- Email: [email protected]
Computer simulation of hydrogen physisorption in a Li-doped single walled carbon nanotube array
CARBON
567
4 8 (2 0 1 0) 5 5 7–57 0
the animals was observed when the carbomix and F953, respectively, was administered after the drug administration, which means that the carbon materials prove to be very effective for the in vivo adsorption of the drug. The higher adsorption capacity of carbomix was also observed in the in vitro adsorption study in simulated gastric and intestinal fluids done by us [7]. The conclusions reached by the in vivo and in vitro studies are in agreement which indicates that the in vitro studies can provide a good evaluation about the in vivo behaviour of the adsorbent. Although both situations are clearly different, being the in vivo drug adsorption much more complex with the possibility of the occurrence of competition for the adsorption sites, it seams that the adsorption mechanisms occurring in vitro also, by some means, are present in vivo, namely the relevance of the electrostatic interactions, pi–pi interactions and porosity to the adsorption mechanism. We did not notice any significant behavioural difference between the animals without intervention, group C3, and the animals of group C1 and C2, which means that the carbon materials tested were not toxic to the animals.
Acknowledgements This work was supported by the Fundac¸a˜o para a Cieˆncia e a Tecnologia (Portugal) with national and European funding (FEDER) (Plurianual Finance Project Centro de Quı´mica de
E´vora (619) and ICAM). We are grateful to Eli Lilly Portugal for providing pure drug compound for developing the assays.
R E F E R E N C E S
[1] Seger D. Position paper: single-dose activated charcoal. Clin Toxicol 2005;43:61–87. [2] Cooney D. Activated charcoal: antidote, remedy, and health aid. Brushton, NY: TEACH Services Inc.; 1999. [3] The world health report 2001 – Mental health: new understanding, new hope. WHO, Geneva, Switzerland; 2001. [4] European pact for mental health and well-being. European Commission, Brussels; 2008. [5] Atta-Politou J, Skopelitis I, Apatsidis I, Koupparis M. In vitro study on fluoxetine adsorption onto charcoal using potentiometry. Eur J Pharm Sci 2001;12:311–9. [6] Carrott PJM, Nabais JMV, Ribeiro Carrott MML, Pajares JA. Preparation of activated carbon fibres from acrylic textile fibres. Carbon 2001;39:1543–55. [7] Valente Nabais JM, Mouquinho A, Galacho C, Carrott PJM, Ribeiro Carrott MML. In vitro adsorption study of fluoxetine in activated carbons and activated carbon fibres. Fuel Process Technol 2008;89:549–55. [8] Stark P, Fuller RW, Wong DT. The pharmacologic profile of fluoxetine. J Clin Psychiatry 1985;46:7–13. [9] Benfield P, Ward A. Fluvoxamine. A review of its pharmacodynamic and pharmacokinetics properties and therapeutic efficacy in depressive illness. Drugs 1986;32:313–34.
Computer simulation of hydrogen physisorption in a Li-doped single walled carbon nanotube array Jinrong Cheng a b c
a,*
, Xinghong Yuan
a,b
, Xing Fang a, Libo Zhang
a,c
School of Physics and Material Science, Anhui University, Hefei 230039, China School of Science, Anhui Agricultural University, Hefei 230036, China Department of Mathematics and Physics, Hefei University, Hefei 230022, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Hydrogen physisorption in a Li-doped single walled carbon nanotube (SWCNT) array is
Received 23 July 2009
investigated by grand canonical Monte Carlo simulation. The optimization of hydrogen
Accepted 30 September 2009
storage capacity at normal temperature and moderate pressure as a function of Li doping
Available online 9 October 2009
arrangement, doping-site position, doping ratio, and SWCNT array configuration is discussed and explained. 2009 Published by Elsevier Ltd.
