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Ab initio insight into graphene nanofibers to destabilize hydrazine borane for hydrogen release
Chemical Physics Letters 669 (2017) 110–114
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Ab initio insight into graphene nanofibers to destabilize hydrazine borane for hydrogen release Zhao Qian a,b,c,⇑, Himanshu Raghubanshi d,e, M. Sterlin Leo Hudson d,f, O.N. Srivastava d, Xiangfa Liu a, Rajeev Ahuja c,g a
Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials (Ministry of Education), Shandong University, China Suzhou Institute of Shandong University, China Condensed Matter Theory, Department of Physics and Astronomy, Ångström Laboratory, Uppsala University, Sweden d Hydrogen Energy Centre, Department of Physics, Banaras Hindu University, India e Applied Chemistry and Nanoscience Laboratory, Department of Chemistry, Vaal University of Technology, South Africa f Department of Physics, Central University of Tamil Nadu, India g Department of Materials Science and Engineering, Royal Institute of Technology (KTH), Sweden b c
a r t i c l e
i n f o
Article history: Received 17 September 2016 In final form 19 December 2016 Available online 21 December 2016 Keywords: Energy storage Destabilization Nanostructured materials Density functional theory
a b s t r a c t We report the potential destabilizing effects of graphene nanofibers on the hydrogen release property of hydrazine borane via state-of-the-art ab initio calculations for the first time. Interactions of a hydrazine borane cluster with two types of graphene patch edges which exist abundantly in our synthesized graphene nanofibers have been investigated. It is found that both zigzag and armchair edges can greatly weaken the H-host bonds (especially the middle NAH bond) of hydrazine borane. The dramatic decrease in hydrogen removal energy is caused by the strong interaction between hydrazine borane and the graphene patch edges concerning the electronic charge density redistribution. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The availability of a sustainable energy resource has become one of the world’s biggest challenges with the development of society and the increasingly larger supply-demand gap of fossil fuels in the long run. Production and storage of clean or green fuels are being addressed and are at the center of attention for many scientists and engineers. Hydrogen fuel is one example discussed widely as a possible solution to the problems we are facing, although at the moment, many hurdles remain to be overcome before a realistic implementation becomes feasible. Besides the unsolved issue of large-scale hydrogen production via renewable energy, requirement of more reliable and economic materials and methods for hydrogen storage is the second biggest technical bottleneck to realize the green hydrogen economy [1–5]. As a kind of inorganic hydride material, Hydrazine borane (N2H4BH3) possesses hydrogen capacity as high as 15.4 wt%, which is a promising candidate for hydrogen storage applications [6–8]. For the direct thermal decomposition of hydrazine borane (HB in abbreviation), there had been no known very effective metal-free ⇑ Corresponding author at: Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials (Ministry of Education), Shandong University, China. E-mail address: [email protected] (Z. Qian). http://dx.doi.org/10.1016/j.cplett.2016.12.043 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
destabilizer reported yet to improve its hydrogen desorption. Carbon based nanomaterials, which generally have large specific surface areas, have been studied in other hydrogen storage systems to enhance the hydrogen sorption [9–11]. While in this letter, using ab initio atomistic modeling, we have theoretically explored the potential destabilizing effects of graphene nanofibers on the hydrogen release of hydrazine borane for the first time.
