Carbon-based nanocomposites in solid-state hydrogen storage technology: An overview

Hydrogen fuel is becoming a hot topic among the scientific community as an alternative energy source. Hydrogen is eco-friendly, renewable, and green. The synthesis and development of materials with great potential for hydrogen storage is still a challenge in research and needs to be addressed to store hydrogen economically and efficiently. Various solid-state materials have been fabricated for hydrogen energy storage; however, carbon-based nanocomposites have gained more attention because of its high surface area, low processing cost, and light weight nature. Carbon materials are easy to modify with various metals, metal oxides (MOs), and other organometallic frameworks because of the functional groups available on the surface and edges that increase the storage capacity of hydrogen. In addition, chemisorption is another way to enhance the hydrogen storage capacity of carbon-based nanocomposites. In this review, we discuss the success achieved thus far and the challenges that remain for the physical and chemical storage of hydrogen in various carbon-based nanocomposites. Various compositions of catalysts (eg, metal, MOs, alloy, metal organic frameworks) and carbon materials are designed for hydrogen storage. Superior energy storage in hybrids and composites as compared with pristine materials (catalysts or carbon nanotubes) is governed by the interaction, activation, and hydrogen adsorption/absorption mechanism of materials in the reaction profile. (Nano)composites comprising carbon material with metals, MOs, or alloys are important in this field, not only because of their potential for hydrogen sorption but also their significant cyclic stability and high efficiency upon successive adsorption-desorption cycles.     

Enhanced hydrogen absorption kinetics for hydrogen storage using Mg flakes as compared to conventional spherical powders

Practical challenges of metal hydrides for hydrogen storage such as magnesium hydride, lie predominantly in improving hydrogen absorption kinetics and capacity. Approaches to improving kinetics of Mg commonly include changing particle geometry, reducing crystallite size and coating with catalytic transition metals such as Ni. This study is aimed at improving the hydrogen storage kinetics and capacity of Mg by a novel approach utilizing high aspect ratio powders (thin metal flakes with large diameters) coated with a Ni nano-catalyst. A high speed orbiting ball media (HSOBM) processor was utilized to fabricate the flake-shaped materials. Mg flakes effectively coated with Ni nano-catalyst using dry mechanical coating were found to possess more favorable hydrogen absorption/desorption characteristics and improved hydrogen storage capacity than traditional spherical particles. Geometric shape was shown to be a vital factor in hydrogen absorption kinetics and its effects proved to be dominant over those of crystallite size.     

Mg-based composites for enhanced hydrogen storage performance

Hydrogen storage in solids of hydrides is advantageous in comparison to gaseous or liquid storage. Magnesium based materials are being studies for solid-state hydrogen storage due to their advantages of high volumetric and gravimetric hydrogen storage capacity. However, unfavorable thermodynamic and kinetic barriers hinder its practical application. In this work, we presented that kinetics of Mg-based composites were significantly improved during high energy ball milling in presence of various types of carbon, including plasma carbon produced by plasma-reforming of hydrocarbons, activated carbon, and carbon nanotubes. The improvement of the kinetics and de-/re-hydrogenation performance of MgH₂ and TiC-catalysed MgH₂ by introduction of carbon are strongly dependent on the milling time, amount of carbon and carbon structure. The lowest dehydrogenation temperature was observed at 180 °C by the plasma carbon–modified MgH₂/TiC. We found that nanoconfinement of carbon structures stabilised Mg-based nanocomposites and hinders the nanoparticles growth and agglomeration. Plasma carbon was found to show better effects than the other two carbon structures because the plasma carbon contained both few layer graphene sheets that served as an active dispersion matrix and amorphous activated carbons that promoted the spill-over effect of TiC catalysed MgH₂. The strategy in enhancing the kinetics and thermodynamics of Mg-based composites is leading to a better design of metal hydride composites for hydrogen storage.     

Hydrogen energy,economy and storage: Review and recommendation

The hydrogen economy is a proposed system where hydrogen is produced and used extensively as the primary energy carrier. Successful development of hydrogen economy means innumerable advantages for the environment, energy security, economy, and final users. One major key to wholly develop hydrogen economy is safe, compact, light and cost-efficient hydrogen storage. The conventional gaseous state storage system as pressurized hydrogen gas and liquid state storage system pose safety and cost problems to onboard applications; therefore, they do not satisfy the future goals for a hydrogen economy. Fortunately, solid-state storage systems based on metal hydrides have demonstrated great potentials to store hydrogen in large quantities in a quite secure, compact, and repeatedly reversible manner and thus, becoming increasingly attractive option for hydrogen applications. However, techno-economic feasibility of hydrogen storage systems is yet to be realized as none of the current metal hydrides fulfill all the essential criteria for a practical hydrogen economy, mainly because of low hydrogen storage capacity, sluggish kinetics and unacceptable temperatures of hydrogen absorption/desorption. This article gives a brief review of hydrogen as an ideal sustainable energy carrier for the future economy, its storage as the stumbling block as well as the current position of solid-state hydrogen storage in metal hydrides and makes a recommendation based on the most promising novel discoveries made in the field in recent times which suggests a prospective breakthrough towards a hydrogen economy.     

