- Email: [email protected]
Enhancement of H2 adsorption on a boron-doped carbon system for hydrogen storage
Solid State Communications 150 (2010) 1959–1962
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Fast-track communication
Enhancement of H2 adsorption on a boron-doped carbon system for hydrogen storage Hoonkyung Lee ∗ Department of Physics and Astronomy, FPRD, and Center for Theoretical Physics, Seoul National University, Seoul 151-747, Republic of Korea
article
info
Article history: Received 19 August 2010 Received in revised form 25 August 2010 Accepted 26 August 2010 by A. Pinczuk Available online 7 September 2010 Keywords: A. Boron-doped polypyrrole D. Hydrogen storage D. Enhancement of H2 adsorption
abstract We study the adsorption of the molecular hydrogen on boron-doped polypyrrole ((–C4 BH3 )n ) using firstprinciples density functional calculations. We find that the binding energy of H2 molecules is slightly reduced to 0.39 eV/H2 from 0.51 eV/H2 as the number of adsorbed H2 molecules increases. This is in sharp contrast to the case of boron-doped fullerenes where the binding energy is drastically reduced as the number of adsorbed H2 molecules increases. We find that the enhancement of H2 adsorption is due to a local charge transfer by H2 adsorption in the B-doped polypyrrole as opposed to a delocalized charge transfer in the B-doped fullerenes. Our finding shows that B-doped carbon systems could be utilized for room temperature hydrogen storage. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogen is one of the most promising alternative energy sources which is clean and renewable [1,2]. Hydrogen storage near room temperature and ambient pressure is crucial to employing hydrogen as a fuel for use on-board vehicles. Nanostructured materials such as carbon nanotubes (CNTs) and metal-organic frameworks (MOFs) have recently attracted much attention because of the potential of swift storage and release, low desorption temperature, and high capacity (large surface area) [3–6]. However, these nanostructured materials have not reached the expected capacity because of the small binding energy of the hydrogen (∼0.07 eV) [7]. In order to achieve hydrogen storage at ambient conditions, the binding energy of hydrogen is required to be ∼0.3–0.5 eV [8,9]. In recent years, it has been proposed theoretically that individually dispersed transition metal-decorated nanostructured materials can adsorb H2 molecules with a binding energy of ∼0.3–0.6 eV/H2 and the hydrogen-storage capacity can reach the gravimetric capacity of ∼9 wt% [8–13]. However, clustering of decorating metal atoms remains a problem to be overcome for achieving hydrogen storage in these materials [14]. Recently, it has been reported that boron (B) or beryllium (Be)-doped fullerenes (C54 B6 or C54 Be6 ) can adsorb a H2 molecule with an enormously enhanced binding energy of 0.31 or 0.56 eV respectively without additional metal decoration as compared
∗
Tel.: +82 2 880 8852; fax: +82 2 876 3973. E-mail address: [email protected].
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.08.028
with 0.07 eV on bare fullerenes [15]. As the number of adsorbed H2 molecules increases up to six on B-doped fullerenes (C54 B6 ), however, the binding energies of H2 molecules are significantly reduced from ∼0.4 to ∼0.1 eV/H2 . In other words, we can only achieve, with B-doping, a ‘‘partial enhancement’’ of the binding energy and the actual capacity near room temperature and ambient pressure is unsatisfactory [9]. In contrast, Bedoped fullerenes (C54 Be6 ) adsorb up to six H2 molecules with a binding energy of ∼0.5 eV/H2 which is not reduced as the number of adsorbed H2 molecules increases, thereby achieving ‘‘full enhancement’’ of the binding energy. In this study, we search for a B-doped hydrogen-storage medium which can maintain the same high hydrogen binding energy as in Be-doped fullerenes. We find the enhancement of H2 adsorption on a B-doped polypyrrole where nitrogen atoms of the polypyrrole are replaced with boron atoms. When H2 molecules are maximally adsorbed on the boron sites, the binding energy of H2 molecules is maintained to be 0.39 eV/H2 and the gravimetric capacity is 3.1 wt%. We will show below that the origin of the enhancement of H2 adsorption comes from a local charge transfer by H2 adsorption as opposed to a delocalized charge transfer in the B-doped fullerenes. We will discuss thermodynamics of H2 adsorption on the B-doped polypyrrole for hydrogen storage. 2. Computational details All our calculations were carried out using first-principles density-functional theory with a plane-wave-based total energy minimization scheme [16]. The exchange correlation energy functional in the local density approximation (LDA) was used [17].