Since the first experimental report of Dillon et al. on hydrogen storage in carbon nanotubes (CNTs) [1], the studies
Corresponding author: Fax: +86 551 5107237. E-mail address: [email protected] (J. Cheng). 0008-6223/$ - see front matter 2009 Published by Elsevier Ltd. doi:10.1016/j.carbon.2009.09.077
on hydrogen storage in various CNTs have attracted considerable experimental and theoretical interest [2,3]. In order to en-
4 8 ( 2 0 1 0 ) 5 5 7 –5 7 0
hance hydrogen storage capacity, the alkali metal doped CNTs are adopted and much progress are made [4,5]. The US DOE has recently established a hydrogen storage target of 6 wt.% for the year 2010 and 9 wt.% for the year 2015.1 In this paper, grand canonical Monte Carlo (GCMC) method [3] with different Li doping schemes is adopted to investigate the influence of Li doping arrangement, doping-site position and doping ratio on hydrogen physisorption in a Li-doped single walled CNT (SWCNT) array at room temperature and moderate pressure. Some new conclusions are given. Three different doping schemes (see Supplementary Data, Fig. S1) are performed to find a better arrangement for Li doping. The 1st scheme is that Li atoms are uniformly located on the outer surface of SWCNTs; the 2nd scheme is that Li atoms are specially located above the hexagon centers of SWCNTs; and the 3rd scheme is that Li atoms are specially located at the hexagon centers of SWCNTs. In the 1st scheme, a Li atom is likely to be located on three different sites: above a carbon hexagon, above a C atom or above a C–C bond [see Supplementary Data, Fig. S1(a)], and all of doping-site positions are uniformly distributed on the outer surface of SWCNTs with a same height to the tube wall. Lennard–Jones (LJ) potential is adopted to model the interaction between two particles. In its simplest form, it is given by 12
/ðrij Þ ¼ 4eij ½ðrij =rij Þ
ðrij =rij Þ6
ð1Þ
where rij denotes the distance between the centers of particle i and particle j. In actual calculation, the spherical cutoff is adopted, and the cutoff radius is set to be 5rij. The calculation of interactions conforms to minimum image convention. The parameter eii of Li atom, C atom and H2 molecule is 567.0, 28.2 and 36.7 kB (Boltzmann constant), and rii is 0.2728, 0.34 and 0.2958 nm, respectively [6,7]. Furthermore, the parameters eij and rij between different particles are calculated by the following Lorentz–Berthelot rules: pffiffiffiffiffiffiffiffi eij ¼ eii ejj ; rij ¼ ðrii þ rjj Þ=2 ð2Þ Before GCMC simulation, the modified Widom test particle method and method of pressure for fluid systems with periodic boundary conditions [8] are applied to determine the relation between the reduced chemical potential and the bulk pressure (unit in MPa), which is l ¼ 74:22053 þ 8:01336 lnP
ð3Þ
During GCMC simulation, four Li-doped SWCNTs with a fixed length of 4.06 nm and the same radius are arranged equidistantly in a cuboid, which is taken as a GCMC simulation cell (see Supplementary Data, Fig. S2). Three types of operations with equal probability are performed randomly in the simulation cell: displacement, creation and deletion of a hydrogen molecule, until the number of hydrogen molecules in the simulation cell comes to the equilibrium [3]. Through 5 · 106 iterative computations, the adsorption isotherms of hydrogen physisorption in a Li-doped (15, 15) SWCNT array with and without Li doping are plotted in Fig. 1. The DBT denotes distance between tubes and H ex-
3rd scheme 2nd scheme (H=0.16nm) 1st scheme (H=0.16nm) Undoping
10 -3
CARBON
H2 average number density/nm
568
8
6
4
2
(15,15) SWCNT array with DBT of 1 nm Li:C = 1:2, Temperature = 293 K 0 0
2
4
6
8
12
14
Fig. 1 – Adsorption isotherms of hydrogen physisorption in a (15, 15) SWCNT array with and without Li doping at 293 K.
