Perspectives and challenges of hydrogen storage in solid-state hydrides

Hydrogen has been widely considered as a clean energy carrier that bridges the energy producers and energy consumers in an efficient and safe way for a sustainable society.Hydrogen can be stored in a gas,liquid and solid states and each method has its unique advantage.Though compressed hydrogen and liquefied hydrogen are mature technologies for industrial applications,appropriate measures are necessary to deal with the issues at high pressure up to around 100 MPa and low temperature at around 20 K.Distinct from those technologies,storing hydrogen in solid-state hydrides can realize a more compact and much safer approach that does not require high hydrogen pressure and cryogenic temperature.In this review,we will provide an overview of the majormaterial groups that are capable of absorbing and desorbing hydrogen reversibly.The main features on hydrogen storage properties of each material group are summarized,together with the discussion of the key issues and the guidance of materials design,aiming at providing insights for new material development as well as industrial applications.

Bimetallic Ru-Ni/TiO2 catalysts for hydrogenation of N-ethylcarbazole:Role of TiO2 crystal structure

Hydrogenation of N-ethylcarbazole(NEC),the hydrogen lean form of a liquid organic hydrogen carrier,on TiO2 supported Ru-Ni bimetallic catalysts is investigated.Crystal structure of TiO2 plays a critical role on the hydrogenation activity and selectivity towards fully hydrogenated product.Ru/anatase catalyst exhibits higher selectivity but lower reactivity compared to Ru/rutile catalyst.Ni addition significantly promotes the performance of Ru/anatase catalyst while causes severe performance deterioration of Ru/rutile catalyst.Commercial P25,a mixture of anatase and rutile phases in approximate ratio A/R1/4,is found to be the best TiO2 support for NEC hydrogenation.Ru/P25 catalyst outperforms both Ru/rutile and Ru/anatase and its activity can be further slightly improved by Ni addition.The unexpected synergism between the two different TiO2 phases for Ru based NEC hydrogenation catalysts is related to metal-support interaction and Ru-Ni interaction.

Hydrogen generation from NaBH_(4) for portable proton exchange membrane fuel cell

Sodium borohydride(NaBH_(4)) is considered as the most potential hydrogen storage material for portable proton exchange membrane fuel cells(PEMFC)because of its high theoretical hydrogen capacity.However,the slow and poor kinetic stability of hydrogen generation from NaBH_(4) hydrolysis limits its application.There are two main factors influencing the kinetics stability of hydrogen generation from NaBH_(4).One factor is that the alkaline byproducts(NaBO_(2)) of the hydrolysis reaction can increase the pH of the solution,thus inhibiting the reaction process.It mainly happens in the NaBH_(4) solution hydrolysis system.Another factor is that the monotonous increase in reaction temperature leads to uncontrollable and unpredictable hydrolysis rates in the solid NaBH_(4) hydrolysis system.This is due to the excess heat generated from this exothermic reaction in the initial reaction of NaBH_(4) hydrolysis.In this perspective,we summarize the latest research progress in hydrogen generation from NaBH_(4) and emphasize the design principles of catalysts for hydrogen generation from NaBH_(4) solution and solid state NaBH_(4).The importance of carbon as catalyst support material for NaBH_(4) hydrolysis is also highlighted.

Determination of elastic and plastic mechanical properties of dentin based on experimental and numerical studies

The aim of this study is to investigate the change of mechanical properties of human dentin due to aging and spatial variation. Sections of coronal dentin are made from human molars in three groups： young, mid-aged, and old patients. A nanoindentation test is conducted from regions near the pulp to the dentin-enamel junction （DE J） to evaluate the load-depth indentation response and determine Young＇s modulus and hardness. Based on the loading and unloading load-displacement curves in nanoindentation, a numerical model of plastic damage is used to study the plastic and the damage behaviors and the contribution to the degradation in the unloading stiffness. The experimental results show that Young＇s modulus of the inner dentin is significantly lower than that of outer dentin in each age group. Compared with the young dentin, the old dentin has greater hardness and Young＇s modulus with similar spatial variations. The magnitudes of the yield strength and the damage variable are also affected by aging and vary with spatial locations. In the same age group, the yield strength in inner dentin is lower than those in middle and outer dentin, more damage occurs with similar spatial variations, and the yield strength of young dentin is generally lower and causes more damage compared with those in both the mid-aged and old groups.

