Chemisorption,physisorption and hysteresis during hydrogen storage in carbon nanotubes

The question of chemisorption versus physisorption during hydrogen storage in carbon nanotubes (CNTs) is addressed experimentally. We utilize a powerful measurement technique based on a magnetic suspension balance coupled with a residual gas analyzer, and report new data for hydrogen sorption at pressures of up to 100 bar at 25 °C. The measured sorption capacity is less than 0.2 wt.%, and there is hysteresis in the sorption isotherms when multi-walled CNTs are exposed to hydrogen after pretreatment at elevated temperatures. The cause of the hysteresis is then studied, and is shown to be due to a combination of weak sorption – physisorption – and strong sorption – chemisorption – in the CNTs. Analysis of the experimental data enables us to calculate separately the individual hydrogen physisorption and chemisorption isotherms in CNTs that, to our knowledge, are reported for the first time here. The maximum measured hydrogen physisorption and chemisorption are 0.13 wt.% and 0.058 wt.%, respectively.     

The optimal adsorption pathway of H2 molecules on Ti-Acetylene/ ethylene compounds: A DFT study

Ti-Acetylene/Ethylene complexes were used to be considered as a potential high capacity hydrogen storage media by physisorption. Here, special attentions have been paid to the optimal adsorption pathway of H₂ molecules on TiC₂H₂/TiC₂H₄ compounds by using CCSD(T) and B3LYP functionals. An interesting result is that some most stable configurations of TiC₂H₂(nH₂)(n = 1–7) complexes are not the structures coordinated by H₂ molecules but plausible hydrogenation intermediates. Based on the potential energy profiles and MD simulations, the optimal adsorption pathway is considered as TiC₂H₂(T) → 1b → 2c → 2b → 2a → 3b → 3a → 4a → 5a → 5b → 6d → C₂H₆ + Ti(H₂)₅ for TiC₂H₂ and TiC₂H₄(1c) → 2a → 3b → 3a → 4a → 5a → 5b → 6d → C₂H₆+Ti(H₂)₅ for TiC₂H₄. It indicates that the adsorptions of H₂ molecules on TiC₂H₂/TiC₂H₄ contain chemisorption and physisorption. The product C₂H₆+Ti(H₂)₅ exhibits 14 wt% uptake of H₂, which is completely consistent with the experimental results.     

The synergistic effect of potassium ions and nitrogen defects on carbon nitride for enhanced photocatalytic hydrogen evolution

Ion doping is an effective method to improve the photocatalytic activity of graphitic carbon nitride (g-C₃N₄) by providing a photocarriers transfer channel. But limited by the bonds in heptazine rings, photoelectrons are still trapped in the structure. Therefore, both potassium ions and nitrogen defects were successfully introduced into g-C₃N₄ by high temperature calcination to accelerate the charges transfer between both interlayers and intralayer of g-C₃N₄. The results showed that the hydrogen production rate of g-C₃N₄ modified simultaneously by nitrogen defects and potassium ions reaches 1722.4 μmol·g⁻¹·h⁻¹, which is 8 times that of pristine g-C₃N₄. Based on various characterization techniques and DFT calculations, we attributed the enhanced photocatalytic hydrogen evolution to the improved light adsorption, more delocalized HOMO-LUMO, and stronger interlayer interactions. This work will provide a promising way to enhance photocatalytic hydrogen evolution of g-C₃N₄ and a possible mechanism was proposed.     

Mechanochemical modification of LiAlH4 with Fe2O3 - A combined DFT and experimental study

LiAlH₄ is a promising material for hydrogen storage, having the theoretical gravimetric density of 10.6 wt% H₂. In order to decrease the temperature where hydrogen is released, we investigated the catalytic influence of Fe₂O₃ on LiAlH₄ dehydrogenation, as a model case for understanding the effects transition oxide additives have in the catalysis process. Quick mechanochemical synthesis of LiAlH₄ + 5 wt% Fe₂O₃ led to the significant decrease of the hydrogen desorption temperature, and desorption of over 7 wt%H₂ in the temperature range 143–154 °C. Density functional theory (DFT)-based calculations with Tran-Blaha modified Becke-Johnson functional (TBmBJ) address the electronic structure of LiAlH₄ and Li₃AlH₆. ⁵⁷Fe Mössbauer study shows the change in the oxidational state of iron during hydrogen desorption, while the ¹H NMR study reveals the presence of paramagnetic species that affect relaxation. The electron transfer from hydrides is discussed as the proposed mechanism of destabilization of LiAlH₄ + 5 wt% Fe₂O₃.     

