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.     

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.     

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.     

A comparative study of the reversible hydrogen storage behavior in several metal decorated graphyne

In virtue of the first-principle calculations, the hydrogen storage behavior in several metal decorated graphyne was investigated. It is found that the hydrogen storage capacity can be as large as 18.6, 10.5, 9.9 and 9.5 wt% with average adsorption energy of about −0.27, −0.36, −0.76 and −0.70 eV/H₂ for Li, Ca, Sc, Ti decorated graphyne, respectively. The results suggest potential candidates for hydrogen storage at ambient condition. The adsorption mechanism for H₂ on metal coated graphyne was mainly attributed to the polarization induced by electrostatic field of metal atoms on graphyne and the hybridization between the metal atoms and hydrogen molecules. Furthermore, the formation of super-molecules of hydrogen can enhance the adsorption energy.     

Sc and Ti-functionalized 4-tert-butylcalix[4]arene as reversible hydrogen storage material

Using the idea of metal functionalized material for H₂ storage, 4-tert-butylcalix[4]arene (CA) functionalized with Sc and Ti atoms are explored. The first principles density functional theory (DFT) with M06 functional and 6-311G(d,p) basis set is used to explore the hydrogen storage properties of metal functionalized CA. Sc and Ti strongly binds with CA by Dewar coordination with high binding energy. It is found that maximum four hydrogen molecules are adsorbed on each metal site in Sc and Ti functionalized CA. Hydrogen molecules are adsorbed on metals by Kubas and Niu-Rao-Jena mechanism. In Sc functionalized CA system all 4 hydrogen molecules on each Sc bind in molecular fashion while on each Ti in Ti functionalized CA, the first hydrogen molecule binds in dissociative fashion and remaining three hydrogen molecules bind in a molecular form. The stability of Sc and Ti functionalized CA is studied by computing conceptual DFT parameters, which obeys maximum hardness and minimum electrophilicity principle. Hirshfeld charge analysis and electrostatic potential map explore the charge transfer mechanism during the hydrogen adsorption. Born-Oppenheimer molecular dynamics simulations are performed at temperature range 200–473 K to study the stability of the system and the reversibility of adsorbed hydrogen from the system. The calculated H wt% is found to be 10.3 and 10.1, respectively for Sc and Ti functionalized CA systems on complete H₂ saturation. This study explores that Sc and Ti functionalized CA systems are efficient reversible hydrogen storage material.     

DFT study of crystal structure and electronic properties of metal-doped AlH3 polymorphs

AlH₃ has been considered for a long time as a hydrogen storage material with suitable gravimetric and volumetric density for practical applications. Among eight AlH₃ polymorphs observed so far, in this work we focus our attention on an investigation of the effects of various metal dopants in α- and β-AlH₃, to perceive a way of enhancing them. Substitutional incorporation of the metal dopants (Li, Sc, Ti, Cu, Cr, Fe, Nb, Mo, Zn, or Zr) is considered, as well as interstitial doping with Li, Sc, Ti, Cu, and Zr. The density functional theory (DFT) (using GGA-PW91) approach is used to address the crystal structure, bonding, dopant stability, and changes in hydrogen desorption energy. In addition, the kinetics of hydrogen desorption is also considered for several interstitially doped cases, by calculating the stability of native point defects. Promising results are presented for Zr, Ti, and Sc – doped hydrides. Doped hydrides, here studied, are considered as n- or p-type semiconducting materials, enabling wider application overcoming hydrogen storage scope.     

Effect of metal atom substitutions in Li based hydrides for hydrogen storage

By using density functional theory and the full-potential linearised augmented plane wave method, the effect of alkali (AM) and transition metal (TM) atom substitutions in Li based hydrides (Li₇XH₈ X (AM) = Na, K, Rb and X (TM) = Ti, V, Cr) was investigated, by studying their formation energies and electron properties, aiming at improving hydrogen storage performance. The calculated formation energy values indicate that there is a gradual degradation in stability due to alkali substitutions from Na to Rb and to transition metal substitutions from Ti to Cr in Li₇XH₈. The found degradations of stability in Li₇XH₈ were better compared concomitantly with corresponding gravimetric hydrogen storage variations. The less stable phase with least variation in gravimetric hydrogen storage was found to be Li₇CrH₈ among all alkali and transition metal atom substitutions. The density of states and the electron density support our observations.     

