Qinfen Gu

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.

Structure and hydrogen storage properties of the first rare-earth metal borohydride ammoniate: Y(BH4)3·4NH3

The ammine complex of yttrium borohydride Y(BH4)3·4NH3, which contains a theoretical hydrogen capacity of 11.9 wt.%, has been successfully synthesized via a simple ball milling of YCl3·4NH3 and LiBH4. The structure of Y(BH4)3·4NH3, determined by high resolution powder X-ray diffraction, crystallizes in the orthorhombic space group Pc21n with lattice parameters a = 7.1151(1) A, b = 11.4192(2) A, c = 12.2710(2) A and V = 997.02(2) A3, in which the dihydrogen bonds with distances in the range of 2.043 to 2.349 A occurred between the NH3 and BH4− units contribute to the hydrogen liberation via the combination reaction of N–H⋯H–B. Thermal gravimetric analysis combined with mass spectrometer results revealed that the decomposition of Y(BH4)3·4NH3 consists of three steps with peaks at 86 °C, 179 °C and 279 °C, respectively, in which the first and second steps mainly release hydrogen accompanied by a fair amount of ammonia emission, while the third one accounts for a pure hydrogen release. Isothermal dehydrogenation results revealed that over 8.7 wt.% hydrogen was released for Y(BH4)3·4NH3 at 200 °C, which are improved significantly in terms of both capacity and kinetics comparing to Y(BH4)3, in which the hydrogen capacity is only 3.2 wt.% at the same temperature. The favorable dehydrogenation properties presented by the Y(BH4)3·4NH3, i.e., lower dehydrogenation temperature and higher nominal hydrogen contents than that of Y(BH4)3, enable it to be a promising candidate for hydrogen storage. In addition, in situ high resolution X-ray diffraction, differential scanning calorimetry, solid-state 11B nuclear magnetic resonance and Fourier transform infrared spectroscopy measurements were employed to understand the dehydrogenation pathway of Y(BH4)3·4NH3.

A New Ammine Dual‐Cation (Li, Mg) Borohydride: Synthesis, Structure, and Dehydrogenation Enhancement

A new ammine dual-cation borohydride, LiMg(BH(4))(3)(NH(3))(2), has been successfully synthesized simply by ball-milling of Mg(BH(4))(2) and LiBH(4)·NH(3). Structure analysis of the synthesized LiMg(BH(4))(3)(NH(3))(2) revealed that it crystallized in the space group P6(3) (no. 173) with lattice parameters of a=b=8.0002(1) Å, c=8.4276(1) Å, α=β=90°, and γ=120° at 50 °C. A three-dimensional architecture is built up through corner-connecting BH(4) units. Strong N-H···H-B dihydrogen bonds exist between the NH(3) and BH(4) units, enabling LiMg(BH(4))(3)(NH(3))(2) to undergo dehydrogenation at a much lower temperature. Dehydrogenation studies have revealed that the LiMg(BH(4))(3)(NH(3))(2)/LiBH(4) composite is able to release over 8 wt% hydrogen below 200 °C, which is comparable to that released by Mg(BH(4))(3)(NH(3))(2). More importantly, it was found that release of the byproduct NH(3) in this system can be completely suppressed by adjusting the ratio of Mg(BH(4))(2) and LiBH(4)·NH(3). This chemical control route highlights a potential method for modifying the dehydrogenation properties of other ammine borohydride systems.

