Generalized model development for a cryo-adsorber and 1-D results for the isobaric refueling period

We have developed 3-D model equations for a cryo-adsorption hydrogen storage tank, where the energy balance accommodates the temperature and pressure variation of all the thermodynamic properties. We then reduce the 3-D model to the 1-D isobaric system and study the isobaric refueling period, for simplified geometry and charging conditions. The hydrogen capacity evolution predicted by the 1-D axial bed model is significantly different than that predicted by the lumped-parameter model because of the presence of sharp temperature gradients during refueling. The 1-D model predicts a higher hydrogen capacity than the lumped-parameter model. This observation can be rationalized by the fact that a bed with temperature gradients on equilibration should desorb gas, whenever the adsorbed phase entropy is lower than the gas phase entropy. The 1-D analysis of the isobaric refueling period does not show any significant difference in hydrogen capacity evolution among the axial, single and multicartridge annular bed designs. Hence, a multicartridge annular design, though giving a slightly lower pressure drop, does not offer any heat and mass transfer enhancement over the single cartridge design. And, the single cartridge annular design appears to be optimal.     

A generalized cryo-adsorber model and 2-D refueling results

Our earlier work [Int J Hyd Energy 2010; 35: 3598-3609], describes a 3-D model for the cryo-adsorber and the 1-D results for the isobaric refueling period using quasi-static adsorption approximation. Herein the isobaric constraint is relaxed by solving the Darcy equation for computing the pressure drop in the bed, the quasi-static approximation is relaxed by introducing the Langmuir adsorption kinetics, and the 2-D refueling results are compared with the 1-D quasi-static isobaric refueling results. In spite of the significant differences in formulation, the two results compare well with each other. The 2-D refueling results show that the pressure transients equilibrate quickly and a nearly steady pressure profile gets established in the bed. This observation justifies the isobaric approximation used earlier. The Langmuir kinetics used here has a desorption rate constant which could vary depending on the diffusional resistance at adsorbent particle level. A sensitivity analysis of this parameter shows that the refueling rate varies negligibly while this parameter is varied over many orders of magnitude. This observation shows that as long as the pellets are small enough, refueling is controlled by macroscopic processes like simultaneous cooling/adsorption in the bed and the movement of the adsorption front out of the bed, rather than the molecular processes like sorption at an adsorbent site or diffusion through the adsorbent lattice. This observation justifies the quasi-static approximation used earlier. The above two approximations offer significant computational advantage to the design and optimization of cryo-adsorber beds with complex geometry.     

Improved design of metal-organic frameworks for efficient hydrogen storage at ambient temperature: A multiscale theoretical investigation

A multiscale theoretical technique is used to examine the combination of different approaches for hydrogen storage enhancement in metal-organic frameworks at room temperature and high pressure by implementation lithium atoms in linkers. Accurate MP2 calculations are performed to obtain the hydrogen binding sites and parameters for the following grand canonical Monte Carlo (GCMC) simulations. GCMC calculations are employed to obtain the hydrogen uptake at different thermodynamic conditions. The results obtained demonstrate that the combination of different approaches can improve the hydrogen uptake significantly. The hydrogen content reaches 6.6 wt% at 300 K and 100 bar satisfying DOE storage targets (5.5 wt%).     

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.     

