Hydrogen adsorption in microporous alkali-doped carbons (single-wall carbon nanotubes and activated carbons)

Transcription

1 Hydrogen adsorption in microporous alkali-doped carbons (single-wall carbon nanotubes and activated carbons) Laurent Duclaux a, Szymon Los b, Michel Letellier c, Philippe Azaïs c, Roland Pellenq d, Thomas Roussel d, Xavier Fuhr d, a LCME, ESIGEC, Université de Savoie, Savoie Technolac, Le Bourget du Lac Cedex, France b IFMPAN, Smoluschowskiego 17, , Poznan, Poland c CRMD, CNRS-Université d Orléans, 1B Rue de la Férollerie Orléans Cedex 2, France d CRMCN, CNRS campus de Luminy, boîte 913, Marseille cedex 09, France ABSTRACT: Doping of microporous carbon by Li or K leads to an increase in the energy of adsorption of H 2 or D 2 molecules. Thus, the room temperature sorption capacities (at P<3 MPa) can be higher than the ones of the raw materials after slight doping. However, the maximum H 2 (or D 2 ) storage uptake 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. The microporous adsorption sites of doped single-walled carbon nanotubes, 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). KEYWORDS : adsorption isotherms, microporous carbons, alkali-metal doping, neutron diffraction, Electron Paramagnetic Resonance Introduction Research on H 2 storage is now focused on new materials that can assume the constraints imposed by the automotive applications. Activated carbons with very high surface area are interesting materials for hydrogen adsorption; but cold temperature are required (80 K, for example). A study on an activated carbon, known as super-activated (microporous carbon), reports an adsorption capacity of 0.3 weight % at 295 K and 3 MPa, and 5 weight % at 77 K and the same pressure [1]. This result is in agreement with the very high specific surface area of this carbon and shows that microporous porosity is required to obtain high specific adsorption capacity of hydrogen. Very high storage capacities were claimed for carbon nanotubes and nanomaterials. However, the more realistic values of H 2 sorption on graphite nanofibers are rather less than 1 weight % [2] instead of the 65 weight % claimed at room temperature [3]. Then, the more promising carbon materials for hydrogen adsorption are those having micropores (pore mean size < 2 nm). Moreover, in the years 70s, several studies on intercalation compounds of graphite (with heavy alkali metals K, Rb, Cs) have shown that these compounds can store hydrogen by chemisorption (at 200 K) or physisorption at lower temperature (77 K and 100 K) in small amount [4]. For example, the second stage compound MC 24 (M=K, Rb, Cs) can physisorb 1.15% weight of H 2 at 77 K [5-6]. The hydrogen molecules are then inserted into the intercalated graphite planes [5]. The charge transfer to the carbon matrix, for example by doping reaction with alkali metals, is supposed to enhance the energy of adsorption [7-8]. In the patent of Petz et al [9], the doping of a microporous activated carbons K or Li was tried and has unfortunately yielded to a decrease in the sorption capacities. The effect of doping by electron donors in porous carbon skeleton implies that the interaction potential between the charged surface and adsorbate show a deeper potential well. Thus, the adsorption energy is higher and the adsorption uptake is expected to be higher at temperature suitable for the automotive applications (close to room temperature). Then, the doping of carbon substrates has been proposed in recent papers as a solution to obtain good hydrogen adsorption properties at appropriate temperatures close to room temperatures [7-10]. Theoretical studies have suggested using Li as a dopant in nanostructured carbons such as Single Wall Nanotubes (SWNTs) or graphite system [10]. The optimized distance between the two layers (i. e; interlayer distance) or intertube distance is described as a critical parameter to control the mean porous size of the system which influences dramatically the hydrogen uptake [8,10]. Thus, we have studied the adsorption properties of microporous carbons (SWNTs and activated carbons) doped with alkali metals. 1/6

