NaBO₂·2H₂O and NaBO₂·4H₂O from SBH/H₂O reacting systems with initial weight ratios, SBH/H₂O = 1/2 and 1/10, respectively, according to the TGA and XRD analyses.     

Recent advances in magnesium-based hydrogen storage materials with multiple catalysts

The slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability of MgH bond are the two major limitations for large-scale utilization of MgH₂. Constructing Mg nanoparticles (NPs) with large specific surface area and short hydrogen diffusion distance and adding effective catalysts to facilitate the sorption kinetics are the main ways to solve the remained shortages of Mg/MgH₂ system. This article reviews the recent advances in the Mg-based nanocomposites with multiple catalysts produced by different approaches. The multiple catalysts was classified into transition metals and/or their hydrides (TM and/or TMH), TM together with metal oxides, TM and/or TMH together with metal hadlies, TM and/or TMH together with metal sulfides, TM and/or TMH together with Mg₂M and other multiple catalysts systems. Compared with single catalyst, the multiple catalysts display enhanced synergistic catalytic effects on the hydrogen absorption and desorption rates.     

Low-temperature hydrogen release through LiAlH4 and NH4F react in Et2O

The application of hydrogen energy urgently requires a high-capacity hydrogen storage technology that can release hydrogen at low temperature. The composite of LiAlH₄ and NH₄F has a hydrogen storage capacity of up to 8.06 wt%, but the release of hydrogen requires a reaction temperature of about 170 °C, and the reaction is difficult to control. In this work, the reaction between LiAlH₄ and NH₄F is proposed to be carried out in diethyl ether to improve its hydrogen release performance. It exhibits good hydrogen release performance over a wide temperature range of −40–25 °C, and the hydrogen release capacity at −40 °C, −20 °C, 0 °C and 25 °C can reach 4.41 wt%, 6.79 wt%, 6.85 wt% and 7.78 wt%, respectively. The activation energy of the reaction is 38.41 kJ mol⁻¹, which is much lower than many previously reported catalytic hydrolysis systems that can release hydrogen at room temperature. Our study demonstrates a high-performance hydrogen storage system with very low operating temperature, which may lay the foundation for the development of practical mobile/portable hydrogen source in the north and the Arctic.     

Desorption properties of LiAlH4 doped with LaFeO3 catalyst

The addition of a catalyst and ball milling process was found to be one of the efficient method to reduce the decomposition temperature and improve the desorption kinetics of lithium aluminium hydride (LiAlH₄). In this paper, a transition metal oxide, LaFeO₃ was used as a catalyst. Decomposition temperature of the 10 wt% of LaFeO₃-doped LiAlH₄ system was found to be lowered from 143 °C to 103 °C (first step) and from 175 °C to 153 °C (second step), respectively. In isothermal desorption kinetics, the amount of hydrogen released of the doped sample was improved to 3.9 wt% in 2.5 h at 90 °C. Meanwhile, the undoped sample had released less than 1.0 wt% of hydrogen under the same condition. The activation energy of the LaFeO₃-doped LiAlH₄ sample was measured to be 73 kJ/mol and 90 kJ/mol for the first two dehydrogenation reactions compared to 107 kJ/mol and 119 kJ/mol for the undoped sample. The improvements of desorption properties were the results from the formation of LiFeO₂, Fe and La or La-containing phase during the heating process.     

Mechanochemical reactions and hydrogen storage capacities in MBH4–SiS2 systems (MLi or Na)

The hydrogen storage properties, and phase compositions of mechanochemically prepared mixtures of xMBH₄-SiS₂ (x = 2–8), where M = Li or Na, were investigated using gas sorption analysis, powder X-ray diffraction, and infrared and solid-state NMR spectroscopic methods. The 2LiBH₄:1SiS₂ system forms an amorphous product that releases ca. 4.3 wt % of H₂ below 385 °C with a T_onset of 88 °C without detectable diborane emission. The dehydrogenated sample reversibly absorbs 1.5 wt % of H₂ at 385 °C under 160 bar pressure. The H₂ release from materials with varying LiBH₄:SiS₂ ratios peaks at 8.2 wt % for the 6LiBH₄:1SiS₂ composition, with a reversible hydrogen storage capacity of 2.4 wt %. The H₂ desorption capacities of the Li-containing systems surpass those of Na-containing systems. Solid-state NMR studies indicate that products of mechanochemical reactions in the LiBH₄SiS₂ system consist of one-dimensional chains of edge-sharing SiS_4/2 tetrahedra in which the non-bridging S-ends are terminated with Li⁺, which are further coordinated to the [BH₄]⁻ anions. A variety of possible polymorphs in the LiSiS-(BH₄) composition space have been identified using first principles and thermodynamic modeling that supports the likelihood of formation of such novel complexes.     

