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First-principles study on hydrogen storage by graphitic carbon nitride nanotubes
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First-principles study on hydrogen storage by graphitic carbon nitride nanotubes Guangyong Koh, Yong-Wei Zhang, Hui Pan* Institute of High Performance Computing, 1 Fusionopolis Way, Fusionopolis 138632, Singapore
article info
abstract
Article history:
First-principles calculations based on density functional theory were carried out to
Received 14 July 2011
investigate the hydrogen storage capacity of graphitic carbon nitride nanotubes. Graphitic
Received in revised form
carbon nitride nanotubes could be attractive hydrogen sorbent for two reasons: firstly, its
30 September 2011
porous structure allows easy access of hydrogen into the interior of the nanotubes; and
Accepted 21 November 2011
secondly, the doubly bonded nitrogen at its pore edges provides active sites for either the
Available online 16 December 2011
adsorption of hydrogen (chemically and physically), or functionalization with metal catalysts. Our calculations show that an isolated nanotube can uptake up to 4.66 wt. %
Keywords:
hydrogen, with an average overall hydrogen adsorption energy of about 0.22 eV per H
Hydrogen storage
atom. In the form of a bulk bundle, the hydrogen storage capacity is enhanced due to the
Graphitic carbon nitride nanotubes
increased availability of space among the tubes. We predict that the hydrogen storage
First-principles calculations
capacity in the bundle is at least 5.45 wt. %. Importantly, hydrogen molecules can easily access the tube’s interior due to the low energy barrier (w0.54 eV) for their passage through the pores, indicating a fast uptake rate at relatively low pressure and temperature. Our findings show that graphitic carbon nitride nanotubes should be applicable to practical hydrogen storage because of the high gravimetric capacity and fast uptake rate. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The search for sustainable alternative energy sources has attracted increasing attention due to the limited supply of the conventional sources of depletable energy (e.g., coal, oil, nuclear) and their detrimental effects on the global climate and environment. Hydrogen is considered as one of the most promising energy sources in the future, due to its abundance, easy synthesis, and non-polluting nature. For hydrogen to be truly used as a clean fuel for mobile applications (e.g., in fuelcell powered vehicles), efficient means for its storage and transport are necessary [1]. Ideally, an efficient hydrogen storage material must have: a) a high volumetric/gravimetric capacity, b) a fast sorption rate at relatively low temperatures,
and c) a high tolerance to recycling [1e3]. Currently, the most common hydrogen storage technique, i.e., the use of pressurized tanks, provides gravimetric storage capacities of only w5e6 wt. % H2 [4]. However, the safety of this storage technique is a major concern. As alternatives to compressed hydrogen storage, three classes of materials, i.e., hydrogen adsorbents, reversible metal hydrides, and chemical hydrogen storage materials, have been identified for high-capacity storage of hydrogen [5]. In particular, nanostructured materials which have a high surface area, contain carbon, boron, lightweight metals, oxygen, and other elements, and possess pore sizes between 0.7 nm and 1.2 nm, may show great potential for breakthrough performance in hydrogen storage [5]. Applications of nanostructured materials to hydrogen
* Corresponding author. Tel.: þ65 64191425; fax: þ65 64674350. E-mail address: [email protected] (H. Pan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.109
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storage have been widely studied since the seminal work on the hydrogen storage of carbon nanotubes (CNTs) in 1997 [6]. However, the storage capacity of single-walled carbon nanotubes (SWNTs) could not be reproduced in different laboratories due to various factors, such as Ti-contamination, measurement methodology, material species, synthesis techniques, structural perfection, and surface and pore characteristics [7,8]. It was predicted that the hydrogen uptake capacity of pure CNTs could be enhanced by increasing the surface area, but the capacity should be saturated (w1 wt. %) due to monolayer adsorption saturation at room temperature [9,10]. To improve the storage capacity of hydrogen in CNTs, numerous theoretical and experimental studies have been carried out, such as engineering the morphology of CNTs [11e13], functionalizing the surface of CNTs [14,15], and doping CNTs [16e18]. Although it was theoretically predicted that the hydrogen storage capacity in metal-functionalized CNTs could reach 8 wt. % [19], the experimental storage capacities were only less than 5 wt. % [20,21]. The improved performance was attributed to the metal-catalyst-driven dissociation of H2, which facilitated both the diffusion of hydrogen into the nanotube-interior, as well as its subsequent adsorption [22e27]. Along with CNTs, other carbon nanomaterials, such as carbon nanofibers (CNFs), activated carbon, and graphene have also been studied for potential use as hydrogen sorbents [28e30]. The study of these highly porous carbon nanomaterials predicted that the optimal pore size for hydrogen storage was about 0.7 nm [31e33]. To achieve high hydrogen storage capacities for practical applications, various other nanostructured materials, such as nanostructured boron nitrides [34e36], sulfides [37,38], carbides [3,39e41], oxides [42e46], and metal-organic frameworks (MOFs) [47e50], have also been experimentally and theoretically investigated as possible hydrogen storage materials. However, high capacity for hydrogen storage has not been achieved due to various reasons. For example, hydrogen stored in oxide nanomaterials can only be partially released at room temperature due to strong chemical adsorption [42e46], and the interior space of the nanotubes cannot be easily accessed. Therefore, it is necessary to design novel nanomaterials to overcome these drawbacks. Carbon nitride has attracted increasing interest and has been used in electronic devices, thermoluminescence dosimeters, humidity sensors, coatings, and catalysts because of its interesting electronic, chemical, mechanical, and tribological properties [51e56]. Carbon nitride can exist in various phases, not only depending on the C to N ratio, but also on atomic arrangements [51]. Among these phases, a heptazine-based form of graphitic carbon nitride (g-C3N4) (Fig. 1a) is regarded as the most stable structure, and can be realized by thermo-condensation of C/N/H-containing precursors [52]. Recently, a novel nanotube formed by curling up the g-C3N4 monolayer (Fig. 1), g-C3N4 nanotube, was proposed as a photocalayst for water splitting [57,58]. The unique porous structure of the nanotube, with a pore diameter of w0.7 nm (Fig. 1), should allow greater hydrogen access to the nanotube-interior, thus increasing its specific surface area, allowing hydrogen storage within and improving the storage capacity accordingly. The doubly bonded N atoms at the pore edges provide active sites for H adsorption
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(chemically and physically) and metal functionalization. In this work, we present our first-principles study on hydrogen storage in g-C3N4 nanotubes. We show that the hydrogen molecules not only bind to the tube’s outside wall, but also easily enter the tube’s interior through the pores on the tube wall, which approximately leads to a hydrogen storage capacity of 5.45 wt. %. We further investigated the effects of functionalization on the storage capacity of the nanotubes.