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.     

DFT study of crystal structure and electronic properties of metal-doped AlH3 polymorphs

AlH₃ has been considered for a long time as a hydrogen storage material with suitable gravimetric and volumetric density for practical applications. Among eight AlH₃ polymorphs observed so far, in this work we focus our attention on an investigation of the effects of various metal dopants in α- and β-AlH₃, to perceive a way of enhancing them. Substitutional incorporation of the metal dopants (Li, Sc, Ti, Cu, Cr, Fe, Nb, Mo, Zn, or Zr) is considered, as well as interstitial doping with Li, Sc, Ti, Cu, and Zr. The density functional theory (DFT) (using GGA-PW91) approach is used to address the crystal structure, bonding, dopant stability, and changes in hydrogen desorption energy. In addition, the kinetics of hydrogen desorption is also considered for several interstitially doped cases, by calculating the stability of native point defects. Promising results are presented for Zr, Ti, and Sc – doped hydrides. Doped hydrides, here studied, are considered as n- or p-type semiconducting materials, enabling wider application overcoming hydrogen storage scope.     

Metal hydride materials for solid hydrogen storage: A review

Hydrogen is an ideal energy carrier which is considered for future transport, such as automotive applications. In this context storage of hydrogen is one of the key challenges in developing hydrogen economy. The relatively advanced storage methods such as high-pressure gas or liquid cannot fulfill future storage goals. Chemical or physically combined storage of hydrogen in other materials has potential advantages over other storage methods. Intensive research has been done on metal hydrides recently for improvement of hydrogenation properties. The present review reports recent developments of metal hydrides on properties including hydrogen-storage capacity, kinetics, cyclic behavior, toxicity, pressure and thermal response. A group of Mg-based hydrides stand as promising candidate for competitive hydrogen storage with reversible hydrogen capacity up to 7.6 wt% for on-board applications. Efforts have been devoted to these materials to decrease their desorption temperature, enhance the kinetics and cycle life. The kinetics has been improved by adding an appropriate catalyst into the system and as well as by ball-milling that introduces defects with improved surface properties. The studies reported promising results, such as improved kinetics and lower decomposition temperatures, however, the state-of-the-art materials are still far from meeting the aimed target for their transport applications. Therefore, further research work is needed to achieve the goal by improving development on hydrogenation, thermal and cyclic behavior of metal hydrides.     

A comprehensive review of hydrogen production and storage: A focus on the role of nanomaterials

Nanomaterials are beginning to play an essential role in addressing the challenges associated with hydrogen production and storage. The outstanding physicochemical properties of nanomaterials suggest their applications in almost all technological breakthroughs ranging from catalysis, metal-organic framework, complex hydrides, etc. This study outlines the applications of nanomaterials in hydrogen production (considering both thermochemical, biological, and water splitting methods) and storage. Recent advances in renewable hydrogen production methods are elucidated along with a comparison of different nanomaterials used to enhance renewable hydrogen production. Additionally, nanomaterials for solid-state hydrogen storage are reviewed. The characteristics of various nanomaterials for hydrogen storage are compared. Some nanomaterials discussed include carbon nanotubes, activated carbon, metal-doped carbon-based nanomaterials, metal-organic frameworks. Other materials such as complex hydrides and clathrates are outlined. Finally, future research perspectives related to the application of nanomaterials for hydrogen production and storage are discussed.     

Spatially resolved hydrogen desorption from aluminum hydride observed by magnetic resonance imaging

Light metal hydrides represent a promising class of materials for hydrogen storage. However, there are several technical challenges to overcome before their potential can be realized. Key among these is the often adverse absorption and desorption kinetics, which are a function of intrinsic reaction rates and practical operating temperatures. Modifying and controlling these kinetics require a thorough understanding of the hydrogen absorption and desorption processes. In this study, we have investigated the thermal decomposition of aluminum hydride, AlH₃, with Magnetic Resonance Imaging in order to visualize spatially the progress and extent of the reaction.     

