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

2LiBH4-MgH2 System Catalytically Modified with a 2D TiNb2O7 Nanoflake for High-Capacity, Fast-Response, and Long-Life Hydrogen Energy Storage

To achieve large-scale hydrogen storage for growing high energy density and long-life demands in end application, the 2LiBH4-MgH2 (LMBH) reactive hydride system attracts huge interest owing to its high hydrogen capacity and thermodynamically favorable reversibility. The sluggish dehydrogenation kinetics and unsatisfactory cycle life, however, remain two challenges. Herein, a bimetallic titanium-niobium oxide with a two-dimensional nanoflake structure (2D TiNb2O7) is selected elaborately as an active precursor that in situ transforms into TiB2 and NbB2 with ultrafine size and good dispersion in the LMBH system as highly efficient catalysts, giving rise to excellent kinetic properties with long-term cycling stability. For the LMBH system added with 5 wt% 2D TiNb2O7, 9.8 wt% H2 can be released within 20 min at 400 °C, after which the system can be fully hydrogenated in less than 5 min at 350 °C and 10 MPa H2. Moreover, a dehydrogenation capacity of 9.4 wt% can be maintained after 50 cycles corresponding to a retention of 96%, being the highest reported to date. The positive roles of TiB2 and NbB2 for kinetics and recyclability are from their catalytic nucleation effects for MgB2, a main dehydrogenation phase of LMBH, thus reducing the apparent activation energy, suppressing the formation of thermostable Li2B12H12 byproducts, and inhibiting the hydride coarsening. This work develops an advanced LMBH system, bringing hope for high-capacity, fast-response, and long-life hydrogen energy storage.

Assisting molybdenum trioxide catalysis by engineering oxygen vacancy for enhancing hydrogen storage performance of magnesium hydride

Magnesium hydride (MgH2) as an ideal hydrogen storage carrier whose hydrogen storage performance can be effectively improved by transition metal-based catalysts. To construct highly active catalysts, much attention has been paid to the regulation of transition metal components while less attention has been paid to non-transition metal components especially oxygen, leading certain limitations. Herein, further improved hydrogen storage performance of MgH2 can be obtained by adjusting oxygen vacancy content in molybdenum trioxide (MoO3) catalyst. Specifically, compared with pure MgH2 (1.1 wt%) and MgH2-10 wt% MoO3 (4.5 wt%), more hydrogen (5.9 wt%) can be released by MgH2-10 wt% MoO3-x (MoO3 with abundant oxygen vacancies) at 300.0 °C within 499.0 s. Besides, superb capacity retention (6.1 wt%, 99.0 %) after 50 isothermal hydrogen ab/desorption cycles can be obtained for MgH2-10 wt% MoO3-x. Through rigorous comparative experiments and theoretical calculations, the excellent catalytic activity of MoO3-x is demonstrated to come from the abundant oxygen vacancies and the active substances (polyvalent Mo and nano-sized MgO) it assists to form during ball milling process. This work verifies the feasibility for further improving the catalytic activity of transition metal-based catalysts by tuning non-transition metal elements and thus provides a new strategy in catalyzed MgH2 system.

Magnesium Magic: State-of-the-Art Nanocrystalline Materials Paving the Way for Hydrogen Storage

Hydrogen has been regarded as a promising alternative to traditional fossil fuels, presenting itself as a viable and environmentally friendly energy choice. The design and fabrication of highly efficient hydrogen storage materials is crucial to the wide utilization of hydrogen-based technologies. Magnesium-based nanocrystalline materials have received significant interest in the field of hydrogen storage due to their remarkable hydrogen storage capabilities and release efficiency. This review emphasizes on the most useful techniques including vapor deposition, sol-gel synthesis, electrochemical deposition, magnetron sputtering, and template-assisted approaches used for the fabrication of Magnesium-based nanocrystalline hydrogen storage materials (Mg-NHSMs), stressing their advantages, limitations, and recent advancements. These cutting-edge techniques demonstrate their significance in offering useful insights into the performance of Mg-NHSMs. Further, this review describes various applications of Mg-NHSMs. In addition, this review highlights the conclusion and future perspectives on the improvement of magnesium based nanocrystalline materials for efficient hydrogen storage.