1960
H. Lee / Solid State Communications 150 (2010) 1959–1962
Fig. 1. (Color online) Optimized atomic structures of the boron-doped polypyrrole with maximally adsorbed H2 molecules where one H2 molecule adsorbs on each side boron atom.
Kim et al. have confirmed that the binding energy of H2 molecules with the LDA is more consistent with that with diffusion quantum Monte Carlo calculations than that with the generalized gradient approximation (GGA) [15]. The kinetic energy cutoff was taken to be 700 eV. The optimized atomic positions were obtained by relaxation until the Hellmann–Feynman force on each atom was less than 0.01 eV/Å. Supercell [18] calculations were employed throughout where the atoms on adjacent polypyrrole was separated by over 10 Å to eliminate spurious interaction. 3. Results and discussion Fig. 1 shows the boron (B)-doped polypyrrole (BDPP) with maximally adsorbed H2 molecules where nitrogen atoms of polypyrrole ((C4 NH3 )n ) are replaced with boron atoms and one H2 molecule adsorbs on each B atom. To investigate the interaction between the H2 molecules attached on the B atoms, we have calculated the total energy for the adsorption of two H2 molecules attached on the BDPP as the distance between adsorbed H2 molecules increases. The interaction between two H2 molecules is repulsive and is negligible when the two H2 molecules are separated by more than ∼15 Å (i.e., four pentagons). The supercell for the minimum length with no interaction for absorption of one H2 molecule consists of four pentagons. From now on, we use the supercell of the B-doped polypyrrole with four pentagons. The calculated binding energy of the H2 molecule is 0.51 eV when a H2 molecule binds to a boron atom in the BDPP. The bond length of the adsorbed H2 molecule is elongated considerably from 0.76 to 0.89 Å and the distance between the B atom and the H atom is 1.33 Å. These results are consistent with Kim et al.’s results for the B-doped fullerene [15]. Up to four H2 molecule per supercell can be adsorbed (i.e., one H2 molecule for each B atom) with various adsorption configurations. To denote up and down configurations of adsorbed H2 molecules with respect to the chain in Fig. 1, we denote the four sites as (x1 , x2 , x3 , x4 ) where xi = u, d, or o which stand for up, down, or vacancy, respectively. In the case of the adsorption of two H2 molecules, the lowest energy configuration is (o, u, o, u). When increased up to four H2 molecules, the lowest energy configurations for three and four H2 adsorption are (o, u, u, d) and (u, d, u, d), respectively. Table 1 shows the calculated binding energy of H2 molecules for each lowest energy configuration. The binding energy of H2 molecules as the number of H2 molecules increases is enhanced to 0.39 eV/H2 , compared with ∼0.1 eV/H2 in the B-doped fullerenes. As a brief summary of the previously reported results [15], when the H2 molecules are consecutively added on boron sites of B-doped fullerenes (C54 B6 ), the binding energies are significantly reduced because of the ‘‘delocalized’’ nature of the charge transfer (i.e., adsorption of one H2 molecule changes the charge of other
Table 1 Calculated binding energy (eV/H2 ) of H2 molecules on the B-doped polypyrrole as a function of the number of adsorbed H2 molecules. Number of H2 molecules
1
2
3
4
The Binding energy per H2
0.51
0.50
0.44
0.39
adsorption sites and weakens their binding energies). In contrast, when the H2 molecules are consecutively added on beryllium sites of Be-doped fullerenes (C54 Be6 ), the binding energies are more or less maintained because the charge transfer is local (i.e., confined to the adsorption site only) and neighboring sites are not affected significantly. Therefore, the binding energy of H2 molecules depends on the nature of the charge transfer induced by already adsorbed H2 molecules. In order to investigate the origin of the enhanced energy on the B-doped polypyrrole, we calculate the charge transfer by the H2 adsorption. Fig. 2 exhibits the charge density difference between the BDPP or the B-doped fullerene and the H2 molecule for comparison, 1ρ = ρ[M + H2 ] − ρ[M] − ρ[H2 ], where ρ[X] is the total charge density for