Perspectives of high entropy alloys as hydrogen storage materials

Storage is a challenging issue that cuts across distribution, delivery, and safe end-uses of hydrogen as fuel. All the fuel cell vehicles are equipped with inefficient and unsafe high-pressure hydrogen cylinders. It is well known that storing such a highly flammable gas at high pressure is not safe. Only hydrogen can be stored safely as a form of metal hydrides, and all the investigated metal hydrides are inefficient in one way or another. Four essential hydrogen parameters for solid-state storage for fuel cell applications are high volumetric storage capacity, excellent heat transfer, and recharge time and feasible charging discharging temperatures. The available metal tanks have good gravimetric storage capacity but did not satisfy the prescribed criterion for good volumetric capacity necessary for mobile applications. Recently, some promising reports are published on the hydrogen storage properties of newly discovered High Entropy Alloys (HEAs). HEAs provide vast composition selection freedom for the formation of favorable simple solid solution phase for hydrogen storage. The four core effects of these alloys may also play a vital role in hydrogen storage properties. Here we reviewed and summarized the published results on hydrogen storage properties of HEAs to date. We underlined different essential aspects for the future development of HEAs as hydrogen storage materials. This review article discusses and describes the perspectives of HEAs in regards to the hydrogen storage applications of these alloys and will provide insight into the future development of hydrogen storage HEAs.     

Enhanced volumetric hydrogen density in sodium alanate by compaction

Powder compaction is a potential process for the enhancement of the volumetric and gravimetric capacities of hydrogen storage systems based on metal hydrides. This paper presents the hydrogen absorption and desorption behaviour of compacts of sodium alanate material prepared under different levels of compaction pressure. It is shown that even at high compaction levels and low initial porosities, hydrogen absorption and desorption kinetics can proceed comparatively fast in compacted material. Furthermore, experimental hydrogen weight capacities of compacted material are higher than the experimental values obtained in case of loose powder. It is demonstrated that the kinetic behaviour of the compacted material during cycling is directly associated to the volumetric expansion of the compact, which is quantitatively measured and analyzed during both hydrogen absorption and desorption processes. The cycling behaviour and dimensional changes of compacted sodium alanate material are a key consideration point if it is used as hydrogen storage materials in practical tank systems.     

Interface effects in NaAlH4–carbon nanocomposites for hydrogen storage

For practical solid-state hydrogen storage, reversibility under mild conditions is crucial. Complex metal hydrides such as NaAlH₄ and LiBH₄ have attractive hydrogen contents. However, hydrogen release and especially uptake after desorption are sluggish and require high temperatures and pressures. Kinetics can be greatly enhanced by nanostructuring, for instance by confining metal hydrides in a porous carbon scaffold. We present for a detailed study of the impact of the nature of the carbon–metal hydride interface on the hydrogen storage properties. Nanostructures were prepared by melt infiltration of either NaAlH₄ or LiBH₄ into a carbon scaffold, of which the surface had been modified, varying from H-terminated to oxidized (up to 4.4 O/nm²). It has been suggested that the chemical and electronic properties of the carbon/metal hydride interface can have a large influence on hydrogen storage properties. However, no significant impact on the first H₂ release temperatures was found. In contrast, the surface properties of the carbon played a major role in determining the reversible hydrogen storage capacity. Only a part of the oxygen-containing groups reacted with hydrides during melt infiltration, but further reaction during cycling led to significant losses, with reversible hydrogen storage capacity loss up to 40% for surface oxidized carbon. However, if the carbon surface had been hydrogen terminated, ∼6 wt% with respect to the NaAlH₄ weight was released in the second cycle, corresponding to 95% reversibility. This clearly shows that control over the nature and amount of surface groups offers a strategy to achieve fully reversible hydrogen storage in complex metal hydride-carbon nanocomposites.     

TM dopant-induced H-vacancy diffusion kinetics of sodium-lithium alanates: Ab initio study for hydrogen storage improvement

We present a hydrogen storage mechanism of the surface and bulk Na–Li–Al hydrides substituted by the transition metal (TM) dopants such as Ni, Cu, Ag, and Zn. The host hydrides of interest, namely, NaAlH₄, LiAlH₄, Na₃AlH₄, Li₃AlH₄, and Na₂LiAlH₄ are found to be stable compositions at ambient pressure. Hydrogen vacancy mechanisms of the host hydrides with the TM dopants are investigated using ab initio calculations. Remarkably, the results show the enhancement of the internal mechanism for hydrogen storage in the Na–Li–Al complex hydrides. Doping of Ni or Zn mainly reduces the energy barrier of diffusion kinetics in the host Na–Li–Al hydrides, leading to the improvement of the hydrogen storage efficiency of the host Na–Li–Al hydrides. Therefore, hydrogen vacancy diffusion kinetics in the Na–Li–Al hydrides can be induced by adding the Ni and Zn dopants.     