2 Experimental Two host carbon materials were studied: activated carbons and SWNTs. The activated carbons (exclusively microporous, with a BET specific surface area of 1600 m 2 /g) were treated by H 2 /N 2 at 700 C in order to remove the oxygenated functionalized groups. Only a 0.5 weight % Oxygen content was determined in these treated activated carbons. Two kinds of SWNTs were used for the doping reactions: SWNTs from an electric-arc origin (Nanoledge, Montpellier [11]) with closed extremities, and SWNTs obtained by the decomposition of CO at high pressure purified by an oxidative treatment (HiPCO process [12]). The latter SWNTs were also opened (extremities and walls) by oxidation in air (12 min. at 400 C) followed by treatment in HCl 5 M (24 h) and annealing at 700 C under H 2 /N 2 atmosphere. The treated activated carbons and SWNTs samples were outgased during one day at 250 C previously to doping reactions. The carbon samples (ie activated carbon and SWNTs) were doped by vapour phase reaction with K or Li; in order to prepare KC 24 activated carbon and KC x SWNTs (x=7, 8, 10 and 24), or LiC x activated carbons (x=6 and 18) and LiC 18 SWNTs respectively. In the case of Li doping, the reaction was carried out in a degassed stainless steel reactor at T=370 C. Doping with K was performed at T=250 C in sealed Pyrex tubes (by the classical two bulbs method) or directly by annealing potassium metal and carbon mixture in an outgased sealed tube. LiC 18 and KC 8 (composition at saturation) compositions were synthesized with HiPCO SWNTs. KC 7 (composition at saturation) and KC 10 stoichiometries were obtained from arc-electric SWNTs. KC 24 sample was prepared from opened HiPCO SWNTs. The doped activated carbons have been characterized by 7 Li Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR). For these experiments the samples were transferred in vacuum sealed Pyrex and quartz tubes respectively. Each steps of Li doping procedure of the activated carbons was controlled by 7 Li nuclear magnetic resonance (NMR) performed at 22 C, using a 8.46 Tesla cryo-magnet and a Bruker Avance DSX 360 spectrometer equipped with a broadband static probe-head. The frequency shifts were referenced to aqueous LiCl (1M). The activated carbons have been also characterised by EPR using X band spectrometer RADIOPAN equipped with ESR900 Oxford cryostat. The spectra were recorded in a wide range of temperature from 300 K to 4.2 K. Hydrogen adsorption measurements were performed in the range [0-3 MPa] on activated carbon samples at room temperature and 77 K using a Baratron gauge (MKS Baratron 120A).The adsorption of molecular D 2 in SWNTs was followed by neutron diffraction in the range [0-0.1 MPa] from room temperature to 10 K, at Institut Laue Langevin (ILL, Grenoble, France) on the D1B experiment (λ=2.52 Å). Simultaneously, the adsorption-desorption isotherms were recorded between 0 and 0.1 MPa. Results Doped Activated Carbons At 77 K, all the isotherms profiles of doped activated carbon are typical of microporous materials (Type I in the IUPAC classification). The values of adsorption capacities of hydrogen at T<77 K (Table 1) of the raw activated carbons (AC) are higher than the ones of the materials doped by Li. Temperature Raw AC LiC 18 AC LiC 6 AC 298 K K Table 1: Adsorption capacities in weight % at 2 MPa of raw and Li-doped activated carbons These differences are probably due to the insertion of Li ions in the micropores sites, hindering the sorption of hydrogen molecules as reported previously [13]. Despite the lower accessible porosity of the doped microporous materials, the most striking effect is the enhancement of the sorption