X and M stands for the Bdoped polypyrrole or B-doped fullerene. When calculating ρ[M] and ρ[H2 ], we have used the coordinates for respective fixed (no relaxed) BDPP and the H2 molecules obtained from the relaxed positions for the ρ[M + H2 ]. The charges are locally transferred around H2 and B in the BDPP whereas the transferred charges are more spread out in the B-doped fullerenes as shown in Fig. 1. The localized transfer after adsorption of H2 molecules does not weaken the additional H2 bonding to neighboring sites (i.e., small perturbation to other B sites). So, the binding energy as the number of adsorbed H2 molecules increases converges to 0.39 eV/H2 . Now, we consider thermodynamics of H2 adsorption on the B-doped polypyrrole. In the equilibrium between adsorbed H2 molecules and H2 gas (reservoir) at given pressure (p) and temperature (T ), the adsorption number (f ) of H2 molecules as a function of p and T is as follows [8]. Nmax
∑
f =
gn nen(µ−εn )/kT
n =0 Nmax
∑
(1) gn en(µ−εn )/kT
n=0
where µ is the chemical potential of H2 gas, and εn (<0) is the energy of the adsorbed H2 per H2 molecule when the number of adsorbed H2 molecules is n in a unit cell, and gn is the degeneracy of the configuration for given the number of adsorbed H2 molecules (n), and Nmax is the maximum adsorption number. We have also calculated the zero-point vibration energy of the H2 molecule with regard to the boron atom using the frozen
H. Lee / Solid State Communications 150 (2010) 1959–1962
1961
a
b
Fig. 2. (Color online) (a) and (b) are the total charge density differences for one H2 adsorption in the boron-doped polypyrrole and C54 B6 at the isosurface value of 0.0005 e/(a.u.)3 , respectively. Red and blue colors indicate electron accumulation and depletion, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
phonon method. The calculated zero-point energy is 0.18 eV, which is as large as 35% of the binding energy. The true binding energy is the calculated energy minus 35% of its value. From now on, in evaluating f in Eq. (1), the zero-point-energy-corrected binding energy will be substituted into −εn . The adsorption number–pressure–temperature (f –p–T ) diagram at given pressure and temperature per supercell is presented in Fig. 3 where the chemical potential µ is employed with experimental values [19]. The degeneracy gn for n = 1, 2, 3, and 4 are 1, 4, 3, and 4, respectively. The adsorption conditions at the time of fueling are chosen to be 30 atm and 25 °C and the desorption conditions at the time of fuel consumption (i.e., releasing H2 from the storage tank) are chosen to be 3 atm and 100 °C based on practical situations as well as the literature [20]. These numbers, of course, may be changed somewhat in the future if general consensus is reached in the community. By taking the difference between the adsorption number f at adsorption conditions (at 30 atm and 25 °C) and the number f at desorption conditions (3 atm and 100 °C), the calculated usable H2 molecules is 2.12 per four pentagons. Therefore, the usable gravimetric capacity is 1.9 wt% out of the maximum gravimetric capacity of 3.1 wt%. The usable number of H2 molecules would reach 2.8 wt% when the temperature at the adsorption conditions is decreased to −73 °C. This result shows that the B-doped polypyrrole can be utilized for room temperature hydrogen storage. To investigate the stability of the B-doped polypyrrole, we calculate the formation energy for the BDPP and other well-known structures for comparison. The calculated formation energies for the BDPP, polypyrrole, cyclopentadiene (C5 H5 ), and benzene (C6 H6 ) are −3.23, −6.86, −1.27, and −4.52 eV, respectively, where graphite, H2 molecule, N2 molecule, and rhombohedral boron bulk for carbon, hydrogen, nitrogen, and boron as reference materials were used, respectively, and the chemical potential of N2 and H2 gases were set to zero. This result shows that the boron-doped polypyrrole is stable as compared with other structures. Therefore, the B-doped polypyrrole we propose may be synthesized experimentally. On the other hand, a further study on dissociative hydrogen chemisorptions on the B-doped polypyrrole is necessary [21].
