DOI: 10.1002/fuce.201200177

ORIGINAL RESEARCH PAPER

Studies of Solid Oxide Fuel Cell Electrode Evolution Using 3D Tomography~ K. Yakal-Kremski1*, J. S. Cronin1, Y.-C. K. Chen-Wiegart2, J. Wang2, S. A. Barnett1 1 2

Department of Materials Science, Northwestern University, Evanston, Illinois 60208, USA Photon Source Directorate, Brookhaven National Laboratory, Upton, New York, USA

Received September 30, 2012; accepted January 21, 2013; published online 䊏䊏䊏

Abstract This paper describes 3D tomographic investigations of the structural evolution of Ni-yttria-stabilized zirconia (Ni-YSZ) and (La,Sr)MnO3-YSZ (LSM-YSZ) composite solid oxide fuel cell (SOFC) electrodes. Temperatures higher than normally used in SOFC operation are utilized to accelerate electrode evolution. Quantitative 3D FIB-SEM and X-ray tomographic imaging contributes to development of mechanistic evolution models needed to accurately predict long-term durability. Ni-YSZ anode functional layers annealed in humidified hydrogen at 900–1,100 °C exhibited microstructural coarsening leading to a decrease in three-phase boundary (TPB)

1 Introduction Long-term (>40,000 h) stability is an important requirement for the commercial viability of SOFCs for stationary power applications. It has previously been demonstrated in the microelectronics industry that long-term predictions of integrated-circuit metal interconnect reliability can be successfully made, and strategies for improving reliability conceived, using models developed based on relatively shortterm “accelerated” tests [1]. Whether similar strategies can be employed in the case of SOFCs, which are arguably more complex than integrated circuit interconnects, is still undetermined. One point made clear by recent studies of long-term tested stacks is that many complex effects occur that impact stack performance [2, 3]. The challenge for SOFC researchers is to de-convolute these effects, develop quantitative models of the specific degradation mechanisms, and then use the results to predict and improve long-term SOFC durability. One approach to this problem is to first tackle relatively simple cases, and then build in more complexity. For example, highly pure gases can be used to avoid degradation which may arise in full stack tests due to impurities in the fuel [4, 5], and idealized cell tests done to avoid contamina-

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Paper presented at the 10th European SOFC Forum 2012, June 26–29, 2012 held in Lucerne, Switzerland. Organized by the European Fuel Cell Forum – www.efcf.com

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density. There was also a change in the fraction of pores that were isolated, which impacted the density of electrochemically active TPBs. The polarization resistance of optimally fired LSM-YSZ electrodes increased upon thermal aging at 1,000 °C, whereas that of under-fired electrodes decreased upon aging. These results are explained in terms of observed 3D microstructural changes. Keywords: Degradation, Electrode, SOFC, Solid Oxide Fuel Cell, Three Dimensional Tomography

tion from stack components such as interconnects and seals [6, 7] or in the SOFC materials themselves [8, 9]. Such studies help reveal the degradation mechanisms that are intrinsic to the SOFC materials – e.g., cation diffusion [2, 10], steam– anode interactions [11–13], and electrode sintering/coarsening [14, 15]. The aim is to develop mechanistic models for these degradation modes by relating changes in electrochemical characteristics to microstructural and microchemical changes. As a practical matter, most experiments should be carried out under conditions that accelerate degradation, facilitating collection of enough data in reasonable times to develop quantitative models. The various models can then be combined and used to predict long-term performance. This paper addresses the morphological stability of the widely used composite SOFC electrodes Ni-Y2O3 stabilized ZrO2 (YSZ) and (La,Sr)MnO3-YSZ (LSM-YSZ). Sintering and/ or coarsening are expected, given that state-of-the-art electrodes engineered for good electrochemical performance have relatively high densities of solid–solid and solid–pore interfaces. For example, SOFC anodes annealed at greater than normal stack temperatures (>800 °C) show an increase in average Ni particle size and a decrease in TPB density [16–18]. Similarly, coarsening of LSM-based cathodes has been observed at elevated temperatures [19–21], and is additionally affected by the presence of a current density [3, 22].