Hydrogen trapping efficiency of Li decorated porous boron fullerene B38: The first-principles study

The demand for clean renewable energy is urgent in current. The hydrogen application is difficult mainly due to the ratively low capacity in the storage medium. In this work, the adsorption and desorption of the hydrogen molecules by the Li atoms decorated B₃₈ cage are studied by the density functional theory. The calculated largest binding energy of one Li atom (2.68 eV and 2.58 eV) is upon the hexagonal hole of the B₃₈ cage, which is much larger than the experimental cohesive energy of bulk Li (1.63 eV). Each Li atom in the outside of the B₃₈ cage can adsorb up to four H₂ molecules. The E_ad of B₃₈(Li-nH₂)₄ decreases from the 0.22 eV for n = 1 to the 0.11 eV for n = 4. The B₃₈(Li–4H₂)₄ structure achieves the 6.85 wt% hydrogen gravimetric density, which is higher than the goal of 5.5 wt% before 2017 set by the United States Department of Energy. The almost the same partial density of states for the fifth H₂ molecule as that of the isolated H₂ molecule, the longer 4.5 Å distance between the fifth H₂ molecule and the Li atom, together with the small NBO charges all reveal the weak electronic field around the Li⁺, which can interpret the weak H₂ adsorption mechanism. Finally, the B₃₈Li₄ structure can easily release 9H₂ molecules at 373 K known from the molecular dynamic simulation and practically trap about 1.08H₂ molecules at 373 K/3 atom condition calculated by the grand partition function. Thus, its reversible practical HGD of B₃₈Li₄-14.34H₂ is 6.18 wt%, which is almost the same value as the theoretical 6.85 wt% for B₃₈(Li–4H₂)₄. Our studies will be the strong theory basis for the future application in hydrogen storage material development.     

Alkali,alkaline earth and transition metal doped B6H6 complexes for hydrogen storage

Alkali, alkaline earth and transition metal doped B₆H₆ complexes are considered for the hydrogen storage. Density functional theory (DFT) and second order Møller–Plesset methods with 6–311++G** basis set have been used for the study. B₆H₆Li, B₆H₆Be, B₆H₆Sc, B₆H₆Li₂, B₆H₆Be₂, B₆H₆Sc₂ complexes can interact with maximum three, two, four, six, four and eight H₂ molecules respectively with respective H₂ uptake capacity of 7.2, 4.8, 6.5, 12.5, 8.3 and 9.1 wt%. This uptake capacity is well above the target set by the U.S. Department of Energy by 2020 except for the B₆H₆Be complex. Thermo chemistry calculations are carried out to estimate the Gibbs free energy corrected H₂ adsorption energy which reveals whether adsorption of hydrogen on these complexes is favourable or not at different temperature. It is observed that H₂ adsorption on all the six complexes are unfavourable at ambient conditions where as it is favourable below 150, 135, 75, and 50 K on B₆H₆Sc, B₆H₆Be, B₆H₆Li and B₆H₆Li₂ complexes respectively. Various interaction energies in H₂ adsorbed complexes are obtained using Many-body analysis approach. The H₂ desorption temperature for the B₆H₆Li, B₆H₆Be, B₆H₆Sc, B₆H₆Li₂, B₆H₆Be₂ and B₆H₆Sc₂ complexes is found to be 25, 165, 265, 10, 265 and 373 K respectively.     

Lithium decoration of boron-doped hybrid fullerenes and nanotubes as a novel 3D architecture for enhanced hydrogen storage: A DFT study