Simulation of hydrogen storage tank packed with metal-organic framework

Hydrogen adsorption in high surface metal-organic framework (MOF) has generated significant interest over the past decade. We studied hydrogen storage processes of MOF-5 hydrogen storage systems with adsorbents of both the MOF-5 powder (0.13 g/cm³) and its compacted tablet (0.30 g/cm³). The charge–discharge cycles of the two MOF-5 adsorbents were simulated and compared with activated carbon. The physical model is based on mass, momentum and energy conservation equations of the adsorbent-adsorbate system composed of gaseous and adsorbed hydrogen, adsorbent bed and tank wall. The adsorption process was modeled using a modified Dubinin–Astakov (D–A) adsorption isotherm and its associated variational heat of adsorption. The model was implemented by means of finite element analysis software Comsol Multiphysics™, and the system simulation platform Matlab/Simulink™. The thermal average temperature from Comsol simulation is used to fill the gap between the system model and the multi-dimensional models. The heat and mass transfer feature of the model was validated by the experiments of activated carbon, the simulated pressure and temperatures are in good agreement with the experimental results. The model was further validated by the metal-organic framework of Cu-BTC and is being extended its application to MOF-5 in this study. The maximum pressure in the powder MOF-5 tank is much higher than that in the activated carbon tank due to the lower adsorbent density of MOF-5 and resulting lower hydrogen adsorption. The maximum pressure in the compacted MOF-5 tank is a little bit lower than that in the activated carbon tank due to the higher adsorbent density and resulting higher hydrogen adsorption. The temperature swings during the charge–discharge cycle of both MOF-5 tanks are higher than that of the activated carbon tank. These are caused mainly by pressure work in the powder MOF-5 tank and by adsorption heat in the compacted MOF-5 tank. For both MOF-5 hydrogen storage systems, the lumped parameter models implemented by Simulink agree well with experimental pressures and with pressures and thermal average temperatures from Comsol simulation.     

Enhanced room-temperature hydrogen storage capacity in Pt-loaded graphene oxide/HKUST-1 composites

Series of Pt-loaded graphene oxide (GO)/HKUST-1 composites were synthesized by the reaction between Pt@GO and precursors of HKUST-1. The parent materials and composites have been characterized by powder X-ray diffraction (XRD), Infrared (IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and gas adsorption analyzer. The XRD and IR analysis showed that the incorporation of Pt@GO did not prevent the formation of HKUST-1 units. SEM, TEM and EDS results revealed that Pt nanoparticles were well-dispersive and anchored tightly into composites. Meanwhile, the percentage of Pt@GO has an obvious effect on morphologies, crystallinities and surface areas of composites. More importantly, the significant enhancement of hydrogen storage capacity at ambient temperature for the composite with low Pt@GO content can be ascribed to the hydrogen spillover mechanism in such system.     

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.     

Review of hydrogen storage techniques for on board vehicle applications

Hydrogen gas is increasingly studied as a potential replacement for fossil fuels because fossil fuel supplies are depleting rapidly and the devastating environmental impacts of their use can no longer be ignored. H₂ is a promising replacement energy storage molecule because it has the highest energy density of all common fuels by weight. One area in which replacing fossil fuels will have a large impact is in automobiles, which currently operate almost exclusively on gasoline. Due to the size and weight constraints in vehicles, on board hydrogen must be stored in a small, lightweight system. This is particularly challenging for hydrogen because it has the lowest energy density of common fuels by volume. Therefore, a lot of research is invested in finding a compact, safe, reliable, inexpensive and energy efficient method of H₂ storage. Mechanical compression as well as storage in chemical hydrides and absorption to carbon substrates has been investigated. An overview of all systems including the current research and potential benefits and issue are provided in the present paper.     

Effect of platinum doping of activated carbon on hydrogen storage behaviors of metal-organic frameworks-5

In this work, we prepared platinum doped on activated carbons/metal-organic frameworks-5 hybrid composites (Pt-ACs-MOF-5) to obtain a high hydrogen storage capacity. The surface functional groups and surface charges were confirmed by Fourier transfer infrared spectroscopy (FT-IR) and zeta-potential measurement, respectively. The microstructures were characterized by X-ray diffraction (XRD). The sizes and morphological structures were also evaluated using a scanning electron microscopy (SEM). The pore structure and specific surface area were analyzed by N₂/77 K adsorption/desorption isotherms. The hydrogen storage capacity was studied by BEL-HP at 298 K and 100 bar. The results revealed that the hydrogen storage capacity of the Pt-ACs-MOF-5 was 2.3 wt.% at 298 K and 100 bar, which is remarkably enhanced by a factor of above five times and above three times compared with raw ACs and MOF-5, respectively. In conclusion, it was confirmed that Pt particles played a major role in improving the hydrogen storage capacity; MOF-5 would be a significantly encouraging material for a hydrogen storage medium as a receptor.     

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.     