Fig. 3. Adsorption number–pressure–temperature (f –p–T ) diagram of the H2 adsorption in the B-doped polypyrrole. The ranges of the pressure and the temperature cover typical conditions of H2 filling and delivering from the storage tank.
4. Summary and conclusion We have shown the mechanism of H2 adsorption enhancement on boron-doped polypyrrole using first-principles density functional calculations. The calculated binding energy of H2 molecules as the number of occupied H2 molecules on the borondoped polypyrrole increases remains in the energy window of ∼0.5–0.4 eV/H2 unlike the case of B-doped fullerenes that the binding energy is decreased as small as ∼0.1 eV/H2 . Our study shows the possibility to achieve reversible hydrogen storage at room temperature and ambient pressure using B-doped carbon systems. Acknowledgements This research was supported by the Center for Nanotubes and Nanostructured Composites funded by the Korean Government
1962
H. Lee / Solid State Communications 150 (2010) 1959–1962
MOST/KOSEF, and the Korean Government MOEHRD, Basic Research Fund No. KRF-2006-341-C000015. Computations are performed through the support of KISTI. References [1] L. Schlapbach, A. Züttel, Nature (London) 414 (2001) 353. [2] G.W. Crabtree, M.S. Dresselhaus, M.V. Buchanan, Phys. Today 57 (12) (2004) 39. [3] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature (London) 386 (1997) 377. [4] N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 300 (2003) 1127. [5] S.-H. Jhi, Y.-K. Kwon, K. Bradley, J.-C.P. Gabriel, Solid State Commun. 129 (2004) 769. [6] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [7] Y. Ye, C.C. Ahn, C. Witham, B. Fultz, J. Lui, A.G. Rinzler, D. Colbert, K.A. Smith, R.E. Smalley, Appl. Phys. Lett. 74 (1999) 2307.
[8] H. Lee, W.I. Choi, J. Ihm, Phys. Rev. Lett. 97 (2006) 056104. [9] Y. Zhao, Y.-H. Kim, A.C. Dillon, M.J. Heben, S.B. Zhang, Phys. Rev. Lett. 94 (2005) 155504. [10] T. Yildirim, S. Ciraci, Phys. Rev. Lett. 94 (2005) 175501. [11] E. Durgun, S. Ciraci, W. Zhou, T. Yildirim, Phys. Rev. Lett. 97 (2006) 226102. [12] H. Lee, M.C. Nguyen, J. Ihm, Solid State Commun. 146 (2008) 431. [13] M.C. Nguyen, H. Lee, J. Ihm, Solid State Commun. 147 (2008) 419. [14] Q. Sun, Q. Wang, P. Jena, Y. Kawazoe, J. Am. Chem. Soc. 127 (2005) 14582. [15] Y.-H. Kim, Y. Zhao, A. Williamson, M.J. Heben, S.B. Zhang, Phys. Rev. Lett. 96 (2006) 016102. [16] J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1976) 5048. [17] J. Ihm, A. Zunger, M.L. Cohen, J. Phys. C 12 (1979) 4409. [18] M.L. Cohen, M. Schlüter, J.R. Chelikowsky, S.G. Louie, Phys. Rev. B 12 (1975) 5575. [19] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 75th ed., CRC Press, New York, 1994. [20] http://www.eere.energy.gov/hydrogenandfuelcells/mypp/. [21] H. Lee, J. Li, G. Zhou, W. Duan, G. Kim, J. Ihm, Phys. Rev. B 77 (2008) 235101.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Fast-track communication
Enhancement of H2 adsorption on a boron-doped carbon system for hydrogen storage Hoonkyung Lee ∗ Department of Physics and Astronomy, FPRD, and Center for Theoretical Physics, Seoul National University, Seoul 151-747, Republic of Korea
article
info
Article history: Received 19 August 2010 Received in revised form 25 August 2010 Accepted 26 August 2010 by A. Pinczuk Available online 7 September 2010 Keywords: A. Boron-doped polypyrrole D. Hydrogen storage D. Enhancement of H2 adsorption