A three dimensional (3D) dumbbell-like nanostructure composed by interconnected fullerenes and nanotubes with Lithium decoration and boron-doping (37Li@C₁₃₉B₃₁) has been proposed in virtue of density functional theory (DFT) and first-principles molecular dynamics (MD) simulations which shows excellent geometric and thermal stability. First-principles calculations are performed to investigate the hydrogen adsorption onto the 37Li@C₁₃₉B₃₁. The results indicate that B substitution can improve the metal binding and the average binding energy of 37 adsorbed Li atoms on the C₁₃₉B₃₁ (2.79 eV) is higher than the cohesive energy of bulk Li (1.63 eV) suppressing the clustering. Meanwhile, the H₂ storage gravimetric density of 178H₂@37Li@C₁₃₉B₃₁ reaches up to 15.9 wt% higher than the year 2020 target from the US department of energy (DOE). The average adsorption energy of H₂ molecules falls in a desirable range of 0.18–0.27 eV. Moreover, grand canonical ensemble Monte Carlo (GCMC) simulations reveal that at room temperature the hydrogen gravimetric density (HGD) adsorbed on 37Li@C₁₃₉B₃₁ reaches up to 11.6 wt% at 100 bars higher than the DOE 2020 target. Our multiscale simulations indicate that our proposed nanostructure provides a promising medium for hydrogen storage.     

Investigation of hydrogen storage on Sc/Ti-decorated novel B24N24

Inspired by the TM−N₄ coordination environment in single-atom catalysts, four novel TM-decorated B₂₄N₂₄ (TM = Sc, Ti) fullerenes with six TM−N₄ or TM−B₄ units are designed. Molecular dynamic simulations confirm that the four TM₆B₂₄N₂₄ fullerenes are thermodynamically stable. Their hydrogen storage properties were investigated using density functional theory calculations. Sc/Ti atoms bind to the N₄/B₄ cavities with an average interaction energy of 6.30–11.96 eV. Hence, the problem of clustering can be avoided. 36H₂ could be adsorbed with average hydrogen adsorption energies of 0.18–0.55 eV. The lowest hydrogen desorption temperatures at atmospheric pressure for Sc₆B₂₄N₂₄(N₄)–36H₂, Sc₆B₂₄N₂₄(B₄)–36H₂, Ti₆B₂₄N₂₄(N₄)–36H₂, and Ti₆B₂₄N₂₄(B₄)–36H₂ are 255 K, 318 K, 243 K, and 408 K, respectively. The maximum hydrogen gravimetric densities of the Sc₆B₂₄N₂₄ and Ti₆B₂₄N₂₄ systems are 7.74 wt% and 7.50 wt%, respectively. Therefore, the novel Sc₆B₂₄N₂₄ and Ti₆B₂₄N₂₄ could be suitable as potential hydrogen storage materials at ambient temperature.     

An alternative approach to the synthesis of NaB3H8 and Na2B12H12 for solid electrolyte applications

Alkaline or alkaline earth octahydrotriborate M(B₃H₈)_x and dodecahydro-closo-dodecaborate M_xB₁₂H₁₂ (M = Li, Na, Mg or Ca with x = 1 or 2) have recently attracted a lot of interest for hydrogen storage and solid electrolyte applications. Nevertheless, their syntheses are still a roadblock for large scale applications. In this paper we propose a novel approach for their syntheses starting from the cheapest borohydride NaBH₄. The process involves first the solvothermal synthesis of tetrabutylammonium octahydrotriborate (C₄H₉)₄NB₃H₈ (TBAB₃H₈) being the basis for the syntheses of the others boranes. Starting from TBAB₃H₈, we have synthesized pure and unsolvated NaB₃H₈ by salt metathesis reaction with sodium tetraphenylborate. Then, we have successfully obtained Na₂B₁₂H₁₂ by solvothermal decomposition of NaB₃H₈. This approach has shown to be quantitative and reproducible, which could lead to the development of these boranes in real life applications.     

Multiple Ti and Li doped carbon nanoring for hydrogen storage

Multiple Ti and Li atom doped carbon nanorings are considered for hydrogen storage using density functional theory for the first time. There are five six membered carbon rings bonded through C–C bond in a carbon nanoring. Formation energy values show that both, Li as well as Ti atom doped carbon nanoring, are thermodynamically stable structures. Cohesive energy values indicate that Li and Ti atom doped carbon nanoring structures are more stable than undoped carbon nanoring. No clustering of metal atoms occurs in metal doped carbon nanorings which usually reduces the hydrogen storage capacity of a material. Li atom doped carbon nanoring is not suitable for hydrogen storage even at very low temperature at 1 atm pressure as well as at high pressure at room temperature. Ti atom doped carbon nanoring is suitable for hydrogen storage below 225 K and 1 atm pressure as well as at high pressure at room temperature. H₂ desorption temperature is found to be 113 and 450 K for Li and Ti atom doped carbon nanoring respectively. H₂ molecules interact strongly with Ti atom doped carbon nanoring than Li atom doped carbon nanoring that results in higher H₂ desorption temperature for the former than the latter.     

