MOLECULAR MODELING OF HYDROGEN AND SELECTED TYPES OF CNT'S INTERACTIONS

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1 MOLECULAR MODELING OF HYDROGEN AND SELECTED TYPES OF CNT'S INTERACTIONS Gražyna SIMHA MARTYNKOVÁ, 1,2 Lucia ROZUMOVÁ, 1,2* Marianna HUNDÁKOVÁ, 1,2 1 Nanotechnology Centre, VŠB Technical University of Ostrava, Ostrava, Czech Republic 2 IT4Innovations Centre of Excellence, VŠB Technical University of Ostrava, Ostrava, Czech Republic * rozumovalucia@gmail.com Abstract State-of-art on hydrogen storage of carbon nanotubes was implemented. Based on current knowledge modeling of carbon nanotube for H2 storage performed using molecular dynamics tools. Several single wall carbon nanotubes associated in nanorope with boron, nitrogen and phosphor heteroatoms were modeled. Physical and chemical properties were investigated in relation to physi- and chemisorption of hydrogen. Keywords: carbon nanotubes, molecular modeling, hydrogen sorption INTRODUCTION More than 20 years after their discovery, carbon nanotubes are still attracting much interest for their potential applications, which largely derives from their exceptional structural, mechanical and electronic properties. Due to their extraordinary properties, carbon nanotubes hold great promise for future technical applications in areas like field emitters, molecular electronics, strength enhancing filler materials, hydrogen storage and absorbent materials. Carbon Nanotubes (CNTs) are rolled graphite sheets, with inner diameter minimum of 0.7 nm up to several nm and a length of micron. Tubes formed by only one single graphite layer are called single wall Nanotubes (SWNT). For tubes contains multiple concentric graphite layers are called Multiwall Nanotubes (MWNT, Fig.1). The interlayer distance in MWNTs is closer then interlayer distance in graphic which has a unit cell parameter c (0.5c = nm). The diameter of SWNTs varies from to 3 nm, where as MWNTs show typical diameters of nm [1, 2]. Fig.1 Molecular model of multiwall carbon nanotubes The interest as energy storage materials owing to their unique properties was attracted almost immediately after discovery. Their hollow morphology and capillary structure may be in favor of electrochemical hydrogen storage. Hydrogen is a very clean and efficient energy source - when it burns the only by-product is waterand it is being looked to as an environmentally friendly next generation fuel. Materials that can store and release hydrogen are therefore highly desirable, but efforts to develop carbon materials for this purpose have only managed modest hydrogen uptake. In 1997, Dillon et al.[3] discovered that SWNTs have a high reversible hydrogen storage capacity. Thereafter, many research groups started to conduct hydrogen storage researches and have made remarkable progresses. In their pioneering work, Dillon et al. showed

2 that hydrogen can condense to high capacity (estimated to 5~10 wt%) inside narrow SWNTs, and predicted that SWNTs with diameters of 16.3 to 20 Å would come close to the target capacity of 6.5 wt%. The adsorption of H2 in SWNT was investigated with temperature programmed desorption (TPD) spectroscopy and it suggested that physical adsorption of hydrogen mainly occurred within the cavities of SWNTs. Efficient storage of hydrogen at room temperature is a bottleneck problem for hydrogen-based energy applications. A simple way of hydrogen storage and release by bending carbon nanotubes (CNTs) at room temperature is demonstrated using molecular dynamics (MD) simulations. A large number of hydrogen molecules can be put in CNTs at low temperatures, and then the hydrogen molecules can be completely encapsulated in the CNTs by bending the CNTs to a critical angle. The critical angle decreases with increasing CNT length, while it increases with increasing hydrogen number and temperature. [10] Hydrogen storage materials can be divided into two categories: 1. dissociation of the hydrogen molecules and chemical binding as hydrides; 2. physisorption of hydrogen molecules on support surfaces. Clearly, the most important characteristics of the latter are a large surface area coupled with a strong binding potential. Theoretical studies have found that the interaction between hydrogen molecules and carbon nanotubes proceeds through the physisorption of hydrogen on the exterior and possibly on the interior surfaces. CNTs appear to be an optimum solution with respect to their chemical stability, low density, and large surface area. METHODS FOR CARBON NANOTUBES ACTIVATION It was proven that activation or other modification of carbon nanotubes leads to higher efficiency of hydrogen absorption as well as storage yield. Table 1 summarizes the most important methods of modification of carbon nanotubes. Table 1 The most important methods of modification of carbon nanotubes Method Chemical activation [11,12] Metal decoration [5,7, 8, 9] Means of modification KOH, acid Pd or V,Ag Pt; Advantage Effect Storage Higher surface area, defect creation introduce hydrogenfavorable active sites Alkali doped [6,14,] K or Li chemical dissociation occurs inside the nanotubes Milling [13] Mechanical activation increase of defects and surface area Heteroatoms [17,18] P, N, B higher redox potential than that of carbon and the lower standard free energy of hydrides formation Adsorption H2 at defect place H2 adsorbs on metal place H2 in inner parts and associated with moisture Shorter nanotubes Chemically and geometrically advantaged places of substitution enhancement Up to 4.7 wt.% at 298 K/100 bar Up to 3x higher at 298 K/100 bar compare original CNT 2 wt% of hydrogen 6x higher compare to original material Up to 2 wt.% of hydrogen storage capacity at 8 MPa/300 K.

