Synthesis,characterization and hydrogen adsorption in mixed crystals of MOF-5 and MOF-177

Mixed MOF crystals with morphology similar to that of pure MOF-5 and pure MOF-177 were synthesized using two organic solvents: dimethylformamide (DMF) and diethylformamide (DEF). The mixed crystals were characterized with XRD, SEM and TGA for their physical properties and also evaluated for their hydrogen adsorption properties. The XRD and SEM results suggest that the mixed crystals are different from pure MOF-5 and pure MOF-177. The DMF-derived mixed MOF crystals have a slightly higher specific surface area, smaller pore diameter and greater pore volume than those of the DEF-derived crystals, and seem to be a better adsorbent than the DEF-derived crystals, which was confirmed by the higher hydrogen and nitrogen adsorption capacities on the DMF-derived crystals. The hydrogen adsorption capacities on the mixed MOF crystals are lower than those of pure MOF-5 and MOF-177. It was also observed that the hydrogen diffusion time constant increases with hydrogen pressure, and the heat of hydrogen adsorption decreases with adsorbed hydrogen amount on both mixed crystals.     

MOF-5 composites exhibiting improved thermal conductivity

The low thermal conductivity of the prototype hydrogen storage adsorbent, metal-organic framework 5 (MOF-5), can limit performance in applications requiring rapid gas uptake and release, such as in hydrogen storage for fuel cell vehicles. As a means to improve thermal conductivity, we have synthesized MOF-5-based composites containing 1–10 wt.% of expanded natural graphite (ENG) and evaluated their properties. Cylindrical pellets of neat MOF-5 and MOF-5/ENG composites with densities of 0.3, 0.5, and 0.7 g/cm³ are prepared and assessed with regard to thermal conductivity, specific heat capacity, surface area, and crystallinity. For pellets of density ∼0.5 g/cm³, we find that ENG additions of 10 wt.% result in a factor of five improvement in thermal conductivity relative to neat MOF-5, increasing from 0.10 to 0.56 W/mK at room temperature. Based on the relatively higher densities, surface areas, and enhanced crystallinity exhibited by the composites, ENG additions appear to partially protect MOF-5 crystallites from plastic deformation and/or amorphization during mechanical compaction; this suggests that thermal conductivity can be improved while maintaining the favorable hydrogen storage properties of this material.     

Heterofullerene C48B12-impregnated MOF-5 and IRMOF-10 for hydrogen storage: A combined DFT and GCMC simulations study

The H₂ storage properties of isoreticular metal-organic framework materials (IRMOFs), MOF-5 and IRMOF-10, impregnated with different numbers and types of heterogeneous C₄₈B₁₂ molecules were investigated using density functional theory and grand canonical Monte Carlo (GCMC) calculations. The excess hydrogen adsorption isotherms of IRMOFs at 77 K within 20 bar indicate that suitable number and type of C₄₈B₁₂ molecules play a crucial role in improving the H₂ storage properties of IRMOFs. Among the studied pure and nC₄₈B₁₂ (n = 1, 2, 4, 8) in Ci symmetry impregnating into MOF-5, at 77 K under 6 bar, MOF-5-4C₄₈B₁₂ with a 3.5 wt% and 29.9 g/L hydrogen storage density, and at 77 K under 12 bar, the pure MOF-5 with a 4.9 wt% and 31.0 g/L hydrogen storage density has the best hydrogen storage properties. Whereas, among the studied pure and nC₄₈B₁₂ (n = 1, 2, 4, 8) in S6 symmetry impregnating into IRMOF-10, IRMOF-10-8C₄₈B₁₂ always shows the best hydrogen storage properties among the pure and C₄₈B₁₂-impregnated IRMOF-10 at 77 K within 20 bar. IRMOF-10-8C₄₈B₁₂ has a 6.0 wt% and 34.6 g/L hydrogen storage density at 77 K under 6 bar, and has a 7.1 wt% and 41.4 g/L hydrogen storage density at 77 K under 12 bar. The confinement effect of IRMOFs on C₄₈B₁₂ molecules, and steric hindrance effect of C₄₈B₁₂ molecules on IRMOFs mainly affects the H₂ uptake capacity by comparing the absolute H₂ molecules in individual IRMOFs units, C₄₈B₁₂ molecules, and IRMOFs-nC₄₈B₁₂ compounds. The absolute hydrogen adsorption profiles show that eight C₄₈B₁₂ molecules impregnating into MOF-5 can exert obvious steric effects for H₂ adsorption. The saturated gravimetric and volumetric H₂ densities of IRMOF-10-8C₄₈B₁₂ higher than those of MOF-5-8C₄₈B₁₂ due to with larger free volume.     

