Enhanced Hydrogen Storage Properties and Reversibility of LiBH4 Confined in Two-Dimensional Ti3C2

Advancing Catalysts by Nanoconfinement and Catalysis for Enhanced Hydrogen Production from Magnesium Borohydride: A Review

Hydrogen storage in solid-state materials represents a promising avenue for advancing hydrogen storage technologies, driven by their potential for high efficiency, reduced risk, and cost-effectiveness. Among the employed materials, magnesium borohydride (Mg(BH4)2) stands out for its exceptional characteristics, with a gravimetric capacity of 14.9 wt% and a volumetric hydrogen density capacity of 146 kg/m3. However, the practical application of Mg(BH4)2 is impeded by challenges such as high desorption temperatures (≥ 270 °C), sluggish kinetics, poor reversibility, and the formation of unexpected byproducts like diborane. To address these limitations, extensive research efforts have been directed towards enhancing the hydrogen storage properties of Mg(BH4)2. Various strategies have been explored, including incorporating catalysts or additives, nanoconfinement of Mg(BH4)2 within porous supports, and modifications involving metal alloys and compositional adjustments. These approaches are actively under investigation for improving the performance of Mg(BH4)2-based hydrogen storage systems. This review provides a comprehensive survey of recent advancements in Mg(BH4)2 research, focusing on experimental findings related to nanoconfined Mg(BH4)2 and modified thermodynamic processes aimed at enabling hydrogen release at lower temperatures by mitigating sluggish kinetics. Precisely, nanostructuring techniques, catalyst-mediated nanoconfinement methodologies, and alloy/compositional modifications will be elucidated, highlighting their potential to enhance hydrogen storage properties and overcome existing limitations. Furthermore, this review also discusses the challenges encountered in utilizing Mg(BH4)2 for hydrogen storage applications and offers insights into the prospects of this material. By synthesizing the latest research findings and identifying areas for further exploration, this review aims to contribute to the ongoing efforts toward realizing the full potential of Mg(BH4)2 as a viable solution for hydrogen storage in diverse applications.

Nanoscale engineering of solid-state materials for boosting hydrogen storage

The development of novel materials capable of securely storing hydrogen at high volumetric and gravimetric densities is a requirement for the wide-scale usage of hydrogen as an energy carrier. In recent years, great efforts via nanoscale tuning and designing strategies on both physisorbents and chemisorbents have been devoted to improvements in their thermodynamic and kinetic aspects. Increasing the hydrogen storage capacity/density for physisorbents and chemisorbents and improving the dehydrogenation kinetics of hydrides are still considered a challenge. The extensive and fast development of advanced nanotechnologies has fueled a surge in research that presents huge potential in designing solid-state materials to meet the ultimate U.S. Department of Energy capacity targets for onboard light-duty vehicles, material-handling equipments, and portable power applications. Different from the existing literature, in this review, particular attention is paid to the recent advances in nanoscale engineering of solid-state materials for boosting hydrogen storage, especially the nanoscale tuning and designing strategies. We first present a short overview of hydrogen storage mechanisms of nanoscale engineering for boosted hydrogen storage performance on solid-state materials, for example, hydrogen spillover, nanopump effect, nanosize effect, nanocatalysis, and other non-classical hydrogen storage mechanisms. Then, the focus is on recent advancements in nanoscale engineering strategies aimed at enhancing the gravimetric hydrogen storage capacity of porous materials, reducing dehydrogenation temperature and improving reaction kinetics and reversibility of hydrogen desorption/absorption for metal hydrides. Effective nanoscale tuning strategies for enhancing the hydrogen storage performance of porous materials include optimizing surface area and pore volume, fine-tuning nanopore sizes, introducing nanostructure doping, and crafting nanoarchitecture and nanohybrid materials. For metal hydrides, successful strategies involve nanoconfinement, nanosizing, and the incorporation of nanocatalysts. This review further addresses the points to future research directions in the hope of ushering in the practical applications of hydrogen storage materials.

