The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage

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The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage Zhi Yang a,b,*, Donghong Wang b, Li-Chun Xu b, Xuguang Liu c,d,**, Xiuyan Li b, Bingshe Xu c a

Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China b College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China c Key Lab of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China d College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

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abstract

Article history:

Based on density functional theory, the capacities of FeC5H5, Fe2C5H5 and one-dimensional

Received 6 August 2014

(FeC5H5)∞ nanowire as hydrogen storage media were investigated. The results show that

Received in revised form

FeC5H5 and Fe2C5H5 can adsorb five and ten H2 molecules, respectively, and form stable

20 September 2014

FeC5H5(H2)5 and Fe2C5H5(H2)10 systems. The hydrogen storage capacities of the two systems

Accepted 22 September 2014

are 7.63 wt% and 10.15 wt%, while the average adsorption energies are 0.49 and 0.73 eV/H2,

Available online xxx

indicating that FeC5H5 and Fe2C5H5 are excellent hydrogen storage media. In addition, (FeC5H5)∞ nanowire can also adsorb H2 molecules (1.62 wt%). Most importantly, the mag-

Keywords:

netic and electrical properties of the nanowire are sensitive to the additional H2, thus

Hydrogen storage

(FeC5H5)∞ can be used for selecting and detecting H2 molecules.

Density functional theory

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Hydrocarbon

Introduction Hydrogen has been considered as a clean alternative energy carrier because of its efficiency, abundance, and environmental friendliness. In traditional methods, hydrogen could be stored by liquefaction, compression or adsorption in complex hydrides and metallic compounds [1]. Although hydrogen capacity may be high through the traditional ways, it will consume a lot of energy in liquefaction and low temperature storage, and probably cause safety problems.

reserved.

The search for promising hydrogen storage materials has attracted increased attention. Materials suitable for hydrogen storage, however, must meet some rigid requirements, such as high gravimetric and volumetric density, and sufficient kinetic and thermal stabilities [2]. On the one hand, the newest target of the US Department of Energy (DOE) for the ideal hydrogen storage material is that the gravimetric storage capacity of hydrogen should reach 5.5 wt% by 2017 [3]. On the other hand, in general, materials that bind hydrogen molecularly with an adsorption energy intermediate between physisorbed and chemisorbed states (0.2e0.6 eV) are necessary.

* Corresponding author. College of Physics and Optoelectronics, Taiyuan University of Technology, Yingze Road, Taiyuan 030024, China. ** Corresponding author. Key Lab of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China. E-mail addresses: [email protected], [email protected] (Z. Yang), [email protected] (X. Liu). http://dx.doi.org/10.1016/j.ijhydene.2014.09.125 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Yang Z, et al., The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.125

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Recently, a large amount of effort has been made to study hydrogen adsorption on carbon-based nanostructures, because of their light weight and high surface-to-volume ratio [4e15]. For example, Li et al. investigated the hydrogen storage of Lidoped fluorinated graphene [4]. They found that such twodimensional (2D) LieF-graphene system may be a good candidate for efficient hydrogen storage. Furthermore, different types of one-dimensional (1D) carbon nanotubes were explored to store hydrogen [7e9]. In addition to 2D and 1D carbon materials, C60 molecule has also been widely studied for the application in hydrogen storage [10e15]. For instance, Chandrakumar and his co-workers found that Na8C60 can adsorb 48 H2 molecules (9.5 wt%) with adsorption energy in the range of 0.149e0.159 eV/H2 [15]. It is well known that transition metal (TM) atoms have novel physical and chemical properties and may interact with H2 to form rich TM-H2 coordinate bonds. C60 sphere coated with TM atom has been found to be able to store large quantities of hydrogen [11]. However, most TM atoms tend to form clusters on the surface of C60 [13], and the segregation process is disadvantageous for hydrogen storage. Instead of TM-C60 systems, very recently, small hydrocarbons decorated with TM atoms have attracted much more attention for the use as possible hydrogen storage materials [16e18]. A typical molecule is C5H5. Lei et al. suggested that LaC5H5, EuC5H5 or HoC5H5 can bind six H2 molecules [16]. Nevertheless, because La, Eu or Ho are rare earth element with large atomic mass, the corresponding gravimetric storage capacities are low, only about 5 wt%. In another study [17], it is found that TiC5Hþ 5 may capture five H2 molecules. Although the gravimetric storage capacity is as high as 8.1 wt%, the positive charge state of the system is unstable in actual materials. Therefore, to find more suitable hydrogen storage medium, new TM-C5H5 system should be further considered. In this paper, the capacity of FeC5H5, Fe2C5H5, and 1D (FeC5H5)∞ nanowire as hydrogen storage media is studied by using density functional theory (DFT). The reason for studying these systems is based on the following consideration. Different types of Fem(C5H5)n (m and n are integers) molecules have been synthesized in experiment [19e21]. Since ferrocene Fe(C5H5)2 is stable and satisfies 18-electron rule [22], it is expected that FeC5H5 and Fe2C5H5 may capture H2 molecules to form stable structures. Indeed, our results show that FeC5H5 and Fe2C5H5 could be used as a new kind of hydrogen storage material with high gravimetric density. Furthermore, (FeC5H5)∞ also has hydrogen storage capability and, most importantly, the magnetic and electrical properties of (FeC5H5)∞ are dependent on the number of H2. Such novel property is very useful for selecting and detecting H2 molecules.

