A critical review on improving hydrogen storage properties of metal hydride via nanostructuring and integrating carbonaceous materials

Hydrogen-based economy has a great potential for addressing the world's environmental concerns by using hydrogen as its future energy carrier. Hydrogen can be stored in gaseous, liquid and solid-state form, but among all solid-state hydrogen storage materials (metal hydrides) have the highest energy density. However, hydrogen accessibility is a challenging step in metal hydride-based materials. To improve the hydrogen storage kinetics, effects of functionalized catalysts/dopants on metal atoms have been extensively studied. The nanostructuring of metal hydrides is a new focus and has enhanced hydrogen storage properties by allowing higher surface area and thus reversibility, hydrogen storage density, faster and tunable kinetics, lower absorption and desorption temperatures, and durability. The effect of incorporating nanoparticles of carbon-based materials (graphene, C60, carbon nanotubes (CNTs), carbon black, and carbon aerogel) showed improved hydrogen storage characteristics of metal hydrides. In this critical review, the effects of various carbon-based materials, catalysts, and dopants are summarized in terms of hydrogen-storage capacity and kinetics. This review also highlights the effects of carbon nanomaterials on metal hydrides along with advanced synthesis routes, and analysis techniques to explore the effects of encapsulated metal hydrides and carbon particles. In addition, effects of carbon composites in polymeric composites for improved hydrogen storage properties in solid-state forms, and new characterization techniques are also discussed. As is known, the nanomaterials have extremely higher surface area (100–1000 time more surface area in m²/g) when compared to the bulk scale materials; thus, hydrogen absorption and desorption can be tuned in nanoscale structures for various industrial applications. The nanoscale tailoring of metal hydrides with carbon materials is a promising strategy for the next generation of solid-state hydrogen storage systems for different industries.     

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.     

Nanostructured hydrogen storage materials prepared by high-energy reactive ball milling of magnesium and ferrovanadium

Hydrogen storage nanocomposites prepared by high energy reactive ball milling of magnesium and vanadium alloys in hydrogen (HRBM) are characterised by exceptionally fast hydrogenation rates and a significantly decreased hydride decomposition temperature. Replacement of vanadium in these materials with vanadium-rich Ferrovanadium (FeV, V80Fe20) is very cost efficient and is suggested as a durable way towards large scale applications of Mg-based hydrogen storage materials. The current work presents the results of the experimental study of Mg–(FeV) hydrogen storage nanocomposites prepared by HRBM of Mg powder and FeV (0–50 mol.%). The additives of FeV were shown to improve hydrogen sorption performance of Mg including facilitation of the hydrogenation during the HRBM and improvements of the dehydrogenation/re-hydrogenation kinetics. The improvements resemble the behaviour of pure vanadium metal, and the Mg–(FeV) nanocomposites exhibited a good stability of the hydrogen sorption performance during hydrogen absorption – desorption cycling at T = 350 °C caused by a stability of the cycling performance of the nanostructured FeV acting as a catalyst. Further improvement of the cycle stability including the increase of the reversible hydrogen storage capacity and acceleration of H₂ absorption kinetics during the cycling was observed for the composites containing carbon additives (activated carbon, graphite or multi-walled carbon nanotubes; 5 wt%), with the best performance achieved for activated carbon.     

Metal-organic-framework based catalyst for hydrogen production: Progress and perspectives

Fossil fuel shortage and global warming have inspired scientists to search for alternative energy sources which are green, renewable, and sustainable. Hydrogen formed from water splitting has been considered as one of the most promising candidates to replace traditional fuels due to its low production cost and zero-emission. Metal-organic frameworks (MOFs) have been considered as potential catalysts for hydrogen production from water splitting account for their flexible structure, ultra-large surface area, and chemical component diversification. This paper reviews different kinds of MOF-related electrocatalysts, involving metals, metal oxides, single atoms, metal phosphides, metal nitrides, and metal dichalcogenides for hydrogen production. Also, MOF-based photocatalysts consisting of pristine MOFs, MOFs as supporters, and MOF-derived heterojunction architectures are reviewed. The finding of MOF-based catalysts for hydrogen generation is summarized. The pros and cons of different MOF-based materials as catalysts for water splitting are discussed. Finally, current challenges and the potential developments of these unique materials as catalysts are also provided.     