Evaluating the impact of pellet densification and graphite addition for design of on-board hydrogen storage in a fixed bed of MOF-5 pellets

On-board storage of hydrogen is a key challenge in the deployment of fuel cell technology for transportation and distributed energy generation. Hydrogen adsorption capacity of up to 6 wt% has been reported for the metal-organic framework MOF-5, at 30 bar and 77 K. However, powders of MOF-5 suffer from low volumetric storage density and poor thermal conductivity for practical use in adsorptive storage systems. Compaction of MOF-5 to form pellets and inclusion of expanded natural graphite (ENG) has been used to address these issues, but their effect on the overall refueling dynamics for a fixed-bed has not been studied. To this end, we use simulations of multiscale pellet-and-bed model (developed in a companion paper) to analyze the impact of pellet modification on the dynamics of hydrogen refueling under cryogenic conditions. We show that a fixed bed with 0.52 g/cc density pellets is recommended, compared to MOF-5 powder or lower-density pellets. In spite of some loss of gravimetric capacity, the former shows good performance in a fixed bed with improved volumetric capacity and reasonable refueling time. Although individual pellet behavior is improved by addition of ENG to the 0.52 g/cc pellets, this has only a minor effect on refueling dynamics of the fixed bed with pellet size of 6 mm or lower. Finally, the effect of pellet size, density and ENG addition is analyzed and recommendations for fixed bed adsorber design are presented.     

MOF-74-immobilized ternary RhNiP nanoparticles as highly efficient hydrous hydrazine dehydrogenation catalysts in alkaline solutions

For the first time, a series of highly efficient RhNiP@MOF-74 nanocomposite catalysts were synthesized by a one-step reduction process, subsequently characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). Synergistic electronic effects between Rh, Ni, and P, along with electron transfer phenomena between RhNiP and MOF-74 resulted in outstanding catalytic activities towards the dehydrogenation of hydrous hydrazine in an alkaline solution (2 M NaOH) at 50 °C. The turnover frequency (TOF) value of Rh₄₇Ni₁₈P₃₅@MOF-74 reached 715.4 mol H₂·h⁻¹(mol metal)⁻¹ with 100% hydrogen selectivity, and there was no significant decrease even after five cycles.     

Investigation of cryo-adsorption hydrogen storage capacity of rapidly synthesized MOF-5 by mechanochemical method

Comparisons were made between the samples mechanochemically (MOF-5(M)) and solvothermally (MOF-5(S)) prepared for the development of efficient hydrogen storage medium. Synthesized samples were undergone structural characterization as well as adsorption equilibrium measurements of hydrogen at temperature-pressure range 77 K–87 K and 0.1–10 MPa. Grand Canonical Monte Carlo (GCMC) simulations were further conducted to study the behaviors of hydrogen molecules adsorbed on MOF-5. It shows that, besides the advantage of large scale synthesis and a lower cost, mechanochemical method respectively brings about 207% and 90.5% increments in the specific surface area and the maximum excess adsorption capacity of hydrogen at 77 K within pressure range 0–10 MPa. Results also reveal that the crystal within MOF-5(M) is regular and distributing uniformly with a mean size only one tenth of that of the MOF-5(S); at 77 K within pressure range 0–10 MPa, Toth equation can predict the adsorption equilibrium data of hydrogen on two MOF-5 samples with a mean relative error less than 1.5%. It suggests that MOF-5(M) is more promising for hydrogen storage by adsorption for practical applications.     

Enhancement of hydrogen adsorption by alkali-metal cation doping of metal-organic framework-5

Over the past decade, metal-organic frameworks (MOFs) have been extensively studied as a novel approach to store hydrogen. The large surface area and volume of micropores that are intrinsic to MOFs make them ideal for gas adsorption. In addition, we chemically reduced MOF-5 by doping it with alkali metals (Li, Na, and K). We found that the H₂ uptake capacity of MOF-5 materials doped with Li, Na, and K exceeded that of a neutral framework by 24%, 68%, and 70%, respectively. Notably, at the same levels of doping, the Li⁺-doped framework exhibited the strongest H₂ binding, and the binding strength decreased sequentially in the order Li⁺ > Na⁺ > K⁺.     

