ARTICLE pubs.acs.org/JPCC
Guided Carbon Nanocapsules for Hydrogen Storage Mikhail V. Suyetin* and Alexander V. Vakhrushev The Institute of Applied Mechanics of the Ural Branch of the Russian Academy of Sciences, T.Baramzinoy str., 34, Izhevsk 426067, Russia ABSTRACT: The storage of hydrogen in the condensed state in high-pressure vessels is dangerous, and it is impossible to store a large amount of hydrogen using adsorbents in normal ambient conditions. In order to overcome these problems, we designed a nanocapsule and investigated it with the help of the molecular dynamics simulation. The nanocapsule combines the advantages of a high-pressure vessel and adsorbents, namely a large hydrogen mass content and safe keeping. The nanocapsule is a system of combined nanotubes. Its outlet is closed by a positively charged endohedral complex K@C601þ. The outlet opening and closing by the K@C601þ ion are induced by the action of an electric field. The processes taking place during the hydrogen adsorption, storage, and desorption from the nanocapsule are analyzed, and the value of the electric field intensity required for transferring the K@C601þ ion into the nanocapsule is determined. The nanocapsule discussed can retain more than 6 wt % of hydrogen under normal conditions and meets the requirements of industrial use.
’ INTRODUCTION The discovery of a fullerene1 and a nanotube2 has aroused significant interest in the adsorption properties of these nanostructures. A significant number of works are devoted to the investigation of the adsorption of hydrogen,38 methane,9,10 ethane,11 and other gases. The results of numerous works show that even various enhancements such as the rearrangement of nanotubes,8 electrification,3 doping with different elements,4 etc., do not improve adsorption properties enough to meet industrial requirements. Therefore, there is the necessity for creating nanostructures that can adsorb hydrogen in adsorption conditions and store it in normal conditions. Such structures could be nanocapsules, which properties and structures have actively been discussed.1217 In the present work the processes of hydrogen adsorption, storage, and desorption by the guided nanocapsule of a complex structural form are studied using molecular dynamics simulations. This paper is the continuation of the authors’ nanocapsules study.1821 The nanocapsule consists of combined nanotubes. Inside the nanocapsule there is a positively charged endohedral complex K@C601þ. The K@C601þ ion is used as a locking particle inside the nanocapsule. The calculations presented show that such nanocapsules are very effective for the hydrogen storage and can be used for the storage of other gases. During the nanocapsule functioning, the self-organization of nanoparticles plays a major role; i.e., in the final stages of the K@C601þ ion displacement, the mutual attraction of the nanocapsule walls and the K@C601þ ion allows to fix the latter in the static position and to avoid the further use of the electric field. Another important advantage of the nanocapsule use is the increasing safety of a gas vessel filled with nanotubes on board a vehicle. Only the nanocapsules which are directly in the deformation area develop leak. This important feature significantly reduces the gas release, and during an accident it prevents the formation of an explosive mixture and, consequently, an explosion itself. r 2011 American Chemical Society
The most promising way of producing nanocapsules is the applications of the techniques of nanostructured-carbon engineering using various energy rays such as electrons, γ-rays, protons, and ions.2239 This results in the changes in the structure and morphology as well as in the electronic, mechanical, and chemical properties of carbon materials. To create a nanocapsule of a desired form and with desired functional properties, it is necessary to produce nanotubes joints and nanotubes with knees, to place charged fullerenes inside the nanotubes and to control the fullerene location via an electric field. In ref 40 it is experimentally shown that “X”, “Y”, and “T” molecular joints between SWNTs can be created by the controlled exposure of crossing nanotubes to an electron beam at elevated temperatures. Moreover, electron microscopes equipped with aberrationcorrected illumination systems allow focusing an electron beam onto 1A spot and displacing preselected atoms. Thus, engineering can be carried out at an atomic scale.41 Also, “T” molecular joints between nanotubes can be created using a CVD technique.42 The irradiation of one wall of a nanotube can be used for controlled bending of the nanotube.36 The electron or ion beam irradiation43 of crossing SWNTs results in the welding of nanotubes, and various stable joints are created. The authors of ref 36 are certain that it is possible to produce nanotube networks. In ref 38 the author is convinced that in the nanotubes nanoengineering the electron beam irradiation leading to thinning of the nanotube preselected areas or bending of nanotubes or creating joints would be useful for making multiterminal nanotube devices. In ref 44 TEM images of carbon nanotubes containing a charged fullerene are shown, and the technique for the production of such structures is provided. The charged fullerene location is controlled by the connecting electrodes supplying the bias voltage. They are integral parts of Received: December 1, 2010 Revised: February 22, 2011 Published: March 11, 2011 5485
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Figure 1. Nanocapsule for the hydrogen molecules storage consists of a holes chamber, a locking chamber, a bent nanotube, and a storage area. Thermodynamic conditions are T = 133 K and P = 30 MPa.
