Electronic properties and hydrogen storage application of designed porous nanotubes from a polyphenylene network

Based on a polyphenylene network, a series of porous graphene nanotubes (PGNTs) are created and optimised via density functional theory calculations. The calculated band dispersion of the two-dimensional porous graphene can be tuned by rolling it into nanotube form. To explore the energy application of PGNTs, we studied H₂ adsorptions on metal (Li, Ca, and Na) decorated structures of PGNTs as well as B-substituted PGNTs. The results indicate that both the curvature effect and B substitution can strengthen the metal binding and prevent the metal atoms from clustering. Particularly for H₂ adsorption, modification of the electronic property by the curvature effect is beneficial to provide more accessible space, leading to much higher adsorption energies of H₂ on PGNTs than that on planar porous graphene, which is promising for the practical application of hydrogen storage.     

High-capacity reversible hydrogen storage properties of metal-decorated nitrogenated holey graphenes

Motivated by the need for an effective way of storing hydrogen (H₂), a promising energy carrier, we have performed density functional theory (DFT) calculations with different van der Waals corrections coupled with the statistical thermodynamic analysis and ab initio molecular dynamics (AIMD) on the light-metal decorated nitrogenated holey graphene (C₂N) monolayers. We have found that the decoration by selected light metals (Na, Mg, Ca) improves the H₂ adsorption on the C₂N to the desired levels (>150 meV/H₂). Moreover, the metal dopants strongly bonded with C₂N even at higher doping concentrations, which invalidates the metal clusters formation. Among considered metals, Na and Mg resulted in H₂ storage capacities of 5.5 and 6.9 wt%, respectively, which exceed the target set by the U.S. Department of Energy's for 2025. Thermodynamic analysis and the AIMD simulations were employed to investigate the H₂ sorption at varied conditions of temperature and pressure for practical applications.     

Density functional theory study on hydrogen storage capacity of metal-embedded penta-octa-graphene

H₂ storage capabilities of penta-octa-graphene (POG) adorned by lightweight alkali metals (Li, Na, K), alkali earth metals (Be, Mg, Ca) and transition metals (Sc, Ti, V, Cr, Mn) are studied by density functional theory. Metals considered, with the exception of Be and Mg, can be stably adsorbed to POG, effectively avoiding metal clustering. The average H₂ adsorption energies are calculated in a range from 0.14 to 0.95 eV for Li (Na, K, Ca, Sc, Ti, V, Cr, Mn) decorated POG. Because the H₂ adsorption energies for reversible physical adsorption lie in the range of 0.15–0.60 eV and the desorption temperatures fall in the range of 233–333 K under the delivery pressure, 4Li@POG and 2Ti@POG are found to be the most suitable for H₂ storage at ambient temperature. By polarization and hybridization mechanisms, up to 3 and 5 hydrogen molecules are stably adsorbed around each Li and Ti, respectively. The H₂ gravimetric densities can reach up to 9.9 wt% and 6.5 wt% for Li and Ti decorated POG, respectively. Our findings suggest that, with metal decoration, such a novel two-dimensional carbon-based structure could be a promising medium for H₂ storage.     

Metal adatoms-decorated silicene as hydrogen storage media

Hydrogen storage properties of 10 different adatom decorated silicene are carried out using density functional theory calculations with long-range van der Waals dispersion correction. It is found that the binding energy between metal adatoms and the silicene is greater than the cohesive energy of bulk metal so that clustering of adatom will not occur once it is bonded with silicene. The adsorption of H₂ on Li, Na, K, Mg, Ca, and Au decorated silicene is a weak physisorption. Differently, a weaker chemisorption is responsible for the adsorption of H₂ on Be, Sc, Ti, and V decorated silicene. In particular, silicene with Na, K, Mg, and Ca decorating on both sides leads to 7.31–9.40 wt% hydrogen storage capacity with desirable adsorption energy, indicating that the metal-decorated silicene can serve as a high capacity hydrogen storage medium.     

