Improved designs of metal-organic frameworks for hydrogen storage.

Hydrogen fuel is considered as a most promising alternative energy source for vehicles and portable electronics. For this technology to be practical for automobiles, the U.S. Department of Energy has established a design target of 6.0 wt% reversible hydrogen storage by 2010, but current materials fall far short of this goal. Consequently, many studies are devoted to metal hydride and carbon-based materials. Furthermore, such novel concepts as BN nanotubes, clathrate hydrates, and metal– organic frameworks (MOFs) are being pursued. In particular MOF materials have attracted attention because of their relatively simple and economic synthesis (compared with chemical vapor deposition), large storage amounts (1.32 wt% hydrogen was reported at 1 bar and 77 K), and high thermal stability up to 400 8C. To provide a fundamental understanding of storage mechanisms and to determine the optimum performance for such MOFs, we report herein first-principles-based calculations on a variety of MOFs. Figure 1 shows the atomistic structures of several MOFs. TheM4O(CO2)6 cluster (whereM=Zn, Be, orMg) serves as a metal oxide node that links organic aromatic units (such as benzene or dibenzo coronene) to form a cubic cage structure. We consider two strategies to enhance the hydrogen storage of MOFs: a) Optimize the metal oxide nodes by using alternative lighter metal elements. We consider replacing M=Zn with Be or Mg. b) Optimize the organic linker to enhance the hydrogen adsorption energy. We consider five organic linkers: BDC (MOF-C6), NDC (MOF-C10), PDC (MOF-C16), PDC1 (MOF-C22), and PDC2 (MOF-C30), where the MOFs are named according to the number of aromatic carbon atoms in the linker.