Solid State Ionics 181 (2010) 1294–1302

Contents lists available at ScienceDirect

Solid State Ionics

j ourna l homepage: www.e lsev ie r.com/ locate /ss i

Evaluation of Co and perovskite Cr-blocking thin films on SOFC interconnects

Robert Lacey a, Abhijit Pramanick a, Jae Chun Lee b, Jae-Il Jung a, Bo Jiang a, Doreen D. Edwards a,Robert Naum c, Scott T. Misture a,⁎a Kazuo Inamori School of Engineering, Alfred University, Alfred, NY 14802, USAb Materials Sci. & Eng., Myongji University, Kyunggi-do, Republic of Koreac Applied Coatings, Inc. Rochester, NY, USA

⁎ Corresponding author. Tel.: +1 (607)871 2438; faxE-mail address: misture@alfred.edu (R. Lacey).

0167-2738/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.ssi.2010.07.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 February 2010Received in revised form 6 July 2010Accepted 16 July 2010

Keywords:SOFCInterconnectChromium blocking

The viability of perovskite (LSC, LSCF, LNF and BSCF) and metallic (Co) thin film coatings to reduce Crmigration from Crofer22, E-Brite and SS430 interconnect materials has been examined. Production-scalephysical vapor deposition (PVD) systems were used to obtain uniform films with thickness near 1 μm. TheLSC, LSCF, LNF and Co films exhibited good adhesion, thermal stability and chemistry similar to the targetmaterials. Although the BSCF films exhibited good initial adhesion, subsequent reactions caused blisterformation upon thermal cycling. Chromium migration upon extended thermal treatments of 168 h wasinvestigated using XPS depth profiling through the films. As a result of interdiffusion of elements across thefilm–substrate interface, intermediate spinel layers were formed spontaneously in all thin films, withconcurrent shifts in compositions of the perovskite films. With the exception of the BSCF films, thecombination of a perovskite or Co coating and the spontaneously-formed, chemically graded intermediatespinel layers was highly effective in blocking Cr diffusion from the interconnect materials.

: +1 (607)871 2354.

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Materials developments enabling operation of SOFCs at lowertemperatures have introduced a new challenge related to chromium(Cr) poisoning of the cathode [1–9]. The Cr source in intermediatetemperature SOFCs is the high-Cr stainless steel interconnect whereinthe Cr content provides good oxidation resistance at high temperature[10]. The most commonly studied commercial alloys of this kindinclude Crofer22, E-Brite, and 430 stainless steel. Some newlydeveloped alloys and several promising oxide dispersion strength-ened (ODS) alloys are also being considered.

The role of Cr poisoning in the degradation of the electrochemicalperformance of SOFCs has been well documented [7–9]. To minimizethe effect of Cr contamination, engineers and component designershave employed protective surface coatings on the interconnectmaterials to act as barriers against outward diffusion of Cr. In recentyears, a variety of coating systems have been investigated, including:perovskite oxides such as (La,Sr)MnO3, (La,Sr)CoO3, (La,Sr)CrO3 ortheir combinations[4,11–16]; spinels such as (Co,Mn)3O4 and (Mn,Cr)3O4 [17–20]; metallic coatings such as Co, Ni and Cu [21]; and othercomplex oxide coatings[22].

A variety of techniques have been employed to deposit thesecoatings including vacuumplasma spraying[15], wet powder spraying[13], electrodeposition [23–26] slurry dip coating, magnetron sputter-

ing[4,12,14,27], and filtered arc deposition[22]. A very recent reviewarticle highlights differences among the performance of variouscoating materials and processes[31], citing electrodeposition asperhaps the most effective method reported to date. The mostcommonly used methods allow deposition of oxide films, butelectrodeposition yields dense metallic films which can subsequentlybe oxidized to produce high-density oxides.

The large-scale production of coated interconnects will require theuse of commercial-scale coating methods for depositing chromiumbarrier layers. In this work we have adopted a commercial scalephysical vapor deposition (PVD) approach to deposit perovskite andmetallic Co coatings on commercial stainless steel substrates, atthicknesses of 1–3 μm. The deposited films are characterized withregard to their microstructure and phase content before and afterthermal treatments in order to define the roles of the chemicalconstituents in mitigating Cr diffusion from the stainless steel.