presses a height of Li atoms to the tube wall in all case. Seen from Fig. 1, hydrogen storage capacity of a SWCNT array can be enhanced availably by Li doping. It is clear that the doping effects of the 2nd and the 3rd schemes are better than that of the 1st scheme, and the 3rd scheme is the best. By varying Li:C ratio, the influence of doping ratio on hydrogen physisorption in a (15, 15) SWCNT array is investigated. As shown in Fig. 2, hydrogen storage capacity of a Lidoped SWCNT array enhances with the increase of Li:C ratio, the best ratio is 1:2 when the 3rd scheme is performed. Our former work [3] indicated that the hydrogen storage capacity of a SWCNT array enhances with the appropriate increase of tube diameter and DBT. In this work, hydrogen physisorption in a Li-doped SWCNT array is investigated by varying tube diameter and DBT synchronously, on condition that the 3rd scheme is adopted with Li:C = 1:2. As shown in Fig. 3, the hydrogen storage capacity has reached 9 wt.% for a Li-doped SWCNT array with both tube diameter and DBT about 7.08 nm at 293 K and 10 MPa or with both tube diameter and DBT about 4.80 nm at 293 K and 15 MPa. Notice that CNT array with an inner diameter more than 10 nm has been synthesized experimentally [9]; therefore, our results suggest that hydrogen storage capacity of a Li-doped SWCNT array can reach and exceed DOE’s 2015 target. Hydrogen storage capacity of a SWCNT array can be enhanced availably by alkali metal doping, because partial charge transfer occurs from alkali metal atoms to the nanotubes [10], this charge transfer can activate nanotubes’ sorption obviously. In order to give a quantificational explanation, the variation of interaction energy between hydrogen molecules and SWCNTs is calculated in the case of with and without Li doping, some results are shown in Fig. 4. Seen from Fig. 4, the depth and width of inner and outer potential wells of nanotube–H2 interactions in a Li-doped SWCNT array are both more than those in an undoped SWCNT array, it is clear that a Li-doped SWCNT array possesses a good capability for hydrogen storage. Because the RþmLi ðRþHÞ ; Li : C ¼ 1 : 2)of a Li-doped equivalent radius (R’ ¼ 2mc2m c þm Li
1
10
Pressure/MPa
http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/h2_stor_mat_work_proceedings.pdf.
CARBON
569
4 8 (2 0 1 0) 5 5 7–57 0
200
Li:C=1:2 Li:C=1:4 Li:C=1:8 Undoping
8
0 -200
Potential/K B
H2 average number density/nm
-3
10
6
4
Inside of tube
-400 -600 -800
-1000
Undoping 1st scheme H=0.16 nm 2nd scheme H=0.16 nm 3rd scheme
Tube wall
2
(15,15) SWCNT array with 3rd scheme DBT = 1 nm, Temperature = 293 K 0 0
2
4
6
8
10
12
-1200
(29,29) SWCNT array DBT=4 nm, Li:C=1:2
-1400
14
0.0
0.5
Pressure/MPa
13 12
-600 -700
Depth of potential well/kB
10 2015 target
8 7 6
2010 target
5
15 MPa 10 Mpa
4
2
3
4
5
2.0
2.5
3.0
3.5
4.0
6
7
(29,29) SWCNT array with 2nd scheme DBT = 4 nm, Li:C=1:2
-800 -900 -1000 -1100 -1200
Inside of tube Out of tube
-1300
3 1
1.5
Fig. 4 – Influence of doping arrangement on LJ potential wells of a (29, 29) SWCNT array.
(n, n) SWCNT array with 3rd scheme Li:C=1:2, Temperature=293 K
11
9
1.0
Distance from the tube center/nm
Fig. 2 – Influence of doping ratio on hydrogen physisorption in a (15, 15) SWCNT array.
Hydrogen/System wt%
Out of tube
8
Tube diameter and DBT/nm Fig. 3 – Influence of tube diameter and DBT on hydrogen physisorption in a Li-doped (n, n) SWCNT array.