Ultrahigh reversible hydrogen storage in K and Ca decorated 4-6-8 biphenylene sheet

By applying density functional theory (DFT) and ab-initio molecular dynamics (AIMD) simulations, we predict the ultrahigh hydrogen storage capacity of K and Ca decorated single-layer biphenylene sheet (BPS). We have kept various alkali and alkali-earth metals, including Na, Be, Mg, K, Ca, at different sites of BPS and found that K and Ca atoms prefer to bind individually on the BPS instead of forming clusters. It was found that 2?2?1 supercell of biphenylene sheet can adsorb eight K, or eight Ca atoms, and each K or Ca atom can adsorb 5H_2, leading to 11.90% or 11.63% of hydrogen uptake, respectively, which is significantly higher than the DOE-US demands of 6.5%. The average adsorption energy of H₂ for K and Ca decorated BPS is ?0.24 eV and ?0.33 eV, respectively, in the suitable range for reversible H₂ storage. Hydrogen molecules get polarized in the vicinity of ionized metal atoms hence get attached to the metal atoms through electrostatic and van der Waals interactions. We have estimated the desorption temperatures of H₂ and found that the adsorbed H₂ can be utilized for reversible use. We have found that a sufficient energy barrier of 2.52 eV exists for the movement of Ca atoms, calculated using the climbing-image nudged elastic band (CI-NEB) method. This energy barrier can prevent the clustering issue of Ca atoms. The solidity of K and Ca decorated BPS structures were investigated using AIMD simulations.     

Hydrogen storage on uncharged and positively charged Mg-decorated graphene

The storage of H₂ molecules was studied theoretically on charged and uncharged Mg decorated graphene surfaces using density-functional theory and by incorporating the van der Waals (vdW) interactions. We found that an increase in the number of Mg atoms and H₂ molecule increases the net interaction of the hydrogen molecule with the surface. The Mg-Gr⁺ has the hydrogen storage capacity of up to nine H₂ molecules, with the average adsorption energy of −0.134 eV/H₂. Also, we found that hydrogen molecules play an important role in the interaction between the graphene surface and the Mg atom. The charge density difference analysis showed that electron transfer occurs from H₂ molecules to Mg atom in uncharged system. However, the Bader charge analysis showed that the positive charges in the Mg-Gr⁺ and nH₂-Mg-Gr⁺ systems are concentrated on the Mg atom. When the number of H₂ molecules reaches more than 4, the charge transfer instead occurs from the Mg atom to H₂ molecules as well as to the graphene surface. This results in better interaction between the Mg atom and the Gr⁺ surface.     

Reconnoitring the nature of interaction and effect of electric field on Pd/Pt/Ni decorated 5-8-5/55–77 defected graphene sheet for hydrogen storage

There is plenty of graphene based Hydrogen storage technologies and studies still few questions like ‘what kind of interaction present between Metal-Metal, Metal-Graphene, Metal-Hydrogen and Graphene-Metal and a possible way of controlling it to enhance H₂ adsorption’ are not revealed properly. Similarly, the chosen metal atoms Pd, Pt and Ni are widely reported as a promising catalyst yet there is no conclusive evidence to show the best among three atoms. Thus, in this present work 5-8-5 and 55–77 defected graphene is decorated with the Pd, Pt and Ni metal atoms to adsorb Hydrogen molecules. The obtained results have shown that the better adsorption of H₂ molecule depends on Metal-Metal and Metal-Graphene interaction. Similarly, the adhesive force between Pt and 5-8-5/55–77 sheets are slightly higher than the Pd and Ni atoms. Pd–Pd (−0.47 eV) and Pt–Pt (−1.99 eV) interaction values on the surface of 5-8-5 sheets are slightly lesser in magnitude than the Pd–C (−1.14 eV, −1.19 eV) and Pt–C (−2.42 eV, −2.55 eV) interactions. The topological analysis results exhibit the partially covalent nature of interaction and it confirms that the adhesive force between Metal-Graphene is higher than the cohesive force between Metal-Metal on 5-8-5 and 55–77 sheets. The electrophilicity results of Pd, Pt and Ni decorated sheets show that the two Pt decorated 5-8-5 sheet has higher electrophilicity value of 16.782 eV which is considerably higher than other sheets and this particular 5-8-5-Pt₂ system has higher H₂ adsorption energy value of −1.696 eV. The overall pattern of H₂ adsorption on chosen three metals are Pt > Ni > Pd and our results show that both strong Metal-Metal and Metal-Graphene interactions lead to poor adsorption activity. The metals are strongly polarizing the H₂ molecules which lead to good adsorption. Further, the results confirm that the π orbitals of Metal and Graphene play a major role in the adsorption of excessive H₂ molecules. In order to enhance and control the H₂ adsorption energy, a positive electric field is applied to the system. This applied electric field enhances the polarization which leads to H–H bond elongation and strong adsorption. From the obtained results, it is conclusive that the 5-8-5-Pt system has shown good response for the supplied electric field with the maximum adsorption energy value of −5.23 eV. Comparatively, the 5-8-5 systems are responding well for the applied electric field by increasing the adsorption energy than 55–77 systems.     

