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Nanoporous adsorbents are a diverse category of solid-state materials that hold considerable promise for vehicular hydrogen storage. Although impressive storage capacities have been demonstrated for several materials, particularly at cryogenic temperatures, materials meeting all of the targets established by the U.S. Department of Energy have yet to be identified. In this Perspective, we provide an overview of the major known and proposed strategies for hydrogen adsorbents, with the aim of guiding ongoing research as well as future new storage concepts. The discussion of each strategy includes current relevant literature, strengths and weaknesses, and outstanding challenges that preclude implementation. We consider in particular metal-organic frameworks (MOFs), including surface area/volume tailoring, open metal sites, and the binding of multiple H2 molecules to a single metal site. Two related classes of porous framework materials, covalent organic frameworks (COFs) and porous aromatic frameworks (PAFs), are also discussed, as are graphene and graphene oxide and doped porous carbons. We additionally introduce criteria for evaluating the merits of a particular materials design strategy. Computation has become an important tool in the discovery of new storage materials, and a brief introduction to the benefits and limitations of computational predictions of H2 physisorption is therefore presented. Finally, considerations for the synthesis and characterization of hydrogen storage adsorbents are discussed.

Small-molecule binding in metal-organic frameworks (MOFs) can be accurately studied both experimentally and computationally, provided the proper tools are employed. Herein, we compare and contrast properties associated with guest binding by means of density functional theory (DFT) calculations using nine different functionals for the M2(dobdc) (dobdc4- = 2,5-dioxido,1,4-benzenedicarboxylate) series, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn. Additionally, we perform Quantum Monte Carlo (QMC) calculations for one system to determine if this method can be used to assess the performance of DFT. We also make comparisons with previously published experimental results for carbon dioxide and water and present new methane neutron powder diffraction (NPD) data for further comparison. All of the functionals are able to predict the experimental variation in the binding energy from one metal to the next; however, the interpretation of the performance of the functionals depends on which value is taken as the reference. On the one hand, if we compare against experimental values, we would conclude that the optB86b-vdW and optB88-vdW functionals systematically overestimate the binding strength, while the second generation of van der Waals (vdW) nonlocal functionals (vdw-DF2 and rev-vdW-DF2) correct for this providing a good description of binding energies. On the other hand, if the QMC calculation is taken as the reference then all of the nonlocal functionals yield results that fall just outside the error of the higher-level calculation. The empirically corrected vdW functionals are in reasonable agreement with experimental heat of adsorptions but under bind when compared with QMC, while Perdew-Burke-Ernzerhof fails by more than 20 kJ/mol regardless of which reference is employed. All of the functionals, with the exception of vdW-DF2, predict reasonable framework and guest binding geometries when compared with NPD measurements. The newest of the functionals considered, rev-vdW-DF2, should be used in place of vdW-DF2, as it yields improved bond distances with similar quality binding energies.

A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu3(btc)2 (btc(3-) = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu3(btc)2, and its isostructural analogue Cr3(btc)2, show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu(2+) or Cr(2+) cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu3(btc)2 are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu3(btc)2 itself.

The well-known frameworks of the type M2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) have numerous potential applications in gas storage and separations, owing to their exceptionally high concentration of coordinatively unsaturated metal surface sites, which can interact strongly with small gas molecules such as H2. Employing a related meta-functionalized linker that is readily obtained from resorcinol, we now report a family of structural isomers of this framework, M2(m-dobdc) (M = Mg, Mn, Fe, Co, Ni; m-dobdc(4-) = 4,6-dioxido-1,3-benzenedicarboxylate), featuring exposed M(2+) cation sites with a higher apparent charge density. The regioisomeric linker alters the symmetry of the ligand field at the metal sites, leading to increases of 0.4-1.5 kJ/mol in the H2 binding enthalpies relative to M2(dobdc). A variety of techniques, including powder X-ray and neutron diffraction, inelastic neutron scattering, infrared spectroscopy, and first-principles electronic structure calculations, are applied in elucidating how these subtle structural and electronic differences give rise to such increases. Importantly, similar enhancements can be anticipated for the gas storage and separation properties of this new family of robust and potentially inexpensive metal-organic frameworks.

Carbon fiber composite overwrapped pressure vessels (COPV) are the industry standard for onboard compressed hydrogen storage tanks. Carbon fiber processing, however, accounts for more than half the cost of these storage tanks. Over the past decade, the US Department of Energy (DOE) has funded research and development (R&D) targeting significant reductions in carbon fiber costs, as well as improvements in overall storage tank performance. This article provides an overview of several DOE-funded R&D projects focused on the development of advanced carbon fibers for onboard hydrogen storage tanks. [ABSTRACT FROM AUTHOR]

Two new ternary uranium selenides, AU2Se6 (A = K, Cs) were prepared using the reactive flux method. Single crystal X-ray diffraction was performed on single crystals, showing the crystallization in the orthorhombic Immm space group.