Recent advances in covalent organic framework (COF) nanotextures with band engineering for stimulating solar hydrogen production: A comprehensive review

Due to the continuous consumption of fossil fuels, natural reserves are depleting and it has been earnest need for developing new sources of energy. Among the several solar energy conversion techniques, photocatalytic hydrogen (H₂) generation is regarded as one of the most promising routes. Till date, several metal-based semiconductor materials have been investigated, however, H₂ generation is not substantial with the notion of sustainable development. Current research trends show the growing interest in advanced and metal free photocatalyst materials such as covalent organic frameworks (COFs) due to their several benefits such as crystalline porous polymers with pre-designed architectures, large surface area, exceptional stability, and ease of molecular functionalization. By combining COFs with other functional materials, composites may be created that display unique characteristics that exceed those of the separate components. This work provides a comprehensive development on COFs as a photocatalysts and their composites/hybrids for photocatalytic hydrogen generation with a focus on visible-light irradiation. To reduce the dependency on novel metals and overcome the drawbacks of individual material, the creation of composite materials based on covalent-organic frameworks (COFs) are systematically discussed. In addition, advantages in terms of performance, stability, durability of composites/hybrids COFs for photocatalytic hydrogen production in reference to traditional catalysts are investigated. Different composites such as metals loading, morphological development, band engineering, and heterojunctions are systematically discussed. Finally, challenges and opportunities associated with constructing COF-based catalysts as future research prospective for chemistry and materials science are highlighted.     

Effect of g-C3N4 amount on green synthesized GdFeO3/g-C3N4 nanocomposites as promising compounds for solid-state hydrogen storage

Hydrogen energy is a key role in novel renewable energy production/consumption technologies. Traditional hydrogen energy systems are suffered from low density, high production cost, low efficiency, and storage complications. With the start of solid-state hydrogen storage technology, many of above deficiencies are fulfilled, however, there are several unknown points, particularly in metal oxides, which need more attention. Hydrogen sorption on the layered materials or inside porous materials is a hopeful key to drawbacks for high-performance hydrogen sorption. Hereupon, layered solids with the merit of hydrogen sorption are introduced, for the first time, including “nanostructured bi-metal oxide (BMO)” and “graphitic carbon nitride (CN)”. Perovskites are ceramic and they are hard materials so they could be a favorable candidate for solid-state hydrogen storage. g-C₃N₄ has attractive features including high surface area, chemical stability, small band gap, and low-cost synthesis methods but also has great potential as an electrode material for energy storage capacitors. The main motivation for this study comes from the potential applications for perovskite materials and graphitic carbon nitride for the solid-state hydrogen storage method. The Perovskite type GdFeO₃ nanostructures (as BMO) synthesized through sol-gel approach in front of natural source of Grape juice as both complexing agent and fuel. The experimental scrutinization ascertains an original fabrication of GdFeO₃ (GF) nanostructures in Grape juice at 800 °C, with an approximately uniform nanosized structure of 70 nm on average. The obtained pure GF nanostructures are then utilized for nanocomposite formation based on g-C₃N₄ (CN) with different amounts. The resulting nanocomposites with the ratio of 1:2 from GF:CN perform a preferable hydrogen sorption capacity, in terms of “maximum discharge capacity of 577 mAhg^?1” in 2 M KOH electrolyte. It should be declared that however, the discharge capacity of the nanostructured GF is 188 mAhg^?1. It can be emphasized that these GF/CN nanocomposites can be utilized as hopeful hosts in an electrochemical hydrogen storage setup due to the synergic effect of g-C₃N₄ with essential characteristics in cooperation with BMO nanostructures as acceptable electrocatalysts.     

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.     

Hydrogen storage in hybrid nanostructured carbon/palladium materials: Influence of particle size and surface chemistry

Hydrogen adsorption on porous materials is one of the possible methods proposed for hydrogen storage for transport applications. One way for increasing adsorption at room temperature is the inclusion of metal nanoparticles to increase hydrogen–surface interactions. In this study, ordered mesoporous carbon materials were synthesized by replication of nanostructured mesoporous SBA-15 silica. The combination of different carbon precursors allowed to tailor the textural, structural and chemical properties of the materials. These carbons were used for the synthesis of hybrid nanostructured carbon/palladium materials with different sizes of metal nanoparticles. The hydrogen sorption isotherms were measured at 77 K and 298 K between 0.1 and 8 MPa. Hydrogen storage capacities strongly correlate with the textural properties of the carbon at 77 K. At room temperature, Pd nanoparticles enhance hydrogen storage capacity by reversible formation of hydride PdH_x and through the spillover mechanism. The hydrogen uptake depends on the combined influences of metal particle size and of carbon chemical properties. Carbons obtained from sucrose precursors lead to the hybrid materials with the highest storage capacities since they exhibits a large microporous volume and a high density of oxygenated surface groups.     

