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14 декабря, 2021
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Dihydrogen
In the long term, H2 is envisaged as a potential energy carrier. However, one of the issues for portable applications of this energy vector relies on its economic and safe pressure storage under the conditions of transport. Although the targets set for 2015 by the U. S. department of energy (DOE) are difficult to reach, several options are extensively investigated. Compressed or liquefied H2 is not suitable for mobile applications, because of low volumetric energy density and safety problems. A promising way for mobile applications is solid state storage. One can differentiate physisorption in porous materials, including zeolites, MOFs and different types of carbons, and chemisorption resulting in the formation of hydrides. INS has been used to characterize various hydrides, starting from transition-metal hydrides, up to complex hydrides composed with light elements (lithium, boron, sodium or aluminium),
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Fig. 2.3 INS spectra from NaAlH^ACF-25: A as prepared, B after hydrogen desorption — absorption cycling (full lines); the dashed line corresponds to bulk NaAlH4. The data were recorded on the instrument TOSCA, at the pulsed neutron and muon source at the Rutherford Appleton Laboratory (ISIS). Reprinted with permission from (D. Colognesi, A. Giannasi, L. Ulivi, M. Zoppi, A. J. Ramirez-Cuesta, A. Roth, M. Fichtner, J. Phys. Chem. A 115, 7503 (2011)) [11] |
which provide high gravimetric H2 density. Catalysts play a role in some cases: for alanates, doping with a titanium catalyst increases the rates of adsorption and desorption of H2 [9], forborohydrides, hydrogen release through a hydrolysis reaction can be controlled catalytically [10].
Since transport can be a limiting step, the use of nanoparticles is an option to improve the kinetics. Raman scattering and INS techniques have been used to find out if the infiltration process of a carbon matrix with NaAlH4 creates new chemical species (e. g., Na3AlH6) and if the nanoparticles of NaAlH4 have a physical state different from the bulk or not [11]. The influence of hydrogen desorption — absorption cycling was also tested. The INS spectra of the melt-infiltrated composite of NaAlH4 and active carbon fibers (ACF-25) are compared to the corresponding spectroscopic data taken from bulk NaAlH4 in Fig. 2.3.
The comparison between spectra (a) and (b) in Fig. 2.3 indicates that INS is not sensitive to the desorption-absorption cycle. On the other hand, the comparison with bulk NaAlH4 shows a broadening of the peaks with some energy shifts, typical of crystal-size effects. The extra intensity observed between 130 and 200 meV was attributed to the presence of a small amount of Na3AlH6, also observed in the Raman spectrum [11].