Increasing hydrogen uptake by increasing the density of adsorbed hydrogen
Yun Liu1,2, Craig M. Brown1, Dan A. Neumann1, Houria Kabbour3, Channing C. Ahn3
1. NIST Center for Neutron Research, National Institute of Standards and Technology, MD, USA.
2. Department of Materials Science and Engineering, University of Maryland, College Park, MD, USA.
3. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA.
Storing hydrogen molecules in nano-porous materials with high surface area is one of several promising approaches to achieve the goals for on-board hydrogen storage. Although efforts to increase the surface area of material have been intensive in past years, the equally important concept of the surface packing density attracts much less attention. In order to increase the surface packing density at temperatures much higher than liquid nitrogen temperature, much stronger hydrogen binding to a framework than that of typical carbon materials is necessary. We show that exposing the coordinatively unsaturated metal centers can greatly enhance the hydrogen binding energy toward the theoretically desired binding energy range. We also demonstrate that the coordinatively unsaturated metal centers can not only increase binding energies but also significantly increase the surface packing density of adsorbed hydrogen molecules on material surfaces. Especially, the adsorbed hydrogen molecule surface packing density in one type of metal-organic framework, MOF-74, can be even packed denser than that of solid hydrogen. This indicates that by optimizing the surface potential of a material, the hydrogen surface packing density can be increased to a very large value without extremely large vapor pressure.