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Hydrogen storage in porous graphene with Al decoration
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
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Hydrogen storage in porous graphene with Al decoration Zhimin Ao a,*, Shixue Dou b, Zhemi Xu c, Quanguo Jiang c, Guoxiu Wang a,* a
Centre for Clean Energy Technology, School of Chemistry and Forensic Science, University of Technology, Sydney, PO Box 123, Broadway, Sydney, NSW 2007, Australia b Institute for Superconducting & Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong, Innovation Campus Squires Way, North Wollongong, NSW 2500, Australia c School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
article info
abstract
Article history:
Porous graphene and Al modified graphene have been reported to be promising hydrogen
Received 27 September 2013
storage materials owing to their possible high hydrogen storage capacity. In this work, Al-
Received in revised form
decorated porous graphene is considered as hydrogen storage material, based on density
18 December 2013
functional calculations. It is found that the hydrogen storage capacity of the Al-decorated
Accepted 9 January 2014
porous graphene is 10.5 wt%, with modest hydrogen adsorption energy from 1.11 to
Available online xxx
0.41 eV/H2 to achieve efficient hydrogen storage/release at ambient conditions. In addition, hydrogen can be released gradually in three stages owing to different adsorption
Keywords:
energy at different adsorption sites, which is desirable in actual hydrogen storage appli-
Graphene
cation. The mechanism for improving the hydrogen storage behavior by the decorated Al
Hydrogen storage materials
atom and its porosity is understood through analyzing the atomic charges, electronic
Density functional theory
distribution, and density of states of the system. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As the global population and the demand for energy increase, fossil-fuel reserves will not be sufficient for the growth in energy consumption. Thus, hydrogen fuel systems are considered to be a highly important topic of research for future energy schemes because hydrogen is abundant, more efficient and environmentally friendly when compared with traditional fossil fuels [1]. There are many different ways to produce hydrogen [2,3]. For example, traditional energy sourcesefossil fuels like natural gas and coal, renewable energy sources such as solar radiation, wind and biomass, and nuclear energy can all be used to produce hydrogen. However,
efficient and safe storage of hydrogen is the primary obstacle to the development of the “hydrogen economy” [1]. The United States Department of Energy (DOE) has set specific performance targets that require the hydrogen storage capacity of materials to be greater than 6 wt% with the smallest possible volume, and the storage system to reversibly charge and release hydrogen near room temperature at the highest possible speed and with the lowest possible cost [4]. As a gas with very small specific volume, hydrogen storage often requires either the use of high-pressure tanks or physical and chemical bonding for solid-state storage [5]. However, compressed hydrogen is a potential hazard because of its high flammability. Liquid hydrogen is a commercial product and is used for rocket fuel, along with oxygen. Cryogenic liquid
* Corresponding authors. Tel.: þ61 2 95141741. E-mail addresses: [email protected] (Z. Ao), [email protected] (G. Wang). 0360-3199/$ e see front matter Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2014.01.044
Please cite this article in press as: Ao Z, et al., Hydrogen storage in porous graphene with Al decoration, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.01.044
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hydrogen (cooled to 20 K) is stored in an insulated cylinder at ambient pressure, which would not be financially viable for domestic users. In addition, the storage of hydrogen in liquid form has two major disadvantages: (i) a 30% energy loss occurs because of refrigeration; and (ii) a 1% boil-off rate [6]. Therefore, it is desirable to store hydrogen in a solid-state material [3]. Although various solid-state hydrogen storage materials have been proposed, none of the materials developed so far have fulfilled the DOE’s requirements. Based on the nature of hydrogen storage in host materials, solid-state hydrogen storage materials can be divided into two categories: (i) atomic hydrogen storage materials, in which hydrogen molecules are first dissociated by the host materials and then stored in the atomic form, that is chemisorption; and (ii) molecular hydrogen storage materials, in which hydrogen molecules bind to the host materials via van der Waals forces, and are thus primarily stored in the molecular form, being physisorption. Metal alloys, such as LaNi5, TiFe, and MgNi, were initially proposed as atomic hydrogen storage media because they form metal hydrides upon hydrogenation [7]. Metal hydrides can be categorized into high- or lowtemperature materials, depending on the temperature at which hydrogen absorption or desorption occurs. La-based and Ti-based alloys are examples of some low-temperature storage materials, but their main drawback is their low hydrogen storage gravimetric capacity (<2 wt%) [8]. Conversely, high-temperature materials, such as Mg-based alloys, can reach a theoretical maximum capacity of 7.6 wt %, although they suffer from poor hydrogenation/dehydrogenation kinetics and thermodynamics [3]. Therefore, metal alloys may not be appropriate for commercial applications in hydrogen-fuel vehicles. Alternatively, carbon nanostructures with dopant metal atoms systems are considered to be promising molecular hydrogen storage materials [9], and they can be designed into three-dimensional structures to further enhance their volumetric hydrogen storage capacity [10]. Graphene, a single layer of carbon atoms, was first fabricated in 2004, and excellent properties were discovered with high intrinsic carrier mobility (200,000 cm2 V1 s1) [11], excellent thermal conductivity (w5000 W m1 K1) [12,13], high optical transmittance (w97.7%) [14], high theoretical specific surface area (2630 m2 g1) [15], and superior mechanical strength [16]. Owing to these unique properties, graphene has wide applications in many fields, such as ultrasensitive gas sensors [17,18], transparent electrodes in liquid display devices [19], and large capacity electrodes in batteries [20,21]. Graphene has high surface areas, thermal stability with unique mechanical properties, and can be an economical and scalable production [22]. The improvement of hydrogen adsorption capacity by suitable modification, such as through strengthening the interaction between H2 molecules and graphene, would be of immense interest [23e26]. For example, Al-doped and Al decorated graphene have been reported to act as a bridge to enhance the interaction between H2 molecules and the graphene layer with high storage capacity of 13.79 wt % [23,24]. Decorating Ca atoms on graphene surface was also reported to improve the hydrogen storage capacity to 8.4 wt% [26], and the adsorption of hydrogen molecules on Li atoms decorated porous graphene is significantly enhanced, with up to a 12 wt% hydrogen storage capacity with adsorption energy
between 0.21 and 0.27 eV/H2, depending on the number of adsorbed H2 molecules [25]. Although the storage capacity of the reported systems is good enough, reaches the target of DOE, the adsorption energy of hydrogen in these materials is not strong enough to achieve efficiently reversible hydrogen adsorption and release. The hydrogen adsorption energy in most of the reported materials is near the lower limitation of the ideal adsorption energy range of 0.2 eV/H2. Owing to the advantages of Al atoms and the porosity in graphene for improving hydrogen storage capacity, the system of porous graphene with Al atoms decorating is considered in this work through density functional theory (DFT) calculations to investigate its hydrogen storage performance to achieve an excellent hydrogen storage material with high hydrogen storage capacity and modest hydrogen adsorption energy. To better understand the effects of the decorated Al atom and the presence of porosity in graphene on hydrogen storage, the electronic distribution and density of states (DOS) will be analyzed.