Improved electrochemical hydrogen storage properties of Mg-Y thin films as a function of substrate temperature

Pd-capped Mg78Y22 thin films have been prepared by direct current magnetron co-sputtering system at different substrate temperatures and their electrochemical hydrogen storage properties have been investigated.It is found that rising substrate temperature to 60 ℃ can coarsen the surface of thin film,thus facilitating the diffusion of hydrogen atoms and then enhancing its discharge capacity to 1725 mAh·g-1.Simultaneously,the cyclic stability is effectively improved due to the increased adhesion force between film and substrate as a function of temperature.In addition,the specimen exhibits a very long and flat discharge plateau at about —0.67 V,at which nearly 60%of capacity is maintained.The property is favorable for the application in metal hydride/nickel secondary batteries.The results indicate that rising optimal substrate temperature has a beneficial effect on the electrochemical hydrogen storage of Mg-Y thin films.

Synthesis and dehydrogenation properties of NaZn(BH4)3·en and NaZn(BH4)3·2en(en= ethylene diamine)

Two new hydrogen storage compounds,Na Zn(BH4)3·en and Na Zn(BH4)3·2en(en=ethylene diamine)are synthesized by a solution method.They can release 6.4 wt%and 6.3 wt%pure hydrogen below 200℃,respectively,demonstrating the potential to be used as hydrogen carriers.Additionally,their dehydrogenation products,thermodynamics and kinetics are well characterized and analyzed.Results shows that the stronger Zn-N interactions and the Hδ+–Hδ-interactions are significant for their stability below 100℃,relatively low decomposition temperatures and elimination of gas impurities.

Hormones Regulate in Vitro Organ Regeneration from Leaf-Derived Explants in <i>Arabidopsis</i>

Plant in vitro organogenesis is well-controlled and thus provides an ideal system for plant propagation and studying mechanisms of plant development. However, the data on systematic in vitro organogenesis from leaf explant with various concentrations and combinations of hormones are limited. Arabidopsis is a very useful model plant species for many aspects of plant biological study. Here, we reported a simple, fast and efficient one-step process for evaluating leaf explant-derived in vitro Arabidopsis organogenesis involving the application of various concentrations and combinations of exogenous hormones. The central portion of the fourth rosette leaf was harvested from the 21-days-old seedling and cultured in vitro on the media containing 216 combinations of exogenous hormones. Different types of organs, including roots, shoots, inflorescences, and leaf-like organs were initiated from leaf explants. Several optimal experimental combinations were selected. A hormone combination, 1.00 μM NAA + 10.00 μM ZT, was considered as the most efficient one for adventitious shoot regeneration. And for adventitious root regeneration, six hormone combinations, [(NAA + ZT: 1.00 + 0.10 μM;10.00 + 0.01 μM;20.00 + 0.10 μM;20.00 + 1.00 μM) and (NAA + 6-BA: 10.00 + 0.10 μM;20.00 + 10.00 μM)], were thought to be the best ones. Further, both auxin and cytokinin ratios and concentrations were crucial for efficient in vitro organogenesis. Our study provides the important information for hormone-regulated organogenesis.

Ultrafine Sn_(4)P_(3)nanocrystals from chloride reduction on mechanically activated Na surface for sodium/lithium ion batteries

Nanostructured metal phosphides are very attractive materials in energy storage and conversion,but their applications are severely limited by complicated preparation steps,harsh conditions and large excess of highly toxic phosphorus source.Here we develop a highly efficient one-step method to synthesize Sn_(4)P_(3)nanostructure based on simultaneous reduction of SnCl_(4)and PCl_(3)on mechanically activated Na surface and in situ phosphorization.The low-toxic PCl3 displays a very high phosphorizing efficiency(100%).Furthermore,this simple method is powerful to control phosphide size.Ultrafine Sn_(4)P_(3)nanocrystals(<5 nm)supported on carbon sheets(Sn_(4)P_(3)/C)are obtained,which is due to the unique bottom-up surface-limited reaction.As the anode material for sodium/lithium ion batteries(SIBs/LIBs),the Sn_(4)P_(3)/C shows profound sodiation/lithiation extents,good phase-conversion reversibility,excellent rate performance and long cycling stability,retaining high capacities of 420 mAh/g for SIBs and 760 mAh/g for LIBs even after 400 cycles at 1.0 A/g.Combining simple and efficient preparation,low-toxic and high-efficiency phosphorus source and good control of nanosize,this method is very promising for low-cost and scalable preparation of high-performance Sn_(4)P_(3)anode.

