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High-capacity hydrogen storage using Li-decorated Li2P sheet
Chemical Physics Letters 614 (2014) 129–135
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
High-capacity hydrogen storage using Li-decorated Li2 P sheet Jun Zhang a , Chunsheng Liu a , Xiaohong Zheng a , Zhi Zeng a,b,∗ , Xin Ju c a
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China University of Science and Technology of China, Hefei 230026, China c Department of Physics, University of Science and Technology of Beijing, Beijing 100083, China b
a r t i c l e
i n f o
Article history: Received 20 July 2014 In final form 8 September 2014 Available online 16 September 2014
a b s t r a c t Using density functional theory, we investigate the hydrogen storage capacity of Li-adsorbed lithium porphyrin sheet (Li2 P·2Li). We show that the central Li atom (Lic ) and the decorated one (Lid ) can bind four or three H2 molecules, respectively. Due to the polarization of hydrogen molecules under the electric field generated by the positively charged Lic (Lid ), the binding energy of the H2 molecules is between 0.12 eV and 0.22 eV, which is beneficial for the storage and release of the H2 molecules. The moderate binding energy and large hydrogen storage capacity (7.95 wt%) indicate that Li2 P·2Li is a promising hydrogen storage material. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has been recognized as an ideal energy carrier. Unfortunately, it is difficult to find efficient storage materials that satisfy the requirements of realistic applications. Metal doped carbon nanostructures have been regarded as good hydrogen storage materials, because of their light weight and high hydrogen adsorption capacity [1–4]. However, the storage method faces the following problems: metal atoms tend to form clusters at the surface of the substrates [5,6]; the binding energy between the metal atoms and the substrates is low [7]; metal-coated carbon nanomaterials are hard to be synthesized [8,9]. Other promising hydrogen storage materials are metal-organic frameworks (MOFs) [10] and covalent organic frameworks (COFs) [11], which are porous crystallines. MOFs (or COFs) have attracted a great deal of interest due to their outstanding properties, such as porous, robust nature, the exceptionally high specific surface area and large pore volume. On the other hand, hydrogen molecules weakly bind to the MOFs and COFs through Van der Waals interactions [12,13]. Therefore the hydrogen adsorption capacity in MOFs (COFs) is not satisfactory at the room temperature. To improve their performance, functionalizing the organic linker with metal atom is one of the effective ways [14,15]. For this purpose, one route is to discover and design materials that have built-in metal atoms with light weight and can
∗ Corresponding author at: Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China. E-mail address: [email protected] (Z. Zeng). http://dx.doi.org/10.1016/j.cplett.2014.09.023 0009-2614/© 2014 Elsevier B.V. All rights reserved.
be easily synthesized. We therefore turn to metalloporphyrins in order to enhance hydrogen storage capacity. In recent years, many theoretical researches have been made on the hydrogen adsorption at metalloporphyrin molecules or metalloporphyrin functionalized COFs. Ryou et al. [16] indicated that the interaction for Ca-porphyrin interacting with hydrogen molecules is electric polarization, while it is Kubas interaction between Ti-porphyrin and hydrogen molecules. The hydrogen storage capacity of metalporphyrin frameworks (MPFs) investigated by Xiong et al. [17] exhibited a higher weight fraction of H2 than that of MOFs because MPFs provide more adsorption sites for H2 . Srepusharawoot et al. [18] reported that the hydrogen adsorption capacity of COF-366 decorated with porphyrin at the corner of the crystal is improved after Li functionalization. Gao et al. [19] investigated Li doped phthalocyanine (or porphyrin) COF, and suggested they could give a high H2 gravimetric density. In addition, the monomeric metalloporphyrins [20] and the sheet-like metalloporphyrin polymers have been synthesized [21,22] and the optical properties of their infinite forms have been theoretically examined [23,24]. A large number of metal ions can also be readily incorporated into the porphyrin center through the coordination mechanism [25]. Though the monomeric metalloporphyrins have been studied for hydrogen storage, the infinitely extended planar metalloporphyrins with periodic structures have never been investigated. Due to its large specific area and abundant built-in metal atoms, we could view it as a potential hydrogen storage material. Here, we choose the lightest metal, Li, as the central atom and theoretically investigate the two-dimensional (2D) Liporphyrin sheet for hydrogen storage. Our main concerns are: (i) Can Li bind strongly enough to the substrate to avoid clustering?
