Surface Science 611 (2013) 5–9

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Surface Science

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A computational study of the effect of alloying additions on the stability ofNi/c-ZrO2 interfaces

Oleksandr I. Malyi a,b,⁎⁎, Vadym V. Kulish a, Kewu Bai c, Ping Wu d,⁎⁎, Zhong Chen a,⁎a School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singaporeb Department of Mechanical Engineering, National University of Singapore, Block EA #02-21, 9 Engineering Drive 1, Singapore 117576, Singaporec Institute of High Performance Computing, 1 Fusionopolis Way, 16-16 Connexis, Singapore 138632, Singapored Engineering Product Development, Singapore University of Technology and Design, 20 Dover Drive Singapore 138682, Singapore

⁎ Correspondence to: Z. Chen, School of Materials ScieTechnological University, 50 Nanyang Avenue, Singapor67904256; fax: +65 67909081.⁎⁎ Corresponding authors.

E-mail addresses: mpeom@nus.edu.sg (O.I. Malyi), wASZChen@ntu.edu.sg (Z. Chen).

0039-6028/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.susc.2013.01.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 November 2012Accepted 3 January 2013Available online 12 January 2013

Keywords:InterfaceNi/c-ZrO2

StabilitySOFCDFT

Design of new anode materials for solid oxide fuel cells (SOFCs) demands the understanding of properties ofNi/cubic-(c-) ZrO2 interfaces. In this work, we investigate the effect of 9 alloying additions (Ag, Au, Cd, Co, Cu,Fe, Sn, Sb, and V) on the stability of Ni/c-ZrO2 interfaces. We provide an analysis of the impact of oxygenpartial pressure on segregation/desegregation behavior of the dopants. Based on the performed calculations,we show that addition of Co, Fe, or V to the classical SOFC anode can improve its stability under typical SOFCoperating conditions. We also predict that Ag, Au, Cd, Cu, Sn, and Sb alloying additions might increase the ag-glomeration rate of the metal particles. Nevertheless, at low doping concentrations and high anode porositythe negative effect might be minimized by segregation of alloying additions at a Ni surface. Predicted resultsare of significant interest for the design of bimetallic cermet for SOFC anode materials.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Adhesion between a metal and a ceramic determines the stabilityof metal–ceramic interfaces and the long-term performance of adevice. It is particularly relevant for solid oxide fuel cells (SOFCs)because triple phase boundary (TPB) plays a key role in hydrogenoxidation [1,2]. Furthermore, weak “wettability” between metal andzirconia is the main cause of long-term degradation of hydrogenSOFC performance [3–7]. This is because the average size of metalparticles in classical Ni/yttria-stabilized zirconia (YSZ) anode mate-rials tends to increase under high temperature operating conditions.For instance, Jiang investigated the microstructure evolution of theanode materials and showed that the agglomeration behaviors ofthe metal are strongly dependent on Ni and YSZ content in the cermet[5]. He observed that for the cermets with the same initial particlesizes of the Ni phase (1.54±0.55 μm), after sintering at 1273 K for2000 h the average size of Ni particles in the cermets increased to3.60±2.13 μm and 2.7±0.84 μm for Ni (70 vol.%)/YSZ (30 vol.%)and Ni (50 vol.%)/YSZ (50 vol.%), respectively. Such microstructuralchanges induce reduction of TPB density and degradation of the

nce and Engineering, Nanyange 639798, Singapore. Tel.: +65

uping@sutd.edu.sg (P. Wu),

rights reserved.

anode materials [3–8]. Hence, it is clear that studies of Ni/zirconia(ZrO2) interfaces and development of methods for their improvementare the basis for the design of new SOFC anode materials. Unfortu-nately, modern experimental methods do not allow measuring themain properties of solid–solid interfaces. Therefore, previous experi-mental works were focused on the investigations of the wettabilitybetween liquid metals and different zirconia polymorphs [9,10]. Al-though, in some extreme cases, these investigations can be used forthe analysis of solid–solid interfaces, the nature of solid–solid andsolid–liquid interfaces can be quite different even for the same mate-rials. Therefore, a lot of different theoretical methods for modeling ofsolid–solid interface have been developed. Among the previous theo-retical studies several most fundamental ones stand out [11–14].These works not only provided guidance for investigation of metal/oxide interfaces, but also analyzed different approaches to study theinterfaces. More recently, the proposed guidance was used for inves-tigations of the effect of vacancies, doping atoms, and impurities onthe stability of metal/oxide interfaces [7,15–19]. Despite these studiesthe effect of alloying additions on the stability of Ni/cubic(c-) ZrO2 in-terfaces is not disclosed. As far as we know, only one paper has beendevoted to the theoretical analysis of Ni-alloys/c-ZrO2 interfaces [15].In their paper, Kulkova and co-workers have analyzed the effectof only Fe addition on the stability of Ni(001)/O-terminatedc-ZrO2(001) (Ni/O) and Ni(001)/Zr-terminated c-ZrO2(001) (Ni/Zr)interfaces. Although their paper provided some information on Ni–Fe/c-ZrO2 interfaces, their work was limited to the investigation ofnon-stoichiometric interface and segregation/desegregation of Feaddition was not discussed.

