ARTICLE pubs.acs.org/IECR
Hydrogen Storage in Perovskite-Type Oxides ABO3 for Ni/MH Battery Applications: A Density Functional Investigation Qiang Wang,*,† Zhiqian Chen,† Yungui Chen,*,‡ Nanpu Cheng,† and Qun Hui† † ‡
School of Materials Science and Engineering, Southwest University, Chongqing 400715, People’s Republic of China School of Materials Science and Engineering, Sichuan University, Chengdu 610064, People’s Republic of China ABSTRACT: Perovskite oxides were typically considered as the electronic and ionic conductors for application in the electrolytes for solid oxides fuel cells (SOFCs). Recently, LaFeO3-based systems were mainly focused on the electrochemical property for the anode of Ni/MH batteries in our previous work, and the exciting results of their electrochemistry capacity and cycle life examination exhibited much scientific values for further investigation. In the present work, the effects of A (La)-site or B (Fe)-site metal ions doped and substituted on the fundamental properties of these materials were calculated by a first-principle approach. In conjunction with the experimental results, the microscopic mechanisms of the doped or substituted effect were discussed and elucidated. On the other hand, the hydrides of LaFe(Cr)O3 were established and computed to explore the nature of electrochemical behaviors of these perovskite oxides.
1. INTRODUCTION Traditionally, ABO3 perovskite oxides have been intensively focused on their ion-exchange ability, which could be considered to be the potential for electrolyte candidates in various electrochemical applications,1 3 since the proton conductivity in doped SrZrO3 and SrCeO3 at elevated temperatures were discovered by Iwahara.4 In particular, proton mobility in Ba(Zr,Ce)O3- and LaBO3-based systems have attracted considerable interest in aspects such as the effects of the various ionic radius of the dopants, the activation energy and the transport mechanism of the proton migration.5 14 It is generally accepted that the proton primarily dissociated from the gas-phase water molecules, and then was incorporated into the vacancies of ABO3 oxides, which thus formed the covalent bonds with the oxygen atoms. Furthermore, the proton conduction occurs through the binding proton rotation and migration in the lattice oxygen ions, which was suggested as the so-called two-state model15 or jump diffusion model.16 Theoretically, the vacancies of lattice oxygen play an important role in promoting proton conductivity of ABO3 perovskite oxides. Numerous theoretical calculations elucidated that the acceptor dopants caused the oxygen vacancies as a charge-compensating effect,17 where the lower-valence ions are substituted for B-site ions in the pure perovskite oxide. The dopants in cubic oxides were widely recognized as discrete “traps”, which could be effectively enhanced the activation energy and trace of the proton mobility.5 To summarize, the ABO3 perovskite oxide, as a promising proton conductor for the solid oxide fuel cell (SOFC), have been attracting various interests in their properties of the proton diffusion at elevated temperature (>700 °C). Recently, the doped pervoskite oxide SrCe0.95Yb0.05O3 σ was first considered by Takao Esaka as anode material of Ni/MH batteries due to its reasonable hydrogen solubility at room temperature.18 Based on the electrochemical measurements, the authors demonstrated that this material, which was prepared via conventional solid-reaction method, provided a considerable electrochemistry capacity of 119 mAh/g (current density = 18.5 mA/g), r 2011 American Chemical Society
corresponding to 1.44 H atoms in one formula unit of material. In our previous work, LaFeO3 powder, prepared by the stearic acid combustion method, presented the exciting capacities values of 329.3, 502.7, 530.3, and 626.0 mAh/g at 25, 40, 60, and 80 °C, respectively, where the applied current density was 31.25 mA/g. Moreover, the electrochemistry capacity conducted a good durability of 20 cycles with some reasonable attenuation.19 According to our experimental results, (La,Sr)(Fe,Cr)O3 perovskite oxides demonstrated higher electrochemistry capacities as the candidates for Ni/MH anode materials in contrast to the traditional hydrogen storage alloys, especially at the temperature range of 25 80 °C.20 22 However, the major challenge for (La,Sr)(Fe,Cr)O3 systems materials application in Ni/MH batteries was the highly sensitive of electrochemistry capacities, depending on the working temperatures, which dramatically reduced as the temperature decreased to room temperature. Even so, this crucial issue was hardly modified through the effect of A- or B-site doping, and the electrochemical mechanism of hydrogen storage in these types of materials was still ambiguous. In the present work, LaFeO3-based systems with H+-ion insertion, which the hydrogen number was corresponding to the electrochemistry capacities in the unit cell of these materials, were established and investigated by the first-principles calculation. Furthermore, the effects of A-site doping and B-site substitution in LaFeO3 were elucidated with the shifting of the electronic structure and chemical bonds as well as modification of the electrochemistry properties. To summarize our previous experimental results, the proton diffusion in electrochemistry tests was also mentioned in this paper to explore the possible Special Issue: Alternative Energy Systems Received: October 6, 2011 Accepted: December 3, 2011 Revised: December 3, 2011 Published: December 04, 2011 11821
dx.doi.org/10.1021/ie202284z | Ind. Eng. Chem. Res. 2012, 51, 11821–11827
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Lattice Parameters of LaFeO3, LaCrO3, and Their Hydrides Crystal Structure Parameters
LaFeO3
a
b
c
volume
error (%)
5.5521
5.5631
7.8432a
242.252
0
5.5022 [LaFeO3H3]3+ [LaFeO3H6]6+
4.7970 4.6684
[LaFeO3H9]9+
4.1412
3.6572
LaCrO3
5.4859
7.7682
5.4878
a
Figure 1. (a) Unit cell of LaFeO3, and different numbers of hydrogen-ion insertions ((b) [LaFeO3H3]3+, (c) [LaFeO3H6]6+, and (d) [LaFeO3H9]9+).
