| AVS 58th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Thursday Sessions |
| Session EN+NS-ThA |
| Session: | Nanostructures for Energy Storage and Fuel Cells II |
| Presenter: | Nour Nijem, University of Texas at Dallas |
| Authors: | N. Nijem, University of Texas at Dallas L. Kong, Rutgers University H. Wu, Rutgers University Y. Zhao, Rutgers University J. Li, Rutgers University D.C. Langreth, Rutgers University Y.J. Chabal, University of Texas at Dallas |
| Correspondent: | Click to Email |
Hydrogen storage is one of the most challenging problems in hydrogen-based energy technologies. One of the goals of hydrogen storage is the ability to store a high volumetric density of hydrogen at room temperature. As a result, studies exploring molecular hydrogen interaction in storage materials are important to facilitate further development of materials. Metal-organic Frameworks (MOFs) are promising candidates for hydrogen storage because their high surface area and porosity facilitate high hydrogen physisorption on specific sites of the structures and because many options are possible to enhance the interaction of molecular hydrogen with the host.
This work explores the incorporation of hydrogen into MOFs using infrared (IR) absorption spectroscopy to characterize its interaction. IR spectroscopy can distinguish possible H2 binding sites based on the perturbation of the internal H2 stretch mode. IR measurements are performed on saturated metal center MOFs varying the ligand and/or the metal center and on unsaturated metal center MOF-74-M (M=Zn, Mg and Ni). We combine room temperature, high pressure with low temperature (20-100K) measurements and theoretical van der Waals density functional (vdW-DF) calculations to derive quantitative information from IR shifts and dipole moment strengths.
Our results show that, in contrast to the current understanding, IR shifts are independent of binding energies and depend instead on the chemical environment of the molecule, including effects such as H2-H2interactions. For example, we see little difference in IR shifts between saturated MOFs with low binding energy (~4kJ/mol), and unsaturated MOFs with higher binding energy (~10kJ/mol) sites at room temperature. Furthermore, we show that dipole moments of adsorbed H2 depends greatly on parameters such as geometry of adsorption site and H2-H2 interactions. Measurements performed at low temperatures on MOF-74 show that IR shifts of H2 is greatly red shifted (an additional ~-30 cm-1) due to H2-H2 interactions on close proximity adsorption sites, and that dipole moments of adsorbed H2 can appreciably vary with loading.
Our analysis indicate that the intensity of H2 IR band cannot always be a measure of the amount of adsorbed H2, therefore methods such as variable temperature IR (VTIR) used to deduce binding energies cannot always be implemented.