Molecular and dissociated adsorption of hydrogen on TiC6H6

The dissociation of H₂ molecule on metal-decorated carbon-based materials is a widespread phenomenon. And it is controversial that the dissociation of H₂ molecule had significant effects on the hydrogen storage capacity. In this paper, comprehensive and detailed researches on the adsorption, dissociation and desorption of H₂ molecules on the complex TiC₆H₆ were performed using Coupled-Cluster theory.We found that the dissociation of the H₂ on TiC₆H₆ almost has no effect on the maximum number of adsorbed H₂ but weakens stepwise adsorption energies of the later H₂ molecules. The adsorption pathway is complicated due to the transformations of adsorption structures. The adsorption pathway along TiC₆H₆→1a→2a→3a pathway will be more favorable thermodynamically at room temperature. The most favorable desorption pathway is 3b→2b→1b→1a→TiC₆H₆, and its rate-determining step is the transition from 1b to 1a. These results indicate that the adsorption and desorption of hydrogen molecules on TiC₆H₆ follow different pathways due to dissociation of H₂ molecules. Once the three H₂ are completely liberated, the TiC₆H₆ system is ready to restart the absorption/desorption process again. In addition, by using CCSD(T) calculations as benchmark, the predictions using the B3LYP functional is valid for hydrogen sorbent materials including TiC₆H₆.     

Li-decorated B2O as potential candidates for hydrogen storage: A DFT simulations study

Two-dimensional (2D) B₂O monolayer is considered as a potential hydrogen storage material owing to its lower mass density and high surface-to-volume ratio. The binding between H₂ molecules and B₂O monolayer proceeds through physisorption and the interaction is very weak, it is important to improve it through appropriate materials design. In this work, based on density functional theory (DFT) calculations, we have investigated the hydrogen storage properties of Lithium (Li) functionalized B₂O monolayer. The B₂O monolayer decorated by Li atoms can effectively improve the hydrogen storage capacity. It is found that each Li atom on B₂O monolayer can adsorb up to four H₂ molecules with a desirable average adsorption energy (E_ave) of 0.18 eV/H₂. In the case of fully loaded, forming B₃₂O₁₆Li₉H₇₂ compound, the hydrogen storage density is up to 9.8 wt%. Additionally, ab initio molecular dynamics (AIMD) calculations results show that Li-decorated B₂O monolayer has good reversible adsorption performance for H₂ molecules. Furthermore, the Bader charge and density of states (DOS) analysis demonstrate H₂ molecules are physically absorbed on the Li atoms via the electrostatic interactions. This study suggests that Li-decorated B₂O monolayer can be a promising hydrogen storage material.     

Open space for the physisorption of H2: Cointercalation of graphite with Li,Ti metal atoms and ethylene molecules

Based on first-principles plane-wave calculations, we explored the method with the ethylene molecules and Ti, Li atoms intercalated into the graphite to open space for the physisorption of hydrogen. And our simulation indicated that the interlayer distance of the graphene is close to the optimal physisorption of hydrogen with this method. From our computation, we got that the type of 3 × 3 supercell has the lowest converge energy and is energetically favorable. The energy barrier of changing the type of 2 × 2 supercell to 3 × 3 supercell is high.     

Hydrogen storage on graphitic carbon nitride and its palladium nanocomposites: A multiscale computational approach

Hydrogen storage capacity (HSC) of multilayer graphitic carbon nitride, d-g-C₃N₄ (d is interlayer spacing), and its palladium nanocomposite, d-Pd@g-C₃N₄, were investigated using multiscale computational techniques including quantum mechanics calculations and grand canonical Monte Carlo (GCMC) simulation. According to the results, the volumetric HSC of 8-g-C₃N₄ and 8-Pd@g-C₃N₄ can reach to DOE target of 30 gH₂/L at 177 K, 5.7 MPa, and 177 K, 4.0 MPa, respectively. The gravimetric HSC of 10-g-C₃N₄ and 12-Pd@g-C₃N₄ meet the DOE target of 4.5 wt% at 150 K, 3.5 MPa, and 125 K, 4.0 MPa, respectively. The incorporation of Pd atoms enhances the delivery volumetric HSC of 6-, 8-, 10-, and 12-g-C₃N₄ by 49, 55, 129, and 146%, respectively at 177 K and 0.5 MPa. On the other hand, the incorporation of Pd atoms has a negative effect on the delivery gravimetric HSC of 6- and 8-g-C₃N₄ and positive effect for 10- and 12-g-C₃N₄. The estimated isostric heat, Q_st, of adsorption is 5.5–8.5 kJ/mol. The maximum value of Q_st for both nanoadsorbents belong to those with d = 8 Å. The structure of adsorbates and possibility of multilayer adsorption occurrence were also investigated using pair correlation functions and density profiles.     