SWCNT increases and DBT decreases correspondingly for the 1st and the 2nd scheme, the interaction between Li-doped SWCNTs and hydrogen molecules changes differently inside and outside the tubes. So the minimum position of the potential well is nearly unchanged for case of inside the tube, but moves outward for case of outside. On condition that Li:C = 1:2 and DBT = 4 nm, the maximal adsorption energy (kB/H2) of a (29, 29) SWCNT array with and without Li doping is calculated. The results (see Supplementary Data, Table S1) show that the maximal adsorption energy of a SWCNT array for H2 can be enhanced availably by Li doping, especially the 3rd scheme is performed. In addition, for a (8, 0) SWCNT array with above three doping schemes, the maximal adsorption energy is 17, 24 and 26 kJ/mol, respectively, which is basically consistent with the result of 10–24 kJ/mol calculated by ab initio density-functional theory [11]. The dependence of H on hydrogen physisorption in a Lidoped (29, 29) SWCNT array is also investigated. It can be seen from Fig. 5 that with H increasing the depth of potential well
-1400 0.00
0.05
0.10
0.15
0.20
Height of Li atoms to tube walls/nm Fig. 5 – Influence of doping-site position on LJ potential wells of a (29, 29) SWCNT array. is decreased observably; this trend inside the tubes is more obvious than that outside the tubes, except that H is less than 0.03 nm. The above results are mostly due to the charge transfer from Li atoms to SWCNTs. The difficulty of charge transfer increases with the augmentation of H, which leads to the decrease of potential wells’ depth in Figs. 4 and 5. So, as the interactions between hydrogen molecules and Li-doped SWCNTs are weakened, the hydrogen storage capacity of a Li-doped SWCNT array is decreased with the increase of H (Fig. 1). In addition, the screen between Li atoms and C atoms weakens the interactions between Li-doped SWCNTs and hydrogen molecules, so the hydrogen storage capacity for uniform Li doping (1st scheme) is less than that for special Li doping (2nd scheme) with the same H. In summary, hydrogen storage capacity can be enhanced efficiently by adopting Li-doped SWCNT array, the influence of doping-site position and doping ratio on hydrogen storage is remarkable. With the best doping scheme and the reasonable control of SWCNT array’s structure and size, the
570
CARBON
4 8 ( 2 0 1 0 ) 5 5 7 –5 7 0
hydrogen storage capacity of a Li-doped SWCNT array can reach and exceed DOE’s 2015 target at normal temperature and moderate pressure.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2009.09.077.
R E F E R E N C E S
[1] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotube. Nature 1997;386:377–9. [2] Liu C, Fan YY, Liu M, Cong HT, Cheng HM. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286:1127–9. [3] Cheng JR, Yuan XH, Zhao L, Huang DC, Zhao M, Dai L, et al. GCMC simulation of hydrogen physisorption on carbon nanotubes and nanotube arrays. Carbon 2004;42:2019–24.
[4] Zhao J, Buldum A, Han J, Lu JP. First-principles study of Li-intercalated carbon nanotube ropes. Phys Rev Lett 2000;85:1706–9. [5] Cabria I, Lo´pez MJ, Alonso JA. Enhancement of hydrogen physisorption on graphene and carbon nanotubes by Li doping. J Chem Phys 2005;123:204721/1–204721/9. [6] Kang JW, Hwang HJ, Song KO, Choi WY, Byun KR, Kwon OK, et al. Ordered phases of cesium in carbon nanotubes. J Korean Phys Soc 2003;43:534–9. [7] Ranganathan S, Pathak KN, Varshni YP. Potential effects on atomic motions in liquid alkali metals. Phys Rev E 1994;49:2835–40. [8] Cheng JR, Zhang LB, Ding R, Ding ZF, Wang X, Wang Z, et al. Influence of chemical potential on the computer simulation of hydrogen storage in single-walled carbon nanotube array. Comput Mater Sci 2008;44:601–4. [9] Pan ZW, Xie SS, Lu L, Chang BH, Sun LF, Zhou WY, et al. Tensile tests of ropes of very long aligned multiwall carbon nanotubes. Appl Phys Lett 1999;74:3152–4. [10] Jo C, Kim C, Lee YH. Electronic properties of K-doped singlewall carbon nanotube bundles. Phys Rev B 2002;65:035420/ 1–035420/5. [11] Wu XJ, Gao Y, Zeng XC. Hydrogen storage in pillared Li-dispersed boron carbide nanotubes. J Phys Chem C 2008;112:8458–63.