the ends of a nanotube to reduce the field screening by the nanotube.45 The possibility of the targeted changes in carbon nanotubes, the welding of nanotubes of different diameters by irradiation, and the use of other carbon nanostructures are determining factors in the process of smart adsorbents formation. The success in the further development of this trend depends considerably on the possibility of the manufacture of nanocapsules for gas storage with improved properties. Such nanocapsules should have a much higher safety level, and there must be the possibility to control the closing and opening of a nanocapsule regardless of the external thermodynamic conditions.
’ COMPUTATIONAL MODEL AND DETAILS The processes of the hydrogen adsorption, storage, and desorption from the nanocapsule and the movement of the K@C601þ ion under the action of an electric field were modeled by the method of molecular dynamics. The computations were made using the NAMD program46 in the force field CHARMM27.46 The computational results obtained were visualized with the VMD program.47 A hydrogen molecule was treated as a monatomic molecule with the parameters indicated in ref 48. The nanocapsule, the structure of which is presented in Figure 1, was used as an investigation object. The simulated system consists of 131 072 hydrogen molecules and 23 520 carbon atoms of the nanocapsule containing one K@C601þ endohedral complex. The atoms at the right end of the nanocapsule (Figure 1) are fixed. During the simulation, the time step is 1 fs. The nanocapsule consists of a holes chamber, a locking chamber, a bent nanotube, and a storage area. The storage area is a nanotube (40,40). The holes chamber consists of a nanotube (10,10) with two holes for the hydrogen molecule penetration. The locking chamber is a nanotube (12,12) with the K@C601þ endohedral complex inside. The nanotube (12,12) radius exceeds the sum of van der Waals radii of the K@C601þ ion and a hydrogen molecule. This is done for the K@C601þ ion to freely move from one side of the locking chamber to the other. The locking chamber and the holes chamber are jointed by a nanotube (8,8). The locking chamber and the storage area jointed by the bent nanotube (10,10). The nanotube (8,8) in the locking chamber serves to restrict the motion of the K@C601þ endohedral complex. During modeling it is imitated that the nanocapsule is located on a substrate; i.e., the nanotube foundation is fixed on the right end of the nanocapsule as is shown in Figure 1. Each hole in the nanocapsule is formed due to the removal of 24 carbon atoms. The holes obtained are sufficiently large for free penetration of hydrogen molecules into the internal space of the nanocapsule. The experiment for obtaining similar holes using electron beams is described in ref 22. It is demonstrated that the rays can be focused onto the section 1 Å in diameter. The dangling
Figure 2. Schematic side view and top view of the nanocapsules bundle. Solid and dashed lines are connecting electrodes.
bonds of carbon atoms are saturated with hydrogen atoms. The hole in the carbon nanotube can exist at the temperatures up to 400 K; after the elevation of the temperature, the diameters of the holes in the nanotubes decrease significantly due to the movement and coalescence of singular vacancies.2224 The K@C601þ ion transfers one electron to the nanocapsule. The chemical nature of C60 is not modified by Kþ; therefore, the charge of þ1|e| is uniformly distributed over the C60 shell during the simulation. The K@C601þ ion displacement is achieved by the effect of the electric field, which is applied between the connecting electrodes supplying the bias voltage. The electrodes are parts of the ends of the holes and the locking chambers. The K@C601þ ion movement under the effect of the electric field is described in detail in ref 44. The direction of the electric field defines the nanocapsule state in the functioning cycle, i.e., adsorption, storage, and desorption. A possible bundle of nanocapsules is shown in Figure 2. The ends of the holes and the locking chamber are connected with nanowire electrodes. The applied voltage between upper and lower crossing electrodes creates an electric field only in the selected nanocapsule. It gives an opportunity to control the condition of each nanocapsule in the bundle.