Hydrogen storage and electrochemical properties of annealed low-Co AB5 type intermetallic compounds

The structure, hydrogen storage and electrochemical properties of annealed low-Co AB₅-type intermetallic compounds have been investigated. La-alloy, Nd-alloy and Cr-alloy are used to represent La_0.8Ce_0.2Ni₄Co_0.4Mn_0.3Al_0.3, La_0.6Ce_0.2Nd_0.2Ni₄Co_0.4Mn_0.3Al_0.3 and La_0.6Ce_0.2Nd_0.2Ni_3.8Co_0.4Mn_0.3Al_0.3Cr_0.2, respectively. The XRD results indicated that annealed samples are all single-phase alloys with CaCu₅ type structure. The maximum of both hydrogen content and discharge capacity is obtained for La-alloy 1.23 wt%H₂ and 321.1 mA h/g, respectively. All the investigated alloys are quiet stable with ΔH of hydrogen desorption about 36–38 kJ/mol H₂. Cycle life of alloy electrode has been improved by partial substitution of La for Nd and Ni for Cr. The highest capacity retention of 92.2% after 100 charge/discharge cycles at 1C has been observed for Nd-alloy. The hydrogen diffusion coefficient measured by PITT is higher at the start of charging process and dramatically reduces by 2–3 order of magnitude with saturation of β-hydride. The highest value 6.9 × 10^?13 cm²/s is observed for La alloy at 100% SOC. Partial substitution La for Nd and Cr for Ni in low-Co AB₅ metal hydride alloys slightly reduces maximum discharge capacity, HRD performance and hydrogen diffusion kinetics. Low-Co alloys show good overall electrochemical properties compared to high-Co alloys and might be perspective materials for various electrochemical applications.     

Hydrogen diffusion and vacancy clusterization in iron

Molecular statics and molecular dynamics simulations were performed to study hydrogen diffusion and vacancy clustering in alpha iron. In particular, it was found that hydrogen atom binds very strongly with vacancies, rather than other hydrogen atoms. The monovacancies were inclined to form the VH4, VH3, VH2 and VH1 complexes, rather than VH6 in the range of our simulated temperatures. The rate of hydrogen diffusion was apparently reduced in the presence of vacancies, while the vacancy trap effect was gradually weakened with increasing temperature. The presence of vacancies changes the diffusion mechanism of H atoms. Moreover, we found that vacancy clusters tended to be formed at the moderate range of temperatures, and fewer clusters were observed at either low or high temperatures. The number of vacancy clusters reduced, while hydrogen-vacancy clusters were gradually created with the increase of hydrogen concentration.     

MOF-5 and activated carbons as adsorbents for gas storage

This paper reports comparatively the capacities of two activated carbons (ACs) and MOF-5 for storing gases. It analyzes, using similar equipments and experimental procedures, the density used to convert gravimetric data to volumetric ones, measuring the density (tap and packing at different pressures). It presents data on porosity, surface area and gas storage (H₂, CH₄ and CO₂) obtained under different temperatures (77 K and RT) and pressures (0.1, 4 and 20 MPa). MOF-5 presents lower volume of narrow micropores than both ACs, making its storage at RT lower, independently of the gas used (H₂, CH₄ and CO₂) and the basis of reporting data (gravimetric or volumetric). For H₂ at 77 K the reliability of the results depends too much on the density used. It is shown that the outstanding volumetric performance of MOF-5, in relation to ACs, is due to the use of an unrealistic high density (crystal density) that, not including the adsorbent inter-particle space, gives an apparently high volumetric gas storage capacity. When a density measured similarly in both types of adsorbents is used (e.g. tap or packing densities) MOF-5 presents, for all gases and conditions studied, lower adsorption capacities on volumetric basis and storage capacities than ACs.     