2. Experimental methods

Thin films were deposited on commercial alloys using commercialelectron-gun PVD coaters, equipped with crystal thickness monitorsand residual gas analyzers, by Applied Coatings, Inc., Rochester, NY.Specimens were coated then cut to size, yielding specimens with acoating on one face only. Eight coating materials were tested,including La0.3Sr0.7CoO3-δ (LSC37), La0.6Sr0.4CoO3-δ (LSC64), La0.8Sr0.2-CoO3-δ (LSC82), La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), Ba0.5Sr0.5Co0.2Fe0.8O3-δ

(BSCF), LaNi0.5Fe0.5O3-δ (LNF55), LaNi0.6Fe0.4O3-δ (LNF64), and Co

1295R. Lacey et al. / Solid State Ionics 181 (2010) 1294–1302

metal. During each coating run, the thin films were deposited on fivedifferent substrates, including E-Brite, Crofer22, SS430, sapphire andborosilicate glass substrates. All coatings were deposited withnominal thickness of ~1000 nm, except the Co coatings which weredeposited at nominal thickness of 3000 nm. Prior to deposition, thesubstrates were sequentially wiped with alcohol, ultrasonicated in amild soap solution, rinsed with hot deionized water (16 MΩresistance), and again wiped with alcohol. During deposition, thesubstrates were held on counter-rotational tooling and positioned inthe coating chamber to control deposition uniformity and maximizecontrol of coating chemistry.

The microstructures of the films were examined using fieldemission gun (FEG) scanning electron microscopy (SEM). Crosssections of the films and substrates were examined using semi-quantitative energy dispersive x-ray spectroscopy (EDS). In-situ x-raydiffraction using a Co Kα sourcewas used to study the evolution of thecrystalline phases, up to a maximum temperature of 1000 °Cwith heating and cooling rates of 30 °C/min in a custom diffractionfurnace [28]. The diffraction data were collected over a 2-theta rangeof 10° to 85°. The crystalline phases were identified using the full scale(10–90°2θ) diffraction patterns using a commercial software packageJADE.

Post-situ analysis was also performed for films heat treated undertwo conditions: 1000 °C for 3 h or 1000 °C for 3 h followed by 168 h(1 week) at 850 °C. X-ray diffraction, SEM, and x-ray photoelectronspectroscopy (XPS) depth profiling were used to characterize thefilms after heat treatment. XPS was performed using monochromaticAl Kα radiation in a Phi Quantera SXM instrument. The spectra werecollected from a region 100 μm in diameter. It was noted that the XPSdepth profiles do not provide high accuracy depth analysis. This is theresult of inherent roughness of the various metal substrates anddifferences in chemical composition and density of the depositedfilms. Nevertheless, the depth profiles provide sufficient informationto understand the general trends in diffusion of the different elementsacross the film–substrate interface.

To verify the expected equilibrium phase assemblage in the thinfilms, bulk reactions of the perovskite phases with Cr2O3 and Crofer22in powder formwere investigated. Powdermixtures at a ratio of 50:50by weight were annealed at 850 °C in air for 168 h and thencharacterized using XRD.

The in-plane electrical conductivity of oxide films (LSC37, LSC64,LSC82, LSCF, LNF55, and LNF64 deposited on glass substrates wasmeasured from 50 °C to 500 °C using impedance spectroscopy with Ptelectrodes. Samples were heated at a rate of 10 °C/min, and heldisothermally at 50 °C increments for 20 min prior to taking theimpedance measurements. Impedance spectra were collected byscanning from 10 MHz to 1 Hz with a 1 V excitation signal using aSolatron 1260 impedance analyzer.

Fig. 1. Cross-sectional view of films of composition LSC82 on (a) glass, (b) sapphire (off-axis view), and (c) Crofer22 substrates.

3. Results and discussion

3.1. Microstructure and electrical behavior of as-deposited films

The PVD process produced uniform, homogeneous films ofsubmicron thicknesses, which was confirmed from SEM micrographsof the coatings as illustrated in Fig. 1 for a thin film coating of LSC82.Coatings deposited on glass or sapphire were dense, uniform andsmooth. Coatings deposited on the steel substrates, which had arougher surface, followed the contour of the substrate.