3 SIMULATION OF THE HYDROGEN SORPTION Many publications have been dedicated to the theoretical study of hydrogen adsorption on CNTs. The chemisorption model calculations have been studied by density functional theory (DFT) based methods, while the prediction of hydrogen storage capacity of CNTs, based on the assumption of physical adsorption have been carried out mainly by Monte Carlo dynamic simulations. The influence of Li-doping arrangement, doping-site position and doping ratio on hydrogen physisorption in a Li-doped SWNT array at room temperature and moderate pressure. The influence of doping-site position and doping ratio on hydrogen storage is remarkable. With the best doping scheme and the reasonable control of SWNT array s structure and size, the hydrogen storage capacity of a Li-doped SWNT array can reach and exceed 9 wt.% at normal temperature and moderate pressure.[15] No essential difference was detected among armchair, zigzag and chiral nanotubes as concerns their ability to host hydrogen molecules inside them. The total amount of the hydrogen inside the nanotubes is very small and H2 molecules outside the nanotubes do not stick to them at higher temperatures. The results of the calculations seem to indicate that high hydrogen content in the nanotubes cannot be achieved through physisorption [16]. SELECTED MODELS OF SWCNT NANOROPE WITH HETEROATOMS The computational study was made using Forcite and Adsorption locator in Accelrys Materials Studio software environment [19]. In order to investigate the adsorption of atomic hydrogen on SWCNTs nanorope, we perform a series of total energy calculations using adsorbate locator module (max force Ha/A, energy Ha, max displacement = A, and max step size = 0.3 A). For substitutional doping, we replace, randomly, some carbon atoms by B, N, P atoms (1.4% of heteroatoms) followed by geometric optimization to study their effects on hydrogen adsorption using Forcite module with the following parameters: (energy = kcal/mol, force = kcal/mol/a, and displacement = A). The geometry optimization process is carried out using an iterative process, in which the atomic coordinates are adjusted until the total energy of a structure is minimized. Adsorbed molecules of hydrogen were differently located in nanoropes with different heteroatom. The lowest system energy was obtained if hydrogen molecules were located inside the SWCNT (Fig.2), while external location increased the energy. Comparing all three atom boron, nitrogen and phosphor the most elevated system was observed in case of nitrogen, where the energy was varying just by 3 kcal/mol, while in case of non-doped SWCNT nanorope or boron doping the difference between the highest and the lowest calculated energy was about 10 kcal/mol.

4 Fig. 1 Molecular models of SWCNT nanorope bundle with 1.4 % of N heretoatoms with lowest energy of system CONCLUSION Several models of SWCNT nanorope with heteroatoms were investigated for adsorption abilities of hydrogen molecules. Comparing all systems the most favorable and stable seems to be system with nitrogen since the energy doesn t fluctuate much. ACKNOWLEDGMENT We are grateful to project CZ.1.05/1.1.00/ IT4Innovations Centre of Excellence for financial support of this work. REFERENCES [1] DRESSELHAUS, M. S., DRESSELHAUS, G., EKLUND, P. C. Science of Fullerenes and Carbon Nanotubes: Their Properties and Applications. Academic Press, Elsevier. 1996, p [2] SAITO, R., DRESSELHAUS, G., DRESSELHAUS, M.S. Physical Properties of Carbon Nanotubes. Imperial College Press, London. 1998, p [3] DILLON, A.C., JONES, K.M., BEKKEDAHL, T.A., KIANG, C.H., BETHUNE, D.S., HEBEN, M.J. Storage of hydrogen in single-walled carbon nanotubes. Nature, 1997, vol. 386, p [4] LIU, C., FAN, Y.Y., LIU, M., CONG, H.T., CHENG, H.M., DRESSELHAUS, M.S. Hydrogen storage in single-walled carbon Nanotubes at room temperature. Science, 1999, vol. 286, p [5] WUA, H., WEXLERB, D., LIU, H. Effects of different palladium content loading on the hydrogen storage capacity of double-walled carbon nanotubes. International Journal of Hydrogen Energy, 2012, vol. 37, Issue 7, p [6] YANG, R.T. Hydrogen storage by alkali-doped carbon nanotubes revisited. Carbon, 2000, vol. 38, p

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