Transition metal decorated covalent triazine-based frameworks as a capacity hydrogen storage medium

From ab initio density functional theory (DFT) calculations, the structural stability and hydrogen adsorption capacity of transition metal (TM, TM = Sc, Ti, V, Cr, Mn) decorated covalent triazine-based framework (CTF) are discussed. It is found that by calculation, these TM atoms can adsorb on the CTF sheet without clusters. The Sc, Ti, V, Cr and Mn decorated CTF are predicated to bind five, four, three, three and two of hydrogen molecules. We found that Sc and Ti decorated CTF are suitable candidates for effective reversible hydrogen storage at near ambient condition, whereas V, Cr and Mn decorated CTF are not promising materials due to too large average bind energies per hydrogen molecule.     

Ethanol steam reforming over cobalt catalysts: Effect of a range of additives on the catalytic behaviors

In this study, we made an effort to modify the Co/Al₂O₃, a typical catalyst for various steam reforming reactions, with Na, Mg, Zr, La, Ce and the elements from K to Zn in the periodic table of elements, in order to obtain a whole picture of impacts of these commonly used additives on catalytic behaviors of cobalt-based catalyst on one basis. The results showed Na, Mg, Zr, Sc, K and Cr addition increased specific area of the catalysts by creating new mesopores or decreasing pore radius while Ca filled the pores. The additives could also react with either cobalt species or alumina. CoV₂O₆, MgAl₂O₄, MnAl₂O₄ NiAl₂O₄ and ZnAl₂O₄ and Co₇Fe₃ alloy were formed during calcination/reduction of the catalysts. Na, K, Cu, Ni or Ce addition promoted the catalytic activity, while Mg, Ca, Sc, Ti, V, Cr, Mn suppressed. As for coking, Na, K, Ca, Fe, Zn, La helped to suppress coking while Cu or Zr enhanced. The coke became more aromatic over the modified catalysts. V or La promoted formation of the coke with graphite structure, while Cu or Fe was the opposite. The additives also affected the morphology of the coke or diameter of the carbon nanotubes formed. The in-situ DRIFTS studies showed that ethanol could be dehydrated to form C=C and dehydrogenated to form acetyl species at 100 °C. The addition of Na suppressed the formation of the acetyl species, while Cu addition promoted the acetyl species, C=O and C=C formation, leading to the high tendencies of Cu–Co/Al₂O₃ towards coking.     

TM dopant-induced H-vacancy diffusion kinetics of sodium-lithium alanates: Ab initio study for hydrogen storage improvement

We present a hydrogen storage mechanism of the surface and bulk Na–Li–Al hydrides substituted by the transition metal (TM) dopants such as Ni, Cu, Ag, and Zn. The host hydrides of interest, namely, NaAlH₄, LiAlH₄, Na₃AlH₄, Li₃AlH₄, and Na₂LiAlH₄ are found to be stable compositions at ambient pressure. Hydrogen vacancy mechanisms of the host hydrides with the TM dopants are investigated using ab initio calculations. Remarkably, the results show the enhancement of the internal mechanism for hydrogen storage in the Na–Li–Al complex hydrides. Doping of Ni or Zn mainly reduces the energy barrier of diffusion kinetics in the host Na–Li–Al hydrides, leading to the improvement of the hydrogen storage efficiency of the host Na–Li–Al hydrides. Therefore, hydrogen vacancy diffusion kinetics in the Na–Li–Al hydrides can be induced by adding the Ni and Zn dopants.     

