Innovative methods to enhance the properties of solid hydrogen storage materials based on hydrides through nanoconfinement: A review

Boosting Hydrogen Storage Performance of MgH2 by Oxygen Vacancy-Rich H-V2O5 Nanosheet as an Excited H-Pump

MgH2 is a promising high-capacity solid-state hydrogen storage material, while its application is greatly hindered by the high desorption temperature and sluggish kinetics. Herein, intertwined 2D oxygen vacancy-rich V2O5 nanosheets (H-V2O5) are specifically designed and used as catalysts to improve the hydrogen storage properties of MgH2. The as-prepared MgH2-H-V2O5 composites exhibit low desorption temperatures (Tonset = 185 °C) with a hydrogen capacity of 6.54 wt%, fast kinetics (Ea = 84.55 ± 1.37 kJ mol-1 H2 for desorption), and long cycling stability. Impressively, hydrogen absorption can be achieved at a temperature as low as 30 °C with a capacity of 2.38 wt% within 60 min. Moreover, the composites maintain a capacity retention rate of ~ 99% after 100 cycles at 275 °C. Experimental studies and theoretical calculations demonstrate that the in-situ formed VH2/V catalysts, unique 2D structure of H-V2O5 nanosheets, and abundant oxygen vacancies positively contribute to the improved hydrogen sorption properties. Notably, the existence of oxygen vacancies plays a double role, which could not only directly accelerate the hydrogen ab/de-sorption rate of MgH2, but also indirectly affect the activity of the catalytic phase VH2/V, thereby further boosting the hydrogen storage performance of MgH2. This work highlights an oxygen vacancy excited "hydrogen pump" effect of VH2/V on the hydrogen sorption of Mg/MgH2. The strategy developed here may pave a new way toward the development of oxygen vacancy-rich transition metal oxides catalyzed hydride systems.

Theoretical prediction of nanosizing effects and role of additives in the decomposition of Mg(BH4)2

The energetic transition towards renewable resources is one of the biggest challenges of this century. In this context, the role of H2 is of paramount importance as a key source of energy that could substitute traditional fossil fuels. This technology, even if available in several manufactures, still needs to be optimized at all levels (production, storage and distribution) to be integrated on a larger scale. Among materials suitable to store H2, Mg(BH4)2 is particularly interesting due to its high content of H2 in terms of gravimetric density. Nanosizing effects and role of additives in the decomposition of Mg(BH4)2 were studied by density functional theory (DFT) modelling. Both effects were analyzed because of their contribution in promoting the decomposition of the material. In particular, to have a quantitative idea of nanosizing effects, we used thin film 2D models corresponding to different crystallographic surfaces and referred to the following reaction: Mg(BH4)2 → MgB2 + 4H2. When moving from bulk to nanoscale (2D models), a remarkable decrease in the decomposition energy (10-20 kJ mol-1) was predicted depending on the surface and the thin film thickness considered. As regards the role of additives (Ni and Cu), we based our analysis on their effect in perturbing neighboring borohydride groups. We found a clear elongation of some B-H bonds, in particular with the NiF2 additive (about 0.1 Å). We interpreted this behavior as an indicator of the propensity of borohydride towards dissociation. On the basis of this evidence, we also explored a possible reaction pathway of NiF2 and CuF2 on Mg(BH4)2 up to H2 release and pointed out the major catalytic effect of Ni compared to Cu.

Nanostructuring of Mg-Based Hydrogen Storage Materials: Recent Advances for Promoting Key Applications

A comprehensive discussion of the recent advances in the nanostructure engineering of Mg-based hydrogen storage materials is presented. The fundamental theories of hydrogen storage in nanostructured Mg-based hydrogen storage materials and their practical applications are reviewed. The challenges and recommendations of current nanostructured hydrogen storage materials are pointed out.

Abstract

With the depletion of fossil fuels and global warming, there is an urgent demand to seek green, low-cost, and high-efficiency energy resources. Hydrogen has been considered as a potential candidate to replace fossil fuels, due to its high gravimetric energy density (142 MJ kg−1), high abundance (H2O), and environmental-friendliness. However, due to its low volume density, effective and safe hydrogen storage techniques are now becoming the bottleneck for the "hydrogen economy". Under such a circumstance, Mg-based hydrogen storage materials garnered tremendous interests due to their high hydrogen storage capacity (~ 7.6 wt% for MgH2), low cost, and excellent reversibility. However, the high thermodynamic stability (ΔH =  − 74.7 kJ mol−1 H2) and sluggish kinetics result in a relatively high desorption temperature (> 300 °C), which severely restricts widespread applications of MgH2. Nano-structuring has been proven to be an effective strategy that can simultaneously enhance the ab/de-sorption thermodynamic and kinetic properties of MgH2, possibly meeting the demand for rapid hydrogen desorption, economic viability, and effective thermal management in practical applications. Herein, the fundamental theories, recent advances, and practical applications of the nanostructured Mg-based hydrogen storage materials are discussed. The synthetic strategies are classified into four categories: free-standing nano-sized Mg/MgH2 through electrochemical/vapor-transport/ultrasonic methods, nanostructured Mg-based composites via mechanical milling methods, construction of core-shell nano-structured Mg-based composites by chemical reduction approaches, and multi-dimensional nano-sized Mg-based heterostructure by nanoconfinement strategy. Through applying these strategies, near room temperature ab/de-sorption (< 100 °C) with considerable high capacity (> 6 wt%) has been achieved in nano Mg/MgH2 systems. Some perspectives on the future research and development of nanostructured hydrogen storage materials are also provided.