abstract We study the adsorption of the molecular hydrogen on boron-doped polypyrrole ((–C4 BH3 )n ) using firstprinciples density functional calculations. We find that the binding energy of H2 molecules is slightly reduced to 0.39 eV/H2 from 0.51 eV/H2 as the number of adsorbed H2 molecules increases. This is in sharp contrast to the case of boron-doped fullerenes where the binding energy is drastically reduced as the number of adsorbed H2 molecules increases. We find that the enhancement of H2 adsorption is due to a local charge transfer by H2 adsorption in the B-doped polypyrrole as opposed to a delocalized charge transfer in the B-doped fullerenes. Our finding shows that B-doped carbon systems could be utilized for room temperature hydrogen storage. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogen is one of the most promising alternative energy sources which is clean and renewable [1,2]. Hydrogen storage near room temperature and ambient pressure is crucial to employing hydrogen as a fuel for use on-board vehicles. Nanostructured materials such as carbon nanotubes (CNTs) and metal-organic frameworks (MOFs) have recently attracted much attention because of the potential of swift storage and release, low desorption temperature, and high capacity (large surface area) [3–6]. However, these nanostructured materials have not reached the expected capacity because of the small binding energy of the hydrogen (∼0.07 eV) [7]. In order to achieve hydrogen storage at ambient conditions, the binding energy of hydrogen is required to be ∼0.3–0.5 eV [8,9]. In recent years, it has been proposed theoretically that individually dispersed transition metal-decorated nanostructured materials can adsorb H2 molecules with a binding energy of ∼0.3–0.6 eV/H2 and the hydrogen-storage capacity can reach the gravimetric capacity of ∼9 wt% [8–13]. However, clustering of decorating metal atoms remains a problem to be overcome for achieving hydrogen storage in these materials [14]. Recently, it has been reported that boron (B) or beryllium (Be)-doped fullerenes (C54 B6 or C54 Be6 ) can adsorb a H2 molecule with an enormously enhanced binding energy of 0.31 or 0.56 eV respectively without additional metal decoration as compared
∗
Tel.: +82 2 880 8852; fax: +82 2 876 3973. E-mail address: [email protected].
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.08.028
with 0.07 eV on bare fullerenes [15]. As the number of adsorbed H2 molecules increases up to six on B-doped fullerenes (C54 B6 ), however, the binding energies of H2 molecules are significantly reduced from ∼0.4 to ∼0.1 eV/H2 . In other words, we can only achieve, with B-doping, a ‘‘partial enhancement’’ of the binding energy and the actual capacity near room temperature and ambient pressure is unsatisfactory [9]. In contrast, Bedoped fullerenes (C54 Be6 ) adsorb up to six H2 molecules with a binding energy of ∼0.5 eV/H2 which is not reduced as the number of adsorbed H2 molecules increases, thereby achieving ‘‘full enhancement’’ of the binding energy. In this study, we search for a B-doped hydrogen-storage medium which can maintain the same high hydrogen binding energy as in Be-doped fullerenes. We find the enhancement of H2 adsorption on a B-doped polypyrrole where nitrogen atoms of the polypyrrole are replaced with boron atoms. When H2 molecules are maximally adsorbed on the boron sites, the binding energy of H2 molecules is maintained to be 0.39 eV/H2 and the gravimetric capacity is 3.1 wt%. We will show below that the origin of the enhancement of H2 adsorption comes from a local charge transfer by H2 adsorption as opposed to a delocalized charge transfer in the B-doped fullerenes. We will discuss thermodynamics of H2 adsorption on the B-doped polypyrrole for hydrogen storage. 2. Computational details All our calculations were carried out using first-principles density-functional theory with a plane-wave-based total energy minimization scheme [16]. The exchange correlation energy functional in the local density approximation (LDA) was used [17].