Recent Progress in Functional Nanomaterials towards the Storage, Separation, and Removal of Tritium

Tritium is a sustainable next-generation prime fuel for generating nuclear energy through fusion reactions to fulfill the increasing global energy demand. Owing to the scarcity-high demand tradeoff, tritium must be bred inside a fusion reactor to ensure sustainability and must therefore be separated from its isotopes (protium and deuterium) in pure form, stored safely, and supplied on demand. Existing multistage isotope separation technologies exhibit low separation efficiency and require intensive energy inputs and large capital investments. Furthermore, tritium-contaminated heavy water constitutes a major fraction of nuclear waste, and accidents like the one at Fukushima Daiichi leave behind thousands of tons of diluted tritiated water, whose removal is beneficial from an environmental point of view. In this review, the recent progress and main research trends in hydrogen isotope storage and separation by focusing on the use of metal hydride (e.g., intermetallic, and high-entropy alloys), porous (e.g., zeolites and metal organic frameworks (MOFs)), and 2-D layered (e.g., graphene, hexagonal boron nitride (h-BN), and MXenes) materials to separate and store tritium based on their diverse functionalities are discussed. Finally, the challenges and future directions for implementing tritium storage and separation are summarized in the reviewed materials.

Progress in the Synthesis Process and Electrocatalytic Application of MXene Materials

With their rich surface chemistry, high electrical conductivity, variable bandgap, and thermal stability, 2D materials have been developed for effective electrochemical energy conversion systems over the past decade. Due to the diversity brought about by the use of transition metals and C/N pairings, the 2D material MXene has found excellent applications in many fields. Among the various applications, many breakthroughs have been made in electrocatalytic applications. Nevertheless, related studies on topics such as the factors affecting the material properties and safer and greener preparation methods have not been reported in detail. Therefore, in this paper, we review the relevant preparation methods of MXene and the safer, more environmentally friendly preparation techniques in detail, and summarize the progress of research on MXene-based materials as highly efficient electrocatalysts in the electrocatalytic field of hydrogen precipitation reaction, nitrogen reduction reaction, oxygen precipitation reaction, oxygen reduction reaction, and carbon dioxide reduction reaction. We also discuss the technology related to MXene materials for hydrogen storage. The main challenges and opportunities for MXene-based materials, which constitute a platform for next-generation electrocatalysis in basic research and practical applications, are highlighted. This paper aims to promote the further development of MXenes and related materials for electrocatalytic applications.

Holey Ti3C2 MXene-Derived Anode Enables Boosted Kinetics in Lithium-Ion Capacitors

Lithium-ion capacitors (LICs) attract enormous attention because of the urgent demands for high power and energy density devices. However, the intrinsic imbalance between anodes and cathodes with different charge-storage mechanisms blocks the further improvement in energy and power density. MXenes, novel two-dimensional materials with metallic conductivity, accordion-like structure, and regulable interlayer spacing, are widely employed in electrochemical energy storage devices. Herein, we propose a holey Ti₃C₂ MXene-derived composite (pTi₃C₂/C) with enhanced kinetics for LICs. This strategy effectively decreases the surface groups (-F and -O) and generates expanded interplanar spacing. The in-plane pores of Ti₃C₂T_x lead to increased active sites and accelerated lithium-ion diffusion kinetics. Benefiting from the expanded interplanar spacing and accelerated lithium-ion diffusion, the pTi₃C₂/C as an anode implements excellent electrochemical property (capacity retention about 80% after 2000 cycles). Furthermore, the LIC fabricated with a pTi₃C₂/C anode and an activated carbon cathode displays a maximum energy density of 110 Wh kg^-1 and a considerable energy density of 71 Wh kg^-1 at 4673 W kg^-1. This work provides an effective strategy to achieve high antioxidant capability and boosted electrochemical properties, which represents a new exploration of structural design and tuneable surface chemistry for MXene in LICs.