2. Methodology We have performed state-of-the-art density functional theory [12,13] calculations using the projector augmented wave (PAW) method [14], which is implemented in the Vienna Ab initio Simulation Package [15,16]. The generalized gradient approximation (PW91) [17] has been used. Geometry optimization has been fully done by minimizing the total energy of the supercell system with a conjugate gradient algorithm converged to less than 0.00001 eV. We have employed a plane wave basis set with an energy cutoff of 520 eV to describe the electronic wave function. Visualization for Electronic and Structural Analysis [18] has been used to visualize and analyze the structures. The supercell of 15 Å 20 Å 10 Å has been tested for convergence and been used. For the k-point, only a single Gamma-point is required. The PAW potentials with
Z. Qian et al. / Chemical Physics Letters 669 (2017) 110–114
the valence states 2s2p for C, 2s2p for B, 2s2p for N and 1s for H have been employed in the work. The results reported here have been successfully tested for convergence in terms of the cutoff energy, k-point samplings of the Brillouin Zone, etc. During the calculation of hydrogen removal energies, the ground state energy of a hydrogen atom has been employed instead of the total energy of a hydrogen molecule (thus what should be noted is, there would occur a data difference with respect to the bonding energy of two hydrogen atoms if the total energy of a hydrogen molecule is used). The hydrogen removal energy has been calculated considering the following equation (All the energies are the ground state values):
DE ¼ Eðthe system after one hydrogen atom is removedÞ þ EðHÞ Eðthe system before hydrogen removalÞ: The differential electron densities have been analyzed by solving the iterative problem through a self-consistent way using the Kohn-Sham ansatz:
qðrÞ ¼
X jwi ðrÞj2
ð1Þ
i
! h2 r2 þ V eff wi ðrÞ ¼ ei wi ðrÞ 2me Z qðr0 Þ 0 dEXC ½qr dr þ V eff ¼ V ext þ jr r0 j dqðrÞ Dq ¼ qðG þ HBÞ qðGÞ qðHBÞ
ð2Þ ð3Þ ð4Þ
In (4), the q(G + HB) stands for the charge density of the interacting system (graphene patch layer + hydrazine borane) after selfconsistent calculations; the q(G) and q(HB) mean the charge densities of the isolated graphene patch layer and hydrazine borane respectively (keeping the same supercell size and respective coordinates as they are in the interacting system). The Bader charge analysis [19–21] has been done to analyze the atomic charge states in the investigated materials. 3. Results and discussion Experimentally, we have already synthesized graphene nanofibers using Fe nanoparticles as growth catalyst (the experimental synthesis method and the morphology of the synthesized graphene nanofibers are placed in the Supplementary Material that includes Fig. S1). The noticeable feature is that the synthesized sample contains both the planar graphene nanofibers (PGNF) and the helical graphene nanofibers (HGNF), which is due to the usage of Fe nanoparticles of non-uniform shape as catalyst during the growth process. The growth of HGNF is due to the uniform distribution of faceted or polygonal shape catalyst particles whereas the growth of PGNF is due to spherical shape catalyst particles. While, it should be noted that both types of GNFs consist of stacked graphene patch layers although they differ in the way of stacking, which would cause the existence of large amounts of graphene edges at the surface of GNFs. Thus in this work, from the perspective of atomistic modeling we have focused on and investigated the effects of graphene patch edges on hydrogen release of hydrazine borane, which would help to predict the potential effects of these graphene nanofibers. Since the graphene edges can have two types (zigzag or armchair) and the edge structures and properties (such as electronic conductivity, etc.) of the two types are different, here in this study we have considered both types. Different local charge densities and electronic structures are also expected due to different atomic arrangements of the edge carbon atoms. To avoid the chemical effect of some heterogeneous species such as H, O and OH (the FTIR study of GNFs didn’t reveal the sig-
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nature of these species), we have employed a graphene patch as our model by fixing some of its edge atoms (represented by the circles ‘‘filled” with gray color in Fig. 1) to simulate their bonding with neighboring carbon atoms further inside the graphene nanofiber restricting their movement. Fig. 1 gives the ground state equilibrium structures of the graphene patch interacting with hydrazine borane cluster and its hydrogen removal products when we consider the effects of graphene zigzag edge. From Fig. 1(a) it can be seen that the hydrazine borane cluster stays far away from the zigzag edge after geometry optimization. When one hydrogen atom is removed from the boron site, after relaxation it can be seen from Fig. 1(b) that the boron atom has interacted with the most nearby carbon atom with a bond distance of 1.60 Å. The evident ‘‘pull-out” of the involving carbon atom and resulting distortion of the hexagon also reveals this. Fig. 1(c) shows the case when one hydrogen atom is removed from the middle nitrogen site of hydrazine borane. Surprisingly, after one hydrogen is removed from the middle nitrogen site, the boron atom also loses one hydrogen atom which finally bonds with one of the carbon atoms. Examination of the bond lengths reveals that the nearest CAN distance is 1.54 Å and the NAB bond length decreases to 1.54 Å compared with 1.62 Å of the case before hydrogen removal. Fig. 1(d) is the case of hydrogen removal from the terminal nitrogen site of hydrazine borane. Also interestingly, the removal has led to the leaving of one hydrogen atom from the middle nitrogen site. The bond length of NAN has found to decrease from the pristine 1.45 Å to 1.26 Å, which is the signature of the double bond forming. Fig. 2 illustrates the cases of hydrazine borane located near an armchair edge of the modeled graphene patch. Fig. 2(a) shows the relaxed configuration before any hydrogen removal has taken place. After one hydrogen atom is removed from the boron site of hydrazine borane, the relaxed