567
4 8 (2 0 1 0) 5 5 7–57 0
the animals was observed when the carbomix and F953, respectively, was administered after the drug administration, which means that the carbon materials prove to be very effective for the in vivo adsorption of the drug. The higher adsorption capacity of carbomix was also observed in the in vitro adsorption study in simulated gastric and intestinal fluids done by us [7]. The conclusions reached by the in vivo and in vitro studies are in agreement which indicates that the in vitro studies can provide a good evaluation about the in vivo behaviour of the adsorbent. Although both situations are clearly different, being the in vivo drug adsorption much more complex with the possibility of the occurrence of competition for the adsorption sites, it seams that the adsorption mechanisms occurring in vitro also, by some means, are present in vivo, namely the relevance of the electrostatic interactions, pi–pi interactions and porosity to the adsorption mechanism. We did not notice any significant behavioural difference between the animals without intervention, group C3, and the animals of group C1 and C2, which means that the carbon materials tested were not toxic to the animals.
Acknowledgements This work was supported by the Fundac¸a˜o para a Cieˆncia e a Tecnologia (Portugal) with national and European funding (FEDER) (Plurianual Finance Project Centro de Quı´mica de
E´vora (619) and ICAM). We are grateful to Eli Lilly Portugal for providing pure drug compound for developing the assays.
R E F E R E N C E S
[1] Seger D. Position paper: single-dose activated charcoal. Clin Toxicol 2005;43:61–87. [2] Cooney D. Activated charcoal: antidote, remedy, and health aid. Brushton, NY: TEACH Services Inc.; 1999. [3] The world health report 2001 – Mental health: new understanding, new hope. WHO, Geneva, Switzerland; 2001. [4] European pact for mental health and well-being. European Commission, Brussels; 2008. [5] Atta-Politou J, Skopelitis I, Apatsidis I, Koupparis M. In vitro study on fluoxetine adsorption onto charcoal using potentiometry. Eur J Pharm Sci 2001;12:311–9. [6] Carrott PJM, Nabais JMV, Ribeiro Carrott MML, Pajares JA. Preparation of activated carbon fibres from acrylic textile fibres. Carbon 2001;39:1543–55. [7] Valente Nabais JM, Mouquinho A, Galacho C, Carrott PJM, Ribeiro Carrott MML. In vitro adsorption study of fluoxetine in activated carbons and activated carbon fibres. Fuel Process Technol 2008;89:549–55. [8] Stark P, Fuller RW, Wong DT. The pharmacologic profile of fluoxetine. J Clin Psychiatry 1985;46:7–13. [9] Benfield P, Ward A. Fluvoxamine. A review of its pharmacodynamic and pharmacokinetics properties and therapeutic efficacy in depressive illness. Drugs 1986;32:313–34.
Computer simulation of hydrogen physisorption in a Li-doped single walled carbon nanotube array Jinrong Cheng a b c
a,*
, Xinghong Yuan
a,b
, Xing Fang a, Libo Zhang
a,c
School of Physics and Material Science, Anhui University, Hefei 230039, China School of Science, Anhui Agricultural University, Hefei 230036, China Department of Mathematics and Physics, Hefei University, Hefei 230022, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Hydrogen physisorption in a Li-doped single walled carbon nanotube (SWCNT) array is
Received 23 July 2009
investigated by grand canonical Monte Carlo simulation. The optimization of hydrogen
Accepted 30 September 2009
storage capacity at normal temperature and moderate pressure as a function of Li doping
Available online 9 October 2009
arrangement, doping-site position, doping ratio, and SWCNT array configuration is discussed and explained. 2009 Published by Elsevier Ltd.