Promotion of H2 adsorption performance on InN monolayer by embedding Cu atom: A first-principles study

Hydrogen energy as a clean energy has great application potential, and finding efficient hydrogen storage materials has become the current research hotspot. This work studied the structure, electronic properties, thermodynamic properties and H₂ adsorption performance of InN, N-defect (V_N–InN), In-defect (V_In–InN), Cu atom substitutes N atom embedded InN (Cu/V_N–InN) and Cu atom substitutes In atom embedded InN (Cu/V_In–InN) by density functional theory (DFT). The results show that all of the five InN materials have good thermal stability at room temperature (300 K), and the structural stability of the defective InN increases after embedding of Cu atom. Meanwhile, the hydrogen interaction on the five InN materials was investigated. Cu/V_In–InN has the best performance for H₂ adsorption among the five InN materials. The adsorption energy for Cu/V_In–InN can reach ?0.769 eV, which is 4.5 times better than original InN nanosheet. After adsorbing 5H₂ molecules, the average adsorption energy is ?0.399 eV that indicates Cu/V_In–InN structure still has possibility of adsorbing more hydrogen molecules and it has the potential to become a new hydrogen storage material.     

A DFT study of H2 adsorption on lithium decorated 3D hybrid Boron-Nitride-Carbon frameworks

Based on density functional theory (DFT) and first-principles molecular dynamics (MD),a new 3D hybrid Boron-Nitride-Carbon–interconnected frameworks (BNCIFs) consisting of organic linkers with Li decoration is created and optimized. Firstly, Li adsorption behaviors on several BNC_xcomplexes are investigated and compared systematically. The results indicate C substitution of N atom in pure BN layer could improve the metal binding energy effectively. Secondly, the BNC layer (BNC_NN) is chosen to model the frameworks of BNCIFs. The average binding energy of adsorbed Li atoms on BNCIFs is 3.6 eV which is much higher than the cohesive energy of bulk Li and avoids the Li clustering problem. Finally, we study the H₂ adsorptions on the Li decorated BNCIFs by DFT. Every Li atom could adsorb four H₂ molecules with an average binding energy of 0.24 eV. The corresponding gravimetric and volumetric storage capacities are 14.09 wt% and 126.2 g/L respectively overpassing the published 2020 DOE target. The excellent thermal stability of 160H₂@40Li@BNCIFs is also proved by MD. This nanostructure could be served as a promising hydrogen storage medium at ambient conditions.     

Synthesis,structural characterization and thermal decomposition studies of (N2H5)2B12H12 and its solvates

A series of salts of the B₁₂H₁₂²⁻ anion has been prepared: a solvent-free (N₂H₅)₂B₁₂H₁₂, its solvates – (N₂H₅)₂B₁₂H₁₂·H₂O, (N₂H₅)₂B₁₂H₁₂·2(CH₃CN), (N₂H₅)₂B₁₂H₁₂·(CH₃OH), and the salt of a protonated azine – [(CH₃)₂CNNHC(CH₃)₂]₂B₁₂H₁₂. These compounds have been synthesized from the commercially available precursors via one- or two-step procedures and fully identified on the basis of single-crystal and powder X-ray diffraction. At room temperature (N₂H₅)₂B₁₂H₁₂ crystallizes in C2/c space group, with a = 18.480(5) Å, b = 6.5344(19) Å, c = 13.106(4) Å and β = 131.911(16)^o, V = 1177.8(7) ų, Z = 4. While this compound nominally contains ca. 10.7 wt% of hydrogen, it thermally decomposes above 200 °C releasing mainly N₂ and NH₃, with H₂ being only the minor gaseous product. Contrary to the recently reported case of hydrazinates of borohydrides, doping with 5 mol% of FeCl₃ does not increase the relative amount of hydrogen significantly, however, it alters the ratio of N₂ and NH₃.     

Y-decorated MoS2 monolayer for promising hydrogen storage: A DFT study

The potential hydrogen storage performance of the constructed Y-decorated MoS₂ was investigated via first-principles density functional theory (DFT) calculations. The Y could be stably decorated on the MoS₂ monolayer with adsorption energy being ?4.82 eV, the absolute value of which was higher than the cohesive energy of bulk Y. The introduced H₂ interacted strongly with the Y-decorated MoS₂ with an elongated bond length and reasonable adsorption energy being 0.792 Å and ?0.904 eV, respectively. There would be four H₂ in maximum adsorbed and stored on the Y-decorated MoS₂ with average adsorption energy being ?0.387 eV. Moreover, the hydrogen gravimetric capacity of the MoS₂ with full Y coverage on each side could be improved to be 4.56 wt% with average adsorption energy being ?0.295 eV. Our study revealed that the MoS₂ decorated with Y could be a potential material to effectively store H₂ with promising gravimetric density.     

