Metal-organic Frameworks For Hydrogen Storage Research Articles

Artificial Intelligence and High-Throughput Computational Workflows Empowering the Fast Screening of Metal-Organic Frameworks for Hydrogen Storage.

Metal-organic frameworks (MOFs) are one of the most promising hydrogen-storing materials due to their rich specific surface area, adjustable topological and pore structures, and modified functional groups. In this work, we developed automatically parallel computational workflows for high-throughput screening of ∼11,600 MOFs from the CoRE database and discovered 69 top-performing MOF candidates with work capacity greater than 1.00 wt % at 298.5 K and a pressure swing between 100 and 0.1 bar, which is at least twice that of MOF-5. In particular, ZITRUP, OQFAJ01, WANHOL, and VATYIZ showed excellent hydrogen storage performance of 4.48, 3.16, 2.19, and 2.16 wt %. We specifically analyzed the relationship between pore-limiting diameter, largest cavity diameter, void fraction, open metal sites, metal elements or nonmetallic atomic elements, and deliverable capacity and found that not only geometrical and physical features of crystalline but also chemical properties of adsorbate sites determined the H2 storage capacity of MOFs at room temperature. It is highlighted that we first proposed the modified crystal graph convolutional neural networks by incorporating the obtained geometrical and physical features into the convolutional high-dimensional feature vectors of period crystal structures for predicting H2 storage performance, which can improve the prediction accuracy of the neural network from the former mean absolute error (MAE) of 0.064 wt % to the current MAE of 0.047 wt % and shorten the consuming time to about 10-4 times of high-throughput computational screening. This work opens a new avenue toward high-throughput screening of MOFs for H2 adsorption capacity, which can be extended for the screening and discovery of other functional materials.

The synthesis, activation, and adsorption capacity of metal-organic frameworks for hydrogen storage

Metal-organic frameworks (MOFs) comprise both organic and inorganic parts, allowing for immense diversity and tunability for specific purposes. Hydrogen has a hopeful prospect of being a clean energy carrier because of its lightness and therefore energy density, and it has been reported that MOFs already effectively adsorb hydrogen at cryogenic temperatures. In this article, the main synthesis methods for MOFs for hydrogen storage will be outlined, including the solvothermal, sonochemical, electrochemical and microwave methods. Methods of activation and stability concerns (thermal, mechanical and moisture) will also be addressed. The main focus of the article will be on increasing hydrogen capacity in MOFs at room temperature as opposed to cryogenic. The main ways to achieve this are adjusting pore size and strengthening hydrogen-MOF interaction. Recent research has shown that the most important effects in this regard are generating active metal sites, the spillover mechanism, and adjusting organic linkers.

Metal-organic frameworks for hydrogen storage: progression in synthetical methods on physical and chemical properties

Metal-organic frameworks (MOFs) with their flexibility give rise to various types that help to improve the capability of MOFs in hydrogen storage. In this article, the synthetical methods as a crucial factor in the advancement in MOFs application are detailly presented. These methods contribute to incrementing the adsorption capacity and the stability of MOFs. The tactic for this approach is mainly increased by the exchange and modification of the metal ion and ligand of the complex. The metal ion increases adsorption enthalpy, while the ligand increases surface area and pore volume. Apart from adsorption enthalpy, stability is another critical property for MOFs in hydrogen storage. The modification and alteration of the structural feature of the MOFs are two main approaches for this target. Chemical stability is primarily related to the adsorption enthalpy, which uses the same methods including the exchange of metal and ligand. The physical stability raised by using penetrated system increases the interaction strength in the MOFs. These techniques discuss the future application in areas such as the working capacity and thermostability of the MOFs. The detailed mechanisms and effects for these general types of synthetical methods are presented. Also, data and analogy with the expected use for these types of the synthetical pathway to evaluate the suitable circumstances to adopt these methods. In the end, reasonable improvements to these methods introduce interpretation to select the appropriate technique for future investigation in MOFs development.

Tuning Metal-Dihydrogen Interaction in Metal-Organic Frameworks for Hydrogen Storage.