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What is the binding mechanism? (ii) How many H2 molecules can be adsorbed by the Li2 P·2Li sheet? What is the binding mechanism between the H2 molecules and the substrate? 2. Model design and computational details Our systematic calculations are performed using density functional theory (DFT) implemented in DMol3 [26,27]. The generalized gradient approximation (GGA) of the Perdew–Wang (PW91) [28] form is adopted for the electronic exchange correlation functional. The spin polarization is also considered in the DFT calculation. To simulate the 2D framework, a vacuum space of 10 A˚ along the zdirection is applied to avoid periodic image interactions. A 3 × 3 × 2 k point grid in the Monkhorst-Pack scheme [29] of meshes is used to sample the Brillouin zone for the 2D polymer structure. All electrons are chosen as the core treatment method and a double numeric basis with polarization (DNP) set is adopted as the basis ˚ All structures set. The orbital cutoff is globally set to a value of 5.1 A. are optimized without symmetry constraint. In the self-consistent field calculations, the electronic density convergence criterion is set to 1 × 10−6 e/A˚ 3 . The convergence criteria for the energy, force ˚ and and maximum displacement are set to 10−5 Ha, 2 × 10−3 Ha/A, ˚ respectively. The accuracy of the computational method is 0.005 A, ˚ and binding energy tested by computing the bond length (0.748 A) of H2 (4.57 eV), which are in good agreement with the experimental values of 0.741 A˚ and 4.533 eV, respectively [5]. To study the diffusion behavior of the doped Li atom on the Li2 P sheet, we calculate the Li diffusion energy barrier (E). The linear synchronous transit (LST) and quadratic synchronous transit (QST) [30], in combination with the conjugate gradient (CG) minimization algorithm are adopted for the subsequent refinement [31]. Additionally, the minimum energy path between the initial structure, transition structure and final structure is investigated using the nudged elastic band (NEB) theory [32]. To construct the 2D infinite lithium porphyrin sheet, we follow the similar strategy to the one introduced by Yamaguchi [24]. Namely, the Li-porphyrin molecules are connected to each other to form a 2D network according to the recently synthesized triply meso-meso-, --, and --linked model. Then, two Li atoms are decorated above and below the carbon octagon. The structural parameters of the Li2 P molecule are consistent with the results observed by Chen and coworkers [23]. The optimized unit cell ˚ The schematic configparameters for the Li2 P sheet are a = b = 8.4A. urations of the Li-porphyrin molecule (Li2 P molecule), Li-porphyrin sheet (Li2 P sheet), and two Li atoms decorated Li-porphyrin sheet (Li2 P·2Li) are shown in Figure 1a–c, respectively. We label the Li atoms in the center of the porphyrin as Lic and the decorated ones as Lid . 3. Results and discussion 3.1. Adsorption of a single Li atom on a Li2 P sheet We first investigate the adsorption behavior of a single Lid atom on a planar Li2 P sheet. The binding energy between the Li atom and the Li2 P sheet is calculated using the following equation: Ebn (Li) = E[Li2 P · (n − 1)Li] + E(Li) − E[Li2 P · nLi],
(1)
where E(Li2 P), E(Li), and E[Li2 P · nLi] are the total energy of the clean pristine Li2 P sheet, a free Li atom, and the Li2 P sheet with n adsorbed Li atoms, respectively. Three stable sites for the Li atom adsorption at Li2 P are indicated in Figure 1b by red triangles. They are L1 (above the carbon octagon), L2 (above the carbon hexagon) and L3 (above the carbon pentagon). The binding energies of Lid for the L1, L2 and
Figure 1. Geometries of Li-porphyrin molecule (a), Li-porphyrin (Li2 P) sheet (b), and two-Li-atom-decorated Li-porphyrin (Li2 ·2Li) sheet (c) with top and side views. The white, gray, purple and blue balls in this figure and hereafter represent H, C, Li and N atoms, respectively. The lithium (Lid ) adsorption sites above the carbon octagon, hexagon and pentagon, are indicated by L1, L2 and L3 respectively. The Li atom in the center of the porphyrin is labeled as Lic and the decorated Li atom is labeled as Lid . Lic (Lid ) and Lic (Lid ) represent the Li atom on the different side of the sheet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
L3 site are very close to 2.650, 2.451 and 2.449 eV, among which we only discuss the first case here. The binding nature between the Lid and the Li2 P sheet can be understood from the charge density difference of the Li2 P·Li system, as illustrated in Figure 2. For the convenience of observation, we only display the region around Lid and its nearest neighbor C atoms. We notice that there is a net loss of electronic charge around the Li atom, indicating a significant charge transfer from the adsorbed Li to its neighbor C atoms. In an effort to quantitatively estimate the amount of charge transfer between the adsorbed Li and the Li2 P substrate, we performed the Mulliken population analysis. The
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Figure 2. Isosurface (0.02 e/A˚ 3 ) of the charge density difference around the adsorbed Li atom and its neighboring C atoms in the Li2 P·Li system: top view (a) and side view (b). The blue and yellow colors represent electron accumulation and depletion, respectively. (c) Partial density of states (PDOS) of isolated Li atom (dashed) and decorated Li atom (solid) – upper panel, and PDOS of C octagon in the Li2 P and Li2 P·Li system – lower panel. The Fermi level (EF ) is set to zero. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Mulliken charge of the Li atom adsorbed on the L1 site is +0.508e, while the average Mulliken charge of the eight neighboring C atoms nearby is −0.071e. All of these results suggest that the interaction between the adsorbed Li atom and its nearest neighbor C atoms is predominately ionic. Figure 2c illustrates the partial density of states (PDOS) of the s and p orbitals of the decorated Li atom and the p orbital of its neighboring C atoms in the Li2 P·Li system. By comparing the PDOS of isolated Li and adsorbed Li atom, we found that, the occupation of the Li s orbital is considerably reduced because the charges transfer to the eight neighboring C atoms, which indicates that the binding between the Li atom and C atoms is mainly ionic. From Figure 2c, after doping Li atom, the PDOS of C octagon changes and a peak appears in the C p orbital around −2.8 eV, which is overlapping with the small peak of Lid p orbital as shown in the inset. Consequently, the binding nature of the Li atom and the substrate is mainly ionic together with partial orbital hybridization. To explore the stability of Lid on the C octagon, we calculate the diffusion barrier of Lid from L1 to L2 and that from L1 to L3, as shown in Figure 3 (where the structures of the initial, transition, and final states are displayed). Compared with the Li diffusion from a hollow site to the nearest one on perfect graphene performed by Zhou et al. [33], the energy barrier E for Li migration from L1 to L2 increases by 0.091 eV and the corresponding backward diffusion barrier decreases by 0.114 eV. A similar trend also exists for the diffusion path from L1 to L3. These results indicate that the Li adatom is more favorable to diffuse toward the C octagon site than to diffuse outward from the C octagon site. Therefore, the Li adatom could be trapped at the hollow site above the octagon ring of Li2 P, which is more stable than the Li adatom on the perfect graphene sheet. 3.2. Adsorption of two Li atoms on a Li2 P sheet We next consider the adsorption of Li atoms on both sides of the Li2 P sheet, 2Li-absorbed case. As observed in Figure 1b, there are three different sites for the second Li atom to be positioned on the