configuration is shown in Fig. 2(b). The boron atom has bonded with the most nearby carbon atom with the length of 1.57 Å; what is interesting is that the removal leads to the leaving of another hydrogen atom from the boron site (also bonded with one carbon atom), which has helped to form the ‘‘hexagonal” shape of the local lattice together with the boron atom. The case of hydrogen removal from the middle nitrogen site of hydrazine borane at the graphene armchair edge is even more interesting. It can be shown in Fig. 2(c) that the removal has even caused the leaving of another two hydrogen atoms: one from the boron site and the other from the terminal nitrogen site. Both of two hydrogen atoms have bonded with carbon. The boron atom has also interacted with the most nearby carbon atom with a distance of 1.62 Å. Fig. 2(d) shows the equilibrium configuration after one hydrogen atom is removed from the terminal nitrogen site of hydrazine borane. Similar to the case in Fig. 1(d), there also occurs the leaving of one hydrogen atom from the middle nitrogen site and the great decrease of NAN bond length to 1.25 Å. The hydrogen removal energy can be used as a rough indicator of possibility and reaction temperature of hydrogen desorption [22,23]. In order to predict or estimate the potential effects of graphene nanofibers, we have calculated the single hydrogen removal energies based on the configurations displayed in the above Figs. 1 and 2. Fig. 3 shows the results compared with the case of pure hydrazine borane cluster (without support of carbon edges). It can be obviously seen that the graphene edges can help to decrease the hydrogen removal energy of hydrazine borane greatly regardless of the type of edges. Especially for the middle nitrogen site, it can be regarded that the graphene edges have weakened the middle NAH bond of hydrazine borane most (the smallest hydrogen removal energy value of 0.52 eV for zigzag case and that of 1.60 eV for armchair case, respectively). We have also performed some test calculations in which the H-saturated carbon atoms replaced the fixed atoms in pure carbon sheet model. The
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Fig. 1. The calculated equilibrium atomic structures of the graphene patch zigzag edges interacting with hydrazine borane and its hydrogen removal product (after one hydrogen atom is removed). (a) Before hydrogen removal from hydrazine borane; (b) after one hydrogen atom is removed from the boron site; (c) after one hydrogen atom is removed from the middle nitrogen site; (d) after one hydrogen atom is removed from the terminal nitrogen site. Gray: C atoms; green: B atoms; blue: N atoms; pink: H atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
results (placed in the Supplementary Materials) support the current model. Thus upon hydrogen desorption with graphene nanofibers, the middle NAH bond will probably become the firststep breaking one of hydrazine borane. The hydrogen removal energies from the BAH bond and from the terminal NAH bond of hydrazine borane have also decreased a lot. All of these results have implied the potential destabilizing effects of graphene nanofibers with large amounts of graphene edges (especially for the HGNF with larger surface areas and carbon edges compared with PGNF) on hydrogen release of the hydrazine borane material. To understand the reason for the decrease of hydrogen removal energies above, first of all we have investigated the bond lengths. While, no much difference has been found on the bond lengths when hydrazine borane is supported by graphene patches or not (Table 1). Then, we have investigated the profiles of the differential electronic charge densities reflecting the interactions between hydrazine borane and two types of graphene patch edges, which is shown in Fig. 4. Whether from Fig. 4a or from Fig. 4b, it can be obviously seen that there occur strong electronic interactions between the two materials. In the space between hydrazine borane and the graphene patch edges, there is a large increase of electronic charge densities (the yellow color stands for the charge accumulation), which means that the existence of graphene patch edges have actually affected the electronic charge distribution of the
systems. Especially in the area facing the middle NAH bond, the electronic charge density is increased most, which corresponds very well with the most weakening of the middle NAH bond already shown in Fig. 3. All of these have been more quantitatively reflected in the Bader charge state analysis (Table 2). Compared with the terminal N atom, the middle N atom is more electronegative, which is probably the underlying origin of the smaller hydrogen removal energy of middle NAH bond than that of terminal NAH bond. The second weakest H-host bond is BAH bond, in which the H atom is hydridic. The quantitative results when considering the circumstances in which the H-saturated carbon atoms replaced the fixed atoms in the current pure carbon sheet model show the same trend (the specific calculated results have been listed in the Supplementary Materials). Thus during hydrogen desorption, the hydridic Hd (from the BAH bond) and the protic Hd+ (from the middle NAH bond) would probably detach from hydrazine borane and combine to form one H2 molecule. 4. Summary and outlook In this letter, we have explored the potential destabilizing effects of graphene nanofibers on hydrazine borane for hydrogen release through state-of-the-art ab initio atomistic modeling for the first time. Using Density Functional Theory, we have calculated
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Z. Qian et al. / Chemical Physics Letters 669 (2017) 110–114
Fig. 2. The calculated equilibrium structures of graphene patch armchair edges interacting with hydrazine borane and its resultants (after removal of one hydrogen atom). (a) Before hydrogen removal; (b) after one hydrogen removal from the boron site; (c) after one hydrogen removal from the middle nitrogen site; (d) after one hydrogen removal from the terminal nitrogen site. Gray: C atoms; green: B atoms; blue: N atoms; pink: H atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The calculated average bond lengths of various H-host bonds in hydrazine borane.