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.     

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.     

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.     

An overview of hydrogen generation by hydrolysis of Al and its alloys with the addition of carbon-based materials

Al and its alloys are studied extensively for hydrogen generation through water splitting. Alloying Al with metal activators such as bismuth, indium, gallium, etc., leads to the formation of micro galvanic cells during hydrolysis reaction, resulting in an improved hydrogen generation rate. Activation of Al by adding carbon-based materials such as graphite, carbon nanotubes (CNTs), graphene, etc., can instantaneously generate hydrogen at room temperature. When carbon particles are desorbed from the Al matrix during hydrolysis, new Al is exposed, resulting in an increased reaction rate. In Al-Graphite composites which form core-shell structures, H₂O molecules penetrate through the graphite layers and break down the core-shell structure during hydrolysis, and the new Al surfaces are exposed to water. It was found that Al with nano bismuth and graphene nanosheets showed better hydrogen generation rate and hydrogen yield. Graphene nanosheets control the agglomeration of Al and enhance the specific surface area for hydrolysis. During the hydrolysis of Al-CNTs composites, CNTs act as a cathode, resulting in galvanic corrosion between CNTs and the Al matrix. CNTs can also effectively control the agglomeration of Al during ball milling. Spark plasma sintered Al–Bi-CNT composites showed an enhanced hydrogen generation rate during hydrolysis. This paper presents an overview of hydrogen generation by hydrolysis of Al and its alloys, emphasising the addition of carbon-based materials such as graphite, graphene, CNTs, etc.     

Electron microscope investigation on hydrogen storage materials: A review

Hydrogen energy is a highly efficient and renewable energy carrier. The rapid and sophisticated development of nanotechnologies has promoted the transition of hydrogen storage systems from gaseous/liquid to solid-state. In order to clarify the intrinsic relationship between structure and performance, and to understand the hydrogen absorption and desorption mechanism of materials, electron microscopy (EM) can effectively help us obtain a series of information such as particle size, phase and composition determination, morphology and structure of the materials at nanoscale. The most recent progress of advanced EM techniques applied in solid-state hydrogen storage materials are summarized, which should also inspire future research on energy storage related materials.     

Improving hydrogen storage properties of magnesium based alloys by equal channel angular pressing

Equal channel angular pressing was applied to a commercial magnesium alloy ZK60 in order to improve its hydrogen storage properties. The microstructure refinement and increase in the density of crystal lattice defects caused by equal channel angular pressing increase hydrogen desorption pressure, change the slope of the pressure plateau in pressure-composition isotherms, decrease the pressure hysteresis, and accelerate the hydrogen desorption kinetics. It is argued that a proper design of the defect structure of materials is a key element in the search for economically viable and environmentally acceptable solutions for mobile hydrogen storage based on metal hydrides.     

Effect of cycling on hydrogen storage properties of Ti2CrV alloy

In the present work, we have studied the hydrogen absorption–desorption properties of the Ti₂CrV alloy, and effect of cycling on the hydrogen storage capacity. The material has been characterized for the structure, morphology, pressure composition isotherms, hydrogen storage capacity, hydrogen absorption kinetics and the desorption profile at different temperatures in detail. The Ti₂CrV crystallizes in body centered cubic (bcc) structure like TiCrV. The pressure composition isotherm of the alloy has been measured at room temperature and at 373K. The Ti₂CrV alloy shows maximum hydrogen storage capacity of 4.37 wt.% at room temperature. The cyclic hydrogen absorption capacity of Ti₂CrV alloy has been investigated at room temperature upto 10th cycle. The hydrogen storage capacity decreased progressively with cycling initially, but the alloy can maintain steady cyclic hydrogen absorption capacity 3.5 wt.% after 5th cycle. To get insight about the desorption behavior of the hydride in-situ desorption has been done at different temperatures and the amount of hydrogen desorbed has been calculated. The TG (Thermo gravimetric) and DTA analysis has been done on uncycled hydride shows that the surface poisoned sample gives a desorption onset temperature of 675K. The DSC measurement of uncycle and multi-cycled saturated hydrides shows that the hydrogen desorption temperature decreasing with cycling.