A model study of effect of M = Li,Na, Be,Mg, and Al ion decoration on hydrogen adsorption of metal-organic framework-5

The effect of light metal ion decoration of the organic linker in metal-organic framework MOF-5 on its hydrogen adsorption with respect to its hydrogen binding energy (ΔB.E.) and gravimetric storage capacity is examined theoretically by employing models of the form MC₆H₆:nH₂ where M = Li⁺, Na⁺, Be²⁺, Mg²⁺, and Al³⁺. A systematic investigation of the suitability of DFT functionals for studying such systems is also carried out. Our results show that the interaction energy (ΔE) of the metal ion M with the benzene ring, ΔB.E., and charge transfer (Q_trans) from the metal to benzene ring exhibit the same increasing order: Na⁺ < Li⁺ < Mg²⁺ < Be²⁺ < Al³⁺. Organic linker decoration with the above metal ions strengthened H₂-MOF-5 interactions relative to its pure state. However, amongst these ions only Mg²⁺ ion resulted in ΔB.E. magnitudes that were optimal for allowing room temperature hydrogen storage applications of MOF-5. A much higher gravimetric storage capacity (6.15 wt.% H₂) is also predicted for Mg²⁺-decorated MOF-5 as compared to both pure MOF-5 and Li⁺-decorated MOF-5.     

Hydrogen permeation and diffusion in densified MOF-5 pellets

The metal-organic framework Zn₄O (BDC)₃ (BDC = 1,4-bezene dicarboxlate), also known as MOF-5, has demonstrated considerable adsorption of hydrogen, up to 7 excess wt.% at 77 K. Consequently, it has attracted significant attention for vehicular hydrogen storage applications. To improve the volumetric hydrogen density and thermal conductivity of MOF-5, prior studies have examined the hydrogen storage capacities of dense MOF-5 pellets and the impact of thermally conductive additives such as expanded natural graphite (ENG). However, the performance of a storage system based on densified MOF-5 powders will also hinge upon the rate of hydrogen mass transport through the storage medium. In this study, we further characterize MOF-5 compacts by measuring their hydrogen transport properties as a function of pellet density (ρ = 0.3–0.5 g cm⁻³) and the presence/absence of ENG additions. More specifically, the Darcy permeability and diffusivity of hydrogen in pellets of neat MOF-5, and composite pellets consisting of MOF-5 with 5 and 10 wt.% ENG additions, have been measured at ambient (296 K) and liquid nitrogen (77 K) temperatures. The experimental data suggest that the H₂ transport in densified MOF-5 is strongly related to the MOF-5 pellet density ρ.     

The enhancement of hydrogen storage capacity in Li,Na and Mg-decorated BC3 graphene by CLICH and RICH algorithms

New hydrogen adsorption states on Li, Na, and Mg-decorated graphene-type BC₃ sheet have been investigated by first-principles calculations. The structural, electronic and binding properties, metal binding and nH₂ (n = 1–10) adsorption states of these systems are studied in detail with the Mulliken analysis, charge density differences, and partial density of states. To enhance the number of the adsorbed H₂ molecules per metal atom, and also to generate the better initial coordinates for reducing the simulation time, we present two masthematical algorithms (CLICH and RICH). The tested results on BC₃ sheet and boron-doped graphene (C₃₀B₂) show that these algorithms can increase the number of adsorbed hydrogen molecules by minimizing the computational time. It is seen that nH₂ (n = 1–10) adsorbed Li,/Na and/Mg-decorated BC₃ single- and double-sided systems are industrial materials for hydrogen storage technology, their adsorption energies fall into the acceptable adsorption energy range (0.20–0.60 eV/H₂). It is concluded from the optimized geometries and charge density differences for the higher number of H₂ adsorbed systems that not only decorated metal atom but also the sheet plays an important role in hydrogen storage process, because the boron atoms in the sheet expand the induced electric field between the adatoms and BC₃ sheet. From Mulliken analysis, there is a charge transfer mechanism between H₂ molecules and metal atoms. Moreover, the resonant peaks for the sheet, metal atoms and H₂ molecules in partial density of states curves indicate the possible hybridizations. Additionally, these adsorption processes are supported by charge density difference plots.     

