A reversible hydrogen storage material of Li-decorated two-dimensional (2D) C4N monolayer: First principles calculations

Two-dimensional (2D) materials can be regarded as potential hydrogen storage candidates because of their splendid chemical stability and high specific surface area. Recently, a new dumbbell-like carbon nitride (C₄N) monolayer with orbital hybridization of sp³ is reported. Motivated from the above exploration, the hydrogen adsorption properties of Li-decorated C₄N monolayer are comprehensively investigated via first principles calculations based on the density functional theory (DFT). It is found that the Dirac points and Dirac cones exists in the Brillouin zone (BZ) from the calculated electronic structure and indicates the C₄N can be used as an excellent topological material. Also, the calculated phonon spectra demonstrate that the C₄N monolayer owns a strong stability. Moreover, the calculated binding energy of decorated Li atom is bigger than its cohesive energy and results in Li atoms disperse over the surface of C₄N monolayer uniformly without clustering. In addition, the Li₈C₄N complex can capture up to 24H₂ molecules with an optimal hydrogen adsorption energy of −0.281 eV/H₂ and achieves the hydrogen storage density of 8.0 wt%. The ab initio molecular dynamics (AIMD) simulations suggest that the H₂ molecules can be desorbed quickly at 300 K. This study reveals that Li-decorated C₄N monolayer can be served as a promising hydrogen storage material.     

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.     

Li decorated heteroborospherene C4B32 as high capacity and reversible hydrogen storage media: A DFT study

In this work, adsorption of H₂ molecules on heteroborospherene C_2v C₄B₃₂ decorated by alkali atoms (Li) is studied by density functional theory calculations. The interaction between Li atoms and C₄B₃₂ is found to be strong, so that it prevents agglomeration of the former. An introduced hydrogen molecule tilts toward the Li atoms and is stably adsorbed on C₄B₃₂. It is obtained that Li₄C₄B₃₂ can store up to 12H₂ molecules with hydrogen uptake capacity of 5.425 wt% and average adsorption energy of ?0.240 eV per H₂. Dynamics simulation results show that 6H₂ molecules can be successfully released at 300 K. Obtained results demonstrate that Li decorated C₄B₃₂ is a promising material for reversible hydrogen storage.     

DFT study of hydrogen storage on the metallic decoration of boron substitution on zeolite templated carbon vacancy

This work reports DFT calculations for the assessment of metallic decoration of boron substitution Zeolite Templated Carbon vacancy for hydrogen adsorption. The boron substitution on Zeolite Templated Carbon vacancy is characterized by the formation of pentagonal and heptagonal rings. Moreover, the boron substitution can be considered as a promising way for hydrogen storage, this way boron substitution is used on Zeolite Templated Carbon vacancy in order to create an active site for metallic decoration. Once that we develop a Boron substitution on Zeolite Templated Carbon vacancy, the decoration with Lithium, Sodium, and Calcium atoms is also carried out. The analysis reveals that the Na decoration has the best performance for hydrogen storage. The results show that boron substitution on Zeolite Templated Carbon vacancy decorated with 3 Sodium atoms can adsorb up to fifteen hydrogen molecules (5 hydrogen molecules per Sodium atom), this gives a gravimetric storage capacity of 6.55 % wt., which is enough for meeting DOE gravimetric targets. In addition, the average binding energies and adsorption energies are calculated in the range 0.2298–0.2144 eV/H₂, which constitute desirable energies for hydrogen adsorption. Besides, the hydrogen adsorption process is carried out by electrostatic interaction between the Na cation and the induced H₂ dipole. The calculation performed in this work reveals that the boron substitution on Zeolite Templated Carbon vacancy decorated with Na atoms is a good candidate as a medium for hydrogen storage.     