described by DFT-D Grimme scheme [28]. Double numericpolarized basis sets (DNP), supplemented with polarization functions, were used for all of the atoms. In the self-consistent field calculations, the electronic-density convergence is set to ˚ 3. The convergence criteria during geometry optimi105 e/A zation without any symmetry constraints are 105 Hartree for ˚ 3 for the force, and 103 A ˚ for atomic the energy, 103 Hartree/A displacements. For small molecules, all possible structures and spin states were considered, and the vibrational frequencies were analyzed to ensure that the optimized structures are stable. For 1D (FeC5H5)∞ nanowire, a simple cubic supercell ˚  20 A ˚ cA ˚ (c is the periodic length of the 1D system) was 20 A employed to ensure that interaction between the supercells could be negligible. After careful test, the Brillouin zone is sampled by 1  1  21 special mesh points in k-space based on the Monkhorst-Pack technology [29]. To test the validity of the above calculated parameters, the structures of free FeC5H5, Fe2C5H5, Fe(C5H5)2 and (FeC5H5)∞ were optimized, as shown in Fig. 1. The calculated ground state of Fe(C5H5)2 is a spin singlet state with D5h symmetry, which is in agreement with previous report [30]. Also, the periodic length c of (FeC5H5)∞ obtained in present study, ˚ , is close to the value of 3.41 A ˚ proposed by Shen and his 3.46 A co-workers [31]. Therefore, present calculated parameters are reasonable and reliable, and can be applied to the larger systems. In addition, because the focus of present study is the adsorption behavior of H2 molecule, the case of H atom is not considered here.

Results and discussion H2 molecule adsorption on FeC5H5 or Fe2C5H5 In order to identify the thermal stabilities of FeC5H5 and Fe2C5H5, the binding energy Eb of two molecules is calculated from the following equation.

Computational method The geometry optimization of FeC5H5, Fe2C5H5, and (FeC5H5)∞ was performed by using spin-polarized DFT method as implemented in DMOL3 package [23,24]. The exchangecorrelation functional was treated within the generalized gradient approximation (GGA) proposed by Perdew, Burke and Ernzerhof (PBE) [25e27]. The noncovalent forces, including hydrogen bond and van der Waals interactions, were

Fig. 1 e The ground-state structures of FeC5H5, Fe2C5 H5, Fe(C5H5)2 and (FeC5H5)∞ c is the periodic length of the nanowire.

Please cite this article in press as: Yang Z, et al., The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.125

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Fig. 2 e (a) The ground state of FeC5H5(H2)n (n ¼ 1e5). (b) The ground state and meta-stable isomers of Fe2C5H5(H2)n (n ¼ 1e10). For a given Fe2C5H5(H2)n, the first isomer is the ground state. DE is the adsorption energy. The point group of each compound is also given.