Since the first experimental report of Dillon et al. on hydrogen storage in carbon nanotubes (CNTs) [1], the studies
Corresponding author: Fax: +86 551 5107237. E-mail address: [email protected] (J. Cheng). 0008-6223/$ - see front matter 2009 Published by Elsevier Ltd. doi:10.1016/j.carbon.2009.09.077
on hydrogen storage in various CNTs have attracted considerable experimental and theoretical interest [2,3]. In order to en-
4 8 ( 2 0 1 0 ) 5 5 7 –5 7 0
hance hydrogen storage capacity, the alkali metal doped CNTs are adopted and much progress are made [4,5]. The US DOE has recently established a hydrogen storage target of 6 wt.% for the year 2010 and 9 wt.% for the year 2015.1 In this paper, grand canonical Monte Carlo (GCMC) method [3] with different Li doping schemes is adopted to investigate the influence of Li doping arrangement, doping-site position and doping ratio on hydrogen physisorption in a Li-doped single walled CNT (SWCNT) array at room temperature and moderate pressure. Some new conclusions are given. Three different doping schemes (see Supplementary Data, Fig. S1) are performed to find a better arrangement for Li doping. The 1st scheme is that Li atoms are uniformly located on the outer surface of SWCNTs; the 2nd scheme is that Li atoms are specially located above the hexagon centers of SWCNTs; and the 3rd scheme is that Li atoms are specially located at the hexagon centers of SWCNTs. In the 1st scheme, a Li atom is likely to be located on three different sites: above a carbon hexagon, above a C atom or above a C–C bond [see Supplementary Data, Fig. S1(a)], and all of doping-site positions are uniformly distributed on the outer surface of SWCNTs with a same height to the tube wall. Lennard–Jones (LJ) potential is adopted to model the interaction between two particles. In its simplest form, it is given by 12
/ðrij Þ ¼ 4eij ½ðrij =rij Þ
ðrij =rij Þ6
ð1Þ
where rij denotes the distance between the centers of particle i and particle j. In actual calculation, the spherical cutoff is adopted, and the cutoff radius is set to be 5rij. The calculation of interactions conforms to minimum image convention. The parameter eii of Li atom, C atom and H2 molecule is 567.0, 28.2 and 36.7 kB (Boltzmann constant), and rii is 0.2728, 0.34 and 0.2958 nm, respectively [6,7]. Furthermore, the parameters eij and rij between different particles are calculated by the following Lorentz–Berthelot rules: pffiffiffiffiffiffiffiffi eij ¼ eii ejj ; rij ¼ ðrii þ rjj Þ=2 ð2Þ Before GCMC simulation, the modified Widom test particle method and method of pressure for fluid systems with periodic boundary conditions [8] are applied to determine the relation between the reduced chemical potential and the bulk pressure (unit in MPa), which is l ¼ 74:22053 þ 8:01336 lnP
ð3Þ
During GCMC simulation, four Li-doped SWCNTs with a fixed length of 4.06 nm and the same radius are arranged equidistantly in a cuboid, which is taken as a GCMC simulation cell (see Supplementary Data, Fig. S2). Three types of operations with equal probability are performed randomly in the simulation cell: displacement, creation and deletion of a hydrogen molecule, until the number of hydrogen molecules in the simulation cell comes to the equilibrium [3]. Through 5 · 106 iterative computations, the adsorption isotherms of hydrogen physisorption in a Li-doped (15, 15) SWCNT array with and without Li doping are plotted in Fig. 1. The DBT denotes distance between tubes and H ex-
3rd scheme 2nd scheme (H=0.16nm) 1st scheme (H=0.16nm) Undoping
10 -3
CARBON
H2 average number density/nm
568
8
6
4
2
(15,15) SWCNT array with DBT of 1 nm Li:C = 1:2, Temperature = 293 K 0 0
2
4
6
8
12
14
Fig. 1 – Adsorption isotherms of hydrogen physisorption in a (15, 15) SWCNT array with and without Li doping at 293 K.