capacity at temperatures higher than 77 K, compared to raw undoped material. For example, in the case of Li doped activated carbons, the sorption capacities at room temperature and 2 MPa of LiC 6 and LiC 18 are higher than the sorption of pristine material (table 1). This is explained by the increase of energy of adsorption due to charge transfer after doping that yield the adsorption to be more efficient at high temperature. From the point of view of 7 Li NMR, a charge transfer occurs from the alkali metal to the activated carbons as Li is mainly found in an ionic state. The ESR signal of raw activated carbon brings out mainly the presence of electron acceptor paramagnetic centres (holes) that can be removed by a reducing treatment (annealing at 700 C in H 2 atmosphere) leading to mainly electron delocalised paramagnetic centres. The LiC 6 and LiC 18 doped activated carbons ESR response show a complex signal indicating the presence of three kinds of 2/6

3 electronic spins: holes, delocalised and localised electrons [14]. This demonstrates that vapour doping (ie. charge transfer from alkali metal) of a heterogenous materials such as AC is only efficient on some peculiar electronic sites that should be correlated to the structure environment of Li insertion sites in the porous carbon skeleton. The Li sites from which charge transfer occurs give an increase in the electrical field at the surface of the adsorbent so that the attractive interaction between adsorbent and hydrogen is more intense. As results, Li-doped AC were found to be more efficient for hydrogen adsorption at low coverage than pristine AC. Doped Single-Wall Nanotubes (SWNTs) In SWNT systems, the sites of adsorption can be determined by the analysis of the neutron diffractograms. The SWNTs are organised in a rope-like structure (Figure 1). Figure 1: Arrangement of the tubes in the section of the ropes. The tubes are arranged in a triangular 2D lattice (a: parameter of the lattice, d: intertube spacing of 0.32 nm, Rnt: radius of the nanotubes). Interstitial cavities (in between the tubes) and central canals are possible adsorption sites. In the ropes, the initial tubes form a 2D triangular lattice (Figure 1) giving a (hk) diffraction pattern.the diffractograms of electric-arc origin KC 10 and KC 7 (saturation composition) SWNTs show that nanotubes are slightly intercalated by the K + ions. Consequently, the cavities remain mainly empty in between the tubes within the bundles and the alkali ions can be localised at the surface of bundles. The adsorption of deuterium in doped SWNTs system leads to a slight shift of the 10 line (at q=0.54 Å -1 ) toward greater distances. This can be explained by an expansion of the 2D lattice (of 0.4 Å) due to the adsorption of D 2 but also by a cross interference between the carbon skeleton and the adsorbate [15]. A careful analysis on the neutron diffractograms, of the intensity of the 10 line as a function of the D 2 sorption ratio (D 2 /C) gives information about the adsorption sites of gas molecules. Indeed, the presence of gas molecules adsorbed at the outside of the tube (for example, in the interstitial voids in between the tubes, or in the groove at the surface of bundles) lead to an increase in the intensity of the (10) line [16, 17, 18]. On the contrary, when the adsorbent molecules are in the inner sites (central canal of the tube), the effect on the (10) line is a decrease of its intensity [16]. As the D 2 molecules are adsorbed, the intensity of the (10) line is found to be increasing in electric-arc doped SWNTs (KC 10, KC 7 ), but remaining constant in HiPCO LiC 18 doped SWNTs (Figure 2). It means that the adsorption occurs exclusively in the interstitial cavities and grooves (micropores) (in between the nanotubes within the bundles) in the SWNTs from electric-arc origins which are closed at their extremities. The weight storage capacities of D 2 (at 1 bar) in the raw nanotubes and those of KC 10 composition can reach respectively 0.45 % and 0.67 %, at 77 K. 3/6