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[*] Corresponding author, [email protected]

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Yakal-Kremski et al.: Studies of Solid Oxide Fuel Cell Electrode Evolution Using 3D Tomography Here preliminary results are presented on accelerated annealing of LSM-YSZ and Ni-YSZ electrodes, characterized using 3D tomographic measurements combined with electrochemical impedance spectroscopy (EIS). Focused ion beamscanning electron microscopy (FIB-SEM) and transmission Xray microscopy (TXM) were employed for the 3D tomography [23–29], yielding quantitative interfacial area and TPB density values [25]. Inherently 3D structural parameters, i.e., phase connectivity and tortuosity, which have been shown to impact electrode performance, are also obtained [24]. The observed microstructural changes are correlated with EIS characteristics. Only the effects of temperature are considered, not polarization/current effects.

2 Experimental The Ni-YSZ anode samples were processed using methods similar to anode-supported cells [30]. 50:50 wt.% NiO-YSZ functional layers and YSZ electrolyte layers were deposited from colloidal solutions on dry-pressed 50:50 wt% NiO (J.T. Baker):YSZ (Tosoh) supports, and the structures were co-sintered at 1,400 °C for 4 h. Another YSZ layer was then deposited by screen printing and fired at 1,200 °C for 2 h, yielding a porous scaffold for subsequent cathode impregnation. Some of the cells were annealed for 100 or 500 h in a 4% H2–3% H2O–93% Ar mixture at 1,100 °C, while non-annealed cells served as a baseline for comparison. The porous YSZ scaffolds were then infiltrated with nitrates using a procedure previously shown to produce low-resistance Sm0.5Sr0.5CoO3–d (SSC) cathodes [31]. The as-infiltrated cells were then mounted and heated for testing; the only calcination of the cathode, the ∼1 h in air at 800 °C at the start of cell testing, was sufficient to form the SSC perovskite phase [31]. Adding the infiltrated cathode after anode annealing assured that the cathodes were similar regardless of the annealing condition. Furthermore, the characteristic frequency of the SSC cathode was well removed from that of the Ni-YSZ anode, facilitating separate observation of the anode response. The LSM-YSZ measurements were done on symmetrical cells to facilitate interpretation of EIS data. Cells were made by screen-printing LSM-YSZ functional layer and LSM current collector inks on dense YSZ electrolyte pellets sintered at 1,400 °C for 4 h. LSM-YSZ ink was produced from a 1:1 weight ratio of A-site deficient (La0.8Sr0.2)0.98MnO3 (Praxair) with Zr0.84Y0.16O2 (Tosoh). LSM-YSZ cathode layers were printed and fired at either 1,075 or 1,175 °C for 1 h, followed by a LSM current collector layer fired at 1,025 °C for 1 h. The functional layer fired at 1,175 °C was similar to that commonly used in many full cells, such that the resulting electrode microstructure and properties should be similar. Some of the cells were analyzed as prepared, and others were annealed in air at 1,000 °C for 100 h. Anode and cathode microstructures were evaluated postmortem via FIB-SEM 3D tomography. Serial sectioning was carried out on a Zeiss NVision 40 dual-beam FIB-SEM, as described elsewhere [32, 33]. Images were collected using a

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2 kV accelerating voltage and an in lens detector. Image resolution was 20 or 25 nm, with a FIB polishing resolution of 30 or 50 nm, depending on the sample. Some anodes were additionally inspected via TXM, using the beamline X8C, National Synchrotron Light Source [34]. The instrument provides a sub-30 nm resolution in 2D and sub-50 nm resolution in 3D [35]. Synchrotron radiation allows for tunability of the incident energy, and thus X-ray absorption edges can be exploited to produce high contrast between phases. Two nano-tomography measurements, at below (8,300 eV) and above (8,332 eV) the Ni K-edge, were carried out on a cylindrical sample (diameter ∼35 lm, height ∼80 lm), with a large field of view of 40 lm. For each tomography, 1,441 X-ray projections were collected over a 180° angular range and aligned automatically using a run-out correction system [35]. A CCD detector with camera binning 2 by 2 was used to record the X-ray projections. A standard filtered back-projection reconstruction algorithm was then used to reconstruct the 3D images. Voxel resolutions of 39.43 and 38.95 nm were achieved in TXM in all orthogonal directions for 100 h and 500 h anneals, respectively. The ratios of the reconstruction side length to the average particle sizes, a widely used measure of statistical accuracy [36], were ∼30–34 for the FIB-SEM data and ∼27–75 for TXM, easily large enough to indicate <5% error [37]. Segmentation procedures are described in detail elsewhere [37, 38]. Note that a “directional” connectivity definition was used requiring that each phase be connected in one direction (e.g., pores must be connected to the current collection side of the measured volume).