Technical assessment of cryo-compressed hydrogen storage tank systems for automotive applications

On-board and off-board performance and cost of cryo-compressed hydrogen storage are assessed and compared to the targets for automotive applications. The on-board performance of the system and high-volume manufacturing cost were determined for liquid hydrogen refueling with a single-flow nozzle and a pump that delivers liquid H₂ to the insulated cryogenic tank capable of being pressurized to 272 atm. The off-board performance and cost of delivering liquid hydrogen were determined for two scenarios in which hydrogen is produced by central steam methane reforming (SMR) or by central electrolysis. The main conclusions are that the cryo-compressed storage system has the potential of meeting the ultimate target for system gravimetric capacity, mid-term target for system volumetric capacity, and the target for hydrogen loss during dormancy under certain conditions of minimum daily driving. However, the high-volume manufacturing cost and the fuel cost for the SMR hydrogen production scenario are, respectively, 2–4 and 1.6–2.4 times the current targets, and the well-to-tank efficiency is well short of the 60% target specified for off-board regenerable materials.     

Crown-like Zn0.5Cd0.5S/M-TiO2 composites derived from metal-organic frameworks for photocatalytic hydrogen production

Photocatalytic hydrogen production has been recognized as one of the most desirable approaches to overcome the worldwide energy and environmental issues. Here, novel sea urchin-like Zn_0.5Cd_0.5S and mesoporous TiO₂ (M-TiO₂) are designed, and a series of crown-like Zn_0.5Cd_0.5S/M-TiO₂ composites with different contents of M-TiO₂ are synthesized by hydrothermal method. The optimum hydrogen production rate of composites reaches 180.4 mmolh^?1g^?1 with the AQE up to 48.9% at 420 nm, which is 3.5 and 216 times that of pure Zn_0.5Cd_0.5S and the M-TiO₂, respectively. The outstanding performance of optimized Zn_0.5Cd_0.5S/M-TiO₂ composite prepared in this work exceeds most reported Cd-S-based catalysts. The improvement on the photocatalytic performance of composites is mainly due to the enlarged specific surface area, the exposure of more active sites, and the enhancement of the electron-hole separation efficiency.     

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⁺.     

Vehicular storage of hydrogen in insulated pressure vessels

This paper describes an alternative technology for storing hydrogen fuel onboard vehicles. Insulated pressure vessels are cryogenic capable vessels that can accept cryogenic liquid hydrogen, cryogenic compressed gas or compressed hydrogen gas at ambient temperature. Insulated pressure vessels offer advantages over conventional storage approaches. Insulated pressure vessels are more compact and require less carbon fiber than compressed hydrogen vessels. They have lower evaporative losses than liquid hydrogen tanks, and are lighter than metal hydrides.

The paper outlines the advantages of insulated pressure vessels and describes the experimental and analytical work conducted to verify that insulated pressure vessels can be safely used for vehicular hydrogen storage. Insulated pressure vessels have successfully completed a series of certification tests. A series of tests have been selected as a starting point toward developing a certification procedure. An insulated pressure vessel has been installed in a hydrogen fueled truck and tested over a six month period.     

Hierarchical methodology for modeling hydrogen storage systems. Part I: Scoping models

Detailed models for hydrogen storage systems provide essential design information about flow and temperature distributions, as well as, the utilization of a hydrogen storage media. However, before constructing a detailed model it is necessary to know the geometry and length scales of the system, along with its heat transfer requirements, which depend on the limiting reaction kinetics. More fundamentally, before committing significant time and resources to the development of a detailed model, it is necessary to know whether a conceptual storage system design is viable. For this reason, a hierarchical system of models progressing from scoping models to detailed analyses was developed. This paper, which discusses the scoping models, is the first in a two part series that presents a collection of hierarchical models for the design and evaluation of hydrogen storage systems.     