Ultrafine Sn nanocrystals in a hierarchically porous N-doped carbon for lithium ion batteries

We report a simple method of preparing a high performance, Sn-based anode material for lithium ion batteries （LIBs）. Adding H2O2 to an aqueous solution containing Sn2＋ and aniline results in simultaneous polymerization of aniline and oxidation of Sn2＋ to SnO2, leading to a homogeneous composite of polyaniline and SnO2. Hydrogen thermal reduction of the above composite yields N-doped carbon with hierarchical porosity and homogeneously distributed, ultrafine Sn particles. The nanocomposite exhibits excellent performance as an anode material for lithium ion batteries, showing a high reversible specific capacity of 788 mAh·g^-1 at a current density of 100 mA·g^-1 after 300 cycles and very good stability up to 5,000 mA·g^-1. The simple preparation method combined with the good electro- chemical performance is highly promising to promote the application of Sn based anode materials.

Surface group directed low-temperature synthesis and self-assembly of Al nanostructures for lithium storage

Nanostructured aluminum recently delivers a variety of new applications of the earth-abundant Al resource due to the unique properties,but its controllable synthesis remains very challenging with harsh conditions and spontaneously flammable precursors.Herein,a surface group directed method is developed to efficiently achieve low-temperature synthesis and selfassembly of zero-dimensional(0D)Al nanocrystals over one-dimensional(1D)carbon fibers(Al@CFs)through non-flammable AlCl3 reduction at 70°C.Theoretical calculations unveil surface‒OLi groups of carbon fibers exert efficient binding effect to AlCl3,which guides intimate adsorption and in-situ self-assembly of the generated Al nanocrystals.The distinctive 0D-over-1D Al@CFs provides long 1D conductive networks for electron transfer,ultrafine 0D Al nanocrystals for fast lithiation and excellent buffering effect for volume change,thus exhibiting high structure stability and superior lithium storage performance.This work paves the way for mild and controllable synthesis of Al-based nanomaterials for new high-value applications.

Plasma-processed homogeneous magnesium hydride/ carbon nanocomposites for highly stable lithium storage

Magnesium hydride （MgH2） is a high-capacity anode material for lithium ion batteries, which suffers from poor cycling stability. In this stud）~ we describe a thermal plasma-based approach to prepare homogeneous MgH2/C nanocomposites with very high cycling stability. In this process, magnesium evaporation is coupled with carbon generation from the plasma decomposition of acetylene, leading to a homogeneous Mg/C nanocomposite, which can be easily converted to MgH2/C by hydrogenation. The MgH2/C nanocomposite achieves a high reversible capacity of up to 620 mAh·g^-1 after 1,000 cycles with an ultralow decay rate of only 0.0036% per cycle, which represents a significantly improved performance compared to previous results.

Nonstoichiometric Yttrium Hydride–Promoted Reversible Hydrogen Storage in a Liquid Organic Hydrogen Carrier

N-Ethylcarbazole(NEC)is one of the most promising liquid organic hydrogen carriers(LOHCs),but its application is limited by sluggish kinetics due to lack of high-efficiency,low-cost catalysts.This work reports a cobalt(Co)-based catalyst promoted by nonstoichiometric yttrium hydride(YH_(3−x))to achieve high-efficiency,reversible hydrogen storage in NEC,with>5.5 wt%reversible hydrogen storage capacity could be achieved below 473 K,and with good kinetics.The YH_(3−x)-promoted Co-based catalyst is the first non-noble metal catalyst with high activity for NEC hydrogenation and 12H-NEC dehydrogenation reactions.A mechanistic study suggests that YH_(3−x)facilitates the reversible hydrogen transfer both in the hydrogenation and the dehydrogenation reactions.The nonstoichiometric YH_(3−x)contained both lattice H and H vacancies with tunable H chemical potential serve as the H donor and H acceptor for reversible hydrogen transfer.Our results support the practical application of LOHCs and inspire new approaches for the utilization of conventional metal hydrides to promote versatile H transfer reactions.