A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu3(btc)2 (btc3- = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu3(btc)2, and its isostructural analogue Cr3(btc)2, show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu2+ or Cr2+ cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu3(btc)2 are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu3(btc)2 itself. [ABSTRACT FROM AUTHOR]

Two isostructural hybrid inorganic-organic frameworks, M2(4,4′-bpy)(tfhba)2·4,4′-bpy, (M = Co (1), Zn (2); 4,4′-bpy = 4,4′-bipyridine; tfhba = 2,3,5,6-tetrafluoro-4-hydroxybenzoate) have been synthesized and their structures elucidated using single crystal X-ray diffraction. These materials are the first hybrid structures synthesized using the tfhba ligand. They contain a one-dimensional chain of corner-sharing MO4N trigonal bipyramids that has not been observed to date for Co2+ units. The magnetic properties of 1 have been investigated and indicate ferromagnetic ordering along the chains, with weak antiferromagnetic interactions between them. Overall the system is antiferromagnetic with a Nel temperature of 3 K, but undergoes a field-induced magnetic phase transition at 3 kOe. [ABSTRACT FROM AUTHOR]

A partially fluorinated metal-organic framework, Zn(bpe)(tftpa)·cyclohexanone [bpe = 1,2-bis(4-pyridyl)ethane; tftpa = tetrafluoroterephthalate], has been synthesized, and its H2 storage properties are reported. The structure is highly interpenetrated yet contains templating cyclohexanone molecules, which can be easily removed to give a porous material with fluorine atoms exposed to the pore surface. The material adsorbs 1.04 wt % H2 at 77 K and 1 atm with an adsorption enthalpy (Qst) of 6.2 kJ/mol, which represents a slight enhancement in the binding strength due to the presence of fluorine atoms over comparable metal-organic frameworks, which bind through purely physisorptive methods. [ABSTRACT FROM AUTHOR]

Two new ternary uranium selenides, AU2Se6 (A = K, Cs), were prepared using the reactive flux method. Single crystal X-ray diffraction was performed on single crystals. The compounds crystallize in the orthorhombic Irnmrn space group, Z = 2. CsU2Se6 has cell parameters of a = 4.046(2) Å, b = 5.559(3) Å, and c = 24.237(12) Å. KU2Se6 has cell parameters of a = 4.058(3) Å, b = 5.556(4) Å, and c = 21.710(17) Å. The compounds are isostructural to the previously reported KTh2Se6. The two-dimensional layered structure is related to ZrSe3 with the alkali metals residing in the interlayer space. The oxidation states of uranium and selenium were evaluated using X-ray photoelectron spectroscopy (XPS). Uranium was found to be tetravalent, while selenium was found to be in two oxidation states, one of which is -2. The other oxidation state is similar to that found in a polyselenide network. While this structure is known, our work examines how the structure changes through the transactinide series. [ABSTRACT FROM AUTHOR]

Small-molecule binding in metal-organic frameworks (MOFs) can be accurately studied both experimentally and computationally, provided the proper tools are employed. Herein, we compare and contrast properties associated with guest binding by means of density functional theory (DFT) calculations using nine different functionals for the M 2 (dobdc) (dobdc 4- = 2,5-dioxido,1,4-benzenedicarboxylate) series, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn. Additionally, we perform Quantum Monte Carlo (QMC) calculations for one system to determine if this method can be used to assess the performance of DFT. We also make comparisons with previously published experimental results for carbon dioxide and water and present new methane neutron powder diffraction (NPD) data for further comparison. All of the functionals are able to predict the experimental variation in the binding energy from one metal to the next; however, the interpretation of the performance of the functionals depends on which value is taken as the reference. On the one hand, if we compare against experimental values, we would conclude that the optB86b-vdW and optB88-vdW functionals systematically overestimate the binding strength, while the second generation of van der Waals (vdW) nonlocal functionals (vdw-DF2 and rev-vdW-DF2) correct for this providing a good description of binding energies. On the other hand, if the QMC calculation is taken as the reference then all of the nonlocal functionals yield results that fall just outside the error of the higher-level calculation. The empirically corrected vdW functionals are in reasonable agreement with experimental heat of adsorptions but under bind when compared with QMC, while Perdew-Burke-Ernzerhof fails by more than 20 kJ/mol regardless of which reference is employed. All of the functionals, with the exception of vdW-DF2, predict reasonable framework and guest binding geometries when compared with NPD measurements. The newest of the functionals considered, rev-vdW-DF2, should be used in place of vdW-DF2, as it yields improved bond distances with similar quality binding energies.