Hydrogen adsorption in doped porous carbons

Full text of publication follows: Hydrogen is a clean fuel that will be used in automotive transport when the problem of storage will be solved. The difficulties of H2 storage (available space, security and performance, etc...) require a material that can store 5 weight % of hydrogen. Research is focused on new materials that can assume the constraints imposed by the automotive applications. Among these materials, the nano-structured carbons (nano-fibers and single walled carbon nano-tubes) were claimed to be promising by numerous authors [1-3]. The more promising carbon materials for hydrogen adsorption are those having micropores (i. e. single walled carbon nano-tubes and activated carbon), for which the energy of sorption of hydrogen molecules is theoretically higher [7-8]. Presently, the best performance of hydrogen adsorption was found in super-activated microporous carbons sorbing 5 weight % at 77 K, and almost 0.5 % at room temperature and 6 MPa [9]. Up to now, the performance of these materials can still be improved as the known mechanism of sorption in these carbon materials: physi-sorption controlled by Van der Waals attractive forces through London interaction is efficient at cryogenic temperatures (77 K) where the interaction between adsorbent and adsorbate becomes stronger. One way to improve the attractive interaction between adsorbent and molecule is to increase the forces due to the interaction of electrical field and induced dipole of the molecule. This can be theoretically tailored in carbon materials through the electron charge transfer by electron donors who can provide an increase in the electrical field at the surface of the adsorbent. Then, the doping of carbon substrates, appearing to be a promising method to increase the energy of adsorption has been proposed in recent papers as a solution to obtain good hydrogen adsorption properties at appropriate temperatures close to room temperatures [10-12]. Thus, we have studied the adsorption properties of doped microporous carbons (SWNTs and activated carbons). The raw nano-structured carbon materials are microporous activated carbons (BET specific surface area: 1600 m2/g), electric-arc closed-end single-walled carbon nano-tubes (SWNTs), and HiPCO SWNTs. They have been doped in the vapour phase by K, and Li (in order to obtain KC7, KC10, LiC18 SWNTs and LiC6, LiC18, KC24 activated carbons). The hydrogen adsorption-desorption isotherms of the doped activated carbons were obtained at room temperature and at 77 K, up to 3 MPa, by a volumetric method. The adsorption of D2 was studied in situ on doped nano-tubes and nano-filaments by neutron diffraction on D1B experiment (ILL), at λ=2.52 Angstroms, as a function of temperature [300 - 20 K] and pressure. Simultaneously the adsorption isotherms were recorded in the range 0 - 0.1 MPa [13]. Doping of microporous carbon by Li or K leads to an increase in the energy of adsorption of H2 or D2 Molecules. Thus, the room temperature sorption capacities (al P≤3 MPa) can be higher than that of the raw materials after slight doping. However, the maximum H2 (or D2) storage measured at T ≤ 77 K is lower than the one of pristine Materials as the sites of adsorption are occupied by alkali ions inserted in the micropores [14]. The microporous adsorption sites of doped single-walled carbon nano-tubes, identified by neutron diffraction, are both the interstitial voids (in electric-arc or HiPCO tubes) in between the tubes and the central canals of the tubes (only in HiPCO tubes) [15]. We have also prepared nano-structured porous doped carbons by ball-milling carbon host materials with some dopant, such as alkali or alkaline earth metal. The new nano-porous carbons obtained by this method have been characterised and tested for their hydrogen-adsorption properties. References: [1]A. Chambers et al, J. Phys. Chem. B, 102, 4253,1998. [2]A. C. Dillon et al, Nature, 386, 377,1997. [3]J. Conard, Ann. Chim. Sci. Mat 26, 107, 2001. [4]A. Zuttel et al, Int J. Hydrogen Energy, 27, 203, 2002. [5]G. G. Tibbetts et al, Carbon 39, 2291, 2001. [6]C. C. Ahn et al, Appl. Phys. Lett, 73, 3378,1998. [7]M. Rzepka, et al, J. Phys. Chem. B, 702 10894,1998. [8]M. Shiraisly et al, Chem. Phys. Lett., 367 633, 2003. [9]P. Benard et al, Langmuir, 17 1950, 2001. [10] G. E. Froudakis, Nanoletters 1, 531 2001. [11] 0. Maresca, et al, accepted in Journal of Chemical Physics, 2004. [12]W.-Q. Deng et al, Phys. Rev Letters 92,166103-1,2004. [13]S. Challet et al, Chemical Physics Letters, 377, 544 2003. [14]S. Challet et al, Journal of Physics and Chemistry of Solids, 65, 541 2004. [15]S Los et al, accepted in Ann. Chim. Sci. Mat., 2005