Processing of 8YSZ and CGO thin film electrolyte layers for intermediate- and low-temperature SOFCs

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ScienceDirect Journal of the European Ceramic Society 35 (2015) 1505–1515

Processing of 8YSZ and CGO thin film electrolyte layers for intermediate- and low-temperature SOFCs Tim Van Gestel ∗ , Doris Sebold, Hans Peter Buchkremer Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, IEK-1: Materials Synthesis and Processing, D-52425 Jülich, Germany Received 24 July 2014; received in revised form 4 November 2014; accepted 17 November 2014 Available online 18 December 2014

Abstract An extensive experimental investigation has been carried out in order to prepare novel thin film electrolytes for enhanced SOFCs. Methods of producing ultra-thin 8 mol% Y2 O3 -doped ZrO2 (8YSZ) electrolytes (<1 ␮m) and thin 10 mol% Gd2 O3 -doped CeO2 (10CGO) electrolytes (∼1 ␮m) are presented. The method deposits such thin dense films onto a highly porous anode substrate. As opposed to conventional powder deposition techniques, the method involves depositing a dispersion of nanoparticles to achieve a thin-film mesoporous layer. After sintering at 1400 ◦ C, the deposited mesoporous layer becomes a dense thin film with a thickness of ∼1 ␮m or even thinner. Such thicknesses are significantly below the limit currently achievable with powder deposition techniques (∼10 ␮m). The electrolyte layer thickness is comparable to the thicknesses found in micro-SOFCs, but here conventional macroporous SOFC substrates are used. Of considerable importance is the use of a spin-coating process, due to its simplicity and the potential ease of further scaling-up. Results from SEM and leakage tests confirmed that the thin-film electrolytes are homogeneous and have a low number of defects after sintering at 1400 ◦ C. The average leak rate for air was 1–2 × 10−5 mbar l s−1 cm−2 for the 8YSZ electrolyte and 10−4 mbar l s−1 cm−2 for the 10CGO electrolyte. © 2014 Elsevier Ltd. All rights reserved. Keywords: Thin-film electrolyte; 8YSZ; CGO; IT-SOFC; LT-SOFC

1. Introduction SOFCs are a class of fuel cells which are characterized by the use of a solid oxide material as the electrolyte. In a working SOFC, the solid electrolyte conducts oxygen ions from the cathode (the air side) to the anode, where oxidation of the oxygen ions with the fuel (usually hydrogen) occurs. SOFCs consist of three main parts, macroporous anode, dense solid electrolyte and macroporous cathode, which are all made of ceramic materials. SOFCs operate at high temperatures, typically between 700 ◦ C and 1000 ◦ C, since the typical materials used in an SOFC are not sufficiently electrically and ionically conducting at lower temperatures. One of the biggest breakthroughs in SOFC research was the development of the anode-supported cell design. This design allowed cells to be manufactured with a much thinner electrolyte in comparison to previous

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http://dx.doi.org/10.1016/j.jeurceramsoc.2014.11.017 0955-2219/© 2014 Elsevier Ltd. All rights reserved.

electrolyte-supported cells. Because of this reduced thickness (typically 10–20 ␮m), it was possible to lower the SOFC operating temperature to the range of 700–800 ◦ C, while providing improved power densities.1–3 To date, one of the major challenges of SOFC research is to reduce the operating temperature to the intermediate(500–700 ◦ C) and low-temperature range (<500 ◦ C).4 The majority of SOFCs use an yttria-stabilized zirconia (YSZ) electrolyte, because of its stability. However, this material displays limited oxygen conductivity, which limits its application at low operating temperatures. Two approaches can in principle be taken to lower the operating temperature of the SOFC. The thickness of the common 8YSZ electrolyte can be significantly reduced to achieve the lowest possible resistance contribution or, alternatively, an electrolyte material with a higher conductivity can be considered. In the state of the art, various techniques permitting the fabrication of thin films have been studied. The most notable are chemical vapour deposition (CVD), physical vapour deposition (PVD), pulsed laser deposition (PLD) and sol-gel.5 With the