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Analysis of prospective SOFC anode materials shows that, in somecases, bimetallic cermets can provide better performance than theclassical anode materials. Several works showed that sulfur and car-bon tolerances of the anode can be improved by adding Sn, Sb, Cu,etc. [20–24]. Nevertheless, the smallest modifications in the metallicpart of the cermet can induce changes of properties of the whole sys-tem. For instance, it is well known that replacement of Ni by Cu leadsto faster agglomeration of the metal particles, thus limiting the poten-tial application of Cu-based anode materials [25]. Therefore, in thispaper, we report the effect of 9 alloying additions (Ag, Au, Cd, Co,Cu, Fe, Sn, Sb, and V) on properties of most stable stoichiometricand non-stoichiometric Ni/c-ZrO2 interfaces, predicting how differentalloying additions can change the interface stability (the rate ofagglomeration of metal particles in real SOFC anode materials). Wealso explain the effect of oxygen partial pressure on the segregationof the alloying additions and suggest some alloying additions thatcan improve the interfaces and the classical anode materials underdifferent oxygen partial pressures. It should be noted that despitethe fact that the classical anode materials contain YSZ, in this study,we investigate the effect of alloying on the stability of Ni/c-ZrO2 inter-face. The details of such treatment have been widely discussed[7,12–17,26,27].

2. Methodology

All calculations were carried out using the PWSCF (Plane-WaveSelf-Consistent Field) computational code and the Perdew–Burke–Ernzerhof (PBE) functional [28] as implemented in the QuantumEspresso package [29]. The ultrasoft pseudopotentials were used fora description of the electron–ion interactions. The plane-wave kineticenergy and charge density cutoffs were set as 40 Ry and 400 Ry, re-spectively. With the selected modeling parameters, the predicted lat-tice constants and formation heats of c-ZrO2 and Ni structures agree

Fig. 1. Side views of different types of interface models: a) metal-absorbed cubic zirconiac-ZrO2(011) interface without a vacuum region; d) top views of Ni(001)/c-ZrO2(001) and Ni(

well with experimental data. The Brillouin-zone integrations wereperformed using Monkhorst–Pack grids [30]: a 3×3×1 mesh formetal/ceramic interfaces and surfaces. The Marzari–Vanderbilt coldsmearing [31] with a smearing factor of 0.025 Ry was used for allspin polarized calculations. All atoms were relaxed by using theBFGS (Broyden–Fletcher–Goldfarb–Shanno) method until the maxi-mum force on each atom was less than 0.01 eV/Å.

Analysis of literature shows that several different models can beused to study properties of metal–zirconia interfaces. In the simplestcase, studies of adsorption of single metal atoms or metal clusters onthe oxide surfaces (Fig. 1a) can provide some information on metal–zirconia interaction [32]. However, such type of model cannot provideany information on the interface stability as a function of oxygen par-tial pressure, interface morphology, etc. Solid–solid interfaces canalso be studied following a supercell approach with periodic bound-ary conditions by stacking two solid surfaces having a small latticemisfit. It should also be noted that two types of models are based onthis approach: 1) model with a vacuum region (typically 10–15 Å,see Fig. 1b); and 2) model without a vacuum region (sandwich slabmodel, see Fig. 1c). The model with a vacuum region is closer to thereal systems, but it has significant limitation in practical application.This is because the size of modeled systems, especially in the firstprinciples calculations, is small and, consequently, the vacuum regionmight affect the solid–solid interface. In the last decade, several re-search groups showed that using the sandwich slab model for predic-tions of interface properties of different metal/oxide systems providesa better agreement with experimental observations than the modelwith a vacuum region [12,15]. Therefore, to study the properties ofideal Ni/c-ZrO2 interfaces, we considered a periodic sandwich slabmodel with seven layers of c-ZrO2 and five layers of the metal(alloy). To avoid large lattice mismatch, we investigated thewell-matched interfaces (see Fig. 1d) [12,14,15,27]. For all consideredinterfaces the predicted relative changes of 2D unit-cell areas are

(001) surface; b) Ni(011)/c-ZrO2(011) interface with a vacuum region; c) Ni(011)/011)/c-ZrO2(011) interfaces. Only three nearest planes to the interfaces are represented.