Paving the Way to the Fuel of the Future-Nanostructured Complex Hydrides

Hydrides have emerged as strong candidates for energy storage applications and their study has attracted wide interest in both the academic and industry sectors. With clear advantages due to the solid-state storage of hydrogen, hydrides and in particular complex hydrides have the ability to tackle environmental pollution by offering the alternative of a clean energy source: hydrogen. However, several drawbacks have detracted this material from going mainstream, and some of these shortcomings have been addressed by nanostructuring/nanoconfinement strategies. With the enhancement of thermodynamic and/or kinetic behavior, nanosized complex hydrides (borohydrides and alanates) have recently conquered new estate in the hydrogen storage field. The current review aims to present the most recent results, many of which illustrate the feasibility of using complex hydrides for the generation of molecular hydrogen in conditions suitable for vehicular and stationary applications. Nanostructuring strategies, either in the pristine or nanoconfined state, coupled with a proper catalyst and the choice of host material can potentially yield a robust nanocomposite to reliably produce H2 in a reversible manner. The key element to tackle for current and future research efforts remains the reproducible means to store H2, which will build up towards a viable hydrogen economy goal. The most recent trends and future prospects will be presented herein.

Directed Stabilization by Air-Milling and Catalyzed Decomposition by Layered Titanium Carbide Toward Low-Temperature and High-Capacity Hydrogen Storage of Aluminum Hydride

AlH₃ is a metastable hydride with a theoretical hydrogen capacity of 10.01 wt % and is very easy to decompose during ball milling especially in the presence of many catalysts, which will lead to the attenuation of the available hydrogen capacity. In this work, AlH₃ was ball milled in air (called "air-milling") with layered Ti₃C₂ to prepare a Ti₃C₂-catalyzed AlH₃ hydrogen storage material. Such air-milled and Ti₃C₂-catalyzed AlH₃ possesses excellent hydrogen storage performances, with a low initial decomposition temperature of just 61 °C and a high hydrogen release capacity of 8.1 wt %. In addition, 6.9 wt % of hydrogen can be released within 20 min at constantly 100 °C, with a low activation energy as low as 40 kJ mol^-1. Air-milling will lead to the formation of an Al₂O₃ oxide layer on the AlH₃ particles, which will prevent continuous decomposition of AlH₃ when milling with active layered Ti₃C₂. The layered Ti₃C₂ will grip on and intrude into the AlH₃ particle oxide layers and then catalyze the decomposition of AlH₃ during heating. The strategy employing air-milling as a synthesis method and utilizing layered Ti₃C₂ as a catalyst in this work can solve the key issue of severe decomposition during ball milling with catalysts economically and conveniently and thus achieve both high-capacity and low-temperature hydrogen storage of AlH₃. This air-milling method is also effective for other active catalysts toward both reducing the decomposition temperature and increasing the available hydrogen capacity of AlH₃.

Destabilization of Boron-Based Compounds for Hydrogen Storage in the Solid-State: Recent Advances

Boron-based materials have been widely studied for hydrogen storage applications. Examples of these compounds are borohydrides and boranes. However, all of these present some disadvantages that have hindered their potential application as hydrogen storage materials in the solid-state. Thus, different strategies have been developed to improve the dehydrogenation properties of these materials. The purpose of this review is to provide an overview of recent advances (for the period 2015–2021) in the destabilization strategies that have been considered for selected boron-based compounds. With this aim, we selected seven of the most investigated boron-based compounds for hydrogen storage applications: lithium borohydride, sodium borohydride, magnesium borohydride, calcium borohydride, ammonia borane, hydrazine borane and hydrazine bisborane. The destabilization strategies include the use of additives, the chemical modification and the nanosizing of these compounds. These approaches were analyzed for each one of the selected boron-based compounds and these are discussed in the present review.

