Hydrogen storage on superalkali NLi4 decorated β12-borophene: A first principles insights

The hydrogen storage properties of superalkali NLi₄ decorated β₁₂-borophene have been comprehensively investigated based on first-principles density functional calculations (DFT). It is found that the NLi₄ cluster can be stably anchored on the surface of β₁₂-borophene because of its large binding energy. The calculated Bader charge population indicates that the charges are transferred from Li atoms to the original monolayer and causes the NLi₄ steady adsorbs onto the surface of β₁₂-borophene. For H₂ storage, two sides of NLi₄ decorated β₁₂-borophene can adsorb up to 24H₂ molecules with an ideal H₂ adsorption energy of ?0.176 eV/H_2. Meanwhile, the hydrogen uptake density achieves 7.66 wt% and surpasses the target of 6.5 wt% from U.S. Department of Energy (DOE). In addition, the adsorption reasons of H₂ molecules include the orbital hybridization between H₂ and β₁₂-borophene from the calculated projected density of states (PDOS) and the polarization effect of electrostatic field from the calculated charge density difference. We hope this theoretical study can encourage the experimental fabrication for hydrogen storage applications in the near future.     

Stability and hydrogen adsorption properties of Mg/Mg2Ni interface: A first principles study

The stability and hydrogen adsorption behaviors of Mg/Mg₂Ni interface were studied by first principles calculations. Results demonstrated that the interaction between Ni (from Mg₂Ni compound) and Mg (from Mg metal) is the key factor stabilizing the interface, and the interface provides a medium to capture hydrogen atoms originating from the accumulation of electrons in the interface zone by the formation of the interface. Hydrogen atoms adsorbed in the interface zone tend to form covalent bonds with metal atoms (Ni and Mg atoms), which deliver negative adsorption energies in the range of ?0.831 to ?0.019 eV for most possible adsorption sites. However, the strength of the H-metal bonds depends on the environment the H located. The present study illustrates that the Mg/Mg₂Ni layered structure could be a potential medium for reversible de/hydrogenation processes.     

A novel hydrogen storage medium of Ca-coated B40: First principles study

Using first principles study, we have investigated the hydrogen storage capacity of Ca-coated B₄₀. Our result shows that Ca prefers to adsorb on the top hollow center of heptagonal ring of B₄₀ due to the large binding energy of ?2.820 eV. Bader charges calculation indicates that charges transfer from Ca to B₄₀ result in an induced electric field so that H₂ molecules are polarized and adsorbed onto the surface of B₄₀ without dissociation. The Ca₆B₄₀ complex can adsorb up to 30 H₂ molecules with average adsorption energy of ?0.177 eV/H₂ and the hydrogen storage gravimetric density reaches up to 8.11 wt.%, higher than the goal from DOE by the year 2020. These findings will suggest a new and potential structure for hydrogen storage in the future.     

Adsorption and diffusion of hydrogen on Ti,Al, and TiAl surfaces

Hydrogen adsorption on HCP Ti (0001), FCC Al (111), and gamma TiAl (100) surfaces is investigated by first principles calculations. Adsorption energy of hydrogen on various sites and energy barrier of diffusion between these sites are derived and compared, to find out the energetically more favorable adsorption site and optimum diffusion path. It is shown that the interaction between H and TiAl surface should be much stronger with lower heat of formation of hydrogen than interaction between H and TiAl bulk, and that the diffusion of hydrogen within the surface layer should be much easier than diffusion from surface to bulk or within bulk layers. Calculation also reveals that it is high energy barrier between various sites which would fundamentally bring about the low diffusion coefficient of H in TiAl bulk. The calculated results are in good agreement with experimental observations in the literature, and could provide a deep understanding of H behavior in gamma TiAl.     

HfNi and its hydrides – First principles calculations

Hydrogen induced modifications to the structural, electronic and bonding properties of HfNi are investigated by performing first principles calculations. The full-potential linearized augmented plane waves (FP-LAPW) code based on the density functional theory (DFT) was used. The charge transfer and bonding between the constituent atoms is examined by means of the Bader's atoms in molecule (AIM) theory. The calculated enthalpies of formation of HfNi, HfNiH and HfNiH₃ are −53.5 kJ/mol atom, −17.3 kJ/molH and −34.6 kJ/molH. They are found to be in a good agreement with the experimental and semi-empirical values. The calculated stability of the hydrides is in agreement with their hydrogen absorption ability.     