2.
Calculation details
All DFT calculations in this work are performed using the Dmol3 code [27]. Local density approximation (LDA) with PWC functional is employed to describe the exchange and correlation effect. The DFT þ D method within the Grimme scheme is used in all the calculations to consider the van der Waals forces [28]. Double numerical plus polarization (DNP) is taken as the basis set, which has been shown to be highly accurate [27]. In our simulation, three-dimensional periodic boundary conditions are applied, and the HeH bond length is set to be 0.74 A, identical to the experimental value [29]. The computational unit cell consists of a 2 2 1 graphene supercell with a vacuum width of 18 A to minimize the interlayer interaction. All atoms are allowed to relax in all calculations. Identical conditions are employed for the isolated H2 molecules, the Al atom and the graphene, and also for the adsorbed graphene system. The k-point is set to 20 20 1 for all slabs, the convergence tolerance of energy is 1.0 106 hartree (1 hartree ¼ 27.21 eV), and that of maximum force is 1.0 104 hartree/ A. The binding energy of Al atoms onto graphene Eb-Al is defined as, EbAl ¼ EnAlgraphene Egraphene þ nEAl n
(1a)
where EnAl-graphene, Egraphene and EAl are the energy of the system with n Al atoms adsorbed on the graphene layer, the energy of the porous graphene layer, and the energy of one Al atom in the same slab respectively. The adsorption energy of each H2 molecule onto Al-decorated graphene layer EadH2 is defined as, EadH2 ¼ EiH2 þAlgraphene Eði1ÞAlgraphene þ EH2
(1b)
where the subscripts iH2þAl-graphene, (i-1)Al-graphene, and H2 denote the Al-decorated porous graphene system with i H2 molecules adsorbed and (i-1) H2 molecules adsorbed (when i ¼ 1, it is isolated Al-decorated porous graphene), and a H2 molecule respectively.
Please cite this article in press as: Ao Z, et al., Hydrogen storage in porous graphene with Al decoration, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.01.044
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Fig. 1 e The relaxed structure of porous graphene (a) and Al decorated porous graphene (b). The grey, pink, and white spheres in this and following figures are C, Al and H atoms respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.
Results and discussion
Recently, Bieri et al. succeeded in synthesizing a well-defined porous graphene for the first time [30]. With the insertion of holes of specific size and distribution into graphene sheets, the electronic structure is greatly modified. Fig. 1(a) shows the geometry of porous graphene, where hydrogen passivation is taken at the edge of the porosity. Based on DFT calculations, it is reported that the presence of porosity in graphene can open the band gap of graphene from 0 to 3.2 eV. More interestingly, the porous graphene can separate the decorated Li atoms to avoid the aggression of Li atoms, thus increasing the hydrogen storage capacity [25]. In addition, transition metal decorated defect graphene with vacancies has also been reported to enhance the adsorption energy of hydrogen EadH2 within the desirable energy window to achieve efficiently reversible hydrogen storage and release, that is, 0.2 w 0.7 eV/H2, between the range of physisorption and chemisorption [31]. Owing to the unique properties of Al element to increase hydrogen storage capacity [23,24], Al atoms are considered to decorate on porous graphene for hydrogen storage application to further strengthen the interaction between H2 molecules and porous graphene here. First, the favorite position of the
decorated Al atom should be determined. We consider different positions of the Al atom on the porous graphene, such as the hollow site of a perfect C ring, the hollow site of a half C ring near the porosity, the top position of a C atom, and the bridge position of a CeC bond. It is found that the favorite configuration is shown in Fig. 1(b), namely, the Al atom prefers to adsorb on the hollow position of a half C ring near the
Table 1 e The comparison between two systems of Aldecorated graphene with and without porosity. Eads_Al is the adsorption of Al atom on graphene, Q is the charge transfer between Al and graphene obtained by Mulliken analysis, dAl-g is the distance between Al and graphene, EadsLH2 is the hydrogen adsorption energy, and dH2 graphene is the distance between H2 molecules and Aldecorated graphene.
Eads_Al Q dAl-g EadsH2 dH2 graphene
Al-decorated perfect graphene
Al-decorated porous graphene
0.824 eV 0.266 e 2.076 A 0.182 eV/H2 2.83 A
1.78 eV 0.367 e 2.059 A 1.11 eV/H2 1.83 A
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porosity. To better understand the effect of porosity in graphene on the adsorption of Al atoms and the subsequent H2 molecule adsorption, a comparison is made with the case of Al-decorated perfect graphene system, and the results are shown in Table 1. From the table, the adsorption energy of the Al atom on the porous graphene is 1.78 eV in terms of Eq. (1a), while that of an Al atom on perfect graphene is reported to be 0.824 eV [23]. Therefore, the presence of porosity in graphene would strengthen the interaction between Al and graphene significantly, which also induces a slight deformation of the porous graphene, as shown in Fig. 1(b). It is reported that the charge of a metal atom would polarize H2 molecules, thus enhancing the interaction between H2 and graphene due to the Coulomb interaction [23]. Through Mulliken analysis, as
shown in Table 1, the atomic charge of the Al atom in the porous graphene is 0.367 e, while that in perfect graphene was reported to be 0.266 e [23]. In addition, as shown in Table 1, a shorter distance between the Al atom and the porous graA is found, while that in perfect graphene phene dAl-g ¼ 2.059 is 2.076 A [23]. Therefore, more electronics transferred from the Al atom to the C atoms and shorter dAl-g in the porous graphene indicate stronger interaction between the Al atom and graphene, which also confirms the result from Eq. (1a). Therefore, stronger interaction between H2 molecules and the Al-decorated porous graphene is expected, because the more positively charged Al atom and the more negatively charged porous graphene can induce stronger polarization of adsorbed H2 molecules.
