Surface Adsorption for Storing Hydrogen in High Density

Alternative possibilities for dense hydrogen storage are by reaction of hydrogen to form chemicals such as metal hydrides and by temperature-dependent adsorption of hydrogen onto lightweight high-area substrates, which may include decorated graphitic surfaces. Hydride storage tanks have been tested in connection with autos and trucks. A figure of merit of such a storage medium is the weight density of hydrogen that can be achieved. A practical benchmark of 6.5% by weight ofhydrogen has been chosen by the U. S. Department of Energy.

A large surface area on which hydrogen adsorbs with a small binding energy is suggested in the cover image (lower left) of this book. The concept shown is a nanoporous structure made purely of carbon, using graphene planes and carbon nanotube pillars. Such a structure is not manufacturable at present, but illustrates properties that are desired. The weak adsorption energy allows hydrogen to be desorbed by a small increase in the temperature, and the surface density is clearly high in such an imagined structure.

A simpler storage model based on graphene layers is shown in Figure 9.7. The modeling associated with Figure 9.7 suggests that graphene layers optimally spaced at 0.8 nm may, with some uncertainty, accommodate hydrogen at the 6.5 wt % level

Подпись: Figure 9.7 Nanometer-scale schematic diagram [117] of one hydrogen storage configuration examined theoretically. The tubular regions, located between graphite

planes of variable spacing, are regions of high probability density for hydrogen molecules. The modeling is based on van der Waals attraction.

at room temperature and moderate pressure. This structure could be described as graphite intercalated with hydrogen. The calculations [117] indicate the hydrogen molecules will be delocalized (i. e., able to freely move, between the graphitic layers), which would promote rapid filling and emptying of the charge. On the other hand, the spacing of the graphene layers is a critical parameter that has to be arranged independently. No obvious route to obtain the desired spacing of graphene layers is known, although there is a large literature of “graphite intercalation compounds.” Any intercalation compound will have additional mass and will block free volume for the intended hydrogen molecules.

The attraction of hydrogen to transition metal atoms, including Ti, is stronger than the attraction to graphene as discussed above. According to Durgun et al. [118a], the bonding strength is about 0.4 eV, which is compatible with adsorption/desorption at room temperature. The interaction can be reliably calculated with advanced methods, leading to predictions, indicated in Figure 9.8, of hydrogen storage in certain simple organic molecules with transition metal adsorbates, in the range of 14 wt %.

Durgun et al. [118a] find that the hydrogen molecules in the right panel of Figure 9.8 are bound with energies varying from 0.29 to 0.49 eV, which are considered suitable for room-temperature storage of hydrogen. This is a reliable theoretical estimate, but synthetic chemistry is needed in order to create the underlying molecule, C2H4Ti2. The molecules would need to be dispersed, in a state more like a gas than a liquid, if hydrogen is to be readily inserted and extracted upon demand. This leaves the problem that the molecules will not reliably remain in the storage

Подпись: Figure 9.8 Diagrams [118] showing hydrogen storage molecules. (a) Structures based on ethylene C2H4; (b) indicates a single Ti atom with bond length 2.04 A from carbon, followed by adsorption of two Ti atoms at binding energy 1.47 eV; (c) shows how this structure can be regarded as a covalent bond between the lowest

unoccupied molecular orbital of the ethylene molecule (large lobes in (d)) with the 3D orbital ofTi (small lobes in (d)). Right-hand side ofthe figure shows binding sites of 10 hydrogen molecules that are provided by the two titanium atoms in this structure. This corresponds to 14 wt % of hydrogen.

tank, but might flow out as the hydrogen fuel is withdrawn. The authors suggest that the molecules should be embedded in a nanoporous matrix. The expected means to bring hydrogen gas in or out of the storage sites is to adjust the temperature. A nanoporous matrix might facilitate this, if it were continuous and could be uniformly heated by passing current through it.

It has been recently shown [118b] that the adsorption sites for H2 shown in Figure 9.8 are blocked by even small amounts of O2. These authors say that to make use of such designs, in practice, where some oxygen will always be present, access to the surface by oxygen would have to be blocked by a polymeric coating or by a nanoporous structure that would not allow molecular oxygen to enter. Palladium – silver alloys are permeable to hydrogen but not other gases, and could, in principle, block entry of oxygen to a cask containing the activated surfaces as shown in Figure 9.8. The idea of a porous structure that will allow a particular chemical element to enter and leave is suggested in Figure 10.9. This figure applies to Li ions, but a similar picture might apply to H2.

Similar results for hydrogen storage in titanium-decorated polymers, notably poly­acetylene (C2H2)n, have been presented by Lee et al. [119]. (Whether oxygen would block these sites is not clear.) Polyacetylene forms sheets, so that containing the material in the “gas tank” while hydrogen is drawn in and out is easier. The authors say that titanium-decorated polyacetylene will store 9% by weight of hydrogen if hydrogen is admitted at 25 °C and initial pressure 30 atm, and is desorbed at 2 atm and 130 °C. These are practical conditions. The bonding geometries are similar to those shown in Figure 9.8.

9.7.1

Updated: October 27, 2015 — 12:10 pm