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current state of the art, thin film deposition has however only been successfully implemented in micro-SOFCs. Designs with YSZ and CGO electrolyte layers with a thickness significantly below 1 ␮m have been described.6–13 The layers are synthesized by vapour deposition techniques (e.g. CVD, PVD, PLD), whereby in each case specially designed substrates are applied. While there has been some success in developing micro-SOFCs with thin films <1 ␮m, scaling the coating technology to common highly porous anode substrates with a greater surface roughness remains problematic. The primary objective of our research work was to develop novel thin film electrolytes suitable for intermediate (IT) and low temperature (LT) SOFCs. Basically, we followed two approaches. One approach was to develop an ultra-thin 8YSZ electrolyte with a thickness <1 ␮m and the other approach was to develop a thin electrolyte composed of gadolinium-doped ceria (10CGO). A specific objective was to prepare such layers on a regular SOFC substrate, consisting of an anode support and a macroporous anode layer, which is typically characterized by a relatively rough surface, high porosity and large pore size. In our research, we introduce wet-coating technology to prepare a dense thinfilm electrolyte membrane. In this way, we avoid the often very complicated experimental methods reported for thin-film SOFCs and we were also able to coat large-area substrates. We are not aware of other systematic studies which report on the deposition of such thin electrolytes on common highly porous and rough SOFC anode substrates. To the best of our knowledge, only one group has managed to achieve a thin film electrolyte on a common SOFC anode substrate.14,15 As opposed to our work, the layers were however deposited on a substrate with a sintered anode functional layer with a dense structure. Furthermore, details on the size of the substrate are not provided. In this work, we use a highly porous substrate which is also used for the preparation of conventional SOFCs in our institute and we provide details on the size and the properties of the substrate and the coating methods. In the first part of this work, the synthesis of an 8YSZ electrolyte with a thickness <1 ␮m was carried out. In our previous work,16 a novel method using dispersions of nanoparticles with a size in the range of 50–100 nm was introduced. Using such dispersions, a dense 8YSZ layer with a thickness in the range of 1–2 ␮m was obtained after sintering at 1400 ◦ C. It became also apparent that the density of the sintered electrolyte improves by using a multiple coating process, which includes successive nano-dispersion and sol-gel coating steps. In our current work, we examined whether our method to produce such 1–2 ␮m thick layers can also provide an even thinner dense 8YSZ layer with a thickness <1 ␮m. Based on previous current density measurements in the range 600–700 ◦ C of a cell with a 1–2 ␮m thick electrolyte,17,18 such a submicrometer electrolyte could be for example promising for the temperature range 500–600 ◦ C. In the second part, we present a method of producing thin-film electrolyte layers composed of 10 mol% Gd2 O3 -doped CeO2 (10CGO). The proposed electrolyte material, 10CGO, is frequently considered to replace 8YSZ for low temperature (LT) SOFCs. At temperatures >500 ◦ C, CGO can in principle not