Fig. 3. The hydrogen removal energies of pure hydrazine borane (HB) and the cases with support of graphene patch edges (considering both zigzag and armchair types).
BAH bond
The middle NAH bond
The terminal NAH bond
1.22 Å
1.03 Å
1.02 Å
the interactions between graphene patch edges (considering both zigzag edges and armchair edges) and the hydrazine borane cluster. The equilibrium structures of them before and after hydrogen removal have been illustrated respectively after geometry optimization. The hydrogen removal energies of the hydrazine borane when interacting with graphene zigzag or armchair edges have been found to decrease greatly compared with those of the pristine hydrazine borane. Especially the middle NAH bond of hydrazine borane, which has been weakened most, would probably cause the first step of hydrogen desorption. These large decrease in hydrogen removal energies, which is caused by the strong electronic interactions between N2H4BH3 and graphene patch edges, can be considered as an indicator of potential destabilizing effects of graphene nanofibers on hydrazine borane for hydrogen release. More experimental investigations of the related materials are proposed and encouraged in the future.
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Z. Qian et al. / Chemical Physics Letters 669 (2017) 110–114
Fig. 4. The differential electronic charge densities on interactions between hydrazine borane and two kinds of graphene patch edges. (a) Zigzag type; (b) armchair type. Yellow: charge accumulation; cyan: depletion. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 The calculated Bader charge states of atoms in hydrazine borane when it is facing graphene zigzag or armchair edge. Atom
Zigzag case
Armchair case
B HB The middle N Hmiddle The terminal N Hterminal
+1.71 0.59 0.90 +0.43 0.75 +0.42
+1.69 0.58 0.92 +0.45 0.79 +0.44
Acknowledgments We would like to thank the support from Jiangsu Province Science Foundation for Youths (BK20160370), the Natural Science Foundation of Shandong Province (ZR2015EQ012), the Fundamental Research Fund of Shandong University (2015TB001), China Postdoctoral Science Foundation (2015M572028) and the Swedish Research Council (VR) and Swedish Research Link (SRL) program. The Natural Science Foundation of China (51571133) and Shandong Postdoctoral Innovation Program (201602019) are also gratefully acknowledged. Appendix A. Supplementary material The experimental synthesis method, the morphology of the synthesized graphene nanofibers and some theoretical results are placed in the Supplementary Materials. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.12.043. References [1] L. Schlapbach, A. Züttel, Hydrogen-storage materials for mobile applications, Nature 414 (2001) 353–358. [2] P. Chen, M. Zhu, Recent progress in hydrogen storage, Mater. Today 11 (2008) 36–43.
[3] G.W. Crabtree, M.S. Dresselhaus, M.V. Buchanan, The hydrogen economy, Phys. Today 57 (2004) 39–49. [4] C. Weidenthaler, M. Felderhoff, Hydrogen storage for mobile applications – Quo Vadis?, Energy Environ Sci. 4 (2011) 2495–2502. [5] S. Takagi, S. Orimo, Recent progress in hydrogen-rich materials from the perspective of bonding flexibility of hydrogen, Scripta Mater. 109 (2015) 1–5. [6] T. Hügle, M.F. Kühnel, D. Lentz, Hydrazine borane: a promising hydrogen storage material, J. Am. Chem. Soc. 131 (2009) 7444–7446. [7] J. Hannauer, O. Akdim, U.B. Demirci, C. Geantet, J.M. Herrmann, P. Miele, et al., High-extent dehydrogenation of hydrazine borane N2H4BH3 by hydrolysis of BH3 and decomposition of N2H4, Energy Environ. Sci. 4 (2011) 3355–3358. [8] R. Moury, U.B. Demirci, Hydrazine borane and hydrazinidoboranes as chemical hydrogen storage materials, Energies 8 (2015) 3118–3141. [9] P.A. Berseth, A.G. Harter, R. Zidan, A. Blomqvist, C.M. Araújo, R.H. Scheicher, R. Ahuja, P. Jena, Carbon nanomaterials as catalysts for hydrogen uptake and release in NaAlH4, Nano Lett. 9 (2009) 1501–1505. [10] R.H. Scheicher, S. Li, C.M. Araujo, A. Blomqvist, R. Ahuja, P. Jena, Theoretical study of C60 as catalyst for dehydrogenation in LiBH4, Nanotechnology 22 (2011) 335401. [11] Z. Qian, S. Li, B. Pathak, C.M. Araujo, R. Ahuja, P. Jena, C60-mediated hydrogen desorption in Li–N–H systems, Nanotechnology 23 (2012) 485406. [12] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Phys. Rev. 136 (1964) B864–B871. [13] W. Kohn, L.J. Sham, Self-consistent equations including exchange and correlation effects, Phys. Rev. 140 (1965) A1133–A1138. [14] P.E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 17953– 17979. [15] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169– 11186. [16] G. Kresse, J. Hafner, Ab initio molecular dynamics for open-shell transition metals, Phys. Rev. B 48 (1993) 13115–13118. [17] J.P. Perdew, Y. Wang, Accurate and simple analytic representation of the electron-gas correlation energy, Phys. Rev. B 45 (1992) 13244–13249. [18] K. Momma, F. Izumi, VESTA: a three-dimensional visualization system for electronic and structural analysis, J. Appl. Crystallogr. 