presses a height of Li atoms to the tube wall in all case. Seen from Fig. 1, hydrogen storage capacity of a SWCNT array can be enhanced availably by Li doping. It is clear that the doping effects of the 2nd and the 3rd schemes are better than that of the 1st scheme, and the 3rd scheme is the best. By varying Li:C ratio, the influence of doping ratio on hydrogen physisorption in a (15, 15) SWCNT array is investigated. As shown in Fig. 2, hydrogen storage capacity of a Lidoped SWCNT array enhances with the increase of Li:C ratio, the best ratio is 1:2 when the 3rd scheme is performed. Our former work [3] indicated that the hydrogen storage capacity of a SWCNT array enhances with the appropriate increase of tube diameter and DBT. In this work, hydrogen physisorption in a Li-doped SWCNT array is investigated by varying tube diameter and DBT synchronously, on condition that the 3rd scheme is adopted with Li:C = 1:2. As shown in Fig. 3, the hydrogen storage capacity has reached 9 wt.% for a Li-doped SWCNT array with both tube diameter and DBT about 7.08 nm at 293 K and 10 MPa or with both tube diameter and DBT about 4.80 nm at 293 K and 15 MPa. Notice that CNT array with an inner diameter more than 10 nm has been synthesized experimentally [9]; therefore, our results suggest that hydrogen storage capacity of a Li-doped SWCNT array can reach and exceed DOE’s 2015 target. Hydrogen storage capacity of a SWCNT array can be enhanced availably by alkali metal doping, because partial charge transfer occurs from alkali metal atoms to the nanotubes [10], this charge transfer can activate nanotubes’ sorption obviously. In order to give a quantificational explanation, the variation of interaction energy between hydrogen molecules and SWCNTs is calculated in the case of with and without Li doping, some results are shown in Fig. 4. Seen from Fig. 4, the depth and width of inner and outer potential wells of nanotube–H2 interactions in a Li-doped SWCNT array are both more than those in an undoped SWCNT array, it is clear that a Li-doped SWCNT array possesses a good capability for hydrogen storage. Because the RþmLi ðRþHÞ ; Li : C ¼ 1 : 2)of a Li-doped equivalent radius (R’ ¼ 2mc2m c þm Li
1
10
Pressure/MPa
http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/h2_stor_mat_work_proceedings.pdf.
CARBON
569
4 8 (2 0 1 0) 5 5 7–57 0
200
Li:C=1:2 Li:C=1:4 Li:C=1:8 Undoping
8
0 -200
Potential/K B
H2 average number density/nm
-3
10
6
4
Inside of tube
-400 -600 -800
-1000
Undoping 1st scheme H=0.16 nm 2nd scheme H=0.16 nm 3rd scheme
Tube wall
2
(15,15) SWCNT array with 3rd scheme DBT = 1 nm, Temperature = 293 K 0 0
2
4
6
8
10
12
-1200
(29,29) SWCNT array DBT=4 nm, Li:C=1:2
-1400
14
0.0
0.5
Pressure/MPa
13 12
-600 -700
Depth of potential well/kB
10 2015 target
8 7 6
2010 target
5
15 MPa 10 Mpa
4
2
3
4
5
2.0
2.5
3.0
3.5
4.0
6
7
(29,29) SWCNT array with 2nd scheme DBT = 4 nm, Li:C=1:2
-800 -900 -1000 -1100 -1200
Inside of tube Out of tube
-1300
3 1
1.5
Fig. 4 – Influence of doping arrangement on LJ potential wells of a (29, 29) SWCNT array.
(n, n) SWCNT array with 3rd scheme Li:C=1:2, Temperature=293 K
11
9
1.0
Distance from the tube center/nm
Fig. 2 – Influence of doping ratio on hydrogen physisorption in a (15, 15) SWCNT array.
Hydrogen/System wt%
Out of tube
8
Tube diameter and DBT/nm Fig. 3 – Influence of tube diameter and DBT on hydrogen physisorption in a Li-doped (n, n) SWCNT array.