be considered, since the reduction of Ce4+ to Ce3+ becomes significant. Our strategy was therefore to develop an ultra-thin 8YSZ electrolyte for the intermediate temperature range and a 10CGO electrolyte for the low temperature range. In the literature, a number of electrolyte materials with greater oxygen conductivity than 8YSZ have been studied. These include mainly Sc-doped ZrO2 (SDZ) and doped CeO2 , Bi2 O3 and LaGaO3 systems.3,4,19–21 SDZ has high ionic conductivity relative to other doped zirconia compounds. Scandium is however a rare element and very expensive. In previous contributions, we show that the low conductivity of YSZ can also be circumvented by using very thin 8YSZ electrolyte layers.18 Thus, it is in principle not necessary to apply SDZ. Bi2 O3 systems have the highest ionic conductivity, but their limited stability in reducing atmosphere limits their applicability. LaGaO3 systems (e.g. LSGM) are also considered as candidates for the temperature range <500 ◦ C, but the conductivity of CGO is in this range slightly higher and Gallium is also a rare and very expensive element. 2. Experimental methods 2.1. Substrates The anode substrate was composed of a highly porous NiO/8YSZ support and a macroporous NiO/8YSZ layer with a pore size of 200–300 nm. The preparation method of this substrate is the same as the method described in our previous paper.16 In summary, the method includes pressing a NiO/8YSZ support plate with a pore size of ∼1 ␮m and vacuum-casting a NiO/8YSZ layer with a suspension. Hereby, commercial NiO (Mallinckrodt Baker) and 8YSZ powders (support plate: Unitec Ceramics, macroporous layer: Tosoh) are employed. The finished substrate has dimensions of ∼250 × 250 mm2 and a thickness of ∼1.5 mm. From this larger substrate, samples of ∼75 × 75 mm2 in size were cut. Based on 3D profilometry (Cybertechnologies CT 350S), the surface shows significant roughness, characterized by strongly curved areas. The roughness data showed a surface roughness (Ra ) of ∼0.5 ␮m and a maximum roughness height (Rmax ) of ∼5 ␮m. Prior to the coating experiments, we performed an additional characterization of the surface of the anode substrate with SEM. As shown in Fig. 1b and c, the substrate shows a very rough surface and also the waviness of the substrate can be clearly recognized, for example in the areas marked in red. In the SEM micrograph with larger magnification (Fig. 1(d)), a typical macroporous structure can be recognized for the NiO/8YSZ anode layer. As can be seen, the pore size distribution is very broad with pores of several 100 nanometers and also scattered smaller and larger pores are visible. 2.2. Preparation of the coating liquids The 8YSZ electrolyte layer with a thickness <1 ␮m was prepared according to a modified version of a method described in our previous contribution.16 This method included the formation of 1–2 ␮m thick 8YSZ electrolyte layers by subsequently

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Fig. 1. (a) Substrate consisting of NiO/8YSZ support plate and macroporous NiO/8YSZ layer (size ∼75 × 75 mm2 ). (b)–(d) Surface micrographs of the NiO/8YSZ layer. A few areas with a large roughness are marked with red arrows. (b) bar = 20 ␮m; (c) bar = 10 ␮m; (d) bar = 1 ␮m.

depositing double layers of an 8YSZ nano-dispersion with a particle size of ∼60 nm, a sol with a particle size of ∼35 nm and a sol with a particle size of ∼6 nm. Thinner layers in this work were made by spin-coating single layers of the same nanodispersions and sols. After each coating step, the layer was calcined at 500 ◦ C. For preparation of a 10CGO electrolyte layer, a novel method using a dispersion of 10CGO nanoparticles with a size of about 100 nm was introduced. The preparation route of the nanodispersion is summarized schematically in Fig. 2. It is prepared by

dispersing a commercial 10CGO nanopowder (Sigma–Aldrich) in a 0.05 M aqueous HNO3 solution by ultrasonication and subsequent separation of larger agglomerates by centrifugation. Fig. 3b shows the particle size distribution of the nanodispersion, measured by a dynamic laser beam scattering method (Horiba LB-550). An average particle size of ∼100 nm can thus be estimated. As can be seen in Fig. 3a, the nanodispersion was opaque in appearance. An important step was the introduction of a polymer to assist the layer deposition process. The addition of polyvinyl alcohol

Fig. 2. Schematic overview of the preparation route of the 10CGO layer. Final sintering is carried out at 1400 ◦ C after deposition of the respective layers.

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Fig. 3. (a) 10CGO coating liquid. (b) Particle size distribution of the 10CGO coating liquid.

(PVA 60,000 g/mol, Merck) to the coating liquids prior to deposition was found to result in a very homogeneous distribution and crack-free layer. None of the previous attempts to deposit the nanodispersion on the macroporous substrate yielded homogeneous layers. The advantage of PVA lies in the fact that PVA forms a hybrid polymeric membrane layer, with the nanoparticles homogeneously distributed in it. Without PVA, strong infiltration of the nano-dispersions was observed. PVA calcines at 500 ◦ C, leaving a mesoporous electrolyte layer which allows further deposition of additional layers. A free-standing material, which is obtained after drying a coating liquid with CGO nanoparticles is shown in Fig. 4. 2.3. Deposition of the electrolyte layer Deposition of all thin film electrolyte layers was carried out by spin-coating (Süss MicroTec D80T2 spin coater). A typical coating step involved dropping 10 ml of the coating liquid onto the substrate (75 × 75 mm2 SOFC anode substrate). The substrate was held by a vacuum chuck and spun at 800 rpm for 1 min. After each coating step, the sample was fired in a conventional furnace at 500 ◦ C in air.