Synergistic effect of {101} crystal facet and bulk/surface oxygen vacancy ratio on the photocatalytic hydrogen production of TiO2

Anatase titanium dioxide (TiO₂) nanocrystals with different percentages (up to 95%) of exposed {101} facet and different concentration ratios of bulk single-electron-trapped oxygen vacancies (SETOVs) to surface oxygen vacancies (SOVs) were prepared by alcohol-thermal method with nanotube titanic acid as the precursor in combination with solid-state reduction by NaBH₄. The as-prepared TiO₂ nanocrystals were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, electron spin resonance spectroscopy, and ultraviolet–visible light spectrometry. The effects of the percentage of crystal facets and the concentration ratio of bulk SETOVs/SOVs on the photocatalytic hydrogen production rate of TiO₂ nanocrystals were investigated with positron annihilation lifetime spectroscopy as well as photocurrent test. Findings indicate that the percentage of the exposed {101} facets of the as-prepared TiO₂ nanocrystals and their concentration ratios of bulk SETOVs/SOVs can be well tuned by properly adjusting the amount of NaBH₄ and the reduction reaction time as well. Increasing percentage of the {101} facet of anatase TiO₂ nanocrystals contributes to improving their photocatalytic hydrogen production activity, because the {101} facets of the anatase TiO₂ nanocrystals possess enriched electrons and can act as the reduction sites to enhance the reduction reaction of H⁺ affording H₂ in the sacrifice system of splitting water. Both the bulk SETOVs and SOVs contribute to the improvement of the light absorption while SOVs can facilitate the separation of photogenerated charges, thereby adding to the photocatalytic activity. However, the bulk SETOVs and excessive SOVs are also the combination centers of photogenerated charges, which means it is essential to maintain a suitable concentration ratio of the bulk SETOVs/SOVs so as to enhance the light absorption and achieve the best separation efficiency of photogenerated charges and achieve the best photocatalytic activity for hydrogen production. Particularly, when anatase TiO₂ nanocrystal with a high percentage (95%) of exposed {101} facet is reduced by NaBH₄ at a mass ratio of 2: 1 for 20 min, the resultant reduced H-TiO₂ nanocrystal (denoted as H-TiO₂-R20(2:1)) provides the highest photocatalytic hydrogen productive rate. Furthermore, the combination of 0.5% Pt/H-TiO₂-R20(2:1) with 0.5% Pt/WO₃ can split water to simultaneously produce H₂ and O₂, showing promising potential for splitting water affording hydrogen and oxygen.     

Influence of the Fe-doping on hydrogen behavior on the ZrCo surface

Ab initio calculations have been carried out to investigate the adsorption, dissociation, and diffusion of atomic and molecular hydrogen on the Fe-doped ZrCo (110) surface. It is found that the adsorption of H₂ on doped surface seems thermodynamically more stable with more negative adsorption energy than that on the pure surface, and the dissociation energy of H₂ on doped surface is much bigger therefore. However, compared with the pure system, there are fewer adsorption sites for spontaneous dissociation. After dissociation, the higher hydrogen adsorption strength sites would promote the H atom diffusion towards them where they can permeate into the bulk further. Furthermore, the ZrCo (110) surface possesses much higher hydrogen permeability and lower hydrogen diffusivity than its corresponding ZrCo bulk. Moreover, further comparison of the present results to analogous calculations for pure surface reveals that the Fe dopant facilitates the H₂ molecule dissociation. Unfortunately, this does not improve the hydrogen storage performance of ZrCo alloy due to the H atom diffusion on the surface and into bulk are prevented with higher reaction energetic barriers by doping Fe. Consequently, ZrCo (110) surface modified with Fe atoms should not be preferred as a result of its terrible hydrogen permeability. A clear and deep comprehending of the inhibiting effect of Fe dopant on the hydrogen storage of ZrCo materials from the perspective of the surface adsorption of hydrogen are obtained from the present results.     