2.
Computational methods
Density functional theory (DFT) [59] with the PerdeweBurkeeErnzerhof [60] generalized gradient approximation (PBE-GGA), as implemented in the Vienna ab initio simulation package (VASP) [61], was used for the investigation. The g-C3N4 nanotube systems were constructed by curling up one sheet into a cylinder along the axial direction (Fig. 1), which is then defined as a zigzag tube with an index of (n, 0) [57,58]. To study the effects of nanotube size on hydrogen adsorption, n was varied from 3 to 10. The g-C3N4 sheet- and nanotube- systems were modeled using supercell, with cell dimensions varied to ensure a wall-to-wall distance of at least ˚ when interactions between the respective systems and 10 A their images in neighbouring cells were meant to be avoided. The Monkhorst-Pack [62] scheme for k-point sampling was used for integration over the first Brillouin zone. The KohnSham energy functional was directly minimized using the conjugate gradient method [63]. A 1 1 1 grid for k-point sampling and an energy cut-off of 450 eV were used in the calculations. The total energy was converged to within ˚ for excellent convergence. As an indication of 0.04 eV/A stability for the hydrogen adsorption, the average binding energy (Eb) was calculated from: Eb ¼ ½Etot ðtube þ HÞ Etot ðtubeÞ nmH =n where Etot (tube þ H), and Etot (tube) are the total energies of the g-C3N4 with and without adsorbed hydrogen, respectively; mH is the chemical potential of the hydrogen, calculated from the hydrogen molecule (mH ¼ ½ E(H2)), and n is the number of hydrogen atoms.
3.
Results and discussion
To determine the stable adsorption site for atomic hydrogen, we calculated the total energies of the monolayer with a single hydrogen atom in the supercell as a function of the distance from the atom to the monolayer, at each of the positions shown in Fig. 1a. These calculations indicate that H atoms are most stably chemisorbed in-plane by the N atoms along the pore edges, with a binding energy of 2.1 eV and a HeN bond ˚ . It should be possible to have more than one length of w1.1 A hydrogen atom chemisorbed in one pore (Fig. 2) because of the presence of 6 doubly bonded N atoms along the edges of each pore (Fig. 1a). To find out the maximum number of hydrogen atoms that can be chemisorbed in one pore, we further calculated the binding energy as a function of the number of H atoms in one pore (Fig. 3). The calculated binding energy is
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Fig. 1 e Representative structure of a heptazine-based graphitic carbon nitride (a) monolayer; and (b) nanotube. C and N atoms are represented in grey and blue respectively. Positions over which H atoms or H2 molecules are possibly sited (i.e., out-of-page) are indicated in red and green on (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2 e Four different configurations with: (a) one-; (b) two-; (c) three-; and (d) four- H atoms chemisorbed at each pore of a graphitic carbon nitride sheet. C, N and H atoms are represented in grey, blue and white respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3 e Average HeN binding energies with 1e4 H atoms chemisorbed at each pore of a graphitic carbon nitride monolayer.