Recent trends in MXene/Metal chalcogenides for electro-/photocatalytic hydrogen evolution reactions

Hydrogen due to high energy density and ecologically benign characteristics can become an excellent energy carrier for a sustainable energy economy and to appease the energy demand of humankind. Moreover, cost-effective and long-lasting photocatalysts can make the hydrogen generating process more economical and suitable. Recently, MXene have become one of the most sought-after composite materials for photocatalytic hydrogen generation. However, the photocstalytic performance can be further enhanced by doping with other semiconductor materials. Transition metal chalcogenides (Transition metals = Cu, Co, Ni, Zn, Cd, Mo, W)/MXene composites and mixed transition metal chalcogenide/MXene nanocomposites have been extensively investigated for the photocatalytic hydrogen generation. These materials possess unique two-dimensional layered structure that ameliorates the photocatalytic water splitting performance by increasing the light adsorption even at low photon flux density. The 2D design assists in reducing the distance necessary to transverse charge carriers to the surface. Because the layered structure tends to trap electrons in the ultrathin layers, 2D materials have unusual optoelectronic properties. In-plane covalent bonding assisted the creation of various heterojunctions and heterostructures in these 2D materials. Water splitting and hydrogen production are aided by the high surface area of these 2D materials. Due to its diverse elemental composition, unique 2D structure, good photoelectronic characteristics, large surface area, and many surface terminations. The design and production of many types of materials used as catalysts for the hydrogen evolution process are discussed in this article.     

Net energy analysis of hydrogen storage options

Hydrogen storage is critical for developing viable hydrogen vehicles. This paper compares compressed hydrogen, cryogenic hydrogen and metal hydride (Mg and FeTi) options using net energy analysis. A simulation of an Indian vehicle with an urban drive cycle using a fuel cell stack is carried out to determine the total hydrogen required per km of travel.Net energy analysis is carried out considering the energy requirements of the storage device and the energy required to produce and store the hydrogen. From net energy analysis compressed hydrogen is the preferred option. The direct energy requirement is more than 55% for magnesium hydride as compared to compressed hydrogen due to the combined effect of increase in weight and higher heat of desorption.In addition to volumetric and gravimetric storage density, it is felt that net energy analysis should be also included as an additional criteria for evaluating any storage option. For metal hydride storage the net energy required to produce the tank should be minimum. This could be used as a selection criterion to design an optimum metal hydride storage. The performance of other materials like porous carbon, carbon nanotubes and hybrids can be evaluated using net energy analysis.     

Investigation of hydrogen storage capacity of various carbon materials

Hydrogen storage capacity of various carbon materials, including activated carbon (AC), single-walled carbon nanohorn, single-walled carbon nanotubes, and graphitic carbon nanofibers, was investigated at 303 and 77 K, respectively. The results showed that hydrogen storage capacity of carbon materials was less than 1 wt% at 303 K, and a super activated carbon, Maxsorb, had the highest capacity (0.67 wt%). By lowering adsorption temperature to 77 K, hydrogen storage capacity of carbon materials increased significantly and Maxsorb could store a large amount of hydrogen (5.7 wt%) at a relatively low pressure of 3 MPa. Hydrogen storage capacity of carbon materials was proportional to their specific surface area and the volume of micropores, and the narrow micropores was preferred to adsorption of hydrogen, indicating that all carbon materials adsorbed hydrogen gas through physical adsorption on the surface.     

Hydrogen sorption studies of palladium decorated graphene nanoplatelets and carbon samples

With the increasing population of the world, the need for energy resources is increasing rapidly due to the development of the industry. 88% of the world's energy needs are met from fossil fuels. Since there is a decrease in fossil fuel reserves and the fact that these fuels cause environmental pollution, there is an increase in the number of studies aimed to develop alternative energy sources nowadays. Hydrogen is considered to be a very important alternative energy source due to its some specific properties such as being abundant in nature, high calorific value and producing only water as waste when burned. An important problem with the use of hydrogen as an energy source is its safe storage. Therefore, method development is extremely important for efficient and safe storage of hydrogen. Surface area, surface characteristics and pore size distribution are important parameters in determining the adsorption capacity, and it is needed to develop new adsorbents with optimum parameters providing high hydrogen adsorption capacity. Until recently, several porous adsorbents have been investigated extensively for hydrogen storage. In this study, it was aimed to develop and compare novel Pd/carbon, Pd/multiwalled carbon nanotube, and Pd/graphene composites for hydrogen sorption. All the palladium/carbon composites were characterized by t-plot, BJH desorption pore size distributions, N₂ adsorption/desorption isotherms, and SEM techniques. The maximum hydrogen storage of 2.25 wt.% at −196 °C was achieved for Pd/KAC composite sample. It has been observed that the spillover effect of palladium increases the hydrogen sorption capacity.     

