Interaction of La2NiO4 (100) Surface with Oxygen Molecule: A First-Principles StudyJun Zhou, Gang Chen,* Kai Wu, and Yonghong Cheng

State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, People’s Republic ofChina

*S Supporting Information

ABSTRACT: Along the way to designing of new cathodematerials for solid oxide fuel cells (SOFCs), an understanding ofthe mechanism of oxygen reduction reaction (ORR) plays a keyrole, especially the interaction between O2 molecule and surfaceof cathode. Recently, La2NiO4 with K2NiF4-type structure hasbeen developed, and it has received great attention as an oxygensensor and a potential cathode for SOFCs. However, thechemical activity of La2NiO4, in particular, the ORR on thesurface, has not been studied so thoroughly. In this report, wepresent the structural and energetic results of O2 adsorbed ontothe perfect and defective La2NiO4 (100) surface to elucidate theinteraction mechanism between O2 molecule and cathode usingatomistic computer simulation based on density functionaltheory. The results show that the surface structure and the adsorbed configurations are vital for O2 adsorption. and activation.The adsorbed species on the perfect surface are energetically less favorable than defective surface. The Ni site is preferred withadsorption energy of −1.25 (Ni-super) and −1.80 eV (Ni-per), much higher than these of La site, supporting the fact thattransition-metal cations are more active than lanthanon metals in K2NiF4-type compounds. Surface oxygen vacancy is found toenhance the adsorption energy of O2 molecule on the La2NiO4 (100) surface; in addition, oxygen vacancy can be an active site inO2 adsorption. The most stable configuration is Ni−O−Ni mode, with the highest adsorption energy being −2.61 eV. This canbe confirmed by the analysis of the local density of states (LDOS) and the difference electron density. These results have animportant implication for understanding the ORR on La2NiO4 (100) surface.

1. INTRODUCTION

To meet the fast-growing global energy demand, solid oxidefuel cells (SOFCs) are one of the promising strategies toproduce clean electricity with high efficiency and fuelflexibility.1−3 The SOFC performance is to a large degreedetermined by the catalytic properties of the cathode.4,5 One ofthe main research targets in the field of SOFC is the attempt toreduce the operating temperature from high temperature(∼1000 °C) to intermediate-temperature (IT) (500 to 800°C). A reduced-temperature operation for SOFC has severaladvantages, which include broadening the material choice,improving the long-term performance, and reducing fuel-cellcosts.3 However, the lower operating temperature results in asignificant decrease in catalytic activity of cathode. Therefore,for the further improvement of the fuel cell performance, thedevelopment of high-performance cathode material is critical.Current highly performing cathodes are based on perovskite

structure (ABO3) and related structures (such as K2NiF4).Lanthanum nikelate, La2NiO4+δ with K2NiF4-type structure(see Figure 1), exhibits high mixed oxide ion and electron holeconductivity in combination with relatively fast surfaceexchange kinetics.6−9 Thus, it is an interesting materialcandidate for cathodes used in IT-SOFCs and for oxygen

gas-separation membranes.10,11 In general, the K2NiF4-typeoxides are structurally related to the perovskite oxides andconsist of ABO3 (perovskite) and AO (rock salt) layersalternating in the c direction.12 In these materials, especially inLa2NiO4+δ, the crystal structure is built of alternating rocksaltLa2O2 and perovskite NiO2 layers and can accommodate asignificant oxygen excess.13−15 The extra O2− anions are charge-compensated by the p-type electronic charge carriers andoccupy interstitial positions in the LaO bilayers, while theconcentration of oxygen vacancies in the perovskite sheets isvery low. The information available on defect formation andtransport mechanisms relevant to the oxygen permeation andcatalytic behavior of La2NiO4-based phases is scarce and oftencontradicting.16

Among many factors affecting the chemical-electrical energyconversion, the oxygen reduction reaction (ORR) on cathode isthe pivot in fuel cell.17−19 The ORR is a kinetically slowprocess,20 which dominates the overall performance of a fuelcell. For the design of novel low-cost cathode materials with

Received: March 28, 2013Revised: May 31, 2013

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high catalytic activity for oxygen reduction in IT-SOFCs, it isvery vital to understand the complicated mechanism of ORR.Some works21−23 suggest that ORR at the surface of a mixedionic−electronic conductor (MIEC) cathode consists of manyelementary steps, which include adsorption of a superoxo-(O2

−) or peroxo-like (O22−) species and dissociation of

diatomic oxygen species into the bulk lattice. Althoughnumerous theoretical investigations on perovskite structures(ABO3) have been done,24−31 detailed mechanistic studies ofORR on the surface of La2NiO4 cathode by means of quantumchemical calculations are still lacking.We report the oxygen interaction mechanisms on La2NiO4

surfaces using first-principles calculations based on DFT andpseudopotential method. The structural sensitivity of La2NiO4was studied by examining the adsorption of O2 molecule onperfect and defective (oxygen vacancy) La2NiO4 (100) surface.It was shown that La2NiO4 surface is indeed active for O2activation, and this reaction is structure-sensitive. A varietypossible binding configurations of O2 on the perfect anddefective La2NiO4 (100) surface in terms of geometries,energies, and electronic structures were investigated. Theadsorbing mechanisms of O2 on these surfaces were alsodiscussed with examination of the local density of states(LDOS) and the difference electron density. We anticipate thatresults from the study of O2 adsorption on the perfect anddefective La2NiO4 (100) surface will allow us to understand ingreat detail the chemical activity of the La2NiO4 surfaces.