Low-index surface investigation of KAlH4: Theoretical attempt to study the surface effect on the hydrogen storage properties

Different preparations of complex hydrides lead to different hydrogen uptake and release. Besides, Potassium Aluminum hydride is a structure with different re/dehydrogenation properties than the rest of alanates. Given these considerations, we investigated nine stable cleavages on the (100), (010), (001), (111), and (101)KAlH₄ surfaces. The results reveal that, while the (010) surface energy is much higher, all the other surfaces are approximately in the same range of energy. Some of these surfaces would be placed on top of the nanocrystallites and the different decomposition pathways may be originated from the different characteristics of these surfaces, which is one of the central issues of the present study. Our results are in accordance with experimental data indicating that long hours milling of alanates just creates fresh surfaces and the structures remain unchanged. Due to the surface effect, huge changes in electronic and geometric characteristics occur. The band gaps are narrowed up to 2eV, which alongside with massive changes in chemical bonds, lead to an improved dehydrogenation relative to the bulk.     

Theoretical study of the synthesis,characterization and hydrogen storage properties of a high-density hydrogen storage material: (CH3NH3)BH4

Inspired by both alkaline metal borohydrides and organic-inorganic hybrid perovskite, we predict a pair of complex structures of (CH₃NH₃)BH₄ with tremendous high hydrogen capacity (21.27 wt.%). Through comparison and analysis of the electronic structures of alkali metal atoms, CH₃NH₃, NH₄, and NH₃BH₃ molecule, it is concluded that similar spatial and electronic structures show the feasibility of synthesizing (CH₃NH₃)BH₄ by a substitution reaction. Firstly, theoretical structures (S₁ and S₂ in P₁) with stable configurations have been reconstructed by cation substitution followed by a series of restrictive structural optimizations, and both the lattice parameters and the position coordinate information of (CH₃NH₃)BH₄ are obtained. Ignoring the relatively mobile hydrogen, the structural symmetries of S₁ and S₂ are I4mm and P4/nmm, respectively. X-ray diffraction characterizations of S₁ and S₂ are consistent with the experimental results. Secondly, the calculated elastic constants of (CH₃NH₃)BH₄ (S₁ and S₂) with P₁ symmetry indicate that angles α, β and γ oscillate at right angles due to the influence of the cation orientation. The calculated spatial dependence of bulk (B), Young's (E), and shear (G) modulus obviously show that the two P₁ phases all have strong elastic anisotropy. Thirdly, the calculated electronic properties show that the protonic amine-H, hydridic borane-H, and neutral methane-H are widely distributed in (CH₃NH₃)BH₄, which allow for weaving in a planar dihydrogen bonding network, which in turn influences the dehydrogenation reaction. Last and most important, we propose the following dehydrogenation process of (CH₃NH₃)BH₄ via the intermediate compounds: 2(CH₃NH₃)BH₄ → CH₃NH₂BH₂NHCH₃BH₃+3H₂. For each dehydrogenation step, the free energy change is negative, which means (CH₃NH₃)BH₄ can decompose spontaneously, similar to ammonium borohydride, which is strongly related to the planar dihydrogen bonding network.     

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.     

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.     

Effect of hydroxyl groups on the oxygen vacancy and hydrogen-sensitive properties of CeO2 in different morphologies

The “hydroxyl-oxygen vacancy model” was of great importance for the catalytic and physicochemical properties of metal oxides. Here, we proposed a simple method to construct a hydroxyl-oxygen vacancy model for one-step reduction of palladium chloride through ascorbic acid rich in hydroxyl, loaded on rod, spherical, nanoparticles and cubic CeO₂, to achieve the Ce⁴⁺/Ce³⁺ transition and the generation of Pd NPs. The surface-loading Pd ions activated the lattice oxygen of CeO₂, and promoted the overflow of reverse oxygen at the Pd–O–Ce interface. The increasing in Ce³⁺ occupancy altered the reduction performance, oxygen vacancy number and active Pd valence state of the catalyst, and greatly contributed to the response and recovery times. Among the obtained sensors, the 0.50 wt % Pd/CeO₂–C/R sensor show a response amplitude of 190/196% to 1000 ppm H₂ at 120 °C with a response/recovery time of only 1/3 s. These excellent results are mainly attributed to the chemisorbed oxygen and Ce³⁺ after ascorbic acid reducing Pd are found to be 4.84 and 1.57 times higher than pure CeO₂. The hydroxyl-oxygen vacancy model may open up a new avenue for detecting hydrogen sensing.     

