Sustainable hydrogen storage
Hydrogen (H2) offers a sustainable energy carrier to replace fossil fuels, but storing the large volumes of H2 needed provides a serious challenge. Researchers in the US and Europe have examined the adsorption of H2 in two different types of material, and come to similar conclusions.
The amount of H2 needed to provide the same energy as gasoline occupies about 3000 times the volume of gasoline at room temperature and pressure, so some type of volume reduction is essential if H2 is to become a viable alternative. One of the most promising areas, rather than compression or liquefaction, is the use of microporous materials, characterised by large and accessible internal volumes.
hydrogen in chabasite
© University of Torino and Oslo University
At the University of California Berkeley, US, Jeffrey Long and Steven Kaye have assessed dehydrated Prussian blue analogues of the type M3[Co(CN)6]2, where M is manganese, iron, chromium, nickel, copper or zinc.1 Their hydrated forms were synthesised and completely dehydrated, leaving the hollow metal-cyanide framework intact.
At 77K and 890Torr, the H2 storage capacities ranged from 1.4 to 1.8wt per cent, which equates to minimum storage densities of 0.018-0.025 kg H2/l. H2 uptake was completely reversible and, for the copper analogue, showed no reduction in capacity after five adsorption/desorption cycles.
However, 'the storage density in these materials is still too low for them to be suitable for real applications,' says Long, who is looking to improve this by replacing the transition metal ions with lighter main group metal ions.
In contrast, Silvia Bordiga and colleagues from the University of Turin, Italy, and the University of Oslo, Norway, have turned to zeolitic materials,2 which are widely employed in gas separation. The researchers studied the protonic chabazite H-SSZ-13 and found it to be the most adsorbent zeolite or zeotype in the literature, with an H2 storage capacity of 1.28 mass per cent at 77K and atmospheric pressure.
At five H2 molecules per cage in the chabazite structure, the capacity is approaching the maximum value of seven H2 molecules that is associated with H2 liquefaction. Bordiga attributes the good performance to the combination of high internal surface areas, small cages and highly dispersed polarising species such as cations. The importance of this work 'lies in showing that H+ can have a reasonably strong binding affinity for H2,' says Long.
While the Prussian blue analogues and chabazite 'show similar properties towards H2, the amount of hydrogen storage is still too low to be considered for applications in mobile transportation,' says Bordiga, 'but could become interesting for applications where weight and volumes are not too restricted, such as sea transportation.'
1 S S Kaye and J R Long, J. Am. Chem. Soc., 2005, 127, 6506
2 A Zecchina et al, J. Am. Chem. Soc., 2005, 127, 6361