negative with up to 3 H atoms in one pore although increasing the number of chemisorbed H at each pore increases the average binding energy, while the energy is positive with 4 H atoms in one pore (Fig. 3). Therefore, the configuration with 1 H atom chemisorbed to each alternate N atom along the pore edges (Fig. 2c) is energetically stable, and will be investigated in the tube’s structures. To investigate the size effect on the hydrogen storage capabilities of g-C3N4 nanotubes, we calculated the binding energy of H-chemisorption as a function of the tube’s diameter (Fig. 4). The H-chemisorption configuration used in these calculations is shown in Fig. 4a. The calculated binding energy for H-chemisorption in nanotubes increases with the increase of the tube’s diameter, and may converge to that in the monolayer (0.31 eV) with further increasing diameter (Fig. 4b). The smallest tube, g-C3N4 (3, 0), has the lowest binding energy (0.82 eV). The binding energy remains at
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˚ about 0.66 eV as the tube’s diameter ranges from 9.5 to 18 A (i.e., g-C3N4 (4, 0) to (8, 0)). Generally, the binding energy for Hchemisorption in nanotubes is significantly lower than that in the monolayer, indicating that the nanotubes are able to store hydrogen in this way more easily. Hydrogen molecules prefer to stay over either side of the pore in the 3H-chemisorbed monolayer (Fig. 2c), with ˚ from a binding energy of 15 meV and at a distance of 2.3 A the centre of the pore. Before studying the hydrogen storage in the nanotubes, we first investigated the possibility for the molecule to enter the tube’s interior by calculating the energy barrier for this process. The diffusion path is shown in Fig. 5, that is, hydrogen molecule enters the tube’s interior from outside via the pore on the tube’s wall. The calculated energy barriers for a molecule penetrating into the tube’s interior are about 0.54 eV and 1.1 eV, for the tubes without and with chemisorbed H respectively (Fig. 5), indicating that the hydrogen molecule can easily pass through the pore and enter the tube interior. The storage and release processes of hydrogen in the graphitic carbon nitride nanotubes should be very fast, and the storage upper-limit should be achievable at a relatively lower pressure and temperature, due to the low energy barrier. Correspondingly, it is unlikely for the H2 molecule to enter the CNT-interior through the tube wall because of the very high energy barrier (12 eV). The process for CNTs should be slow, and a higher pressure should be required to reach the storage upper-limit, because hydrogen can enter the tube-interior only via the open ends or defects. To investigate the size effect on H2 adsorption, nanotubes with n from 3 to 10 were studied. The hydrogen adsorption was modeled using the optimal H-chemisorption configuration (Fig. 4a), and positioning the H2 molecules (with the HeH bond parallel to the tube axis) over both sides of each pore (i.e., one molecule inside the nanotube and one outside) (Fig. 6a), because the molecules can easily access the tube’s interior. The results show that the average binding energy for physisorption in tubes with n ¼ 3 is positive due to the strong H2eH2 interaction within the tubes, indicating that the
Fig. 4 e (a) Representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) with three H atoms chemisorbed at each pore. C, N and H atoms are represented in grey, blue and white respectively. (b) Average HeN chemisorption binding energy (Echem) as a function of nanotube diameter. The corresponding binding energy for the g-C3N4-sheet system is presented here in red for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5 e Energy profile of a single H2 molecule as it passes through the pore of a g-C3N4 (5, 0) nanotube; the tube wall ˚ , with negative distances representing the tube is at 0 A interior, and vice versa. The insets show the diffusion path via the pore of pristine nanotube. The diffusion path of H2 in H-chemisorbed nanotube is as same as that in pristine nanotube. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
reaction is endothermic (with a binding energy of about 87 meV) (Fig. 6b). As the tube size increases (n 4), the energy becomes negative, indicating that the reactions in larger tubes are exothermic and the adsorption is stable (Fig. 6b). For ˚, example, the g-C3N4 (5, 0) nanotube with a diameter of w11 A provides the most energetically stable H2 physisorption, with an average binding energy of 22 meV and a binding distance
˚ to the centre of the pore. To provide a basis for of w2.6 A comparison, we also calculated a system consisting of a CNT ˚ ) and a lone H2 molecule sited over an (diameter w7.8 A aromatic ring on the tube’s external surface. The calculated ˚ binding energy and binding distance are 9.8 meV and w3.4 A respectively. Therefore, both the relatively higher binding energy and the shorter binding distance observed with g-C3N4 suggest that g-C3N4 nanotubes are more suitable for hydrogen storage than pure CNTs. The theoretical gravimetric storage capacity of the g-C3N4 nanotube for stable adsorption shown in Fig. 6a is estimated to be 3.66 wt. % H2 (1.57 wt. % chemisorbed, 2.09 wt. % physisorbed). The average overall hydrogen adsorption energies for nanotubes of the various sizes that were studied are shown in Fig. 7. The adsorption energy remains at about 0.29 eV per H atom as the tube’s diameter ˚ (i.e., g-C3N4 (4, 0) to (8, 0)). ranges from 9.5 to 18 A The H2 loading configuration as shown in Fig. 6a results in a long H2eH2 distance between H2 molecules located outside ˚ for g-C3N4 (5, 0)), which is significantly the tube (e.g., w7.8 A larger than the calculated van der Waals H2eH2 binding ˚ , with a H2eH2 interaction energy of about distance (w2.9 A 6.3 meV), and suggests the possibility of additional H2 loading on the external surface of the nanotube. To examine this, calculations were performed with additional H2 molecules positioned over the N atoms at the vertices of each pore (i.e., the triply bonded N atoms) (Fig. 8). The calculations show that this additional H2 loading is stable with physisorption binding ˚, energy of about 0.02 eV and binding distance of about 2.4 A resulting in a hydrogen storage capacity of 4.66 wt. % (1.57 wt. % chemisorbed, 3.09 wt. % physisorbed). The average overall hydrogen adsorption energy in this case is about 0.22 eV per H atom. Our previous calculations suggest that isolated nanotubes promise better performance at hydrogen storage than CNTs. Experimentally, we can expect that these nanotubes form bulk bundles, which should affect the storage performance. To determine the theoretical hydrogen storage capacity of