Hydrogen permeation in a Cr–Mo–V medium-carbon steel: Effect of the quenching medium and tempering temperature

Hydrogen permeation tests are carried out to evaluate the effect of the quenching medium and tempering temperature on the permeation parameters and density of hydrogen traps, of a Cr–Mo–V low-alloy medium-carbon steel. Three types of steel membranes are tested: 1) in the as-quenched condition, 2) tempered at 235 °C and 3) tempered at 530 °C; each one quenched in two different media: oil or brine. From the as-quenched condition, the apparent concentration of hydrogen and hydrogen flux, tend to decrease as the tempering temperature increases. The membranes in the as-quenched condition and tempered at 530 °C, show lower hydrogen diffusivity and higher density of hydrogen traps than membranes tempered at 235 °C. It is concluded that tempering at 235 °C, promotes hydrogen induced cracking, which is contrary to what has been previously determined. The cracking is related to a higher hydrogen diffusivity and lower density of hydrogen traps.     

Sieverts law pressure exponent for hydrogen permeation through Pd-based membranes: Coupled influence of non-ideal diffusion and multicomponent external mass transfer

This paper focuses on the suitability of using a Sieverts-type empirical law to describe hydrogen permeation through Pd-based membranes in the presence of both concentration-dependent hydrogen transport through the selective layer – referred to as “non-ideal” behaviour – and external mass transfer resistance. In particular, the functionality of the pressure exponent with temperature, pressure and membrane thickness is provided, showing that the Sieverts-type empirical law can be used to incorporate the external mass transfer influence just up to a moderate external resistance. The explanation for this fact is recognised in the inadequacy of the Sieverts-type empirical law to describe the behaviour of the flux limited by external resistance for a sufficiently large permeation driving force. The methodology described in this paper is effective also when the permeation-determining steps cannot be established, helping researchers to a correct interpretation of permeation tests in the presence of severe mass transfer resistance.     

Hydrogen permeation in anisotropic Nb–TiNi two-phase alloys formed by forging and rolling

The formation of an anisotropic microstructure by forging and rolling of a Nb–TiNi two-phase alloy and the effects of direction and annealing on hydrogen permeability were investigated. After forging and rolling, the granular (Nb, Ti) phase was strongly elongated along the rolling direction (RD) and compressed along the normal direction (ND). Hydrogen permeability along the RD (ND) increased (decreased) dramatically. The hydrogen permeability of this anisotropic microstructure can be explained by the law of mixtures using the hydrogen permeabilities of (Nb, Ti) and TiNi single-phase alloys. The hydrogen permeabilities along RD and ND correspond to parallel- and series-type hydrogen permeability, respectively. The 94-μm-thick RD sample shows a large hydrogen flux of 0.57 mol H₂ m^?2 s^?1 (77 ccH₂ cm^?2 min^?1) without hydrogen embrittlement. Phase boundary between (Nb, Ti) and TiNi phases, aligned parallel to the hydrogen flux, is one of the hydrogen permeation path.     

Hydrogen diffusion and the effect of hydrogen on structural transformations in binary TiNi based alloys

The paper analyzes the effect of electrolytic hydrogenation on martensite transformation temperatures in binary TiNi alloys. The analysis shows that this effect can be strong or weak depending on the phase state of TiNi. Research data are presented on the diffusivity of hydrogen in binary TiNi alloys in martensite and austenite states. The diffusion coefficient of hydrogen is estimated from its distribution measured by glow discharge spectroscopy in TiNi after hydrogenation. The experimental results about the formation of TiNiH hydride in the martensitic and austenitic state in binary TiNi based alloys are also presented.     

Static and dynamic hydrogen adsorption on Pt/AC and MOF-5

Hydrogen adsorption has been studied by static and dynamic methods on activated carbon (AC), platinum/activated carbon (Pt/AC), metal organic frameworks (MOF-5), and Pt/AC_MOF-5.The static method showed that all of adsorbents used in this study exhibited a Langmuir (type I) adsorption isotherm at 77 K and a linear function of hydrogen partial pressure at 298 K. The dynamic method produced breakthrough curves, indicating (i) slow rate of hydrogen diffusion in the densely packed activated carbon and Pt/AC beds and (ii) high rate of hydrogen diffusion in the loosely packed bed with large MOF-5 crystallites. Temperature variable adsorption resulted in the higher hydrogen uptake on Pt/AC than other adsorbents. The results suggested that temperature variable adsorption enhanced the hydrogen storage process by (i) initiating hydrogen dissociation at high temperature and (ii) facilitating spillover at low temperature on Pt/AC.     