reasons for the temperature-caused decrease in electrochemistry capacities, in conjunction with the density function calculation.
2. THEORY, METHODS, AND COMPUTATIONAL DETAILS 2.1. Theory and Methods. Hydrogen, normally as a proton, in perovskite oxides exhibits high proton conductivity at elevated temperature, which was applied in various electrochemistry devices, such as solid oxide fuel batteries and other batteries. It is clear that hydrogen was dissolved in the +1 oxidation state, and that the proton H+ is the hydrogen species involved in these perovskite oxides. Therefore, the hydrogen in perovskite oxides was usually treated as the proton with a charge +1,7 and the same was done in this paper. Moreover, according to our previous electrochemical capacity tests for Ni/MH batteries, the corresponding number of H+ ions in LaFeO3-based systems was estimated. Thus, the calculated unit cell with charges n was established as the mediate active species involved in the electrochemistry reaction, and this could be denoted as [LaFeO3Hn]n+. The proton diffusion coefficient of perovskite oxides in our electrochemical tests elucidated the improved proton conductivity as the temperature arising in all the tested samples, which were demonstrated in our previous work more thoroughly.19 21 In this paper, the proton diffusion coefficients of LaFeO3 with various dopants were exhibited in different temperatures, and the effects of dopants on their performances also were addressed. 2.2. Computational Details. The first-principle calculations were performed with the CASTEP computer code23 in the
7.7361
5.4534 4.6880 3.5080
6.5442 6.2510
232.127
4.2
147.168 102.371
39.3 57.7
5.6006
84.822
65.0
5.5247b
235.438
0
5.5275
7.6051
230.692
2.0
[LaCrO3H3]3+
4.7816
6.4391
4.7088
144.980
38.4
[LaCrO3H6]6+
5.5982
5.3561
3.2572
97.666
58.5
From ref 28. b From ref 29.
framework of density functional theory (DFT), which is based on the plane-wave pseudopotential24,25 approach and implements in Materials studio software from Accelrys. The generalized gradient approximation (GGA) of Perdew et al. (PBE)26 with ultrasoft pseudopotential27 was employed to include the exchange correlation energy in the total energy, and projector-augmented wave potentials were described for the electron-ion interaction. Brillouin zone sampling was done using a 6 6 6 k-point grid for the five-atom LaFeO3 primitive cell. For the doped system calculations, we used the supercells consisting of 2 2 1 unit cell, which was expressed as La16Fe16O48. All the calculations were 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 residue force and stress in equilibrium geometry is on the order of 3 10 3 eV/Å and 0.01 GPa, respectively. As mentioned above, the number of H+ ions in the unit cell of perovskite oxides, corresponding to the electrochemistry capacities, was calculated as 3, 6, or even possibly 9 for one formula of LaFeO3. Hence, it is suggested that there were one, two, or even three protons bound to one O atom in the unit cell of the crystal structure, and then the models of these systems were successfully relaxed to obtain the stable structure. In view of the hydrogen reaction involved in the Ni/MH battery application, it is greatly different to the traditional proton conductivity calculation; rather, it is more similar to proton storage in perovskite oxides in our work than the one-proton mobility in them.