The oxide films were amorphous and remained so upon heating to~600 °C as determined by XRD, shown later. The electrical conduc-tivity of the amorphous films at 50 °C ranged from 2×10−4 S/cm forLSC37 to 3 S/cm to LNF55. At 500 °C the conductivities ranged from4 S/cm for LSC37 to 102 S/cm for LSCF. The conductivity of theamorphous LSC film is some 2.5 orders of magnitude lower than for

bulk crystalline LSC at 500 °C[29]. However, both crystalline andamorphous LSCF display similar total conductivity at 500 °C [30].

The amorphous LNF, LSC, and LSCF films exhibited thermallyactivated conductivity with either one or two activation energies. TheLSC37 and LSC64 exhibited a single activation energy (~0.4–0.5 eV)whereas LSC82 exhibited two activation energies (0.2 eV below200 °C and 1.2 eV above). LNF and LSCF behaved similarly, withactivation energies of 0.16 or 0.17 eV below 200 °C and 0.55 and 0.74above, respectively. The structural origin of the change in activationenergy near 200 °C remains unclear, and additional work is underwayto establish the interfacial resistance of the crystalline coatings afterlong term annealing.

1296 R. Lacey et al. / Solid State Ionics 181 (2010) 1294–1302

3.2. Short term crystallization and phase evolution

Fig. 2(a) shows a cross section and composition gradient of anLSC82 thin film on a Crofer22 substrate after heat treatment at1000 °C for 3 h. The thickness of the film is ~1 μm, as can be estimatedfrom the contrast in the cross-sectional micrograph and thedistribution of the chemical elements shown in the EDS line scan.The particle size is in the range of ~100 nm, as can be observed from

Fig. 2. (a) Cross-sectional view of LSC82 film on Crofer22 substrate, along with line EDS scan(B) after crystallization at 1000 °C, (C) after isothermal heat treatment at 850 °C for 168 h; (spinel, M2O3 where M is transition metal, and (La,Sr)CoO3-δ – in the deposited film on Crof

the top-view of the microstructure shown in Fig. 2(b). The evolutionof the different phases during heat treatment is shown in the in-situdiffraction data in Fig. 2(c). At room temperature, the films areamorphous, and the perovskite phase (La,Sr)CoO3 starts to crystallizeat ~600 °C. The XRD patterns are typical of a random powder,demonstrating that the amorphous films crystallize without anypreferred crystallite orientation. A spinel phase begins to form at~800 °C, which is most likely of the form (Cr,Co,Mn)3O4, though the

across the film–substrate interface; (b) microstructure of LSC82 film: (A) as-deposited,c) in situ X-ray diffraction showing evolution of crystalline reaction products – SrCrO4,er22 substrate.

Fig. 3. (a) Cross-sectional view of LNF64 film on Ebrite substrate, along with line EDSscan across the film–substrate interface; (b) in situ X-ray diffraction pattern of LNF55film on Crofer22 substrate, showing evolution of crystalline reaction products – spinel,M2O3 where M is transition metal, and La(Ni,Fe)O3-δ – in the deposited film on Crofer22substrate.

1297R. Lacey et al. / Solid State Ionics 181 (2010) 1294–1302

exact composition of the spinel phase cannot be determined usingonly diffraction data. In addition to the primary phases of (La,Sr)CoO3

and spinel phase, the formation of an appreciable fraction of SrCrO4

was also observed, beginning at ~750 °C.Phase evolution for the LSC37 and LSC64 films was similar to

LSC82, with the first crystallization of the perovskite phase alsooccurring at ~600 °C. For LSCF, formation of (La,Sr)(Co,Fe)O3, a spinelphase, and SrCrO4 from the amorphous film was observed upon heattreatment, again similar to the LSC films.

The results for LSC, in a general sense, agree with earlier results ofFujita et al. [11] who also used a PVD process but found only the spinelphase as a reaction product after heating to 750 °C. The moreaggressive heat treatment used in the current work shows thatSrCrO4 also forms given ample time at elevated temperatures.