3 Results 3.1 Ni-YSZ Anodes Anode-supported cells were annealed at 1,100 °C for 100 and 500 h. The Ni-YSZ anode functional layer microstructures before and after annealing were compared. Figure 1 depicts 3D images from FIB-SEM data of the pore phase in the as-reduced anode (a) and after 100 h annealing (b), and the corresponding TPB line images (c and d). In all images, the electrolyte is to the left, and the current collector to the right. In the pore phase images, green signifies the pores that were part of a pore network that is connected to the fuel gas supply, whereas red indicates isolated pores and yellow unknown. The “unknown” portion corresponds to networks that appear isolated but extend outside of the measured volume, where their connectivity is not known. Only TPBs on connected networks are electrochemically active, and so the same color scheme was used for TPBs: green indicates active TPB, red inactive (on at least one isolated phase), and yellow unknown. Two annealing-induced changes in structure are apparent in Figure 1. First, there is a general increase in feature size after annealing, resulting in a decreased total TPB density. Second, there is a substantial increase in the fraction of isolated (from 2 to 7 vol.%) and unknown (from 5 to 52 vol.%)

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Yakal-Kremski et al.: Studies of Solid Oxide Fuel Cell Electrode Evolution Using 3D Tomography

ORIGINAL RESEARCH PAPER Fig. 1 3D images showing the pore phase and portion of TPB of the non-annealed (a and c) and annealed (b and d) anode functional layers. The connectivity is indicated by color – red represents isolated features, yellow represents features of unknown connectivity, and green represents features that are connected. These thin (2.5 lm) sections were taken from near the center of the measured volumes. This provides a more representative visual comparison than images showing the entire measured volumes, which tend to accentuate the unknown networks that are found mostly near the measured volume edges.

pores after annealing. As expected, the isolated and unknown networks are most prevalent on the left-hand side of the images (near the electrolyte) based on the directional definition of connectivity and the fact that these are farthest from the pores in the anode current collector. The “true” fraction connected was estimated by dividing the unknowns into connected and isolated, by assuming the same connected/isolated ratio as in the overall population. This yielded ∼98% connected pores in the non-annealed anode, and ∼85% connected pores in the annealed anode (Table 1). Although there is clearly considerable error in these values, the isolated pore fraction increase after annealing was also verified by the observation of pores that had not filled with epoxy during FIB-SEM sample preparation. The measurements described below provide more accurate information on the isolated pore fraction. Additional 3D measurements were collected with a full field X-ray imaging technique – transmission X-ray microscopy (TXM) – on Ni-YSZ anode samples annealed at 1,100 °C for 100 and 500 h. Besides providing new data for a longer annealing time, the TXM-based data sets had larger volumes, ≥3,000 lm3 [39], compared to ∼1,000 lm3 for the above FIBSEM data. Larger 3D volumes generally yield better statistical

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accuracy, particularly a lower fraction of “unknown” networks, since the “unknown” pores are associated with the faces of the reconstruction volume. For example, the percentage of unknown pores in the 100 h annealed anode was only ∼15% in the TXM data, much less than the ∼60% value in the smaller FIB-SEM data set. Figure 2 shows typical reconstructed cross-sectional images from the TXM data sets, taken both below and above the Ni absorption K-edge to enhance the Ni contrast, for anodes annealed at 1,100 °C for 100 and 500 h. The corresponding segmented images, obtained using both types of images [34], are also shown. Coarsening of the structure annealed for the longer time is apparent – particularly noticeable are the larger Ni feature sizes. Table 1 shows averaged structural data derived from the full 3D data sets. Note that both TXM and FIB-SEM results were obtained for the 100 h annealed anode, while the volume fraction and surface area values agreed well between the two measurements, active TPB densities were ∼20% lower in the TXM results. This may be a result of the slightly lower resolution of the TXM measurement, as coarser voxel grids will tend to under-estimate lengths if they are comparable in size to the TPB radius of curvature.