SWCNT increases and DBT decreases correspondingly for the 1st and the 2nd scheme, the interaction between Li-doped SWCNTs and hydrogen molecules changes differently inside and outside the tubes. So the minimum position of the potential well is nearly unchanged for case of inside the tube, but moves outward for case of outside. On condition that Li:C = 1:2 and DBT = 4 nm, the maximal adsorption energy (kB/H2) of a (29, 29) SWCNT array with and without Li doping is calculated. The results (see Supplementary Data, Table S1) show that the maximal adsorption energy of a SWCNT array for H2 can be enhanced availably by Li doping, especially the 3rd scheme is performed. In addition, for a (8, 0) SWCNT array with above three doping schemes, the maximal adsorption energy is 17, 24 and 26 kJ/mol, respectively, which is basically consistent with the result of 10–24 kJ/mol calculated by ab initio density-functional theory [11]. The dependence of H on hydrogen physisorption in a Lidoped (29, 29) SWCNT array is also investigated. It can be seen from Fig. 5 that with H increasing the depth of potential well
-1400 0.00
0.05
0.10
0.15
0.20
Height of Li atoms to tube walls/nm Fig. 5 – Influence of doping-site position on LJ potential wells of a (29, 29) SWCNT array. is decreased observably; this trend inside the tubes is more obvious than that outside the tubes, except that H is less than 0.03 nm. The above results are mostly due to the charge transfer from Li atoms to SWCNTs. The difficulty of charge transfer increases with the augmentation of H, which leads to the decrease of potential wells’ depth in Figs. 4 and 5. So, as the interactions between hydrogen molecules and Li-doped SWCNTs are weakened, the hydrogen storage capacity of a Li-doped SWCNT array is decreased with the increase of H (Fig. 1). In addition, the screen between Li atoms and C atoms weakens the interactions between Li-doped SWCNTs and hydrogen molecules, so the hydrogen storage capacity for uniform Li doping (1st scheme) is less than that for special Li doping (2nd scheme) with the same H. In summary, hydrogen storage capacity can be enhanced efficiently by adopting Li-doped SWCNT array, the influence of doping-site position and doping ratio on hydrogen storage is remarkable. With the best doping scheme and the reasonable control of SWCNT array’s structure and size, the
570
CARBON
4 8 ( 2 0 1 0 ) 5 5 7 –5 7 0
hydrogen storage capacity of a Li-doped SWCNT array can reach and exceed DOE’s 2015 target at normal temperature and moderate pressure.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2009.09.077.
R E F E R E N C E S
[1] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotube. Nature 1997;386:377–9. [2] Liu C, Fan YY, Liu M, Cong HT, Cheng HM. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286:1127–9. [3] Cheng JR, Yuan XH, Zhao L, Huang DC, Zhao M, Dai L, et al. GCMC simulation of hydrogen physisorption on carbon nanotubes and nanotube arrays. Carbon 2004;42:2019–24.
[4] Zhao J, Buldum A, Han J, Lu JP. First-principles study of Li-intercalated carbon nanotube ropes. Phys Rev Lett 2000;85:1706–9. [5] Cabria I, Lo´pez MJ, Alonso JA. Enhancement of hydrogen physisorption on graphene and carbon nanotubes by Li doping. J Chem Phys 2005;123:204721/1–204721/9. [6] Kang JW, Hwang HJ, Song KO, Choi WY, Byun KR, Kwon OK, et al. Ordered phases of cesium in carbon nanotubes. J Korean Phys Soc 2003;43:534–9. [7] Ranganathan S, Pathak KN, Varshni YP. Potential effects on atomic motions in liquid alkali metals. Phys Rev E 1994;49:2835–40. [8] Cheng JR, Zhang LB, Ding R, Ding ZF, Wang X, Wang Z, et al. Influence of chemical potential on the computer simulation of hydrogen storage in single-walled carbon nanotube array. Comput Mater Sci 2008;44:601–4. [9] Pan ZW, Xie SS, Lu L, Chang BH, Sun LF, Zhou WY, et al. Tensile tests of ropes of very long aligned multiwall carbon nanotubes. Appl Phys Lett 1999;74:3152–4. [10] Jo C, Kim C, Lee YH. Electronic properties of K-doped singlewall carbon nanotube bundles. Phys Rev B 2002;65:035420/ 1–035420/5. [11] Wu XJ, Gao Y, Zeng XC. Hydrogen storage in pillared Li-dispersed boron carbide nanotubes. J Phys Chem C 2008;112:8458–63.