Effect of nitrogen induced defects in Li dispersed graphene on hydrogen storage

As a candidate for hydrogen storage medium, Li decorated graphene with experimentally realizable nitrogen defects was investigated for geometric stability and hydrogen capacity using density functional theory (DFT) calculations. Among the three types of defective structures, it is expected that Li metal atoms are well dispersed on the graphene sheets with pyridinic and pyrrolic defects without clustering as the bond strength of Li on pyridinic and pyrrolic N-doped graphene layers is higher than the cohesive energy of the Li metal bulk. The two stable structures were found to exhibit hydrogen uptake ability up to three H₂ per Li atom. The binding energies of the hydrogen molecules for these structures were in the range of 0.12–0.20 eV/H₂. These results demonstrate that a Li/N-doped graphene system could be used as a hydrogen storage material.     

Computational insights of alkali metal (Li / Na / K) atom decorated buckled bismuthene for hydrogen storage

The mechanism of hydrogen molecule adsorption on 2D buckled bismuthene (b-Bi) monolayer decorated with alkali metal atoms was studied using density functional theory based first principles calculations. The decorated atoms Li, Na and K exhibited distribution on surface of b-Bi monolayer with increasing binding energy of 2.6 eV, 2.9 eV and 3.6 eV respectively. The adsorption of H₂ molecule on the slabs appeared stable which was further improved upon inclusion of van der Waals interactions. The adsorption behaviour of H₂ molecules on the decorated slabs is physisorption whereas the slabs were able to bind up to five H₂ molecules. The average adsorption energy per H₂ molecules are in range of 0.1–0.2 eV which is good for practical applications. The molecular dynamics simulation also confirmed the thermodynamic stabilities of five H₂ molecules adsorbed on the decorated slabs. The storage capacity values are found 2.24 wt %, 2.1 wt %, and 2 wt %, for respective cases of Li, Na and K atoms decorated b-Bi. The analysis of the adsorbed cases pointed to electrostatic interaction of Li and H₂ molecule. The adsorption energies, binding energies, charge analysis, structural stability, density of states, and hydrogen adsorption percentage specifies that the decorated b-Bi may serve as an efficient hydrogen storage material and could be an effective medium to interact with hydrogen molecules at room temperature.     

The reversible hydrogen storage abilities of metal Na (Li,K, Ca,Mg, Sc,Ti, Y) decorated all-boron cage B28