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Yakal-Kremski et al.: Studies of Solid Oxide Fuel Cell Electrode Evolution Using 3D Tomography

Fig. 2 Example reconstructed cross-sectional 2D images from TXM data sets taken above (a) and below (b) the Ni absorption edge, for anodes annealed at 1,100 °C for 100 and 500 h. The segmented images (c) show the Ni (white), YSZ (gray), and pore (black) phases.

The volume percentages of the phases in Table 1 were in good agreement for the as-reduced and the annealed anodes. The fact that the pore volume fraction did not change, within measurement accuracy of 1–2%, indicates that there was little sintering. On the other hand, the specific surface areas tended to decrease with increasing annealing time, especially between the 100 h and 500 h anneals, indicating coarsening of the microstructure. The observed Ni coarsening has been observed previously [18, 38–40]. The dominance of surfacediffusion-driven coarsening over grain-boundary-diffusiondriven sintering is expected to continue to temperatures <1,100 °C, as the former process generally has a lower activation energy than the latter. This suggests that accelerated microstructural data may, in the future, be useful for making predictions of microstructural evolution at SOFC operating temperatures of ∼800 °C. Table 1 also shows that total TPB densities decreased substantially with increasing anneal time. On the other hand, the fraction of connected porosity was lower after the 100 h anneal, and then increased to nearly the as-reduced value after 500 h annealing. This surprising behavior needs to be verified by further measurements. In any case, the electrochemically active TPB density decreased with increasing anneal time despite the changes in pore connectivity (Table 1). The EIS results for full cells with as-reduced and 100 h annealed

anodes, detailed elsewhere, showed a substantial increase in anode polarization resistance, from 0.39 to 0.75 X cm2 [33]. This correlates well with the substantial decrease in active TPB density – from 2.89 lm–2 for the non-annealed anode to 1.75 lm–2 for the annealed anode as measured by FIB-SEM – due to a combination of coarsening and decreased pore connectivity as discussed above. However, more recent EIS measurements comparing cells with 100 h and 500 h annealed anodes showed relatively little change in the anode polarization resistance, which remained below 0.3 X cm2 at 800 °C. Note that this low polarization resistance was verified for a few different annealed anode cells. This is surprising given the lower active TPB density for the longer annealing time, although further study is needed to model the polarization behavior of these structurally complex cermet anodes. It should also be noted that one Ni-YSZ patterned electrode study [42] reported a relatively weak dependence of polarization resistance on TPB density. 3.2 LSM-YSZ Cathodes The symmetrical cells used in annealing studies were produced with LSM-YSZ cathodes fired either at an optimal condition of 1,175 °C for 1 h, or “under-fired” at 1,075 °C for 1 h. Some of these cells were then subjected to an anneal at 1,000 °C for 100 h; this temperature is well above normal ∼800 °C operating temperatures in order to accelerate structural evolution. EIS measurements on both as-prepared and annealed cells showed responses similar to those reported earlier for LSM-YSZ cathode symmetric cells [32, 43, 44]. For the normally processed, 1,175 °C fired cells, EIS measurements made in air at 800 °C showed an increase in the cathode polarization resistance from 0.48 to 0.61 X cm2. On the other hand, for the cathode that had been fired at 1,075 °C, annealing caused a decrease in polarization resistance from 0.74 to 0.41 X cm2. These results can be understood based on EIS and structural measurements of LSM-YSZ cathodes fired at different temperatures Tf. As shown in Figure 3, a plot of measured polarization resistance versus Tf, there was a minimum at Tf ≈ 1,175 °C. Noting that the dominant response from LSMYSZ cathodes has been associated with the TPB process [37], we can focus on TPBs in explaining the results in Figure 3. Figure 4 shows 3D images of the TPB lines in cathodes fired

Table 1 Representative structural data from Ni-YSZ anodes annealed at 1,100 °C for 100 and 500 h. FIB-SEM and TXM data are presented. Microstructural parameter Ni volume (%) YSZ volume (%) Pore volume (%) Connected pore (%) Ni specific surface area (lm–1) YSZ specific surface area (lm–1) Pore specific surface area (lm–1) Total TPB density (lm–2) Active TPB density (lm–2)