41 (2008) 653–658. [19] R.F.W. Bader, Atoms in Molecules: A Quantum Theory, International Series of Monographs on Chemistry, Clarendon, Oxford, 1990. [20] G. Henkelman, A. Arnaldsson, H. Jónsson, A fast and robust algorithm for Bader decomposition of charge density, Comput. Mater. Sci. 36 (2006) 354–360. [21] E. Sanville, S.D. Kenny, R. Smith, G. Henkelman, An improved grid-based algorithm for Bader charge allocation, J. Comput. Chem. 28 (2007) 899–908. [22] C.R. Miranda, G.J. Ceder, Ab initio investigation of ammonia-borane complexes for hydrogen storage, J. Chem. Phys. 126 (2007) 184703–184711. [23] C. Zhang, A. Alavi, A first-principles investigation of LiNH(2) as a hydrogenstorage material: effects of substitutions of K and Mg for Li, J. Phys. Chem. B 110 (2006) 7139–7143.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Ab initio insight into graphene nanofibers to destabilize hydrazine borane for hydrogen release Zhao Qian a,b,c,⇑, Himanshu Raghubanshi d,e, M. Sterlin Leo Hudson d,f, O.N. Srivastava d, Xiangfa Liu a, Rajeev Ahuja c,g a
Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials (Ministry of Education), Shandong University, China Suzhou Institute of Shandong University, China Condensed Matter Theory, Department of Physics and Astronomy, Ångström Laboratory, Uppsala University, Sweden d Hydrogen Energy Centre, Department of Physics, Banaras Hindu University, India e Applied Chemistry and Nanoscience Laboratory, Department of Chemistry, Vaal University of Technology, South Africa f Department of Physics, Central University of Tamil Nadu, India g Department of Materials Science and Engineering, Royal Institute of Technology (KTH), Sweden b c
a r t i c l e
i n f o
Article history: Received 17 September 2016 In final form 19 December 2016 Available online 21 December 2016 Keywords: Energy storage Destabilization Nanostructured materials Density functional theory
a b s t r a c t We report the potential destabilizing effects of graphene nanofibers on the hydrogen release property of hydrazine borane via state-of-the-art ab initio calculations for the first time. Interactions of a hydrazine borane cluster with two types of graphene patch edges which exist abundantly in our synthesized graphene nanofibers have been investigated. It is found that both zigzag and armchair edges can greatly weaken the H-host bonds (especially the middle NAH bond) of hydrazine borane. The dramatic decrease in hydrogen removal energy is caused by the strong interaction between hydrazine borane and the graphene patch edges concerning the electronic charge density redistribution. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The availability of a sustainable energy resource has become one of the world’s biggest challenges with the development of society and the increasingly larger supply-demand gap of fossil fuels in the long run. Production and storage of clean or green fuels are being addressed and are at the center of attention for many scientists and engineers. Hydrogen fuel is one example discussed widely as a possible solution to the problems we are facing, although at the moment, many hurdles remain to be overcome before a realistic implementation becomes feasible. Besides the unsolved issue of large-scale hydrogen production via renewable energy, requirement of more reliable and economic materials and methods for hydrogen storage is the second biggest technical bottleneck to realize the green hydrogen economy [1–5]. As a kind of inorganic hydride material, Hydrazine borane (N2H4BH3) possesses hydrogen capacity as high as 15.4 wt%, which is a promising candidate for hydrogen storage applications [6–8]. For the direct thermal decomposition of hydrazine borane (HB in abbreviation), there had been no known very effective metal-free ⇑ Corresponding author at: Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials (Ministry of Education), Shandong University, China. E-mail address: [email protected] (Z. Qian). http://dx.doi.org/10.1016/j.cplett.2016.12.043 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
destabilizer reported yet to improve its hydrogen desorption. Carbon based nanomaterials, which generally have large specific surface areas, have been studied in other hydrogen storage systems to enhance the hydrogen sorption [9–11]. While in this letter, using ab initio atomistic modeling, we have theoretically explored the potential destabilizing effects of graphene nanofibers on the hydrogen release of hydrazine borane for the first time.