Fig. 2 e The atomic structures of the Al-decorated porous graphene with one H2 molecule (a), two H2 molecules (b), three H2 molecules (c), four H2 molecules (d), five H2 molecules (e), six H2 molecules (f), and twelve molecules (g) adsorbed. In panel (g), both sides of graphene are decorated with an Al atom, and six H2 molecules are adsorbed at each side. Panel (h) shows a 4 3 4 supercell of the system with six H2 molecules adsorbed on one side of the graphene for better demonstrating the position of H2 molecules. The two triangles are plotted here to guide the eyes. H1 and H2 indicate the sites for the seventh and eighth H2 molecules. Please cite this article in press as: Ao Z, et al., Hydrogen storage in porous graphene with Al decoration, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.01.044
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Fig. 3 e The electrons distribution [panel (a)] and partial density of states (PDOS) [panel (b)] of the systems of one H2 molecule adsorbed on Al-decorated graphene with and without porosity. In panel (b), the Fermi level is indicated by the dashed line and is located at 0.
When one H2 molecule is adsorbed on the Al-decorated porous graphene, the H2 molecule prefers to adsorb at the hollow position of porosity, as shown in Fig. 2(a). In terms of Eq. (1b), the corresponding adsorption energy Ead_H2 is 1.11 eV/H2, which is stronger than the desirable range of 0.2 to e0.7 eV/H2 [31]. In the system of Al decorated graphene without porosity, Ead_H2 was reported to be 0.182 eV/H2 for the first H2 molecule adsorption [23]. Therefore, the presence of porosity in graphene would strengthen the interaction of H2 and Al-decorated graphene significantly. From Fig. 2(a), the H2 molecule locates at the center of the porosity and almost in the same plane of the graphene layer, which is different from the case of Al-decorated graphene without porosity, where the first H2 molecule is adsorbed at the top site of a C atom and also the center of a triangle of Al atoms, as shown in Fig. 4(a) in Ref. [23]. The H2 molecule is out of the graphene plane with a A. In this case, dH2 graphene ¼ 1.83 A distance dH2 graphene ¼ 2.83 is much shorter. Therefore, stronger hydrogen adsorption energy is expected owing to the shorter dH2 graphene and more positively charged Al atom. To further understand the effect of porosity in graphene on the adsorption of H2 molecules, electronic distribution and partial density of states (PDOS) of the systems of Al-decorated porous graphene and Al-decorated graphene with first H2 molecule adsorbed are analyzed, and the results are shown in Fig. 3. From the electronic distribution in Fig. 3(a), electronic clouds of the Al atom and the porous graphene overlap, indicating electron transfer and relatively strong interaction between them. This also confirms the results that 0.276 e is transferred from the Al atom to the porous graphene from Mulliken analysis and strong adsorption energy 1.78 eV in terms of Eq. (1a). Therefore, the adsorption of the Al atom on the porous graphene is between physical and chemical adsorption, but more like chemical adsorption. For the electronic cloud of the H2 molecule, there is no overlap with that of the Al or the graphene, but the clouds of H2 and graphene are close to each other, which indicates that the adsorption of the H2 molecule is not so strong, compared with the Al adsorption. However, the interaction is much stronger than the H2
molecule adsorption of Al-decorated graphene without porosity, due to larger distances in the system without porosity, as shown for the electronic clouds of H2 and graphene, or those of H2 and Al atom. The adsorption energy Ead_H2 is 1.11 eV/H2 in Al-decorated porosity graphene and 0.182 eV/H2 in Al-decorated graphene without porosity [23]. From PDOS of the Al-decorated systems with and without porosity in Fig. 3(b), the H2 molecule has weak interaction with both graphene and Al atom owing to the small weight of overlap bands of peaks. But in the Al-decorated porosity graphene system, the overlap of the bands between H2 and graphene is more than between those of H2 and Al, especially below the Fermi level. Therefore, the first H2 molecule adsorption is mainly contributed by the interaction between H2 and graphene in the porous system, while the H2 molecule has weak interaction with both Al and graphene in the graphene system without porosity. In addition, PDOS of the porous graphene shows that the conductive band and valence band open at the Dirac point, which is also consistent with the
Fig. 4 e The adsorption energy of each H2 molecule Eb_H2 on the Al-decorated porous graphene.
