A Self-Catalyzing Hydrogen-Storage Material

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.

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.

Kinetic alteration of the 6Mg(NH2)2-9LiH-LiBH4 system by co-adding YCl3 and Li3N

The 6Mg(NH₂)₂-9LiH-LiBH₄ composite system has a maximum reversible hydrogen content of 4.2 wt% and a predicted dehydrogenation temperature of about 64 °C at 1 bar of H₂. However, the existence of severe kinetic barriers precludes the occurrence of de/re-hydrogenation processes at such a low temperature (H. Cao, G. Wu, Y. Zhang, Z. Xiong, J. Qiu and P. Chen, J. Mater. Chem. A, 2014, 2, 15816-15822). In this work, Li₃N and YCl₃ have been chosen as co-additives for this system. These additives increase the hydrogen storage capacity and hasten the de/re-hydrogenation kinetics: a hydrogen uptake of 4.2 wt% of H₂ was achieved in only 8 min under isothermal conditions at 180 °C and 85 bar of H₂ pressure. The re-hydrogenation temperature, necessary for a complete absorption process, can be lowered below 90 °C by increasing the H₂ pressure above 185 bar. Moreover, the results indicate that the hydrogenation capacity and absorption kinetics can be maintained roughly constant over several cycles. Low operating temperatures, together with fast absorption kinetics and good reversibility, make this system a promising on-board hydrogen storage material. The reasons for the improved de/re-hydrogenation properties are thoroughly investigated and discussed.

Clarifying the dehydrogenation pathway of catalysed Li4(NH2)3BH4-LiH composites

The effect of different metal oxides (Co₃O₄ and NiO) on the dehydrogenation reaction pathways of the Li₄(NH₂)₃BH₄-LiH composite was investigated. The additives were reduced to metallic species i.e. Co and Ni which act as catalysts by breaking the B-H bonds in the Li-B-N-H compounds. The onset decomposition temperature was lowered by 32 °C for the Ni-catalysed sample, which released 8.8 wt% hydrogen below 275 °C. It was demonstrated that the decomposition of the doped composite followed a mechanism via LiNH₂ and Li₃BN₂ formation as the end product with a strong reduction of NH₃ emission. The sample could be partially re-hydrogenated (∼1.5 wt%) due to lithium imide/amide transformation. To understand the role of LiH, Li₄(NH₂)₃BH₄-LiH-NiO and Li₄(NH₂)₃BH₄-NiO composites were compared. The absence of LiH as a reactant forced the system to follow another path, which involved the formation of an intermediate phase of composition Li₃BN₂H₂ at the early stages of dehydrogenation and the end products LiNH₂ and monoclinic Li₃BN₂. We provided evidence for the interaction between NiO and LiNH₂ during heating and proposed that the presence of Li facilitates a NH_x-rich environment and the Ni catalyst mediates the electron transfer to promote NH_x coupling.

The effect of Sr(OH)2 on the hydrogen storage properties of the Mg(NH2)2–2LiH system

Sr(OH)₂ influences both the thermodynamics and kinetics of the Mg(NH₂)₂–2LiH system, lowering the dehydrogenation onset and peak temperatures by ca. 70 °C and 13 °C.

Improvement of hydrogen storage property of three-component Mg(NH2)2-LiNH2-LiH composites by additives

The three-component Mg(NH₂)₂-LiNH₂-4LiH composite reversibly stores hydrogen exceeding 5 wt% at a temperature as low as 150 °C. In this work, a number of additives such as CeF₄, CeO₂, TiCl₃, TiH₂, NaH, KBH₄ and KH are added to the Mg(NH₂)₂-LiNH₂-4LiH composite in order to improve its kinetics, thermodynamics and cycling properties. Addition of 3 wt% of KH reduces the dehydrogenation onset temperature of the Mg(NH₂)₂-LiNH₂-4LiH composite to below 90 °C without emission of NH₃ during the whole dehydrogenation process up to 450 °C. Moreover, the dehydrogenation kinetics and cycling ability are remarkably enhanced upon KH-addition. The reaction model of the Mg(NH₂)₂-LiNH₂-4LiH composite is altered upon KH-addition with the active molecule density improved by about 200 times. In addition, by optimization of the ratio of Mg²⁺ to Li⁺ in the Mg(NH₂)₂-LiNH₂-LiH system, several novel composites, e.g., Mg(NH₂)₂-2LiNH₂-5.9LiH-0.1KH and Mg(NH₂)₂-LiNH₂-5.9LiH-0.1KH, with the hydrogen storage capacity exceeding 6 wt% without emission of NH₃ below 250 °C are developed. Our study demonstrates that there are various undiscovered candidates with promising hydrogen storage properties in the three-component Li-Mg-N-H system.

Effective participation of Li4(NH2)3BH4 in the dehydrogenation pathway of the Mg(NH2)2–2LiH composite

The presence of Li₄(NH₂)₃BH₄ in the MgNH₂–LiH composite enhances the hydrogen sorption kinetics and its cycling stability.

Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art

One of the limitations to the widespread use of hydrogen as an energy carrier is its storage in a safe and compact form. Herein, recent developments in effective high-capacity hydrogen storage materials are reviewed, with a special emphasis on light compounds, including those based on organic porous structures, boron, nitrogen, and aluminum. These elements and their related compounds hold the promise of high, reversible, and practical hydrogen storage capacity for mobile applications, including vehicles and portable power equipment, but also for the large scale and distributed storage of energy for stationary applications. Current understanding of the fundamental principles that govern the interaction of hydrogen with these light compounds is summarized, as well as basic strategies to meet practical targets of hydrogen uptake and release. The limitation of these strategies and current understanding is also discussed and new directions proposed.

Mechanochemistry of lithium nitride under hydrogen gas

This article unveils reaction paths and chemical kinetics during mechanochemical hydrogenation of lithium nitride, a key material for hydrogen storage.

Hydrogen storage materials discovery via high throughput ball milling and gas sorption

The lack of a high capacity hydrogen storage material is a major barrier to the implementation of the hydrogen economy. To accelerate discovery of such materials, we have developed a high-throughput workflow for screening of hydrogen storage materials in which candidate materials are synthesized and characterized via highly parallel ball mills and volumetric gas sorption instruments, respectively. The workflow was used to identify mixed imides with significantly enhanced absorption rates relative to Li2Mg(NH)2. The most promising material, 2LiNH2:MgH2 + 5 atom % LiBH4 + 0.5 atom % La, exhibits the best balance of absorption rate, capacity, and cycle-life, absorbing >4 wt % H2 in 1 h at 120 °C after 11 absorption-desorption cycles.

A prospect for LiBH4 as on-board hydrogen storage

In contrast to the traditional metal hydrides, in which hydrogen storage involves the reversible hydrogen entering/exiting of the host hydride lattice, LiBH4 releases hydrogen via decomposition that produces segregated LiH and amorphous B phases. This is obviously the reason why lithium borohydride applications in fuel cells so far meet only one requirement — high hydrogen storage capacity. Nevertheless, its thermodynamics and kinetics studies are very active today and efficient ways to meet fuel cell requirements might be done through lowering the temperature for hydrogenation/dehydrogenation and suitable catalyst. Some improvements are expected to enable LiBH4 to be used in on-board hydrogen storage.

Optical cell for combinatorial in situ Raman spectroscopic measurements of hydrogen storage materials at high pressures and temperatures

An optical cell is described for high-throughput backscattering Raman spectroscopic measurements of hydrogen storage materials at pressures up to 10 MPa and temperatures up to 823 K. High throughput is obtained by employing a 60 mm diameter × 9 mm thick sapphire window, with a corresponding 50 mm diameter unobstructed optical aperture. To reproducibly seal this relatively large window to the cell body at elevated temperatures and pressures, a gold o-ring is employed. The sample holder-to-window distance is adjustable, making this cell design compatible with optical measurement systems incorporating lenses of significantly different focal lengths, e.g., microscope objectives and single element lenses. For combinatorial investigations, up to 19 individual powder samples can be loaded into the optical cell at one time. This cell design is also compatible with thin-film samples. To demonstrate the capabilities of the cell, in situ measurements of the Ca(BH(4))(2) and nano-LiBH(4)-LiNH(2)-MgH(2) hydrogen storage systems at elevated temperatures and pressures are reported.

Advanced materials for energy storage

Popularization of portable electronics and electric vehicles worldwide stimulates the development of energy storage devices, such as batteries and supercapacitors, toward higher power density and energy density, which significantly depends upon the advancement of new materials used in these devices. Moreover, energy storage materials play a key role in efficient, clean, and versatile use of energy, and are crucial for the exploitation of renewable energy. Therefore, energy storage materials cover a wide range of materials and have been receiving intensive attention from research and development to industrialization. In this Review, firstly a general introduction is given to several typical energy storage systems, including thermal, mechanical, electromagnetic, hydrogen, and electrochemical energy storage. Then the current status of high-performance hydrogen storage materials for on-board applications and electrochemical energy storage materials for lithium-ion batteries and supercapacitors is introduced in detail. The strategies for developing these advanced energy storage materials, including nanostructuring, nano-/microcombination, hybridization, pore-structure control, configuration design, surface modification, and composition optimization, are discussed. Finally, the future trends and prospects in the development of advanced energy storage materials are highlighted.

Titanium and native defects in LiBH(4) and NaAlH(4)

We report combined density functional studies and thermodynamic considerations on Ti-related and native defects in lithium borohydride and sodium alanate. Ti atoms introduced into the bulk of LiBH(4) are thermodynamically unfavorable for all their oxidation states, while high oxidation states of Ti(n+) cations may become thermodynamically stable in the bulk of NaAlH(4) at certain thermodynamic conditions. Neutral hydrogen vacancies and interstitials or cation vacancies are less stable than their charged counterparts in both compounds. In sodium alanate, the formation of native defects leads to changes of the coordination number of aluminum, while in lithium borohydride BH(4) groups change their mutual orientation but B-H bonds remain intact. The electronic band alignment in LiBH(4) and NaAlH(4) is different.