3. RESULTS AND DISCUSSIONS 3.1. Crystal Structure. LaFeO3 is orthorhombic at room temperature with the space group of Pbnm, which demonstrates that O2 ions move on the perovskite surface and are the corners of the FeO3 octahedra. As seen in Figure 1a, some O2 ions get closer while others become further apart, because of the octahedral FeO3 tilting, which indicates the available excess space for hydrogen storage. Optimum lattice parameters for LaFeO3 are slightly deviated from the experimental values of the volume of the unit cell shrinking 4.2%, because the parameters a, b, and c get shorter, as can be seen in Table 1. Surprisingly, this happens in all of the optimization of the hydrides of oxides such as [ABO3Hn]n+, as shown in Figures 1b, 1c, and 1d, and they are extremely decreased in volume (>30%) as the number of hydrogens (n) increased. This has never been mentioned in the DFT calculation 11822
dx.doi.org/10.1021/ie202284z |Ind. Eng. Chem. Res. 2012, 51, 11821–11827
Industrial & Engineering Chemistry Research
ARTICLE
Table 2. Electrochemistry Capacities and Proton Diffusion Coefficient of LaFeO3 at Various Temperaturesa Electrochemistry Capacity (mA h/g)
a
temperature (°C)
experimenta
theory
[LaFeO3Hn]n+
25
329.3
331.2
n=3
0.808
40
502.7
662.4
n=6
2.02
60
530.3
662.4
n=6
4.04
80
626.0
662.4
n=6
6.02
993.6
n=9
proton diffusion coefficient, D ( 10
17
cm2 s 1)b
Note: discharged current density 31.5 mA/g. b From ref 19.
Table 3. Atomic Populations and Bond Populations of LaFeO3 and LaCrO3
La LaFeO3 LaCrO3
Figure 2. Total and partial density states of LaFeO3 and LaCrO3.
of perovskite oxides before, for usually it was neglected that the single proton in the lattice was established to elucidate the mobility of the proton in perovskite oxides. To identify the phase structure of protons store in the lattice unit cell (La4Fe4O12), [LaFeO3Hn]n+ (n = 3, 6, 9) are precisely calculated and achieved the stable structure; these are illustrated in Figures 1b, 1c, and 1d, while the [LaFeO3H12]12+ could not to be computed as a unit cell and achieved a stable crystal structure. In conjunction with the electrochemistry capacity of this material, the number of H atoms in one unit cell is reasonable in our calculation, although they do not precisely correspond to each other. For this reason, two options were attempted. First, the excess electrochemistry current is not fully released, because of the dull reactive of anode material, which is analogous to the occasion of [LaFeO3Hn]n+ (n = 6 or 9) at the highest working temperature of 80 °C. The other option is that the functional capacity of [LaFeO3Hn]n+ (n = 3, 6, and 9) is restricted by the working temperature such as room temperature, which was described by the knowledge of the phonon vibration. Positively, these two assumptions are negligible, relative to the fundamental properties of the hydrides, and, consequently, are used to elucidate the phase structure of hydrogen storage in ABO3 oxides for Ni/MH battery applications. 3.2. LaFeO3. Although considerable proton conduction in perovskite oxides at elevated temperature was observed and draws much attention to the SOFC applications, this character was not fairly noticeable at the lower temperatures, even at room
1.61 1.74
Sr
Atomic Populations
Bond Populations
(Mulliken)
[M O1 (O2)]
Fe(Cr) 0.46 0.25
O1 (O2) 0.7 ( 0.67) 0.67 ( 0.65)
Fe
Cr
0.21 (0.18) 0.25, 0.24
temperature. This is due to the well-known conduction ability, which sharply receded as the temperature got lower, and where the ion conduction at room temperature was rarely taken into account in perovskite oxides. Nevertheless, a rare-earth transition-metal oxide (LaFeO 3 ) demonstrated a very acceptable capability of hydrogen storage and conduction at room temperature. As shown in Table 2, the higher the temperature, the greater the hydrogen storage and the faster the proton diffusion coefficient in the electrochemistry test. Moreover, LaFeO3 also exhibited higher electrochemistry capacities than the report of ACe1 xMxO3 σ (A = Sr or Ba, M = rare-earth element) by Takao Esaka.18To our knowledge, the first reason for this is the lighter metallic element used and smaller molecular weight in LaFeO3, and the other factor is the greater H-atom storage in one formula of this material. The detailed theory, experimental capacities, and proton diffusion coefficient (D) at various temperatures are comparatively presented in Table 2. It indicates that the higher proton diffusion coefficient