4 (a) (b) Intensity [a. u.] vacuum 10 SWNTs D2/C=0.03 P=108.5 mbar D2/C=0.1 P= mbar 001 KC 8 (GIC) Intensity [a. u.] SWNTs vacuum D2/C=0.097 P=0.9 mbar D2/C=0.14 P= mbar θ ( ) θ ( ) Figure 2: Neutrons diffractograms at low θ angle of (a) KC 10 SWNTs (arc-electric origin) at 33 K and (b) LiC 18 SWNTs (HiPCO origin) as a function of D 2 /C ratio at 25 K. The D 2 /C ratios and the corresponding equilibrium pressure P are mentioned. The (10) of the 2D lattice of doped SWNTs and the 001 line due to KC 8 graphite intercalation impurities are labelled. In doped HiPCO SWNTs, the adsorption sites are both grooves, interstitial and inner sites. Indeed, the purified pristine tubes were partially opened by oxidation treatment during purification.the accessible porosity is then greater in HiPCO SWNTs and can be again increased by oxidative annealing in air in order to open totally the nanotubes. As a result, the deuterium adsorption storage capacity of opened tubes (3 weight % at 77 K and 0.1 MPa) is twice the one of pristine purified HiPCO SWNTs (1.5 weight % at 77 K and 0.1 MPa). By contrast to arc-electric SWNTs, The purified HiPCO nanotubes doped by saturation (KC 8 ) are a non adsorbent system. As a matter of fact, neither the cavities in between the bundles, nor the inner sites (in the central canal) are accessible [19]. Indeed, the K ions fully occupy all the porosity of the material so that no additional adsorption of hydrogen gas is allowed. This result means that insertion sites of the dopant in the two kinds of SWNTs (HiPCO and electric-arc) are different. In electric-arc nanotubes, the amorphous carbon deposit at the surface of bundles can hinder the diffusion of alkali metals within the ropes. 30 Hads (kj/mol) HiPCO nanotubes LiC18 HiPCO SWNTs KC7 electric-arc SWNTs electric-arc SWNTs 0 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 coverage (mmol/g) Figure 3: adsorption isosteric heats as a function of coverage for pristine (HiPCO and electric-arc) and doped (KC 7 electric-arc et LiC 18 HiPCO) nanotubes. 4/6

5 Whatever the types of nanotubes, the adsorption capacity of deuterium at T<77 K decreases as the dopant content increases. For example, the KC 7 saturation doped sample adsorbs (at 1 bar) 0.35 % at 77 K instead of 0.67 % for the KC 10 stoichiometry. After doping, the alkali ions are occupying adsorption sites limiting the accessible porosity of microporous carbons (nanotubes and activated carbons). However, at high temperature (close to room temperature), the adsorption capacities of the microporous carbons doped at appropriate stoichiometries (KC 24 activated carbon or LiC 18 single-wall nanotubes) can reach higher values than the pristine materials [19]. This effect is explained by an increase of the adsorption energy due to charge transfer by electron donors. The energies of adsorption are generally comparable to the sorption isosteric heats which were determined in doped SWNTs by using the Clausius-Clapeyron law from the isotherm network at T ~80 K. The values found for KC 7 (arc electric-origin) and LiC 18 SWNTs (HiPCO) are close to 10 kjmol -1 and 20 kjmol -1, respectively (at coverage =0.2 mmole/g) instead of 5 kjmol -1 for the raw material at similar coverage (Figure 3). Conclusion Despite promising properties for hydrogen adsorption, carbon SWNTs have still lower sorption capacities compared to activated carbons due to the presence of macropores, mesopores and some closed porosity. Doping by alkali metals of microporous carbon gives an increase in the energy of adsorption through the electron charge transfer. The energy of adsorption is more than twice the one in raw carbon porous matrix (5 kj.mol -1 ). Thus, the effect of doping is an increase in sorption capacity at high temperature, only if some