Enhancement of hydrogen storage properties of metal-organic framework-5 by substitution (Zn,Cd and Mg) and decoration (Li,Be and Na)

Two strategies of decoration by three elements Z = Li, Be and Na in cyclic site, and substitution of Zn by Mg and Cd in unit cell were used in the framework of functional density theory to tune the hydrogen storage properties of metal-organic framework-5 (MOF-5) based on Zn whose decomposition temperature and initial gravimetric capacity are 300 K and 1.57 wt% respectively.Based on the adsorption of hydrogen molecules in the crystal surface at three different adsorption sites with three orientations of H₂, we show that our system may reach a maximum gravimetric storage capacity of 4.09 wt% for multiple hydrogen molecules. Moreover, the functionalization of Z combined to the substitution, shows an exceptional improvement of hydrogen storage properties. For example, Mg-MOF-5 decorated with Li₂ has a capacity up to 5.41 wt% and 513 K as desorption temperature.     

Ab initio study of hydrogen adsorption on Zn2(NDC)2(diPyTz) metal-organic framework decorated with alkali and alkaline earth metal cations

We have studied effect of alkali and alkaline earth metal cations (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺) decoration on hydrogen adsorption of the organic linker of Zn₂(NDC)₂(diPyTz) by employing three cluster models: diPyTz:mLi⁺ (m = 1–4), diPyTz:mLi⁺:nH₂ (m = 0,1,2 and n = 1–5) and diPyTz:1M⁺:1H₂ (M⁺ = Na⁺, K⁺, Be²⁺, Mg²⁺) complexes, using density functional theory (DFT) and second-order Moller–Plesset perturbation theory (MP2). The calculated binding energies show that decoration of the organic linker with alkali and alkaline earth metal cations enhanced H₂ interaction with diPyTz when compared with the pristine diPyTz. The atomic charges were derived by Mulliken, ChelpG and ESP methods. Finally, the atoms in molecules theory (AIM) were also applied to get more insight into the nature of the interaction of diPyTz and Li⁺. Results of AIM analysis show that N–Li⁺ bond in diPyTz organic linker's complex appears as shared electron interaction.     

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.     

Efficient synthesis for large-scale production and characterization for hydrogen storage of ligand exchanged MOF-74/174/184-M (M = Mg2+, Ni2+)

A semitechnical route (optimized by BASF SE) to synthesize MOF-74/174-M (M = Mg²⁺, Ni²⁺) efficiently in ton-scale production is presented with the goal of mobile and stationary gas storage applications especially for hydrogen as future energy carrier. In addition, a new member of these series of materials, MOF-184-M (M = Mg²⁺, Ni²⁺) is introduced using ligand exchange strategy in order to produce a more porous analogue (possessing large aperture) without loss of crystallinity. This family comprising MOF-74/174/184 are characterized systematically for hydrogen adsorption properties by volumetric measurements with a Sieverts’ apparatus. Replacing the linker by a longer one results in an increase of the BET area from 984 to 3154 m²/g and an enhancement of the excess cryogenic (77 K) hydrogen storage capacity from 1.8 to 4.7 wt%. The heat of adsorption of linker exchanged MOF-174/184 (as a function of uptake) shows similar values to the parent MOF-74, indicating successful construction of expanded MOFs in large scale production. Finally, a usable capacity of these MOFs is investigated for mobile application, revealing that the increasing surface area without strong binding metal sites through longer linker exchange is one of important parameters for improving usable capacity.     

Equilibrium,kinetics and enthalpy of hydrogen adsorption in MOF-177

Metal–organic framework (MOF-177) was synthesized, characterized and evaluated for hydrogen adsorption as a potential adsorbent for hydrogen storage. The hydrogen adsorption equilibrium and kinetic data were measured in a volumetric unit at low pressure and in a magnetic suspension balance at hydrogen pressure up to 100 bar. The MOF-177 adsorbent was characterized with nitrogen adsorption for pore textural properties, scanning electron microscopy for morphology and particle size, and X-ray powder diffraction for phase structure. The MOF-177 synthesized in this work was found to have a uniform pore size distribution with median pore size of 12.7 Å, a higher specific surface area (Langmuir: 5994 m²/g; BET: 3275 m²/g), and a higher hydrogen adsorption capacity (11.0 wt.% excess adsorption, 19.67 wt.% absolute adsorption) than previously reported values on MOF-177. Freundlich equation fits well the hydrogen adsorption isotherms at low and high pressures. Diffusivity and isosteric heat of hydrogen adsorption were estimated from the hydrogen adsorption kinetics and equilibrium data measured in this work.     