Ammine bimetallic (Na, Zn) borohydride for advanced chemical hydrogen storage

The synthesis and dehydrogenation performance of purified NaZn(BH4)3 with a new phase and its novel ammine metal borohydride, NaZn(BH4)3·2NH3, were first reported. Structure analysis shows that NaZn(BH4)3·2NH3 crystallizes in an orthorhombic structure with lattice parameters of a = 7.2965(2) A, b = 10.1444(2) A and c = 12.9714(3) A and space group P21nb, in which the Zn atoms are located in a tetrahedral coordination environment with two NH3 molecules and two BH4− units, presenting a novel 3D framework comprised of isolated BH4−1 units and [NaZn(BH4)2(NH3)2]+ complexes. Dehydrogenation results showed that the ZnCl2 assisted NaZn(BH4)3·2NH3 is able to release 7.9 wt% hydrogen at 110 °C without the concomitant release of undesirable gases such as ammonia and/or boranes, thereby demonstrating the potential of the ammoniated Zn-based borohydrides to be used as solid hydrogen storage materials.

Carbon-Coated Na3.32Fe2.34(P2O7)2 Cathode Material for High-Rate and Long-Life Sodium-Ion Batteries

Rechargeable sodium-ion batteries are proposed as the most appropriate alternative to lithium batteries due to the fast consumption of the limited lithium resources. Due to their improved safety, polyanion framework compounds have recently gained attention as potential candidates. With the earth-abundant element Fe being the redox center, the uniform carbon-coated Na3.32 Fe2.34 (P2 O7 )2 /C composite represents a promising alternative for sodium-ion batteries. The electrochemical results show that the as-prepared Na3.32 Fe2.34 (P2 O7 )2 /C composite can deliver capacity of ≈100 mA h g-1 at 0.1 C (1 C = 120 mA g-1 ), with capacity retention of 92.3% at 0.5 C after 300 cycles. After adding fluoroethylene carbonate additive to the electrolyte, 89.6% of the initial capacity is maintained, even after 1100 cycles at 5 C. The electrochemical mechanism is systematically investigated via both in situ synchrotron X-ray diffraction and density functional theory calculations. The results show that the sodiation and desodiation are single-phase-transition processes with two 1D sodium paths, which facilitates fast ionic diffusion. A small volume change, nearly 100% first-cycle Coulombic efficiency, and a pseudocapacitance contribution are also demonstrated. This research indicates that this new compound could be a potential competitor for other iron-based cathode electrodes for application in large-scale Na rechargeable batteries.

A novel aided-cation strategy to advance the dehydrogenation of calcium borohydride monoammoniate

The crystal structure of a promising hydrogen storage material, calcium borohydride monoammoniate (Ca(BH4)2·NH3), is reported. Structural analysis revealed that this compound crystallizes in an orthorhombic structure (space group Pna21) with unit-cell parameters of a = 8.4270 A, b = 12.0103 A, c = 5.6922 A and V = 576.1121 A3, in which the Ca atom centrally resides in a slightly distorted octahedral environment furnished by five B atoms from BH4 units and one N atom from the NH3 unit. As Ca(BH4)2·NH3 tends to release ammonia rather than hydrogen when heated in argon, a novel aided-cation strategy via combining this compound with LiBH4 was employed to advance its dehydrogenation. It shows that the interaction of the two potential hydrogen storage substances upon heating, based on a promoted recombination reaction of BH and NH groups, enables a significant mutual dehydrogenation improvement beyond them alone, resulting in more than 12 wt% high-pure H2 (>99%) released below 250 °C. The synergetic effect of associating the dihydrogen reaction with mutually aided-metal cations on optimizing the dehydrogenation of this kind of composites may serve as an alternative strategy for developing and expanding the future B–N–H systems with superior and tuneable dehydrogenation properties.