Effect of Different Amounts of TiF3 on the Reversible Hydrogen Storage Properties of 2LiBH4-Li3AlH6 Composite

Hydrogen is a potential green alternative to conventional energy carriers such as oil and coal. Compared with the storage of hydrogen in gaseous or liquid phases, the chemical storage of hydrogen in solid complex hydrides is safer and more effective. In this study, the complex hydride composite 2LiBH4-Li₃AlH₆ with different amounts of TiF₃ was prepared by simple ball-milling and its hydrogen storage properties were investigated. Temperature programmed desorption and differential scanning calorimetry were used to characterize the de/rehydrogenation performance, and X-ray diffraction and scanning electron microscopy (SEM) were used to explore the phase structure and surface topography of the materials. The dehydrogenation temperature decreased by 48°C in 2LiBH4-Li₃AlH₆ with 15 wt% TiF₃ composites compared to the composite without additives while the reaction kinetics was accelerated by 20%. In addition, the influence of hydrogen back pressure on the 2LiBH4-Li₃AlH₆ with 5 wt% TiF₃ composite was also investigated. The results show that hydrogen back pressure between 2.5 and 3.5 bar can improve the reversible performance of the composite to some extent. With a back pressure of 3.5 bar, the second dehydrogenation capacity increased to 4.6 wt% from the 3.3 wt% in the 2LiBH₄-Li₃AlH₆ composite without hydrogen back pressure. However, the dehydrogenation kinetics was hindered. About 150 h, which is 100 times the time required without back pressure, was needed to release 8.7 wt% of hydrogen at 3.5 bar hydrogen back pressure. The SEM results show that aluminum was aggregated after the second cycle of dehydrogenation at the hydrogen back pressure of 3 bar, resulting in the partial reversibility of the 5 wt% TiF₃-added 2LiBH4-Li3AlH₆ composite.

Combinations of V2C and Ti3C2 MXenes for Boosting the Hydrogen Storage Performances of MgH2

Two-dimensional vanadium carbide (V₂C) and titanium carbide (Ti₃C₂) MXenes were first synthesized by exfoliating V₂AlC or Ti₃AlC₂ and then introduced jointly into magnesium hydride (MgH₂) to tailor the hydrogen desorption/absorption performances of MgH₂. The as-prepared MgH₂-V₂C-Ti₃C₂ composites show much better hydrogen storage performances than pure MgH₂. MgH₂ with addition of 10 wt % of 2V₂C/Ti₃C₂ initiates hydrogen desorption at around 180 °C; 5.1 wt % of hydrogen was desorbed within 60 min at 225 °C, while 5.8 wt % was desorbed within 2 min at 300 °C. Under 6 MPa H₂, the dehydrided MgH₂-2V₂C/Ti₃C₂ can start to recover hydrogen at room temperature, and 5.1 wt % of H₂ is obtained within 20 s at a constant temperature of 40 °C. The reversible capacity (6.3 wt %) does not decline for up to 10 cycles, which shows excellent cycling stability. The addition of 2V₂C/Ti₃C₂ can remarkably lower the activation energy for the hydrogen desorption reaction of MgH₂ by 37% and slightly reduce the hydrogen desorption reaction enthalpy by 2 kJ mol^-1 H₂. It was demonstrated that the combination of V₂C and Ti₃C₂ promotes the hydrogen-releasing process of MgH₂ compared with addition of only V₂C or Ti₃C₂, while Ti₃C₂ impacts MgH₂ more significantly than V₂C in the hydrogen absorption process of MgH₂ at ambient temperatures. A possible mechanism in the hydrogen release and uptake of the MgH₂-V₂C-Ti₃C₂ system was proposed as follows: hydrogen atoms or molecules may preferentially transfer through the MgH₂/V₂C/Ti₃C₂ triple-grain boundaries during the desorption process and through the Mg/Ti₃C₂ interfaces during the absorption process. Microstructure studies indicated that V₂C and Ti₃C₂ mainly act as efficient catalysts for MgH₂. This work provides an insight into the hydrogen storage behaviors and mechanisms of MgH₂ boosted by a combination of two MXenes.

