MOFs For Hydrogen Storage
Considerable interest has been shown in the development of non-petroleum energy carriers for use in transportation. Hydrogen is an attractive option because it has a high energy content (120 MJ/kg compared to 44 MJ/kg for gasoline), produces clean exhaust product (water vapor without CO2 or NOx), and can be derived from a variety of primary energy sources. However, the specific energy of uncompressed hydrogen gas is very low, and considerable attention must be given to denser storage methods if hydrogen is to emerge as a serious option for energy storage.
Proposed forms of reversible hydrogen storage include: compressed gas, cryogenic liquid, adsorption to high surface-area materials, chemical storage as metal hydrides, and various reactions of liquid fuels high in hydrogen content (whose products must be collected and recycled after use). Of these, compressed and liquid hydrogen are the most mature technologies and are the most suitable for immediate deployment. The United States Department of Energy (USDOE) projects that with further technological development, adsorptive or chemical storage may prove most effective for storage.
Metal Organic Frameworks (MOFs) attract attention as materials for adsorptive hydrogen storage because of their exceptionally high specific surface areas and chemically tunable structures. MOFs can be thought of as a three-dimensional grid in which the vertices are metal ions or clusters of metal ions that are connected to each other by organic molecules called linkers. Hydrogen molecules are stored in a MOF by adsorbing to its surface. Compared to an empty gas cylinder, a MOF-filled gas cylinder can store more gas because of adsorption that takes place on the surface of MOFs. (Note that wikt:molecular hydrogen adsorbs to the surface, not wikt:atomic hydrogen.) Furthermore, MOFs are free of dead-volume, so there is almost no loss of storage capacity as a result of space-blocking by non-accessible volume. Also, MOFs have a fully reversible uptake-and-release behavior: since the storage mechanism is based primarily on physisorption, there are no large activation barriers to be overcome when liberating the adsorbed hydrogen. The storage capacity of a MOF is limited by the liquid-phase density of hydrogen because the benefits provided by MOFs can be realized only if the hydrogen is in its gaseous state.
In order to realize the benefits provided, such as adsorption, by MOFs hydrogen cannot be stored in them at densities greater than its liquid-phase density. The extent to which a gas can adsorb to a MOF's surface depends on the temperature and pressure of the gas. In general, adsorption increases with decreasing temperature and increasing pressure (until a maximum is reached, typically 20-30 bar, after which the adsorption capacity decreases)., However, MOFs to be used for hydrogen storage in automotive fuel cells need to operate efficiently at ambient temperature and pressures between 1 and 100 bar, as these are the values that are deemed safe for automotive applications.
In 2012, the lab led by William A. Goddard III predicted that MOF-210 will have Hydrogen storage capacity of 2.90 delivery wt% (1-100 bar) at 298 K and 100 bar. Also that MOF-200 will have a Hydrogen storage capacity of 3.24 delivery wt% (1-100 bar) at 298 K and 100 bar. They also proposed new strategies to obtain higher interaction with H2. Such strategy consist on metalating the COF with alkaline metals such as Li. These complexes composed of Li, Na and K bound to benzene ligands (such as 1,3,5-benzenetribenzoate, the ligand used in MOF-177) have been synthesized by Krieck et al. and Goddard showed that the THF is important of their stability. If the metalation with alkaline is performed in the COFs, Goddard et al. calculated that two MOFs will reach the 2015 DOE target of 5.5 wt % at 298 K: MOF200-Li (6.34 delivery wt%) and MOF200-Na (5.94 6.34 delivery wt%) at 100 bar. Other strategies to increase the interaction of MOFs with molecular hydrogen have been reviewed recently.
Read more about this topic: Metal-organic Framework
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