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reported result that porosity can induce the band gap of graphene to open to about 3.2 eV [25]. The adsorption of the Al atom leads to an N-type doping of the graphene where the Fermi level moves towards the conductive band. When more H2 molecules are adsorbed on the Al-decorated porous graphene, the atomic structures are shown in Fig. 2. From this figure, it is found that H2 molecules prefer to occupy the two hollow sites of the fine C rings, and then they take the hollow sites of the half C rings near the porosity. To better demonstrate the positions of each H2 molecule, Fig. 2(h) shows a 4 4 supercell for the case of six H2 molecules adsorbed on one side of the graphene in the original calculation cell, as shown in Fig. 2(f). As guided by the two triangles in Fig. 2(h), all H2 molecules take the hollow positions of the C rings or the porosity, and the six H2 molecules form two triangles with the Al atom locating at the center of the triangles. Fig. 4 shows the adsorption energy of each H2 molecule on graphene, with the
number of H2 molecules increasing. It shows that Ead_H2 increases when more H2 molecules are adsorbed. Therefore, the first H2 molecule has the strongest Ead_H2 ¼ 1.11 eV/H2 at the hollow site of the porosity, the second and third H2 molecules would take the two hollow sites of the fine C rings with Ead_H2 ¼ 0.64 eV/H2, and the following three H2 molecules would take the three hollow sites of the half C rings near the porosity with Ead_H2 ¼ 0.41 eV/H2. If two more H2 molecules are adsorbed on the two hollow sites of the C rings near the porosity, but away from the Al atoms, indicated as H1 and H2 in Fig. 2(f), Ead_H2 would be 0.18 eV/H2, which is too weak for hydrogen storage application [31]. Therefore, one side of the porous graphene can adsorb six H2 molecules. If both sides of the porous graphene are decorated with Al atoms, similar H2 molecule adsorption behavior can be found, that is, the other side can also adsorb six H2 molecules, first at the hollow site of the porosity, then the two hollow sites of the fine C rings, and
Fig. 5 e The electronic cloud distribution of the Al-decorated porous graphene system with two H2 (a), three H2 (b), and four H2 molecules (c) adsorbed. Please cite this article in press as: Ao Z, et al., Hydrogen storage in porous graphene with Al decoration, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.01.044
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lastly, the three hollow sites of the half C rings near the porosity. The corresponding atomic structure is shown in Fig. 2(g). Note that the two H2 molecules (one molecule on each side) at the hollow site of the porosity are not in the plane of graphene any more owing to the repulsive interaction between the two H2 molecules, where slightly weaker Ead_H2 ¼ 1.01 eV/H2 is found. In this case, the hydrogen storage capacity is 10.5 wt%, which is over DOE’s target of 6 wt %. In addition, the adsorption energies are all almost in the desirable range of 0.2 w 0.7 eV/H2 to achieve efficiently reversible hydrogen storage and release at ambient conditions, except for the two H2 molecules at the hollow site of the porosity that are adsorbed relatively firmly. It is believed that the two H2 molecules can be released by slightly increasing the temperature. Why are different Eb_H2 obtained at different sites, as shown in Fig. 4? From the top part of Fig. 3(a), it is shown that the first H2 molecule interacts with all the atoms at the porosity owing to the short distance between the electronic clouds of the H2 and graphene, while the interaction with the Al only contributes a little to the adsorption energy. When the second and third H2 molecules adsorbed at the hollow sites of the two fine C rings, both the interactions between H2 molecules and the Al atom, and between H2 molecules and graphene, equally contribute to Ead_H2 . This can be confirmed by the relative longer distances between the electronic cloud of the second or the third H2 and that of the Al atom or graphene in Fig. 5(a and b). Therefore, the Eb_H2 at these two sites is weaker owing not only to a longer distance between the electronic clouds that weaken the interaction caused by Coulomb force, but also longer distances between the corresponding atoms, which is believed to decrease the interaction caused by the van der Waals force. When more H2 molecules are adsorbed at the hollow sites of the three half C ring near the porosity, the electronic cloud gap between the three H2 molecules and the Al atom, and that of between the three H2 molecules and graphene are similar to the case of the adsorption of the second and the third H2 molecules, as shown in Fig. 5(c). However, the electronic cloud gap between the fourth H2 molecule and the second H2 molecule is small, which induces intermolecular interaction. This interaction would decrease the interaction of the fourth H2 molecule with the graphene substrate. A similar effect has been reported on a graphene/SiO2 interface [32] and graphite on a transitionmetal carbide surface [33]. Therefore, Ead_H2 of these three H2 molecules is weaker than those of the second and third H2 molecules. For the seventh and eighth H2 molecules adsorption, as indicated in Fig. 3(h), the distance between the two H2 molecules and the graphene substrate is similar as the adsorption of fourth H2 molecule, but they are away from the Al atom, which further weakens Ead_H2 . In addition, the short distance to nearby H2 molecules strengthens the intermolecular interaction, which would reduce Ead_H2 , as discussed above. Thus, the adsorption of the seventh and eighth H2 molecules is the weakest, with even the corresponding Ead_H2 out of the desired range for hydrogen storage. In this case, the adsorption after the sixth molecule on each side is not strong enough to enable hydrogen storage at ambient conditions; thus the corresponding hydrogen storage capacity is as high as 10.5 wt%, much higher than 6 wt% of DOE’s target. When
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hydrogen releasing, H2 molecules can be desorbed gradually owing to different EadH2 , as shown in Fig. 4. The weakest adsorbed fourth, fifth and sixth H2 molecules should be desorbed first, then the second and third H2 molecules, and lastly, the first H2 molecule. The gradual releasing of H2 is desirable for hydrogen storage application where hydrogen is consumed slowly.
4.
Conclusion
The adsorption of H2 molecules on Al-decorated porous graphene is investigated by using density functional theory calculations. It is found that the presence of Al atoms and the porosity in graphene could enhance the interaction of H2 molecules with graphene, which results in the adsorption energies Ead_H2 from 1.11 to 0.41 eV/H2, depending on the number of H2 molecules adsorbed. The obtained Ead_H2 is modest for hydrogen storage at ambient conditions. The hydrogen storage capacity of this Al-decorated porous graphene can reach 10.5 wt%. For hydrogen releasing, hydrogen would be released gradually owing to different Ead_H2 being at different adsorption positions, which is desirable for actual hydrogen storage application. To understand the enhance mechanism of the decorated Al atom and the porosity in graphene for hydrogen adsorption, electronics distribution and density of states of Al-decorated graphene systems with and without porosity are analyzed. It is found that the porosity can induce more electronics transferred from the decorated Al atom to graphene, which further polarizes the adsorbed H2 molecules. Thus, stronger H2 adsorption is present in the porous graphene system.