Please cite this article in press as: Yang Z, et al., The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.125

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Eb ¼ mEðFeÞ þ EðC5 H5 Þ  EðmoleculeÞ

(1)

where E($) is the total energy of Fe atom, free C5H5 and FeC5H5 (m ¼ 1) or Fe2C5H5 (m ¼ 2). The calculated binding energies of FeC5H5 and Fe2C5H5 are 3.71 and 5.10 eV, indicating the two molecules have high thermal stabilities and can be used as hydrogen storage media. The ground-state structures of FeC5H5(H2)n and Fe2C5H5(H2)n are depicted in Fig. 2. Generally speaking, the C5H5 ring is still planar after H2 adsorption, and the structural deformation can be neglected. We considered all possible spin states and found that FeC5H5(H2)n and Fe2C5H5(H2)n are spin doublet states, independent of n. Correspondingly, the magnetic moments of two systems are 1 mB and are not affected by additional H2. Next we first discuss the hydrogen storage capability of FeC5H5. From Fig. 2, one can see that FeC5H5 can adsorb at most five H2 molecules and form a stable C5v FeC5H5(H2)5 compound. When the sixth H2 is attached to FeC5H5(H2)5, we found the separation distance between the Fe atom and the H2 molecule ˚ , thus the sixth H2 molecule may not be increases to about 5.60 A useful as far as hydrogen storage is concerned. Nevertheless, adsorption of five H2 molecules corresponds to 7.63 wt% hydrogen, which is higher than the value of LaC5H5(H2)6 (5.56 wt %), EuC5H5(H2)6 (5.24 wt%) or HoC5H5(H2)6 (4.94 wt%) [16]. As mentioned before, an ideal storage material would not only have high gravimetric density but also involve molecular adsorption of hydrogen with a binding-energy intermediate between that of physisorption and chemisorption. Here, the adsorption energy DE by successive additions of H2 molecules to FeC5H5 was calculated from the following equation.     DE ¼ E FeC5 H5 ðH2 Þn1 þ EðH2 Þ  E FeC5 H5 ðH2 Þn

(2)

where E($) is the total energy of FeC5H5(H2)n1, H2 and FeC5H5(H2)n. The calculated DE is given in Fig. 2. It is obvious that DE is in a reasonable and required range, from 0.23 to 0.84 eV. To further illustrate this point, according to Equation (2), the average adsorption energy DE is defined as DE ¼ SDE=nmax

(3)

where nmax is the maximum number of H2 molecule in the system. For FeC5H5, nmax is five. The calculated DE of FeC5H5 is 0.49 eV/H2, well in the ideal energy window of 0.2e0.6 eV. In addition, the positive DE and DE also ensure that FeC5H5(H2)n is thermal stable. The gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is given in Fig. 3. Generally speaking, HOMO-LUMO gap can reflect the kinetic stability of a molecule or cluster [32]. Although the gap of FeC5H5(H2)n gradually decreases with n, on the whole, FeC5H5(H2)n has large gap (> 1 eV). It is well known that at a given temperature T, the energy fluctuation due to thermal excitation is kBT, where kB is Boltzmann's constant. At room temperature T ¼ 300 K, the energy fluctuation is only about 0.026 eV. Therefore, FeC5H5(H2)n has relatively high kinetic stability even at room temperature. In addition, based on the frequency calculation, the average Gibbs free energy DG of FeC5H5(H2)5 at 300 K was predicted to be 11.79 kcal/mol. The negative sign of the free energy indicates that the spontaneous formation of the FeC5H5(H2)5 is permitted.

Fig. 3 e The HOMO-LUMO gap of (a) FeC5H5(H2)n (n ¼ 1e5) and (b) Fe2C5H5(H2)n (n ¼ 1e10).