2. COMPUTATIONAL DETAILS AND SURFACECONFIGURATIONS

All calculations were carried out by periodic density functionaltheory (DFT) with the projector-augmented wave (PAW)method,32 as implemented in the CASTEP.33,34 The correlationinteractions were described using the generalized gradientapproximation (GGA)35 proposed by Perdew and Wang(PW91).36 Electronic wave functions were expanded in aplane-wave basis set, and ionic cores were described by ultrasoftpseudopotentials.37 Here a 340 eV plane-wave basis set thecutoff, and the cutoff energy used the whole calculations toensure convergence. The Monkhorst-Pack38 grids of 3 × 3 × 1

k-points was used for the bulk unit cell, 3 × 2 × 1 k-points forthe (100) surface. A relaxation is performed for the constructedslabs by using Broyden−Fletcher−Goldfarb−Shanno (BFGS)algorithm39 to minimize the energy with respect to atomicposition. The tolerances for self-consistence are set at 1.0 ×10−6 eV/atom for total energy, 0.1 eV/Å for force, 0.2 GPa formaximum stress, 1.0 × 10−5 eV/atom for band energy, and0.005 Å for the maximum displacement, respectively.Experimental surface energies, which are difficult to measure,

are unavailable for La2NiO4. Previous work40 showed that the

order of stability for the relaxed surfaces of undoped La2NiO4is: {100} > {111} > {110} > {001} > {011}. Therefore, onlythe (100) surface of La2NiO4 was considered in this work. The(100) surface has the same termination, which includes alltypes of atoms (La, Ni, and O). To avoid the macroscopicdipole moment perpendicular to the polar surface, we usedsymmetrical slabs terminated on both sides in the same way.For 3D periodic boundary conditions, the metal oxide surfacescomprising the atomic layers were separated by a vacuum spaceequivalent of 10 Å in the direction perpendicular at the builtsurface. In all calculations, the atoms in the bottom layers werefixed, but the atoms in the three topmost layers were allowed torelax. Convergence with respect to k-point sampling, kineticenergy cutoff, and slab thickness was tested and found to besatisfactory. The geometry of the isolated O2 molecule wasoptimized using a large cubic cell of 8 Å × 8 Å × 8 Å, similar toprevious study.41 The predicted O−O bond length was equal to1.24 Å and was in good agreement with the experimental valuesof 1.21 Å. The calculated binding energy of free O2 molecule is6.52 eV. After optimization, the LDOS and the differenceelectron density analyses were performed. These analyses wereused to help to understand the mechanism of the bonding andthe interaction between O2 and the La2NiO4 surfaces.In this work, the adsorption energies (Eads) were calculated

via the following equation

± = − −−E E E Eads sub ad sub ad (1)

where Esub and Ead are the total energy of substrate andadsorbate and Esubad is the total energy of the subad system inthe equilibrium state, respectively. According to the aboveequation, a negative Eads value corresponds to an exothermicadsorption, and the more negative the Eads, the stronger theadsorption. Here a coverage of 10.5% was used for the oxygenmolecule adsorbed on the perfect and defective La2NiO4 (100)surface. To estimate the vacancy formation energy of theoxygen vacancy in the crystal, we used the following expression

= + −−E E E E(F) (F) 1/2 (O ) (perfect)O vac 2 (2)

where E(O2) is the predicted electronic energy of triplet O2 in a8 Å cubic box. E(F) and E(perfect) are energies of the defectiveand perfect crystal, respectively.

3. RESULTS AND DISCUSSION

3.1. Bulk Properties. The stoichiometric nicklate La2NiO4belongs to the Ruddlesden−Popper series with general formulaAn+1MnO3n+1 (n = 1, 2, 3, ...). In such a structure, n AMO3perovskite layers alternate with AO rocksalt-type layer along thec crystallographic direction.42 As for the perovskite structure,the Goldschmidt tolerance factor43 can be used to account forthe effects of the size mismatch between anions and cations inthe A2MO4 structure. It is written as

Figure 1. Crystal structure of bulk La2NiO4.