Acknowledgments Financial support of the Chancellor’s Postdoctoral Research Fellowship Program from the University of Technology, Sydney is acknowledged.
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Hydrogen storage in porous graphene with Al decoration Zhimin Ao a,*, Shixue Dou b, Zhemi Xu c, Quanguo Jiang c, Guoxiu Wang a,* a
Centre for Clean Energy Technology, School of Chemistry and Forensic Science, University of Technology, Sydney, PO Box 123, Broadway, Sydney, NSW 2007, Australia b Institute for Superconducting & Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong, Innovation Campus Squires Way, North Wollongong, NSW 2500, Australia c School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
article info
abstract
Article history:
Porous graphene and Al modified graphene have been reported to be promising hydrogen
Received 27 September 2013
storage materials owing to their possible high hydrogen storage capacity. In this work, Al-
Received in revised form
decorated porous graphene is considered as hydrogen storage material, based on density
18 December 2013
functional calculations. It is found that the hydrogen storage capacity of the Al-decorated
Accepted 9 January 2014
porous graphene is 10.5 wt%, with modest hydrogen adsorption energy from 1.11 to
Available online xxx
0.41 eV/H2 to achieve efficient hydrogen storage/release at ambient conditions. In addition, hydrogen can be released gradually in three stages owing to different adsorption
Keywords:
energy at different adsorption sites, which is desirable in actual hydrogen storage appli-
Graphene
cation. The mechanism for improving the hydrogen storage behavior by the decorated Al
Hydrogen storage materials
atom and its porosity is understood through analyzing the atomic charges, electronic
Density functional theory
distribution, and density of states of the system. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As the global population and the demand for energy increase, fossil-fuel reserves will not be sufficient for the growth in energy consumption. Thus, hydrogen fuel systems are considered to be a highly important topic of research for future energy schemes because hydrogen is abundant, more efficient and environmentally friendly when compared with traditional fossil fuels [1]. There are many different ways to produce hydrogen [2,3]. For example, traditional energy sourcesefossil fuels like natural gas and coal, renewable energy sources such as solar radiation, wind and biomass, and nuclear energy can all be used to produce hydrogen. However,
efficient and safe storage of hydrogen is the primary obstacle to the development of the “hydrogen economy” [1]. The United States Department of Energy (DOE) has set specific performance targets that require the hydrogen storage capacity of materials to be greater than 6 wt% with the smallest possible volume, and the storage system to reversibly charge and release hydrogen near room temperature at the highest possible speed and with the lowest possible cost [4]. As a gas with very small specific volume, hydrogen storage often requires either the use of high-pressure tanks or physical and chemical bonding for solid-state storage [5]. However, compressed hydrogen is a potential hazard because of its high flammability. Liquid hydrogen is a commercial product and is used for rocket fuel, along with oxygen. Cryogenic liquid
* Corresponding authors. Tel.: þ61 2 95141741. E-mail addresses: [email protected] (Z. Ao), [email protected] (G. Wang). 0360-3199/$ e see front matter Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2014.01.044
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hydrogen (cooled to 20 K) is stored in an insulated cylinder at ambient pressure, which would not be financially viable for domestic users. In addition, the storage of hydrogen in liquid form has two major disadvantages: (i) a 30% energy loss occurs because of refrigeration; and (ii) a 1% boil-off rate [6]. Therefore, it is desirable to store hydrogen in a solid-state material [3]. Although various solid-state hydrogen storage materials have been proposed, none of the materials developed so far have fulfilled the DOE’s requirements. Based on the nature of hydrogen storage in host materials, solid-state hydrogen storage materials can be divided into two categories: (i) atomic hydrogen storage materials, in which hydrogen molecules are first dissociated by the host materials and then stored in the atomic form, that is chemisorption; and (ii) molecular hydrogen storage materials, in which hydrogen molecules bind to the host materials via van der Waals forces, and are thus primarily stored in the molecular form, being physisorption. Metal alloys, such as LaNi5, TiFe, and MgNi, were initially proposed as atomic hydrogen storage media because they form metal hydrides upon hydrogenation [7]. Metal hydrides can be categorized into high- or lowtemperature materials, depending on the temperature at which hydrogen absorption or desorption occurs. La-based and Ti-based alloys are examples of some low-temperature storage materials, but their main drawback is their low hydrogen storage gravimetric capacity (<2 wt%) [8]. Conversely, high-temperature materials, such as Mg-based alloys, can reach a theoretical maximum capacity of 7.6 wt %, although they suffer from poor hydrogenation/dehydrogenation kinetics and thermodynamics [3]. Therefore, metal alloys may not be appropriate for commercial applications in hydrogen-fuel vehicles. Alternatively, carbon nanostructures with dopant metal atoms systems are considered to be promising molecular hydrogen storage materials [9], and they can be designed into three-dimensional structures to further enhance their volumetric hydrogen storage capacity [10]. Graphene, a single layer of carbon atoms, was first fabricated in 2004, and excellent properties were discovered with high intrinsic carrier mobility (200,000 cm2 V1 s1) [11], excellent thermal conductivity (w5000 W m1 K1) [12,13], high optical transmittance (w97.7%) [14], high theoretical specific surface area (2630 m2 g1) [15], and superior mechanical strength [16]. Owing to these unique properties, graphene has wide applications in many fields, such as ultrasensitive gas sensors [17,18], transparent electrodes in liquid display devices [19], and large capacity electrodes in batteries [20,21]. Graphene has high surface areas, thermal stability with unique mechanical properties, and can be an economical and scalable production [22]. The improvement of hydrogen adsorption capacity by suitable modification, such as through strengthening the interaction between H2 molecules and graphene, would be of immense interest [23e26]. For example, Al-doped and Al decorated graphene have been reported to act as a bridge to enhance the interaction between H2 molecules and the graphene layer with high storage capacity of 13.79 wt % [23,24]. Decorating Ca atoms on graphene surface was also reported to improve the hydrogen storage capacity to 8.4 wt% [26], and the adsorption of hydrogen molecules on Li atoms decorated porous graphene is significantly enhanced, with up to a 12 wt% hydrogen storage capacity with adsorption energy
between 0.21 and 0.27 eV/H2, depending on the number of adsorbed H2 molecules [25]. Although the storage capacity of the reported systems is good enough, reaches the target of DOE, the adsorption energy of hydrogen in these materials is not strong enough to achieve efficiently reversible hydrogen adsorption and release. The hydrogen adsorption energy in most of the reported materials is near the lower limitation of the ideal adsorption energy range of 0.2 eV/H2. Owing to the advantages of Al atoms and the porosity in graphene for improving hydrogen storage capacity, the system of porous graphene with Al atoms decorating is considered in this work through density functional theory (DFT) calculations to investigate its hydrogen storage performance to achieve an excellent hydrogen storage material with high hydrogen storage capacity and modest hydrogen adsorption energy. To better understand the effects of the decorated Al atom and the presence of porosity in graphene on hydrogen storage, the electronic distribution and density of states (DOS) will be analyzed.