1960
H. Lee / Solid State Communications 150 (2010) 1959–1962
Fig. 1. (Color online) Optimized atomic structures of the boron-doped polypyrrole with maximally adsorbed H2 molecules where one H2 molecule adsorbs on each side boron atom.
Kim et al. have confirmed that the binding energy of H2 molecules with the LDA is more consistent with that with diffusion quantum Monte Carlo calculations than that with the generalized gradient approximation (GGA) [15]. The kinetic energy cutoff was taken to be 700 eV. The optimized atomic positions were obtained by relaxation until the Hellmann–Feynman force on each atom was less than 0.01 eV/Å. Supercell [18] calculations were employed throughout where the atoms on adjacent polypyrrole was separated by over 10 Å to eliminate spurious interaction. 3. Results and discussion Fig. 1 shows the boron (B)-doped polypyrrole (BDPP) with maximally adsorbed H2 molecules where nitrogen atoms of polypyrrole ((C4 NH3 )n ) are replaced with boron atoms and one H2 molecule adsorbs on each B atom. To investigate the interaction between the H2 molecules attached on the B atoms, we have calculated the total energy for the adsorption of two H2 molecules attached on the BDPP as the distance between adsorbed H2 molecules increases. The interaction between two H2 molecules is repulsive and is negligible when the two H2 molecules are separated by more than ∼15 Å (i.e., four pentagons). The supercell for the minimum length with no interaction for absorption of one H2 molecule consists of four pentagons. From now on, we use the supercell of the B-doped polypyrrole with four pentagons. The calculated binding energy of the H2 molecule is 0.51 eV when a H2 molecule binds to a boron atom in the BDPP. The bond length of the adsorbed H2 molecule is elongated considerably from 0.76 to 0.89 Å and the distance between the B atom and the H atom is 1.33 Å. These results are consistent with Kim et al.’s results for the B-doped fullerene [15]. Up to four H2 molecule per supercell can be adsorbed (i.e., one H2 molecule for each B atom) with various adsorption configurations. To denote up and down configurations of adsorbed H2 molecules with respect to the chain in Fig. 1, we denote the four sites as (x1 , x2 , x3 , x4 ) where xi = u, d, or o which stand for up, down, or vacancy, respectively. In the case of the adsorption of two H2 molecules, the lowest energy configuration is (o, u, o, u). When increased up to four H2 molecules, the lowest energy configurations for three and four H2 adsorption are (o, u, u, d) and (u, d, u, d), respectively. Table 1 shows the calculated binding energy of H2 molecules for each lowest energy configuration. The binding energy of H2 molecules as the number of H2 molecules increases is enhanced to 0.39 eV/H2 , compared with ∼0.1 eV/H2 in the B-doped fullerenes. As a brief summary of the previously reported results [15], when the H2 molecules are consecutively added on boron sites of B-doped fullerenes (C54 B6 ), the binding energies are significantly reduced because of the ‘‘delocalized’’ nature of the charge transfer (i.e., adsorption of one H2 molecule changes the charge of other
Table 1 Calculated binding energy (eV/H2 ) of H2 molecules on the B-doped polypyrrole as a function of the number of adsorbed H2 molecules. Number of H2 molecules
1
2
3
4
The Binding energy per H2
0.51
0.50
0.44
0.39
adsorption sites and weakens their binding energies). In contrast, when the H2 molecules are consecutively added on beryllium sites of Be-doped fullerenes (C54 Be6 ), the binding energies are more or less maintained because the charge transfer is local (i.e., confined to the adsorption site only) and neighboring sites are not affected significantly. Therefore, the binding energy of H2 molecules depends on the nature of the charge transfer induced by already adsorbed H2 molecules. In order to investigate the origin of the enhanced energy on the B-doped polypyrrole, we calculate the charge transfer by the H2 adsorption. Fig. 2 exhibits the charge density difference between the BDPP or the B-doped fullerene and the H2 molecule for comparison, 1ρ = ρ[M + H2 ] − ρ[M] − ρ[H2 ], where ρ[X] is the total charge density for