Session H20: Focus Session: Physics of Energy Storage Materials III -- Hydrogen Storage Adsorbents

Strategies for Hydrogen Storage in Nanoporous Metal-Organic Framework Materials

Storing hydrogen by physisorption in porous materials is a challenging problem of great interest for future vehicle technology. Metal-organic frameworks (MOFs) are a new class of nanoporous materials that have demonstrated exciting potential for solving this problem. MOFs are synthesized by the self-assembly of metal nodes and connecting organic linker molecules to create stable, porous frameworks. The synthetic chemistry opens the possibility to create an almost unlimited number of MOFs and to tailor them for particular applications, such as hydrogen storage. The diversity of MOFs also creates an opportunity to learn more about the fundamentals of hydrogen adsorption in porous materials. We have used a combination of classical Monte Carlo simulations and quantum mechanical approaches to investigate fundamental questions about hydrogen storage in MOFs and to design new materials with improved storage capabilities. Relationships have been elucidated between hydrogen uptake and properties such as the MOF surface area, void volume, degree of catenation, enthalpy of adsorption, and cation content. Introduction of cations is a promising strategy to improve hydrogen uptake at room temperature, and different metal cations and different strategies for introducing them into MOFs have been screened computationally.   

Increased 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 H$_{2}$ by mass, up to 10 wt.{\%} absolute at 70 bar at 77K. 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 to 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/cm$^{3}$ by mechanical compaction. We find that optimal hydrogen storage properties are achieved for $\rho \quad \sim $ 0.5 g/cm$^{3}$, yielding a 350{\%} increase in volumetric H$_{2}$ density with only a modest 15{\%} reduction in gravimetric H$^{2}$ 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/cm$^{3}$, as confirmed from powder XRD.   

First-principles study of hydrogen adsorption in metal-doped COF-10

Covalent organic frameworks (COFs), due to their low-density, high-porosity, and high-stability, have promising applications in gas storage. In this study we have explored the potential of COFs doped with Li and Ca metal atoms for storing hydrogen under ambient thermodynamic conditions. Using density functional theory we have performed detailed calculations of the sites Li and Ca atoms occupy in COF-10 and their interaction with hydrogen molecules. The binding energy of Li atom on COF-10 substrate is found to be about 1.0 eV and each Li atom can adsorb up to three H2 molecules. However, at high concentration, Li atoms cluster and, consequently, their hydrogen storage capacity is reduced due to steric hindrance between H2 molecules. On the other hand, due to charge transfer from Li to the substrate, O sites provide additional enhancement for hydrogen adsorption. With increasing concentration of doped metal atoms, the COF-10 substrate provides an additional platform for storing hydrogen. Similar conclusions are reached for Ca doped COF-10.   

Enhanced Dihydrogen-Metal Interaction in Transition Metal Exposed Paddle-Wheel Frameworks

The experimentally observed enhancement of hydrogen adsorption in Cu$_{2}$-tetracarboxylate paddle-wheel frameworks is investigated by first-principles density-functional theory calculations [1]. We reveal that the puzzling enhancement is due to the effective orbital coupling between the occupied H$_{2}$ \textit{$\sigma $} and the unoccupied Cu 4$s$-derived states. The nontrivial dihydrogen-metal \textit{$\sigma $s} interaction is enabled by a strong localization of the Cu 4$s$ orbital after hybridizing with the neighboring oxygen 2$p$ orbitals. Based on this understanding, we predict that the dihydrogen-metal interaction can be further increased by alloying Cu with $s$-orbital element Zn or Mg. We will also discuss on the enhanced dihydrogen adsorption on other 3$d$-transition-metal paddle wheel frameworks.\\[4pt] [1] Y.-H. Kim, J. Kang, and S.-H. Wei, Phys. Rev. Lett., in press (2010).   

Iron decorated - functionalized MOF for high-capacity hydrogen storage: First-principles calculations

We perform electronic structure calculations for the Fe-decorated, OH-functionalized isoreticular metal organic framework 16 (IRMOF16) to investigate the hydrogen storage capacity. Because of the relatively strong Kubas interaction between Fe and H$_{2}$, hydrogen molecule can be adsorbed on the proposed MOF even at room temperature. The reversibly usable storage capacity under ambient conditions reaches 6.0 wt{\%}. Fe has a much lower oxidation tendency than other metals (e.g., Ti, Ca, or Li) used for decorating backbone structures and therefore far more convenient in practical implementation. We also find that the spin flip, which comes from the competition between exchange field splitting and ligand field splitting, plays a significant role in the interaction between Fe and H$_{2}$.   

Study on Ca32C60 Cluster for Hydrogen Storage

Using first-principles calculations within density functional theory (DFT), we study the assembly of Ca32C60, the most desirable metal-coating fullerene as hydrogen storage medium. We first explore possible structures of Ca32C60 dimer with different initial configurations, and find a surprisingly large binding energy up to 2.8 eV. Our further analysis on electronic structures indicates that such a large binding strength stems from the enhanced chemical reactivity of Ca due to the Ca-3s valence electrons partially transferred to the fullerene. We then systematically investigate the alkali and alkali earth elements coated on fullerene, and find that the chemical reactivity of these metal elements can be tuned due to the large electron affinity of C60. Based on this finding, we then extend our studies to the bulk form and two-dimensional structures of Ca32C60, and propose an optimum assemble structure for hydrogen storage. These results shall facilitate designing and optimizing carbon-based materials for hydrogen storage.   