Fig. 6 e (a) Representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) with three H atoms chemisorbed at each pore, and with one H2 molecule each over both the inner and outer tube surfaces, centred above each pore. C, N and H atoms are represented in grey, blue and white respectively. (b) Average H2 physisorption binding energy (Ephys) as a function of nanotube diameter. The corresponding binding energy for the g-C3N4-sheet system is presented here in red for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7 e Average overall hydrogen adsorption energy (Eoverall) as a function of nanotube diameter. The corresponding hydrogen adsorption energy for the g-C3N4-sheet system is presented here in red for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bulk g-C3N4 (5, 0) nanotubes, calculations were performed on nanotube bundles. A hexagonal bulk arrangement of the nanotubes (Fig. 9a) was used in our calculations to investigate this effect. To determine the stable hexagonal arrangement, we first calculated the energy as a function of wall-to-wall distance (see supporting data, S1-S2). The results indicate ˚ is that a wall-to-wall van der Waals binding distance of w4.6 A most stable in both the pristine nanotube (i.e., one with no adsorbed hydrogen) bundle, and the bundle of nanotubes bearing only chemisorbed H. With the nanotubes then being loaded with physisorbed H2 as in Fig. 8, the shortest inter-tube ˚ (S3), which is partially attributdistance increased to w6.2 A able to the DFT method not being able to correctly estimate
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the van der Waals interactions. With an inter-tube distance of ˚ , the average binding energy of H2 in the nanotube w6.2 A bundle is about 26 meV, which is less than that in the isolated tubes (19 meV), indicating that the adsorption of hydrogen in the bundle is more stable. The enhanced binding energy may results in the improvement of the storage capacity in bundle. Because of the arrangement of the nanotubes in a bundle, additional H2 molecules can also be fitted in the inter-tube spaces. For example, by adding 8 more H2 molecules into the space among the nanotubes (Fig. 9b), the storage capacity was increased to 5.45 wt. %, while the average H2 physisorption binding energy remained as low as 25 meV. This indicates that the storage capacity can be further improved in the bundles and may reach the practical requirement. The particular porous structure and the doubly bonded nitrogen at the edge of the pore in the graphitic carbon nitride nanotube provide a unique superiority for its application to hydrogen storage compared to other nanotubes, such as carbon and boron nitride nanotubes because of the much lower diffusion barrier and improved physical and chemical adsorption. The functionalization of nanostructures with metal atoms, especially Ti, has been considered as an important strategy for enhancing hydrogen storage capacity [19,64]. The photocatalytic ability and magnetic properties of metalfunctionalized g-C3N4 nanotubes has been reported [57,58]. The functionalization energy of Ti is lower than those of other metal elements [58]. In the following, we investigated the hydrogen storage property of Ti-functionalized g-C3N4 nanotubes. We used g-C3N4 (6, 0) as a model, with the Ti atoms at the centre of each pore. The calculated energy barrier for a molecule penetrating into the tube’s interior is about 9.7 eV for the Ti-functionalized tubes, which is much higher than that of the pristine tubes, indicating that it is more difficult for the hydrogen molecule to enter the tube-interior via the pore. Therefore, adsorption was only simulated on the outer tube surface, where one H2 molecule with the HeH bond parallel to the tube axis was positioned over each Ti atom (Fig. 10). The average HeTi binding energy is 0.23 eV and the average
Fig. 8 e (a) End-on view, and (b) Side-view, of: a representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) with three H atoms chemisorbed at each pore, with one H2 molecule each over both the inner and outer tube surfaces, centred above each pore, and with one additional H2 molecule over the outer tube surface, centred above each of the N atoms at the vertices of each pore. C, N and H atoms are represented in grey, blue and white respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9 e (a) Representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) bundle. (b) Representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) bundle displaying a hydrogen storage capacity of 5.45 wt. %. C, N and H atoms are represented in grey, blue and white respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
˚ . Notably, the length of the HeH binding distance is w1.9 A ˚ , which is consistent with the bond is extended to w0.83 A literature [19,64]. The extended bond length indicates that the Ti functionalization improves the possibility of the
dissociation of H2 and facilitates the diffusion of hydrogen atoms into the nanotube-interior, which may result in the enhancement of the storage capacity.
4.
Conclusions
The first-principles calculations predict that the hydrogen storage capacity of g-C3N4 nanotube bundles is at least 5.45 wt. % H2 in both chemisorbed and physisorbed form. Importantly, the H2 molecule can easily enter the tube’s interior due to the low energy barrier (0.54 eV), resulting in fast uptake/release processes at relatively low temperature and pressure. By functionalizing the nanotubes with Ti, the dissociation of H2 into H atoms is possible, indicating the possibility of the catalyst-facilitated diffusion of hydrogen atoms into the nanotube-interior, although at the cost of restricting access through the pores into the nanotube-interior. This suggests that by lightly doping (i.e., not saturating) g-C3N4 nanotubes with Ti, it may be possible to induce hydrogen spillover (and take advantage of the attendant storage gains) without substantially increasing the mass of the sorbent, and while still allowing hydrogen to gain access to the tube interior easily. Nevertheless, the present first-principles calculations show that g-C3N4 nanotubes are a promising high performance material for hydrogen storage. With suitable material engineering, its storage capacity can be further enhanced. It is expected that the present work provides a new routine and framework for experimentalists to explore this interesting material for hydrogen storage and other possible applications. Fig. 10 e Representative structure of Ti-functionalized graphitic carbon nitride (g-C3N4 (6, 0)), with one H2 molecule adsorbed at each pore. C, N, H and Ti atoms are represented in grey, blue, white and silver respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgement The DFT calculations were performed at the Agency for Science, Technology and Research (A*STAR, Singapore) Computational Resource Center (A*CRC).
