Alain686 wrote:This discussion regarding hydrogen has raised a question, I am not sure if it has been answered. Why would you need to store hydrogen in it's molecular form? Even liquid hydrogen has very low density and water has a lot more hydrogen per unit volume than pure hydrogen. Water is a lot easier to store and transport and has other uses besides just reactor mass.
I don't recall anyone saying that it needed to be stored in its molecular form. I could have missed that post. However, how it's stored would depend on its specific application and the logistics of the system.
What readily comes to mind is its application to hydrogen powered automobile fuel-cells, where a target density, reactive temperature and reversibility is important.
I hope enough of an excerpt to explain ...
Hydrogen is viewed as a clean energy alternative that could one day replace fossil fuels. The U.S. Department of Energy has determined that a hydrogen storage density of 9 wt% will be required for fuel-cell powered vehicles to be able to replace petroleum-fueled vehicles on a large scale [1]. Very high pressure vessels are capable of storing hydrogen at 9 wt%, but the various practical considerations indicate that other solutions will be required. A few solid-state materials, such as Li3N [2] and Li3Be2H7 [3], can absorb up to 9 wt% hydrogen, but, at the present time, these systems are reversible only near 250C.
Alanates such as LiAl4 and TiAlH44 contain more than 9 wt% hydrogen, but these systems have not yet exhibited reversibility. The related NaAlH4 material has shown 5 wt% reversibility at a temperature of 180C [4]. Compounds such as NaBH4 contain a great deal of hydrogen that can be liberated upon reaction with water, but these materials must be regenerated by a chemical process off board [5].
In previous work, we measured the binding energies for nondissociative hydrogen adsorption on both carbon single-wall (20 kJ=mol) [6] and multiwall nanotubes (54 kJ=mol) [7] when nanosized transition metal (TM) species were present. These binding energies are significantly higher than what is expected for simple van der Waals (vdW) adsorption of H2 on the carbon, and substantially lower than if atomic hydrogen were chemisorbed on the metal surfaces. The amount adsorbed and the first-order desorption process were inconsistent with dissociative hydrogen uptake by either surface chemisorption or bulk metal hydride formation.
The observation that a trace amount of transition metal can lead to an enhanced capacity with a moderate binding energy caused us to explore the ways in which carbon and metals could be combined to construct new adsorbents capable of storing large amounts of hydrogen.
Here, we report an entirely new concept for storing hydrogen in its molecular form using rationally designed, novel organometallic molecules based on C60. We show theoretically that the amount of H2 that can be retrieved reversibly at room temperature (RT) and near ambient pressure can approach 9 wt%. The design is based on the fact that both H2 and cyclopentadiene rings (Cp C5H5) can act as ligands for TMs. ATM atom interacts with a Cp ring through Dewar coordination [8] and with a dihydrogen ligand through a Kubas interaction [9]. Although each type of bonding is of historical importance in coordination chemistry [9,10], the combination of the two as a solution for hydrogen storage has never been seriously considered. Others have shown that isolated TM atoms can store up to six dihydrogen species [11,12], but the metal atoms are expected to coalesce and form bulk materials when hydrogen is removed. We show that a CpScH2 complex can store 6.7 wt% of nondissociated H2, but these complexes may polymerize after hydrogen has been removed, making the process irreversible. However, when the complexes are arranged symmetrically on a buckyball, species such as C60ScH212 and C48B12ScH12 are stable and can reversibly adsorb additional hydrogen, resulting in capacities of 7.0 and 8.77 wt%, respectively. Moreover, the reversible hydrogen is stored with a binding energy that is ideal for vehicular applications, 0.3–0.4 eV. Notice that stable TMcoated buckyballs [13] and nanotubes [14] have been recently synthesized.
We use a spin-polarized first-principles calculation as implanted in the Vienna Ab-initio Simulation Package (VASP) [15]. Ultrasoft pseudopotential with the PerdewWang 1991 [16] generalized gradient approximation yields practically the same results to an all-electron-like projector augmented-wave potential with the Perdew-BurkeErnzerhof exchange-correlation functional [17]. An energy cutoff of 400 eV was used. Two cubic unit-cell sizes of dimensions 16 and 25 A˚ were used for complexes with Cp rings and C60 ligands, respectively, to maintain a similar vacuum region. As a benchmark, we calculated CpMCp (the so-called metallocene molecule) from Sc to Ni. The M-C bond length of 2.05 A˚ for ferrocene is found to be nearly identical to the experimental value [18]. Also the calculated Sc cohesive energy of 4.08 eV is in reasonable agreement with the measured value of 3.9 eV [19].