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=+

+t

r rr r2 ( )

A 0

M O (3)

where rA, rM, and rO are the ionic radii of the An+, Mm+, and O2−

ions, respectively. t = 1 corresponds to an ideal radii match. ForLa2NiO4, t = 0.89,44 indicating that the NiO2 plane is underpressure as a result of the stretched La2O2 layer. This strain mayinfluence the oxygen vacancy formation of either NiO6octahedral or La2O2 layers. Thus, two different oxygen vacancysites were considered in this work. In our computingconfigurations, we calculated the formation energy of anoxygen vacancy based on the reaction of La2NiO4 →La2NiO4−δ + (1/2)δO2(g).To test the accuracy of our approach, we compared the

optimized unit cell parameters obtained using PW91 with theexperimental data and other GGA methods (PBE and rPBE).As shown in Table 1, the predicted unit cell parameters atPW91 level are a = b = 3.769 Å, c = 13.88 Å, and α = β = γ =90°, which is in agreement with the experimental data42 moreclosely than the PBE and rPBE levels. For this reason, in thefollowing sections, only GGA-PW91 energies are used in thediscussion.The migration of the interstitial oxygen (OI) in La2NiO4 has

been by far the most investigated because of the intrinsicfeatures governing ionic conductivity for a direct interstitialmechanism of oxygen ions (OI).

45 However, the oxygenvacancies are important for the ORR in SOFCs as they arelikely to couple strongly to oxygen dissociation, transportation,and incorporation. Meanwhile, an oxygen vacancy has twoeffective positive charges. For bulk, two kinds of oxygen

vacancy sites of NiO6 octahedral were considered here, andthey are a vacancy on the equatorial site (VO1) and a vacancyon the apical site (VO2), respectively. Their calculatedformation energies are 3.73 and 4.01 eV, which are similar toother K2NiF4 structural material.40 However, the oxygenvacancy formation energies for bulk La2NiO4 are larger thanthose of perovskite-type cathodes-based LaMnO3.

25−27

3.2. Structure of La2NiO4 (100) Surface. The surfaceenergies and structures of the La2NiO4 (100) surface were firstinvestigated using slabs of four (3.769 Å × 13.886 Å × 15.654Å), six (3.769 Å × 13.886 Å × 19.423 Å), and eight (3.769 Å ×13.886 Å × 23.541 Å) atomic layers (Figure 2), maintaining thestoichiometric balance. The surface energy was calculated usingthe equation S = (Eslab

N − NEbulk)/2A,where A is the area of thesurface, Eslab

N is the total energy of the surface slabs, N is thenumber of La2NiO4 units in the cell, and Ebulk is the energy perstoichiometric unit of the bulk.46

In Table 2, we report results for slab modes consisting offour, six, and eight layers, respectively. The predicted surfaceenergies of various layers are in agreement with Read’s research(0.98 J/m2) of (100) surface.39 It is found that the surfaceenergies of (100) converged to within 0.12 to 0.14 J/m2,suggesting that four layers for (100) surface is thick enough incalculated modes. Thus, to save computational time, we choseonly a four-layer slab to study O2 adsorption in this work.To determine the surface relaxation of the La2NiO4 (100)

surface, we optimized the clean surface, and no reconstructionwas observed. (See the Supporting Information.) In general,only the uppermost one or two atomic layers exhibit the slightrelaxation, indicating that our optimized modes can be used forfurther research. The oxygen vacancies are vital for the ORR inMIECs cathodes as they are likely to couple strongly to oxygenadsorption, dissociation, and transportation. To study the effectof oxygen vacancies on the O2−cathode interactions on theLa2NiO4 (100) surface with different layers modes, we locatedan oxygen vacancy on the top layers. There are two differentkinds of oxygen vacancy (VO1 and VO2). In the present work,we calculated the formation energies of the oxygen vacanciesbased on the reaction of La2NiO4 (bulk and surface) →La2NiO4−δ + 1/2O2(g). The calculated oxygen vacancy

Table 1. Calculated and Experimental Lattice Parameters ofLa2NiO4

computing methods a = b (Å) c (Å) α = β = γ (deg) V (Å3)

PW91 3.769 13.88 90 197.30PBE 3.775 13.89 90 198.12rPBE 3.975 13.08 90 206.51experimenta 3.869 12.60 90 188.61

aSee ref 45.

Figure 2. Side (a) and top (b) views of the La2NiO4 (100) surface. (Green circle denotes an oxygen vacancy.)

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formation energies for the La2NiO4 (100) surface with differentlayers are listed in Table 2. However, the EVO2 of all layers arelarger than EVO1 for the La2NiO4 (100) surface, totally different

from the oxygen vacancy formation energy of La2NiO4 bulkstructure. Meanwhile, Minervini et al.47 reported that severaldetailed mechanisms of charged defects including oxygen

Table 2. Surface Energy and Oxygen Vacancy Formation Energy of Various La2NiO4 (100) Layersa

layers surface energy (J/m2) EVO1 (eV) EVO2 (eV)

(100) surface plane4 0.37 8.89 5.926 0.49 8.72 6.048 0.51 8.75 6.11

aEVO1 and EVO2 represent the oxygen vacancy formation energy of the La2NiO4 (100) surface.

