Phosphorene: A promising candidate for H2 storage at room temperature

Owing to the existence of periodic channels in phosphorene, this 2D material can be a good candidate for room temperature reversible hydrogen storage. The density functional theory calculations (DFT), including van der Waals interactions (vdW-DF2) coupled with the cooper exchange functional (C09), has been applied to study the potential of phosphorene as a new 2D material for hydrogen storage. Our results show that the adsorption energy (−292 to −277 meV) of H₂ on phosphorene is appropriate for storage. The analysis of diffusion pathways between different physisorbed states on phosphorene shows that a single hydrogen molecule diffuses very easily along the open channel (less than 1 meV along the zigzag direction), as compared to 14 meV for diffusion across the channels (along the armchair direction). The potential energy surfaces for the dissociative chemisorption of H₂ was computed on highly symmetric sites of phosphorene and the highest activation barrier was found to be 2.77 eV. The very large dissociation energy coupled with a strong physisorption of H₂ on phosphorene and facile diffusion, makes this 2D material a promising candidate for H₂ storage at room temperature.     

Study of the reversible hydrogen storage performance of Ti-decorated hexagonal B36 by DFT calculations with van der Waals corrections

In this work, we report on the study of the hydrogen storage capability of titanium (Ti) decorated B₃₆ nanosheets using density functional theory (DFT) calculations with van der Waals corrections. Ti atoms are strongly bonded to the surface of B₃₆ with a binding energy of 6.23 eV, which exceeds the bulk cohesive energy of crystalline Ti. Ti-decorated B₃₆ (2Ti@B₃₆) can reversibly adsorb up to 12 H₂ molecules with a hydrogen storage capacity of 4.75 wt % and average adsorption energy between 0.361 and 0.674 eV/H₂. The values of desorption temperature and the results of molecular dynamics simulations enable to conclude that 2Ti@B₃₆ is a perspective reversible material for hydrogen storage under real conditions.     

Li-decorated graphyne as high-capacity hydrogen storage media: First-principles plane wave calculations

Taking into account the van der Waals correction, the characteristics of the Li-decorated graphyne as the hydrogen storage medium have been explored using first-principles plane wave calculations. We find that Li atom can be adsorbed not only over the center of large hexagon (H_L site) but also over the center of small hexagon (H_S site). For double-side Li decorations, there are 14H₂ molecules can be adsorbed on Li-decorated graphyne primitive cell with the adsorption energy of 0.19 eV/H₂. As a result, the hydrogen storage capacity of 13.0 wt% can be obtained. This suggests that the Li-decorated graphyne system can serve as a high-capacity hydrogen storage medium.     

Enhancing energy storage efficiency of lithiated carbon nitride (C7N6) monolayers under co-adsorption of H2 and CH4

There is a great interest in the design of innovative concepts and strategies of nitrogen rich carboneous materials for exploring their hydrogen (H₂) storage properties. Methane (CH₄) storage can be an alternative to H₂ because the combustion energy of the former is around three times higher than the latter. However, strong inter-molecular repulsion between the CH₄ molecules is a major bottleneck to achieve a high gravimetric density. In this study, we use first principles density functional calculations to investigate the coadoption of H₂ and CH₄ on Li decorated carbon nitride (Li–C₇N₆) monolayer. The repulsion between CH₄ molecules has been avoided by keeping them in asymmetric configuration whereas the repulsion between CH₄–H₂ is in moderation due to the exploitation of open Li doped sites on C₇N₆ surface. Though Li–C₇N₆ has a lower H₂ or CH₄ storage capacity due to weak van der Waals interactions, the capacity could be doubled with a novel strategy of co-mixing of H₂ with CH₄ which results into a significantly high gravimetric density of 8.1 wt%. This clearly shows that the CH₄–H₂ co-mixing strategies have the potential to further propel the prospects of C₇N₆ monolayers for reversible clean energy storage applications.     

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.     

Wood cellulose as a hydrogen storage material

Hydrogen has become a strong candidate to be a future energy storage medium but there are technological challenges both in its production and storage. For storage, a search for lightweight, abundant and non-toxic materials is on the way. An abundant natural material such as wood cellulose would make an ideal storage medium from a sustainability perspective. Here, using a combination of static DFT calculations and ab initio molecular dynamics simulations at different temperatures, it is shown that wood cellulose has the ability to uptake H₂ via a physisorption mechanism based on dispersion interactions of the van der Waals type involving the O-atoms of the d-glucose rings. The absorption causes little to no disturbances on the cellulose structure and H₂ is highly mobile in the material. At an external pressure of H₂(g) of 0.09 atm and T = 25 °C, cellulose has a theoretical gravimetric density of hydrogen storage of ≈1%.     