Fig. 2. Lateral views of a Ni(001)/Zr terminated c-ZrO2(001) (Ni/Zr) interface supercell used for calculations of segregation energy. a) Ni/Zr interface supercell with alloying addi-tions at the interface. b) Ni/Zr interface supercell with alloying additions at the central layer. Green, red, gray, and brown circles are zirconium, oxygen, nickel, and alloying additionatoms, respectively.

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within 1.5% of those for equilibrium structures. According to theabove studies for such range of mismatch between ceramic, metal,and interface cross areas, the actual interface structures containlarge coherent regions separated by misfit dislocations or small in-commensurate disordered regions. Hence, predicted results can beused to analyze the real interfaces.

To understand the effect of alloying additions on stability of themetal (alloy)/c-ZrO2 interfaces, we predicted the work of separation(Wsep, according to Eq. (1))

Wsep ¼ Em þ Eс−ZrO2−Em=с−ZrO2

� �=2A ð1Þ

where Em=c−ZrO2refers to the total energy of a relaxed interface system;

Em and Ec−ZrO2refer to the total energies of the same supercells

containing a single slab of the metal (alloy) and c-ZrO2, respectively; Ais the area of the interface and the factor 2 represents two interfacesin the supercell. Herewith, to avoid interaction between alloying addi-tion and its periodic images, 2×2 interface supercells were used (seeFig. 2–3). To verify the proposed models, some of the calculationsreported below have been recalculated for larger 3×3 and 3×2 inter-face supercells. Predicted changes for segregation energies and the

Fig. 3. Top views of metal doped Ni(001)/Zr-terminated c-ZrO2(001) (a), Ni(001)/O-termiinterfaces are represented. Green, red, gray, and brown circles are zirconium, oxygen, nicke

work of separations were within 0.25% of the results for 2×2 interfacesupercells.

3. Results and discussion

First, we studied the segregation/desegregation behavior of thealloying additions by calculating their segregation energies as a differ-ence of energies for two systems: (1) alloying atom located at theinterface and (2) alloying atom located at the central layer (seeFig. 2). Predicted results, listed in Table 1, suggest that segregation/desegregation behavior of alloying additions is sensitive to both inter-face stoichiometry and interface orientation. Keeping in mind that ithas been shown thatNi/Zr, Ni/O, and stoichiometric interfaces are stableunder oxygenpoor (−3.781 eV>ΔμO), oxygen rich (ΔμO>−1.912 eV),and intermediate conditions (where ΔμO represents difference of oxy-gen chemical potential in zirconia and reference state (O2)), respectively[7], we conclude that oxygen partial pressure determines behavior ofalloying additions.

Predicted segregation energies at the Ni/O and stoichiometric inter-faces suggest that alloying additions with less occupied d-states thanthat of Ni (Co, Fe, and V) tend to segregate at the interface layers, or atleast “feel” a segregation effect reducing the segregation energy. Thisbehavior is different with that for a Ni surface, we remind that for

nated c-ZrO2(001) (a), and Ni(011)/c-ZrO2(011) (b). Only three nearest planes to thel, and alloying addition atoms, respectively.

Table 1Predicted segregation energies (in eV) for different alloying additions at Ni/O, Ni/Zr,and Ni(011)/c-ZrO2(011) interfaces.