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Appendix. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ijhydene.2011.11. 109
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First-principles study on hydrogen storage by graphitic carbon nitride nanotubes Guangyong Koh, Yong-Wei Zhang, Hui Pan* Institute of High Performance Computing, 1 Fusionopolis Way, Fusionopolis 138632, Singapore
article info
abstract
Article history:
First-principles calculations based on density functional theory were carried out to
Received 14 July 2011
investigate the hydrogen storage capacity of graphitic carbon nitride nanotubes. Graphitic
Received in revised form
carbon nitride nanotubes could be attractive hydrogen sorbent for two reasons: firstly, its
30 September 2011
porous structure allows easy access of hydrogen into the interior of the nanotubes; and
Accepted 21 November 2011
secondly, the doubly bonded nitrogen at its pore edges provides active sites for either the
Available online 16 December 2011
adsorption of hydrogen (chemically and physically), or functionalization with metal catalysts. Our calculations show that an isolated nanotube can uptake up to 4.66 wt. %
Keywords:
hydrogen, with an average overall hydrogen adsorption energy of about 0.22 eV per H
Hydrogen storage
atom. In the form of a bulk bundle, the hydrogen storage capacity is enhanced due to the
Graphitic carbon nitride nanotubes
increased availability of space among the tubes. We predict that the hydrogen storage
First-principles calculations
capacity in the bundle is at least 5.45 wt. %. Importantly, hydrogen molecules can easily access the tube’s interior due to the low energy barrier (w0.54 eV) for their passage through the pores, indicating a fast uptake rate at relatively low pressure and temperature. Our findings show that graphitic carbon nitride nanotubes should be applicable to practical hydrogen storage because of the high gravimetric capacity and fast uptake rate. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The search for sustainable alternative energy sources has attracted increasing attention due to the limited supply of the conventional sources of depletable energy (e.g., coal, oil, nuclear) and their detrimental effects on the global climate and environment. Hydrogen is considered as one of the most promising energy sources in the future, due to its abundance, easy synthesis, and non-polluting nature. For hydrogen to be truly used as a clean fuel for mobile applications (e.g., in fuelcell powered vehicles), efficient means for its storage and transport are necessary [1]. Ideally, an efficient hydrogen storage material must have: a) a high volumetric/gravimetric capacity, b) a fast sorption rate at relatively low temperatures,
and c) a high tolerance to recycling [1e3]. Currently, the most common hydrogen storage technique, i.e., the use of pressurized tanks, provides gravimetric storage capacities of only w5e6 wt. % H2 [4]. However, the safety of this storage technique is a major concern. As alternatives to compressed hydrogen storage, three classes of materials, i.e., hydrogen adsorbents, reversible metal hydrides, and chemical hydrogen storage materials, have been identified for high-capacity storage of hydrogen [5]. In particular, nanostructured materials which have a high surface area, contain carbon, boron, lightweight metals, oxygen, and other elements, and possess pore sizes between 0.7 nm and 1.2 nm, may show great potential for breakthrough performance in hydrogen storage [5]. Applications of nanostructured materials to hydrogen
* Corresponding author. Tel.: þ65 64191425; fax: þ65 64674350. E-mail address: [email protected] (H. Pan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.109
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storage have been widely studied since the seminal work on the hydrogen storage of carbon nanotubes (CNTs) in 1997 [6]. However, the storage capacity of single-walled carbon nanotubes (SWNTs) could not be reproduced in different laboratories due to various factors, such as Ti-contamination, measurement methodology, material species, synthesis techniques, structural perfection, and surface and pore characteristics [7,8]. It was predicted that the hydrogen uptake capacity of pure CNTs could be enhanced by increasing the surface area, but the capacity should be saturated (w1 wt. %) due to monolayer adsorption saturation at room temperature [9,10]. To improve the storage capacity of hydrogen in CNTs, numerous theoretical and experimental studies have been carried out, such as engineering the morphology of CNTs [11e13], functionalizing the surface of CNTs [14,15], and doping CNTs [16e18]. Although it was theoretically predicted that the hydrogen storage capacity in metal-functionalized CNTs could reach 8 wt. % [19], the experimental storage capacities were only less than 5 wt. % [20,21]. The improved performance was attributed to the metal-catalyst-driven dissociation of H2, which facilitated both the diffusion of hydrogen into the nanotube-interior, as well as its subsequent adsorption [22e27]. Along with CNTs, other carbon nanomaterials, such as carbon nanofibers (CNFs), activated carbon, and graphene have also been studied for potential use as hydrogen sorbents [28e30]. The study of these highly porous carbon nanomaterials predicted that the optimal pore size for hydrogen storage was about 0.7 nm [31e33]. To achieve high hydrogen storage capacities for practical applications, various other nanostructured materials, such as nanostructured boron nitrides [34e36], sulfides [37,38], carbides [3,39e41], oxides [42e46], and metal-organic frameworks (MOFs) [47e50], have also been experimentally and theoretically investigated as possible hydrogen storage materials. However, high capacity for hydrogen storage has not been achieved due to various reasons. For example, hydrogen stored in oxide nanomaterials can only be partially released at room temperature due to strong chemical adsorption [42e46], and the interior space of the nanotubes cannot be easily accessed. Therefore, it is necessary to design novel nanomaterials to overcome these drawbacks. Carbon nitride has attracted increasing interest and has been used in electronic devices, thermoluminescence dosimeters, humidity sensors, coatings, and catalysts because of its interesting electronic, chemical, mechanical, and tribological properties [51e56]. Carbon nitride can exist in various phases, not only depending on the C to N ratio, but also on atomic arrangements [51]. Among these phases, a heptazine-based form of graphitic carbon nitride (g-C3N4) (Fig. 1a) is regarded as the most stable structure, and can be realized by thermo-condensation of C/N/H-containing precursors [52]. Recently, a novel nanotube formed by curling up the g-C3N4 monolayer (Fig. 1), g-C3N4 nanotube, was proposed as a photocalayst for water splitting [57,58]. The unique porous structure of the nanotube, with a pore diameter of w0.7 nm (Fig. 1), should allow greater hydrogen access to the nanotube-interior, thus increasing its specific surface area, allowing hydrogen storage within and improving the storage capacity accordingly. The doubly bonded N atoms at the pore edges provide active sites for H adsorption
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(chemically and physically) and metal functionalization. In this work, we present our first-principles study on hydrogen storage in g-C3N4 nanotubes. We show that the hydrogen molecules not only bind to the tube’s outside wall, but also easily enter the tube’s interior through the pores on the tube wall, which approximately leads to a hydrogen storage capacity of 5.45 wt. %. We further investigated the effects of functionalization on the storage capacity of the nanotubes.