Inspired by a recently reported metal-organic framework (MOF), V2Cl2.8(btdd) [H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin], that shows a greatly improved H2 adsorption enthalpy, we employ density functional theory to probe how the number of d electrons and the mixed valences influence the M-H2 interaction inside the M2Clx(btdd) MOFs. We find a cliff in the H2 adsorption energy: the interaction strength remains strong from Sc to V and then falls sharply at Cr. Our results confirm V2Cl2.8(btdd) as one of the best performing hydrogen adsorbents and predict that Ti2Cl2.8(btdd) is equally promising while Sc2Cl2(btdd) and Ti2Cl2(btdd) may be even better. Our analysis indicates that an empty dx2-y2 orbital is the key to the much stronger binding of H2 at the open M(II) site (M = Sc, Ti, or V), whereas a partially filled dx2-y2 orbital in Cr(II) and later M(II) greatly weakens H2 binding. Our findings will be useful in designing MOFs to enhance H2 adsorption.

Metal organic framework for hydrogen storage

Hydrogen is considered to be a clean energy alternative to non-renewable fossil fuels. Hydrogen storage using metal-organic frameworks (MOFs) has received more attention in recent years. Their higher pore volume, high density of interaction sites, and a wide variety of structures contribute to their greater versatility. MOFs for hydrogen storage was discussed in this article, including their mechanism, preparation, and factors. In this study, several conventional and distinct methods for the synthesis of MOFs were demonstrated in detail, including slow diffusion, hydrothermal (solvothermal), electrochemical, mechanochemical, and microwave-assisted methods. In addition, hydrogen can be adsorbed by physical and chemical adsorption. In particular, physical sorption is facilitated by relatively low heat and pressure by van der Waal’s force. To increase the uptake of hydrogen storage, strategies to improve MOFs by altering pore structure and isosteric heat were addressed. The article discusses how MOFs have the potential to meet DOE targets for hydrogen storage and explains why that is so through examples and comparisons.

Porous metal-organic frameworks for hydrogen storage.

The high gravimetric energy density and environmental benefits place hydrogen as a promising alternative to the widely used fossil fuels, which is however impeded by the lack of safe, energy-saving and cost-effective H2 storage systems. The use of solid adsorbents as candidate materials offers a less energy-intensive way of storing hydrogen. The exceptional diversity and tunability of the chemical composition, topological structure, and surface chemistry together with large surface area position porous metal-organic frameworks as promising hydrogen storage material candidates. In this review, we first introduce several classes of important metal-organic frameworks for hydrogen storage, and then highlight the progress associated with the key challenges to be addressed, including the improvement of hydrogen-framework interaction required for enhancing room-temperature hydrogen storage capacities, and the optimization/balance of both gravimetric and volumetric storage/working capacities. In particular, the strategies used to tune and enhance hydrogen binding energies have been comprehensively reviewed. Future development prospects and related challenges of using porous metal-organic frameworks as hydrogen storage materials are also outlined. This feature review provides a wide perspective and insightful thoughts and suggestions for hydrogen storage using metal-organic frameworks, and promotes the further development of hydrogen storage materials to realize a hydrogen economy.

(Invited) Computational Discovery of Metal-Organic Frameworks for Hydrogen Storage: Combining High-Throughput Screening, Machine Learning, and Experimental Demonstration

Metal organic frameworks (MOFs) are promising materials for the storage of hydrogen fuel due to their high surface areas, tunable properties, and reversible gas adsorption. Although several MOFs are known to exhibit high hydrogen densities on a gravimetric basis, realizing high volumetric capacities a critical attribute for maximizing the driving range of fuel cell vehicles remains a challenge. Here, MOFs that achieve high gravimetric and volumetric H2 densities simultaneously are identified computationally, and demonstrated experimentally. The hydrogen capacities of ~100,000 MOFs drawn from databases of known and hypothetical compounds were predicted using empirical correlations and direct atomistic simulations. Based on these predictions, promising MOF candidates were synthesized and evaluated with respect to their usable H2 capacities. Several MOFs predicted to exhibit high capacities displayed low surface areas upon activation, highlighting the need to understand the factors that control stability. Consistent with the computational predictions, several MOFs were experimentally demonstrated to exhibit an uncommon combination of high usable volumetric and gravimetric capacities. Importantly, the measured capacities exceed those of the benchmark compound MOF-5, the record-holder for combined volumetric/gravimetric performance. Multiple machine learning algorithms were trained on this computationally-generated database. The relative accuracy of these algorithms were compared, with the most promising algorithm applied to predict H2 capacities in an additional 400,000 MOFs. Our study illustrates the value of computational screening in pinpointing materials that optimize overall storage performance.