Effect of Few-Layer Ti3C2Tx Supported Nano-Ni via Self-Assembly Reduction on Hydrogen Storage Performance of MgH2

For the first time, few-layer Ti₃C₂T_x (FL-Ti₃C₂T_x) supporting highly dispersed nano-Ni particles with an interconnected and interlaced structure was elaborated through a self-assembly reduction process. FL-Ti₃C₂T_x not only acts as a supporting material but also self-assembles with Ni²⁺ ions through the electrostatic interaction, assisting in the reduction of nano-Ni. After ball milling with MgH₂, Ni₃₀/FL-Ti₃C₂T_x (few-layer Ti₃C₂T_x supported 30 wt % nano-Ni via self-assembly reduction) shows superior catalytic activity for MgH₂. For example, MgH₂-5 wt % Ni₃₀/FL-Ti₃C₂T_x can release approximately 5.83 wt % hydrogen within 1800 s at 250 °C and absorb 5 wt % hydrogen within 1700 s at 100 °C. The combined effects of finely dispersed nano-Ni in situ-grown on FL-Ti₃C₂T_x, large specific area of FL-Ti₃C₂T_x, multiple-valence Ti (Ti⁴⁺, Ti³⁺, Ti²⁺, and Ti⁰) derived from FL-Ti₃C₂T_x, and the electronic interaction between Ni and FL-Ti₃C₂T_x can explain the superb hydrogen storage performance. Our results will attract more attention to the elaboration of the metal/FL-Ti₃C₂T_x composite via self-assembly reduction and provide a guideline to design high-efficiency composite catalysts with MXene in hydrogen storage fields.

A Unique Double-Layered Carbon Nanobowl-Confined Lithium Borohydride for Highly Reversible Hydrogen Storage

Poor reversibility and high desorption temperature restricts the practical use of lithium borohydride (LiBH₄ ) as an advanced hydrogen store. Herein, a LiBH₄ composite confined in unique double-layered carbon nanobowls prepared by a facile melt infiltration process is demonstrated, thanks to powerful capillary effect under 100 bar of H₂ pressure. The gradual formation of double-layered carbon nanobowls is witnessed by transmission electron microscopy (TEM) observation. Benefiting from the nanoconfinement effect and catalytic function of carbon, this composite releases hydrogen from 225 °C and peaks at 353 °C, with a hydrogen release amount up to 10.9 wt%. The peak temperature of dehydriding is lowered by 112 °C compared with bulk LiBH₄ . More importantly, the composite readily desorbs and absorbs ≈8.5 wt% of H₂ at 300 °C and 100 bar H₂ , showing a significant reversibility of hydrogen storage. Such a high reversible capacity has not ever been observed under the identical conditions. The usable volumetric energy density reaches as high as 82.4 g L^-1 with considerable dehydriding kinetics. The findings provide insights in the design and development of nanosized complex hydrides for on-board applications.

Catalytic effect of sandwich-like Ti3C2/TiO2(A)-C on hydrogen storage performance of MgH2

A sandwich-like Ti₃C₂/TiO₂(A)-C prepared through a facile gas-solid method was doped into MgH₂ by ball milling. Ti₃C₂/TiO₂(A)-C shows a far superior catalytic effect on the hydrogen storage of MgH₂ than individual Ti₃C₂ or TiO₂(A)-C, assigning as a synergistic catalysis between Ti₃C₂ and TiO₂(A)-C. For example, the peak dehydrogenation temperature of MgH₂-5 wt% Ti₃C₂/TiO₂(A)-C is reduced to 308 °C, much lower than that of MgH₂-5 wt% Ti₃C₂ (340 °C) or MgH₂-5 wt% TiO₂(A)-C (356 °C). After dehydrogenation, the dehydrogenated MgH₂-5 wt% Ti₃C₂/TiO₂(A)-C can uptake approximately 4 wt% of hydrogen within 800 s at 125 °C, while for the dehydrogenated MgH₂-5 wt% Ti₃C₂ and MgH₂-5 wt% TiO₂(A)-C, only 3 wt% and 2.65 wt% hydrogen content can be obtained, respectively. Besides this, MgH₂-5 wt% Ti₃C₂/TiO₂(A)-C exhibits the lowest apparent activation energies (42.32 kJ mol^-1 H₂ for the hydrogen absorption and 77.69 kJ mol^-1 H₂ for the hydrogen desorption), which can explain the excellent hydrogen ab/desorption kinetic properties. The synergetic effects between the special layered structure and multiple valence titanium compounds (Ti⁴⁺, Ti³⁺, Ti²⁺, Ti⁰) verified by the x-ray photoelectron spectroscopy results are responsible for the catalytic mechanism on the hydrogen storage of MgH₂. This study also supplies innovative insights into designing high efficiency MXene derivative catalysts in hydrogen storage.