Sc/Ti-decorated and B-substituted defective C60 as efficient materials for hydrogen storage

Doping heteroatoms and producing defects are perfect methods to improve the hydrogen storage property of TM-decorated carbon materials. In this view, four novel Sc/Ti-decorated and B- substituted defective C₆₀ fullerenes (B₂₄C₂₄) are explored. The special stability, large specific surface, uniform distribution of the metal and positively charged states make these four fullerenes have high hydrogen storage capacities. Especially, each Sc atom in Sc₆B₂₄C₂₄(B₄) can adsorb up to five H₂ molecules with a storage capacity of 6.80 wt %. The adsorbed H₂ molecules in Sc₆B₂₄C₂₄(B₄)–30H₂ begin to relax at 190 K and are 100% released at 290 K. Moreover, a comparative study is carried out for hydrogen storage properties of Sc-decorated B₄, C₄, or N₄ coordination environments. These results provide a new focus on the nature of B-, and N-substituted defective carbon nanomaterials.     

Application of chemical geothermometers to thermal springs of the Maghreb,North Africa

In order to assess the geothermal potential in the Maghrebian region, several studies have been undertaken in the three countries concerned, Morocco, Algeria and Tunisia, during the past decade. Research programmes have considered the surface evidence (thermal springs) and underground thermal information from deep and shallow wells. The main chemical characteristics of the sampled thermal springs and the results of the application of geothermometers as result from these studies are presented. Of the 238 inventoried thermal springs, 169 have been selected, on the basis of complete water analyses and acceptable ionic balances. Measured temperatures range from 22.5 to 98°C, thermal indexes from 0.5 to 78°C and salinities from 0.13 to 52.5 g/L. Most studied springs are sodium-chloride type waters. These basic data allow identification of the main thermal anomalies in the Maghrebian zone, which are located in regions of the Libyan-Tunisian, Algerian-Moroccan and Algerian-Tunisian frontiers, of northern Tunisia, the Eastern Rif and the northern part of the Saharan Atlas.Several chemical geothermometers have been applied to selected springs: NaK, NaKCa, NaKCaMg, Na/Li, Mg/Li, K²/Mg, quarts, chalcedony (Fournier) and chalcedony (Arnorsson). The NaK, NaKCa, NaKCaMg, Na/Li and Mg/Li geothermometers seem to give unreliable results, while K²/Mg and silica temperatures are apparently reasonable. However, dissolved silica seems to be governed by quartz solubility for some thermal springs and by chalcedony solubility for others. The results are tentatively compared with known geothermal gradients and geological features.     

In-situ formation of ultrafine MgNi3B2 and TiB2 nanoparticles: Heterogeneous nucleating and grain coarsening retardant agents for magnesium borate in Li–Mg–B–H reactive hydride composite

Li–Mg–B–H reactive hydride composite (RHC) has attracted extensive attention over the past decades for its extremely high hydrogen storage capacity (11.5 wt%). But the sluggish desorption kinetics for the second step dehydrogenation reaction need to be further improved. Herein, short rod-like TMTiO₃ (TM = Co, Ni) bimetallic oxides, which contain two kinds of transition metal elements, were synthesized and introduced into Li–Mg–B–H RHC for the first time. The NiTiO₃ exhibits excellent catalytic effect on the hydrogen desorption kinetic performance of Li–Mg–B–H RHC, and the incubation period for the second step dehydrogenation reaction is eliminated completely by reducing the apparent activation energy for the generation of MgB₂ from 296 kJ/mol to 269 kJ/mol. The NiTiO₃ doped Li–Mg–B–H RHC can desorb about 9.0 wt% H₂ without obvious attenuation of kinetic performance in five cycles. Mechanism analyses reveal that the in-situ generated nano-sized MgNi₃B₂ and TiB₂ species (∼5 nm) both meet the critical value ( < 10%) of the edge-to-edge matching model (5.77% for MgNi₃B₂ and 2.22% for TiB₂), which play a significant role in supporting the nucleation of MgB₂. Meanwhile, the extremely fine MgNi₃B₂ and TiB₂ heterogeneous nucleation sites can inhibit the excessive growth for a single crystal nucleus of MgB₂. The heterogeneous nucleation and grain refinement mechanisms caused by the novel bimetallic oxide could provide alternative insights into designing an in-situ generated nano-sized catalytic hydrogen storage system with enhanced kinetics and cyclic stability for hydrogen-fueled applications.