D, because of the temperature elevation, causes the tested electrochemistry capacities to become similar to the theoretical values of hydrogen storage. It also means that there is considerable room to improve these materials for high-capacity battery applications. Lattice oxygen vacancies in perovskite oxides were the key role for the proton and ion exchange in fuel cells, which were effectively influenced by the B-site element that was substituted or doped. For LaFeO3, the electronic density of states indicates that Fe 3d states are strongly hybridized with the O 2p in the range of 8 eV to 4 eV, as can be seen in Figure 2, and these are clearly the main electronic states that contribute to the entire states of the unit cell. When oxygen vacancies are introduced in the lattice crystal of LaFeO3, the electrons of Fe 3d are compensated from the B-site metal ion to oxygen, and this results in the various covalent states of metal Fe ion, which is considered as the origin of redox in pervoskite oxides. Moreover, the smalleratom-radius element of Cr is substituted for Fe ions in LaMO3, and the electronic states in Figure 2 also illustrate the hybridization of Cr 3d and O 2p states. However, it is noted that there is much overlap between La 5d, and Cr 3d states in the conduction 11823
dx.doi.org/10.1021/ie202284z |Ind. Eng. Chem. Res. 2012, 51, 11821–11827
Industrial & Engineering Chemistry Research
ARTICLE
Table 4. Electrochemistry Capacities and Proton Diffusion Coefficients of La1 xSrxFeO3 (x = 0.2, 0.4, and 0.6) at Various Temperaturesa Electrochemistry Capacity (mAh/g) temperature (°C)
experimentb,c
theory
[La1 ySryFeO3Hn]n+
25
324.4
345.8
n=3
1.62
40 60
467.8 543.0
691.6 691.6
n=6 n=6
2.02 4.04
25
147.0
361.8
n=3
0.8
40
388.5
723.6
n=6
1.82
60
501.5
723.6
n=6
2.02
25
153.4
379.3
n=3
0.81
40
198.1
758.6
n=6
1.64
60
502.6
758.6
n=6
2.02
proton diffusion coefficient, D ( 10
17
cm2 s 1)b,c
La0.8Sr0.2FeO3
La0.6Sr0.4FeO3
La0.4Sr0.6FeO3
a
Note: discharged current density = 31.5 mA/g. b From ref 20. c From ref 21.
band, while these are not taking place between La and Fe atoms in the LaFeO3. Meanwhile, the atomic Mulliken charges and average overlaps population for bonds in LaFeO3 and LaCrO3, as shown in Table 3, also indicate that the charges were compensated from La to Cr in the LaCrO3 relative this in LaFeO3, and it is in good agreement with the hybridization states between La 5d and Cr 3d. Subsequently, the strong covalent states of LaCrO3 resulted in stronger bonds between Cr and O atoms, and cause a more-compact unit cell of the crystal structure. 2.3. La1 xSrxFeO3. Typically, numerous experiments demonstrated that the lower-valence Sr doped in the A-site ion of LaBO3 was beneficial to both electrical conduction and oxygen ion conduction.30 32 In an attempt to elucidate the A site of the doped effect, various strontium contents doped in LaFeO3 were introduced and tested in electrochemistry, relative to the intrinsic LaFeO3. In our previous experimental works, La1 xSrxFeO3 (x = 0.2, 0.4, and 0.6) were prepared and exhibited the crystal structure transition from orthorhombic Pnma (x = 0.2) to rhombohedral R3c (x = 0.4 and 0.6) as the strontium content increased, which agreed well with the report by S. E. Dann.33 For the electrochemical properties of these doped materials, which are listed in Table 4, they theoretically hold higher electrochemistry capacities as the strontium content increases, while the tested capacities decrease, and these are likewise occurred in the proton diffusion coefficient examinations. Compared with LaFeO3, La0.8Sr0.2FeO3 has many similar results of electrochemistry capacities and proton diffusion testified, except the predominantly 2D value improved at room temperature, whereas the other doped systems display slightly disappointing outcomes. To some extent, these are the correlations of the structure that the crystal structure of the doped system (x = 0.2) shows the same type of orthorhombic structure, while the others (La1 xSrxFeO3 (x = 0.4 and 0.6)) transitioned to rhombohedral with increasing strontium content. To elucidate the effects of Sr doping and the types of crystal structure on the fundamental properties of Fe-based perovskite oxides (LaFeO3, La0.5Sr0.5FeO3, and SrFeO3), mainly corresponding to the orthorhombic, rhombohedral, and cubic structure types, respectively, were established for the theoretical calculation. As
Figure 3. Total and partial density states of LaFeO3, La0.5Sr0.5FeO3, and SrFeO3.