When the first H2 molecule is adsorbed on FeC5H5, two H atoms both tend to interact with Fe atom, forming two FeeH ˚ , and bonds. The lengths of the FeeH bonds are 1.61 and 1.62 A are nearly the same. Recently, Xie et al. investigated the adsorption behavior of H2 on body-centered cubic (bcc) Fe (110) surface [33]. They found that, at very low coverage, the average ˚ , which is close to our result, indicating FeeH distance is 1.603 A that present model can give a reasonable description of the FeeH interaction. Mulliken charge analysis shows that only about 0.036e transfers from FeC5H5 to H2, thus the ionic interaction in FeeH bond is weak. On the other hand, we also performed Mayer bond order (MBO) to analyze the covalent bond of the system [34]. The MBO is a very useful and explicit physical quantity in describing covalent bond. For example, the double bond in H2 CO would have a CeO MBO close to 2.00. The MBOs of two FeeH bonds in FeC5H5 H2 are 0.57 and 0.54, thus the bonds have outstanding covalent component. In addition, the MBO of H2 decreases from 1.00 to 0.40, suggesting that the s bond in H2 is weakened. In fact, as a result of the formation of new FeeH bonds, for all the FeC5H5(H2)n and Fe2C5H5(H2)n considered here, the HeH bond length becomes longer, as compared with free H2 molecule. In Fig. 4, the frontier molecular orbitals, i.e. HOMO and LUMO, as well as the deformation density of FeC5H5(H2)n are

Please cite this article in press as: Yang Z, et al., The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.125

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Fig. 4 e (a) The HOMO and LUMO of FeC5H5(H2)n (n ¼ 1e5). (b) The deformation density of FeC5H5(H2)n. The isovalue of ˚ ¡3. isosurface is 0.05 eA

given. The deformation density was calculated as the difference between the total molecule electron density and the density of the isolated atoms. Obviously, additional electron density is distributed around the FeeH bonds of FeC5H5 H2, exhibiting covalent characteristics. The HOMO of FeC5H5 H2 is mainly comprised of 3dyz of Fe, 1s of H and some 2pz of C. Such sd-p hybridization has significant contribution to the FeeH bonds for FeC5H5 H2 and other FeC5H5(H2)n systems. If we add the second H2 molecule to FeC5H5 H2, the case is slightly different. On the one hand, Fe atom of FeC5H5 H2 tends to capture two H atoms of the second H2 to form more FeeH bonds and stablize the system. On the other hand, the first H2

will repel the second one owing to the overlap of the orbitals. As a result, for FeC5H5(H2)2, although the two H2 molecules both bind with Fe atom, they are separated from each other, finally leading to the decrease of DE from 0.84 to 0.63 eV (see Fig. 2a). Similar behavior was also observed in FeC5H5(H2)3. It is very interesting that, in FeC5H5(H2)4 or FeC5H5(H2)5, only one H atom of H2 adsorbs on the Fe atom. Therefore, different from n ¼ 1e3 cases, the H2 molecules have new coordination mode in the two systems. The structural transformation should arise from the repulsion among the H2 molecules. If the sixth H2 molecule is added to the system, as mentioned before, the H2 molecule will be excluded.

Fig. 5 e (a) The ground-state structures of [Fe2(C5H5)2]∞, [Fe2(C5H5)2 H2]∞ and [Fe2(C5H5)2(H2)2]∞. (b) The spin densities of ˚ ¡3. [Fe2(C5H5)2]∞ and [Fe2(C5H5)2H2]∞. Mt is the total magnetic moment of the system. The isovalue of isosurface is 0.05 eA Please cite this article in press as: Yang Z, et al., The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.125

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Because Fe2C5H5 can be approximately viewed as two fused FeC5H5, the results show that Fe2C5H5 can capture ten H2 molecules, and the hydrogen storage capacity is as high as 10.15 wt%. The calculated DE of Fe2C5H5 is 0.73 eV/H2, and is close to the upper limit of ideal energy window, 0.6 eV. Just like FeC5H5(H2)n, positive DE, DE and large HOMO-LUMO gap ensure Fe2C5H5(H2)n has high thermal and kinetic stabilities. The hydrogen storage capacities of FeC5H5 (7.63 wt%) and Fe2C5H5 (10.15 wt%) are close to or higher than that of TiC5Hþ 5 (8.1 wt%) [17]. But the electric neutrality makes the two molecules stable in actual materials. Furthermore, for a given n, we found Fe2C5H5(H2)n often has different structures (see Fig. 2b). The symmetric isomer has high adsorption energy, while the adsorption energy of the asymmetric one is low. For example, Fe2C5H5(H2)2 has two isomers. The two H2 molecules can simultaneously adsorb on

