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.     

Concepts for improving hydrogen storage in nanoporous materials

Hydrogen storage in nanoporous materials has been attracting a great deal of attention in recent years, as high gravimetric H₂ capacities, exceeding 10 wt% in some cases, can be achieved at 77 K using materials with particularly high surface areas. However, volumetric capacities at low temperatures, and both gravimetric and volumetric capacities at ambient temperature, need to be improved before such adsorbents become practically viable. This article therefore discusses approaches to increasing the gravimetric and volumetric hydrogen storage capacities of nanoporous materials, and maximizing the usable capacity of a material between the upper storage and delivery pressures. In addition, recent advances in machine learning and data science provide an opportunity to apply this technology to the search for new materials for hydrogen storage. The large number of possible component combinations and substitutions in various porous materials, including Metal-Organic Frameworks (MOFs), is ideally suited to a machine learning approach; so this is also discussed, together with some new material types that could prove useful in the future for hydrogen storage applications.     

Thermally stable Ni MOF catalyzed MgH2 for hydrogen storage

The poor kinetics is the main issue hindering MgH₂ for practical hydrogen storage application. In this work, the tricarboxybenzene was used to construct the stable Ni MOF (Ni-BTC300) as the catalyst for MgH₂. The prepared MOFs maintained their chemical structure after 300 °C calcining and doped to MgH₂ by ball milling. The dispersed, uniformly bonded Ni atoms can improve the kinetics of the composites, which could desorb 5.14 wt% H₂ within 3 min at 300 °C. And the stable MOF structure leading to good cycle stability in both kinetics and capacity, with retention of 98.2% after 10 cycles.     

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.     

A fast procedure for the estimation of the hydrogen storage capacity by cryoadsorption of metal-organic framework materials from their available porous properties

In order to identify the best porous materials for the cryogenic physisorption of hydrogen, high-throughput calculations are performed starting, i.e., from the collected information in crystallographic databases. However, these calculations, like molecular simulations, require specific training and significant computational cost. Herein, a relatively simple procedure is proposed to estimate and compare hydrogen uptakes at 77 K and pressure values from 40 bar starting from the porous properties of MOF materials, without involving simulation tools. This procedure uses definitions for adsorption and considers the adsorbed phase as an incompressible fluid whose pressure-density change is that for the liquid phase at 19 K. For the 7000 structures from the CoRE MOF database, the average error of the predictions is only of 1% from reference values at 100 bar, with an SD of ±8%. This accuracy is lower than that from simulation tools, but involving lower computational cost and training.     

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.     

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.     

Potential application of metal-organic frameworks (MOFs) for hydrogen storage: Simulation by artificial intelligent techniques

Metal-organic frameworks are a new class of materials for hydrogen adsorption/storage applications. The hydrogen storage capacity of this structure is typically related to pressure, temperature, surface area, and adsorption enthalpy. Literature provides no reliable correlation for estimating the hydrogen uptake capacity of MOFs from these easy-measured variables. Therefore, this study introduces several straightforward and accurate artificial intelligence (AI) techniques to fill this gap, initially determining the appropriate topology of AI-based methods, then comparing their performances by statistical criteria, and introducing the most accurate. This study used artificial neural networks, hybrid neuro-fuzzy systems, and support vector machines as estimators. The general regression neural networks (GRNN) with a spread of 7.92 × 10⁻⁴ shows the highest correlation with the literature data and provides a relative absolute deviation of 5.34%, mean squared error of 0.059, and coefficient of determination of 0.9946.     

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.     