Significantly improved high k dielectric performance:Rare earth oxide as a passivation layer laminated with TiO_(2) film

In order to achieve a super gate dielectric performance,rare earth oxides featuring for large band gap,good thermodynamic stability and relatively high k value were selected to be laminated with TiO_(2)film to prepare bilayer dielectric films.As an example,the microstructure,morphology,band gap structure and electrical performance of TiO_(2)-Y_(2)O_(3)bilayer films were systematically investigated.Results show that stacking sequence of TiO_(2)and Y_(2)O_(3)sublayers has a significant impact on the dielectric performance and Y_(2)O_(3)film as a passivation layer can effectively improve electrical properties.Besides,the electrical behaviors analysis of TiO_(2)-Y_(2)O_(3),Y_(2)O_(3)-TiO_(2),Y_(2)O_(3)and TiO_(2)samples was carried out by impedance spectra and equivale nt circuit.The result shows that TiO_(2)-Y_(2)O_(3)/Si sample holds the largest internal re sistance of 74665Ωamong four samples.Moreover,the most outstanding properties of Pt/TiO_(2)-Y_(2)O_(3)/Si capacitor are achieved by varying the thickness of sublayers and annealing temperature.500℃-annealed bilayer film with 17 nm-TiO_(2)and 3-nm Y_(2)O_(3)displays a k value of 28.24,which is more than 1.4 times that of current commercial HfO_(2).Further,Schottky emission was determined to be leakage current transport mechanism for TiO_(2)-Y_(2)O_(3)bilayer films.Inspired by this result,the electrical performance of more general Pt/TiO_(2)-REOs/Si MOS capacitors(RE=Sc,La,Ce,Gd and Pr)was measured.The combination of TiO_(2)film and REOs passivation layer with the satisfying performance provides promising candidates for future Si-based integrated circuit(IC).

A review of rare-earth oxide films as high k dielectrics in MOS devices——Commemorating the 100th anniversary of the birth of Academician Guangxian Xu

Recently,rare-earth oxide films have attracted more and more attention as gate dielectrics in metaloxide-semiconductor(MOS)devices,showing the advantages of high dielectric constant(k value),large band gap(Eg)and outstanding physical and chemical stability in contact with silicon substrates.This paper reviews the recent development of rare earth oxide-based gate dielectric films.Aiming at the problem that k value of rare earth oxides(REOs)is generally inversely proportio nal to the band gap value,one of the biggest technical obstacles of high k films,we reviewed three strategies reported in recent papers,namely doping modification,nitriding treatment and multilayer composite,which can provide some insights for long-term development of MOS devices in integrated circuit(IC).

Ta-doped modified Gd2O3 film for a novel high k gate dielectric

Gadolinium oxide(Gd2O3) film has potential as a candidate gate dielectric to replace Hf O2. In this work,we provide a simple method by trace Ta(~1%) doping to significantly improve the dielectric properties of Gd2O3 film. And effects of annealing temperatures of Ta-doped Gd2O3(GTO) films are investigated in detail. Results show that GTO film annealed at 500℃ exhibits excellent performance as a novel gate dielectric material for integrated circuit, showing a small surface roughness of 0.199 nm, a large band gap of 5.45 e V, a high dielectric constant(k) of 21.2 and a low leakage current density(Jg) of 2.10 × 10^-3A/cm^2.All properties of GTO films are superior to pure Gd2O3 films and these GTO films meet the requirements for next-generation gate dielectrics. In addition, impedance spectrum is first used to analyze the equivalent circuit of GTO based metal-oxide-semiconductor(MOS) capacitors, which represents a new insight to understand observed electrical behaviors.