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.     

Hydrogen storage on calcium-decorated BC7 sheet: A first-principles study

The adsorption behavior of hydrogen molecules on the calcium-decorated BC₇ sheet has been investigated using first-principles calculations. Our calculations demonstrate that the van der Waals interactions are crucial for the hydrogen storage in the calcium-decorated BC₇ sheet. We find that the average adsorption energy per hydrogen molecules decreases with the number of adsorbed hydrogen molecule increasing. When six hydrogen molecules adsorb, the average adsorption energy is 0.26 eV. In this case, the gravimetric density for hydrogen storage on two sides of calcium-decorated BC₇ sheet is about 4.96 wt%. These features indicate that the calcium-decorated BC₇ sheet has potential application in hydrogen storage.     

Development of a novel simulator for modelling underground hydrogen and gas mixture storage

Underground hydrogen storage can store grid-scale energy for balancing both short-term and long-term inter-seasonal supply and demand. However, there is no numerical simulator which is dedicated to the design and optimisation of such energy storage technology at grid scale. This study develops novel simulation capabilities for GPSFLOW (General Purpose Subsurface Flow Simulator) for modelling grid-scale hydrogen and gas mixture (e.g., H₂–CO₂–CH₄–N₂) storage in cavern, deep saline aquifers and depleted gas fields.The accuracy of GPSFLOW is verified by comparisons against the National Institute of Standard and Technology (NIST) online thermophysical database and reported lab experiments, over a range of temperatures from 20 to 200 °C and pressure up to 1000 bar. The simulator is benchmarked against an existing model for modelling pure H₂ storage in a synthetic aquifer. Several underground hydrogen storage scenarios including H₂ storage in a synthetic salt cavern, H₂ injection into a CH₄-saturated aquifer experiment, and hydrogen storage in a depleted gas field using CO₂ as a cushion gas are used to test the GPSFLOW's modelling capability. The results show that GPSFLOW offers a robust numerical tool to model underground hydrogen storage and gas mixture at grid scale on multiple parallel computing platforms.     

Hydrogen storage in Ca-decorated carbyne C10-ring on either Dnh or D(n/2)h symmetry. DFT study

We computationally investigate the hydrogen storage properties of carbyne C₁₀-ring structure on either D_nh or D_(n/2)h symmetry decorated with calcium (Ca) atoms adsorbed on its outer surface. The calculations are carried out on DFT-GGA-PW91 and DFT-GGA-PBE levels of theory as implemented in Biovia Materials Studio modeling and simulation software. To account for van der Waals interactions we also carried out calculations using DFT-D method of Grimme. Dmol³ is used to calculate total energies, HOMO-LUMO electronic charge density, Mulliken population analysis, and electrostatic potential fitting charges (ESP). Based on these results: i) the average binding energy of Ca atom doping to C₁₀-ring is ~2.3 eV (PW91) and ~2.1 eV (PBE). ii) Up to seven H₂ molecules per Ca atom can be physically adsorbed with an average energy of ~0.2 eV per H₂ molecule. iii) This physisorption leads to 8.09 wt percentage (wt. %) for the gravimetric storage capacity. According to these results, calcium-decorated carbyne C₁₀-ring structure is excellent candidate for hydrogen storage at ambient conditions with application to fuel cells.     

Hydrogen and chemical energy storage in gas hydrate at mild conditions

Hydrogen storage in clathrate hydrates is a promising approach for industry-scale utilizations. However, extreme operation conditions such as high pressure (about GPa) limit the development. In this work hydrogen hydrate phase equilibrium in addition of methane, tert-butyl alcohol (tBA), trichloromethane (CHCl₃) and 1,1-dichloro-1-fluoroethane (HCFC-141 b) are reported at 6 MPa–20 MPa and 274 K–286 K, which including 21 points in total. Mechanism studies using Raman spectroscopy show that tBA and H₂O form metastable hydrate cages via hydrogen bonds, then form stable sII hydrates with the help of CH₄. Hydrate-based hydrogen storage capacity in 5.6 mol%HCFC-141 b-water mixture could reach 46 V/V (0.36 wt%) at 273 K and 10 MPa. Combing with chemical energy of HCFC-141 b, this work achieved high capacity of hydrogen and chemical energy storage in gas hydrate at mild conditions. This study will provide guidance on hydrate-chemical hybrid hydrogen storage technology, and leads to the next generation of hybrid hydrate-based hydrogen technology in the future.     