As mentioned above, single spin-coating – calcination steps were employed for the formation of the 8YSZ electrolyte. For the formation of the 10CGO electrolyte, a single spin-coating and calcination step was first tried out. Based on the obtained result from this experiment, a triple spin-coating – calcination procedure was employed for the 10CGO electrolyte. After performing the different spin-coating and calcination steps, the 8YSZ and 10CGO samples were exposed to a heat treatment at 1400 ◦ C for 5 h in air. It should be noticed that the same heat treatment is also applied for sintering conventional 10 ␮m thick 8YSZ electrolyte layers at our institute. In analogy with the samples with a conventional electrolyte layer, a “flattening” step was also employed. This included an additional firing at 1350 ◦ C for 1 h in a furnace which is equipped with movable SiC plates. When the furnace achieves 1350 ◦ C, a SiC plate is pressed automatically on the sample to flatten it. This creep deformation treatment produces perfectly flat samples, which can be mounted in the holder for gas-tightness testing. Furthermore, the formation of a stack of cells in a later phase of the research will require perfectly flat samples. A well-known effect is that at such high temperatures the macroporous NiO/8YSZ layer is also (partially) sintered. In order to avoid this effect which can influence the characterization of the gas-tightness of the deposited thin-film layers, the samples were subsequently treated at 900 ◦ C in Ar/H2 (4% H2 ). During this treatment, NiO was reduced to Ni, which created a porous structure in the anode layer. Finally, the samples were cut with a laser to a size of 50 × 50 mm2 . Fig. 5 shows photographs of an 8YSZ and a CGO sample after sintering and after reduction and laser cutting. 2.4. Characterization methods

Fig. 4. Free-standing hybrid membrane consisting of a mixture of polyvinyl alcohol (PVA) and 10CGO nanoparticles.

Microscopical observations of the fracture and the surface of the uncoated and coated substrates were performed with a Zeiss Ultra 55 FEG-SEM in a similar way as in our previous paper. The gas-tightness of the layers was measured with a commercial leak testing system (dr.wiesner INTEGRA) which

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Fig. 5. (a) Photograph of a sample with 8YSZ electrolyte after firing at 1400 ◦ C. (b) Sample after reduction at 900 ◦ C in Ar/4%H2 and laser cutting. (c) Photograph of a sample with 10CGO electrolyte after firing at 1400 ◦ C. (d) Sample after reduction at 900 ◦ C in Ar/4%H2 and laser cutting.

uses ambient air as the test gas. The module for testing the samples was 50 × 50 mm2 in size and the dimensions of the test area inside the sealing were 40 × 40 mm2 .

3. Results 3.1. Formation of dense thin-film 8YSZ layer with a thickness <1 μm Figs. 6 and 7 show fracture and surface SEM images of a sample with an 8YSZ electrolyte layer, which was prepared according to the method described in Sections 2.2 and 2.3. The sample was thermally treated at 1400 ◦ C in air for 5 h and at 900 ◦ C in Ar/4% H2 for 3 h. In the overview micrograph (Fig. 6a), the upper part of the Ni/8YSZ anode support plate and the macroporous Ni/8YSZ layer can be recognized. It can be clearly seen that a porous anode layer was obtained and it appeared that exposure to a reducing environment reduces NiO in the anode layer into Ni, without affecting the integrity of the dense thin film electrolyte. Furthermore, a very homogeneous dense thin 8YSZ layer can be observed. In the detail micrographs (Fig. 6c and d), a layer thickness in the range 500 nm to 1 ␮m can be estimated. Fig. 7 shows SEM top views of the same sample. As seen from Fig. 7a and b, the thin 8YSZ electrolyte layer covers the surface of the substrate homogeneously. As can be seen in the detail micrographs (Fig. 7c and d), the film exhibits large grains with clear grain boundaries and no pores are visible. The size of the grains varies from several hundred nanometers to significantly greater than 1 ␮m. Since the layer thickness is <1 ␮m,