2.
Calculation details
All DFT calculations in this work are performed using the Dmol3 code [27]. Local density approximation (LDA) with PWC functional is employed to describe the exchange and correlation effect. The DFT þ D method within the Grimme scheme is used in all the calculations to consider the van der Waals forces [28]. Double numerical plus polarization (DNP) is taken as the basis set, which has been shown to be highly accurate [27]. In our simulation, three-dimensional periodic boundary conditions are applied, and the HeH bond length is set to be 0.74 A, identical to the experimental value [29]. The computational unit cell consists of a 2 2 1 graphene supercell with a vacuum width of 18 A to minimize the interlayer interaction. All atoms are allowed to relax in all calculations. Identical conditions are employed for the isolated H2 molecules, the Al atom and the graphene, and also for the adsorbed graphene system. The k-point is set to 20 20 1 for all slabs, the convergence tolerance of energy is 1.0 106 hartree (1 hartree ¼ 27.21 eV), and that of maximum force is 1.0 104 hartree/ A. The binding energy of Al atoms onto graphene Eb-Al is defined as, EbAl ¼ EnAlgraphene Egraphene þ nEAl n
(1a)
where EnAl-graphene, Egraphene and EAl are the energy of the system with n Al atoms adsorbed on the graphene layer, the energy of the porous graphene layer, and the energy of one Al atom in the same slab respectively. The adsorption energy of each H2 molecule onto Al-decorated graphene layer EadH2 is defined as, EadH2 ¼ EiH2 þAlgraphene Eði1ÞAlgraphene þ EH2
(1b)
where the subscripts iH2þAl-graphene, (i-1)Al-graphene, and H2 denote the Al-decorated porous graphene system with i H2 molecules adsorbed and (i-1) H2 molecules adsorbed (when i ¼ 1, it is isolated Al-decorated porous graphene), and a H2 molecule respectively.
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Fig. 1 e The relaxed structure of porous graphene (a) and Al decorated porous graphene (b). The grey, pink, and white spheres in this and following figures are C, Al and H atoms respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.
Results and discussion
Recently, Bieri et al. succeeded in synthesizing a well-defined porous graphene for the first time [30]. With the insertion of holes of specific size and distribution into graphene sheets, the electronic structure is greatly modified. Fig. 1(a) shows the geometry of porous graphene, where hydrogen passivation is taken at the edge of the porosity. Based on DFT calculations, it is reported that the presence of porosity in graphene can open the band gap of graphene from 0 to 3.2 eV. More interestingly, the porous graphene can separate the decorated Li atoms to avoid the aggression of Li atoms, thus increasing the hydrogen storage capacity [25]. In addition, transition metal decorated defect graphene with vacancies has also been reported to enhance the adsorption energy of hydrogen EadH2 within the desirable energy window to achieve efficiently reversible hydrogen storage and release, that is, 0.2 w 0.7 eV/H2, between the range of physisorption and chemisorption [31]. Owing to the unique properties of Al element to increase hydrogen storage capacity [23,24], Al atoms are considered to decorate on porous graphene for hydrogen storage application to further strengthen the interaction between H2 molecules and porous graphene here. First, the favorite position of the
decorated Al atom should be determined. We consider different positions of the Al atom on the porous graphene, such as the hollow site of a perfect C ring, the hollow site of a half C ring near the porosity, the top position of a C atom, and the bridge position of a CeC bond. It is found that the favorite configuration is shown in Fig. 1(b), namely, the Al atom prefers to adsorb on the hollow position of a half C ring near the
Table 1 e The comparison between two systems of Aldecorated graphene with and without porosity. Eads_Al is the adsorption of Al atom on graphene, Q is the charge transfer between Al and graphene obtained by Mulliken analysis, dAl-g is the distance between Al and graphene, EadsLH2 is the hydrogen adsorption energy, and dH2 graphene is the distance between H2 molecules and Aldecorated graphene.
Eads_Al Q dAl-g EadsH2 dH2 graphene
Al-decorated perfect graphene
Al-decorated porous graphene
0.824 eV 0.266 e 2.076 A 0.182 eV/H2 2.83 A
1.78 eV 0.367 e 2.059 A 1.11 eV/H2 1.83 A
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porosity. To better understand the effect of porosity in graphene on the adsorption of Al atoms and the subsequent H2 molecule adsorption, a comparison is made with the case of Al-decorated perfect graphene system, and the results are shown in Table 1. From the table, the adsorption energy of the Al atom on the porous graphene is 1.78 eV in terms of Eq. (1a), while that of an Al atom on perfect graphene is reported to be 0.824 eV [23]. Therefore, the presence of porosity in graphene would strengthen the interaction between Al and graphene significantly, which also induces a slight deformation of the porous graphene, as shown in Fig. 1(b). It is reported that the charge of a metal atom would polarize H2 molecules, thus enhancing the interaction between H2 and graphene due to the Coulomb interaction [23]. Through Mulliken analysis, as
shown in Table 1, the atomic charge of the Al atom in the porous graphene is 0.367 e, while that in perfect graphene was reported to be 0.266 e [23]. In addition, as shown in Table 1, a shorter distance between the Al atom and the porous graA is found, while that in perfect graphene phene dAl-g ¼ 2.059 is 2.076 A [23]. Therefore, more electronics transferred from the Al atom to the C atoms and shorter dAl-g in the porous graphene indicate stronger interaction between the Al atom and graphene, which also confirms the result from Eq. (1a). Therefore, stronger interaction between H2 molecules and the Al-decorated porous graphene is expected, because the more positively charged Al atom and the more negatively charged porous graphene can induce stronger polarization of adsorbed H2 molecules.
