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.     

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.     

Hydrogen dissociation and diffusion on transition metal (= Ti,Zr, V,Fe, Ru,Co, Rh,Ni, Pd,Cu, Ag)-doped Mg(0001) surfaces

The kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H₂ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab initio density functional theory investigation of H₂ dissociation and subsequent atomic H diffusion on TM (= Ti, V, Zr, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TMs on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TMs on the right of the periodic table do not bind H, however, they do not reduce the barrier to dissociate H₂ significantly. Our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice.     

First-principles studies of lithium hydride series for hydrogen storage

The application of hydrogen as a clean energy source is based on storage of hydrogen. In metal hydrides is possible, since many metals react readily with hydrogen forming a stable metal hydride. Thus, saline hydrides such as lithium hydride have appeared as new alternatives to this, because of their high reactivity and reversibility. The first principles calculations based on density functional theory (DFT) have been used to study the physical properties of several Li–H compounds. The crystal structure, electronic properties and internal optimization parameters are treated by the LAPW method implemented in the WIEN2k code. In the present study we show the comparison of three different phases of lithium hydride compounds, in six different crystal structures, with the purpose of comparing the formation energies in all cases, and determine which is the structure, with the best structural properties for applications as hydrogen reservoir. The comparisons between the results obtained in the structures of lithium–hydride are discussed in this work.     

Storing-hydrogen processes on graphene activated by atomic-vacancies

We investigated the minimum energy pathways and energy barriers of reversible reaction (V₁₁₁ + H₂?V₂₂₁) based upon calculations using density functional theory. We find a comparable activation barrier of around 1.3 eV for both the dissociative chemisorption and desorption processes. The charge transfer rate from a reacting hydrogen atom to the graphene is around 0.18 e per hydrogen atom in the final state. A subsequent reaction path to recover the initial structure of V₁₁₁ is realized by the migration of hydrogen atoms from V₂₂₁ onto the graphene surface. The comparable energy barrier of 1.3 eV for both adsorption and desorption suggests that this novel storage and release concept has the potential to act as a hydrogen storage system for certain applications.     

InSe based Janus Ⅵ-Ⅲ-Ⅳ-Ⅴ monolayers as water-splitting photocatalysts: Role of vacuum level difference

Two-dimensional Janus monolayers have great potential to solve the increasing energy crisis and environmental pollution. In this work, we mainly study the structural, electronic and photocatalytic properties of InSe based Janus Ⅵ-Ⅲ-Ⅳ-Ⅴ monolayers, i.e. InSe-MQ (M = Si, Ge, Sn; Q = P, As) monolayers via first-principles calculations. Theoretical results reveal that they satisfy dynamic, energetic and mechanic stability. Their band gaps range from 1.76 to 2.28 eV, so that abundant visible light could be absorbed. The band edges of single-layer InSe-MQ except InSe–SiP straddle the redox potential of water, and thus the InSe-MQ monolayers except InSe–SiP are promising water-splitting photocatalysts at pH = 0. Particularly, most of InSe-MQ monolayers still have water splitting ability at less acid conditions with higher pH values. On the other hand, our theoretical results also indicate the band edges of group Ⅵ-Ⅲ-Ⅳ monolayers are to a large extent modified as the vacuum level difference between top and bottom surfaces is included, which has been neglected before and firmly collaborated by recalculations on the band arrangements of explored GaS-SnP and InS–SnP monolayers. Overall, this work not only defines several water-splitting photocatalysts but also paves the way to obtain reliable photocatalytic properties of the large group Ⅵ-Ⅲ-Ⅳ-Ⅴ (Ⅲ = Al, Ga, In; Ⅵ = O, S, Se, Te; Ⅳ = Si, Ge, Sn; Ⅴ = N, P, As) monolayers.     

Two-dimensional C3N/WS2 vdW heterojunction for direct Z-scheme photocatalytic overall water splitting

The development and design of efficient photoelectric catalysts is of great significance for environmental friendliness. This paper is devoted to finding a new two-dimensional van der Waals heterojunction to realize hydrogen production from water splitting. Based on first-principles calculations, a direct type-Z C₃N/WS₂ heterojunction was successfully designed by combining highly active two-dimensional transition metal dichalcogenides (TMDs) with C₃N similar in structure to graphene. The staggered band structure of the direct Z-scheme and high carrier mobility facilitates the efficient separation of photogenerated electron and hole pairs. More importantly, the C₃N/WS₂ direct Z-type heterojunction can perfectly realize total water splitting from pH = 0 to pH = 7. What's more, the Gibbs free energy and overpotential demonstrate the excellent hydrogen evolution capability and the oxygen evolution capacity of the material. In summary, these studies provide new ideas for designing high-performance photoelectric catalysts for visible-light water splitting.     

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.