this implies that the grains have a flattened shape with a width that exceeds significantly the thickness. In order to describe the formation of such unique grains, we refer to the works reported by Butz et al.22 and Scherrer et al.23 As opposed to our work, they considered dense substrates with a very homogeneous surface. The thickness of their layers is however in the same range and the same sintering phenomena can be recognized. Butz et al. deposited 8YSZ films on sapphire substrates using a multiple sol-gel dip-coating procedure (10 coating and calcination steps). In analogy with our results, they reported an acceleration of the grain growth and densification for the samples fired in the range 1250 ◦ C and 1350 ◦ C. The average grain size increased with increasing annealing temperature from around 5 nm after 650 ◦ C to 335 ± 117 nm at 1250 ◦ C and 548 ± 187 nm at 1350 ◦ C. They showed that for a firing temperature of 1350 ◦ C, the grain size exceeded the size of the layer thickness and that the films consisted of one monolayer of YSZ grains only. After annealing at 1600 ◦ C, some grains reach lateral dimensions up to 20 times that of the film thickness, which resulted in a disruption of the YSZ film. Scherrer et al. investigated the microstructures of YSZ (0–15 mol% Y) thin films deposited by spray pyrolysis at 370 ◦ C on sapphire substrates. The average grain size increased from around 17 ± 5 nm after 800 ◦ C for 20 h to 1.4 ± 0.7 ␮m at 1400 ◦ C for 15 h. They reported that for annealing temperatures above 1200 ◦ C, the grain size grew to the size of the film thickness and therefore the films consist of one monolayer of YSZ grains only. As mentioned above in the experimental part, the thin-film samples in this work were end-sintered using the standard temperature program for conventional electrolytes in our institute (1400 ◦ C, 5 h). In our previous contribution, we studied the

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Fig. 6. 8YSZ layer after sintering in air at 1400 ◦ C and reduction at 900 ◦ C in Ar/4%H2 . (a) and (b) Overview fracture micrographs. (c) and (d) Detailed fracture micrographs. (a) bar = 10 ␮m, (b) and (c) bar = 2 ␮m, (d) bar = 1 ␮m.

densification of an 8YSZ thin-film with a thickness of ∼1–2 ␮m at 1200 ◦ C, 1300 ◦ C and 1400 ◦ C. After firing at 1200 ◦ C, the thin film 8YSZ layer was not sufficiently gas tight. After firing at 1300 ◦ C, 8YSZ grains grew to the micrometer range, which resulted in the formation of mainly dense 8YSZ electrolyte

layers. At this point, the layer appeared as a monolayer of 8YSZ grains. The first 1300 ◦ C samples showed a He leakage of 4.9 × 10−4 and 6.5 × 10−4 mbar l s−1 cm−2 , which confirmed the suitability of the developed coating processes. However, in our continuing work on the deposition and sintering of

Fig. 7. 8YSZ layer after sintering in air at 1400 ◦ C and reduction at 900 ◦ C in Ar/4%H2 . (a) Overview surface micrograph. (b) Overview surface micrograph in the back-scattering mode. (c) and (d) Detailed surface micrographs. (a) and (b) bar = 20 ␮m, (c) bar = 2 ␮m, (d) bar = 1 ␮m.

T. Van Gestel et al. / Journal of the European Ceramic Society 35 (2015) 1505–1515 Table 1 Air leak test results for a series of 3 samples with spin-coated <1 ␮m thick 8YSZ layer.

S1 S2 S3

Specific air leak rate after sintering at 1400 ◦ C for 5 h (mbar l s−1 cm−2 )

Specific air leak rate after reduction at 900 ◦ C in Ar/H2 for 3 h (mbar l s−1 cm−2 )

1.70 E−05 1.50 E−05 2.55 E−05

6.45 E−04 9.18 E−04 7.11 E−04

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As shown in Table 1, the gas-tightness in terms of air-leakage was ∼1–2 × 10−5 mbar l cm2 s for the samples fired at 1400 ◦ C and ∼7 × 10−4 mbar l cm2 s after reducing the same samples. These values, which are the average values of 3 samples, are in line with the values obtained in our previous work for similar, but thicker, layers made with double coating – calcination steps. Thus, it is shown that in principle these additional coating steps are not necessary and can be omitted when a thinner electrolyte layer is desired.