2. Methodology We have performed state-of-the-art density functional theory [12,13] calculations using the projector augmented wave (PAW) method [14], which is implemented in the Vienna Ab initio Simulation Package [15,16]. The generalized gradient approximation (PW91) [17] has been used. Geometry optimization has been fully done by minimizing the total energy of the supercell system with a conjugate gradient algorithm converged to less than 0.00001 eV. We have employed a plane wave basis set with an energy cutoff of 520 eV to describe the electronic wave function. Visualization for Electronic and Structural Analysis [18] has been used to visualize and analyze the structures. The supercell of 15 Å 20 Å 10 Å has been tested for convergence and been used. For the k-point, only a single Gamma-point is required. The PAW potentials with
Z. Qian et al. / Chemical Physics Letters 669 (2017) 110–114
the valence states 2s2p for C, 2s2p for B, 2s2p for N and 1s for H have been employed in the work. The results reported here have been successfully tested for convergence in terms of the cutoff energy, k-point samplings of the Brillouin Zone, etc. During the calculation of hydrogen removal energies, the ground state energy of a hydrogen atom has been employed instead of the total energy of a hydrogen molecule (thus what should be noted is, there would occur a data difference with respect to the bonding energy of two hydrogen atoms if the total energy of a hydrogen molecule is used). The hydrogen removal energy has been calculated considering the following equation (All the energies are the ground state values):
DE ¼ Eðthe system after one hydrogen atom is removedÞ þ EðHÞ Eðthe system before hydrogen removalÞ: The differential electron densities have been analyzed by solving the iterative problem through a self-consistent way using the Kohn-Sham ansatz:
qðrÞ ¼
X jwi ðrÞj2
ð1Þ
i
! h2 r2 þ V eff wi ðrÞ ¼ ei wi ðrÞ 2me Z qðr0 Þ 0 dEXC ½qr dr þ V eff ¼ V ext þ jr r0 j dqðrÞ Dq ¼ qðG þ HBÞ qðGÞ qðHBÞ
ð2Þ ð3Þ ð4Þ
In (4), the q(G + HB) stands for the charge density of the interacting system (graphene patch layer + hydrazine borane) after selfconsistent calculations; the q(G) and q(HB) mean the charge densities of the isolated graphene patch layer and hydrazine borane respectively (keeping the same supercell size and respective coordinates as they are in the interacting system). The Bader charge analysis [19–21] has been done to analyze the atomic charge states in the investigated materials. 3. Results and discussion Experimentally, we have already synthesized graphene nanofibers using Fe nanoparticles as growth catalyst (the experimental synthesis method and the morphology of the synthesized graphene nanofibers are placed in the Supplementary Material that includes Fig. S1). The noticeable feature is that the synthesized sample contains both the planar graphene nanofibers (PGNF) and the helical graphene nanofibers (HGNF), which is due to the usage of Fe nanoparticles of non-uniform shape as catalyst during the growth process. The growth of HGNF is due to the uniform distribution of faceted or polygonal shape catalyst particles whereas the growth of PGNF is due to spherical shape catalyst particles. While, it should be noted that both types of GNFs consist of stacked graphene patch layers although they differ in the way of stacking, which would cause the existence of large amounts of graphene edges at the surface of GNFs. Thus in this work, from the perspective of atomistic modeling we have focused on and investigated the effects of graphene patch edges on hydrogen release of hydrazine borane, which would help to predict the potential effects of these graphene nanofibers. Since the graphene edges can have two types (zigzag or armchair) and the edge structures and properties (such as electronic conductivity, etc.) of the two types are different, here in this study we have considered both types. Different local charge densities and electronic structures are also expected due to different atomic arrangements of the edge carbon atoms. To avoid the chemical effect of some heterogeneous species such as H, O and OH (the FTIR study of GNFs didn’t reveal the sig-
111
nature of these species), we have employed a graphene patch as our model by fixing some of its edge atoms (represented by the circles ‘‘filled” with gray color in Fig. 1) to simulate their bonding with neighboring carbon atoms further inside the graphene nanofiber restricting their movement. Fig. 1 gives the ground state equilibrium structures of the graphene patch interacting with hydrazine borane cluster and its hydrogen removal products when we consider the effects of graphene zigzag edge. From Fig. 1(a) it can be seen that the hydrazine borane cluster stays far away from the zigzag edge after geometry optimization. When one hydrogen atom is removed from the boron site, after relaxation it can be seen from Fig. 1(b) that the boron atom has interacted with the most nearby carbon atom with a bond distance of 1.60 Å. The evident ‘‘pull-out” of the involving carbon atom and resulting