An alternative platform of solid-state hydrides with polymers as composite/encapsulation for hydrogen storage applications: Effects in intermetallic and complex hydrides

Hydrogen energy proved the remarkable benefits from the perspective of future energy needs and environmental concerns. However, the existing challenges in hydrogen energy technologies, mainly hydrogen storage need to be overcome. In hydrogen storage systems, metal hydrides are highly suffered by the protic species and oxygen because of their reactivity in ambient conditions, leading to surface contamination and affecting the hydrogen sorption behaviors. Moreover, the hydrogen storage in metal hydrides can be hindered by impurities such as CO and CO₂ in hydrogen gas. The new concepts of hydrides with polymers are vital for overcoming the hydrides’ contamination-related issues. This comprehensive review spotlight primarily on the alternative ways to avoid the contamination of hydrides with polymers. The researchers worldwide developed different concepts of polymer-hydrides (intermetallic and complex hydrides) systems, mainly composites and polymer encapsulation methods, to avoid contamination from the air/moisture/gas impurities and improve hydrogen sorption properties in hydrides. Interestingly, selective gas properties of polymers with hydrides show tremendous advantages because of their selective permeability of hydrogen, which influences the easier path for hydrogen diffusion to form the hydrides through the polymer matrix and prohibit the contamination of hydride materials from the other gases/protic species. Notably, this review bridges the understanding between the polymers and hydrides (intermetallic, complex and reactive hydride composite) for hydrogen storage systems.     

Mechanism for electrochemical hydrogen insertion in carbonaceous materials

The mechanism for safe and reversible storage of hydrogen in porous carbonaceous materials by electrochemical decomposition of water in alkaline electrolyte is proposed. Atomic H was found to be inserted into the microdomains of defective graphene layers. Hydrogen storage capacity increases with increasing interlayer distance between carbon sheets. Hydrogen insertion in carbonaceous materials occurs at ambient conditions. Static potential acts as an electrochemical valve which can retain the hydrogen in the carbon structure, thus preventing leakage during storage.     

Theoretical discovery of high capacity hydrogen storage metal tetrahydrides

Hydrogen energy is attractive energy carrier due to its high energy density, abundant, environmentally friendly and renewable etc. However, the search for the high capacity hydrogen storage material is still a great challenge. In addition, the hydrogen storage materials should have excellent catalytic activity and superior mechanical properties to meet dehydrogenation and transportation. Here, we report on a novel metal tetrahydride that can effectively improve the hydrogen storage capacity. We obtain two novel metal tetrahydrides: TiH₄ and VH₄ based on the phonon dispersion and thermodynamically, respectively. In particular, those metal tetrahydrides not only exhibit good dehydrogenation behavior but also show superior mechanical properties. We demonstrate that the high hydrogen storage capacity of those tetrahydrides derives from the alternative stacking of metal layer and hydrogen layer. However, the excellent dehydrogenation process is attributed to the van der Waals interaction between hydrogen layers. Finally, the thermodynamic properties of TiH₄ and VH₄ are discussed.     

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.     

氢存储技术

日益严峻的能源危机和环境污染，使得发展清洁的可再生能源成为各个国家的重要议题。氢能源以其可再生性和良好的环保效应成为未来最具发展潜力的能源载体。氢的储存是发展氢能技术的难点之一。文章介绍了高压、液化、金属氢化物和碳质吸附等储氢技术的研究现状，并对储氢技术的发展趋势进行了讨论。     

First-principles study on hydrogen storage by graphitic carbon nitride nanotubes

First-principles calculations based on density functional theory were carried out to investigate the hydrogen storage capacity of graphitic carbon nitride nanotubes. Graphitic carbon nitride nanotubes could be attractive hydrogen sorbent for two reasons: firstly, its porous structure allows easy access of hydrogen into the interior of the nanotubes; and secondly, the doubly bonded nitrogen at its pore edges provides active sites for either the adsorption of hydrogen (chemically and physically), or functionalization with metal catalysts. Our calculations show that an isolated nanotube can uptake up to 4.66 wt. % hydrogen, with an average overall hydrogen adsorption energy of about −0.22 eV per H atom. In the form of a bulk bundle, the hydrogen storage capacity is enhanced due to the increased availability of space among the tubes. We predict that the hydrogen storage capacity in the bundle is at least 5.45 wt. %. Importantly, hydrogen molecules can easily access the tube’s interior due to the low energy barrier (∼0.54 eV) for their passage through the pores, indicating a fast uptake rate at relatively low pressure and temperature. Our findings show that graphitic carbon nitride nanotubes should be applicable to practical hydrogen storage because of the high gravimetric capacity and fast uptake rate.     

Progress of electrochemical capacitor electrode materials: A review

The electrode is the key part of the electrochemical capacitors (ECs), so the electrode materials are the most important factors to determine the properties of ECs. In this paper, the storage principles and characteristics of electrode materials, including carbon-based materials, transition metal oxides and conductive polymers for ECs are depicted briefly. Among them, more work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and organic electrolytes. But the composites of pseudocapacitive and carbonaceous materials are promising electrode materials for ECs because of their good electrical conductivity, low cost and high mass density.