Fig. 2 e The atomic structures of the Al-decorated porous graphene with one H2 molecule (a), two H2 molecules (b), three H2 molecules (c), four H2 molecules (d), five H2 molecules (e), six H2 molecules (f), and twelve molecules (g) adsorbed. In panel (g), both sides of graphene are decorated with an Al atom, and six H2 molecules are adsorbed at each side. Panel (h) shows a 4 3 4 supercell of the system with six H2 molecules adsorbed on one side of the graphene for better demonstrating the position of H2 molecules. The two triangles are plotted here to guide the eyes. H1 and H2 indicate the sites for the seventh and eighth H2 molecules. Please cite this article in press as: Ao Z, et al., Hydrogen storage in porous graphene with Al decoration, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.01.044
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Fig. 3 e The electrons distribution [panel (a)] and partial density of states (PDOS) [panel (b)] of the systems of one H2 molecule adsorbed on Al-decorated graphene with and without porosity. In panel (b), the Fermi level is indicated by the dashed line and is located at 0.
When one H2 molecule is adsorbed on the Al-decorated porous graphene, the H2 molecule prefers to adsorb at the hollow position of porosity, as shown in Fig. 2(a). In terms of Eq. (1b), the corresponding adsorption energy Ead_H2 is 1.11 eV/H2, which is stronger than the desirable range of 0.2 to e0.7 eV/H2 [31]. In the system of Al decorated graphene without porosity, Ead_H2 was reported to be 0.182 eV/H2 for the first H2 molecule adsorption [23]. Therefore, the presence of porosity in graphene would strengthen the interaction of H2 and Al-decorated graphene significantly. From Fig. 2(a), the H2 molecule locates at the center of the porosity and almost in the same plane of the graphene layer, which is different from the case of Al-decorated graphene without porosity, where the first H2 molecule is adsorbed at the top site of a C atom and also the center of a triangle of Al atoms, as shown in Fig. 4(a) in Ref. [23]. The H2 molecule is out of the graphene plane with a A. In this case, dH2 graphene ¼ 1.83 A distance dH2 graphene ¼ 2.83 is much shorter. Therefore, stronger hydrogen adsorption energy is expected owing to the shorter dH2 graphene and more positively charged Al atom. To further understand the effect of porosity in graphene on the adsorption of H2 molecules, electronic distribution and partial density of states (PDOS) of the systems of Al-decorated porous graphene and Al-decorated graphene with first H2 molecule adsorbed are analyzed, and the results are shown in Fig. 3. From the electronic distribution in Fig. 3(a), electronic clouds of the Al atom and the porous graphene overlap, indicating electron transfer and relatively strong interaction between them. This also confirms the results that 0.276 e is transferred from the Al atom to the porous graphene from Mulliken analysis and strong adsorption energy 1.78 eV in terms of Eq. (1a). Therefore, the adsorption of the Al atom on the porous graphene is between physical and chemical adsorption, but more like chemical adsorption. For the electronic cloud of the H2 molecule, there is no overlap with that of the Al or the graphene, but the clouds of H2 and graphene are close to each other, which indicates that the adsorption of the H2 molecule is not so strong, compared with the Al adsorption. However, the interaction is much stronger than the H2
molecule adsorption of Al-decorated graphene without porosity, due to larger distances in the system without porosity, as shown for the electronic clouds of H2 and graphene, or those of H2 and Al atom. The adsorption energy Ead_H2 is 1.11 eV/H2 in Al-decorated porosity graphene and 0.182 eV/H2 in Al-decorated graphene without porosity [23]. From PDOS of the Al-decorated systems with and without porosity in Fig. 3(b), the H2 molecule has weak interaction with both graphene and Al atom owing to the small weight of overlap bands of peaks. But in the Al-decorated porosity graphene system, the overlap of the bands between H2 and graphene is more than between those of H2 and Al, especially below the Fermi level. Therefore, the first H2 molecule adsorption is mainly contributed by the interaction between H2 and graphene in the porous system, while the H2 molecule has weak interaction with both Al and graphene in the graphene system without porosity. In addition, PDOS of the porous graphene shows that the conductive band and valence band open at the Dirac point, which is also consistent with the
Fig. 4 e The adsorption energy of each H2 molecule Eb_H2 on the Al-decorated porous graphene.