can be seen in Figure 3, the states of rhombohedral La0.5Sr0.5FeO3 are principally similar to LaFeO3, whereas partial states of Sr atoms occurred in the range of 4 8 eV, because of the doped Sr atoms. For SrFeO3, it is considerably different in the states of Fe atoms in the area of the conduction band, where there are not only the Fe 3d electrons but also the electrons in other orbits of Fe atom hybridization with O 2p electrons. Furthermore, because of the doped lower covalent states of the Sr ion, Fe atoms contribute more electrons to form covalent bonds with the O ions, and this causes stronger chemical bonds between the Fe ion and the O ion as the doped Sr content increased. It is also noted that the cubic SrFeO3 demonstrated the strongest strength of Fe O bonds, as can be seen in Table 5, when the La atom is fully substituted by lower valence Sr atoms. 2.4. LaCrO3. Generally, the B-site ions were key roles in the proton conduction investigation of Ba- or Zr-based systems, in view of the strong hybridization with lattice oxygen and caused various vacancies for proton incorporation, and this attracts much more attention for the electrolyte application of the 11824
dx.doi.org/10.1021/ie202284z |Ind. Eng. Chem. Res. 2012, 51, 11821–11827
Industrial & Engineering Chemistry Research
ARTICLE
SOFCs. Recently, LaFeO3 exhibited a considerable exciting result of electrochemistry capacities in our attempt to explore perovskite oxide anodes for Ni/MH batteries, the substituted B-site ion was also considered to improve their properties in our previous experiment works. Comparatively, the electrochemistry properties of LaFeO3 and LaCrO3 are shown in Table 6; it was noted that the latter material demonstrated slightly higher theoretical capacities than the former, and there was a considerable improvement in the practical experimental results at various temperatures. Table 5. Atomic Populations and Bond Populations of LaFeO3, La0.5Sr0.5FeO3, and SrFeO3 Atomic Populations (Mulliken) bond populations La LaFeO3
1.61
La0.5Sr0.5FeO3
0.96
SrFeO3
Sr
Fe
O1,2 (O3)
[Fe O1,2 (O3)]
0.46
0.7 ( 0.67)
0.54
0.53
0.68 ( 0.67)
0.21 (0.18) 0.22, 0.21, 0.19
1.31
0.67
0.66
0.40
Table 6. Theoretical and Experimental Electrochemistry Capacity of LaFeO3 and LaCrO3a Electrochemistry Capacity (mAh/g) temperature (°C)
experimentb,c
theory
[LaMO3Hn]n+
25
51.8
331.2
n=3
40 60
330.7 443.2
662.4 662.4
n=6 n=6
25
194.8
336.5
n=3
40
436.8
673.1
n=6
60
484.1
673.1
n=6
LaFeO3
LaCrO3
a
Note: discharge current density = 125 mA/g. b From ref 19. c From ref 22.
In an attempt to explore the hydrides and their elementary properties of perovskite oxides, lattices of [LaMO3Hn]n+ (M = Fe, Cr) were established and calculated by the first principle. Since the hydride of perovskite oxide LaSrCoO3H0.7, where the hydride ion replaced some of the oxide anions in their role of linking metal cation to form a two-dimensional network, was synthesized by soft chemistry method and demonstrated the ability of proton transport, the hydrides of perovskite oxides were thus studied as a existed substance, which posed a unprecedented structure.34,35 Unlike the crystal structure of LaSrCoO3H0.7, the hydrides of LaFeO3 and LaCrO3 were established on account of the majority report of the proton bound to the lattice oxygen, and it was further supported by the report that the hydride BaMnO3H3 was constructed as the hydrogen insertion product of BaMnO3/Pt system hydrogenation.36 To our knowledge, it is seemly reasonable and suitable to construct hydrides of LaFeO3 and LaCrO3, as mentioned in section 3.1. As the volume of the hydrides of the LaFeO3 and LaCrO3 contract, the distance between various types of atoms decreases and gets closer, when three or six protons are inserted into the unit cell of intrinsic oxides. As a result, the energy levels of each atom are broadened and consecutive in density of states, as can be seen in Figure 4; this occurred significantly in the total and partial states of each atom. In addition, the atomic Mulliken charges and average overlap population for bonds in these hydrides are also calculated to qualitative analyze the mechanism of the substituted B-site atoms, as listed in Table 7. It is noted that the extreme value of atomic populations for H and O atoms decreased, whereas that of the Cr atom increases when Fe was substituted by Cr. This is because the Cr O bond is stronger and the O H bonds are weaker, and the more hydrogen is inserted in the lattice, the weaker the chemical bonds between them. According to the results of proton diffusion coefficient examination, the weaker strength of O H bonds clearly facilitates the formation of free H+ ion and then transport. To some extent, it reduces the activation energy for H+ ion diffusion and exhibits higher practical electrochemistry capacities in LaCrO3. Certainly, the working temperatures of these oxide anodes is one of the crucial factors for the proton diffusion modified due to the thermodynamics of lattice and the phonon vibration, and the quantum-mechanic tunnel is usually considered as an import role