3D-Printed MOF Monoliths: Fabrication Strategies and Environmental Applications

Metal-organic frameworks (MOFs) have been extensively considered as one of the most promising types of porous and crystalline organic-inorganic materials, thanks to their large specific surface area, high porosity, tailorable structures and compositions, diverse functionalities, and well-controlled pore/size distribution. However, most developed MOFs are in powder forms, which still have some technical challenges, including abrasion, dustiness, low packing densities, clogging, mass/heat transfer limitation, environmental pollution, and mechanical instability during the packing process, that restrict their applicability in industrial applications. Therefore, in recent years, attention has focused on techniques to convert MOF powders into macroscopic materials like beads, membranes, monoliths, gel/sponges, and nanofibers to overcome these challenges.Three-dimensional (3D) printing technology has achieved much interest because it can produce many high-resolution macroscopic frameworks with complex shapes and geometries from digital models. Therefore, this review summarizes the combination of different 3D printing strategies with MOFs and MOF-based materials for fabricating 3D-printed MOF monoliths and their environmental applications, emphasizing water treatment and gas adsorption/separation applications. Herein, the various strategies for the fabrication of 3D-printed MOF monoliths, such as direct ink writing, seed-assisted in-situ growth, coordination replication from solid precursors, matrix incorporation, selective laser sintering, and digital light processing, are described with the relevant examples. Finally, future directions and challenges of 3D-printed MOF monoliths are also presented to better plan future trajectories in the shaping of MOF materials with improved control over the structure, composition, and textural properties of 3D-printed MOF monoliths.

In Situ Construction of Interface with Photothermal and Mutual Catalytic Effect for Efficient Solar-Driven Reversible Hydrogen Storage of MgH2

Hydrogen storage in MgH2 is an ideal solution for realizing the safe storage of hydrogen. High operating temperature, however, is required for hydrogen storage of MgH2 induced by high thermodynamic stability and kinetic barrier. Herein, flower-like microspheres uniformly constructed by N-doped TiO2 nanosheets coated with TiN nanoparticles are fabricated to integrate the light absorber and thermo-chemical catalysts at a nanometer scale for driving hydrogen storage of MgH2 using solar energy. N-doped TiO2 is in situ transformed into TiNxOy and Ti/TiH2 uniformly distributed inside of TiN matrix during cycling, in which TiN and Ti/TiHx pairs serve as light absorbers that exhibit strong localized surface plasmon resonance effect with full-spectrum light absorbance capability. On the other hand, it is theoretically and experimentally demonstrated that the intimate interface between TiH2 and MgH2 can not only thermodynamically and kinetically promote H2 desorption from MgH2 but also simultaneously weaken Ti─H bonds and hence in turn improve H2 desorption from the combination of weakened Ti─H and Ti─H bonds. The uniform integration of photothermal and catalytic effect leads to the direct action of localized heat generated from TiN on initiating the catalytic effect in realizing hydrogen storage of MgH2 with a capacity of 6.1 wt.% under 27 sun.

Interfacial Engineering of Fluorinated TiO2 Nanosheets with Abundant Oxygen Vacancies for Boosting the Hydrogen Storage Performance of MgH2

The interaction between fluorinated surface in the partially reduced nano-crystallite titanium dioxide (TiO2-x(F)) and MgH2 is studied for the first time. Compared with pristine MgH2 (416 °C), the onset desorption temperature of MgH2+5 wt.% TiO2-x(F) composite can be dramatically lowered to 189 °C. In addition, the composite exhibits remarkable dehydrogenation kinetics, which can release 6.0 wt.% hydrogen thoroughly within 6 min at 250 °C. The apparent activation energy for dehydriding is decreased from 268.42 to 119.96 kJ mol-1. Structural characterization and theoretical calculations indicate that the synergistic effect between multivalent Ti species, and the in situ formed MgF2 and MgF2-xHx is beneficial for improving the hydrogen storage performance of MgH2. Moreover, oxygen vacancies can accelerate the electron transportation and facilitate hydrogen diffusion. The study provides a novel perspective on the modification of MgH2 by fluorinated transition metal oxide catalyst.

