Tailoring Thermodynamics and Kinetics for Hydrogen Storage in Complex Hydrides towards Applications

Hydrogen Storage Performance of Mg/MgH2 and Its Improvement Measures: Research Progress and Trends

Due to its high hydrogen storage efficiency and safety, Mg/MgH₂ stands out from many solid hydrogen storage materials and is considered as one of the most promising solid hydrogen storage materials. However, thermodynamic/kinetic deficiencies of the performance of Mg/MgH₂ limit its practical applications for which a series of improvements have been carried out by scholars. This paper summarizes, analyzes and organizes the current research status of the hydrogen storage performance of Mg/MgH₂ and its improvement measures, discusses in detail the hot studies on improving the hydrogen storage performance of Mg/MgH₂ (improvement measures, such as alloying treatment, nano-treatment and catalyst doping), and focuses on the discussion and in-depth analysis of the catalytic effects and mechanisms of various metal-based catalysts on the kinetic and cyclic performance of Mg/MgH₂. Finally, the challenges and opportunities faced by Mg/MgH₂ are discussed, and strategies to improve its hydrogen storage performance are proposed to provide ideas and help for the next research in Mg/MgH₂ and the whole field of hydrogen storage.

Multielement synergetic effect of NiFe2O4 and h-BN for improving the dehydrogenation properties of LiAlH4

NiFe₂O₄@h-BN composites significantly improved the dehydrogenation and rehydrogenation properties of LiAlH₄. The Al₄Ni₃ and LiFeO₂ found in doped LiAlH₄, and Al_1.1Ni_0.9 in the process of heating, improved the dehydrogenation properties of LiAlH₄.

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.

Heterostructures Built in Metal Hydrides for Advanced Hydrogen Storage Reversibility

Hydrogen storage is a vital technology for developing on-board hydrogen fuel cells. While Mg(BH₄ )₂ is widely regarded as a promising hydrogen storage material owing to its extremely high gravimetric and volumetric capacity, its poor reversibility poses a major bottleneck inhibiting its practical applications. Herein, a facile strategy to effectively improve the reversible hydrogen storage performance of Mg(BH₄ )₂ via building heterostructures uniformly inside MgH₂ nanoparticles is reported. The in situ reaction between MgH₂ nanoparticles and B₂ H₆ not only forms homogeneous heterostructures with controllable particle size but also simultaneously decreases the particle size of the MgH₂ nanoparticles inside, which effectively reduces the kinetic barrier that inhibits the reversible hydrogen storage in both Mg(BH₄ )₂ and MgH₂ . More importantly, density functional theory coupled with ab initio molecular dynamics calculations clearly demonstrates that MgH₂ in this heterostructure can act as a hydrogen pump, which drastically changes the enthalpy for the initial formation of BH bonds by breaking stable BB bonds from endothermic to exothermic and hence thermodynamically improves the reversibility of Mg(BH₄ )₂ . It is believed that building heterostructures provides a window of opportunity for discovering high-performance hydrogen storage materials 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.

Nanosheet-like Lithium Borohydride Hydrate with 10 wt % Hydrogen Release at 70 °C as a Chemical Hydrogen Storage Candidate

A nanosheet-like lithium borohydride hydrate (LiBH₄·H₂O) measuring 20-30 nm in thickness is successfully synthesized for the first time by a facile, scalable freeze-drying strategy. The prepared LiBH₄·H₂O nanosheets start releasing hydrogen below 50 °C and release an amount up to approximately 10 wt % at 70 °C because of the strong affinity of H⁺ in the H₂O ligand and H^- in the BH₄ group. The reported dehydrogenation properties here are superior to those of all known complex hydrides, indicating applicability as an advanced chemical hydrogen storage medium.

Catalytic Tuning of Sorption Kinetics of Lightweight Hydrides: A Review of the Materials and Mechanism

Hydrogen storage materials have been a subject of intensive research during the last 4 decades. Several developments have been achieved in regard of finding suitable materials as per the US-DOE targets. While the lightweight metal hydrides and complex hydrides meet the targeted hydrogen capacity, these possess difficulties of hard thermodynamics and sluggish kinetics of hydrogen sorption. A number of methods have been explored to tune the thermodynamic and kinetic properties of these materials. The thermodynamic constraints could be resolved using an intermediate step of alloying or by making reactive composites with other hydrogen storage materials, whereas the sluggish kinetics could be improved using several approaches such as downsizing and the use of catalysts. The catalyst addition reduces the activation barrier and enhances the sorption rate of hydrogen absorption/desorption. In this review, the catalytic modifications of lightweight hydrogen storage materials are reported and the mechanism towards the improvement is discussed.