S1, sample 1; S2, sample 2; S3, sample 3.

3.2. Formation of dense thin-film 10CGO layer nanoparticle 8YSZ layers we decided to thermally treat the samples at 1400 ◦ C for 5 h, which is the standard temperature program for conventional thicker electrolytes at our institute. The gas tightness of the 8YSZ thin-film electrolyte was significantly better at 1400 ◦ C and the high temperature had no negative influence on the appearance of the layer. This improved gas tightness is likely due to the significant shrinkage of the substrate at 1400 ◦ C, as described, for example, by Mücke et al.24 and Tikanen et al.25 Of considerable importance in our work is also the possibility to use the same “flattening” step after sintering as used in the standard processing method of samples which have a 10 ␮m thick 8YSZ electrolyte layer. As mentioned in the experimental part, the samples are subjected to an additional firing treatment at 1350 ◦ C, while a SiC plate is pressed on the sample to flatten it. Perfectly flat samples are e.g. required for our gas-tightness testing set-up and for making a fuel-cell stack.

Fig. 8 shows detailed SEM and back-scattering SEM images of two different 10CGO samples fabricated using one (Fig. 8a and b) and three (Fig. 8c and d) coating – calcination steps. As shown in Figs. 8a and b, a continuous mesoporous layer with a thickness in the range of 500 nm to 1 ␮m was achieved for a single coating step. Furthermore, infiltration of the CGO nanoparticles into the pores of the macroporous NiO/8YSZ anode layer could be largely prevented using the coating liquid composition with PVA as additive. As seen, the addition of PVA to the nanodispersion prior to the deposition prevented infiltration of the CGO nanoparticles and created a homogeneous distribution. Images of a triple layer (Fig. 8c and d) show that the thickness of the layer increases with the number of coating steps. In the back-scattering image of the triple layer system, a separation line is also clearly visible between the first and second, and second and third calcined layer, respectively. It should be mentioned that we aimed at a thickness of 2–3 ␮m for the

Fig. 8. Mesoporous 10CGO layer, obtained by spin coating the 10CGO nanodispersion on a macroporous SOFC substrate. (a) and (b) Detail of the fracture and backscattering image after 1 coating step and firing at 500 ◦ C. (c) and (d) After 3 coating steps and firing at 500 ◦ C. (a) and (b) bar = 200 nm, (c) and (d) bar = 1 ␮m.

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Fig. 9. Mesoporous 10CGO layer, obtained by spin coating the 10CGO nanodispersion on a macroporous SOFC substrate. (a) and (b) Overview of the fracture and backscattering image after 3 coating steps and firing at 500 ◦ C. (c) and (d) Overview and detailed surface images. (a) and (b) bar = 2 ␮m, (c) bar = 20 ␮m, (d) bar = 1 ␮m.

Fig. 10. 10CGO layer after sintering in air at 1400 ◦ C and reduction at 900 ◦ C in Ar/4%H2 . (a) Overview fracture micrograph. (b) and (c) Fracture micrograph and back-scattering fracture micrograph. (d) Detailed fracture micrograph. (a) bar = 10 ␮m, (b) and (c) bar = 2 ␮m, (d) bar = 1 ␮m.

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Fig. 11. 10CGO layer after sintering in air at 1400 ◦ C and reduction at 900 ◦ C in Ar/4%H2 . (a)–(c) Detail and overview surface micrographs. (d) Overview surface micrograph in the backscattering mode. (e) Surface micrograph showing a large defect. (a) bar = 1 ␮m, (b) and (e) bar = 10 ␮m, (c) and (d) bar = 20 ␮m.