one or two Fe, giving rise to different results. Because the difference of adsorption energies of the two systems is small, only about 0.13 eV, they can both exist stably. For n ¼ 10 case, we only found one structure with D5h symmetry. This stable drum-like molecule has the most FeeH bonds and largest hydrogen storage capacity. Similar to FeC5H5(H2)5, the DG of Fe2C5H5(H2)10 is negative, 17.22 kcal/mol. All attempts to add an eleventh H2 molecule failed.

H2 molecule adsorption on (FeC5H5)∞ nanowire Ferrocene Fe(C5H5)2 has closed-shell electronic configuration and satisfies 18-electron rule, it is not a good hydrogen storage medium. However, the calculated results suggest that each Fe atom in 1D (FeC5H5)∞ nanowire can adsorb one additional H2 molecule. Because the unit cell of the nanowire is FeC5H5

Fig. 6 e (a) The spin-polarized density of states of [Fe2(C5H5)2]∞, [Fe2(C5H5)2H2]∞ and [Fe2(C5H5)2(H2)2]∞. (b) The projected density of states of three systems. The vertical dashed line indicates the Fermi level. The smearing width is 0.05 eV. Please cite this article in press as: Yang Z, et al., The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.125

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other than Fe(C5H5)2, it is not surprising Fe atom in (FeC5H5)∞ can capture H2. (FeC5H5)∞ may be used as hydrogen storage material and deserves further investigation. We first considered a supercell that consists of two FeC5H5 unit cells. For simplicity, we use [Fe2(C5H5)2]∞ to represent the supercell, and the corresponding periodic length ˚ , where c ¼ 3.46 A ˚ (see Fig. 1). The ground state of c' ¼ 2c ¼ 6.92 A free [Fe2(C5H5)2]∞ is a ferromagnetic (FM) half metal and the total magnetic moment Mt is 2.00 mB (1.00 mB per unit cell). The predicted ground state of the free nanowire is in agreement with previous study [31]. The local magnetic moments on Fe atom and C5H5 ring are 1.16 and 0.16 mB, respectively, and can be qualitatively understood from spin density plot as given in Fig. 5b. If we add one H2 to the supercell of the free nanowire, according to the periodicity, the obtained system is

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[Fe2(C5H5)2H2]∞. The hydrogen storage capacity is 0.82 wt%. It is interesting that, since the formation of FeeH bond, the local magnetic moment on the Fe atom is quenched and the corresponding value of C5H5 changes to positive. As a result, the Mt of the supercell decreases to 1.00 mB. When the second H2 is introduced to the supercell of [Fe2(C5H5)2H2]∞ and form [Fe2(C5H5)2(H2)2]∞, the hydrogen storage capacity is 1.62 wt% and is the maximal. At the same time, the Mt changes to zero, i.e. [Fe2(C5H5)2(H2)2]∞ is non-magnetic (NM). Obviously, the formation of FeeH bond will suppress the magnetic moment of (FeC5H5)∞ nanowire. Therefore, the magnetic properties of the nanowire are sensitive to H2 molecules. In addition, for (FeC5H5)∞ and [(Fe2(C5H5)2H2]∞, the results show that there are no anti-ferromagnetic solutions for the two systems, and the FM state is the ground state. For [(Fe2(C5H5)2(H2)2]∞, only the NM state is stable. Recently, Lee

Fig. 7 e (a) The ground-state structures of [Fe4(C5H5)4H2]∞, [Fe4(C5H5)4(H2)2]∞ and [Fe4(C5H5)4(H2)3]∞ nanowires. The [Fe4(C5H5)4(H2)2]∞ has two different isomers. (b) The ground-state structures of [Fe8(C5H5)8H2]∞ and [Fe8(C5H5)8(H2)2]∞ nanowires. Only one isomer of [Fe8(C5H5)8(H2)2]∞ is given. c00 and c000 are the periodic lengths of the supercells. Please cite this article in press as: Yang Z, et al., The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.125