other side of Li2 P, which are below L1, L2 and L3. We denote these sites as L1 , L2 , and L3 , respectively. The configurations of the three cases associated with their corresponding Lic −Lid distances and the binding energies of the second Li adatom are shown in Figure 4. The L1 + L2 and L1 + L3 adsorptions have a slightly higher binding energy than the L1 + L1 adsorption, because the Couloumb repulsion in the L1 − L2 and L1 − L3 configurations is smaller than that in the L1 − L1 case. However, even if in the L1 − L2 and L1 − L3 configurations, the binding energies of the second Li are higher, they are not beneficial for the adsorption of H2 molecules. Since in these two cases, on one side of the sheet, the Lic − Lid distance is only 4.4 A˚ and 3.4 A˚ respectively (Figure 4b and c), which is too close for H2 adsorption. As a matter of fact, our calculations indicate that, in the L1 − L2 and L1 − L3 cases, Lic is only able to adsorb 3 H2 molecules due to the steric repulsion between Lic and L2 or L3 . However, in the L1 − L1 case, each Lic adsorbs 4 H2 molecules. Therefore, we will only consider the L1−L1 case in this work. The average charge of the C octagon in the 2Li adsorbed cases is now nearly doubled (−0.141e) compared with the previous single Li adsorption case because the second Li atoms contribute the charge transfer. 3.3. Hydrogen adsorption on the Li2 P·2Li complex We now investigate the interaction of a hydrogen molecule with the Li2 P·2Li sheet by adsorbing H2 . The hydrogen adsorption energy is defined as: En (H2 ) = E[Li2 P · 2Li@(n − 1)H2 ] + E(H2 ) − E[Li2 P · 2Li@nH2 ],
(2)
where En (H2 ) is the binding energy of the nth H2 molecule in the Li2 P·2Li system, E[Li2 P · 2Li @ nH2 ] is the total energy of the Li2 P·2Li system with n adsorbed H2 molecules, E(H2 ) is the total energy of a hydrogen molecule, and n indicates the number of H2 molecules. The configurations of H2 adsorbed at the Li2 P·2Li sheet are illustrated in Figure 5. The average distance between H2 molecule and Li, the binding energy of the nth H2 molecule, the average bond length of H2 and the Mulliken charge on the Li atom are shown in Figure 5 as well. Our calculations indicate that each Lic (Lid ) atom can attach
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Figure 3. The energy barriers E along the Li migration path of (a) L1 → L2 (b) L1 → L3 on Li2 P. In the paths, the initial, saddle-point, and final states are labeled with stars.
Figure 4. The configurations of two lithium (Lid ) atoms adsorbed at both sides of the Li2 P sheet with top view and side view: (a) L1 + L1 ; (b) L1 + L2 and (c) L1 + L3 . The ˚ are also shown. binding energies of Li and the Lic − Lid distances (in A)
Figure 5. The optimized atomic geometries of the Li2 P·2Li sheet with H2 molecules gradually adsorbed. Average distance between the center of the H2 molecules and Lic /Lid ˚ binding energy of the nth adsorbed H2 molecules En in eV, average bond length of H2 molecules (LH−H in A), ˚ and the Mulliken charge of Lic /Lid atoms (QLi in e) (L¯ H2 −Li in A), are given in the figure as well.
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Figure 6. H1 and H2 represent hydrogen atoms in one H2 molecule where the H1 atom is closer to the Li atoms. n(H2 ) indicates the nth H2 molecule attracted around Lic (Lid ) atom. (a) and (b) show the values of charges on H1 and H2 in Li2 P·2Li system with H2 molecules adsorbed on Lic or Lid site. H1 (H2) and H1 (H2 ) represent the hydrogen atom in the adsorbed H2 molecules on the two sides of the sheet.
Figure 7. (a) Variation of the charge on Lic (Lid ) when hydrogen molecules gradually attached. Lic (Lid ) and Lic (Lid ) represents the Lic (Lid ) atom on the two sides of the sheet.(b) Adsorption energy of hydrogen molecules. The line with magenta triangle is the result of prior work (Ref. [34]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
up to four (three) H2 molecules. The binding energies of the hydrogen molecules on Li2 P·2Li system (Figure 5) are in the range of 0.12–0.22 eV, which is excellent for hydrogen storage applications. As the number of H2 molecules adsorbed at each Li atom increases, the binding energy of the H2 molecule decreases and the average distance between Li and H2 molecules increases. The bond length ˚ We of the H2 molecule is slightly elongated from 0.749 A˚ to 0.755 A. also carried out the Mulliken population analysis on H2 molecules around Lic and Lid as shown in Figure 6, where the hydrogen atoms of the adsorbed H2 molecule at Lic and Lid are denoted as H1 and H2, and the corresponding Mulliken charges are denoted as Q(H1) and Q(H2). We now turn to the binding nature between H2 and Lic /Lid . Figure 5 exhibits that the bond length of each adsorbed H2 molecule is slightly elongated. This result occurs because the H2 molecules are polarized by the electric field of the positively charged Li atoms. However, does the polarization mechanism only work in the binding of H2 molecules? If a molecule is polarized in the electric field, its charge will redistribute and give rise to zero net charge. However, from Figure 6, besides charge redistribution (Q(H1) > 0, Q(H2) < 0; except for the first adsorbed H2 molecule), there is a net positive charge on H2 molecules (Q(H1) + Q(H2) > 0) both around Lic and Lid , which demonstrates that a small fraction of electrons transferred from H2 orbital to Li empty s orbital. This can be confirmed by Figure 7a that, the Lic /Lid becomes less positively charged by accepting electrons from the H2 molecules adsorbed on it. Therefore, the orbital interaction is another factor responsible for the binding of H2 molecule, though it is very weak. Furthermore, from Figure 7b, the binding energy of H2 molecules around Lic is apparently lower than that of H2 molecules around Lid . Consequently, the H2 adhesion mechanism should be different for Lic and Lid , which will be discussed separately.