Hydrogen storage capacity of alkali and alkaline earth metal ions doped carbon based materials: A DFT study

The hydrogen storage (H-storage) capacity of alkali (Li⁺, Na⁺ and K⁺) and alkaline earth metal ion (Mg²⁺ and Ca²⁺) doped cubane, cyclohexane and adamantane has been investigated using Density Functional Theory (DFT) based M05-2X functional employing 6-31+G∗∗ basis set. The adsorption of number of H₂ molecules on the metal ion doped complexes depends on ionic radii and charge of the metal ions. Among the 15 complexes investigated in this study, Mg²⁺ ion doped cubane, cyclohexane and adamantane complexes have higher H-storage capacity when compared to other complexes. The calculated binding energy (BE) of 5H₂@Cub-Mg²⁺ complex is 46.85 kcal/mol with binding energy per H₂ molecule (BE/nH₂) of 9.37 kcal/mol. The corresponding gravimetric density of the complexes is 7.3 wt%. In the case of 4H₂@Cyc-Mg²⁺ complex, the BE is 32.19 kcal/mol (BE/nH₂ is 8.05 kcal/mol with 6.9 wt% in gravimetric density). The Adm-Mg²⁺ complexes adsorb 4H₂ molecules with BE of 33.33 kcal/mol, the BE of per H₂ molecule is 8.33 kcal/mol. The corresponding gravimetric density of the complex is around 4.8 wt%, respectively. A new linker modified MOP-9 has been constructed based on the results and their H-storage capacity has also estimated.     

Al-decorated carbon nanotube as the molecular hydrogen storage medium

Al-decorated, single-walled carbon nanotube has been investigated for hydrogen storage applications by using Density Functional Theory (DFT) based calculations. Single Al atom-decorated on (8,0) CNT adsorbs upto six H₂ molecules with a binding energy of 0.201 eV/H₂. Uniform decoration of Al atom is considered for hydrogen adsorption. The first Al atom has a binding energy of 1.98 eV on (8,0) CNT and it decreases to 1.33 eV/Al and 0.922 eV/Al respectively, when the number of Al atoms is increased to four and eight. Each Al atom in (8,0) CNT-8Al adsorbs four H₂ molecules, without clustering of Al atoms, and the storage capacity reaches to 6.15 wt%. This gravimetric storage capacity is higher than the revised 2015 target of U.S Department of Energy (DOE). The average adsorption binding energy of H₂ in (8,0) CNT-8(Al+4H₂), i.e. 0.214 eV/H₂, lies between 0.20 and 0.60 eV/H₂ which is required for adsorbing and desorbing H₂ molecules at near ambient conditions. Thus, Al-decorated (8,0) CNT is proposed as a good hydrogen storage medium which could be useful for onboard automobile applications, at near ambient conditions.     

A first principles study of hydrogen storage in lithium decorated defective phosphorene

In this study, we studied defect-engineering and lithium decoration of 2D phosphorene for effective hydrogen storage using density functional theory. Contrary to graphene, it is found that the presence of point-defects is not preferable for anchoring of H₂ molecules over defective phosphorene. According to previous research, strategies such as defect engineering, metal decoration, and doping enhance the hydrogen storage capacity of several 2D materials. Our DFT simulations show that point defects in phosphorene do not improve the hydrogen storage capacity compared to pristine phosphorene. However, selective lithium decoration over the defective site significantly improves the hydrogen adsorption capacity yielding a binding energy of as high as ?0.48 eV/H₂ in Li-decorated single vacancy phosphorene. Differential charge densities and projected density of states have been computed to understand the interactions and charge transfer among the constituent atoms. Strong polarization of the H₂ molecule is evidenced by the charge accumulation and depletion. The PDOS shows that the presence of Li leads to enhanced charge transfer. The maximum gravimetric density was investigated by sequentially adding H₂ molecules to the Li-decorated single vacancy defective phosphorene. The Li-decorated single vacancy phosphorene is found to possess a gravimetric density of around 5.3% for hydrogen storage.     

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.     