Interaction potential and IR absorption of endohedral H$_2$ in C$_{60}$

We measured the IR spectra of a H$_2$ molecule trapped inside a C$_{60}$ cage at temperatures from 6 to 300\,K and analyzed the spectra by using a model of a vibrating rotor in a spherical potential. The electric dipole moment of IR transitions is induced by the translational motion of H$_2$. The rotation of H$_2$ is unhindered but coupled to the translational motion. The isotropic and translation-rotation coupling part of the potential are anharmonic and different in the ground and excited vibrational states of H$_2$. The vibrational frequency and the rotational constant of endohedral H$_2$ are smaller than in the gas phase. The assignment of IR lines to ortho- and para-H$_2$ is confirmed by measuring spectra of a para enriched H$_2$@C$_{60}$ and is consistent with the earlier interpretation of the low temperature IR spectra [ S. Mamone \textit{et al.}, J. Chem. Phys.\textbf{ 130}, 081103 (2009) ].   

Metallacarboranes: Towards promising hydrogen storage metal organic framework

Using first principles calculations we show the high hydrogen storage capacity of metallacarboranes,\footnote{A. K. Singh, A. Sadrzadeh, and B. I. Yakobson, Metallacarboranes: Toward Promising Hydrogen Storage Metal Organic Frameworks, JACS 132,14126 (2010).} where the transition metal (TM) atoms bind hydrogen via Kubas interaction. The average binding energy of $\sim $0.3 eV/H favorably lies within the reversible adsorption range The Sc and Ti are found to be the optimum metal atoms maximizing the number of stored H$_{2}$ molecules. Depending upon the structure, metallacarboranes can adsorb up to 8 wt{\%} of hydrogen, which exceeds DOE goal for 2015. Being integral part of the cage, TMs do not suffer from the aggregation problem. Furthermore, the presence of carbon atom in the cages permits linking the metallacarboranes to form metal organic frameworks (MOF), thus able to adsorb hydrogen via Kubas interaction, in addition to van der Waals physisorption.   

Anomalous Characteristics of a PVDC Carbon Adsorbant

Nanoporous carbon produced by the pyrolisis of poly(vinylidene chloride-co-vinyl chloride) shows remarkably high adsorption of molecular hydrogen despite its relatively low surface area. In particular, its room temperature volumetric storage is significantly higher than other carbons with surface areas four times higher. In this talk we will present experimental hydrogen adsorption isotherms (and low-temperature isosteric heats of adsorption), subcritical nitrogen adsorption, real space images (TEM), and inelastic neutron scattering. In all cases, the sample characteristics are quite unusual. Whereas the sample under consideration is quite unusual in its high hydrogen sorption capacity, other samples in the literature also show similar unusual characteristics, suggesting the presence of phenomena not fully understood by standard adsorption theory.   

Inelastic Neutron Scattering from Hydrogen Adsorbed in Carbon

Inelastic neutron scattering (INS) from adsorbed hydrogen offers a powerful tool to probe the local adsorption environment of storage material. We will show recently measured INS spectra of hydrogen adsorbed on four different carbon samples and discuss the interpretation of their spectral features, using previous theoretical calculations [1]. Both rotational and vibrational transitions can be observed, along with free recoil scattering parallel to the adsorption plane. The spectra from carbon nanotubes and activated carbon are well explained by theory. However, the spectra from PVDC carbon is quite unusual. \\[4pt] [1] R. Olsen, L. Firlej, B. Kuchta, H. Taub, P. Pfeifer, and C. Wexler; Sub-Nanometer Characterization of Activated Carbon by Inelastic Neutron Scattering; Carbon (under review).   

Analysis of hydrogen sorption characteristics of boron-doped activated carbons

There is significant interest in the properties of boron-doped activated carbons for their potential to improve hydrogen storage.\footnote{See http://all-craft.missouri.edu} Boron-doped activated carbons have been produced using a novel process involving the pyrolysis of a boron containing compound and subsequent high-temperature annealing. In this talk we will present a systematic study of the effect of different boron doping processes on the samples' surface area, micropore structure, and hydrogen sorption. Experimental results include boron content from prompt gamma neutron activation analysis, boron-carbon chemistry from Fourier transform infrared spectroscopy (FTIR), nitrogen subcritical adsorption, and 80K and 90K hydrogen adsorption isotherms which allow us to evaluate the hydrogen binding energy for each sorptive material.   

The effect of KOH:C and activation temperature on hydrogen storage capacities of activated carbons

The Alliance for Collaborative Research in Alternative Fuel Technologies (ALL-CRAFT\footnote{See http://all-craft.missouri.edu}) has been producing high surface area activated carbons. Here we will investigate the effect of the ratio of activating agent to carbon and activation temperature on hydrogen sorption characteristics and sample structure. Results show that a ratio of 3:1 KOH:C and an activation temperature of 790 C are the ideal activation conditions for hydrogen storage applications. Hydrogen sorption measurements are completed using a volumetric instrument that operates at pressures up to 100 bar and at temperatures of 80 K, the sublimation temperature of dry ice (-78.5 C), and room temperature. Specific surface area and pore size distributions are measured using subcritical nitrogen isotherms.