As presented in Figure 6a, the net charge on the hydrogen molecules around Lic is less than 0.06e, and when four H2 molecules are adsorbed around Lic , the net charge of each H2 molecule is nearly zero. This result is consistent with the change of positive charge carried by Lic . In Figure 7a, when H2 molecules are gradually attached to Lic , the positive charge on Lic is almost unchanged, which is around 0.53e. It means that little charge is transferred from H2 molecules to Lic . This is also reflected in the very weak hybridization between H s and Lic p orbitals shown in Figure 8a. When the number of attached H2 molecules increases to 4, the hybridization between the H2 molecules and Lic nearly disappears. Thus, the binding energy of H2 molecules around Lic is lower than that of H2 molecules around Lid . That also makes the H2 − Lic distance slightly larger than H2 − Lid distance. On the contrary, for Lid , it bears more charge than Lic initially, which provides stronger polarization to H2 molecule resulting a higher binding energy of H2 . In addition, the charge donated from each attached H2 molecule to Lid is nearly unchanged (Figure 6b), i.e., 0.08 electrons. This result matches the variation of charge carried by Lid as displayed in Figure 7a, in which the positive charge of Lid decreases linearly with increasing the number of the adsorbed H2 molecules. This can be further proved by the PDOS of H2 and Lid in Figure 8b, in which stronger hybridization between H s and Lid p orbitals than that between H s and Lic p orbitals is exhibited. Due to such a slightly stronger hybridization, the H2 adsorption energy is higher than that of H2 molecules around Lid . In addition, the H s orbital broadens as the number of H2 molecules increases due to the interactions between H2 molecules. This phenomenon is not clearly observed for H2 adsorbed near Lic because of the large distance between H2 molecules. The binding mechanism of H2 molecules to Lid is very similar to other Li doped carbon based nanomaterials. In a previous work [34], the doped Li atom also can adsorb a
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Figure 8. The partial density of states (PDOS) of (a) one to four H2 molecules adsorbed to Lic and (b) one to three H2 molecules adsorbed to Lid in the Li2 P·Li system. The Fermi level (EF ) is set to zero.
maximum of three H2 molecules, and the pattern of H2 binding energy variation is similar as shown in Figure 7a. We now analyze the change of the bond length of the adsorbed H2 molecules. From Figure 5, we notice that the H2 bond length is
first increased and then decreased. This is due to the different H2 location pattern of the first adsorbed H2 molecule from the other. We know that the H2 bond elongation is due to the polarization by positively charged Li and arises from the different electrostatic
J. Zhang et al. / Chemical Physics Letters 614 (2014) 129–135
force on the two hydrogen atoms. As shown in Figure 5, in the case of single H2 adsorption, the adsorbed molecule is parallel to the substrate and the hydrogen atoms are equidistant from Li atoms. As a result, the electrostatic forces on the two hydrogen are similar, so that the bond elongation is not obvious. Once the number of H2 adsorbed by each Li atom exceeds one, the adsorbed H2 molecules tend to tilt toward Li atoms. Consequently, the electrostatic forces on the two hydrogen atoms are different, resulting in the bond elongation. In addition, the bond elongation is slightly alleviated when the number of H2 molecules is more than two. That is because as the number of H2 molecules increases, the polarization on each H2 molecules is diminished and the elongation of the H2 bond length becomes less obvious. 4. Conclusions In summary, we have investigated the hydrogen storage capacity of Li-adsorbed lithium porphyrin sheet (Li2 P·2Li) by firstprinciples calculations. It is found that Li can bind strongly to the lithium-porphyrin sheet with a very large binding energy and further H2 molecules can be adsorbed on the Li atoms with a moderate binding energy between 0.12 eV and 0.22 eV. The binding nature of H2 around Lic (Lid ) is mainly electric polarization with a small part of orbital interaction. The binding of H2 molecules around Lic is weaker than that of H2 molecules around Lid due to fewer charge transfer from H2 to Lic than H2 to Lid or weaker hybridization of the H s and Lic p orbitals than that of H s and Lid p orbitals. Each Li atom can adsorb three or four H2 molecules, which leads to a hydrogen storage capacity of 7.95 wt%. The moderate binding energy and the large hydrogen storage capacity indicate that Li-adsorbed lithium porphyrin sheet is a very promising hydrogen storage material. Acknowledgements This work was supported by the special Funds for Major State Basic Research Project of China (973) under grant no. 2012CB933702, the NSFC under grant nos. 11104278, 11174284 and U1230202 (NSAF), Hefei Center for Physical Science and