Hierarchical methodology for modeling hydrogen storage systems. Part II: Detailed models

There is significant interest in hydrogen storage systems that employ a media which either adsorbs, absorbs or reacts with hydrogen in a nearly reversible manner. In any media based storage system the rate of hydrogen uptake and the system capacity is governed by a number of complex, coupled physical processes. To design and evaluate such storage systems, a comprehensive methodology was developed, consisting of a hierarchical sequence of models that range from scoping calculations to numerical models that couple reaction kinetics with heat and mass transfer for both the hydrogen charging and discharging phases. The scoping models were presented in Part I [Hardy BJ, Anton DL. Hierarchical methodology for modeling hydrogen storage systems, Part I: scoping models. Int J Hydrogen Energy 2009;34(5):2269–77.] of this two part series of papers. This paper describes a detailed numerical model that integrates the phenomena occurring when hydrogen is charged and discharged. A specific application of the methodology is made to a system using NaAlH₄ as the storage media.     

Technical assessment of compressed hydrogen storage tank systems for automotive applications

The performance and cost of compressed hydrogen storage tank systems has been assessed and compared to the U.S. Department of Energy (DOE) 2010, 2015, and ultimate targets for automotive applications. The on-board performance and high-volume manufacturing cost were determined for compressed hydrogen tanks with design pressures of 350 bar (∼5000 psi) and 700 bar (∼10,000 psi) capable of storing 5.6 kg of usable hydrogen. The off-board performance and cost of delivering compressed hydrogen was determined for hydrogen produced by central steam methane reforming (SMR). The main conclusions of the assessment are that the 350-bar compressed storage system has the potential to meet the 2010 and 2015 targets for system gravimetric capacity but will not likely meet any of the system targets for volumetric capacity or cost, given our base case assumptions. The 700-bar compressed storage system has the potential to meet only the 2010 target for system gravimetric capacity and is not likely to meet any of the system targets for volumetric capacity or cost, despite the fact that its volumetric capacity is much higher than that of the 350-bar system. Both the 350-bar and 700-bar systems come close to meeting the Well-to-Tank (WTT) efficiency target, but fall short by about 5%.     

Finite element-based simulation of a metal hydride-based hydrogen storage tank

In this paper, a novel 3D flexible tool for simulation of metal hydrides-based (LaNi₅) hydrogen storage tanks is presented. The model is Finite Element-Based and considers coupled heat and mass transfer resistance through a non-uniform pressure and temperature metal hydride reactor. The governing equations were implemented and solved using the COMSOL Multiphysics simulation environment. A cylindrical reactor with different cooling system designs was simulated. The shortest reactor fill time (15 min) was obtained for a cooling design configuration consisting of twelve inner cooling tubes and an external cooling jacket. Additional simulations demonstrated that an increase of the hydride thermal conductivity can further improve the reactor dynamic performance, provided that the absorbent bed is sufficiently permeable to hydrogen.     

Increased volumetric hydrogen uptake of MOF-5 by powder densification

The metal-organic framework MOF-5 has attracted significant attention due to its ability to store large quantities of H₂ by mass, up to 10 wt.% absolute at 70 bar and 77 K. On the other hand, since MOF-5 is typically obtained as a bulk powder, it exhibits a low volumetric density and poor thermal conductivity—both of which are undesirable characteristics for a hydrogen storage material. Here we explore the extent to which powder densification can overcome these deficiencies, as well as characterize the impact of densification on crystallinity, pore volume, surface area, and crush strength. MOF-5 powder was processed into cylindrical tablets with densities up to 1.6 g/cm³ by mechanical compaction. We find that optimal hydrogen storage properties are achieved for ρ ∼ 0.5 g/cm³, yielding a 350% increase in volumetric H₂ density with only a modest 15% reduction in gravimetric H₂ excess in comparison to the powder. Higher densities result in larger reductions in gravimetric excess. Total pore volume and surface area decrease commensurately with the gravimetric capacity, and are linked to an incipient amorphization transformation. Nevertheless, a large fraction of MOF-5 crystallinity remains intact in densities up to 0.75 g/cm³, as confirmed from powder XRD. Predictably, the radial crush strength of the pellets is enhanced by densification, increasing by a factor of 4.3 between a density of 0.4 g/cm³ and 0.6 g/cm³. Thermal conductivity increases slightly with tablet density, but remains below the single crystal value.     

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.