Molecular modelling and machine learning for high-throughput screening of metal-organic frameworks for hydrogen storage

ABSTRACTHydrogen is an appealing energy storage solution for electric vehicles due to its low environmental impact and faster recharge times compared to batteries. However, there are many engineering challenges involved in safely storing a sufficient amount of hydrogen onboard a vehicle with a reasonable volumetric density. Nanoporous materials such as metal–organic frameworks (MOFs) have the potential to store hydrogen at high density and only moderate pressure. Considerable research has been devoted to finding new MOFs for hydrogen storage in recent years; however, a MOF that provides sufficient hydrogen density and is suitable to commercial applications has not yet been found. Much of this research makes use of molecular modelling to screen thousands of materials in a high-throughput way. Computational screening can be an effective tool for gaining insight into structure-performance relationships as well as finding specific candidates for an application. Recently, some research groups have also used machine learning to analyze data more effectively and accelerate the screening process. In this review, we discuss some recent advances in using molecular modelling and machine learning to find materials for hydrogen storage. We also discuss and compare some popular models for the hydrogen molecule and the accuracy of different equations of state, which are important considerations for accurate molecular simulations.

Structure–property relationships in metal-organic frameworks for hydrogen storage

Experimental hydrogen isotherms on several metal-organic frameworks (IRMOF-1, IRMOF-3, IRMOF-9, ZIF-7, ZIF-8, ZIF-9, ZIF-11, ZIF-12, ZIF-CoNIm, MIL-101 (Cr), NH2-MIL-101 (Cr), NH2-MIL-101 (Al), UiO-66, UiO-67 and HKUST-1) synthesized in-house and measured at 77 K and pressures up to 18 MPa are presented, along with N2 adsorption characterization. The experimental isotherms together with literature high pressure hydrogen data were analyzed in order to search for relationships between structural properties of the materials and their hydrogen uptakes. The total hydrogen capacity of the materials was calculated from the excess adsorption assuming a constant density for the adsorbed hydrogen. The surface area, pore volumes and pore sizes of the materials were related to their maximum hydrogen excess and total hydrogen capacities. Results also show that ZIF-7 and ZIF-9 (SOD topology) have unusual hydrogen isotherm shapes at relatively low pressures, which is indicative of “breathing”, a phase transition in which the pore space increases due to adsorption. This work presents novel correlations using the modelled total hydrogen capacities of several MOFs. These capacities are more practically relevant for energy storage applications than the measured excess hydrogen capacities. Thus, these structural correlations will be advantageous for the prediction of the properties a MOF will need in order to meet the US Department of Energy targets for the mass and volume capacities of on-board storage systems. Such design tools will allow hydrogen to be used as an energy vector for sustainable mobile applications such as transport, or for providing supplementary power to the grid in times of high demand.

Neutron powder diffraction of metal-organic frameworks for hydrogen storage

We review recent structural studies that we have undertaken aimed at elucidating the fundamental properties of metal-organic framework materials and their interactions with hydrogen. We have shown that exposing coordinatively unsaturated metal centers can greatly enhance the hydrogen binding energy and that they result in a significant increase of the surface packing density of adsorbed hydrogen molecules on materials’ surface. We will review some of the structural aspects of these materials, especially the adsorbed hydrogen molecule surface packing density in one type of metal-organic framework, MOF-74, which can be packed even denser than that in solid hydrogen.