2.
Computational methods
Density functional theory (DFT) [59] with the PerdeweBurkeeErnzerhof [60] generalized gradient approximation (PBE-GGA), as implemented in the Vienna ab initio simulation package (VASP) [61], was used for the investigation. The g-C3N4 nanotube systems were constructed by curling up one sheet into a cylinder along the axial direction (Fig. 1), which is then defined as a zigzag tube with an index of (n, 0) [57,58]. To study the effects of nanotube size on hydrogen adsorption, n was varied from 3 to 10. The g-C3N4 sheet- and nanotube- systems were modeled using supercell, with cell dimensions varied to ensure a wall-to-wall distance of at least ˚ when interactions between the respective systems and 10 A their images in neighbouring cells were meant to be avoided. The Monkhorst-Pack [62] scheme for k-point sampling was used for integration over the first Brillouin zone. The KohnSham energy functional was directly minimized using the conjugate gradient method [63]. A 1 1 1 grid for k-point sampling and an energy cut-off of 450 eV were used in the calculations. The total energy was converged to within ˚ for excellent convergence. As an indication of 0.04 eV/A stability for the hydrogen adsorption, the average binding energy (Eb) was calculated from: Eb ¼ ½Etot ðtube þ HÞ Etot ðtubeÞ nmH =n where Etot (tube þ H), and Etot (tube) are the total energies of the g-C3N4 with and without adsorbed hydrogen, respectively; mH is the chemical potential of the hydrogen, calculated from the hydrogen molecule (mH ¼ ½ E(H2)), and n is the number of hydrogen atoms.
3.
Results and discussion
To determine the stable adsorption site for atomic hydrogen, we calculated the total energies of the monolayer with a single hydrogen atom in the supercell as a function of the distance from the atom to the monolayer, at each of the positions shown in Fig. 1a. These calculations indicate that H atoms are most stably chemisorbed in-plane by the N atoms along the pore edges, with a binding energy of 2.1 eV and a HeN bond ˚ . It should be possible to have more than one length of w1.1 A hydrogen atom chemisorbed in one pore (Fig. 2) because of the presence of 6 doubly bonded N atoms along the edges of each pore (Fig. 1a). To find out the maximum number of hydrogen atoms that can be chemisorbed in one pore, we further calculated the binding energy as a function of the number of H atoms in one pore (Fig. 3). The calculated binding energy is
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Fig. 1 e Representative structure of a heptazine-based graphitic carbon nitride (a) monolayer; and (b) nanotube. C and N atoms are represented in grey and blue respectively. Positions over which H atoms or H2 molecules are possibly sited (i.e., out-of-page) are indicated in red and green on (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2 e Four different configurations with: (a) one-; (b) two-; (c) three-; and (d) four- H atoms chemisorbed at each pore of a graphitic carbon nitride sheet. C, N and H atoms are represented in grey, blue and white respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3 e Average HeN binding energies with 1e4 H atoms chemisorbed at each pore of a graphitic carbon nitride monolayer.
