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First-Principle Study of H2 Adsorption on LaFeO3(110) Surface
The adsorption of H2 molecule on LaFeO3(110) surface was studied by first-principle calculations. Based on the adsorption sites, adsorption energies, and electronic structures, it can be found that one H atom can be adsorbed on O atom and form –OH with the O atom, which is the most stable structure. One H atom can be adsorbed on one Fe atom, which makes Fe3+ turn to Fe2+. Two H atoms can form H2O molecule with O atom, which makes it possible to form oxygen vacancy on the surface.
Nickel-metal (NI-MH) batteries have been widely studied for its favorable property such as high capacity, fast charge and discharge rate, environmental compatibility, and long stable periods . Cathode material has a great effect on the performance of NI-MH batteries. Traditional negative materials are hydrogen storage alloys including AB5, AB2, AB, and Mg-based alloys [2, 3]. Although AB5-type alloys have been widely applied in various devices, it has high cost and low capacity. Therefore, a lot of researches have been carried out to develop new negative materials to reduce the cost and improve the capacity. In recent years, Perovskite ABO3 has attracted a lot of attention as potential negative materials [4, 5].
The capacity of perovskite ABO3 as cathode material is much higher than that of traditional materials. Mandal et al.  have developed an unprecedented intake of hydrogen by BaMnO3/Pt to the extent of 1.25 w.% at moderate temperatures (190–260°C) and ambient pressure. Esaka et al.  have proposed perovskite-type oxides (A = Sr or Ba, M = rare earth element) prepared by a conventional solid-state reaction method as innovative electrode materials for Ni-MH batteries. Deng et al.  have reported that the reversible capacity of perovskite was more than 500 mAh/g at a discharge current density of 31.25 mA/g when the temperature rises to 333 K, which is much higher than that of AB5 alloys . Considering the high capacity and abundance of La, perovskite LaFeO3 has been studied widely as potential negative materials for Ni-MH batteries. Deng et al.  have reported that before the 20th cycle, the discharge capacity of LaFeO3 keeps steady at about 80 mAh/g, 160 mAh/g, and 350 mAh/g at 298 K, 313 K, and 333 K, respectively. However, the electrochemical hydrogen storage mechanism of the perovskite oxide still remains uncertain, and it requires more investigation to know how the H atoms combine with the perovskite oxide. In this research, we employed density functional theory (DFT) to investigate the slab character of LaFeO3 and hydrogen adsorption on the slab aiming to explain the adsorption mechanism of reaction and provide theoretical guidance for correlative experiment.
2. Models and Computational Methods
In this paper, we employed the slab model which is widely used in surface calculation. The structure of LaFeO3 belongs to orthorhombic crystal system, whose space group is Pbnm . As for LaFeO3(110) slab, there are two kinds of models which we defined as Model I and Model II. Taking accuracy and speed in consideration, we simulate the LaFeO3(110) surface with a 4-layer model and for further discussion a 7-layer model will be studied for improving and comparison. The structure parameter of H2 molecule which is about to be adsorbed is 0.741 Å and the thickness of vacuum is set at 20 Å. When the model is set up, 8 favorable sites are taken in calculation, which is shown in Figure 1. H2 molecule is put on all the sites separately parallel to the crystal surface.
2.2. Computational Methods
The first-principle calculations are performed with Cambridge serial total energy package (CASTEP) in the framework of density functional theory (DFT), which is based on the plane-wave pseudopotential. The exchange and correlation energy were treated within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE). The electron-ion interaction is described with ultrasoft pseudopotential (UUSP). After testing the model with -point grid set , it is decided to use the supercells consisting of 5 × 5 × 1 unit cell for the doped system calculations. All the calculations are carried out non-spin-polarized with a kinetic energy cutoff of 340 eV, and the convergence criteria for energy and displacement are 2 × 10−6 eV/atom and 10−3 Å, respectively. The calculated structure parameters of H2 molecule are 0.746 Å, which agrees well with experimental results .
Moreover, the structure parameters of LaFeO3(110) surface is , which is the same as experimental results.
3. Results and Discussions
3.1. Decision of the Adsorption Sites of H2 Molecule on LaFeO3(110) Surface
Table 1 lists the energies and structure parameters of all possible sites. It is defined that , , , and are the shortest distances of H atoms, H atom and O atom, H atom and Fe atom, and H atom and La atom, respectively. The dissociation energy of H2 molecule is defined as the following equation: and represent the energy of H2 molecule and H atom, respectively. The adsorption energy of H2 molecule is defined in the following equation: and represent the energy of clean crystal surface and the surface with two adsorbed H atoms, respectively. According to the definitions above, it can be inferred that the structure of LaFeO3(110)/H2 is stable when is positive, and the bigger the is, the more stable the structure will be; it means that the H2 molecule has been dissociated when is positive.
According to the results in Table 1, the adsorption of H2 molecule on sites a, b, c, h, and f belong to chemical adsorptions, and they are physical adsorptions when H2 molecule is put on the other sites.
All the chemical adsorption sites are shown in Figure 2. The data in the figure are representative of distances between H atoms, whose unit is Å. By analysing the data in Table 1 and adsorption in Figure 2, we can find the following.(1)On both site a and site b, the H2 molecule is dissociated into two H atoms and then each H atom is absorbed by O atom, which is a chemical adsorption.(2)On both site h and site f, after dissociation of the the H2 molecule, each H atom is absorbed by Fe atom, which is chemical adsorption.(3)On site c, after adsorption, the structure parameters ( = 1.613 Å, = 1.007 Å) are close to the experimental parameters that Gu et al.  have reported ( = 1.545 Å, = 0.978 Å). We can infer that H2O molecule is formed, which makes it possible to form an oxygen vacancy if the H2O molecule gets rid of the crystal surface .