X and M stands for the Bdoped polypyrrole or B-doped fullerene. When calculating ρ[M] and ρ[H2 ], we have used the coordinates for respective fixed (no relaxed) BDPP and the H2 molecules obtained from the relaxed positions for the ρ[M + H2 ]. The charges are locally transferred around H2 and B in the BDPP whereas the transferred charges are more spread out in the B-doped fullerenes as shown in Fig. 1. The localized transfer after adsorption of H2 molecules does not weaken the additional H2 bonding to neighboring sites (i.e., small perturbation to other B sites). So, the binding energy as the number of adsorbed H2 molecules increases converges to 0.39 eV/H2 . Now, we consider thermodynamics of H2 adsorption on the B-doped polypyrrole. In the equilibrium between adsorbed H2 molecules and H2 gas (reservoir) at given pressure (p) and temperature (T ), the adsorption number (f ) of H2 molecules as a function of p and T is as follows [8]. Nmax
∑
f =
gn nen(µ−εn )/kT
n =0 Nmax
∑
(1) gn en(µ−εn )/kT
n=0
where µ is the chemical potential of H2 gas, and εn (<0) is the energy of the adsorbed H2 per H2 molecule when the number of adsorbed H2 molecules is n in a unit cell, and gn is the degeneracy of the configuration for given the number of adsorbed H2 molecules (n), and Nmax is the maximum adsorption number. We have also calculated the zero-point vibration energy of the H2 molecule with regard to the boron atom using the frozen
H. Lee / Solid State Communications 150 (2010) 1959–1962
1961
a
b
Fig. 2. (Color online) (a) and (b) are the total charge density differences for one H2 adsorption in the boron-doped polypyrrole and C54 B6 at the isosurface value of 0.0005 e/(a.u.)3 , respectively. Red and blue colors indicate electron accumulation and depletion, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
phonon method. The calculated zero-point energy is 0.18 eV, which is as large as 35% of the binding energy. The true binding energy is the calculated energy minus 35% of its value. From now on, in evaluating f in Eq. (1), the zero-point-energy-corrected binding energy will be substituted into −εn . The adsorption number–pressure–temperature (f –p–T ) diagram at given pressure and temperature per supercell is presented in Fig. 3 where the chemical potential µ is employed with experimental values [19]. The degeneracy gn for n = 1, 2, 3, and 4 are 1, 4, 3, and 4, respectively. The adsorption conditions at the time of fueling are chosen to be 30 atm and 25 °C and the desorption conditions at the time of fuel consumption (i.e., releasing H2 from the storage tank) are chosen to be 3 atm and 100 °C based on practical situations as well as the literature [20]. These numbers, of course, may be changed somewhat in the future if general consensus is reached in the community. By taking the difference between the adsorption number f at adsorption conditions (at 30 atm and 25 °C) and the number f at desorption conditions (3 atm and 100 °C), the calculated usable H2 molecules is 2.12 per four pentagons. Therefore, the usable gravimetric capacity is 1.9 wt% out of the maximum gravimetric capacity of 3.1 wt%. The usable number of H2 molecules would reach 2.8 wt% when the temperature at the adsorption conditions is decreased to −73 °C. This result shows that the B-doped polypyrrole can be utilized for room temperature hydrogen storage. To investigate the stability of the B-doped polypyrrole, we calculate the formation energy for the BDPP and other well-known structures for comparison. The calculated formation energies for the BDPP, polypyrrole, cyclopentadiene (C5 H5 ), and benzene (C6 H6 ) are −3.23, −6.86, −1.27, and −4.52 eV, respectively, where graphite, H2 molecule, N2 molecule, and rhombohedral boron bulk for carbon, hydrogen, nitrogen, and boron as reference materials were used, respectively, and the chemical potential of N2 and H2 gases were set to zero. This result shows that the boron-doped polypyrrole is stable as compared with other structures. Therefore, the B-doped polypyrrole we propose may be synthesized experimentally. On the other hand, a further study on dissociative hydrogen chemisorptions on the B-doped polypyrrole is necessary [21].