negative with up to 3 H atoms in one pore although increasing the number of chemisorbed H at each pore increases the average binding energy, while the energy is positive with 4 H atoms in one pore (Fig. 3). Therefore, the configuration with 1 H atom chemisorbed to each alternate N atom along the pore edges (Fig. 2c) is energetically stable, and will be investigated in the tube’s structures. To investigate the size effect on the hydrogen storage capabilities of g-C3N4 nanotubes, we calculated the binding energy of H-chemisorption as a function of the tube’s diameter (Fig. 4). The H-chemisorption configuration used in these calculations is shown in Fig. 4a. The calculated binding energy for H-chemisorption in nanotubes increases with the increase of the tube’s diameter, and may converge to that in the monolayer (0.31 eV) with further increasing diameter (Fig. 4b). The smallest tube, g-C3N4 (3, 0), has the lowest binding energy (0.82 eV). The binding energy remains at
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˚ about 0.66 eV as the tube’s diameter ranges from 9.5 to 18 A (i.e., g-C3N4 (4, 0) to (8, 0)). Generally, the binding energy for Hchemisorption in nanotubes is significantly lower than that in the monolayer, indicating that the nanotubes are able to store hydrogen in this way more easily. Hydrogen molecules prefer to stay over either side of the pore in the 3H-chemisorbed monolayer (Fig. 2c), with ˚ from a binding energy of 15 meV and at a distance of 2.3 A the centre of the pore. Before studying the hydrogen storage in the nanotubes, we first investigated the possibility for the molecule to enter the tube’s interior by calculating the energy barrier for this process. The diffusion path is shown in Fig. 5, that is, hydrogen molecule enters the tube’s interior from outside via the pore on the tube’s wall. The calculated energy barriers for a molecule penetrating into the tube’s interior are about 0.54 eV and 1.1 eV, for the tubes without and with chemisorbed H respectively (Fig. 5), indicating that the hydrogen molecule can easily pass through the pore and enter the tube interior. The storage and release processes of hydrogen in the graphitic carbon nitride nanotubes should be very fast, and the storage upper-limit should be achievable at a relatively lower pressure and temperature, due to the low energy barrier. Correspondingly, it is unlikely for the H2 molecule to enter the CNT-interior through the tube wall because of the very high energy barrier (12 eV). The process for CNTs should be slow, and a higher pressure should be required to reach the storage upper-limit, because hydrogen can enter the tube-interior only via the open ends or defects. To investigate the size effect on H2 adsorption, nanotubes with n from 3 to 10 were studied. The hydrogen adsorption was modeled using the optimal H-chemisorption configuration (Fig. 4a), and positioning the H2 molecules (with the HeH bond parallel to the tube axis) over both sides of each pore (i.e., one molecule inside the nanotube and one outside) (Fig. 6a), because the molecules can easily access the tube’s interior. The results show that the average binding energy for physisorption in tubes with n ¼ 3 is positive due to the strong H2eH2 interaction within the tubes, indicating that the
Fig. 4 e (a) Representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) with three H atoms chemisorbed at each pore. C, N and H atoms are represented in grey, blue and white respectively. (b) Average HeN chemisorption binding energy (Echem) as a function of nanotube diameter. The corresponding binding energy for the g-C3N4-sheet system is presented here in red for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5 e Energy profile of a single H2 molecule as it passes through the pore of a g-C3N4 (5, 0) nanotube; the tube wall ˚ , with negative distances representing the tube is at 0 A interior, and vice versa. The insets show the diffusion path via the pore of pristine nanotube. The diffusion path of H2 in H-chemisorbed nanotube is as same as that in pristine nanotube. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
reaction is endothermic (with a binding energy of about 87 meV) (Fig. 6b). As the tube size increases (n 4), the energy becomes negative, indicating that the reactions in larger tubes are exothermic and the adsorption is stable (Fig. 6b). For ˚, example, the g-C3N4 (5, 0) nanotube with a diameter of w11 A provides the most energetically stable H2 physisorption, with an average binding energy of 22 meV and a binding distance
˚ to the centre of the pore. To provide a basis for of w2.6 A comparison, we also calculated a system consisting of a CNT ˚ ) and a lone H2 molecule sited over an (diameter w7.8 A aromatic ring on the tube’s external surface. The calculated ˚ binding energy and binding distance are 9.8 meV and w3.4 A respectively. Therefore, both the relatively higher binding energy and the shorter binding distance observed with g-C3N4 suggest that g-C3N4 nanotubes are more suitable for hydrogen storage than pure CNTs. The theoretical gravimetric storage capacity of the g-C3N4 nanotube for stable adsorption shown in Fig. 6a is estimated to be 3.66 wt. % H2 (1.57 wt. % chemisorbed, 2.09 wt. % physisorbed). The average overall hydrogen adsorption energies for nanotubes of the various sizes that were studied are shown in Fig. 7. The adsorption energy remains at about 0.29 eV per H atom as the tube’s diameter ˚ (i.e., g-C3N4 (4, 0) to (8, 0)). ranges from 9.5 to 18 A The H2 loading configuration as shown in Fig. 6a results in a long H2eH2 distance between H2 molecules located outside ˚ for g-C3N4 (5, 0)), which is significantly the tube (e.g., w7.8 A larger than the calculated van der Waals H2eH2 binding ˚ , with a H2eH2 interaction energy of about distance (w2.9 A 6.3 meV), and suggests the possibility of additional H2 loading on the external surface of the nanotube. To examine this, calculations were performed with additional H2 molecules positioned over the N atoms at the vertices of each pore (i.e., the triply bonded N atoms) (Fig. 8). The calculations show that this additional H2 loading is stable with physisorption binding ˚, energy of about 0.02 eV and binding distance of about 2.4 A resulting in a hydrogen storage capacity of 4.66 wt. % (1.57 wt. % chemisorbed, 3.09 wt. % physisorbed). The average overall hydrogen adsorption energy in this case is about 0.22 eV per H atom. Our previous calculations suggest that isolated nanotubes promise better performance at hydrogen storage than CNTs. Experimentally, we can expect that these nanotubes form bulk bundles, which should affect the storage performance. To determine the theoretical hydrogen storage capacity of