In-situ one-step synthesis of activated Carbon@MIL-101 (Cr) composites for hydrogen storage

Metal-organic frameworks (MOFs) unlocked new prospects of developing novel adsorbing materials for H₂ storage. However, MOF porosity is not yet fully utilized. To compensate for that disadvantage, we synthesized MIL-101(Cr) MOF-based activated carbon AC@MIL-101 (Cr) composites using in situ hydrothermal method. Different amounts of activated carbon (AC) derived from fir bark were added to adjust the pore structure of the resulting MOF-based composites. The pore number and their sizes increased and decreased, respectively, after pristine MIL-101(Cr) was combined with AC. The surface area and pore volume of pristine MIL-101(Cr) were equal to 2299 m²/g and 1.06 cm³/g, respectively. These values became equal to 3367 m²/g and 1.64 cm³/g after AC was combined with MIL-101(Cr) to form AC@MIL-101(Cr) composite. The highest H₂ uptake by AC@MIL-101(Cr) was equal to 6.93 wt % at 77 K and 40 bar. Such excellent hydrogen storage performance (a 32.3% increase than what was observed for unmodified MIL-101(Cr) material) was attributed to a synergy between AC and MIL-101(Cr).     

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.     

Synthesis and hydrogen adsorption properties of a new iron based porous metal-organic framework

A new metal-organic framework [Fe₃O(OOC-C₆H₄-COO)₃(H₂O)₃]Cl·(H₂O)_x was synthesized with a specific surface area of 2823 m²/g and a lattice parameter of 88.61 Å. Isostructural with MIL-101, this compound exhibits similar hydrogen adsorption properties, with maximum adsorption capacity of 5.1wt.% H at 77 K. The adsorption enthalpy of hydrogen for MIL-101 and ITIM-1 (MIL-101Fe) at zero coverage was calculated for a wide temperature range of 77 K ÷ 324 K, considering corrections for the variation of hydrogen gas entropy with the temperature. The resulted adsorption enthalpy is 9.4 kJ/mol for MIL-101, in excellent agreement with the value reported in literature from microcalorimetric measurements, and a value of 10.4 kJ/mol at zero coverage was obtained for ITIM-1 (MIL-101Fe).     

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.     

Physical model of onboard hydrogen storage tank thermal behaviour during fuelling

A physical model to simulate thermal behaviour of an onboard storage tank and parameters of hydrogen inside the tank during fuelling is described. The energy conservation equation, Abel-Noble real gas equation of state, and the entrainment theory are applied to calculate the dynamics of hydrogen temperature inside the tank and distribution of temperature through the wall to satisfy requirements of the regulation. Convective heat transfer between hydrogen, tank wall and the atmosphere are modelled using Nusselt number correlations. An original methodology, based on the entrainment theory, is developed to calculate changing velocity of the gas inside the tank during the fuelling. Conductive heat transfer through the tank wall, composed of a load-bearing carbon fibre reinforced polymer and a liner, is modelled by employing one-dimensional unsteady heat transfer equation. The model is validated against experiments on fuelling of Type III and Type IV tanks for hydrogen onboard storage. Hydrogen temperature dynamics inside a tank is simulated by the model within the experimental non-uniformity of 5 °C. The calculation procedure is time efficient and can be used for the development of automated hydrogen fuelling protocols and systems.     

Metal hydride hydrogen storage tank for fuel cell utility vehicles

The “low-temperature” intermetallic hydrides with hydrogen storage capacities below 2 wt% can provide compact H₂ storage simultaneously serving as a ballast. Thus, their low weight capacity, which is usually considered as a major disadvantage to their use in vehicular H₂ storage applications, is an advantage for the heavy duty utility vehicles. Here, we present new engineering solutions of a MH hydrogen storage tank for fuel cell utility vehicles which combines compactness, adjustable high weight, as well as good dynamics of hydrogen charge/discharge. The tank is an assembly of several MH cassettes each comprising several MH containers made of stainless steel tube with embedded (pressed-in) perforated copper fins and filled with a powder of a composite MH material which contains AB₂- and AB₅-type hydride forming alloys and expanded natural graphite. The assembly of the MH containers staggered together with heating/cooling tubes in the cassette is encased in molten lead followed by the solidification of the latter. The tank can provide >2 h long H₂ supply to the fuel cell stack operated at 11 kWe (H₂ flow rate of 120 NL/min). The refuelling time of the MH tank (T = 15–20 °C, P(H₂) = 100–150 bar) is about 15–20 min.