The LNF64 and LNF55 thin films were similar in microstructure tothe LSC thin films, regardless of the interconnect alloy used as asubstrate. Fig. 3(a) shows a cross section of an LNF64 film after heattreatment at 1000 °C for 3 h on E-Brite and the composition gradientof the elements present. The contrast in the cross-sectional micro-graph and the distribution of the thin film composition elements inthe EDS plot suggest that the film thickness is~1 μm. Fig. 3(b) showsthe in-situ diffraction patterns for LNF55 during thermal treatment ofthe thin film on Crofer22 to 1000 °C. Similar to the phase formationsequence in LSC films, perovskite La(Ni,Fe)O3 begins to form at~600 °C and a spinel phase begins to form at ~800 °C. In addition,minor diffraction peaks begin to appear from the formation of M2O3

(M=Cr, Fe, Ni, Mn, Co, etc.) at ~800 °C. The EDS line scans show someCr enrichment at the metal-oxide interface (also clearly detectedusing XPS as discussed later) which is likely due to the formation ofCr2O3.

Fig. 4(a) shows the microstructure of the BSCF thin film after heattreating at 1000 °C for 3 h. The blisters formed on the surface are likelythe result of thermal mismatch between the reaction products formedand the substrate. The in-situ diffraction patterns at varioustemperatures are shown in Fig. 4(b). In this case, the large relativecontent of Ba and/or Sr results in crystallization of major fractions of(Ba,Sr)CrO4, in addition to the expected spinel phase. It is likely thatthe large quantity of (Ba,Sr)CrO4, with its large CTE mismatch, isresponsible for the blistering.

LSC, LSCF, and BSCF all react to form chromate phases of the form(Ba,Sr)CrO4 because of the divalent A-site cations, but LNF does not.To verify the expected equilibrium phase assemblage in the thin filmswith longer heat treatments, bulk reactions of the perovskite phaseswith Cr2O3 and Crofer22 in powder form were investigated. Theresults for mixtures with LSC37, LSC64, LSC82, LSCF6428, BSCF5582and LNF46 are summarized in Table 1. With the exception of BSCF, allof the reaction mixtures produced the same phases observed in thethin films. In the case of BSCF, several additional phases were noted inthe bulk samples that were not detected in the thin film. Twoadditional observations can be made from the data in Table 1. First,the phase assemblage for reaction with Crofer22 and Cr2O3 is nearlyidentical in all cases (ignoring any Fe in solid solution in the variousphases). Second, the LNF is nearly inert to Cr, forming only traceamounts of spinel and NiO.

Examination of the Co thin film indicates that the microstructureand phase evolution sequences differ appreciably from the perovs-kites. A cross section of the film after heat treatment at 1000 °C for 3 his shown in Fig. 5(a). The thickness of the film on average was ~3 μm,as estimated from the contrast in the cross-sectional micrograph andthe distribution of Co in the EDS plot. The heat treated film showed adendritic morphology with subsequent transformation to equiaxedgrains as shown in Fig. 5(b). The evolution of the crystalline phases inthe film upon heat treatment was determined from the in-situdiffraction patterns, as shown in Fig. 5(c). The as-deposited filmcontained some crystalline Co, and upon heating in air oxidationoccurs to form first CoO, beginning at ~500 °C and then Co3O4. On

Fig. 4. (a) Microstructure of crystallized BSCF film showing formation of blisters on the film surface (marked by dotted circles). A close-up of the blisters is shown on the right,whereas the inset in the picture on the left shows crystalline nature of the films. (b) In situ X-ray diffraction pattern showing evolution of crystalline reaction products – (Ba,Sr)CrO4,spinel, (Ba,Sr)MO3 and M2O3 where M is transition metal – in the deposited film on Crofer22 substrate.

1298 R. Lacey et al. / Solid State Ionics 181 (2010) 1294–1302

further heating, Co3O4 reacts with the Crofer22 to form a spinel-typephase and excess CoO, which is chemically reasonable from a massbalance analysis.

3.3. Long-term stability and chemical interdiffusion

LSC82, LNF55 and Co films on Crofer22, E-Brite, and SS430substrates were studied in detail using XPS depth profiling. The

Table 1Thin film and bulk reaction products of different coating compositions with Cr2O3 andCrofer22.