calcined mesoporous precursor layer, since it was expected that this would yield an overall thickness of ∼1 ␮m for the dense layer after the final sintering step. The triple layer system showed an overall thickness of 2–3 ␮m and was thus selected for further coating and sintering experiments. The overview fracture and surface micrographs in Fig. 9 confirm the formation of a homogeneous 10CGO mesoporous layer on the macroporous anode substrate. In the backscattering Image (Fig. 9b), the upper part of the NiO/8YSZ anode support plate, the macroporous NiO/8YSZ layer and the mesoporous 10CGO triple layer can be recognized. The 10CGO layer is here clearly visible as a brighter film, due to its much smaller pore size. In the overview surface SEM Image (Fig. 9c), it appears that the layer was crack-free, despite the waviness of the substrate. Fig. 9d shows a detailed surface image of the 10CGO layer. Comparison of this image with the image in Fig. 1b of the macroporous anode layer before the coating

process clearly indicates that homogeneous coverage was achieved. The layer shown in Fig. 9d shows however scattered also a number of larger pores, which have in some cases a slit shape. Figs. 10 and 11 show fracture and surface SEM images of a 10CGO sample which was further fired at 1400 ◦ C in air for 5 h and treated at 900 ◦ C in Ar/4% H2 for 3 h. In the overview fracture micrographs (Fig. 10a, b and c), it can be clearly seen that a porous anode layer was obtained, homogeneously coated with a dense 10CGO layer. As can be seen in the back-scattering micrograph (Fig. 10c), the thin 10CGO electrolyte layer covers the surface of the substrate homogeneously. However, it should be noticed that we found also areas which might not be completely gas-tight. In this image, such an area is marked with a red arrow. Afterwards, we also found similar areas in the surface micrographs (Fig. 11). As shown in the detailed fracture micrograph (Fig. 10d), a thickness of ∼1 ␮m was achieved for the

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Table 2 Air leak test results for 3 series of 3 samples with spin-coated 1 ␮m thick 10CGO layer. Specific air leak rate after sintering at 1400 ◦ C for 5 h (mbar l s−1 cm−2 )

Specific air leak rate after reduction at 900 ◦ C in Ar/H2 for 3 h (mbar l s−1 cm−2 )

S11 S12 S13

8.66 E−05 5.33 E−05 6.07 E−05

6.87 E−04 5.36 E−04 6.26 E−04

S21 S22 S23

1.44 E−04 1.35 E−04 3.65 E−04

7.41 E−04 1.54 E−03 3.38 E−03

S31 S32 S33

1.71 E−04 2.38 E−04 3.26 E−04

1.47 E−03 1.86 E−03 1.33 E−03

S11, sample 1 of series 1; S12, sample 2 of series 1, . . .; S21, sample 1 of series 2, . . .

10CGO layer. Furthermore, it is clear that the films consisted of one monolayer of 10CGO grains only. The surface micrographs (Fig. 11a and b) show that such a layer contains also in this case comparatively large grains, with a size clearly exceeding 1 ␮m. This suggests that also here some of the grains in the layer should have a flattened shape, with a width exceeding the thickness of the grain. Subsequently, the gas-tightness of the thin film 10CGO layer was investigated. To check the reproducibility of the novel synthesis route, 3 series of 3 samples were prepared in the same way and tested. As seen in Table 2, the 10CGO thin films showed a comparable leak rate, with values in the range of 5 × 10−5 to 3 × 10−4 mbar l s−1 cm−2 (average value 1.8 × 10-4 mbar l s−1 cm−2 ) after sintering at 1400 ◦ C. After treatment in a reducing atmosphere, the leak rate was 5 × 10−4 – 1 × 10−3 mbar l s−1 cm−2 , and thus higher by a factor of 10. This confirmed that an effective novel coating process had been developed for the reproducible deposition of thinfilm 10CGO electrolyte layers with a thickness of ∼1 ␮m on a conventional macroporous anode substrate. It became however apparent that the gas tightness of these first series of samples is still significantly lower as the values obtained for 8YSZ samples. After its successful introduction, development work is under way to prepare improved 10CGO electrolyte layers. The composition of the coating liquid may be refined to improve the coverage of the substrate during the coating step. Another important aspect is the particle size of the coating liquids. The first 10CGO prototypes were made by spin-coating triple layers of a nano-dispersion. For the 8YSZ samples, the density of the sintered electrolyte improves by using a multiple coating process, with successive nanodispersion and sol coating steps. Comparable CGO sols are however not available yet. It is most likely that a comparable improvement of the gas tightness will be achieved as soon as suitable CGO sols are available and a graded system can be deposited. Another important aspect is the quality of the anode substrate in our work. Scanning the surface with SEM indicated