Figure 3. Optimized adsorption configurations of O2 molecule at the perfect or defective (oxygen vacancy) La2NiO4 (100) surfaces.

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vacancy and oxygen interstitial were predicted. Only one typeof oxygen vacancy (8e site) in bulk was considered in theirstudy. Here only VO2 was considered in our calculations fordefective configurations.3.3. O2 Adsorption on Perfect La2NiO4 (100) Surface.

In general, for O2 adsorption on metal oxide surface, there areseveral adsorbed configurations. On the basis of various bondinteractions, modes can be obtained: (1) O2 only binds X (Laor Ni) sites as superoxo-like species. (2) O2 forms a bond withX sites as peroxo-like species. (3) O2 interacts with oxygen

atoms and oxygen vacancy site. (4) O2 interacts with both Laand Ni sites, respectively. The optimized stable configurationsare shown in Figure 3. The adsorption of O2 on thestoichiometric La2NiO4 (100) surface was investigated ontwo different adsorption sites: the first-layer La atoms and Niatoms. For clarity, the two oxygen atoms of the adsorbed O2were labeled as O1 (shorter bond length for O−X) and O2(longer bond length for O−X) if they were not in equivalentpositions. When O2 molecule is adsorbed on the stoichiometricperfect La2NiO4 (100) surface, the adsorption energy is acriterion to determine the stability of the adsorption. Theoxygen sites were shown to be much less reactive with therespect to O2 molecule adsorption, and only super config-uration was considered. The calculated results of adsorptionenergy (Eads), the equilibrium distance (dO‑X), and the O−Obond length (dO−O) are shown in Table 3. Calculated datashow that the adsorption energy of La-super is −0.49 eV, whichis smaller than that of La-per (−0.71 eV); meanwhile, theadsorption energy of Ni-super (−1.25 eV) is bigger than that ofNi-per (−1.80 eV). This is to say that per-mode has the biggeradsorption energy than the supermode including all of the atomsites, indicating that when O2 molecule is adsorbed on thestoichiometric La2NiO4 (100) surface the per-configuration ismore stable. Moreover, for all adsorption configurations westudied, the Ni-per mode has the highest adsorption energy.

Table 3. Properties of the O2 Adsorption on the La2NiO4(100) Surface

configuration ΔEads (eV) d(O1−X) (Å) d(O1−O2) (Å)

O2 1.214La-super −0.49 2.767 1.275La-per −0.71 2.793 1.279Ni-super −1.25 1.841 1.281Ni-per −1.80 2.004 1.327O-super 0.60 2.506 1.272O-per 0.87 2.599 1.274Ni−O−La −2.35 1.677 3.689Ni−O−Ni −2.61 1.698 2.929Ov-super −1.37 1.924 1.435

Figure 4. Local density of states (LDOS) for the adsorption of O2 on the prefect La2NiO4 (100) surface with various configurations.

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This shows that the Ni site is the preferential adsorption site forstoichiometric La2NiO4 (100) surface, consistent with othersimilar prediction results in some cathodes with perovskitestructures.26,27,40 Interestingly, for O-per and O-super config-urations, the positive value of adsorption energy calculatedfrom the eq 1 showed that it is difficult for oxygen moleculeadsorbed on oxygen site.For the Ni-super configuration, the calculated equilibrium

distance (d(O1−La)) between adsorbed O2 and the La site is1.841 Å, which is short enough to form the Ni−O bond. TheO−O bond length (d(O1−O2)) of the absorbed O2 is found tobe 1.281 Å. However, the bond length of Ni−O elongated to2.004 Å (d (O1−Ni)) for La-per configuration, while the bond

length of the O−O bond is 1.327 Å. Compared with the O−Obond length of the free O2 is 1.214 Å, it is in agreement withthe experimental value of about 1.207 Å.48 However, they areelongated after adsorption on the stoichiometric La2NiO4(100) surface for both configurations (Ni-super and Ni-per).This phenomenon is related to the charge transfer between theO2 and the absorbed clusters, leading to the elongation of thebond.49 Such charge transfer is confirmed by the analysis of theelectronic structure in the following parts.We also calculated the density of states (DOS) for the