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.     

Potassium-doped PC71BM for hydrogen storage: Photoelectron spectroscopy and first-principles studies

We consider the fullerene C₇₀ derivative PC₇₁BM as a potential onboard hydrogen storage material. As the cases of many sorbents, metal-decoration is needed to increase the adsorption energy of H₂. We study the metal-decoration by intercalating K atoms into a PC₇₁BM film and performing photoelectron spectroscopy measurements. The results reveal that the maximum stoichiometry is K₉–₁₀PC₇₁BM. Then we study the hydrogen storage property of an isolated K₉PC₇₁BM molecule with density functional theory calculations. A K₉PC₇₁BM molecule can compactly adsorb 45H₂ (6.22 wt%); the compact adsorption structure does not require large interstitial space in solid phase and thus should be able to be realized in experiments. Considering the poor crystallinity of solid PC₇₁BM (with many large interstitial spaces), a gravimetric capacity >7 wt% can be anticipated. The adsorption energy meets the DOE target of onboard operating temperatures, and van der Waals interaction contributes more than 40% of the adsorption energy.     

Reversible hydrogen storage for NLi4-Decorated honeycomb borophene oxide

The boron-based two-dimensional (2D) materials decorated with functional groups NLi₄ has been numerically investigated for hydrogen storage via first principles calculations method. Strain-energy analysis and molecular dynamics simulations shows the pristine planar honeycomb B₂O has strong mechanical and thermal stability. Crystal Orbital Hamiltonian Population analysis confirmed that there exist stronger B–B/B–O covalent bonds within B₂O monolayer. In functional material, a local electric field around each lithium atom can be formed and the overall electronic structure is favorably changed for gas adsorptions. Both electrostatic forces and the van der Waals interaction are the dominant hydrogen-attached mechanisms of lithium cation. An anchored functional group NLi₄ can adsorb at most 11 hydrogen molecules, and the average adsorption energy per hydrogen molecules is around ?0.20 eV, indicating high hydrogen storage capacity and reversible applicability. The highest hydrogen storage capacity can reach to 9.1 wt%. The study shows the investigated material is a good candidate for 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.     

H2 permeation and its influence on gases through a SAPO-34 zeolite membrane

This work analysed the permeation of binary and ternary H₂-containing mixtures through a SAPO-34 membrane, aiming at investigating how hydrogen influences and its permeation is influenced by the presence of the other gaseous species, such as CO₂ and CH₄. We considered the behaviour of various gas mixtures in terms of permeability and selectivity at various temperatures (25–300 °C), feed pressures (400–1000 kPa) and compositions by means of an already validated mass transport model, which is based on surface and gas translation diffusion. We found that the presence of CO₂ and CH₄ in the H₂-containing mixtures influences in a similar way the H₂ permeation, reducing its permeability of about 80% compared to the single-gas value because of their stronger adsorption. On the other hand, H₂ promotes the permeation of CO₂ and CH₄, causing an increment of their permeability with respect to those as single gases. These combined effects reflected in interesting selectivity values in binary mixture (e.g., CO₂/H₂ about 11 at 25 °C, H₂/CH₄ about 9 at 180 °C), which showed the potential of SAPO-34 membranes in treating of H₂-containing mixtures.     

Mesoporous g-C3N4/Zn–Ti LDH laminated van der Waals heterojunction nanosheets as remarkable visible-light-driven photocatalysts