Facile Synthesis and Superior Catalytic Activity of Nano-TiN@N-C for Hydrogen Storage in NaAlH4

Herein, we synthesize successfully ultrafine TiN nanoparticles (<3 nm in size) embedded in N-doped carbon nanorods (nano-TiN@N-C) by a facile one-step calcination process. The prepared nano-TiN@N-C exhibits superior catalytic activity for hydrogen storage in NaAlH₄. Adding 7 wt % nano-TiN@N-C induces more than 100 °C reduction in the onset dehydrogenation temperature of NaAlH₄. Approximately 4.9 wt % H₂ is rapidly released from the 7 wt % nano-TiN@N-C-containing NaAlH₄ at 140 °C within 60 min, and the dehydrogenation product is completely hydrogenated at 100 °C within 15 min under 100 bar of hydrogen, exhibiting significantly improved desorption/absorption kinetics. No capacity loss is observed for the nano-TiN@N-C-containing sample within 25 de-/hydrogenation cycles because nano-TiN functions as an active catalyst instead of a precursor. A severe structural distortion with extended bond lengths and reduced bond strengths for Al-H bonding when the [AlH₄]^- group adsorbs on the TiN cluster is demonstrated for the first time by density functional theory calculations, which well-explains the reduced de-/hydrogenation temperatures of the nano-TiN@N-C-containing NaAlH₄. These findings provide new insights into designing and synthesizing high-performance catalysts for hydrogen storage in complex hydrides.

Mechanisms of partial hydrogen sorption reversibility in a 3NaBH4/ScF3 composite

A new hydrogen storage composite containing NaBH4 and a 3d transition metal fluoride, 3NaBH4/ScF3, was synthesized via ball milling. The composite shows no reaction during milling and its dehydriding process can be divided into three steps upon heating: (i) partial substitution of H- by F- in NaBH4 to form NaBH x F4-x at the early stage, releasing about 0.19 wt% of hydrogen; (ii) formations of Na3ScF6, NaBF4 and ScB2 through the reaction between NaBH4 and ScF3, with 2.52 wt% of hydrogen release and a dehydriding activation energy of 162.67 kJ mol-1 H2; (iii) further reaction of residual NaBH4 and Na3ScF6 to form NaF, B and ScB2, with a dehydriding activation energy of 169.37 kJ mol-1 H2. The total hydrogen release of the composite reaches 5.54 wt% at 530 °C. The complete dehydrided composite cannot be rehydrogenated while the products after the second dehydriding step can be hydrogenated with an absorption activation energy of 44.58 kJ mol-1 H2. These results demonstrate that by adding 3d transition metal fluorides into NaBH4, a partial reversibility in NaBH4 can be achieved.

In Situ Synthesis of 3D Flower-Like Nanocrystalline Ni/C and its Effect on Hydrogen Storage Properties of LiAlH4

Lithium alanate (LiAlH₄ ) is of particular interest as one of the most promising candidates for solid-state hydrogen storage. Unfortunately, high dehydrogenation temperatures and relatively slow kinetics limit its practical applications. Herein, 3D flower-like nanocrystalline Ni/C, composed of highly dispersed Ni nanoparticles and interlaced carbon flakes, was synthesized in situ. The as-synthesized nanocrystalline Ni/C significantly decreased the dehydrogenation temperature and dramatically improved the dehydrogenation kinetics of LiAlH₄ . It was found that the LiAlH₄ sample with 10 wt % Ni/C (LiAlH₄ -10 wt %Ni/C) began hydrogen desorption at approximately 48 °C, which is very close to ambient temperature. Approximately 6.3 wt % H₂ was released from LiAlH₄ -10 wt %Ni/C within 60 min at 140 °C, whereas pristine LiAlH₄ only released 0.52 wt % H₂ under identical conditions. More importantly, the dehydrogenated products can partially rehydrogenate at 300 °C under 4 MPa H₂ . The synergetic effect of the flower-like carbon substrate and Ni active species contributes to the significantly reduced dehydrogenation temperatures and improved kinetics.

Enhanced hydrogen storage properties of 1.1MgH2–2LiNH2–0.1LiBH4 system with LaNi5-based alloy hydrides addition

The weakening of N–H bond and the homogeneous distribution of LaNi₅-based alloy hydrides in the Li–Mg–B–N–H composite enhance its hydrogen storage properties.

Improved overall hydrogen storage properties of a CsH and KH co-doped Mg(NH2)2/2LiH system by forming mixed amides of Li–K and Cs–Mg

Further improved overall hydrogen storage properties of a Mg(NH₂)₂/2LiH system is achieved by codoping CsH and KH.

Controllable decomposition of Ca(BH4)2 for reversible hydrogen storage

The formation of CaB₆ from the thermal decomposition of Ca(BH₄)₂ goes along two distinct routes, i.e. via CaB₂H₆ or elemental boron as a reaction intermediate, depending on temperature.