First-principle investigation for the hydrogen storage properties of NaXH3 (X= Mn,Fe, Co) perovskite type hydrides

In the present study, NaXH₃ (X = Mn, Fe, Co) perovskite type hydrides have been investigated by performing first-principles calculation. The results of the structural optimizations show that all these compounds have negative formation energy implying the thermodynamic stability and synthesisability. The mechanical stability of these compounds has been studied with the elastic constants. Moreover, the polycrystalline properties like bulk modulus, Poisson's ratio, etc. have been obtained using calculated elastic constants of interest compounds. The electronic properties have been studied and band structures have been drawn with the corresponding partial density of states. These plots indicated that NaXH₃ hydrides show metallic characteristics. The charge transfer characteristics in these compounds have been studied with the Bader partial charge analysis. The phonon dispersion curves and corresponding density of states indicated that NaXH₃ compounds are dynamically stable compounds. The investigation on hydrogen storage characteristics of NaXH₃ compounds resulted in hydrogen storage capacities of 3.74, 3.70 and 3.57 wt% for X = Mn, Fe and Co, respectively. The present study is the first investigation of NaXH₃ perovskite type hydrides as known up to date and may provide remarkable contribution to the future researches in hydrogen storage applications.     

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.     

Effect of hydrogen concentration on various properties of gamma TiAl

First principles calculation reveals that the TiAlH phases become relatively less stable after the addition of more H atoms, and that the H migration from tetrahedral interstitial site to octahedral site in gamma TiAl should be much easier than that between octahedral sites. Calculation also shows that H concentration has an important effect on mechanical properties of TiAlH phases, and that the energetically favorable TiAlH phase should possess bigger E, G, and G/B values as well as lower elastic anisotropy. Moreover, it is found that the heat capacities of TiAlH phases increase with the increase of H concentration, and that the coefficients of thermal expansion of TiAl and TiAlH phases decrease with the increase of pressure.     

A first-principles simulation of the metal borohydride ammonia borane complex (LiBH4)2(NH3BH3) and the decomposition reaction pathway for hydrogen storage

In this work, we predict a range of favorable functional properties of (LiBH₄)₂NH₃BH_3, a relatively new member of the boron-containing metal borohydride ammonia borane family, by means of ab initio calculations, as well as its parent compounds, LiBH₄ and NH₃BH₃, for comparisons. Both the mechanical and dynamical stabilities of this new compound have been demonstrated theoretically for the first time. Results from elastic modulus calculations show that the mechanical properties of (LiBH₄)₂NH₃BH₃ are remarkably improved compared with its parent compounds. Secondly, Electronic structure results show that it remains to an insulator with large band gap typical of the boron-containing hydrogenous family, but the band gap can be tuned by the compositions of NH₃BH₃ and LiBH₄. Charge analysis demonstrates that charge transfers in individual layers of LiBH₄ and NH₃BH₃ are similar to LiBH₄ and NH₃BH₃, respectively. A measurable amount of charge transfers from the LiBH₄ layers to the NH₃BH₃ layers result in enhanced activation properties for hydrogenation and dehydrogenation in (LiBH₄)₂NH₃BH₃. Thirdly, Free energies of six possible dehydrogenation reactions have been calculated from 0 K to 700 K, and the results show that combination of NH₃BH₃ and LiBH₄ can reduce the dehydrogenation energies compared with the parent compounds, a result consistent with recent experiments. Meanwhile, the N–B bond strengths increase and thereby borazine and diborane formation are reduced upon dehydrogenation.     