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
High-capacity hydrogen storage using Li-decorated Li2 P sheet Jun Zhang a , Chunsheng Liu a , Xiaohong Zheng a , Zhi Zeng a,b,∗ , Xin Ju c a
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China University of Science and Technology of China, Hefei 230026, China c Department of Physics, University of Science and Technology of Beijing, Beijing 100083, China b
a r t i c l e
i n f o
Article history: Received 20 July 2014 In final form 8 September 2014 Available online 16 September 2014
a b s t r a c t Using density functional theory, we investigate the hydrogen storage capacity of Li-adsorbed lithium porphyrin sheet (Li2 P·2Li). We show that the central Li atom (Lic ) and the decorated one (Lid ) can bind four or three H2 molecules, respectively. Due to the polarization of hydrogen molecules under the electric field generated by the positively charged Lic (Lid ), the binding energy of the H2 molecules is between 0.12 eV and 0.22 eV, which is beneficial for the storage and release of the H2 molecules. The moderate binding energy and large hydrogen storage capacity (7.95 wt%) indicate that Li2 P·2Li is a promising hydrogen storage material. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has been recognized as an ideal energy carrier. Unfortunately, it is difficult to find efficient storage materials that satisfy the requirements of realistic applications. Metal doped carbon nanostructures have been regarded as good hydrogen storage materials, because of their light weight and high hydrogen adsorption capacity [1–4]. However, the storage method faces the following problems: metal atoms tend to form clusters at the surface of the substrates [5,6]; the binding energy between the metal atoms and the substrates is low [7]; metal-coated carbon nanomaterials are hard to be synthesized [8,9]. Other promising hydrogen storage materials are metal-organic frameworks (MOFs) [10] and covalent organic frameworks (COFs) [11], which are porous crystallines. MOFs (or COFs) have attracted a great deal of interest due to their outstanding properties, such as porous, robust nature, the exceptionally high specific surface area and large pore volume. On the other hand, hydrogen molecules weakly bind to the MOFs and COFs through Van der Waals interactions [12,13]. Therefore the hydrogen adsorption capacity in MOFs (COFs) is not satisfactory at the room temperature. To improve their performance, functionalizing the organic linker with metal atom is one of the effective ways [14,15]. For this purpose, one route is to discover and design materials that have built-in metal atoms with light weight and can
∗ Corresponding author at: Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China. E-mail address: [email protected] (Z. Zeng). http://dx.doi.org/10.1016/j.cplett.2014.09.023 0009-2614/© 2014 Elsevier B.V. All rights reserved.
be easily synthesized. We therefore turn to metalloporphyrins in order to enhance hydrogen storage capacity. In recent years, many theoretical researches have been made on the hydrogen adsorption at metalloporphyrin molecules or metalloporphyrin functionalized COFs. Ryou et al. [16] indicated that the interaction for Ca-porphyrin interacting with hydrogen molecules is electric polarization, while it is Kubas interaction between Ti-porphyrin and hydrogen molecules. The hydrogen storage capacity of metalporphyrin frameworks (MPFs) investigated by Xiong et al. [17] exhibited a higher weight fraction of H2 than that of MOFs because MPFs provide more adsorption sites for H2 . Srepusharawoot et al. [18] reported that the hydrogen adsorption capacity of COF-366 decorated with porphyrin at the corner of the crystal is improved after Li functionalization. Gao et al. [19] investigated Li doped phthalocyanine (or porphyrin) COF, and suggested they could give a high H2 gravimetric density. In addition, the monomeric metalloporphyrins [20] and the sheet-like metalloporphyrin polymers have been synthesized [21,22] and the optical properties of their infinite forms have been theoretically examined [23,24]. A large number of metal ions can also be readily incorporated into the porphyrin center through the coordination mechanism [25]. Though the monomeric metalloporphyrins have been studied for hydrogen storage, the infinitely extended planar metalloporphyrins with periodic structures have never been investigated. Due to its large specific area and abundant built-in metal atoms, we could view it as a potential hydrogen storage material. Here, we choose the lightest metal, Li, as the central atom and theoretically investigate the two-dimensional (2D) Liporphyrin sheet for hydrogen storage. Our main concerns are: (i) Can Li bind strongly enough to the substrate to avoid clustering?