Fig. 3. Adsorption number–pressure–temperature (f –p–T ) diagram of the H2 adsorption in the B-doped polypyrrole. The ranges of the pressure and the temperature cover typical conditions of H2 filling and delivering from the storage tank.
4. Summary and conclusion We have shown the mechanism of H2 adsorption enhancement on boron-doped polypyrrole using first-principles density functional calculations. The calculated binding energy of H2 molecules as the number of occupied H2 molecules on the borondoped polypyrrole increases remains in the energy window of ∼0.5–0.4 eV/H2 unlike the case of B-doped fullerenes that the binding energy is decreased as small as ∼0.1 eV/H2 . Our study shows the possibility to achieve reversible hydrogen storage at room temperature and ambient pressure using B-doped carbon systems. Acknowledgements This research was supported by the Center for Nanotubes and Nanostructured Composites funded by the Korean Government
1962
H. Lee / Solid State Communications 150 (2010) 1959–1962
MOST/KOSEF, and the Korean Government MOEHRD, Basic Research Fund No. KRF-2006-341-C000015. Computations are performed through the support of KISTI. References [1] L. Schlapbach, A. Züttel, Nature (London) 414 (2001) 353. [2] G.W. Crabtree, M.S. Dresselhaus, M.V. Buchanan, Phys. Today 57 (12) (2004) 39. [3] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature (London) 386 (1997) 377. [4] N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 300 (2003) 1127. [5] S.-H. Jhi, Y.-K. Kwon, K. Bradley, J.-C.P. Gabriel, Solid State Commun. 129 (2004) 769. [6] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [7] Y. Ye, C.C. Ahn, C. Witham, B. Fultz, J. Lui, A.G. Rinzler, D. Colbert, K.A. Smith, R.E. Smalley, Appl. Phys. Lett. 74 (1999) 2307.
[8] H. Lee, W.I. Choi, J. Ihm, Phys. Rev. Lett. 97 (2006) 056104. [9] Y. Zhao, Y.-H. Kim, A.C. Dillon, M.J. Heben, S.B. Zhang, Phys. Rev. Lett. 94 (2005) 155504. [10] T. Yildirim, S. Ciraci, Phys. Rev. Lett. 94 (2005) 175501. [11] E. Durgun, S. Ciraci, W. Zhou, T. Yildirim, Phys. Rev. Lett. 97 (2006) 226102. [12] H. Lee, M.C. Nguyen, J. Ihm, Solid State Commun. 146 (2008) 431. [13] M.C. Nguyen, H. Lee, J. Ihm, Solid State Commun. 147 (2008) 419. [14] Q. Sun, Q. Wang, P. Jena, Y. Kawazoe, J. Am. Chem. Soc. 127 (2005) 14582. [15] Y.-H. Kim, Y. Zhao, A. Williamson, M.J. Heben, S.B. Zhang, Phys. Rev. Lett. 96 (2006) 016102. [16] J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1976) 5048. [17] J. Ihm, A. Zunger, M.L. Cohen, J. Phys. C 12 (1979) 4409. [18] M.L. Cohen, M. Schlüter, J.R. Chelikowsky, S.G. Louie, Phys. Rev. B 12 (1975) 5575. [19] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 75th ed., CRC Press, New York, 1994. [20] http://www.eere.energy.gov/hydrogenandfuelcells/mypp/. [21] H. Lee, J. Li, G. Zhou, W. Duan, G. Kim, J. Ihm, Phys. Rev. B 77 (2008) 235101.