’ RESULTS AND DISCUSSION The molecular dynamics simulation of adsorption of hydrogen molecules into the nanocapsule is carried out at a temperature of 133 K and under a pressure of 10.0, 20.0, 30.0, 40.0, and 50.0 MPa. The position of the K@C601þ ion shown in Figure 1 allows the hydrogen molecules to freely penetrate through the holes from the environment into the nanocapsule. The results of the molecular dynamics simulation of the nanocapsule hydrogen adsorption are shown in Figure 3. It is clearly seen that 25 ns is sufficient to complete the adsorption process on the nanocapsule storage area. 5486
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Figure 3. Hydrogen adsorption into the nanocapsule. Thermodynamic conditions are as follows: the pressure is 10.0, 20.0, 30.0, 40.0, and 50.0 MPa, and the temperature is 133 K.
Figure 4. K@C601þ ion position with respect to the initial (time = 0 ps) ion position is shown as a function of time at the stage of closing. The change in the K@C601þ ion kinetic energy is shown as a function of time.
The computation of the hydrogen mass content in the nanocapsule is based on the following formula: Wt ¼
m H2 100% mnanocapsule þ mK@C60 1þ þ mH2
ð1Þ
where mH2 is the mass of adsorbed hydrogen molecules, mnanocapsule the mass of the nanocapsule, and mK@C601þ the mass of the K@C601þ ion. The nanocapsule adsorbs 6.12 wt % of hydrogen at P = 30 MPa and T = 133 K and 6.97 wt % at P = 50 MPa and T = 133 K. In order to start the hydrogen storage stage, it is necessary to close the locking chamber. In this case the K@C601þ ion moves under the action of the electric field and blocks the open end of
the locking chamber. The value of the intensity of the electric field required for moving the K@C601þ ion equals to 2.66 108 V/M. This value is extremely large, but we must take into account that the length of the locking chamber is extremely small; therefore, the voltage of the electric current, which is essential for this electric field creation, is very small as well. The results of our simulation of closing the locking chamber are shown in Figure 4. The graphs of the K@C601þ ion displacement and the change in the K@C601þ ion kinetic energy in the locking chamber under the action of the electric field at the stage of closing are shown. On the whole, the K@C601þ ion displacement in the locking chamber is almost uniform, and the velocity reaches the maximum value of 301 m/s. The dramatic decrease of the K@C601þ ion 5487
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kinetic energy by the second picosecond after strong acceleration at the moment of 0.5 ps due to the action of the electric field is explained by the K@C601þ ion deceleration; i.e., the K@C601þ ion kinetic energy is converted into the vibrational temperature of the hydrogen molecules, located in the locking chamber. Further, we can see how the acceleration of the K@C601þ ion and damped oscillations take place when the ion reaches the left end of the locking chamber. The K@C601þ ion kinetic energy is also damping reaching almost a zero value. Further small oscillations can be simply explained by the thermal vibrations of the atoms of the K@C601þ ion. In Figure 5, the nanocapsule with hydrogen molecules locked in it is shown. The K@C601þ ion is pressed by the hydrogen molecules to the outlet of the locking chamber after the electric field has been switched off. The K@C601þ ion cannot penetrate into the holes chamber because the diameter of the nanotube (8,8) is smaller than the diameter of the K@C601þ ion. The K@C601þ ion blocks the outlet, and the gas molecules cannot leave the nanocapsule. Now the external hydrogen gas pressure can be greatly reduced down to 0.1 MPa, and the temperature can be raised up to 300 K. Then, we perform the molecular dynamics simulation of the whole system during 500 ns. The simulation shows that there is no hydrogen leakage and the nanocapsule structure is stable. The hydrogen content in the nanocapsule under ambient conditions equals to 6.12 wt %, which meets the industrial use requirements. If the desorption of the hydrogen molecules from the nanocapsule is needed, it is necessary to open the outlet of the locking chamber by the K@C601þ ion under the action of the electric field. The value of the electric field intensity required for