The factors affecting the diffusion properties of hydrogen in palladium copper alloys: Ab initio study

The experiments showed that the addition of copper (Cu) in palladium (Pd) membrane reduces the permeability of hydrogen (H) in Pd, and the permeability of H in body-centered cubic (BCC) PdCu alloy is larger than that in face-centered cubic (FCC) PdCu alloy. The underlying reasons are still not well understood. In the present work, systematical ab initio calculations are carried out to investigate the factors affecting the diffusion properties of H in PdCu alloys. It is found that the lowest diffusion barrier of H with zero-point energy correction in BCC PdCu alloy (0.016 eV) is relatively low than that in FCC PdCu alloy (0.346 eV). The interactions among interstitial H atoms are significantly repulsive in BCC PdCu alloy, while it is attractive in FCC PdCu alloy. The results suggest that it is energetically favorable to form interstitial H cluster in FCC PdCu alloy rather than that in BCC PdCu alloy. The formation of interstitial H clusters impedes the fast movement of H in FCC PdCu alloy. The presence of intrinsic vacancy in FCC PdCu alloy further hinders the migration of H. It is found that the binding strength of vacancy with H in FCC PdCu alloy is much larger than that in BCC PdCu. The stronger the combination of H and vacancy, the greater the obstacle to the movement of H in PdCu alloy. All these results elucidate the experimental phenomena why H penetration ability in FCC PdCu alloy is weaker than that in BCC PdCu alloy.     

Concentration-dependent hydrogen diffusion in hydrogenation and dehydrogenation of vanadium-coated magnesium nanoblades

We carry out a computational investigation to show how the exponential concentration dependence of hydrogen diffusion, which was recently verified in a combined experimental and analytical study, could affect the characteristics of hydrogenation and dehydrogenation of a vanadium-coated magnesium nanoblade. A reaction model is built that separates hydrogen surface sorption and interior diffusion during the hydrogenation/dehydrogenation process. For the hydrogenation process, the hydrogen surface adsorption is much faster than the hydrogen diffusion, resulting in high hydrogen concentration buildup at the surface at a relatively low temperature. With increasing temperature, the hydrogen diffusion time decreases more rapidly than the hydrogen surface adsorption time. This leads to a relatively low-gradient diffusion field in the nanoblade during most time of the hydrogenation process, and no shell-core structure with a finite hydride layer is observed. However, for the dehydrogenation process, when hydrogen molecules are released at the surface, a hydride core is formed inside the nanoblade and the interface recedes gradually. The receding rate of the hydride core is determined by the hydrogen molecule release rate. In a two-dimensional simulation with decorated vanadium catalyst islands on the surface, isolated interior hydride islands are sometimes observed before the hydride core entirely fades away. The hydride core boundary is sharper at lower temperature when the surface reaction rate is high relative to the interior diffusion rate.     

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 permeation dynamics across a palladium membrane in a varying pressure environment

Permeation dynamic of hydrogen through a palladium (Pd) membrane in an environment of varying pressure is investigated and analyzed experimentally. By monitoring the instantaneous pressure and mass transfer rate of hydrogen in the conducted system, the present study provides a comprehensive and precise measurement on the permeance of the membrane. It is found that a threshold of pressure difference between the both sides of the membrane for hydrogen permeation is exhibited. That is, when the driving force of the mass transfer is below the minimum pressure difference, hydrogen permeation will not occur. Accordingly, a modified equation accounting for the hydrogen permeation flux through the membrane is suggested. As a whole, the hydrogen permeation flux versus the pressure difference is characterized by a linear relationship, regardless of what the pressure exponent is. Nevertheless, the optimal pressure exponent is located between 0.5 and 0.7. A dimensionless time, the permeation number, is derived to describe the permeation process. The characteristic time of hydrogen permeation depends on the pressure exponent. The experiments reveal that the permeation number is around 7–13 for the hydrogen permeation flux in the system reaching the quasi-steady state.     

Modelling of hydrogen diffusion in a steel containing micro-porosity. Application to the permeation experiment

Two numerical models based on non-equilibrium and local equilibrium approaches respectively were developed to simulate hydrogen transport in porous metals, taking account of gaseous hydrogen trapping inside the micro-porosities. They were applied to the case of hydrogen permeation in a cast steel at room temperature. Numerical simulations revealed that the two models are equivalent under certain conditions. A parametric analysis was performed to explore the effect of external hydrogen fugacity, hydrogen solubility and porosity fraction on the hydrogen diffusion behavior. A comparison between experimental permeation data and the numerical results showed reasonable agreement considering no input parameter was adjusted.