distortion of the hexagon also reveals this. Fig. 1(c) shows the case when one hydrogen atom is removed from the middle nitrogen site of hydrazine borane. Surprisingly, after one hydrogen is removed from the middle nitrogen site, the boron atom also loses one hydrogen atom which finally bonds with one of the carbon atoms. Examination of the bond lengths reveals that the nearest CAN distance is 1.54 Å and the NAB bond length decreases to 1.54 Å compared with 1.62 Å of the case before hydrogen removal. Fig. 1(d) is the case of hydrogen removal from the terminal nitrogen site of hydrazine borane. Also interestingly, the removal has led to the leaving of one hydrogen atom from the middle nitrogen site. The bond length of NAN has found to decrease from the pristine 1.45 Å to 1.26 Å, which is the signature of the double bond forming. Fig. 2 illustrates the cases of hydrazine borane located near an armchair edge of the modeled graphene patch. Fig. 2(a) shows the relaxed configuration before any hydrogen removal has taken place. After one hydrogen atom is removed from the boron site of hydrazine borane, the relaxed configuration is shown in Fig. 2(b). The boron atom has bonded with the most nearby carbon atom with the length of 1.57 Å; what is interesting is that the removal leads to the leaving of another hydrogen atom from the boron site (also bonded with one carbon atom), which has helped to form the ‘‘hexagonal” shape of the local lattice together with the boron atom. The case of hydrogen removal from the middle nitrogen site of hydrazine borane at the graphene armchair edge is even more interesting. It can be shown in Fig. 2(c) that the removal has even caused the leaving of another two hydrogen atoms: one from the boron site and the other from the terminal nitrogen site. Both of two hydrogen atoms have bonded with carbon. The boron atom has also interacted with the most nearby carbon atom with a distance of 1.62 Å. Fig. 2(d) shows the equilibrium configuration after one hydrogen atom is removed from the terminal nitrogen site of hydrazine borane. Similar to the case in Fig. 1(d), there also occurs the leaving of one hydrogen atom from the middle nitrogen site and the great decrease of NAN bond length to 1.25 Å. The hydrogen removal energy can be used as a rough indicator of possibility and reaction temperature of hydrogen desorption [22,23]. In order to predict or estimate the potential effects of graphene nanofibers, we have calculated the single hydrogen removal energies based on the configurations displayed in the above Figs. 1 and 2. Fig. 3 shows the results compared with the case of pure hydrazine borane cluster (without support of carbon edges). It can be obviously seen that the graphene edges can help to decrease the hydrogen removal energy of hydrazine borane greatly regardless of the type of edges. Especially for the middle nitrogen site, it can be regarded that the graphene edges have weakened the middle NAH bond of hydrazine borane most (the smallest hydrogen removal energy value of 0.52 eV for zigzag case and that of 1.60 eV for armchair case, respectively). We have also performed some test calculations in which the H-saturated carbon atoms replaced the fixed atoms in pure carbon sheet model. The
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Fig. 1. The calculated equilibrium atomic structures of the graphene patch zigzag edges interacting with hydrazine borane and its hydrogen removal product (after one hydrogen atom is removed). (a) Before hydrogen removal from hydrazine borane; (b) after one hydrogen atom is removed from the boron site; (c) after one hydrogen atom is removed from the middle nitrogen site; (d) after one hydrogen atom is removed from the terminal nitrogen site. Gray: C atoms; green: B atoms; blue: N atoms; pink: H atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
results (placed in the Supplementary Materials) support the current model. Thus upon hydrogen desorption with graphene nanofibers, the middle NAH bond will probably become the firststep breaking one of hydrazine borane. The hydrogen removal energies from the BAH bond and from the terminal NAH bond of hydrazine borane have also decreased a lot. All of these results have implied the potential destabilizing effects of graphene nanofibers with large amounts of graphene edges (especially for the HGNF with larger surface areas and carbon edges compared with PGNF) on hydrogen release of the hydrazine borane material. To understand the reason for the decrease of hydrogen removal energies above, first of all we have investigated the bond lengths. While, no much difference has been found on the bond lengths when hydrazine borane is supported by graphene patches or not (Table 1). Then, we have investigated the profiles of the differential electronic charge densities reflecting the interactions between hydrazine borane and two types of graphene patch edges, which is shown in Fig. 4. Whether from Fig. 4a or from Fig. 4b, it can be obviously seen that there occur strong electronic interactions between the two materials. In the space between hydrazine borane and the graphene patch edges, there is a large increase of electronic charge densities (the yellow color stands for the charge accumulation), which means that the existence of graphene patch edges have actually affected the electronic charge distribution of the