isolated O2 molecule (free O2) and for oxygen species absorbedon the more stable adsorption site on the Ni atom (Ni-per andNi-super). Figure 4 shows the DOS of the Ni atom before andafter adsorption with Ni-super and Ni-per configurations. Afteroxygen molecule adsorption, the d, p, and s orbits of Ni atomexhibit significant changes around the Fermi level for both ofNi-per and Ni-super configurations. The d orbits of Ni atom inthese configurations split more orbits near the Fermi level. Theintensity of p orbits of Ni atom decreases in the energy regionof 0−5 eV but becomes stronger in the lower energy levels.Interestingly, the s, p, and d orbits of the Ni atom all participatein the bonding of the Ni−O. In addition, the overlap of DOSpeaks of Ni-d and O-p orbit depicts a stronger hybridizationbetween them, obviously illustrating that the d orbit of the Niatom plays the main role for the bonding of the Ni-super andNi-per configurations. We can also analyze the adsorptionenergies for adsorbed oxygen from electronic effects point ofview. The hybridization can be seen more clearly in the caseswhere the oxygen is in the Ni-per configuration. What makesthe Ni-per mode more favorable than the Ni-super mode is thefact that there is more occupation in the lower bonding state ofthe DOS for the Ni-per mode. Ni-per mode also seems to bemore stable with a lower value in energy.Figure 5 shows the difference electron density maps due to

the adsorption of O2 on La2NiO4 (100) surface with respect to

Figure 5. Side views of the difference electron density (a for Ni-per, c for Ni-super) and the difference electron density maps (b for (010) plane ofNi-super mode, d for the (100) plane of Ni-per mode) highlighting the electron charge density redistribution due to the O2 adsorption.

Figure 6. Local density of states (LDOS) for the adsorption of O2 onthe defective La2NiO4 (100) surface with various configurations (a)Ni−O−La, (b) Ni−O−Ni, and (c) Ov-super.

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the isolated molecule and clean surface. It clearly exhibits thatthe electron density increases between Ni atom and oxygenmolecule in Ni-per and Ni-super configurations. Meanwhile,some electron density depletion is visible near the Ni atom,which oxygen molecule adsorbed on. The density isosurfacesclearly show charge transfer from Ni atom into the adsorbedoxygen species, with charge transfer in Ni-super configurationhaving a lesser extent into the oxygen than that in Ni-per. Itshows that the more the charge transfer the more the O−Obond with lengthening the bond (Table 3). Such structuralchanges illustrate that these molecular states are likelyprecursors for oxygen molecule dissociation.3.4. O2 Adsorption on Defective La2NiO4 (100)

Surface. In this section, we studied O2 molecule adsorptionon the defective La2NiO4 (100) surface with an oxygen vacancy(VO2). To study the defective (VO2) surface, one oxygen atomof the outermost surface is removed from the supercellconfiguration, which is shown in Figure 3. In this Figure, thelocation of VO2 is indicated by a green cycle. From ourcalculated results, the adsorption energy of the defective-O-super configurations (Ov-super) is −1.37 eV, which is biggerthan that of perfect La-super (−0.49 eV), La-per (−0.71 eV),and Ni-super (−1.25 eV). This result shows that oxygenvacancies are also active sites on the surface of metal oxides.Thus, in the following, we consider O2 adsorption at a surface

with oxygen vacancy. In the case of one atomic O adsorbed onan oxygen vacancy and another atomic O binding to Ni site, wefound that configurations shown in Figure 3g (Ni−O−La) andFigure 3h (Ni−O−Ni) are much more active. For theseconfigurations, the adsorption energy increased distinctly to therange from −2.35 to −2.61 eV, both larger than those of otherconfigurations in this work. Compared with the perfect-configurations, the Ni−O bond lengths (dNi−O1) of Ni−O−La and Ni−O−Ni configurations decreased to 1.677 and 1.698Å after adsorption, but the O−O bond lengths distinctlyelongated to 3.689 and 2.929 Å. It also proved that the Ni−O−La and Ni−O−Ni modes are more stable. Moreover, thisphenomenon is associated with the charge transfer between theO2 molecule and the absorbate. This charge transfer weakensthe O−O bond by lengthening its bond in the process ofadsorption. For all adsorption configurations, the Ni−O−Nihas the highest adsorption energy and smallest dNi−O1, showingthat this adsorbed configuration is most stable.Furthermore, the adsorption mechanism was examined by

local densities of states (LDOS) calculations for the adsorbedoxygen species on an oxygen vacancy and another atomic Obinding to Ni site, including Ni−O−La, Ni−O−Ni, and Ov-super configurations. As shown in Figure 6, after adsorption,the d orbits of Ni atom and the p orbits of O atom showsignificant hybridization for these configurations. However, a

Figure 7. Side views of the difference electron density and the difference electron density highlighting the electron charge density redistribution in(a,b) the Ni−O−Ni mode, (c,d) the Ni−O−La mode, and (e,f) the Ov-super mode, respectively.

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stronger hybridization can be found in Ni−O−Ni config-uration, indicating that this configuration is more stable, inagreement with the adsorption energy analysis. This is alsoinvestigated by plotting the difference electron density andexhibiting at the regions of charge accumulation and depletionupon adsorption, as shown in Figure 7. Similarly, we can seethat there is a distinct charge accumulation between the speciesand the Ni atoms in these configurations. It confirms thepresence of a strong covalent bond between O2 and the surface.Atomic oxygen adsorption causes significant electron redis-tribution. In addition, an obvious charge accumulation occurredon the Ni−O−Ni configuration.