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What is the binding mechanism? (ii) How many H2 molecules can be adsorbed by the Li2 P·2Li sheet? What is the binding mechanism between the H2 molecules and the substrate? 2. Model design and computational details Our systematic calculations are performed using density functional theory (DFT) implemented in DMol3 [26,27]. The generalized gradient approximation (GGA) of the Perdew–Wang (PW91) [28] form is adopted for the electronic exchange correlation functional. The spin polarization is also considered in the DFT calculation. To simulate the 2D framework, a vacuum space of 10 A˚ along the zdirection is applied to avoid periodic image interactions. A 3 × 3 × 2 k point grid in the Monkhorst-Pack scheme [29] of meshes is used to sample the Brillouin zone for the 2D polymer structure. All electrons are chosen as the core treatment method and a double numeric basis with polarization (DNP) set is adopted as the basis ˚ All structures set. The orbital cutoff is globally set to a value of 5.1 A. are optimized without symmetry constraint. In the self-consistent field calculations, the electronic density convergence criterion is set to 1 × 10−6 e/A˚ 3 . The convergence criteria for the energy, force ˚ and and maximum displacement are set to 10−5 Ha, 2 × 10−3 Ha/A, ˚ respectively. The accuracy of the computational method is 0.005 A, ˚ and binding energy tested by computing the bond length (0.748 A) of H2 (4.57 eV), which are in good agreement with the experimental values of 0.741 A˚ and 4.533 eV, respectively [5]. To study the diffusion behavior of the doped Li atom on the Li2 P sheet, we calculate the Li diffusion energy barrier (E). The linear synchronous transit (LST) and quadratic synchronous transit (QST) [30], in combination with the conjugate gradient (CG) minimization algorithm are adopted for the subsequent refinement [31]. Additionally, the minimum energy path between the initial structure, transition structure and final structure is investigated using the nudged elastic band (NEB) theory [32]. To construct the 2D infinite lithium porphyrin sheet, we follow the similar strategy to the one introduced by Yamaguchi [24]. Namely, the Li-porphyrin molecules are connected to each other to form a 2D network according to the recently synthesized triply meso-meso-, --, and --linked model. Then, two Li atoms are decorated above and below the carbon octagon. The structural parameters of the Li2 P molecule are consistent with the results observed by Chen and coworkers [23]. The optimized unit cell ˚ The schematic configparameters for the Li2 P sheet are a = b = 8.4A. urations of the Li-porphyrin molecule (Li2 P molecule), Li-porphyrin sheet (Li2 P sheet), and two Li atoms decorated Li-porphyrin sheet (Li2 P·2Li) are shown in Figure 1a–c, respectively. We label the Li atoms in the center of the porphyrin as Lic and the decorated ones as Lid . 3. Results and discussion 3.1. Adsorption of a single Li atom on a Li2 P sheet We first investigate the adsorption behavior of a single Lid atom on a planar Li2 P sheet. The binding energy between the Li atom and the Li2 P sheet is calculated using the following equation: Ebn (Li) = E[Li2 P · (n − 1)Li] + E(Li) − E[Li2 P · nLi],
(1)
where E(Li2 P), E(Li), and E[Li2 P · nLi] are the total energy of the clean pristine Li2 P sheet, a free Li atom, and the Li2 P sheet with n adsorbed Li atoms, respectively. Three stable sites for the Li atom adsorption at Li2 P are indicated in Figure 1b by red triangles. They are L1 (above the carbon octagon), L2 (above the carbon hexagon) and L3 (above the carbon pentagon). The binding energies of Lid for the L1, L2 and
Figure 1. Geometries of Li-porphyrin molecule (a), Li-porphyrin (Li2 P) sheet (b), and two-Li-atom-decorated Li-porphyrin (Li2 ·2Li) sheet (c) with top and side views. The white, gray, purple and blue balls in this figure and hereafter represent H, C, Li and N atoms, respectively. The lithium (Lid ) adsorption sites above the carbon octagon, hexagon and pentagon, are indicated by L1, L2 and L3 respectively. The Li atom in the center of the porphyrin is labeled as Lic and the decorated Li atom is labeled as Lid . Lic (Lid ) and Lic (Lid ) represent the Li atom on the different side of the sheet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
L3 site are very close to 2.650, 2.451 and 2.449 eV, among which we only discuss the first case here. The binding nature between the Lid and the Li2 P sheet can be understood from the charge density difference of the Li2 P·Li system, as illustrated in Figure 2. For the convenience of observation, we only display the region around Lid and its nearest neighbor C atoms. We notice that there is a net loss of electronic charge around the Li atom, indicating a significant charge transfer from the adsorbed Li to its neighbor C atoms. In an effort to quantitatively estimate the amount of charge transfer between the adsorbed Li and the Li2 P substrate, we performed the Mulliken population analysis. The
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Figure 2. Isosurface (0.02 e/A˚ 3 ) of the charge density difference around the adsorbed Li atom and its neighboring C atoms in the Li2 P·Li system: top view (a) and side view (b). The blue and yellow colors represent electron accumulation and depletion, respectively. (c) Partial density of states (PDOS) of isolated Li atom (dashed) and decorated Li atom (solid) – upper panel, and PDOS of C octagon in the Li2 P and Li2 P·Li system – lower panel. The Fermi level (EF ) is set to zero. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Mulliken charge of the Li atom adsorbed on the L1 site is +0.508e, while the average Mulliken charge of the eight neighboring C atoms nearby is −0.071e. All of these results suggest that the interaction between the adsorbed Li atom and its nearest neighbor C atoms is predominately ionic. Figure 2c illustrates the partial density of states (PDOS) of the s and p orbitals of the decorated Li atom and the p orbital of its neighboring C atoms in the Li2 P·Li system. By comparing the PDOS of isolated Li and adsorbed Li atom, we found that, the occupation of the Li s orbital is considerably reduced because the charges transfer to the eight neighboring C atoms, which indicates that the binding between the Li atom and C atoms is mainly ionic. From Figure 2c, after doping Li atom, the PDOS of C octagon changes and a peak appears in the C p orbital around −2.8 eV, which is overlapping with the small peak of Lid p orbital as shown in the inset. Consequently, the binding nature of the Li atom and the substrate is mainly ionic together with partial orbital hybridization. To explore the stability of Lid on the C octagon, we calculate the diffusion barrier of Lid from L1 to L2 and that from L1 to L3, as shown in Figure 3 (where the structures of the initial, transition, and final states are displayed). Compared with the Li diffusion from a hollow site to the nearest one on perfect graphene performed by Zhou et al. [33], the energy barrier E for Li migration from L1 to L2 increases by 0.091 eV and the corresponding backward diffusion barrier decreases by 0.114 eV. A similar trend also exists for the diffusion path from L1 to L3. These results indicate that the Li adatom is more favorable to diffuse toward the C octagon site than to diffuse outward from the C octagon site. Therefore, the Li adatom could be trapped at the hollow site above the octagon ring of Li2 P, which is more stable than the Li adatom on the perfect graphene sheet. 3.2. Adsorption of two Li atoms on a Li2 P sheet We next consider the adsorption of Li atoms on both sides of the Li2 P sheet, 2Li-absorbed case. As observed in Figure 1b, there are three different sites for the second Li atom to be positioned on the