moving the K@C601þ ion equals to 3.2625 108 V/M, and the direction of the electric field changes to opposite. The results of our simulation of the locking-chamber opening are shown in Figure 6, where the graphs of the K@C601þ ion displacement and the change in the K@C601þ ion kinetic energy in the locking chamber under the action of the electric field are presented. The kinetic energy of the K@C601þ ion grows rapidly during the first few picoseconds, reaching the maximum value of ∼1.5 eV at 0.5 ps. Then the kinetic energy sharply decreases. The kinetic energy is transformed into the vibrational temperature of the hydrogen molecules situated on the right part of the locking chamber. The K@C601þ ion displacement is carried out smoothly. The K@C601þ ion achieves equilibrium by the eighth picosecond and after that the electric field is switched off. The K@C601þ ion opens the outlet under the action of the electric field, and hydrogen molecules leave the nanocapsule freely under the excess pressure inside it. The nanocapsule at the stage of the hydrogen molecules desorption is shown in Figure 7. The K@C601þ ion is kept at the right end of the locking chamber by the van der Waals forces, and the hydrogen molecules stream from the storage area does not contact with it. The diagrams of the hydrogen density distribution in the layers in the nanocapsule storage area under different storage conditions T = 133 K, P = 30 MPa and T = 300 K, P = 0.1 MPa are shown in parts a and b of Figure 8, respectively. In (a), the hydrogen density is 25 kg/m3 in the storage-area volume, and at the wall the density value increases to 36.9 kg/m3. In (b), the hydrogen density is nearly the same throughout the volume of the storage area and is ∼30 kg/m3. In (a), the increase in the hydrogen density at the wall is explained by adsorption; however, when the temperature increases up to 300 K (b), the adsorption
Figure 5. Nanocapsule is in the beginning of the stage of the hydrogen molecules storage. The K@C601þ ion is on the left part of the locking chamber and blocks the outlet. The thermodynamic conditions are T = 133 K and P = 30 MPa.
Figure 7. Nanocapsule is at the stage of hydrogen molecules desorption. The K@C601þ ion is on the right part of the locking chamber. The hydrogen molecules escape from the nanocapsule freely. Thermodynamic conditions are as follows: T = 300 K and P = 0.1 MPa.
Figure 6. K@C601þ ion position with respect to the initial (time = 0 ps) ion position as a function of time at the stage of opening. Change in the K@C601þ ion kinetic energy as a function of time. 5488
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Figure 8. Diagrams of the hydrogen density distribution in the layers in the nanocapsule storage area under different storage conditions: (a) T = 133 K, P = 30 MPa; (b) T = 300 K, P = 0.1 MPa.
Figure 9. Hydrogen molecules desorption from the nanocapsule under thermodynamic conditions: P = 0.1 MPa and T = 300 K.
significantly decreases and the hydrogen density at the wall is comparable with the hydrogen density in the volume of the nanocapsule storage area. The hydrogen molecules desorption process is shown in Figure 9. The thermodynamic conditions of the desorption are T = 300 K and P = 0.1 MPa. The desorption process takes 50 ns; it is sufficient for complete discharge. The electric field is switched off during the process of the hydrogen molecules desorption from the nanocapsule. Thirty-one (0.022 wt %) molecules remain in the nanocapsule.
without the effect of an electric field under normal external conditions; however, considerable deviations in temperature and pressure are possible. The nanocapsules can be improved further, and a better effectiveness in the hydrogen storage can be reached. However, the synthesizing of similar nanostructures is a rather complicated task.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ CONCLUSIONS Using the molecular dynamics simulation, we demonstrated the operation of the nanocapsule. The use of such guided nanocapsules, the construction of which is described, is a new approach to the hydrogen storage. The nanocapsules functioning scheme allows reaching a significant hydrogen content —6.12 wt % under normal conditions, which is suitable for industrial use. The hydrogen is stored in the nanocapsule
’ ACKNOWLEDGMENT Calculations were performed in the Supercomputer Center of the Institute of Mathematics and Mechanics of the Ural Branch of the Russian Academy of Sciences. The work was supported by “Grant for young scientist - 2010” of the Ural Branch of the Russian Academy of Sciences. 5489
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