systems. Especially in the area facing the middle NAH bond, the electronic charge density is increased most, which corresponds very well with the most weakening of the middle NAH bond already shown in Fig. 3. All of these have been more quantitatively reflected in the Bader charge state analysis (Table 2). Compared with the terminal N atom, the middle N atom is more electronegative, which is probably the underlying origin of the smaller hydrogen removal energy of middle NAH bond than that of terminal NAH bond. The second weakest H-host bond is BAH bond, in which the H atom is hydridic. The quantitative results when considering the circumstances in which the H-saturated carbon atoms replaced the fixed atoms in the current pure carbon sheet model show the same trend (the specific calculated results have been listed in the Supplementary Materials). Thus during hydrogen desorption, the hydridic Hd (from the BAH bond) and the protic Hd+ (from the middle NAH bond) would probably detach from hydrazine borane and combine to form one H2 molecule. 4. Summary and outlook In this letter, we have explored the potential destabilizing effects of graphene nanofibers on hydrazine borane for hydrogen release through state-of-the-art ab initio atomistic modeling for the first time. Using Density Functional Theory, we have calculated
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Fig. 2. The calculated equilibrium structures of graphene patch armchair edges interacting with hydrazine borane and its resultants (after removal of one hydrogen atom). (a) Before hydrogen removal; (b) after one hydrogen removal from the boron site; (c) after one hydrogen removal from the middle nitrogen site; (d) after one hydrogen removal from the terminal nitrogen site. Gray: C atoms; green: B atoms; blue: N atoms; pink: H atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The calculated average bond lengths of various H-host bonds in hydrazine borane.
Fig. 3. The hydrogen removal energies of pure hydrazine borane (HB) and the cases with support of graphene patch edges (considering both zigzag and armchair types).
BAH bond
The middle NAH bond
The terminal NAH bond
1.22 Å
1.03 Å
1.02 Å
the interactions between graphene patch edges (considering both zigzag edges and armchair edges) and the hydrazine borane cluster. The equilibrium structures of them before and after hydrogen removal have been illustrated respectively after geometry optimization. The hydrogen removal energies of the hydrazine borane when interacting with graphene zigzag or armchair edges have been found to decrease greatly compared with those of the pristine hydrazine borane. Especially the middle NAH bond of hydrazine borane, which has been weakened most, would probably cause the first step of hydrogen desorption. These large decrease in hydrogen removal energies, which is caused by the strong electronic interactions between N2H4BH3 and graphene patch edges, can be considered as an indicator of potential destabilizing effects of graphene nanofibers on hydrazine borane for hydrogen release. More experimental investigations of the related materials are proposed and encouraged in the future.
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Fig. 4. The differential electronic charge densities on interactions between hydrazine borane and two kinds of graphene patch edges. (a) Zigzag type; (b) armchair type. Yellow: charge accumulation; cyan: depletion. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 The calculated Bader charge states of atoms in hydrazine borane when it is facing graphene zigzag or armchair edge. Atom
Zigzag case
Armchair case
B HB The middle N Hmiddle The terminal N Hterminal
+1.71 0.59 0.90 +0.43 0.75 +0.42
+1.69 0.58 0.92 +0.45 0.79 +0.44
Acknowledgments We would like to thank the support from Jiangsu Province Science Foundation for Youths (BK20160370), the Natural Science Foundation of Shandong Province (ZR2015EQ012), the Fundamental Research Fund of Shandong University (2015TB001), China Postdoctoral Science Foundation (2015M572028) and the Swedish Research Council (VR) and Swedish Research Link (SRL) program. The Natural Science Foundation of China (51571133) and Shandong Postdoctoral Innovation Program (201602019) are also gratefully acknowledged. Appendix A. Supplementary material The experimental synthesis method, the morphology of the synthesized graphene nanofibers and some theoretical results are placed in the Supplementary Materials. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.12.043. References [1] L. Schlapbach, A. Züttel, Hydrogen-storage materials for mobile applications, Nature 414 (2001) 353–358. [2] P. Chen, M. Zhu, Recent progress in hydrogen storage, Mater. Today 11 (2008) 36–43.
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