The density functional theory is used to study the hydrogen storage abilities of alkali metal Li (Na, K), alkaline-earth metal Mg (Ca), and transition metal Ti (Ti, Sc, Y) decorated B₂₈, which is the possible smallest all-boron cage and contains one hexagonal hole and two octagonal holes. The most stable structure of B₂₈ explored by the calypso search is as same as that explored by Zhao et al. [Nanoscale 7(2015)15086]. It is calculated that the hollow sites outside of the cavities should be the most stable for all metals except for Ti. The average adsorption energy of H₂ molecules (E_ad) adsorbed by each Na (Ca, K, Mg, Sc, Y and Li) atom outside of the B₂₈ cage are in the range from 0.2 to 0.6 eV, which is suitable for hydrogen storage under near-ambient conditions. However, the largest hydrogen gravimetric density (HGD) for the B₂₈Sc₃-12H₂ structure is smaller than the target of 5.5 wt% by the year 2017 specified by the US Department of Energy (DOE). Therefore, the metal Ti (Sc) decorated all-boron cage B₂₈ should not be good candidates for hydrogen storage. The calculated desorption temperature and the molecular dynamic simulation indicate that the B₂₈M₃-nH₂ (M = Na, Li, Ca, K, Mg, Y) structures are easy to desorb the H₂ molecules at the room temperature (T = 300 k). Furthermore, the B₂₈ cages bridged by the sp²-terminated B₅ chain can hold Na (Li, Ca, K, Mg, Y) atoms to capture hydrogen molecules with moderate E_ad and HGD. These findings suggest a new route to design hydrogen storage materials under the near-ambient conditions.     

Ca and K decorated germanene as hydrogen storage: An ab initio study

The hydrogen storage capacity and performance of Ca and K decorated germanene were studied using density functional theory calculation. The Ca and K adatoms were found to be sufficiently bonded to the germanene without clustering at the hollow site. Further investigation has shown an ionic bonding is apparent based on the charge density difference and Bader charge analysis. Upon adsorption of H₂ on the decorated germanene, it was found that the Ca and K decorated systems could adsorb 8 and 9 H₂ molecules, respectively. The adsorption energies of H₂ molecules were within the Van der Waals energy (400–435 meV), suggesting weak physisorption. The charge density profile revealed that the electron of H₂ moved toward the adatom decoration without leaving the local region of H₂. This suggests that a dipole-dipole interaction was apparent and consistent with the energy range found. Finally, the gravimetric density obtained from the adsorption of H₂ on the decorated germanene shows that this material is a potential for H₂ storage media.     

Superalkali NLi4 decorated graphene: A promising hydrogen storage material with high reversible capacity at ambient temperature

This paper investigates the decoration of superalkali NLi₄ on graphene and the hydrogen storage properties by using first principles calculations. The results show that the NLi₄ units can be stably anchored on graphene while the Li atoms are strongly bound together in the superalkali clusters. Decoration using the superalkali clusters not only solve the aggregation of metal atoms, it also provide more adsorption sites for hydrogen. Each NLi₄ unit can adsorb up to 10 H₂ molecules, and the NLi₄ decorated graphene can reach a hydrogen storage capacity 10.75 wt% with an average adsorption energy ?0.21 eV/H₂. We also compute the zero-point energies and the entropy change upon adsorption based on the harmonic frequencies. After considering the entropy effect, the adsorption strengths fall in the ideal window for reversible hydrogen storage at ambient temperatures. So NLi₄ decorated graphene can be promising hydrogen storage material with high reversible storage capacities.     

Light metal functionalized two-dimensional siligene for high capacity hydrogen storage: DFT study

In this work, the hydrogen storage capacities of two-dimensional siligene (2D-SiGe) functionalized with alkali metal (AM) and alkali-earth metal (AEM) atoms were studied using density functional theory calculations. One AM (Li, Na, K) or AEM (Be, Mg, Ca) atom was placed on the 2D-SiGe surface, and several H₂ molecules were placed in the vicinity of the adatom. The results demonstrate that the most favorable siligene site for the adsorption of Li, Na, K and Be atoms is the hollow site, while for the Mg and Ca atoms is the down site. The AM atoms are the only ones with considerable binding energies on the SiGe nanosheets. Pristine 2D-SiGe slightly adsorbs one H₂ molecule per hollow site and, therefore, it is not suitable for hydrogen storage. In some of the AM- and AEM-decorated 2D-SiGe, several hydrogen molecules can be physisorbed. In particular, the Na-, K- and Ca-functionalized 2D-SiGe can adsorb six hydrogen molecules, whereas Li and Mg atoms adsorbed three hydrogen molecules, and the Be adatom only adsorbed one hydrogen molecule. The complexes formed by hydrogen molecules adsorbed on the analyzed metal decorated 2D-SiGe are energetically stable, indicating that functionalized 2D-SiGe could be an efficient molecular hydrogen storage media.     