Stability and hydrogen storage properties of Sc6O8 and Y6O8 cage-like complexes

To study the dihydrogen storage capacity of Sc₆O₈ and Y₆O₈ complexes, the stability and hydrogen adsorption behavior have been investigated by using density functional theoretical calculations. The lowest-lying isomers are cage-like complexes Sc₆O₈ 01 and Y₆O₈ 01, which are energetically much low-lying by at least 40.43 kcal/mol than the other isomers, respectively. Sc₆O₈ 01 can adsorb 26H₂ with gravimetric uptake capacity of 11.64 wt%. The average adsorption energy (ΔE_ave) is 0.12 eV/H₂, which is in the reversible adsorption range. The Y₆O₈ 01 seem have little ability to adsorb hydrogen molecules, because the ΔE_ave Y₆O₈ 01 (1H₂) is just only 0.065 eV. However, the binding capacity increases with the number of adsorbed H₂ increasing. Y₆O₈ 01 can adsorb 32H₂ with ΔE_ave of 0.11 eV/H₂, and the gravimetric uptake capacity is 8.89 wt%. Various characterization methods indicate that both transition metals and nonmetals in Sc₆O₈ 01 and Y₆O₈ 01 can effectively adsorb hydrogen molecules, and these two compounds can be regarded as candidate materials of dihydrogen adsorption under suitable condition.     

Structures and hydrogen storage properties of AeVH3 (Ae = Be,Mg, Ca,Sr) perovskite hydrides by DFT calculations

The capability of hydrogen to be an energy source has made the hydrogen storage as one of the most investigated research fields during the recent years, and novel perovskite materials have become the current focus for hydrogen storage applications. Here we study the AeVH₃ (Ae = Be, Mg, Ca, Sr) perovskite-type hydrides to explorer their potential for hydrogen storage applications using the density functional theory (DFT) implemented CASTEP code along with exchange correlation potential. The study examines the electronic structure, optical properties, elastic features and mechanical stability of the materials. The crystal structure of AeVH₃ compounds is found to be cubic with lattice constant as 3.66, 3.48, 3.76 and 3.83 for Ae = Be, Mg, Ca and Sr compounds, respectively. The calculated electronic structures of these compounds show ionic bonding and no energy bandgap. The mechanical characteristics of compounds are also investigated as to meet the Born stability criterion, these compounds should be mechanically stable. The Cauchy pressure and Pugh criteria revealed that these materials have a brittle character and rather hard. In low energy range, all optical properties are found to be suitable as needed for storing the hydrogen. Furthermore, the gravimetric ratios suggested that all the compounds are suitable for hydrogen storage as a fuel for a longer time and may provide remarkable contributions in diversity of power and transportation applications.     

Density Functional Theory study of the hydrogen storage in a vacancy zone of an iron–nickel cell

Calculations using the SIESTA code have been performed to study the location of one and two hydrogens in a vacancy zone of a Fe₅₀Ni₅₀ cell. H debilitates the original metal–metal bonds by forming strong interactions with the metallic matrix. The Fe–H interaction is stronger than the Ni–H interaction. The H–metal exchange contributes to this process. After first H atom adsorption, the strength of the nearest Fe–Fe, Fe–Ni and Ni–Ni bonds decreases to about 89%, 15% and 1%, respectively. Then, the Fe–Fe bond is the most affected. The adsorption of an additional H atom modified the metal–metal strength in a lesser percent. Then, no additional decohesion is observed in the metallic bonds when two H atoms are present but in this case more metallic bonds are affected. The H–H interaction is small; an H₂ molecule is not formed in the vacancy zone of the Fe₅₀Ni₅₀ cell.     

Micro-mechanism study on the effect of O2 poisoned ZrCo surface on hydrogen absorption performance

Hydrogen storage alloys are usually susceptible to poisoning by O₂, CO, CO₂, etc., which decreases the hydrogen storage property sharply. In this paper, the adsorption characteristics of oxygen on the ZrCo(110) surface were investigated, and the effect of oxygen occupying an active site on the surface on the hydrogen adsorption behavior was discussed. The results show that the dissociation barrier of H₂ is increased by more than 26% after O occupies the active sites on the ZrCo(110) surface, and the probability of H₂ adsorption and dissociation decreases significantly. The adsorption energy of H atoms on the O–ZrCo(110) surface decreased by 18–56%, and the adsorption stability of H decreased. In addition, H atom diffusion on the surface and into bulk are prevented with higher reaction energetic barriers by O occupying active sites. Eventually, the ability of the ZrCo surface to adsorb hydrogen is seriously reduced.