Vibrational and thermodynamic properties of LiBH4 polymorphs from first-principles calculations

The study of phonons describes the thermodynamic properties behavior of compounds with small atoms because phonons have an important influence on its properties. Lithium borohydride, LiBH₄, is one of the suitable materials for hydrogen storage solid state. Although the transformations of Lithium borohydride LiBH₄ were repeatedly studied by experiments and fundamental side, these transformations are still under discussion. In the present work, the mode vibrational analysis of orthorhombic and hexagonal LiBH₄ structures were considered with ab initio lattice-dynamics based on the quasi-harmonic approximation approach as implemented in Phonopy code. The results show that the orthorhombic structure is thermodynamically stable, while the hexagonal structure is unstable owing to the presence of negative mode frequency. The thermal expansion behavior and various thermodynamic properties stability like heat capacity, entropy and Helmholtz energy were also studied and the obtained results are in good agreement with experiments. This shows a deep connection between stability and strength and helps researchers to estimate accurately the thermodynamic performance of LiBH₄ materials.     

Parameter optimization of ZrCo–H system for durable tritium storage and delivery system of ITER

Cycling stability of ZrCo–H system is extremely important for the long-term operation of the storage and delivery system (SDS) in ITER. Herein, the optimal cycling operation parameters were systematically investigated. It indicates that various parameters, such as hydrogen pressure, temperature, composition, and stoichiometric ratio of H atoms, will all affect the cycling performance of the ZrCo–H system significantly. The decline rate of the hydrogen capacity of the ZrCo–H system is positively correlated with the hydrogen pressure. The experimental result shows that 54% of hydrogen capacity decreases under 28.1 kPa hydrogen pressure, while 30% of attenuation is obtained when the pressure is decreased to 8.1 kPa after 14 cycles. In terms of temperature, the lowest cycling attenuation can be maintained at about 25% after 14 cycles when the dehydrogenation temperature at 550 °C. The effects of doping elements, Hf and Ti, on the cycling stability of ZrCo–H system are also compared. The Zr_0.8Ti_0.2Co sample exhibits higher cycling capacity than ZrCo and Zr_0.8Hf_0.2Co samples. The extremely excellent behavior can be achieved when all ZrCo alloys are continuously evacuated during the hydrogen release process, and the attenuation of only 1.1% is observed for Zr_0.8Ti_0.2Co after 15 cycles. Besides, the cycling attenuation is related to residual stoichiometric ratio of H atoms in ZrCo alloy during the cycling test. When the residual H atoms proportion exceeds 1 in ZrCo during dehydrogenation, hydrogen cycling capacity hardly fades. The XRD results reveal that the disproportionation of ZrCo is directly associated with the cycling degradation, yielding the more stable products of ZrCo₂ and ZrH₂, However, the disproportionation can be avoided during the cycling process by controlling the stoichiometric ratio of H atoms remained in ZrCo above 1. This study demonstrates that the cycling performance of ZrCo can be substantially improved when the operation parameters are properly adjusted, which provides a significant important reference for durable running of SDS in ITER.     

First principles study towards the influence of interstitial nitrogen on the hydrogen storage properties of the Mg2Ni (0 1 0) surface

Doping light elements into Mg-based alloy has been viewed as an effective method for improving the hydrogen storage properties without remarkably reducing hydrogen capacity. The influences of interstitial nitrogen doping on the crystal structure, thermal stability, hydrogen adsorption energy and electronic properties of Mg₂Ni (0 1 0) surface were investigated by first principles calculations. The calculation results showed that the addition of interstitial N results in an anisotropic expansion in the crystal structure and a better improvement effect on lowering thermal stability of the Mg₂Ni surface than the commonly used transition metal. Three stable sites including the NiNi bridge site, the top sites of Mg and Ni atoms, were determined to take in hydrogen in the pure surface. When the nonmetal N is doped into the pure surface, the number of the stable adsorption sites is increased and the adsorption energy of H in the NiNi bridge site is also increased from ?0.9614 eV for the pure to ?0.5441 eV for the N-doped counterpart. The increases in both the stable adsorption sites and the energy caused by the addition of N indicate that more hydrogen could be adsorbed in the weaker NiH bonds of the N-doped Mg₂Ni alloy, thereby improving the hydrogen storage behaviors of Mg-based alloy.     