4. CONCLUSIONS

For the understanding of the mechanism of ORR, periodicDFT calculations have been performed to study the oxygenmolecule adsorption on the La2NiO4 (100) surface. We haveconsidered the interaction of O2 with stoichiometric perfectand containing oxygen vacancy of La2NiO4 (100) surface. Thesurface structure, the vacancy formation energy, the most stableadsorption configuration, and the adsorption energy as well asthe LDOS and the difference electron density for theinteraction of O2 on the perfect- and defective-La2NiO4(100) surface were systematically investigated. Comparedwith the oxygen vacancy in O1 site (VO1), the oxygen vacancyin O2 site (VO2) was created much more easily in La2NiO4(100) surface by predicted the oxygen vacancy formationenergy. For the perfect (100) surface, the most favorableoxygen adsorption sites are observed to be atop surface Niatoms (Ni-super and Ni-per configurations), with adsorptionenergies being −1.25 and −1.80 eV, respectively. For defectiveLa2NiO4 (100) surface, surface oxygen vacancy was found toplay an important role and can significantly enhance theadsorption energy of O2 molecule with the metal-oxide’ssurface: the intermediates on the defective surface areenergetically more favorable than perfect surface for allconfigurations. Moreover, oxygen vacancies are also activesites on the surface of La2NiO4. The highest adsorption energy(−2.61 eV) can be obtained in the Ni−O−Ni configuration.Analysis of the LDOS and the difference electron density mapsfor all adsorbed O2 on La2NiO4 (100) surfaces shows that O2accepted electrons from the surface, especially Ni atoms. This isalso confirmed from the strong hybridization between Ni (3d)and O (2p) and shorter Ni−O bond length. We may concludethat our results demonstrate that the surface structure ofLa2NiO4 is vital for O2 adsorption. Oxygen vacancy not onlywas observed to distinctly modify the adsorption energies of O2on La2NiO4 (100) surface but also participated to be an activesite in O2 reduction reaction. This result is useful for furtherresearch of new cathode design in SOFC and gas sensor.

■ ASSOCIATED CONTENT

*S Supporting InformationIon displacement for the top layer of the La2NiO4 (100)surface. This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: chainway@mail.xjtu.edu.cn. Tel: +86 029 82668493.Fax: +86 029 82668493.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This project was financially supported by the FundamentalResearch Funds for the Central Universities and the State KeyLaboratory of Electrical Insulation and Power Equipment, Xi’anJiaotong University.