Alloying addition Ni/O Ni/Zr Ni(011)/c-ZrO2(011)

Ag 0.346 −0.271 −0.195Au 0.896 −0.562 −0.759Cd −0.110 0.067 −0.613Co −0.601 0.312 −0.597Cu 0.114 0.051 −0.847Fe −0.514 0.651 −0.456Sb −0.037 −0.010 −0.931Sn −0.215 0.191 −0.266V −2.177 1.201 −0.654

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ideal Ni surfaces Co, Fe, and V additions have desegregation proper-ties [20]. Consideration of the interface cells (see Fig. 1d) can givesome understanding of these observations. For both the Ni/O and stoi-chiometric interfaces, the interface stabilities are determined by Ni–Ointeractions, but the strengths of the interactions are different for the in-terfaces. Analysis of Bader charges shows that atNi/O interfaceNi atomshave a charge reduction of 0.48e, indicating the existence of a strongNi\O bond. Similar results were obtained for stoichiometric Ni(011)/c-ZrO2(011) interface, however, in this case the charge transfer is small-er (~0.09e). It is also in agreement with the predicted workof separations; the work of separation for stoichiometric interface(1.57 J m−2) is more than three times smaller than that of Ni/O inter-face (5.74 J m−2). Since Ni/O bonds are stronger at the Ni/O interfacecompared to those for the stoichiometric interface, the interface effectis more noticeable for the Ni/O interface (see Table 1). For instance,the segregation energy of V is−2.177 eV (for the stoichiometric inter-face it is −0.654 eV), suggesting that at low doping concentrations Vmight form Ni–V alloys within the first interface layer. Analysis of theBader charges indicates that the segregation effect is caused by increas-ing the charge reduction at the Ni–V layers. V atoms have charge reduc-tions of ~0.55e and ~0.25e at the Ni/O and stoichiometric interfaces,respectively, while the total charge transfers from Ni–V slabs to the

Fig. 4. Predicted work of separations for pure and alloyed Ni(001)/O-terminated c-ZrO2(00color shows alloying additions having tendencies to increase the work of separation for the

oxide slabs are 0.06e and 0.02e larger than those for ideal Ni/O and stoi-chiometric interfaces, respectively. For both theNi/O and stoichiometricinterfaces alloying additions with more occupied d-states (Ag, Au, Cd,Cu, Sb, and Sn) “feel” a desegregate effect increasing the segregation en-ergies. So, Ag, Au, and Cu have a desegregation behavior at the Ni/O in-terface. This is differentwith that of pureNi surfaces [20]. To understandthis observation, the segregation behavior of a Ag atom at different in-terfaces and Ni surfaces were studied in detail. We found that Agatoms segregate at a pure Ni surface (the segregation energy of aAg atom at a Ni(001) surface is −1.36 eV [20]), while at the Ni/Oand stoichiometric interfaces the segregation energies are 0.346 eVand −0.195 eV, respectively. The increase in the segregation energy iscaused by the reduction of charge transfer from Ni–Ag layers to theoxide. Ag atoms have charge reductions of 0.37e and 0.01e at theNi–Ag/O and Ni–Ag stoichiometric interfaces, respectively, while thetotal charge transfers from Ni–Ag slabs to the oxide slabs are 0.08eand 0.07e smaller than those for the ideal Ni/O and stoichiometric inter-faces, respectively. These results imply that the interface effect is stron-ger for the Ni/O interface than for the stoichiometric interface, hence,the Ag atom has a larger segregation energy at the Ni/O interface.

As stability of the Ni/Zr interface is determined by strong Ni–Zrmetallic bonds, therefore, substitution of one Ni atom by alloying ad-dition leads to the formation of new Ni–M andM–Zr bonds. Herewith,it is expected that this increases the bonding within the alloy layer(first interface Ni–M layer) and weakens the interface stability [15];hence, most alloying additions (except Ag, Au, and Sb) have desegre-gation properties.

To understand howalloying additions can change adhesion betweenNi and c-ZrO2, the effect of alloying additions on the work of separationwas investigated. We used the same sizes of interface supercells as forpredictions of segregation energies with alloying atoms located at theinterfaces. To avoid significant interface reconstructions, the interfacemodels contained the same concentration of alloying additions at bothinterfaces (see Fig. 3 and Fig. S1).

Predicted results for the interface stability (see Fig. 4) are of signif-icant interest for practical applications. For instance, we observed that

1) (a), Ni(001)/Zr-terminated c-ZrO2(001) (b), and Ni(011)/c-ZrO2(011) (c). The redcorresponding interfaces.

