Hydrogen Storage

Climate change and global warming dominate the news today. Due to the high consumption of hydrocarbons which results in the emission of carbon dioxide and other pollutants, it is said that global warming will be the highest threat to the environment in the near future.68,69 Considering these threats, the palpable

combustion product of hydrogen with air is water, hydrogen emphasizes the advocates of hydrogen economy. The development of hydrogen-fueled vehicles and portable electronics will require new materials that can store large amounts of hydrogen at ambient temperature and relatively low pressures with small volume, low weight, and fast kinetics for recharging.13 The U.S. Department of Energy targets for a hydrogen storage system are as follows,13,70

 A capacity of 45 g H2per L

 Re-fuelling time of 10 min or less

 Life time of 1000 refuelling cycles

 Ability to operate within the temperature range of -30 to 50 ºC

These targets are set for the entire storage system. Therefore practically a material should have a higher capacity to store hydrogen when considering the weight and the regeneration system. However the size and the weight of these components depend on the operational conditions such as whether it goes through chemisorption or physisorption mechanism.13 Physisorption correlates with surface area, with greater gas uptake favored by higher surface area. As MOFs have higher surface areas and low densities, they are one of the attractive candidates for hydrogen storage. However since dispersive forces cannot assist to uptake considerable amount of hydrogen, MOFs need to accommodate new functionalities to enhance the storage properties.

A vast variety of research has been done on computational studies to model H2 adsorption in MOFs.71-73 MOF-5 and its isoreticular series have received much attention for this purpose. These computed isotherms and binding energies usually agree with the experimental results. Van der Waals type interactions between H2 and frameworks are employed in this regard. These types of materials where physisorption dominates the adsorption, storage capacity depends on the size of the pore. In graphites and carbon nanotubes it was predicted that the maximal uptake takes place with the materials having pores around 7 Å wide.13 This will allow one layer of H2 molecules to adsorb on opposing surface without leaving any space in between.

Increasing the H2binding energy within MOFs is the most important challenge for creating hydrogen adsorbents that operate at 298 K. The desired binding energy to be achieved is ca. 20 kJ mol-1.74 Therefore mostly in MOF systems strong orbital interactions will be avoided and move to simple charge-induced dipole interactions. The H2 adsorption enthalpy in MOF systems can probably be increased by introducing open metal sites on the surfaces.75

MOFs that consist of large pores may not be good enough for H2 storage due to the poor attraction from the surface of pore walls experienced by the H2molecules in the center of the pores. Therefore MOFs with pores just bigger than the kinetic diameter of the H2would be an ideal storage material.76Interpenetration is a tool which can be used to reduce the number of large voids in a given structure. But it

material. There is only one example known in the literature which is Cu3(tatb)2 (tatb3-= 4,4’,4’’-s-triazine-2,4,6-triyltribenzoate) where the catenated and the noncatenated versions of the same framework is compared for H2 storage.77 The catenated version adsorb 1.9 wt% of H2 at 1 bar and 77 K whereas the non catenated version adsorbs only 1.3 wt%.77

One of the best examples of MOFs for H2storage reported today is MOF-5, which is a cubic Zn-terephthalate based network (Figure 1.12).9 At 77 K it shows a gravimetric uptake of about 1.3 wt% at 1 bar and 5.1 wt% at 5 bar.78,79 This variation is due to the incomplete evacuation of guest molecules from the channels as well as partial decomposition of the framework upon exposure to air. It was reported that upon complete activation and protection of the sample from water and air, the H2 uptake observed was 7.1 wt% at 77 K and 40 bar. This amount increased to 10 wt% at 100 bar (Figure 1.13) which corresponds to the volumetric storage density of 66 g L-1.80 More interestingly it was reported that hydrogen can be loaded into a cold sample of the compound within 2 minutes, and 24 cycles of adsorption and desorption has taken place without loss of capacity.80

Figure 1.12. Crystal structure of MOF-5. Yellow, gray and red spheres represent Zn, C, and O atoms, respectively. H atoms are omitted for clarity.13

Figure 1.13. Excess (squares) and total (circles) hydrogen uptake for MOF-5 at 77 K. The solid line represents the density of compressed hydrogen over the given

Although MOF-5 shows high storage capacity at 77 K, it doesn’t show a good uptake at 298 K due to weak interactions between H2 and the framework. The volumetric capacity of MOF-5 at 298 K and 100 bar is about 9.1 g L-1 whereas compressed H2has a value of 7.7 g L-1under the same conditions.80This suggests that MOF -5 is not a good storage material for H2at room temperature.

Mn3[(Mn4Cl)3(BTT)8]2, which contains open Mn2+ coordination sites has a volumetric storage capacity of 60 g L-1at 77 K and 90 bar and 12.1 g L-1at 298 K and 90 bar which is 77 % greater than the density of compressed gas under same conditions.81 This is the highest storage capacity reported for MOFs under these conditions. The isosteric heat of adsorption of H2for this MOF is 10.1 kJ mol-1at zero coverage.81 According to the powder neutron diffraction data, strong Mn-D2 interactions can be seen with a separation of 2.27 Å between metal and the center of the D2 molecule.82 This is a much shorter distance observed for physisorbed hydrogen and may have an effect on efficient packing of hydrogen molecules in channels.

MOFs have a number of key characteristics which show promise for exceptional hydrogen storage properties such as high surface area and fine tunable pore architectures. However so far none of them meet all the criteria set by U.S. Department of Energy and the hydrogen economy is yet to flourish.