■ REFERENCES(1) Haile, S. M. Fuel Cell Materials and Components. Acta Mater.2003, 51, 5981−6000.(2) Fleig, J.; Maier, J. The Polarization of Mixed Conducting SOFCCathodes: Effects of Surface Reaction Coefficient, Ionic Conductivityand Geometry. J. Eur. Ceram. Soc. 2004, 24, 1343−1347.(3) Steele, B.; Heinzel, A. Materials for Fuel-Cell Technologies.Nature 2001, 414, 345−352.(4) Perez-Coll, D.; Aguaderoa, A.; Escuderoa, M. J.; Daza, L. Effect ofDC Current Polarization On the Electrochemical Behaviour ofLa2NiO4+δ and La3Ni2O7+δ-Based Systems. J. Power Sources 2009,192, 2−13.(5) Tarancon, A.; Burriel, M.; Santiso, J.; Skinner, S. J.; Kilner, J. A.Advances in Layered Oxide Cathodes for Intermediate TemperatureSolid Oxide Fuel Cells. J. Mater. Chem. 2010, 20, 3799−3813.(6) Mauvy, F.; Lalanne, C.; Bassat, J. M.; Grenier, J. C.; Zhao, H.Oxygen Reduction on Porous Ln2NiO4+δ Electrodes. J. Eur. Ceram.Soc. 2005, 25, 2669−2672.(7) Skinner, S. J.; Kilner, J. A. Oxygen Diffusion and SurfaceExchange in La2−xSrxNiO4+δ. Solid State Ionics 2000, 135, 709−712.(8) Skinner, S. J.; Kilner, J. A. A Comparison of the TransportProperties of La2−xSrxNi1−yFeyO4+δ Where 0< x < 0.2 and 0< y < 0.2.Ionics 1999, 5, 171−174.(9) Boehm, E.; Bassat, J. M.; Steil, M. C.; Dordor, P.; Mauvy, F.;Grenier, J. C. Oxygen Transport Properties of La2Ni1−xCuxO4+δ MixedConducting Oxides. Solid State Sci. 2003, 5, 973−981.(10) Zhou, J.; Chen, G.; Wu, K.; Chen, Y. H. La0.8Sr1.2CoO4+δ−CGOComposite as Cathode on La0.9Sr0.1Ga0.8Mg0.2O3−δ Electrolyte forIntermediate Temperature Solid Oxide Fuel Cells. J. Power Sources2013, 232, 332−337.(11) Boehm, E.; Bassat, J. M.; Dordor, P.; Mauvy, F.; Grenier, J. C.;Stevens, P. Oxygen Diffusion and Transport Properties in Non-Stoichiometric Ln2‑xNiO4+δ Oxides. Solid State Ionics 2005, 176, 2717−2725.(12) Munnings, C. N.; Skinner, S. J.; Amow, G.; Whitfield, P. S.;Davidson, I. J. Oxygen Transport in the La2Ni1−xCoxO4+δ System. SolidState Ionics 2005, 176, 1895−1901.(13) Jorgensen, J. D.; Dabrowski, B.; Pei, S.; Richards, D. R.; Hinks,D. G. Structure of the Interstitial Oxygen Defect in La2NiO4+δ. Phys.Rev. B 1989, 40, 2187−2199.(14) Demourgues, A.; Wattiaux, A.; Grenier, J. C.; Pouchard, M.;Soubeyroux, J. L.; Dance, J. M.; Hagenmuller, P. ElectrochemicalPreparation and Structural Characterization of La2NiO4+δ Phases (0 ≤δ ≤ 0.25). J. Solid State Chem. 1993, 105, 458−468.(15) Naumovich, E. N.; Patrakeev, M. V.; Kharton, V. V.;Yaremchenko, A. A.; Logvinovich, D. I.; Marques, F. M. B. OxygenNonstoichiometry in La2Ni(M)O4+δ (M = Cu, Co) Under OxidizingConditions. Solid State Sci. 2005, 7, 1353−1362.(16) Kharton, V. V.; Tsipis, E. V.; Naumovich, E. N.; Thursfield, A.;Patrakeev, M. V.; Kolotygin, V. A.; Waerenborgh, J. C.; Metcalfe, I. S.Mixed Conductivity, Oxygen Permeability and Redox Behavior ofK2NiF4-type La2Ni0.9Fe0.1O4+δ. J. Solid State Chem. 2008, 181, 1425−1433.(17) Choi, Y.; Mebane, D. S.; Wang, J.; Liu, M. Continuum andQuantum-Chemical Modeling of Oxygen Reduction on the Cathodein a Solid Oxide Fuel Cell. Top Catal 2007, 46, 386−401.(18) Adler, S. B. Factors Governing Oxygen Reduction in SolidOxide Fuel Cell Cathodes. Chem Rev 2004, 104, 4791−4844.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp403094x | J. Phys. Chem. C XXXX, XXX, XXX−XXXH

(19) Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reactionon Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011,115, 11170−11176.(20) Liu, M.; Winnick, J. Fundamental Issues in Modeling of MixedIonic-Electronic Conductors (MIECs). Solid State Ionics 1999, 118,11−21.(21) Steele, B. C. H. Survey of Materials Selection for Ceramic FuelCells II. Cathodes and Anodes. Solid State Ionics 1996, 86, 1223−1234.(22) Liu, M. Equivalent Circuit Approximation to Porous Mixed-Conducting Oxygen Electrodes in Solid-State Cells. J. Electrochem. Soc.1998, 145, 142−154.(23) Wang, Y.; Cheng, H. P. Oxygen Reduction Activity onPerovskite Oxide Surfaces: A Comparative First-Principles Study ofLaMnO3, LaFeO3, and LaCrO3. J. Phys. Chem. C 2013, 117, 2106−2112.(24) Kotomin, E.; Mastrikov, Y.; Heifets, E.; Maier, J. Adsorption ofAtomic and Molecular Oxygen on the LaMnO3(001) Surface: Ab initioSupercell Calculations and Thermodynamics. Phys. Chem. Chem. Phys.2008, 10, 4644−4649.(25) Mastrikov, Y.; Merkle, R.; Heifets, E.; Kotomin, E.; Maier, J.Pathways for Oxygen Incorporation in Mixed Conducting Perovskites:A DFT-Based Mechanistic Analysis for (La, Sr)MnO3−δ. J. Phys. Chem.C 2010, 114, 3017−3027.(26) Choi, Y.; Liu, M. C.; Liu, M. Computational Study on theCatalytic Mechanism of Oxygen Reduction Reaction onLa0.5Sr0.5MnO3 in Solid Oxide Fuel Cells. Angew. Chem., Int. Ed.2007, 46, 7214−7219.(27) Chen, H.; Raghunath, P.; Liu, M. C. ComputationalInvestigation of O2 Reduction and Diffusion on 25% Sr-DopedLaMnO3 Cathodes in Solid Oxide Fuel Cells. Langmuir 2011, 27,6787−6793.(28) Choi, Y.; Liu, M. C.; Liu, M. Rational Design of Novel CathodeMaterials in Solid Oxide Fuel Cells Using First-Principles Simulations.J. Power Sources 2010, 195, 1441−1445.(29) Lee, Y.; Kleis, J.; Rossmeisl, J.; Horn, Y. S.; Morgan, D.Prediction of Solid Oxide Fuel Cell Cathode Activity with First-Principles Descriptors. Energy Environ. Sci. 2011, 4, 3966−3970.(30) Wang, L.; Merkle, R.; Mastrikov, Y. A.; Kotomin, E. A.; Maier, J.Oxygen Exchange Kinetics on Solid Oxide Fuel Cell CathodeMaterials-General Trends and Their Mechanistic Interpretation. J.Mater. Res. 2012, 27, 2000−2008.(31) Kuklja, M. M.; Kotomin, E. A.; Merkle, R.; Mastrikov, Y. A.;Maier, J. Combined Theoretical and Experimental Analysis ofProcesses Determining Cathode Performance in Solid Oxide FuelCells. Phys. Chem. Chem. Phys. 2013, 15, 5443−5471.(32) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B1994, 50, 17953−17979.(33) Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M.C.; Akhmataskaya, E. V.; Nobes, R. H. Electronic Structure, Properties,and Phase Stability of Inorganic Crystals: A Pseudopotential Plane-Wave Study. Int. J. Quantum Chem. 2000, 77, 895−910.(34) Segall, M. D.; Lindan, P. J. 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. Matter2002, 14, 2717−2744.(35) Anisimov, V. I.; Aryasetiawan, F.; Lichtenstein, A. I. First-Principles Calculations of the Electronic Structure and Spectra ofStrongly Correlated Systems: the LDA+ U Method. J. Phys.: Condens.Matter 1997, 9, 767−808.(36) Wang, L.; Maxisch, T.; Ceder, G. Oxidation Energies ofTransition Metal Oxides within the GGA+ U Framework. Phys. Rev. B2006, 73, 1951071−1951076.(37) Vanderbilt, D. Soft Self-Consistent Pseudopotentials in aGeneralized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892−7895.(38) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-ZoneIntegrations. Phys. Rev. B 1976, 13, 5188−5192.(39) Pfrommer, B. G.; Cote, M.; Louie, S. G.; Cohen, M. L.Relaxation of Crystals with the Quasi-Newton Method. J. Comput.Phys. 1997, 131, 233−240.