Stable region of ZrCo under hydrogen atmosphere at high temperature

In this work, the unusual reproportionation phenomenon, which occurs under hydrogen atmosphere, has been studied for the first time by combing temperature-programmed dehydrogenation and XRD characterization. The phase transition from ZrCo₂–ZrH₂ to ZrCo for reproportionation was clearly observed without vacuum pumping. It was shown that the reproportionation could take place even under hydrogen pressure of 0.65 bar at 640 °C. In addition, the reproportionation behaviors under different initial hydrogen pressures were investigated and the onset boundary for reproportionation was successfully established. The original insight into the unusual reproportionation phenomenon reveals that there is a stable region for ZrCo under hydrogen atmosphere at high temperature. On the basis of this work, it is hopeful that the application of ZrCo at high temperature will be substantially broadened.     

Remarkable enhancement in durability of ZrCo alloy against hydrogen induced disproportionation by Ti and Nb co-substitution

Considering the thermodynamic stability of various hydrides, a strategy has been employed to improve the hydrogen isotope storage properties of ZrCo alloy which involves partial co-substitution of Zr with Ti and Nb. Herein, alloys of composition Zr_0.8Ti_0.2-xNb_xCo (x = 0.05, 0.1, 0.15) is prepared, characterized and the effect of Ti and Nb doping on hydrogen storage properties of parent ZrCo alloy is investigated. XRD analysis confirmed the formation of desired pure cubic phase of all the synthesized alloys similar to ZrCo phase. The presence of a single plateau in hydrogen desorption pressure-composition isotherms confirms single step hydrogen absorption-desorption behavior in Zr_0.8Ti_0.2-xNb_xCo alloys. The equilibrium pressure of hydrogen desorption decreases marginally with increasing Nb content in Zr_0.8Ti_0.2-xNb_xCo alloys which is further corroborated by differential scanning calorimetry measurements. Investigation of hydrogen induced disproportionation behavior in ITER-simulating condition revealed substantial impact of co-substitution of Ti and Nb on anti-disproportionation properties of ZrCo alloy. These remarkable properties make the Ti and Nb co-substituted quaternary alloys a desirable material for hydrogen isotope storage and delivery application.     

Effect of catalytic Pd coating on the hydrogen storage performances of ZrCo alloy by electroless plating method

The effects of Pd coating with different deposition concentration (PdCl₂ 0.2 g L^?1, 0.6 g L^?1, 1.0 g L^?1) on the surface morphology, microstructure and hydrogen storage performances of ZrCo alloy have been investigated. Results show that spherical Pd particles have been deposited on the surface of ZrCo alloy successfully, which transfer from sparse arrangement to continuous and compact film with increasing deposition concentration of PdCl₂. The hydriding kinetic property of all Pd coated alloys is improved compared with the bare alloy, which is due to the catalyst effect of Pd coating. The hydriding rate of the samples firstly increases and then decreases with increasing deposition concentration, which is closely related to the surface morphology and thickness of Pd coating. The hydriding kinetic property of the samples is greatly improved after 5 cycles, although Pd particles on the alloy surface peel off to some extent. This phenomenon indicates that the accumulated fresh surface during cycling makes a greater contribution to the improved hydriding kinetic property and the catalyst effect of Pd coating is weakened during cycling.     