The role of differently distributed vanadium nanocatalyst in the hydrogen storage of magnesium nanostructures

Using a glancing angle (co)deposition technique, ∼4.6 at.% V has been coated on the surface of individual Mg nanoblades and doped into Mg nanostructures fabricated at different deposition angles. The hydrogen storage properties of the formed V-decorated and V-doped Mg nanostructures depend strongly on how the nanocatalyst V is surrounded by the host Mg. The V-doped Mg sample has lower activation energies for hydrogen absorption (E_a^a = 35.3 ± 0.9 kJ/mol H₂) and hydrogen desorption (E_a^d = 38.9 ± 0.3 kJ/mol H₂) than the V-decorated Mg sample when deposited at the same deposition angle of θ = 70°. The activation energies of the doped samples increase gradually with the decrease of the θ angle. We also find that the porosity of the Mg nanostructures plays a secondary role. A phenomenological model based on a heterogeneous reaction is proposed to explain the different hydrogen desorption activation energies obtained for different V–Mg nanostructured samples.     

Nickel-decorated single vacancy phosphorene – A favourable candidate for hydrogen storage

This work studies the effect of nickel decoration on the hydrogen adsorption properties of single vacancy (SV) defective phosphorene. First principles simulations of Ni decoration show that the SV defective surfaces relax to a doped-like structure with the Ni atom in the place of the vacant phosphorus atom. The functionalised surface shows excess negative charge on neighbouring P atoms, making it suitable for sensing purposes. Additionally, the chemical activity of Ni is reduced due to strong bond formation with phosphorus. Both Ni-decorated SV phosphorene systems have H₂ adsorption energies more than 3 times than that of defective phosphorene, with values between ?0.594 eV and ?0.6 eV. The adsorption mechanism of H₂ is a two-fold process involving a small charge transfer from the surface P atoms and weak dipole-dipole interactions between the H₂ molecule and the Ni atom, as the reduced chemical activity of Ni prevents bond formation with H₂. The results demonstrate Ni-decorated SV Phosphorene as a promising candidate for Hydrogen storage and gas sensing applications. Further, decoration on defective phosphorene surfaces can be regarded as a method to control the chemical activity of transition metals for use in applications such as catalysis.     

Studies on metal hydride electrodes containing no binder additives

Electrochemical properties of hydrogen storage alloys (AB₅ type: LaMm-Ni_4.1Al_0.3Mn_0.4Co_0.45) were studied in 6 M KOHaq using Limited Volume Electrode (LVE) method. Working electrodes were prepared by pressing alloy powder (without binding and conducting additives) into a metal net wire serving as a support and as a current collector. Cyclic voltammetry curves reveal well defined hydrogen sorption and desorption peaks which are separated from other faradic processes, such as surface oxidation. Voltammograms of LVE resemble the curves obtained by various authors for single particle metal alloy electrodes. Hydrogen diffusion coefficient calculated at room temperature for LV electrodes and for 100% state of charge reaches a constant value of ca. 3.3 × 10⁻⁹ and 2.1 × 10⁻¹⁰ cm² s⁻¹, for chronoamperometric and chronopotentiometric measurements, respectively. A comparison of the electrodes with average alloy particle sizes of ca. 50 and 4 μm allows us to conclude that at room temperature hydrogen storage capability of AB₅ alloy studied is independent on the alloy particle size. On the other hand, reduction of the particle size increases alloy capacity at temperatures below −10 °C and reduces time of electrochemical activation of the electrode.     

Interpreting the hydrogen adsorption on organic groups functionalized MOF-5s by statistical physics model

The hydrogen adsorption isotherms at equilibrium on four adsorbents (MOF-5 and three modified MOF-5s named, CH₃-MOF-5, Br-MOF-5 and Cl-MOF-5) were studied using a monolayer model with four adsorption sites energies. The analytical expression of this model was developed using the grand canonical ensemble in statistical physics by taking some working hypotheses involving some physicochemical parameters which can describe the adsorption process. These parameters are: four numbers of hydrogen adsorbed molecules per site (n₁, n₂, n₃ and n₄), four receptor site densities (N_M1, N_M2, N_M3 and N_M4), four saturation adsorbed quantities (Q₁, Q₂, Q₃ and Q₄) and four adsorption energies (??₁, ??₂, ??₃ and ??₄). The evolutions of these parameters in relation with temperature were discussed to understand and interpret the adsorption process at different temperatures. Fitting results revealed that the adsorption of hydrogen on MOF-5 is an exothermic physisorption process. The adsorption surface is inhomogeneous with many site energies. The fitting of the adsorption site is achieved by an aggregate of hydrogen molecules. The adopted model expression is used to derive the thermodynamic potential functions which govern the sorption mechanism such as entropy S_a, free enthalpy of Gibbs G and internal energy E_int.     