Hydrogen storage on Ti decorated SiC nanostructures: A first principles study

In the present study we report the hydrogen adsorption behavior of two SiC nanostructures; a planar sheet and a nanotube (10, 0) of 1 nm diameter decorated by Ti atoms on it. All calculations have been performed using a plane-wave based pseudopotential method. The lowest energy structure of the Ti adsorbed SiC sheet shows that Ti atom distorts the sheet in such a way that one of the Si atoms goes down the plane and the Ti atom bind with nearest three C atoms. The interaction of this Ti decorated sheet with hydrogen suggests that each Ti atom can bind up to four hydrogen molecules (all hydrogens are adsorbed in the molecular form) with an average binding energy of 0.37 eV. For SiC nanotube, the adsorption of Ti favors the hexagonal hollow site. Moreover, on interaction of this Ti decorated tube with hydrogen leads to dissociation of the first hydrogen molecule in the atomic form and thereafter adsorbs hydrogen in the molecular form. The average binding energy of hydrogen molecules on this Ti decorated tube is estimated to be 0.65 eV. Based on these results we infer that the Ti decorated SiC nanostructures moderately bind with hydrogen molecules (within the energy window for hydrogen storage materials) and therefore, can be considered as one of the potential hydrogen storage material.     

Hydrogen adsorption and storage behaviors of Li-decorated PdS2 monolayer: An extended tight-binding study based on DFT

The hydrogen adsorption and storage of the lithium-decorated PdS₂ monolayer at nano-size has been investigated by using extended tight-binding (GFN1-xTB) based on density functional theory (DFT). The calculation results demonstrate that the average adsorption energies of 1–5H₂ change in 0.47–0.20 eV/H₂ range which decreases with increasing of adsorbed hydrogen molecule number. The gravimetric density for hydrogen storage adsorption with 12Li atom and 60H₂ molecules of Li-decorated PdS₂ monolayer is about 6.98 wt% considered as possible application in hydrogen storage. The examination of the hydrogen store mechanism between the monolayer and hydrogen molecules is presented by polarization between Pd and H₂, which can be effect on the adsorption behavior.     

Role of Ti-based catalysts in the dehydrogenation mechanism of magnesium borohydride: A cluster approach

The role of Ti and Ti-based catalysts such as TiCl₃ and TiF₃ in the dehydrogenation of Mg(BH₄)₂ has been studied using a cluster approach and density functional theory. The optimized geometry of Mg(BH₄)₂ clusters mimics the structure it has in the crystalline form, but undergoes significant geometric distortion as well as changes in the natural bond orbital (NBO) charge after the addition of the catalysts. While all the catalysts lower the hydrogen desorption energy, elemental Ti appears to be the best catalyst followed by TiCl₃ and TiF₃. The lowering of the hydrogen desorption is shown to be due to the weakening of the BH bond which is caused by the interaction of Ti with H and Mg.     

Adsorption of hydrogen molecule on alkali metal-decorated hydrogen boride nanotubes: A DFT study

Adsorption of Li, Na, and K atom on surfaces of armchair (5,5) and zigzag (10,0) hydrogen boride nanotubes (HBNTs) was investigated using the periodic-DFT method. It was found that the average diameter (5,5) HBNT is shorter than the (10,0) HBNT by 1.246 Å and the (5,5) HBNT is more stable than the (10,0) HBNT by 0.991 eV. Adsorption strength of the (5,5) HBNT on alkali metals was found to be higher than the (10,0) HBNT. Adsorption abilities of H₂ on the (5,5) HBNT and (5,5) HBNT are in the same order: Li > Na > K. The adsorption energies of H₂ on Li-, Na-, and K-(5,5) HBNTs are −0.242, −0.165, and −0.121 eV, respectively, and on Li-, Na-, and K-(10,0) HBNTs are −0.277, −0.168, and −0.094 eV, respectively. The Li-HBNTs, Li-(5,5) HBNT, and (10,0) HBNT are the highest adsorption abilities on H₂ adsorption and the most significant change of metal charges. Therefore, the Li-(5,5) HBNT and (10,0) HBNT used as H₂ storage materials were suggested.     