other side of Li2 P, which are below L1, L2 and L3. We denote these sites as L1 , L2 , and L3 , respectively. The configurations of the three cases associated with their corresponding Lic −Lid distances and the binding energies of the second Li adatom are shown in Figure 4. The L1 + L2 and L1 + L3 adsorptions have a slightly higher binding energy than the L1 + L1 adsorption, because the Couloumb repulsion in the L1 − L2 and L1 − L3 configurations is smaller than that in the L1 − L1 case. However, even if in the L1 − L2 and L1 − L3 configurations, the binding energies of the second Li are higher, they are not beneficial for the adsorption of H2 molecules. Since in these two cases, on one side of the sheet, the Lic − Lid distance is only 4.4 A˚ and 3.4 A˚ respectively (Figure 4b and c), which is too close for H2 adsorption. As a matter of fact, our calculations indicate that, in the L1 − L2 and L1 − L3 cases, Lic is only able to adsorb 3 H2 molecules due to the steric repulsion between Lic and L2 or L3 . However, in the L1 − L1 case, each Lic adsorbs 4 H2 molecules. Therefore, we will only consider the L1−L1 case in this work. The average charge of the C octagon in the 2Li adsorbed cases is now nearly doubled (−0.141e) compared with the previous single Li adsorption case because the second Li atoms contribute the charge transfer. 3.3. Hydrogen adsorption on the Li2 P·2Li complex We now investigate the interaction of a hydrogen molecule with the Li2 P·2Li sheet by adsorbing H2 . The hydrogen adsorption energy is defined as: En (H2 ) = E[Li2 P · 2Li@(n − 1)H2 ] + E(H2 ) − E[Li2 P · 2Li@nH2 ],
(2)
where En (H2 ) is the binding energy of the nth H2 molecule in the Li2 P·2Li system, E[Li2 P · 2Li @ nH2 ] is the total energy of the Li2 P·2Li system with n adsorbed H2 molecules, E(H2 ) is the total energy of a hydrogen molecule, and n indicates the number of H2 molecules. The configurations of H2 adsorbed at the Li2 P·2Li sheet are illustrated in Figure 5. The average distance between H2 molecule and Li, the binding energy of the nth H2 molecule, the average bond length of H2 and the Mulliken charge on the Li atom are shown in Figure 5 as well. Our calculations indicate that each Lic (Lid ) atom can attach
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Figure 3. The energy barriers E along the Li migration path of (a) L1 → L2 (b) L1 → L3 on Li2 P. In the paths, the initial, saddle-point, and final states are labeled with stars.
Figure 4. The configurations of two lithium (Lid ) atoms adsorbed at both sides of the Li2 P sheet with top view and side view: (a) L1 + L1 ; (b) L1 + L2 and (c) L1 + L3 . The ˚ are also shown. binding energies of Li and the Lic − Lid distances (in A)
Figure 5. The optimized atomic geometries of the Li2 P·2Li sheet with H2 molecules gradually adsorbed. Average distance between the center of the H2 molecules and Lic /Lid ˚ binding energy of the nth adsorbed H2 molecules En in eV, average bond length of H2 molecules (LH−H in A), ˚ and the Mulliken charge of Lic /Lid atoms (QLi in e) (L¯ H2 −Li in A), are given in the figure as well.
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Figure 6. H1 and H2 represent hydrogen atoms in one H2 molecule where the H1 atom is closer to the Li atoms. n(H2 ) indicates the nth H2 molecule attracted around Lic (Lid ) atom. (a) and (b) show the values of charges on H1 and H2 in Li2 P·2Li system with H2 molecules adsorbed on Lic or Lid site. H1 (H2) and H1 (H2 ) represent the hydrogen atom in the adsorbed H2 molecules on the two sides of the sheet.
Figure 7. (a) Variation of the charge on Lic (Lid ) when hydrogen molecules gradually attached. Lic (Lid ) and Lic (Lid ) represents the Lic (Lid ) atom on the two sides of the sheet.(b) Adsorption energy of hydrogen molecules. The line with magenta triangle is the result of prior work (Ref. [34]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
up to four (three) H2 molecules. The binding energies of the hydrogen molecules on Li2 P·2Li system (Figure 5) are in the range of 0.12–0.22 eV, which is excellent for hydrogen storage applications. As the number of H2 molecules adsorbed at each Li atom increases, the binding energy of the H2 molecule decreases and the average distance between Li and H2 molecules increases. The bond length ˚ We of the H2 molecule is slightly elongated from 0.749 A˚ to 0.755 A. also carried out the Mulliken population analysis on H2 molecules around Lic and Lid as shown in Figure 6, where the hydrogen atoms of the adsorbed H2 molecule at Lic and Lid are denoted as H1 and H2, and the corresponding Mulliken charges are denoted as Q(H1) and Q(H2). We now turn to the binding nature between H2 and Lic /Lid . Figure 5 exhibits that the bond length of each adsorbed H2 molecule is slightly elongated. This result occurs because the H2 molecules are polarized by the electric field of the positively charged Li atoms. However, does the polarization mechanism only work in the binding of H2 molecules? If a molecule is polarized in the electric field, its charge will redistribute and give rise to zero net charge. However, from Figure 6, besides charge redistribution (Q(H1) > 0, Q(H2) < 0; except for the first adsorbed H2 molecule), there is a net positive charge on H2 molecules (Q(H1) + Q(H2) > 0) both around Lic and Lid , which demonstrates that a small fraction of electrons transferred from H2 orbital to Li empty s orbital. This can be confirmed by Figure 7a that, the Lic /Lid becomes less positively charged by accepting electrons from the H2 molecules adsorbed on it. Therefore, the orbital interaction is another factor responsible for the binding of H2 molecule, though it is very weak. Furthermore, from Figure 7b, the binding energy of H2 molecules around Lic is apparently lower than that of H2 molecules around Lid . Consequently, the H2 adhesion mechanism should be different for Lic and Lid , which will be discussed separately.