LSC LSCF BSCF LNF

Cr2O3 (La,Sr)CoO3-δ (La,Sr)FeO3-δ BaCrO4 La(Ni,Fe)O3-δ

SrCrO4 SrCrO4 Sr3Cr2O8 Trace spinelSpinel Spinel Spinel Trace NiO

Ba12Co11O33

(Sr,Ba)2M2O5

Crofer22 (La,Sr)CoO3-δ (La,Sr)FeO3-δ BaCrO4 La(Ni,Fe)O3-δ

SrCrO4 SrCrO4 Sr3Cr2O8 SpinelSpinel Spinel Spinel Trace NiO

Ba12Co11O33

(Sr,Ba)2M2O5

inherent rough surface finish of the substrates (see for example Fig. 1(c)) prevents accurate determinations of the sputtered depth, but theprofiles nonetheless show trends in chemical interdiffusion.

Fig. 6 shows the XPS depth profiles for several samples, whichhighlights similarities in the Cr diffusion behavior:

• Cr enrichment at the oxide–metal interface, with oxygen present• The Cr diffusion profile from the metal into the oxide is well-defined, and reaches a concentration of zero at a depth of 350–400 nm below the surface of the oxide (~1100 nm for the thicker Cofilm)

• Small Cr concentrations are noted on the surface of each specimen,whichwe attribute to vapor phase transport of Cr from the uncoatedregions of the specimens rather than from Cr diffusion through thecoatings.

Fig. 6(a) shows the concentration depth profile for the LSC82 thinfilm on Crofer 22 substrate after heat treating at 1000 °C for 3 h and850 °C for 168 h. Outward diffusion of Fe, Mn and Cr from the Crofer22 substrate toward the film is clear, even after the short periods ofexposure to high temperature environments. Concurrent inwarddiffusion of Co from the film toward the substrate is also evident.

The composition profiles for the LSC82 films treated at 168 and 3 hare remarkably similar, with no significant amount of Cr noted at ornear the surface and indistinguishable Cr concentration profiles. The

Fig. 5. (a) Cross-sectional view of Co film on Crofer22 substrate, along with line EDS scan across the film–substrate interface; (b) microstructure of Co film: after crystallization at1000 °C (A), and after isothermal heat treatment at 850 °C for 168 h (B); (c) in situ X-ray diffraction pattern showing evolution of crystalline reaction products – Co3O4, spinel, andCoO – in the deposited film on Crofer22 substrate.

1299R. Lacey et al. / Solid State Ionics 181 (2010) 1294–1302

1300 R. Lacey et al. / Solid State Ionics 181 (2010) 1294–1302

primary differences between the depth profiles for long and shortheat treatments are the increased Mn content below the film and thenearly complete diffusion of Co from the film toward the substrate. An

Fig. 6. XPS profiles for (a)LSC 82 on Crofer22, (b)LNF55 on Crofer22, (c)LNF64 on EBrite, (dtreated at 1000 °C for 3 h, the plots marked B are for samples heat treated at 850 °C for 168

analysis of the La profiles suggests that the initial reaction of the filmwith Crofer22 results in a largely stable phase assemblage that trapsthe Cr below the initial thin film boundary.

)LNF64 on 430SS, and (e)Co films on Crofer22. The plots marked A are for sample heath subsequent to treatment at 1000 °C for 3 h.

Fig. 6 (continued).

1301R. Lacey et al. / Solid State Ionics 181 (2010) 1294–1302

Qualitative XRD analyses of the LSC82 films before and after the168 h heat treatments show that identical phases are present. Fromthis result, it is clear that the diffusion of a fraction of the Co towardthe Crofer22 substrate, in concert with outward diffusion of Mn, doesnot require decomposition of the perovskite coating. Instead, thechemical composition of the initial LSC82 film changes with time,

incorporating some Mn in place of Co, while concurrently shiftingtoward lower Sr content as a result of the partial reaction to formSrCrO4. Indeed, it is also possible that the SrCrO4 layer mayincorporate some Mn, as hexavalent Mn and Cr have very similarionic radii in tetrahedral coordination (0.255 vs. 0.250 Å). Heating thesamples for 168 h does not result in any notable changes inmicrostructure, as shown in Fig. 2(b). The microstructures are similarto the as-deposited films, even maintaining the nano-scale grain sizeafter one week at 850 °C.