Hydrogen diffusion and hydrogen influenced critical stress intensity in an API X70 pipeline steel welded joint – Experiments and FE simulations

An API X70 pipeline steel has been investigated with respect to hydrogen diffusion and fracture mechanics properties. A finite element cohesive element approach has been applied to simulate the onset of hydrogen-induced fracture. Base metal, weld simulated heat affected zone and weld metal have been investigated. The electrochemical permeation technique was used to study hydrogen diffusion properties, while in situ fracture mechanics testing was performed in order to establish the hydrogen influenced threshold stress intensity. The average effective diffusion coefficient at room temperature was 7.60 × 10⁻¹¹ m²/s for the base metal, 4.01 × 10⁻¹¹ m²/s for weld metal and 1.26 × 10⁻¹¹ m²/s for the weld simulated heat affected zone. Hydrogen susceptibility was proved to be pronounced for the heat affected zone samples. Fracture toughness samples failed at a net section stress level of 0.65 times the yield strength; whereas the base metal samples did not fail at net section stresses lower than the ultimate tensile strength. The initial cohesive parameters which best fitted the experimental results were σ_c = 1500 MPa (3.1·σ_y) for the base metal, σ_c = 1800 MPa (3.0·σ_y) for weld metal and σ_c = 1840 MPa (2.3·σ_y) for heat affected zone. Threshold stress intensities K_Ic,HE were in the range 143–149 MPa√m.     

Hydrogen storage properties of TiMn1.5V0.2-based alloys for application to fuel cell system

To meet the requirements of fuel cell power system for electric bike, the influence of partial substitution of Zr and Cr on hydrogen storage performance of TiMn_1.5V_0.2-based alloys is investigated first, and a hydrogen storage tank is then built using the developed TiMn_1.5V_0.2-based alloy as metal hydride bed and its hydrogen supply ability is further evaluated. It is found that for TiMn_1.5V_0.2-based alloys, the Zr substitution for Ti effectively reduces the plateau pressure but increases the plateau slope, while the partial substitution of Mn by Cr decreases the absorption plateau pressure, leading to a smaller hysteresis factor. After the optimization of components, 6 kg of Ti_0.95Zr_0.05Mn_1.4Cr_0.1V_0.2 alloy powder with 5 wt.% aluminum foam is mixed uniformly to form a metal hydride bed inside the tank. The measurements show that the tank releases up to 82 g of hydrogen to produce a 200 W fuel cell output for 300 min and has a stable cyclic capacity, indicating that hydrogen storage system of TiMn_1.5V_0.2-based alloys for fuel cell power system of electric bike is applicable.     

Lab-size rechargeable metal hydride-air cells

Lab-size rechargeable metal hydride-air (MH-air) cells with a gas management device were designed in order to minimize the loss of electrolyte. An AB₅-type hydrogen storage alloy was used as anode materials of the MH-air. The thickness of the metal hydride electrodes was in the range of 3.0-3.4 mm. Porous carbon-based air electrodes with Ag₂O catalysts were used as bi-functional electrodes for oxygen reduction and generation. The electrodes were first examined in half-cells to evaluate their performance and then assembled into one MH-air cell. The results showed the good cycling stability of the rechargeable MH-air cell with a capacity of 1990 mAh. The discharge voltage was 0.69 V at 0.05-0.1 C. The charge efficiency was about 90%. The specific and volumetric energy densities were about 95Wh kg⁻¹ and 140 Wh L⁻¹, respectively.     

High density hydrogen storage in nanocavities: Role of the electrostatic interaction

High pressure H₂ adsorption isotherms at N₂ liquid temperature were recorded for the series of cubic nitroprussides, Ni_1−xCo_x[Fe(CN)₅NO] with x = 0, 0.5, 0.7, 1. The obtained data were interpreted according to the effective polarizing power for the metal found at the surface of the cavity. The cavity volume where the hydrogen molecules are accumulated was estimated from the amount of water molecules that are occupying that available space in the as-synthesized solids considering a water density of 1 g/cm³. The calculated cavity volume was then used to obtain the density of H₂ storage in the cavity. For the Ni-containing material the highest storage density was obtained, in a cavity volume of 448.5 Å³ up to 10.4 hydrogen molecules are accumulated, for a local density of 77.6 g/L, above the density value corresponding to liquid hydrogen (71 g/L). Such high value of local density was interpreted as related to the electrostatic contribution to the adsorption potential for the hydrogen molecule within the cavity.