Temperature-hydrogen pressure phase boundaries and corresponding thermodynamics for ZrCoH system

The thermodynamically and kinetically stable regions of the temperature–H₂ pressure phase boundaries for the ZrCoH system were established using the Temperature-Concentration-Isobar (TCI) method. Based on this, the enthalpy change and entropy change values of dehydrogenation and disproportionation reactions were successfully obtained. The average enthalpy change (ΔH) and entropy change (ΔS) estimated from the phase boundaries for dehydrogenation of ZrCoH₃ to ZrCo are respectively 103.07 kJ mol^?1H₂ and 148.85 J mol^?1 H₂ K^?1, which are well agreement with the data reported in literature. The average ΔH and ΔS were estimated to be ?120.91 kJ mol^?1H₂ and -149.32 J mol^?1 H₂ K^?1 for the disproportionation of ZrCoH₃, whereas the ΔH and ΔS were calculated to be ?84.6 kJ mol^?1H₂ and -92.29 J mol^?1 H₂ K^?1 for disproportionation of ZrCo. In addition, it was found from the established phase boundaries that the anti-disproportionation property of ZrCo alloy can be enhanced if the phase boundaries of hydrogenation/dehydrogenation are far away from the phase boundaries of disproportionation by adjusting the thermodynamics. Meanwhile, it is possible to keep ZrCo away from disproportionation even at high temperature of 650 °C under hydrogen atmosphere, if the temperature-H₂ pressure trajectory is carefully controlled without crossing the phase boundaries of disproportionation. Therefore, the established phase boundaries can be used as a guide to the eye avoiding disproportionation and improving the anti-disproportionation property of ZrCo alloy.     

Effects of Mo substitution on the kinetic and thermodynamic characteristics of ZrCo1-xMox (x = 0–0.2) alloys for hydrogen storage

In this study, ZrCo_1-xMo_x (x = 0, 0.05, 0.1, 0.15, 0.2) alloys were prepared via vacuum arc-melting method. The effects of substituting Co with Mo on the structure, initial activation behaviors, and thermodynamic properties of the afore-mentioned alloys were systematically investigated. The results showed that ZrCo_1-xMo_x (x = 0, 0.05, 0.1, 0.15) alloys exhibited a single ZrCo phase and their corresponding hydrides, a ZrCoH₃ phase. Furthermore, ZrCo_0.8Mo_0.2 alloy consisted of ZrCo phase and a trace of ZrMo₂ phase, and the hydride contained ZrCoH₃ and ZrH phases. As the Mo content was increased, the initial activation period decreased significantly from 19277 s for ZrCo to 576 s for ZrCo_0.8Mo_0.2, which was closely related to the catalytic effect of ZrMo₂. The plateau width of pressure composition temperature curves were shortened, and the equilibrium pressures of hydrogen desorption decreased slightly as Mo content increased. Additionally, the experiments showed that the anti-disproportionation performance was greatly improved by Mo substitution. The extent of disproportionation decreased from 64.28% for ZrCo to 24.11% for ZrCo_0.8Mo_0.2. The positive effect of Mo substitution on improving the anti-disproportionation property of ZrCo alloy was attributed to the reduction of hydrogen atom in 8f₂ and 8e sites, which decreased the driving force of the disproportionation reaction.     

Mechanisms of dopants influence on hydrogen uptake in COF-108: A first principles study

Mechanisms of dopants (Li, Na, Mg, and Al) influence on hydrogen uptake in COF-108 were investigated by means of first principles. The binding energy of dopants in COF-108 was estimated from the first principles total energy calculations. All doped systems are shown positive binding energies with the metallic state of the dopant as the reference. The lowest binding energy of 0.518 eV appeared in the Na-doped system while a large amount of energy (2.692 eV) is required for Al to dope into COF-108. Electronic structure analysis shows that dopants Li and Na move the conduction band crossing the Fermi energy level and introduce weakly bonded electrons near the Fermi energy, which may polarize the hydrogen molecules. It is expectable that interaction between hydrogen molecule and the host COF-108 could be enhanced by the polarization of hydrogen molecule. Therefore the hydrogen uptake will be improved in the doped systems. Dopant Mg slightly reduces the band gap between the valence and conduction bands, but is hard to build chemical bonds with the host atoms owing to the less overlaps between the bond peaks of Mg and the COF-108. It hardly affects the electron distributions of the COF-108 and therefore weakly changes the chemical interactions between atoms in COF-108.