Figure 4. Total and partial densities of states of the hydrides of LaFeO3 and LaCrO3. 11825
dx.doi.org/10.1021/ie202284z |Ind. Eng. Chem. Res. 2012, 51, 11821–11827
Industrial & Engineering Chemistry Research
ARTICLE
Table 7. Atomic Populations and Bond Populations of the Hydrides of LaFeO3 and LaCrO3 Atomic Populations (Mulliken) La
Bond Populations
H1,2 (H3)
Fe(Cr)
3.05
0.11 (0.09)
0.83
0.39 ( 0.40)
0.27, 0.44, 0.36
0.97
5.77
0.09( 0.03)
0.11
0.06
0.25, 0.41, 0.37
0.91, 0.40
[LaCrO3H3]3+
2.68
0.05(0.06)
1.21
0.34 ( 0.36)
[LaCrO3H6]6+
5.36
0.03( 0.20)
0.93
0.05( 0.07)
3+
[LaFeO3H3]
[LaFeO3H6]6+
for proton mobility near the room temperature,37 especially in this work of shrinking in volume of unit cell. In summary, which one of these reasons primarily causes the higher proton diffusion coefficient as the temperature increases is beyond the scope of this paper; these works will be further investigated in our future work.
4. CONCLUSIONS Typically, perovskite oxide LaFeO3-based systems were considered as a new type of electrolyte material for proton and oxygen conduction in solid oxide fuel cell (SOFC) application, while it was rarely considered these materials as a type of ion conductor at room temperature, based on the fact that it was a negligible value of proton diffusion coefficient, based on the Arrhenius plot calculation. Recently, LaFeO3 exhibited impressive electrochemistry capacities and cycle life results in the electrochemistry test for Ni/MH batteries application, this indicated the proton conduction in perovskite oxides at room temperature was still an important and complicated object for its practical application. Based on our previous experimental works and first principle calculation in the present work, the conclusions are summarized as follows. First, the hydrides of perovskite oxides, hydrogen ion insertion in the unit cell of LaFeO3 and LaCrO3, were established and computed by the first principle calculation in view of the related report of the investigation. All the H+ ions were bonded with the lattice oxygen, and the chemical bond between them became weaker as the bond between transition metals (Fe, Cr) and O atoms became stronger; the greater the number of H+ ions inserted into the unit cell of perovskite oxides, the weaker the O H chemical bonds. Thus, it reduced the activation energy necessary to break the O H bond and then make the free hydrogen ion transportation; this agreed well with the results of the proton diffusion coefficient test. Second, the crystal structures of La1 xSrxFeO3 transformed from orthorhombic Pnma (x = 0.2) to rhombohedral R3c (x = 0.4, 0.6), which demonstrated different electrochemical behaviors for Ni/MH batteries. For La0.8Sr0.2FeO3, it seemed comparable in electrochemical capacity and proton diffusion coefficient at various temperatures with LaFeO3, because they have the same structure, while La1 xSrxFeO3 (x = 0.4, 0.6) demonstrated lower values. Moreover, LaFeO3 materials were substituted in B-site metal for Cr and exhibited much improvement in their electrochemical capacities. The theoretical calculation of their hydrides indicated that the substitution caused a much stronger relationship between Cr and O atoms, and subsequently induced weaker strength in O H chemical bonds. Therefore, the lower activation energy needed to break O H bonds and cause the transport of the liberated H+ ions further resulted in higher practical electrochemistry capacities. In conclusion, the present work has demonstrated that the A- and B-site metal-ion doped or substitution effects on the
O1,2 (O3)
Fe O1,2 (O3)
Cr O1,2 (O3)
O H
0.30, 0.54, 0.58
0.85, 0.72
0.49, 0.34
0.78, 0.54
fundamental and electrochemistry properties of perovskite oxides LaFeO3 for the Ni/MH application. For practical applications, there are many primary problems such as the sensitive relationship between electrochemical properties and temperature to be solved, and this will be further investigated and the details will be elucidated in our future work.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86-23-68253204. Fax: +86-23-68254373. E-mail: wysnu@ swu.edu.cn (Q.W.),
[email protected] (Y.C.).
’ ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 51101130), Natural Science Foundation project of CQ CSTC (No. 2010BB4119), and Doctor Foundation of Southwest University (No. SWU109031). ’ REFERENCES (1) Haile, S. M. Fuel cell materials and components. Acta Mater. 2003, 51, 5981. (2) Park, C. Y.; Azzarello, F. V.; Jacobson, A. J. The oxygen nonstoichiometry and electrical conductivity of La0.7Sr0.3Cu0.2Fe0.8O3 δ. J. Mater. Chem. 2006, 16, 3624. (3) Mai, A.; Haanappel, V. A. C.; Uhlenbruck, S.; Tietz, F.; St€over, D. Ferrite-based perovskites as cathode materials for anode-supported solid oxide fuel cells: Part I. Variation of composition. Solid State Ionics 2005, 176, 1341. (4) Iwahara, H.; Esaka, T.; Uchida, H.; Maeda, N. Proton conduction in sintered oxides and its application to steam electrolysis for hydrogen production. Solid State Ionics 1981, 359, 3. (5) Bj€orketun, M. E.; Sundell, P. G.; Wahnstr€om, G. Effect of acceptor dopants on the proton mobility in BaZrO3: A density functional investigation. Phys. Rev. B 2007, 76, 054307. (6) Ahmed, I.; Eriksson, S.-G.; Ahlberg, E.; Knee, C. S.; Karlsson, M.; Matic, A.; Engberg, D.; B€orjesson, L. Proton conductivity and low temperature structure of In-doped BaZrO3. Solid State Ionics 2006, 177, 2357. (7) Shi, T. C.; Yoshino, M.; Morinaga, M. First-principles study of protonic conduction in In-doped AZrO3 (A = Ca, Sr, Ba). Solid State Ionics 2005, 176, 1091. (8) Gomez, M. A.; Chunduru, M.; Chigweshe, L.; Fletcher, K. M. The effect of dopant at the Zr site on the proton conduction pathways of SrZrO3: An orthorhombic perovskite. J. Chem. Phys. 2010, 133, 064701. (9) Gomez, M. A.; Chunduru, M.; Chigweshe, L.; Foster, L.; Fensin, S. J.; Fletcher, K. M.; Fernandez, L. E. The effect of yttrium dopant on the proton conduction pathways of BaZrO3, a cubic perovskite. J. Chem. Phys. 2010, 132, 214709. (10) Kuhn, J. N.; Matter, P. H.; Millet, J.-M. M.; Watson, R. B.; Ozkan, U. S. Oxygen Exchange Kinetics over Sr- and Co-Doped LaFeO3. J. Phys. Chem. C 2008, 112, 12468. 11826
dx.doi.org/10.1021/ie202284z |Ind. Eng. Chem. Res. 2012, 51, 11821–11827
Industrial & Engineering Chemistry Research (11) Bouwmeester, H. J. M.; Den Otter, M. W.; Boukamp., B. A. Oxygen transport in La0.6Sr0.4Co1 yFeyO3 δ. J. Solid State Electrochem. 2004, 8, 599. (12) Patrakeev, M. V.; Bahteeva, J. A.; Mitberg, E. B.; Leonidov, I. A.; Kozhevnikov, V. L.; Poeppelmeier, K. R. Electron/hole and ion transport in La1 xSrxFeO3 δ. J. Solid State Chem. 2003, 172, 219. (13) Saiful Islam, M. Ionic transport in ABO3 perovskite oxides: A computer modelling tour. J. Mater. Chem. 2000, 10, 1027. (14) Jones, A.; Saiful Islam, M. Atomic-Scale Insight into LaFeO3 Perovskite: Defect Nanoclusters and Ion Migration. J. Phys. Chem. C 2008, 112, 4455. (15) Matzke, T.; Stimming, U.; Kramonik, C.; Soetratmo, M.; Hempelmann, R.; G€uthoff, F. Quasielastic thermal neutron scattering experiment on the proton conductor SrCe0.95Yb0. 