Considering the accuracy, a model consisting of 7 layers of atoms is adopted. Table 2 lists the structure parameters and energies of the crystal surface. According to the data in the table, the surface with H atoms absorbed on O atoms is more stable than that with H atoms absorbed on Fe atoms and both are chemical adsorptions.
3.2. The Adsorption Mechanism of H2 Molecule on O Short Bridge in Model I
The following discussions are about the properties on O short bridge in Model I including Mulliken charge population, density of states, and electron localization function.
3.2.1. Mulliken Charge Population
It shows the information of interaction between the H2 molecule and the crystal surface by analysing Mulliken charge population. Table 3 lists the Mulliken Charge population before and after adsorption, respectively. The H atoms get positive charges and the crystal surface is negatively charged after adsorption. The change of charges on the surface takes 69.5% of total change, which indicates that the H2 molecules mainly interact with the atoms on the surface. Moreover, surface potential falls after adsorption, which means the structure is more stable.
3.2.2. Density of States (DOS)
The information of interaction between atoms can be obtained by analysing the change of valance states and energies. Figure 3 lists density of states of H2 molecule adsorption on O short bridge in Model I before and after adsorption, respectively. By analysing the data, we can find that the energy of highest occupied states (HOS) is between −6.5 eV and 3.0 eV, which is mainly caused by O 2p electrons and Fe 3d electrons. In contrast, the energy of HOS has changed to that which is between −8.0 eV and 0.5 eV after adsorption, which is mainly caused by H 1s electron, O 2p electrons, and Fe 3d electrons. The HOS of electrons moved to deeper levels after adsorption. That is to say, the structure becomes more stable after adsorption. And the H2 molecule is dissociated absolutely after adsorption. In addition, H 1s not only overlaps but also resonates with O 2s and O 2p; it means that H atom forms–OH with O atom after adsorption.
3.2.3. Electron Localization Function (ELF)
Properties of bonds between atoms can be analysed by ELF. The range of ELF is the interval from 0 to 1. The closer ELF to 1, the deeper the electron localization degree. On the other hand, if ELF is close to zero, the electrovalent bond will be strong.
Figure 5(a) lists the ELF of LaFeO3(110) surface with H atoms absorbed on O atoms and on the initial position of O short bridge in Model I. It is obvious that the ELF is close to 1, which means the covalent bond between the H atom and the O atom is very strong.
3.3. Adsorption Mechanism of H2 Molecule on O Top in Model II
The following discussions are about the properties on O top in Model II including Mulliken charge population, density of states, and electron localization function.
3.3.1. Mulliken Charge Population
Table 4 lists the Mulliken charge population before and after adsorption, respectively. The H atoms get negative charges and the crystal surface gets positive charges after adsorption. The Fe atoms, which absorb H atoms, lose charges after adsorption. It makes Fe3+ turn to Fe2+. And that agrees with what Hoffmann et al.  have reported on change of valance states of B in perovskite ABO3. Surface potential falls after adsorption, which means that the structure is more stable. Moreover, it is observed that the La atoms got charges after adsorption, which indicates that the valance states of La atoms have changed.
3.3.2. Density of States (DOS)
Figure 4 lists the density of states of H2 molecule adsorption on O top in Model II before and after adsorption, respectively. By analysing the data, we get three conclusions. First, the energy of HOS of H atoms is decreased after adsorption and in contrast the energy of HOS of Fe atoms is increased after adsorption, which means that the H atoms get more stable and the Fe atoms get more excited after adsorption. Second, the peaks of density of states falls after adsorption, which means the energy of the surface is lower and the structure is more stable after adsorption. Third, after adsorption, the DOS of Fe atoms have changed, which means the charge population of Fe atoms have changed.
3.3.3. Electron Localization Function (ELF)
Figure 5(b) lists the ELF of LaFeO3(110) surface with H atoms absorbed on Fe atoms with the initial position of the H2 molecule being O top in Model II. It is obvious that the ELF between the H atom and the Fe atom is about 0.5, which means that a metallic bond is formed after adsorption. And that agree with what Mandal et al.  have reported on the change of valance states of Mn.
The adsorption H2 on LaFeO3(110) surface is studied. H2 is put on one symmetrical adsorption site, parallel to the surface, and then is calculated with first-principle methods. After analysing and discussing the results, we can find the following.(1)H atom can be absorbed on O atom and form –OH. There is a strong covalent bond between them after adsorption. The adsorption energy is 3.507 eV, which is higher than that on other sites.(2)Two H atoms can form H2O molecule with an O atom. The adsorption energy is 1.220 eV, which is lower than that when H atom is absorbed on O atom or Fe atom. An Oxygen vacancy will be formed on the surface if the H2O molecule gets rid of the surface.(3)H atoms can be absorbed on Fe atom and forms a metallic bond with it, which not only makes Fe3+ turn to Fe2+ but also makes the H atoms more stable and the Fe atoms more excited.(4)Although no H atom is absorbed on La atom, it is observed that the charge population has changed after adsorption, which indicates that the valance states of La may have changed.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the financial support from the Basic Scientific Research Foundation for Gansu Universities of China (Grant no. 05-0342), the Science and Technology Project of Lanzhou City (Grant no. 2011-1-10), and the Doctoral Foundation of Lanzhou University of Technology (Grant no. BS200901).
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