Flexible, Water-Resistant and Air-Stable LiBH4 Nanoparticles Loaded Melamine Foam With Improved Dehydrogenation

Flexible, water-resistant, and air-stable hydrogen storage material (named PMMA-LiBH4/GMF), consisting of LiBH4 nanoparticles confined by poly (methylmethacrylate) (PMMA) and reduced graphene oxide (rGO) modified melamine foam (GMF), were prepared by a facile method. PMMA-LiBH4/GMF can recover original shape after compression at the strain of 50% and exhibits highly hydrophobic property (water contact angle of 123°). Owing to the highly hydrophobic property and protection of PMMA, PMMA-LiBH4/GMF demonstrates outstanding water-resistance and air-stability. Significantly, the onset dehydrogenation temperature of PMMA-LiBH4/GMF at first step is reduced to 94°C, which is 149°C less than that of LiBH4/GMF, and the PMMA-LiBH4/GMF desorbs 2.9 wt% hydrogen within 25 min at 250°C, which is obviously more than the dehydrogenation amount of LiBH4/GMF under the same conditions. It's our belief that the flexible, water-resistant and air-stable PMMA-LiBH4/GMF with a simple preparation route will provide a new avenue to the research of hydrogen storage materials.

Complex Hydrides for Energy Storage, Conversion, and Utilization

Functional materials are the key enabling factor in the development of clean energy technologies. Materials of particular interest, which are reviewed herein, are a class of hydrogenous compound having the general formula of M(XH_n )_m , where M is usually a metal cation and X can be Al, B, C, N, O, transition metal (TM), or a mixture of them, which sets up an iono-covalent or covalent bonding with H. M(XH_n )_m is generally termed as a complex hydride by the hydrogen storage community. The rich chemistry between H and B/C/N/O/Al/TM allows complex hydrides of diverse composition and electronic configuration, and thus tunable physical and chemical properties, for applications in hydrogen storage, thermal energy storage, ion conduction in electrochemical devices, and catalysis in fuel processing. The recent progress is reviewed here and strategic approaches for the design and optimization of complex hydrides for the abovementioned applications are highlighted.

A Facile Method to Construct MXene/CuO Nanocomposite with Enhanced Catalytic Activity of CuO on Thermal Decomposition of Ammonium Perchlorate

In this work, a mixing-calcination method was developed to facilely construct MXene/CuO nanocomposite. CuO and MXene were first dispersed in ethanol with sufficient mixing. After solvent evaporation, the dried mixture was calcinated under argon to produce a MXene/CuO nanocomposite. As characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and X-ray photoelectron spectra (XPS), CuO nanoparticles (60⁻100 nm) were uniformly distributed on the surface and edge of MXene nanosheets. Furthermore, as evaluated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA), the high-temperature decomposition (HTD) temperature decrease of ammonium perchlorate (AP) upon addition of 1 wt% CuO (hybridized with 1 wt% MXene) was comparable with that of 2 wt% CuO alone, suggesting an enhanced catalytic activity of CuO on thermal decomposition of AP upon hybridization with MXene nanosheets. This strategy could be further applied to construct other MXene/transition metal oxide (MXene/TMO) composites with improved performance for various applications.