05H0.02O2.985. Solid State Ionics 1996, 86 88, 621. (16) Bj€orketun, M. E.; Sundell, P. G.; Wahnstr€om, G.; Engberg, D. A kinetic Monte Carlo study of proton diffusion in disordered perovskite structured lattices based on first-principles calculations. Solid State Ionics 2005, 176, 3035. (17) Pardo, V.; Pickett, W. E. Compensated magnetism by design in double perovskite oxides. Phys. Rev. B 2009, 80, 054415. (18) Esaka, T.; Sakaguchi, H.; Kobayashi, S. Hydrogen storage in proton conductive perovskite type oxides and their application to nickel hydrogen batteries. Solid State Ionics 2004, 166, 351. (19) Deng, G.; Chen, Y.; Tao, M.; Wu, C.; Shen, X.; Yang, H.; Liu, M. Electrochemical properties and hydrogen storage mechanism of perovskite-type oxide LaFeO3 as a negative electrode for Ni/MH batteries. Electrochim. Acta 2010, 55, 1120. (20) Deng, G.; Chen, Y.; Tao, M.; Wu, C.; Shen, X.; Yang, H. Electrochemical properties of La1 xSrxFeO3 (x = 0.2, 0.4) as negative electrode of Ni MH batteries. Electrochim. Acta 2009, 54, 3910. (21) Deng, G.; Chen, Y.; Tao, M.; Wu, C.; Shen, X.; Yang, H.; Liu, M. Preparation and electrochemical properties of La0.4Sr0.6FeO3 as negative electrode of Ni/MH batteries. Int. J. Hydrogen Energy 2009, 34, 5568. (22) Deng, G.; Chen, Y.; Tao, M.; Wu, C.; Shen, X.; Yang, H.; Liu, M. Study of the electrochemical hydrogen storage properties of the proton-conductive perovskite-type oxide LaCrO3 as negative electrode for Ni/MH batteries. Electrochim. Acta 2010, 55, 884. (23) Segall, M. D.; Lindan, P. L. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys.: Condens. Matter. 2002, 14, 2717. (24) Kresse, G.; Hafner, J. Ab initio molecular-dynamics simulation of the liquid metal amorphous semiconductor transition in germanium. Phys. Rev. B 1994, 49, 14251. (25) Kresse, G.; Furthmu€ller, J. Efficiency of Ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (27) Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 1990, 41, 7892. (28) Sangaletti, L.; Depero, L. E.; Allieri, B.; Nunziante, P.; Traversa, E. An X-ray study of the trimetallic LaxSm1 xFeO3 orthoferrites. J. Eur. Ceram. Soc. 2001, 21, 719. (29) Oikawa, K.; Kamiyama, T.; Hashimoto, T.; Shimojyo, Y.; Morii, Y. Structural phase transition of orthorhombic LaCrO3 studied by neutron powder diffraction. J. Solid State Chem. 2000, 154, 524. (30) Simner, S. P.; Shelton, J. P.; Anderson, M. D.; Stevenson, J. W. Interaction between La(Sr)FeO3 SOFC cathode and YSZ electrolyte. Solid State Ionics 2003, 161, 11. (31) Wærnhus, I.; Vullum, P. E.; Holmestad, R.; Grande, T.; Wiik, K. Electronic properties of polycrystalline LaFeO3. Part I: Experimental results and the qualitative role of Schottky defects. Solid State Ionics 2005, 176, 2783. (32) Wærnhus, I.; Sakai, N.; Yokokawa, H.; Grande, T.; Einarsrud, M.-A.; Wiik, K. Mass transport in La1 xSrxFeO3 (x = 0 and 0.1) measured by SIMS. Solid State Ionics 2004, 175, 69.
ARTICLE
(33) Fossdal, A.; Menon, M.; Wærnhus, I.; Wiik, K.; Einarsrud, M.-A.; Grande, T. Crystal Structure and Thermal Expansion of La1 xSrxFeO3 δ Materials. J. Am. Ceram. Soc. 2004, 87, 1952. (34) Hayward, M. A.; Cussen, E. J.; Claridge, J. B.; Bieringer, M.; Rosseinsky, M. J.; Kiely, C. J.; Blundell, S. J.; Marshall, I. M.; Pratt, F. L. The Hydride Anion in an Extended Transition Metal Oxide Array: LaSrCoO3H0.7. Science 2002, 295, 1882. (35) Bridges, C. A.; Fernandez-Alonso, F.; Goff, J. P.; Rosseinsky, M. J. Observation of Hydride Mobility in the Transition-Metal Oxide Hydride LaSrCoO3H0.7. Adv. Mater. 2006, 18, 3304. (36) Mandal, T. K.; Sebastian, L.; Gopalakrishnan, J.; Abrams, L.; Goodenough, J. B. Hydrogen uptake by barium Manganite at atmospheric pressure. Mater. Res. Bull. 2004, 39, 2257. (37) Sundell, P. G.; Bj€orketun, M. E.; Wahnstr€om, G. Densityfunctional calculations of prefactors and activation energies for H diffusion in BaZrO3. Phys. Rev. B 2007, 76, 094301.
11827
dx.doi.org/10.1021/ie202284z |Ind. Eng. Chem. Res. 2012, 51, 11821–11827