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FIB-SEM

TXM

800 °C control

1,100 °C, 100 h

1,100 °C, 100 h

1,100 °C, 500 h

27.8 54.1 18.1 97.6 5.66 4.83 9.76 3.37 2.89

26.5 53.8 19.7 84.6 5.76 4.51 7.30 2.50 1.75

27.5 53.3 19.2 91.8 5.56 4.67 6.84 1.87 1.45

26.8 53.4 19.8 96.4 4.62 4.15 6.05 1.17 1.03

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Yakal-Kremski et al.: Studies of Solid Oxide Fuel Cell Electrode Evolution Using 3D Tomography (1,075 °C) cathode had a decreased RP after annealing, because the increased LSM connectivity dominated losses from coarsening/sintering.

4 Conclusions FIB-SEM and TXM 3D tomography studies showed that high temperature annealing of Ni-YSZ anode functional layers resulted in a reduction in the electrochemically active TPB density. The change in active TPB density resulted mainly from the decrease in total TPB density. Pore connectivity also played a significant role, because the present lowNi anodes, made using high sintering temperature with no pore formers, had a relatively low porosity of ∼20%. This made it more likely for pores to become isolated. On the other hand, the low pore volume and high YSZ fraction in these anodes presumably helped to limit Ni coarsening. The LSM-YSZ results suggest that under-fired cathodes may approach a more optimal microstructure, with better LSM-phase intra-connectivity, after extended cell operation. After longer operation times, however, coarsening of the microstructure is expected to gradually degrade performance. These ideas suggest that under-fired cathodes may provide extended cathode performance life. The present electrode microstructure changes are characteristic of annealing at temperatures higher than normal for SOFC operation. Further work, especially tests at different temperatures and times, is needed to verify that the same basic mechanisms apply at normal stack temperatures and, if so, to allow predictions of long-term degradation. Ultimately, 3D data sets can also be used as starting structures for detailed simulations of microstructural evolution, and coupled with a 3D electrochemical simulation to yield accurate predictions of long-term performance [25, 43].

Fig. 3 Polarization resistance RP of LSM-YSZ cathodes, measured in air at 800 °C, vs. firing temperature Tf.

Fig. 4 3D image representations of the TPBs in cathodes fired at different temperatures, with active TPBs shown in green and inactive TPBs shown in red. These thin (1.5 lm) sections were taken from near the center of the measured volumes. This provides a more representative visual comparison than images showing the entire measured volumes, which tend to accentuate the unknown networks that are found mostly near the measured volume edges.

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at 1,075, 1,175, and 1,325 °C. Although the cathodes in Figure 4 have stoichiometric LSM, they are expected to show similar trends as the A-site deficient LSM cathodes shown in Figure 3. Indeed, a similar variation of polarization resistance with Tf was observed for the stoichiometric LSM cathodes [45]. The most obvious change is a decrease in the density of TPB lines with increasing Tf, a result of coarsening of LSM and YSZ particles. The cathode also sintered more as Tf increased, indicated by the decrease in the pore volume fraction from 54% at 1,075 °C to 23% at 1,325 °C. This probably also contributed to the decreased TPB density. On the other hand, LSM networks in the cathode became better connected with increasing Tf, going from ∼50 vol.% connected at Tf = 1,075 °C to nearly 100% connected at 1,325 °C. The pore and YSZ connectivity remained near 100% at all Tf. The opposing effects with increasing Tf – the decrease in total TPB density and the increase in LSM connectivity – led to a maximum in the active TPB density at 1,175 °C, effectively explaining a minimum in RP in Figure 3. The LSM-YSZ annealing results can now be explained, assuming that increasing firing temperature has a similar effect as annealing for increased time. Thus, the cathode fired at the optimal Tf value of 1,175 °C showed an increase in RP after annealing, as more coarsening and sintering decreased the active TPB density. On the other hand, the under-fired

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Yakal-Kremski et al.: Studies of Solid Oxide Fuel Cell Electrode Evolution Using 3D Tomography

5 Acknowledgments The authors gratefully acknowledge financial support from the National Science Foundation Ceramics program through grant DMR-0907639. The FIB-SEM was accomplished at the Electron Microscopy Center for Materials Research at Argonne National Laboratory, a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. The FIB lift-out sample preparation for TXM was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Use of the National Synchrotron Light Source to obtain the TXM data is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

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