A comparison of the concentration depth profiles for the LNF55thin film before and after the long heat treatment is shown in Fig. 6(b). The diffusion behavior is similar to that found for the LSC82 thinfilm, with outward diffusion of Fe, Mn and Cr toward the film from theCrofer 22 substrate, and inward diffusion of Ni from the film towardthe substrate. A large increase in Mn concentration is detected nearthe surface of the film. Similar to LSC82, the LNF55 thin film is shownto be effective in mitigating diffusion of Cr from the substrate to thesample surface.

For both LSC82 and LNF55, the La profile exhibited an exceedinglysharp concentration boundary between La on the surface and theunderlying oxides, even after 168 h at 850 °C. X-ray diffraction datafor the films before and after the 168 h heat treatments arequalitatively identical, with the formation of a perovskite phase, aspinel phase, and either M2O3 or SrCrO4. Evidence that the diffusion ofthe various cations simply changes the compositions of the varioussolid solutions of the perovskite, spinel, and M2O3 phases can beconfirmed with an XRD measurement that probes the entire depth ofthe film. Again, the XRD results show substantial mass transportthrough the depth without changing the phases present.

The XPS depth profiles of LNF films on E-Brite and SS430 substratesafter heating for 3 h at 1000 °C are qualitatively similar, as shown inFig. 6(c) and (d), respectively. This result suggests that minor changesin the alloy composition have little effect on cation diffusion and thatchemical differences can be accommodated by shifting solid solutioncompositions without altering the phase assemblage. Fig. 6(e) showsthe concentration profile through the Co thin film on Crofer 22substrate. The Co film is observed to be highly effective in preventingdiffusion of Cr to the surface of the film, in agreement with recentwork by Stanislowski et al. [21] who studied films of ~10 μm thicknessprepared by sputtering. The effectiveness of the PVD Co coating isclearly demonstrated, with only 3 μm resulting in a spinel phasecapped by CoO/ Co3O4 oxides.

4. Conclusions

The PVD process produced LSC, LSCF, LNF and Co films on Crofer22,SS430 and E-Brite that are each highly effective in blocking Crdiffusion from the steel substrate. BSCF films were reactive andformed blisters upon thermal cycling in air, so were not consideredfurther. All of the perovskite films formed a similar phase assemblageafter heat treatment, consisting of the target perovskite, a spinelintermediate layer, and minor fractions of M2O3 or SrCrO4 phases,depending on the initial chemistry. Reactions of the as-depositedfilms with the substrate occurred rapidly, with little or no differencein the chemical diffusion, microstructure, or phase assemblage foundbetween samples heat treated for 3 vs. 168 h.

The chemical depth profiles clearly demonstrate that diffusion of theperovskite B-site cations (Ni, Fe, Co) and the transition metals from thesteel (Fe, Cr, Mn) occurs rapidly at high temperature to spontaneouslyform chemically graded films with stable spinel and perovskite phasesthat inhibit outward diffusion of Cr. The wide range of solid solutionaccommodated by the spinel, perovskite, and M2O3 phases allows thisextensive chemical interdiffusionwithout changing the phases present.

In the case of the Co metal coating, heat treatment results inoxidation and reaction to form a compositionally graded spinel layer

1302 R. Lacey et al. / Solid State Ionics 181 (2010) 1294–1302

and a CoO/ Co3O4 surface. This film is also highly effective in blockingCr diffusion.

Overall, the results suggest that these PVD films are effective formitigating diffusion of Cr from SOFC metal interconnects and thatrefinements in film chemistry and processing might improve theirperformance even further.

Acknowledgments

Financial support for this work was provided by the New YorkState Foundation for Science, Technology and Innovation, NYSTAR,under contract C030093, and Applied Coatings, Inc.

References

[1] T. Brylewski, M. Nanko, T. Maruyama, K. Przybylski, Solid State Ionics 143 (2001)131.