Enhancing the High Rate Capability and Cycling Stability of LiMn2O4 by Coating of Solid-State Electrolyte LiNbO3

To study the influence of solid-state electrolyte coating layers on the performance of cathode materials for lithium-ion batteries in combination with organic liquid electrolyte, LiNbO3-coated Li1.08Mn1.92O4 cathode materials were synthesized by using a facile solid-state reaction method. The 0.06LiNbO3-0.97Li1.08Mn1.92O4 cathode exhibited an initial discharge capacity of 125 mAh g(-1), retaining a capacity of 119 mAh g(-1) at 25 °C, while at 55 °C, it exhibited an initial discharge capacity of 130 mAh g(-1), retaining a capacity of 111 mAh g(-1), both at a current density of 0.5 C (where 1 C is 148 mAh g(-1)). Very good rate capability was demonstrated, with the 0.06LiNbO3-0.97Li1.08Mn1.92O4 cathode showing more than 85% capacity at the rate of 50 C compared with the capacity at 0.5 C. The 0.06LiNbO3-0.97Li1.08Mn1.92O4 cathode showed a high lithium diffusion coefficient (1.6 × 10(-10) cm(2) s(-1) at 55 °C), and low apparent activation energy (36.9 kJ mol(-1)). The solid-state electrolyte coating layer is effective for preventing Mn dissolution and maintaining the high ionic conductivity between the electrode and the organic liquid electrolyte, which may improve the design and construction of next-generation large-scale lithium-ion batteries with high power and safety.

Stabilization of NaZn(BH4)3via nanoconfinement in SBA-15 towards enhanced hydrogen release

In the present work, the decomposition behaviour of NaZn(BH4)3 nanoconfined in mesoporous SBA-15 has been investigated in detail and compared to bulk NaZn(BH4)3 that was ball milled with SBA-15, but not nanoconfined. The successful incorporation of nanoconfined NaZn(BH4)3 into mesopores of SBA-15 was confirmed by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, 11B nuclear magnetic resonance, nitrogen absorption/desorption isotherms, and Fourier transform infrared spectroscopy measurements. It is demonstrated that the dehydrogenation of the space-confined NaZn(BH4)3 is free of emission of boric by-products, and significantly improved hydrogen release kinetics is also achieved, with pure hydrogen release at temperatures ranging from 50 to 150 °C. By the Arrhenius method, the activation energy for the modified NaZn(BH4)3 was calculated to be only 38.9 kJ mol−1, a reduction of 5.3 kJ mol−1 compared to that of bulk NaZn(BH4)3. This work indicates that nanoconfinement within a mesoporous scaffold is a promising approach towards stabilizing unstable metal borohydrides to achieve hydrogen release with high purity.

A GBH/LiBH4 coordination system with favorable dehydrogenation

A novel combined hydrogen storage system LiBH4/[C(NH2)3]+[BH4]− (GBH) complexes were reported. By a short time ball milling of LiBH4 and guanidinium chloride, a series of new LiBH4/GBH complexes were produced. It was found that the two potential hydrogen storage materials exhibited a mutual dehydrogenation improvement, releasing >10.0 wt.% of fairly pure H2 from LiBH4/GBH below 250 °C. Further investigations revealed that balancing the protic and hydridic hydrogens, and the complexation between LiBH4 and GBH, are two important roles in the improvement of the dehydrogenation of this system, which may serve as an alternative strategy for developing a new metal borohydride/B–N–H system with favourable dehydrogenation.

Multiangular Rod-Shaped Na0.44MnO2 as Cathode Materials with High Rate and Long Life for Sodium-Ion Batteries

The tunnel-structured Na0.44MnO2 is considered as a promising cathode material for sodium-ion batteries because of its unique three-dimensional crystal structure. Multiangular rod-shaped Na0.44MnO2 have been first synthesized via a reverse microemulsion method and investigated as high-rate and long-life cathode materials for Na-ion batteries. The microstructure and composition of prepared Na0.44MnO2 is highly related to the sintering temperature. This structure with suitable size increases the contact area between the material and the electrolyte and guarantees fast sodium-ion diffusion. The rods prepared at 850 °C maintain specific capacity of 72.8 mA h g-1 and capacity retention of 99.6% after 2000 cycles at a high current density of 1000 mA g-1. The as-designed multiangular Na0.44MnO2 provides new insight into the development of tunnel-type electrode materials and their application in rechargeable sodium-ion batteries.