(40) Read, M. S.; Islam, M. S.; Watsonb, G. W.; Hancock, F. E.Surface Structures and Defect Properties of Pure and Doped La2NiO4.J. Mater. Chem. 2001, 11, 2597−2602.(41) Zhou, J.; Chen, G.; Wu, K.; Cheng, Y. H.; Peng, B.; Guo, J. J.;Jiang, Y. Z. Density Functional Theory Study on Oxygen Adsorptionin LaSrCoO4: An Extended Cathode Material for Solid Oxide FuelCells. Appl. Surf. Sci. 2012, 258, 3133−3138.(42) Rodriguez-Carvajal, J.; Fernandez-Diaz, M. T.; Martinez, J. L.Neutron Diffraction Study on Structural and Magnetic Properties ofLa2NiO4. J. Phys.: Condens. Matter 1991, 3, 3215−3234.(43) Tang, J. P.; Dass, R. I.; Manthiram, A. Comparison of theCrystal Chemistry and Electrical Properties of La2−xAxNiO4 (A = Ca,Sr, and Ba). Mater. Res. Bull. 2000, 35, 411−424.(44) Frayret, C.; Villesuzanne, A.; Pouchard, M. Application ofDensity Functional Theory to the Modeling of the Mixed Ionic andElectronic Conductor La2NiO4+δ: Lattice Relaxation, Oxygen Mobility,and Energetics of Frenkel Defects. Chem. Mater. 2005, 17, 6538−6544.(45) Read, M.; Islam, M.; King, F.; Hancock, F. Defect Chemistry ofLa2Ni1‑xMxO4 (M = Mn, Fe, Co, Cu): Relevance to Catalytic Behavior.J. Phys. Chem. B 1999, 103, 1558−1562.(46) Liu, L.; Zhao, W.; Sun, H.; Li, P.; Sun, L. Surface Dependence ofCO2 Adsorption on Zn2GeO4. Langmuir 2012, 28, 10415−10424.(47) Minervini, L.; Grimes, R. W.; Kilner, J. A.; Sickafus, K. E.Oxygen Migration in La2NiO4-δ. J. Mater. Chem. 2000, 10, 2349−2354.(48) Bielanski, A.; Haber, J. Oxygen in Catalysis; Marcel Dekker: NewYork, 1991.(49) Zhou, Y.; Lu, Z.; Wei, B.; Zhu, X.; Huang, X.; Jiang, W.; Su, W.Oxygen Adsorption on the Ag/La1‑xSrxMnO3(0 0 1) CatalystsSurfaces: A First-Principles Study. J. Power Sources 2012, 209, 158−162.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp403094x | J. Phys. Chem. C XXXX, XXX, XXX−XXXI