[2] K. Huang, P.Y. Hou, J.B. Goodenough, Solid State Ionics 129 (2000) 237.[3] T. Horita, Y. Xiong, K. Yamaji, N. Sakai, H. Yokokawa, Journal of The

Electrochemical Society 150 (2003) A243.[4] J.H. Zhu, Y. Zhang, A. Basu, Z.G. Lu, M. Paranthaman, D.F. Lee, E.A. Payzant, Surface

and Coatings Technology 177–178 (2004) 65.[5] S.P.S. Badwal, R. Deller, K. Foger, Y. Ramprakash, J.P. Zhang, Solid State Ionics 99

(1997) 297.[6] S. Taniguchi, M. Kadowaki, H. Kawamura, T. Yasuo, Y. Akiyama, Y. Miyake, T.

Saitoh, Journal of Power Sources 55 (1995) 73.[7] S.P. Jiang, J.P. Zhang, K. Foger, Journal of The Electrochemical Society 147 (2000)

3195.[8] S.C. Paulson, V.I. Birss, Journal of The Electrochemical Society 151 (2004) A1961.[9] S.P. Jiang, S. Zhang, Y.D. Zhen, Journal of The Electrochemical Society 153 (2006)

A127.[10] Z. Yang, K.S. Weil, D.M. Paxton, J.W. Stevenson, Journal of The Electrochemical

Society 150 (2003) A1188.

[11] K. Fujita, K. Ogasawara, Y. Matsuzaki, T. Sakurai, Journal of Power Sources 131(2004) 261.

[12] C. Johnson, R. Gemmen, N. Orlovskaya, Composites Part B: Engineering 35 (2004)167.

[13] S. Linderoth, Surface and Coatings Technology 80 (1996) 185.[14] N. Orlovskaya, A. Corotalo, C. Johnson, R. Gemmen, Journal of the American

Ceramic Society 87 (2004) 1981.[15] W.J. Quadakkers, H. Greiner, M. Hänsel, A. Pattanaik, A.S. Khanna, W. Malléner,

Solid State Ionics 91 (1996) 55.[16] Y. Larring, T. Norby, Journal of the Electrochemical Society 147 (2000) 3251.[17] X. Chen, P.Y. Hou, C.P. Jacobson, S.J. Visco, L.C. De Jonghe, Solid State Ionics 176

(2005) 425.[18] W. Qu, L. Jian, J.M. Hill, D.G. Ivey, Journal of Power Sources 153 (2006) 114.[19] Z. Yang, G. Xia, J.W. Stevenson, Electrochemical and Solid-State Letters 8 (2005)

A168.[20] Z. Yang, G. Xia, S.P. Simmer, J.W. Stevenson, Journal of The Electrochemical Society

152 (2005) A1896.[21] M. Stanislowski, J. Froitzheim, L. Niewolak, W.J. Quadakkers, K. Hilpert, T. Markus,

L. Singheiser, Journal of Power Sources 164 (2007) 578.[22] P. Gannon, M. Deibert, P. White, R. Smith, H. Chen, W. Priyantha, J. Lucas, V.

Gorokhovsky, International Journal of Hydrogen Energy 33 (2008) 3991.[23] W. Wei, W. Chen, D.G. Ivey, Chemistry of Materials 19 (2007) 2816.[24] W. Wei, W. Chen, D.G. Ivey, Journal of Power Sources 186 (2009) 428.[25] X. Deng, P. Wei, M.R. Bateni, A. Petric, Journal of Power Sources 160 (2006) 1225.[26] J. Wu, C.D. Johnson, R.S. Gemmen, X. Liu, Journal of Power Sources 189 (2009)

1106.[27] D.O. Klenov, W. Donner, L. Chen, A.J. Jacobson, Journal of Materials Research 18

(2003) 188.[28] S.T. Misture, Measurement Science and Technology 14 (2003) 1091.[29] V.V. Kharton, E.V. Tsipis, A.A. Yaremchenko, I.P. Marozau, A.P. Viskup, J.R. Frade, E.

N. Naumovich, Materials Science and Engineering: B 134 (2006) 80.[30] L.W. Tai, M.M. Nasrallah, H.U. Anderson, D.M. Sparlin, S.R. Sehlin, Solid State Ionics

76 (1995) 273.[31] N. Shaigana, W. Qua, D.G. Iveyb, W. Chen, Journal of Power Sources 195 (2010)

1529.