New materials capable of storing hydrogen at high gravimetric and volumetric densities are required if hydrogen is to be widely employed as a clean alternative to hydrocarbon fuels in cars and other mobile applications. With exceptionally high surface areas and chemically-tunable structures, microporous metal–organic frameworks have recently emerged as some of the most promising candidate materials. In this critical review we provide an overview of the current status of hydrogen storage within such compounds. Particular emphasis is given to the relationships between structural features and the enthalpy of hydrogen adsorption, spectroscopic methods for probing framework–H2 interactions, and strategies for improving storage capacity (188 references).

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Hydrogen storage in metal–organic frameworks

Increased attention is being focused on metal-organic frameworks as candidates for hydrogen storage materials. This is a result of their many favorable attributes, such as high porosity, reproducible and facile syntheses, amenability to scale-up, and chemical modification for targeting desired properties. A discussion of several strategies aimed at improving hydrogen uptake in these materials is presented. These strategies include the optimization of pore size and adsorption energy by linker modification, impregnation, catenation, and the inclusion of open metal sites and lighter metals.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200462786

Strategies for hydrogen storage in metal--organic frameworks.

Dihydrogen, methane, and carbon dioxide isotherm measurements were performed at 1−85 bar and 77−298 K on the evacuated forms of seven porous covalent organic frameworks (COFs). The uptake behavior and capacity of the COFs is best described by classifying them into three groups based on their structural dimensions and corresponding pore sizes. Group 1 consists of 2D structures with 1D small pores (9 A for each of COF-1 and COF-6), group 2 includes 2D structures with large 1D pores (27, 16, and 32 A for COF-5, COF-8, and COF-10, respectively), and group 3 is comprised of 3D structures with 3D medium-sized pores (12 A for each of COF-102 and COF-103). Group 3 COFs outperform group 1 and 2 COFs, and rival the best metal−organic frameworks and other porous materials in their uptake capacities. This is exemplified by the excess gas uptake of COF-102 at 35 bar (72 mg g−1 at 77 K for hydrogen, 187 mg g−1 at 298 K for methane, and 1180 mg g−1 at 298 K for carbon dioxide), which is similar to the performance of COF...

http://yaghi.berkeley.edu/pdfPublications/Furukawa09.pdf

Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications

Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

The prototypical metal-organic framework Zn4O(BDC)3 (MOF-5, BDC2- = 1,4-benzenedicarboxylate) decomposes gradually in humid air to form a nonporous solid. Recognizing this, improved procedures for its synthesis and handling were developed, leading to significant increases in N2 and H2 gas adsorption capacities. Nitrogen adsorption isotherms measured at 77 K reveal an enhanced maximum N2 uptake of 44.5 mmol/g and a BET surface area of 3800 m2/g, compared to the 35.8 mmol/g and 3100 m2/g obtained for a sample prepared using previous methods. High-pressure H2 adsorption isotherms show improvements from 5.0 to 7.1 excess wt % at 77 K and 40 bar. The total H2 uptake was further observed to climb to 11.5 wt % at 170 bar, corresponding to a volumetric storage density of 77 g/L. Thus, the air-free compound exhibits the highest gravimetric and volumetric H2 uptake capacities yet demonstrated for a cryogenic hydrogen storage material. Moreover, no loss of capacity was apparent during 24 complete adsorption−desorpti...

Impact of preparation and handling on the hydrogen storage properties of Zn4O(1,4-benzenedicarboxylate)3 (MOF-5).

We have investigated the storage capability of microporous carbon materials for gaseous hydrogen both theoretically and experimentally. In the grand canonical Monte Carlo calculation the hydrogen molecules are physisorbed by van der Waals interactions with the surface atoms of carbon slitpores and carbon nanotubes. At room temperature the optimum pore geometry is a slitpore consisting of two graphite platelets separated by a distance that corresponds approximately to two times the diameter of a hydrogen molecule. In this case for a storage pressure of 10 MPa a maximum adsorbed hydrogen density of 14 kg/m3 can be reached, which corresponds to a gravimetric storage capacity of 1.3 wt %. Only for low gas pressure a cylindrical geometry like that in carbon nanotubes can exceed the storage density of carbon slitpores owing to capillary forces.

Physisorption of Hydrogen on Microporous Carbon and Carbon Nanotubes

Advances in the study of methane storage in porous carbonaceous materials

Adsorption of CO2 at 273 K up to 4 MPa has been studied in activated carbons and carbon molecular sieves of different origins and pore size distribution. The materials selected for this study include carbon molecular sieves with a pore size (i.e., pore width) between 0.3 and 0.5 nm, activated carbons with supermicroporosity (pore size between 0.7 and 2 nm), and mesoporous and macroporous activated carbons. The relative fugacities covered in the experiments ranges from 10-4 to nearly 1. Additionally, N2 adsorption at 77 K at subatmospheric pressures has been also done. The experimental conditions used allow us to compare both measurements at similar adsorption potentials, in which both gases adsorb in the different ranges of porosity. The results obtained show that CO2 adsorbs at 273 K in the different ranges of porosity following a mechanism similar to that of N2 at 77 K. CO2 is sensitive to narrow micropores not accessible to N2 at 77 K, and hence, it is an adequate complement to N2 at 77 K. This is espe...

CO2 As an Adsorptive To Characterize Carbon Molecular Sieves and Activated Carbons

Hydrogen storage has been studied in a large variety of activated carbons and activated carbon fibers and a wide range of pressures (up to 70 MPa). The experimental technique used has good reliability, and the experiments performed have a small error and high reproducibility. This seems to be essential to get trustworthy conclusions. In these samples, we have not found large amounts of hydrogen adsorbed. In any case, an activated carbon derived through a simple preparation method provides hydrogen storage values at 10 MPa close to 1 wt % (i.e., a value close to the target from an application point of view). The experimental results have been compared with theoretical work found in the literature, and an important agreement can be observed. From this study, we conclude that the optimum pore size for hydrogen storage is that which can hold two layers of adsorbed hydrogen. This work also considers practical aspects related to hydrogen storage in activated carbons and activated carbon fibers.

M. A. De La Casa-Lillo

Papers

Physisorption of Hydrogen on Microporous Carbon and Carbon Nanotubes

Advances in the study of methane storage in porous carbonaceous materials

CO2 As an Adsorptive To Characterize Carbon Molecular Sieves and Activated Carbons

Hydrogen Storage in Activated Carbons and Activated Carbon Fibers

Methane storage in activated carbon fibres