Hydrogen storage capacities of alkali and alkaline-earth metal atoms on SiC monolayer: A first-principles study

A detailed theoretical Density-Functional-Theory-based investigation of hydrogen adsorption on silicon carbide monolayers (SiC-ML) decorated with alkali and alkaline-earth metal atoms is presented. The results show that the favourable position for all adsorbed metal atoms is above a Si atom. These metal atoms are chemisorbed to the SiC-ML, except for Mg which is physisorbed. The adsorbed atoms act in turn as adsorption sites for H₂ molecules. The single-sided K-functionalized SiC-ML can store up to six H₂ molecules. For double-side K-decorated SiC-ML, up to ten H₂ molecules can be captured. In all cases, the H₂ molecules are physisorbed. This is beneficial because the breaking of chemical bonds, which otherwise would be needed to make use of the stored H₂, is energetically expensive. These results find decorated SiC-ML as a promising material for hydrogen storage systems.     

Ammonia decomposition over nickel catalysts supported on alkaline earth metal aluminate for H2 production

The Ni catalysts supported on alkaline earth metal aluminate compounds, Ni/AM-Al-O (AM = Mg, Ca, Sr, Ba) were synthesized to investigate the influence of their basic property on NH₃ decomposition activity. The basic strength of the catalysts was confirmed to correspond to that of added alkaline earth metal in the support materials (Ni/Mg–Al–O < Ni/Ca–Al–O < Ni/Sr–Al–O < Ni/Ba–Al–O) from CO₂-TPD measurement. This basic strength of the catalysts could influence the catalytic activity for NH₃ decomposition, which increased in order of the Ni/Mg–Al–O < Ni/Ca–Al–O < Ni/Sr–Al–O < Ni/Ba–Al–O catalysts. NH₃-TPSR showed that the strong basic property weakened H₂ adsorption but slightly strengthened N₂ adsorption for the catalysts except for the Ni/Mg–Al–O catalyst. From the kinetic analysis, the absolute value of the H₂ reaction order decreased with increasing basic strength of the catalysts, indicating that the strong basic property of the catalysts could alleviate the H₂ inhibition in ammonia decomposition.     

Effect of defects and dopants in graphene on hydrogen interaction in graphene-supported NaAlH4

Carbon-based materials have attracted great attention over the past few years due to their role as a support for sodium alanate improving the kinetics of H₂ release/uptake. Herein, we used graphene with defects and various dopants to simulate the carbon materials and performed a periodic density functional theory study on the impact of the modifications in the graphene substrates on the hydrogen interaction in, and hydrogen desorption from, the highly dispersed sodium alanate. Our results showed that the impact of various defects and dopants can be categorized in groups: (i) Pristine graphene and pentagon–heptagon (5–7) pairs defective graphene, as well as N- and S-doped graphene substrates show a weak interaction with the supported sodium alanate cluster, as reflected in the geometry change of the supported cluster and charge transfer between the supported cluster and the substrate. These defects and dopants do not promote H₂ formation and desorption. (ii) Carbon vacancies, as well as B and Cl dopants, cause instantaneous H₂ formation in supported NaAlH₄ upon relaxation. (iii) O-, P-, F- and OH-doped graphene substrates led to the formation of a meta-stable di-hydrogen state with a H–H distance of ∼0.96 Å. There is an activation barrier between the meta-stable di-hydrogen state and the most stable state with H₂ being formed. Furthermore, our results with the optB88-vdW functional show that van der Waals interaction strengthens the binding of the cluster on the substrates by 0.9–1.4 eV over the PBE results but does not alter the relative stability of the system.     