Fig. 6 e (a) Representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) with three H atoms chemisorbed at each pore, and with one H2 molecule each over both the inner and outer tube surfaces, centred above each pore. C, N and H atoms are represented in grey, blue and white respectively. (b) Average H2 physisorption binding energy (Ephys) as a function of nanotube diameter. The corresponding binding energy for the g-C3N4-sheet system is presented here in red for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7 e Average overall hydrogen adsorption energy (Eoverall) as a function of nanotube diameter. The corresponding hydrogen adsorption energy for the g-C3N4-sheet system is presented here in red for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bulk g-C3N4 (5, 0) nanotubes, calculations were performed on nanotube bundles. A hexagonal bulk arrangement of the nanotubes (Fig. 9a) was used in our calculations to investigate this effect. To determine the stable hexagonal arrangement, we first calculated the energy as a function of wall-to-wall distance (see supporting data, S1-S2). The results indicate ˚ is that a wall-to-wall van der Waals binding distance of w4.6 A most stable in both the pristine nanotube (i.e., one with no adsorbed hydrogen) bundle, and the bundle of nanotubes bearing only chemisorbed H. With the nanotubes then being loaded with physisorbed H2 as in Fig. 8, the shortest inter-tube ˚ (S3), which is partially attributdistance increased to w6.2 A able to the DFT method not being able to correctly estimate
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the van der Waals interactions. With an inter-tube distance of ˚ , the average binding energy of H2 in the nanotube w6.2 A bundle is about 26 meV, which is less than that in the isolated tubes (19 meV), indicating that the adsorption of hydrogen in the bundle is more stable. The enhanced binding energy may results in the improvement of the storage capacity in bundle. Because of the arrangement of the nanotubes in a bundle, additional H2 molecules can also be fitted in the inter-tube spaces. For example, by adding 8 more H2 molecules into the space among the nanotubes (Fig. 9b), the storage capacity was increased to 5.45 wt. %, while the average H2 physisorption binding energy remained as low as 25 meV. This indicates that the storage capacity can be further improved in the bundles and may reach the practical requirement. The particular porous structure and the doubly bonded nitrogen at the edge of the pore in the graphitic carbon nitride nanotube provide a unique superiority for its application to hydrogen storage compared to other nanotubes, such as carbon and boron nitride nanotubes because of the much lower diffusion barrier and improved physical and chemical adsorption. The functionalization of nanostructures with metal atoms, especially Ti, has been considered as an important strategy for enhancing hydrogen storage capacity [19,64]. The photocatalytic ability and magnetic properties of metalfunctionalized g-C3N4 nanotubes has been reported [57,58]. The functionalization energy of Ti is lower than those of other metal elements [58]. In the following, we investigated the hydrogen storage property of Ti-functionalized g-C3N4 nanotubes. We used g-C3N4 (6, 0) as a model, with the Ti atoms at the centre of each pore. The calculated energy barrier for a molecule penetrating into the tube’s interior is about 9.7 eV for the Ti-functionalized tubes, which is much higher than that of the pristine tubes, indicating that it is more difficult for the hydrogen molecule to enter the tube-interior via the pore. Therefore, adsorption was only simulated on the outer tube surface, where one H2 molecule with the HeH bond parallel to the tube axis was positioned over each Ti atom (Fig. 10). The average HeTi binding energy is 0.23 eV and the average
Fig. 8 e (a) End-on view, and (b) Side-view, of: a representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) with three H atoms chemisorbed at each pore, with one H2 molecule each over both the inner and outer tube surfaces, centred above each pore, and with one additional H2 molecule over the outer tube surface, centred above each of the N atoms at the vertices of each pore. C, N and H atoms are represented in grey, blue and white respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9 e (a) Representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) bundle. (b) Representative structure of a graphitic carbon nitride nanotube (g-C3N4 (5, 0)) bundle displaying a hydrogen storage capacity of 5.45 wt. %. C, N and H atoms are represented in grey, blue and white respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
˚ . Notably, the length of the HeH binding distance is w1.9 A ˚ , which is consistent with the bond is extended to w0.83 A literature [19,64]. The extended bond length indicates that the Ti functionalization improves the possibility of the
dissociation of H2 and facilitates the diffusion of hydrogen atoms into the nanotube-interior, which may result in the enhancement of the storage capacity.
4.
Conclusions
The first-principles calculations predict that the hydrogen storage capacity of g-C3N4 nanotube bundles is at least 5.45 wt. % H2 in both chemisorbed and physisorbed form. Importantly, the H2 molecule can easily enter the tube’s interior due to the low energy barrier (0.54 eV), resulting in fast uptake/release processes at relatively low temperature and pressure. By functionalizing the nanotubes with Ti, the dissociation of H2 into H atoms is possible, indicating the possibility of the catalyst-facilitated diffusion of hydrogen atoms into the nanotube-interior, although at the cost of restricting access through the pores into the nanotube-interior. This suggests that by lightly doping (i.e., not saturating) g-C3N4 nanotubes with Ti, it may be possible to induce hydrogen spillover (and take advantage of the attendant storage gains) without substantially increasing the mass of the sorbent, and while still allowing hydrogen to gain access to the tube interior easily. Nevertheless, the present first-principles calculations show that g-C3N4 nanotubes are a promising high performance material for hydrogen storage. With suitable material engineering, its storage capacity can be further enhanced. It is expected that the present work provides a new routine and framework for experimentalists to explore this interesting material for hydrogen storage and other possible applications. Fig. 10 e Representative structure of Ti-functionalized graphitic carbon nitride (g-C3N4 (6, 0)), with one H2 molecule adsorbed at each pore. C, N, H and Ti atoms are represented in grey, blue, white and silver respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgement The DFT calculations were performed at the Agency for Science, Technology and Research (A*STAR, Singapore) Computational Resource Center (A*CRC).
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Appendix. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ijhydene.2011.11. 109
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