Linear complex HCC-TMH (TM=Sc–Ni): A simple and efficient adsorbent for hydrogen molecules

The structure and hydrogen adsorption properties of linear HCC-TMH (TM = Sc–Ni) complexes were systematically investigated using a density functional method. The ground states of the HCC-TMH (TM = Sc–Ni) complexes are 2, 3, 4, 3, 4, 3, 2, and 3. The gravimetric H₂ uptake capacities of these ground state HCC-TMH (TM = Sc–Ni) complexes are 4.54–14.56%. Ab initio molecular dynamic simulations indicated that maximal reversible hydrogen storage densities at 77–300 K of the ground state HCC-TMH (TM = Ti–Co) complexes are 6.65–12.00 wt%. The corresponding average adsorption energies are 0.07–0.49 eV. Due to reasonable superior storage capacity and ideal binding energy, HCC-TMH (TM = Ti–Co) complexes are proposed as a suitable hydrogen storage medium at ambient conditions.When HCC-TMH (TM = Ti, V, Cr) are nonground state structures, they can adsorb one more H₂ molecule than their ground states. Unfortunately, the first adsorbed H₂ molecules are dissociated and the adsorption energies are too large. Therefore, the importance of the multiplicity of sorbents for hydrogen storage is emphasized in this work.     

Adsorption of hydrogen molecules on the alkali metal ion decorated boric acid clusters: A density functional theory investigation

The hydrogen storage capacity of alkali metal ion decorated boric acid (BA) based bowl, sheet and ball structures have been investigated using B3LYP method employing 6-31+G∗∗ basis set. The maximum gravimetric density has been observed for the bowl shaped clusters. These values for Li⁺, Na⁺ and K⁺ decorated clusters are 8.3, 8.8 and 7.8 wt.%, respectively. The range of the calculated binding energy per H₂ molecule (BE/H₂) for Li⁺, Na⁺ and K⁺ decorated bowl shaped clusters are 2.57-3.59, 1.88-2.11 and 0.76-1.00 kcal/mol, respectively. The same for the sheet clusters are 3.18-3.73, 1.68-2.40 and 0.73-0.97 kcal/mol, respectively. Similarly, BE/H₂ of Na⁺ decorated ball clusters ranges from 1.88 kcal/mol to 2.62 kcal/mol. It has been shown in earlier studies that the BE/H₂ should be in between the physisorption and chemisorption limits for realizing the practical applications of different class of materials. In this context, both BE/H₂ and gravimetric density of Na⁺ decorated clusters indicate that these systems have appropriate properties. Hence Na⁺ decorated (BA)_n structures are suitable for hydrogen storage applications.     

Comparison between hydrogen production via H2S and H2O splitting on transition metal-doped TiO2 (101) surfaces as potential photoelectrodes

Doping is an effective way to engineer the electronic band structure of semiconductor materials and consequently their photocatalytic activity for hydrogen generation. In this work, periodic Density Functional Theory (DFT) was employed to compare the adsorption of H₂S and H₂O molecules on TiO₂(101) anatase surfaces compared to four transition metal-doped TiO₂(101) anatase surfaces; Cr⁴⁺-TiO₂, V⁴⁺-TiO₂, Mn⁴⁺-TiO₂, and Nb⁴⁺-TiO₂. The defect formation energy, molecular adsorption energy, hydrogen splitting energies, geometrical changes, electronic structure and charge transfer characteristics were investigated to determine and compare the changes in adsorption of H₂S and H₂O on the pristine vs. doped surfaces. The defect formation energy calculations revealed the Nb⁴⁺-TiO₂ surface resulted in the highest stability, smallest change in neighboring bond lengths and the highest dopant to surface charge transfer. However, upon H₂S and H₂O adsorption, the calculations concluded that the V⁴⁺-TiO₂ surface resulted in the most stable structure for adsorbed H₂S and lowest hydrogen splitting energy requiment compared to the other dopant metals and the lowest for H₂S vs H₂O, indicating its potential catalytic activity for facile dehydrogenation for industrial applications.     

First-principles study of the H2 splitting processes on pure and transition-metal-doped Al (111) surfaces

By performing first-principles calculations, H₂ splitting processes on pure and transition metal (TM) atom substituted Al (111) surfaces were examined. Corrected reaction pathways with splitting energy barriers (0.99 eV) lower than those in previous studies (1.28 eV) were obtained. By further analyzing the H₂ splitting process on the 3d-TM-atom-doped Al (111) surface, the relationship of the catalysis effect and the electron donation-back donation process on TM 3d orbitals were examined in detail. Finally, to confirm the possibility of reducing the partially oxidized Al (111) surface with an H₂ molecule, the surface reduction process was studied by using the climb-image nudged elastic band (CI-NEB) method systematically.