As presented in Figure 6a, the net charge on the hydrogen molecules around Lic is less than 0.06e, and when four H2 molecules are adsorbed around Lic , the net charge of each H2 molecule is nearly zero. This result is consistent with the change of positive charge carried by Lic . In Figure 7a, when H2 molecules are gradually attached to Lic , the positive charge on Lic is almost unchanged, which is around 0.53e. It means that little charge is transferred from H2 molecules to Lic . This is also reflected in the very weak hybridization between H s and Lic p orbitals shown in Figure 8a. When the number of attached H2 molecules increases to 4, the hybridization between the H2 molecules and Lic nearly disappears. Thus, the binding energy of H2 molecules around Lic is lower than that of H2 molecules around Lid . That also makes the H2 − Lic distance slightly larger than H2 − Lid distance. On the contrary, for Lid , it bears more charge than Lic initially, which provides stronger polarization to H2 molecule resulting a higher binding energy of H2 . In addition, the charge donated from each attached H2 molecule to Lid is nearly unchanged (Figure 6b), i.e., 0.08 electrons. This result matches the variation of charge carried by Lid as displayed in Figure 7a, in which the positive charge of Lid decreases linearly with increasing the number of the adsorbed H2 molecules. This can be further proved by the PDOS of H2 and Lid in Figure 8b, in which stronger hybridization between H s and Lid p orbitals than that between H s and Lic p orbitals is exhibited. Due to such a slightly stronger hybridization, the H2 adsorption energy is higher than that of H2 molecules around Lid . In addition, the H s orbital broadens as the number of H2 molecules increases due to the interactions between H2 molecules. This phenomenon is not clearly observed for H2 adsorbed near Lic because of the large distance between H2 molecules. The binding mechanism of H2 molecules to Lid is very similar to other Li doped carbon based nanomaterials. In a previous work [34], the doped Li atom also can adsorb a
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Figure 8. The partial density of states (PDOS) of (a) one to four H2 molecules adsorbed to Lic and (b) one to three H2 molecules adsorbed to Lid in the Li2 P·Li system. The Fermi level (EF ) is set to zero.
maximum of three H2 molecules, and the pattern of H2 binding energy variation is similar as shown in Figure 7a. We now analyze the change of the bond length of the adsorbed H2 molecules. From Figure 5, we notice that the H2 bond length is
first increased and then decreased. This is due to the different H2 location pattern of the first adsorbed H2 molecule from the other. We know that the H2 bond elongation is due to the polarization by positively charged Li and arises from the different electrostatic
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force on the two hydrogen atoms. As shown in Figure 5, in the case of single H2 adsorption, the adsorbed molecule is parallel to the substrate and the hydrogen atoms are equidistant from Li atoms. As a result, the electrostatic forces on the two hydrogen are similar, so that the bond elongation is not obvious. Once the number of H2 adsorbed by each Li atom exceeds one, the adsorbed H2 molecules tend to tilt toward Li atoms. Consequently, the electrostatic forces on the two hydrogen atoms are different, resulting in the bond elongation. In addition, the bond elongation is slightly alleviated when the number of H2 molecules is more than two. That is because as the number of H2 molecules increases, the polarization on each H2 molecules is diminished and the elongation of the H2 bond length becomes less obvious. 4. Conclusions In summary, we have investigated the hydrogen storage capacity of Li-adsorbed lithium porphyrin sheet (Li2 P·2Li) by firstprinciples calculations. It is found that Li can bind strongly to the lithium-porphyrin sheet with a very large binding energy and further H2 molecules can be adsorbed on the Li atoms with a moderate binding energy between 0.12 eV and 0.22 eV. The binding nature of H2 around Lic (Lid ) is mainly electric polarization with a small part of orbital interaction. The binding of H2 molecules around Lic is weaker than that of H2 molecules around Lid due to fewer charge transfer from H2 to Lic than H2 to Lid or weaker hybridization of the H s and Lic p orbitals than that of H s and Lid p orbitals. Each Li atom can adsorb three or four H2 molecules, which leads to a hydrogen storage capacity of 7.95 wt%. The moderate binding energy and the large hydrogen storage capacity indicate that Li-adsorbed lithium porphyrin sheet is a very promising hydrogen storage material. Acknowledgements This work was supported by the special Funds for Major State Basic Research Project of China (973) under grant no. 2012CB933702, the NSFC under grant nos. 11104278, 11174284 and U1230202 (NSAF), Hefei Center for Physical Science and
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