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Co, V, and Fe alloying additions, at least for low doping concentra-tions, can increase the work of separation for the Ni/O and stoichio-metric interfaces. Thus, for a Ni–V/O interface the predicted work ofseparation (6.19 J m−2) is 0.45 J m−2 higher than that for the idealNi/O interface. For the stoichiometric interface the predicted effectof alloying additions is less visible. Nevertheless, Co addition at thestoichiometric interface induces increasing the work of separationfrom 1.57 J m−2 to 1.63 J m−2. These results indicate that for highand intermediate oxygen partial pressures Co, V, and Fe alloying addi-tions can improve the stability of the Ni/c-ZrO2 interfaces. Takinginto account that the oxide is in the equilibrium with H2/H2O atΔμO≈−2.7 eV [33], it becomes clear that the above dopants can re-duce the rate for particle agglomeration under SOFC operating condi-tions. Because it is well known that the reduction of TPB is the mainreason for long term degradation of SOFC performance, additionsof Co, V, and Fe can improve the stability of the anode materials.Hence, the above dopants are attractive for the design of SOFCanode materials operating on hydrogen. Co addition is especially rel-evant, as it can improve the catalytic properties of the anode mate-rials [21]. This prediction is also of significant interest for the designof bimetallic anode materials. This is because a relatively high con-centration of Co alloying additions within the top surface layers isnecessary to improve the catalytic properties of Ni [21]. Since our cal-culations show that at low dopant concentrations Co tends to segre-gate at the Ni/c-ZrO2 interfaces, we hypothesize that the catalyticproperties of Ni–Co/c-ZrO2 based anode materials are extremely sen-sitive to the method of anode preparation.

Alloying additions reducing the interface stability are expected toincrease the agglomeration rate of the metal particles, resulting intodegradation of SOFC performance. Nevertheless, it is important to an-alyze the effect of alloying additions in the light of their segregation/desegregation behavior at the interfaces and Ni surfaces. The fact isthat due to the minimization of the Gibbs free energy for some sys-tems it is thermodynamically unfavorable to realize a high concentra-tion of the alloying additions at the interfaces. For instance, Cd, Sn,and Sb alloying additions have a strong tendency to segregate at Nisurfaces (Esegb−1.68) [20], while the segregation energies at the in-terfaces are about 1 eV higher, indicating that for the alloying ele-ments it is thermodynamically more favorable to segregate at the Nisurfaces. Since it is well known that SOFC anode materials, in partic-ular Ni/YSZ cermets, have significant porosity, it is clear that at lowdoping concentrations, the additions tend to segregate at the Ni sur-faces, consequently, their effect on the interface stabilities is limited.However, the reduction of the system porosity, resulting into a reduc-tion of the Ni surface area—“sink” for the alloying additions, or an in-crease in the doping concentrations at the interface layer mightenhance the effect. This prediction is of significant interest for practi-cal application as it has been reported that the addition of Sb or Sn cansignificantly improve sulfur tolerance of Ni [20,22,23]. Hence, theaddition of low concentrations of Sn or Sb to Ni can improve thesulfur tolerance of the anode materials and, most likely, it does notprovide a significant reduction of the interface stability.

4. Conclusions

Based on DFT calculations the effect of 9 alloying additions(Ag, Au, Co, Cd, Cu, Fe, Sn, Sb, and V) on the stability of Ni(001)/O-terminated c-ZrO2(001), Ni(001)/Zr-terminated c-ZrO2(001), andstoichiometric Ni(011)/c-ZrO2(011) interfaces were analyzed. Pre-dicted results showed that segregation/desegregation properties ofalloying additions are sensitive to oxygen partial pressure. For Ni/Oand the stoichiometric interfaces the behavior of alloying additionsand their effect on stability of interfaces are determined by the

formation of new bonds between alloying additions and oxygen,while for Ni/Zr interfaces those are determined by the formation ofnew metallic bonds. Co, Fe, and V alloying additions can improvethe stability of Ni/O and the stoichiometric interfaces, and therefore,such additions are attractive for designing the new anode materialsoperating on hydrogen. By contrast, Ag, Au, Cd, Cu, Sn, and Sb alloyingadditions have a tendency to reduce the work of separations. Never-theless, since the alloying additions prefer to segregate at a Ni surfaceand Ni/YSZ anode materials have significant porosity, it is clear that atlow doping concentrations the alloying additions have a limited effecton the interface stability.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.susc.2013.01.001.

Acknowledgments

We gratefully acknowledge the use of the supercomputers inA*STAR Computational Resource Center (ACRC) and support fromthe Institute of High Performance Computing of A-STAR Singapore.We also thank Prof. Sergei Manzhos from the National University ofSingapore for helpful discussions.

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