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Such is the scenario presented by hydrogen sorption in activated carbons and metal-organic-frameworks (MOFs). Both these materials seem to present a reasonable potential for hydrogen (H2) storage, a desirable if not necessary step towards full commercialization of fuel cell technology.
Pores Accessible to HydrogenThere exists a need, therefore, for rapid characterization tools-tools that are available now so as not to suffer any further delay in advancing the development of useful nanoscale materials. Thankfully, that technology does already exist in the gas sorption arena. Cryogenic gas sorption analyzers have been characterizing microporous and mesoporous materials such as zeolites and activated carbons for generations. Until now however, pore size and pore volume measurements have been almost exclusively limited to the adsorption of nitrogen and argon. But some pores (or parts of pores) accessible to H2 might not be accessible to other molecules because of size restrictions or due to very slow diffusion. Therefore it only seems sensible to use H2 for the pore size distribution (PSD) analysis of porous materials considered for H2 applications.
Appreciable AdsorptionCurrent state-of-the-art volumetric adsorption equipment (such as the Autosorb-1-MP, supplied by Quantachrome Instruments, Boynton Beach, Fla.) is already being used to measure hydrogen adsorption isotherms, since at cryogenic temperatures appreciable adsorption of hydrogen begins at about 10-4 atm for microporous materials. It is important to note that the critical temperature of hydrogen is much lower, around 33 K. Hence, even though measurements at temperatures of liquid nitrogen (77 K) or liquid argon (87 K) might seem far from forecast/actual storage temperature, both are at supercritical conditions. In fact, lowering the temperature has a similar effect of increasing the amount adsorbed (over room temperature amount) as does increasing pressure (at room temperature or above), the latter being conceivably the practical solution for mobile fuel cell applications.
Therefore, hydrogen adsorption experiments performed even at subatmospheric pressures provide important information about the hydrogen storage potential of an adsorbent.
New Models DevelopedAdsorption data measured at different temperatures (see Figure 1) can be used to calculate the isosteric heat of adsorption, Qst.1 Materials showing high Qst values over a wide range of adsorbed amount will have high adsorption capacity at ambient temperatures.
PSD calculations can also be done from the sub atmospheric data, but not using classical models. Therefore, new models have been developed, and analyses using Density Functional Theory (DFT) applied to H2 adsorption isotherms measured for several porous carbons were presented recently.2
A Tool for Other Nanoscale MaterialsThis new development in size analysis and characterization of sub-nano pores (small micropores), though created to meet the need to investigate the hydrogen storage potential of various materials, will undoubtedly be adopted for the characterization of other material structures bearing nanospace and/or for different nanoscale applications. Such applications might include nanoparticle-enhanced filters for separating compounds at the molecular level, or porous frameworks used as nanoflasks for polymer synthesis.
In his testimony to the Senate Committee on Energy and Natural Resources, Nobel Prize-winning Professor Richard Smalley foretold of massive electrical power transmission over continental distances. He envisages that nanotechnology in the form of single-walled carbon nanotubes, forming what he calls the Armchair Quantum Wire, may play a big role in this new electrical transmission system.
For More InformationFor more information about size analysis and characterization of sub-nano pores for hydrogen storage or other applicatoins, contact Quantachrome Instruments at email@example.com, call Jacek Jagiello at (561) 731-4999, or visit http://www.quantachrome.com.
References1. A. Ansón, M.A. Callejas, A.M. Benito, W.K. Maser, M.T. Izquierdo, B. Rubio, J. Jagiello, M.Thommes, J.B. Parra and M.T. Martínez (2004), "Hydrogen Adsorption Studies on Single Wall Carbon Nanotubes," Carbon, Vol. 42, pp. 1243-1248.
2. J. Jagiello and M. Thommes (2004), "Comparison of DFT Characterization Methods Based on N2, Ar, CO2, and H2 Adsorption Applied to Carbons with Various Pore Size Distributions," Carbon, Vol. 42, pp. 1227-1232.
An Abbreviated Autosorb-1/Nanocarbon Bibliography1. A. Ansón, M. Benham, J. Jagiello, M.A. Callejas, A.M. Benito, W.K. Maser, A. Züttel, P.Sudan and M.T. Martínez (2004) "Hydrogen adsorption on a single-walled carbon nanotube material: a comparative study of three different adsorption techniques." Nanotechnology 15, 1503-1508.
2. A.V. Neimark, S. Ruetsch, K.G. Kornev, P.I. Ravikovitch, P. Poulin, S. Badaire and M. Maugey (2003) "Hierarchical Pore Structure and Wetting Properties of Single-Wall Carbon Nanotube Fibers" Nano Lett. 3, 419-423
3. E. Bekyarova, K.Kaneko, M. Yudasaka, D. Kasuya, S. Iijima, A. Huidobro, and F. Rodriguez-Reinoso (2003) "Controlled opening of single-wall carbon nanohorns by heat treatment in carbon dioxide" J Phys Chem B. 107. 4479-4484
4. E.Poirier, R. Chahine, P. Bénard, D. Cossement, L. Lafi, E. Mélançon, T.K. Bose and S. Désilets (2004) "Storage of hydrogen on single-walled carbon nanotubes and other carbon structures"Appl. Phy. A: Mat. Sci. Process. 78, 961 - 967.
5. W.Z. Zhu, D.E. Miser, W.G. Chan and M.R. Hajaligol (2003) "Characterization of multiwalled carbon nanotubes prepared by carbon arc cathode deposit" Mat. Chem. Phys. 82, 638-647
6. Z. Ma, T. Kyotani and A. Tomita (2002) "Synthesis methods for preparing microporous carbons with a structural regularity of zeolite Y" Carbon 40, 2367-2374