Graphical Abstract

Recent Development in Nanoconfined Hydrides for Energy Storage

Hydrogen is the ultimate vector for a carbon-free, sustainable green-energy. While being the most promising candidate to serve this purpose, hydrogen inherits a series of characteristics making it particularly difficult to handle, store, transport and use in a safe manner. The researchers’ attention has thus shifted to storing hydrogen in its more manageable forms: the light metal hydrides and related derivatives (ammonia-borane, tetrahydridoborates/borohydrides, tetrahydridoaluminates/alanates or reactive hydride composites). Even then, the thermodynamic and kinetic behavior faces either too high energy barriers or sluggish kinetics (or both), and an efficient tool to overcome these issues is through nanoconfinement. Nanoconfined energy storage materials are the current state-of-the-art approach regarding hydrogen storage field, and the current review aims to summarize the most recent progress in this intriguing field. The latest reviews concerning H₂ production and storage are discussed, and the shift from bulk to nanomaterials is described in the context of physical and chemical aspects of nanoconfinement effects in the obtained nanocomposites. The types of hosts used for hydrogen materials are divided in classes of substances, the mean of hydride inclusion in said hosts and the classes of hydrogen storage materials are presented with their most recent trends and future prospects.

From Iron to Copper: The Effect of Transition Metal Catalysts on the Hydrogen Storage Properties of Nanoconfined LiBH4 in a Graphene-Rich N-Doped Matrix

Incipient wetness impregnation was employed to decorate two N-doped graphene-rich matrixes with iron, nickel, cobalt, and copper nanoparticles. The N-doped matrix was wetted with methanol solutions of the corresponding nitrates. After agitation and solvent evaporation, reduction at 800 °C over the carbon matrix promoted the formation of nanoparticles. The mass of the metal fraction was limited to 5 wt. % to determine if limited quantities of metallic nanoparticles catalyze the hydrogen capture/release of nanoconfined LiBH₄. Isotherms of nitrogen adsorption afforded the textural characterization of the matrixes. Electronic microscopy displayed particles of definite size, evenly distributed on the matrixes, as confirmed by X-ray diffraction. The same techniques assessed the impact of LiBH₄ 50 vol. % impregnation on nanoparticle distribution and size. The hydrogen storage properties of these materials were evaluated by differential scanning calorimetry and two cycles of volumetric studies. X-ray diffraction allowed us to follow the evolution of the material after two cycles of hydrogen capture-release. We discuss if limited quantities of coordination metals can improve the hydrogen storage properties of nanoconfined LiBH₄, and which critical parameters might restrain the synergies between nanoconfinement and the presence of metal catalysts.

Ammonia Borane: An Extensively Studied, Though Not Yet Implemented, Hydrogen Carrier

Ammonia borane H3N−BH3 (AB) was re-discovered, in the 2000s, to play an important role in the developing hydrogen economy, but it has seemingly failed; at best it has lagged behind. The present review aims at analyzing, in the context of more than 300 articles, the reasons why AB gives a sense that it has failed as an anodic fuel, a liquid-state hydrogen carrier and a solid hydrogen carrier. The key issues AB faces and the key challenges ahead it has to address (i.e., those hindering its technological deployment) have been identified and itemized. The reality is that preventable errors have been made. First, some critical issues have been underestimated and thereby understudied, whereas others have been disproportionally considered. Second, the potential of AB has been overestimated, and there has been an undoubted lack of realistic and practical vision of it. Third, the competition in the field is severe, with more promising and cheaper hydrides in front of AB. Fourth, AB has been confined to lab benches, and consequently its technological readiness level has remained low. This is discussed in detail herein.

Tuning LiBH4 for Hydrogen Storage: Destabilization, Additive, and Nanoconfinement Approaches

Hydrogen technology has become essential to fulfill our mobile and stationary energy needs in a global low–carbon energy system. The non-renewability of fossil fuels and the increasing environmental problems caused by our fossil fuel–running economy have led to our efforts towards the application of hydrogen as an energy vector. However, the development of volumetric and gravimetric efficient hydrogen storage media is still to be addressed. LiBH₄ is one of the most interesting media to store hydrogen as a compound due to its large gravimetric (18.5 wt.%) and volumetric (121 kgH₂/m³) hydrogen densities. In this review, we focus on some of the main explored approaches to tune the thermodynamics and kinetics of LiBH₄: (I) LiBH₄ + MgH₂ destabilized system, (II) metal and metal hydride added LiBH₄, (III) destabilization of LiBH₄ by rare-earth metal hydrides, and (IV) the nanoconfinement of LiBH₄ and destabilized LiBH₄ hydride systems. Thorough discussions about the reaction pathways, destabilizing and catalytic effects of metals and metal hydrides, novel synthesis processes of rare earth destabilizing agents, and all the essential aspects of nanoconfinement are led.