Effect of boron substitution on hydrogen storage in Ca/DCV graphene: A first-principle study

By using density functional calculations, the effects of boron are investigated in the new hydrogen storage systems, which are formed by substituting different numbers of boron atoms to the first (BDDCV-F) and the second (BDDCV-S) neighbor of double carbon-vacancy (DCV). The layered host systems of boron-substituted DCV graphene are decorated with Ca metal to increase the number of adsorbed H₂ molecules. Storing of H₂ applications are performed by using two coordinate algorithms as CLICH (Cap-Like Initial Conditions for Hydrogens) and RICH (Rotational Initial Conditions for Hydrogens). The adsorption properties of (1–14) H₂ molecules on the constructed systems are examined. The results for BDDCV-F and BDDCV-S boron-doped systems are compared with each other and those of the pure-DCV graphene. To compare the stabilities of BDDCV-systems, the formation energies are calculated. It is concluded from Mulliken charge analysis, the partial density of states and electron density differences that boron substitution process to different locations of the DCV graphene plays a crucial role on the charge transfer between Ca atom the layered host system, ionic nature and the binding properties of the systems. The herringbone-like anisotropic electron density is transformed to isotropic density with the substitution of the boron atoms. Then, the electric field, which is induced by ionic interactions and governs H₂ adsorption processes, is changed and intensified along with the sheet. In this way, it can be achieved more effective H₂ adsorption. It is seen from the adsorption energies of single- and double-side Ca-decorated systems that the processes of boron-substitution and Ca-decoration can considerably improve the hydrogen storage capability of pure-DCV graphene system, thus (8 and 10)H₂ can be adsorbed per Ca-atom in these-type systems. The high gravimetric density of 5.80% is calculated, although larger cell and empty surface states. Moreover, the average desorption temperatures are calculated by using van't Hoff equation, and it is seen that the DCV including boron-substituted systems have closer desorption temperatures to the room temperature than pure-DCV. To check the H₂ desorption of the systems, molecular dynamics simulations are performed at 200 K and 300 K temperatures.     

Li-decorated double vacancy graphene for hydrogen storage application: A first principles study

Lithium decoration is an effective strategy for improving the hydrogen adsorption binding energy and the storage capacity in carbon nanostructures. Here, it is shown that Li-decorated double carbon vacancy graphene (DVG) can be used as an efficient hydrogen storage medium by means of Density Functional Theory (DFT) based calculations. The Li binding energy in DVG is 4.04 eV, which is much higher than that of pristine graphene. A maximum of four hydrogen molecules adsorb on Li decorated on one side of DVG and this leads to a gravimetric storage capacity of 3.89 wt% with an average adsorption binding energy of 0.23 eV/H₂. When Li is decorated on both sides of DVG, the gravimetric storage capacity reaches 7.26 wt% with a binding energy of 0.26 eV/H₂ which shows that desorption would take place at ambient conditions.     

Hydrogen storage capacity of Al,Ca, Mg,Ni, and Zn decorated phosphorus-doped graphene: Insight from theoretical calculations

In this work, adsorption of molecular hydrogen on five different metals: Aluminum, Calcium, Magnesium, Nickel and Zinc decorated phosphorus-doped graphene have been investigated using density functional theory (DFT) computation at the PBE0-D3BJ/def2svp method. From literature reviews, phosphorus doped graphene are potential candidates for hydrogen storage. Herein, theoretical investigation on the changes in structural and electronic properties of the studied materials was conducted. Natural bond orbital (NBO) analysis was employed to study the intermolecular and intra-molecular interactions arising from chemical bonds in the studied systems. In addition, the density of states (DOS) plots shows notable individual orbital contribution and hybridization between the decorated metals and the phosphorus-doped graphene which is also responsible for the adsorption of hydrogen. Based on the frontier molecular orbital analysis, results indicates that Al and Ni surfaces possess excellent structural and electronic properties with lower values of chemical hardness and ionization with adsorption energy values of 1.924eV and 1.236eV obtained for both surfaces potential indicating better conductivity and excellent H₂ adsorption potential. The obtained results shows the suitability of the Al and Ni decorated phosphorus-doped graphene for hydrogen storage.