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et al. proposed that the absorption of hydrogen onto the 2D Feterminated TiFe (001) surface results in a decrease in the magnetic moment [35]. While Chaboy and his co-workers found hydrogen can greatly influence the magnetic properties of bulk Nd2 Fe14 B [36]. Our results further suggest that the adsorption of H2 can also change the magnetic properties of 1D organic systems. More interestingly, not only the magnetic properties but also the electrical properties of the nanowire are dependent on H2. The spin-polarized density of states (DOS) and the projected density of states (PDOS) of [Fe2(C5H5)2]∞, [Fe2(C5H5)2H2]∞ and [Fe2(C5H5)2(H2)2]∞ are depicted in Fig. 6. For the free nanowire, the spin-up states at the Fermi level are non-zero and are metallic, while the spin-down states open a band gap of 0.44 eV at the Fermi level and are semiconducting. As mentioned before, it is a typical FM half metal. If one or two H2 are introduced in the system, [Fe2(C5H5)2H2]∞ and [Fe2(C5H5)2(H2)2]∞ both change to semiconductors. The spinup and spin-down band gaps of [Fe2(C5H5)2H2]∞ or [Fe2(C5H5)2(H2)2]∞ are both non-zero. Such transition of half metal to semiconductor will greatly influence the electrical properties, e.g. electrical resistivity, of the nanowire. From the PDOS of the system, one can see that, for the free nanowire, the Fe 3d states cross the Fermi level and are the origination of the half metallicity. However, because of the hybridization of the Fe 3d states and H2 1s states, the spin-up band of [Fe2(C5H5)2H2]∞ or [Fe2(C5H5)2(H2)2]∞ will open a gap near the Fermi level, giving rise to the transition of electrical properties. Similar phenomena were also observed in larger systems. To illustrate this point, larger supercells [Fe4(C5H5)4]∞ and [Fe8(C5H5)8]∞ were explored and the results are listed in Fig. 7. For [Fe8(C5H5)8]∞, only [Fe8(C5H5)8H2]∞ and [Fe8(C5H5)8(H2)2]∞, are given in the figure for simplicity. The total magnetic moments of [Fe4(C5H5)4 H2]∞ and [Fe8(C5H5)8H2]∞ are 3.00 mB and 7.00 mB, respectively, and they are both FM semiconductors. Therefore, a small amount of H2 will cause significant change of magnetic and electrical properties of the nanowire. By measuring the magnetization and resistance, (FeC5H5)∞ can be used for finding and detecting H2 molecules. We further considered some other common diatomic molecules such as NO, CO, N2 and O2. Because C, N and O atoms have large atomic radii, these molecules can not be inserted in the nanowire. They only stay on the ’surface’ of the nanowire and the related interactions are very weak. These weak interactions will not significantly change the magnetic and electrical properties of the nanowire. In other words, these properties are only affected by H2 molecules.

Conclusions We have theoretically investigated the capacity of FeC5H5, Fe2C5H5, and 1D (FeC5H5)∞ nanowire as hydrogen storage media. The maximal hydrogen storage capacities of FeC5H5 and Fe2C5H5 are 7.63 wt% and 10.15 wt%, respectively. The average adsorption energies of FeC5H5(H2)5 and Fe2C5H5(H2)10 are 0.49 and 0.73 eV/H2, and they both have high thermal and kinetic stabilities. These results show that FeC5H5 and Fe2C5H5 are excellent hydrogen storage materials. 1D (FeC5H5)∞

nanowire could also adsorb H2 molecules (1.62 wt%). Most importantly, the magnetic and electrical properties of the nanowire are sensitive to the additional H2, thus (FeC5H5)∞ can also be used for finding and detecting H2 molecules.

Acknowledgments This work was supported by National Natural Science Foundation of China (11104199), Program for Changjiang Scholar and Innovative Research Team in University (IRT0972), Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011), and Natural Science Foundation of Shanxi Province (2012011021-3).

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Please cite this article in press as: Yang Z, et al., The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.09.125