Mesoporous g-C₃N₄/Zn–Ti layered double hydroxide (LDH)-laminated van der Waals heterojunction nanosheets were prepared by a facile one-step in situ hydrothermal method. Due to the strong electrostatic interactions between the positively charged Zn–Ti LDH and negatively charged g-C₃N₄ nanocrystal, a laminated van der Waals heterostructure was successfully formed between Zn–Ti LDH and g-C₃N₄. The mesoporous g-C₃N₄/Zn–Ti LDH-laminated van der Waals heterojunction, which had a narrow bandgap of 2.41 eV extended the photoresponse to the visible light region. The obtained heterojunctions showed excellent visible-light-driven photocatalytic performance for the complete removal of ceftriaxone sodium (up to ∼97%) and a high hydrogen production rate (∼161.87 μmol h⁻¹ g⁻¹). This was mainly attributed to the formation of the laminated van der Waals heterojunctions, which favoured charge separation and the absorption of visible light, and the mesoporous structure, which provided more surface active sites. This facile strategy for preparing mesoporous g-C₃N₄/Zn–Ti LDH-laminated van der Waals heterojunctions offers new insights for the fabrication of high-performance van der Waals heterojunction photocatalytic materials.     

Biohydrogen production from wheat straw hydrolysate using Caldicellulosiruptor saccharolyticus followed by biogas production in a two-step uncoupled process

A two-step, un-coupled process producing hydrogen (H₂) from wheat straw using Caldicellulosiruptor saccharolyticus in a ‘Continuously stirred tank reactor’ (CSTR) followed by anaerobic digestion of its effluent to produce methane (CH₄) was investigated. C. saccharolyticus was able to convert wheat straw hydrolysate to hydrogen at maximum production rate of approximately 5.2 L H₂/L/Day. The organic compounds in the effluent collected from the CSTR were successfully converted to CH₄ through anaerobic digestion performed in an ‘Up-flow anaerobic sludge bioreactor’ (UASB) reactor at a maximum production rate of 2.6 L CH₄/L/day. The maximum energy output of the process (10.9 kJ/g of straw) was about 57% of the total energy, and 67% of the energy contributed by the sugar fraction, contained in the wheat straw. Sparging the hydrogenogenic CSTR with the flue gas of the UASB reactor ((60% v/v) CH₄ and (40% v/v) CO₂) decreased the H₂ production rate by 44%, which was due to the significant presence of CO₂. The presence of CH₄ alone, like N₂, was indifferent to growth and H₂ production by C. saccharolyticus. Hence, sparging with upgraded CH₄ would guarantee successful hydrogen production from lignocellulosic biomass prior to anaerobic digestion and thus, reasonably high conversion efficiency can be achieved.     

Augmented hydrogen adsorption on metal (Mg,Mn) doped α-phase TeO2: A DFT investigation

TeO₂, as a promising gas sensor material, has been extensively studied for its capacity to detect hydrogen with high sensitivity. First-principles calculations were applied to explore the adsorption properties of hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), and hydrogen sulfide (H₂S) on TeO₂ doped with either Mg or Mn to explore this compound's potential as hydrogen sensors. Hydrogen is more readily adsorbed on pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ than CO₂, CH₄ and H₂S molecules by calculating their adsorption energy and charge transfer; the sequence of adsorption strength is H₂>H₂S > CO₂>CH₄. The hydrogen molecules and pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ form H–O bonds with lengths of 0.98, 0.98 and 0.99 Å, respectively, indicating that chemical adsorption is dominant between them. The adsorption of hydrogen leads to significant changes in the density of states (DOSs) of pure-TeO₂, Mg–TeO₂ and Mn–TeO₂, which may lead to changes in their electrical conductivity. Moreover, the larger diffusion coefficients for hydrogen on the surfaces of pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ relative to other gases indicates that hydrogen diffuses readily in TeO₂-based sensing materials, and the higher gas concentration contributes to improvements in response performance. This finding offers a theoretical basis for experimental explorations of the influence of metal dopants on TeO₂ hydrogen sensing performance.     

Catenated metal-organic frameworks: Promising hydrogen purification materials and high hydrogen storage medium with further lithium doping

Based on the first-principles derived force fields and grand canonical Monte Carlo simulations, we find that the catenated metal-organic frameworks outperform the noncatenated structures, in terms of H₂ separation from other gases (CH₄, CO and CO₂) and H₂ adsorption by Li doping. A system utilizing IRMOF-11 (or IRMOF-13) for hydrogen separation and Li-doped IRMOF-9 for hydrogen storage is therefore proposed, with hydrogen uptake of 4.91 wt% and 36.6 g/L at 243 K and 100 bar for Li-doped IRMOF-9, which is close to the 2017 DOE target. It is promising to find appropriate microporous materials for hydrogen purification and storage at ambient conditions with structure catenated.