adsorption sites are accessible and not totally blocked by alkali metals. Actually, during the doping reaction, the alkali metals are also inserted in the microporous adsorption sites leading to a decrease in the maximum adsorption capacity observed at low temperature. Attempts at doping the adsorbent without any decrease of the specific surface area still need to be performed. References: [1] P. Bénard et R. Chahine, Determination of the adsorption isotherms of hydrogen activated carbons above the critical temperature of the adsorbate over wide temperature and pressure ranges, Langmuir, 17, , [2] A. Züttel et al., Hydrogen storage in carbon nanostructures, int. J. Hydrogen Energy, 27, , [3] A. Chambers, C. Park, R. Terry, K. Baker, N. Rodriguez, Hydrogen storage in graphite nanofibers, J. Phys. Chem. B, 102, , [4] T. Enoki, S. Miyajima, M. Sano, H. Inokuchi, Hydrogen-alkali-metal-graphite ternary intercalation compound, Journal of Material Research, 5, , [5] K. Watanabe, T. Kondow, M. Soma, T. Onishi and K. Tamaru, Molecular sieve type sorption on alkali graphite intercalation compounds Proc. Roy. Soc. Lond., A 333, 51, [6] K Ichimura, E. Takamura and M. Sano, Hydrogen in alkali-metal-graphite intercalation compounds, Synthetic Metals, 40, , [7] G. E. Froudakis, Why Alkali-Metal Doped Carbon Nanotubes possess high hydrogen uptake, Nanoletters 1, , [8] O. Maresca, R. J.-M. Pellenq, F. Marinelli and J. Conard, A search for a strong physisorption site for H2 in Li-doped porous carbons, Journal of Chemical Physics, 121, , [9] G. D. Petz, W. A. Steyert, Method for adsorbing and storing hydrogen at cryogenic temperatures, US patent,us , [10] W.-Q. Deng, X. Xu, W. A. Goddard., New alkali doped pillared carbon materials designed to achieve practical reversible hydrogen storage for transportation, Phys. Rev Lett. 92, [11] C. Journet, W. K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant, P. Deniard, R.Lee, J. E. Fischer, Large scale production of single walled carbon nanotubes by the electric-arc technique, Nature, 388 (21), , [12] P. Nikolaev, M. J. Bronikowski, R. K. Bradley, F. Rohmund, D. T. Colbert, K. A. Smith, R. E. Smalley, Gas phase catalytic growth of single-walled carbon nanotubes from carbon monoxide, Chemical Physics Letters, 313, 91-97, [13] S. Challet, P. Azaïs, R. J-M. Pellenq, O. Isnard, J-L. Soubeyroux, L. Duclaux, Hydrogen adsorption in microporous alkali-doped carbons (activated carbons and single wall nanotubes) Journal of Physics and Chemistry of Solids 65, , /6

6 [14] S. Łoś, M. Letellier, P. Azaïs, L., Li doped carbons (activated microporous carbons and graphite): characterisation by resonance spectroscopies (ESR and 7Li NMR) and their potentiality for hydrogen adsorption, Journal of Physics and Chemistry of Solids, in press, [15] M. Bienfait, P. Zeppenfeld, N. Dupont-Pavlosky, M. Muris, M. R. Johnson, T. Wilson, M. DePies and O.E. Vilches, Thermodynamics and structure of hydrogen, methane, argon, oxygen, and carbon dioxide adsorbed on single-wall carbon nanotubes bundles, Phys. Rev. B 70, , [16] A. Fujiwara, K. Ishii, H. Suematsu, H. Kataura, Y. Maniwa, S. Suzuki, Y. Achiba, Gas adsorption in the inside and outside of single-walled carbon nanotubes, Chem. Phys. Lett., , [17] M. R. Johnson, S. Rols, P. Wass, M. Muris, M. Bienfait, P. Zeppenfeld, N. Dupont-Pavlovsky, Neutron diffraction and numerical modelling investigation of methane adsorption on bundles of carbon nanotubes, Chemical Physics, 293, , [18] S. Challet, P. Azaïs, R.J-M. Pellenq, L. Duclaux, Deuterium adsorption in potassium-doped single wall carbon nanotubes: a neutron diffraction and isotherms study, Chemical Physics Letters, 377, , [19] S. Los, P. Azaïs, R. Pellenq, Y. Breton, O. Isnard, L. Duclaux, Confining H 2 and D 2 by adsorption in microporous carbon (single walled carbon nanotubes and activated carbons) doped by K or Li, Ann. Chim. Sci. Mat., 30, , /6