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reported result that porosity can induce the band gap of graphene to open to about 3.2 eV [25]. The adsorption of the Al atom leads to an N-type doping of the graphene where the Fermi level moves towards the conductive band. When more H2 molecules are adsorbed on the Al-decorated porous graphene, the atomic structures are shown in Fig. 2. From this figure, it is found that H2 molecules prefer to occupy the two hollow sites of the fine C rings, and then they take the hollow sites of the half C rings near the porosity. To better demonstrate the positions of each H2 molecule, Fig. 2(h) shows a 4 4 supercell for the case of six H2 molecules adsorbed on one side of the graphene in the original calculation cell, as shown in Fig. 2(f). As guided by the two triangles in Fig. 2(h), all H2 molecules take the hollow positions of the C rings or the porosity, and the six H2 molecules form two triangles with the Al atom locating at the center of the triangles. Fig. 4 shows the adsorption energy of each H2 molecule on graphene, with the
number of H2 molecules increasing. It shows that Ead_H2 increases when more H2 molecules are adsorbed. Therefore, the first H2 molecule has the strongest Ead_H2 ¼ 1.11 eV/H2 at the hollow site of the porosity, the second and third H2 molecules would take the two hollow sites of the fine C rings with Ead_H2 ¼ 0.64 eV/H2, and the following three H2 molecules would take the three hollow sites of the half C rings near the porosity with Ead_H2 ¼ 0.41 eV/H2. If two more H2 molecules are adsorbed on the two hollow sites of the C rings near the porosity, but away from the Al atoms, indicated as H1 and H2 in Fig. 2(f), Ead_H2 would be 0.18 eV/H2, which is too weak for hydrogen storage application [31]. Therefore, one side of the porous graphene can adsorb six H2 molecules. If both sides of the porous graphene are decorated with Al atoms, similar H2 molecule adsorption behavior can be found, that is, the other side can also adsorb six H2 molecules, first at the hollow site of the porosity, then the two hollow sites of the fine C rings, and
Fig. 5 e The electronic cloud distribution of the Al-decorated porous graphene system with two H2 (a), three H2 (b), and four H2 molecules (c) adsorbed. Please cite this article in press as: Ao Z, et al., Hydrogen storage in porous graphene with Al decoration, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.01.044
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lastly, the three hollow sites of the half C rings near the porosity. The corresponding atomic structure is shown in Fig. 2(g). Note that the two H2 molecules (one molecule on each side) at the hollow site of the porosity are not in the plane of graphene any more owing to the repulsive interaction between the two H2 molecules, where slightly weaker Ead_H2 ¼ 1.01 eV/H2 is found. In this case, the hydrogen storage capacity is 10.5 wt%, which is over DOE’s target of 6 wt %. In addition, the adsorption energies are all almost in the desirable range of 0.2 w 0.7 eV/H2 to achieve efficiently reversible hydrogen storage and release at ambient conditions, except for the two H2 molecules at the hollow site of the porosity that are adsorbed relatively firmly. It is believed that the two H2 molecules can be released by slightly increasing the temperature. Why are different Eb_H2 obtained at different sites, as shown in Fig. 4? From the top part of Fig. 3(a), it is shown that the first H2 molecule interacts with all the atoms at the porosity owing to the short distance between the electronic clouds of the H2 and graphene, while the interaction with the Al only contributes a little to the adsorption energy. When the second and third H2 molecules adsorbed at the hollow sites of the two fine C rings, both the interactions between H2 molecules and the Al atom, and between H2 molecules and graphene, equally contribute to Ead_H2 . This can be confirmed by the relative longer distances between the electronic cloud of the second or the third H2 and that of the Al atom or graphene in Fig. 5(a and b). Therefore, the Eb_H2 at these two sites is weaker owing not only to a longer distance between the electronic clouds that weaken the interaction caused by Coulomb force, but also longer distances between the corresponding atoms, which is believed to decrease the interaction caused by the van der Waals force. When more H2 molecules are adsorbed at the hollow sites of the three half C ring near the porosity, the electronic cloud gap between the three H2 molecules and the Al atom, and that of between the three H2 molecules and graphene are similar to the case of the adsorption of the second and the third H2 molecules, as shown in Fig. 5(c). However, the electronic cloud gap between the fourth H2 molecule and the second H2 molecule is small, which induces intermolecular interaction. This interaction would decrease the interaction of the fourth H2 molecule with the graphene substrate. A similar effect has been reported on a graphene/SiO2 interface [32] and graphite on a transitionmetal carbide surface [33]. Therefore, Ead_H2 of these three H2 molecules is weaker than those of the second and third H2 molecules. For the seventh and eighth H2 molecules adsorption, as indicated in Fig. 3(h), the distance between the two H2 molecules and the graphene substrate is similar as the adsorption of fourth H2 molecule, but they are away from the Al atom, which further weakens Ead_H2 . In addition, the short distance to nearby H2 molecules strengthens the intermolecular interaction, which would reduce Ead_H2 , as discussed above. Thus, the adsorption of the seventh and eighth H2 molecules is the weakest, with even the corresponding Ead_H2 out of the desired range for hydrogen storage. In this case, the adsorption after the sixth molecule on each side is not strong enough to enable hydrogen storage at ambient conditions; thus the corresponding hydrogen storage capacity is as high as 10.5 wt%, much higher than 6 wt% of DOE’s target. When
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hydrogen releasing, H2 molecules can be desorbed gradually owing to different EadH2 , as shown in Fig. 4. The weakest adsorbed fourth, fifth and sixth H2 molecules should be desorbed first, then the second and third H2 molecules, and lastly, the first H2 molecule. The gradual releasing of H2 is desirable for hydrogen storage application where hydrogen is consumed slowly.
4.
Conclusion
The adsorption of H2 molecules on Al-decorated porous graphene is investigated by using density functional theory calculations. It is found that the presence of Al atoms and the porosity in graphene could enhance the interaction of H2 molecules with graphene, which results in the adsorption energies Ead_H2 from 1.11 to 0.41 eV/H2, depending on the number of H2 molecules adsorbed. The obtained Ead_H2 is modest for hydrogen storage at ambient conditions. The hydrogen storage capacity of this Al-decorated porous graphene can reach 10.5 wt%. For hydrogen releasing, hydrogen would be released gradually owing to different Ead_H2 being at different adsorption positions, which is desirable for actual hydrogen storage application. To understand the enhance mechanism of the decorated Al atom and the porosity in graphene for hydrogen adsorption, electronics distribution and density of states of Al-decorated graphene systems with and without porosity are analyzed. It is found that the porosity can induce more electronics transferred from the decorated Al atom to graphene, which further polarizes the adsorbed H2 molecules. Thus, stronger H2 adsorption is present in the porous graphene system.
Acknowledgments Financial support of the Chancellor’s Postdoctoral Research Fellowship Program from the University of Technology, Sydney is acknowledged.
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Please cite this article in press as: Ao Z, et al., Hydrogen storage in porous graphene with Al decoration, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.01.044