Atomic reconstruction for realizing stable solar-driven reversible hydrogen storage of magnesium hydride

Reversible solid-state hydrogen storage of magnesium hydride, traditionally driven by external heating, is constrained by massive energy input and low systematic energy density. Herein, a single phase of Mg2Ni(Cu) alloy is designed via atomic reconstruction to achieve the ideal integration of photothermal and catalytic effects for stable solar-driven hydrogen storage of MgH2. With the intra/inter-band transitions of Mg2Ni(Cu) and its hydrogenated state, over 85% absorption in the entire spectrum is achieved, resulting in the temperature up to 261.8 °C under 2.6 W cm-2. Moreover, the hydrogen storage reaction of Mg2Ni(Cu) is thermodynamically and kinetically favored, and the imbalanced distribution of the light-induced hot electrons within CuNi and Mg2Ni(Cu) facilitates the weakening of Mg-H bonds of MgH2, enhancing the "hydrogen pump" effect of Mg2Ni(Cu)/Mg2Ni(Cu)H4. The reversible generation of Mg2Ni(Cu) upon repeated dehydrogenation process enables the continuous integration of photothermal and catalytic roles stably, ensuring the direct action of localized heat on the catalytic sites without any heat loss, thereby achieving a 6.1 wt.% H2 reversible capacity with 95% retention under 3.5 W cm-2.

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.

Insights into Nano- and Micro-Structured Scaffolds for Advanced Electrochemical Energy Storage

Adopting a nano- and micro-structuring approach to fully unleashing the genuine potential of electrode active material benefits in-depth understandings and research progress toward higher energy density electrochemical energy storage devices at all technology readiness levels. Due to various challenging issues, especially limited stability, nano- and micro-structured (NMS) electrodes undergo fast electrochemical performance degradation. The emerging NMS scaffold design is a pivotal aspect of many electrodes as it endows them with both robustness and electrochemical performance enhancement, even though it only occupies complementary and facilitating components for the main mechanism. However, extensive efforts are urgently needed toward optimizing the stereoscopic geometrical design of NMS scaffolds to minimize the volume ratio and maximize their functionality to fulfill the ever-increasing dependency and desire for energy power source supplies. This review will aim at highlighting these NMS scaffold design strategies, summarizing their corresponding strengths and challenges, and thereby outlining the potential solutions to resolve these challenges, design principles, and key perspectives for future research in this field. Therefore, this review will be one of the earliest reviews from this viewpoint.

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.

Solid-solution MAX phase TiVAlC assisted with impurity for enhancing hydrogen storage performance of magnesium hydride

Although MXene catalysts etched from precursor MAX have greatly improved the hydrogen storage performance of magnesium hydride (MgH2), the use of dangerous and polluting etchers (such as hydrofluoric acid) and the direct removal of potentially catalytically active A-layer substances (such as Al) present certain limitations. Here, solid-solution MAX phase TiVAlC catalyst without etching treatment has been directly introduced into MgH2 system to improve the hydrogen storage performance. The optimal MgH2-10 wt% TiVAlC can release about 6.00 wt% hydrogen at 300 °C within 378 s and absorb about 4.82 wt% hydrogen at 175 °C within 900 s. After 50 isothermal hydrogen ab/desorption cycles, the excellent cyclic stability and capacity retention (6.4 wt%, 99.6%) can be found for MgH2-10 wt% TiVAlC. The superb catalytic activity of TiVAlC catalyst can be explained by abundant electron transfer at external interfaces with MgH2/Mg, which can be further enhanced by impurity phase Ti3AlC2 due to strong H affinity brought from abundant electron transfer at internal interfaces (Ti3AlC2/TiVAlC). The influence of impurity phase which is common in MAX phase on the overall activity of catalysts has been firstly studied here, providing a unique method for designing composite catalyst to improve hydrogen storage performance of MgH2.

Pd Decoration with Synergistic High Oxygen Mobility Boosts Hydrogen Sensing Performance at Low Working Temperature on WO3 Nanosheet

Pd-based materials have received remarkable attention and exhibit excellent H2 sensing performance due to their superior hydrogen storage and catalysis behavior. However, the synergistic effects originated from the decoration of Pd on a metal oxide support to boost the sensing performance are ambiguous, and the deep investigation of metal support interaction (MSI) on the H2 sensing mechanism is still unclear. Here, the model material of Pd nanoparticle-decorated WO3 nanosheet is synthesized, and individual fine structures can be achieved by treating it at different temperatures. Notably, the Pd-WO3-300 materials display superior H2 sensing performance at a low working temperature (110 °C), with a superior sensing response (Ra/Rg = 40.63 to 10 ppm), high sensing selectivity, and anti-interference ability. DFT calculations and detailed structural investigations confirm that the moderate MSI facilitates the generation of high mobility surface O2- (ad) species and a proper ratio of surface Pd0-Pd2+ species, which can significantly boost the desorption of intermediate PdHx species at low temperatures and contribute to enhanced sensing performance. Our work illustrates the effect of MSI on sensing performance and provides insight into the design of advanced sensing materials.