Thermally stable Ni MOF catalyzed MgH2 for hydrogen storage

The poor kinetics is the main issue hindering MgH₂ for practical hydrogen storage application. In this work, the tricarboxybenzene was used to construct the stable Ni MOF (Ni-BTC300) as the catalyst for MgH₂. The prepared MOFs maintained their chemical structure after 300 °C calcining and doped to MgH₂ by ball milling. The dispersed, uniformly bonded Ni atoms can improve the kinetics of the composites, which could desorb 5.14 wt% H₂ within 3 min at 300 °C. And the stable MOF structure leading to good cycle stability in both kinetics and capacity, with retention of 98.2% after 10 cycles.     

Model-based design of an automotive-scale,metal hydride hydrogen storage system

Sandia and General Motors have successfully designed, fabricated, and experimentally operated a vehicle-scale hydrogen storage demonstration system using sodium alanates. The demonstration system module design and the system control strategies were enabled by experiment-based, computational simulations that included heat and mass transfer coupled with chemical kinetics. Module heat exchange systems were optimized using multi-dimensional models of coupled fluid dynamics and heat transfer. Chemical kinetics models were coupled with both heat and mass transfer calculations to design the sodium alanate vessels. Fluid flow distribution was a key aspect of the design for the hydrogen storage modules and computational simulations were used to balance heat transfer with fluid pressure requirements.     

Recent advances in hydrogen production by thermo-catalytic conversion of biomass

Hydrogen production from biomass is a green, clean, and zero emissions technology that has attracted increasing attention. This technology has been considered to possess a long-term growth potential, and it is expected to gradually reduce environmental pollution and over-exploitation of resources. In this context, we holistically review this technology and focused on the conversion of biomass into hydrogen using chemical methods (i.e., those with potential for obtaining H₂-rich producer gas streams). Several reaction parameters were discussed and classified herein including biomass conversion methods and conditions, hydrogen production and carbon conversion ratios, the effect of different catalysts types, the catalytic properties of these materials, and their related mechanisms. The overall findings provide new insights for the selection of highly effective and suitable hydrogen production catalysts by biomass conversion applications.     

Macroscale evaluation and testing of chemically hydrogenated graphene for hydrogen storage applications

The effective storage of H₂ gas represents one of the major challenges in the wide spread adoption of hydrogen powered fuel cells for light vehicle transportation. Here, we investigate the merits of chemically hydrogenated graphene (graphane) as a means to store high-density hydrogen fuel for on demand delivery. In order to evaluate hydrogen storage at the macroscale, 75 g of hydrogenated graphene was synthesized using a scaled up Birch reduction, representing the largest reported synthesis of this material to date. Covalent hydrogenation of the material was characterized via Raman spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). We go on to demonstrate the controlled release of H₂ gas from the bulk material using a sealed pressure reactor heated to 600 °C, identifying a bulk hydrogen storage capacity of 3.2 wt%. Additionally, we demonstrate for the first time, the successful operation of a hydrogen fuel cell using chemically hydrogenated graphene as a power source. This work demonstrates the utility of chemically hydrogenated graphene as a high-density hydrogen storage medium, and will be useful in the design of prototype hydrogen storage systems moving forward.     

Effect of adding different percentages of HfCl4 on the hydrogen storage properties of MgH2

Magnesium hydride (MgH₂) is the best candidate material to store hydrogen in the solid-state form owing to its advantages such as good reversibility, high hydrogen storage capacity (7.6 wt%), low raw material cost and abundance in the earth. Nevertheless, slow desorption/absorption kinetics and high thermodynamic stability are two issues that have constrained the commercialization of MgH₂ as a solid-state hydrogen storage material. So, to boost the desorption/absorption kinetics and to alter the thermodynamics of MgH₂, hafnium tetrachloride (HfCl₄) was used as a catalyst in this study. Different percentages of HfCl₄ (5, 10, 15 and 20 wt%) were added to MgH₂ and their catalytic influences on the hydrogen storage properties of MgH₂ were investigated. Results showed that the 15 wt% HfCl₄-doped MgH₂ sample was the best composite to enhance the hydrogen storage performance of MgH₂. The onset decomposition temperature of the 15 wt% HfCl₄-doped MgH₂ composite was decreased by ~75 °C compared to as-milled MgH₂. Meanwhile, the desorption/absorption kinetic measurements showed an improvement compared to the undoped MgH₂. From the Kissinger analysis, the apparent dehydrogenation activation energy was 167.0 kJ/mol for undoped MgH₂ and 102.0 kJ/mol for 15 wt% HfCl₄-doped MgH₂. This shows that the HfCl₄ addition reduced the activation energy of the hydrogen decomposition of MgH₂. The desorption enthalpy change calculated by the van't Hoff equation showed that the addition of HfCl₄ to MgH₂ did not affect the thermodynamic properties. Scanning electron microscopy showed that the size of the MgH₂ particles decreased and there was less agglomeration after the addition of HfCl₄. It is believed that the decrease in the particle size and in-situ generated MgCl₂ and Hf-containing species had synergistic catalytic effects on enhancing the hydrogen storage properties of the HfCl₄-doped MgH₂ composite.