the presence of large defects in the 10CGO layers, which can be related to substrate. Examples of such defects are shown in Fig. 11(c)–(e). First, as shown in Fig. 11(c) and the backscattering Image (Fig. 11d), the layer contains a number of untight areas. These areas can be found in particular where the waviness of the surface is large. Possible problem areas are marked with a red arrow in Fig. 11(c) and (d). Second, larger defects were also found as shown in Fig. 11(e). It is likely that the use of substrates with improved surface quality can help to prevent such defects. A tender point is also that the surface properties may differ between different batches. In principle, the substrate is the same NiO/8YSZ anode substrate in all examples. In practice, however, varying substrate quality may also have an influence on the results. In essence, it can be concluded that different aspects need improvement. First, forthcoming work should be directed towards the fabrication of substrates with a smoother surface and the lowest possible amount of defects in a reproducible way. We also suggest to characterise the surface quality/roughness of each individual substrate prior to the coating experiment. In this way, problems related to the substrate can be identified more accurately. Second, in the case of 10CGO, similar sols as used for the deposition of 8YSZ films should be developed. However, despite these shortcomings, it is clear that our research yielded a very important novel processing method to fabricate a CGO thin-film electrolyte. 4. Conclusion Very thin 8YSZ electrolyte layers (<1 ␮m) were obtained on a common SOFC substrate in the first part of this work. Our previously published method for fabricating 1–2 ␮m thick layers on a conventional macroporous substrate could be successfully modified to yield such layers. The prepared ultrathin 8YSZ electrolytes have a leak rate for air in the range of 1.5–2.5 × 10−5 mbar l s−1 cm−2 after sintering at 1400 ◦ C. After treatment in a reducing atmosphere, the leak rate was 6.5 × 10−4 –9 × 10−4 mbar l s−1 cm−2 . Successful synthesis of a 10CGO thin-film electrolyte with a thickness of ∼1 ␮m was demonstrated in the second part of this work. Such thin films have a leak rate for air in the range of 5 × 10−5 –3 × 10−4 mbar l s−1 cm−2 after sintering at 1400 ◦ C. After treatment in a reducing atmosphere, the leak rate was 5 × 10−4 –1 × 10−3 mbar l s−1 cm−2 , and thus higher by a factor of 10. In the samples under consideration, a few scattered defects were found, which limit the gas-tightness. It is clear that this work introduces a major breakthrough in low- and intermediate-temperature SOFC research. The electrolyte thickness achieved is significantly below the limit currently possible with powder deposition techniques (∼10 ␮m), which is the state of the art. The thickness of the electrolytes developed here is comparable to the thickness of micro-SOFC electrolytes described in the introduction. In this work, such electrolytes are obtained with a very simple, practical and scalable spin-coating process on a conventional macroporous anode substrate.

T. Van Gestel et al. / Journal of the European Ceramic Society 35 (2015) 1505–1515

References 1. Wachsman ED, Singhal SC. Solid oxide fuel cell commercialization, research and challenges. Am Ceram Soc Bull 2010;89(3):22–32. 2. Yamamoto O. Solid oxide fuel cells: fundamentals and prospects. Electrochim Acta 2000;45:2423–35. 3. Steele BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345–52. 4. Wachsman ED, Lee KT. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935. 5. Beckel D, Bieberle-Hütter A, Harvey A, Infortuna A, Muecke UP, Prestat M, Rupp JLM, Gauckler LJ. Thin films for micro solid oxide fuel cells. J Power Sources 2007;173:325–45. 6. Noh H-S, Son J-W, Lee H, Song H-S, Lee H-W, Lee J-H. Low temperature performance improvement of SOFC with thin film electrolyte and electrodes fabricated by pulsed laser deposition. J Electrochem Soc 2009;156(12):B1484–90. 7. Heiroth S, Lippert T, Wokaun A, Döbeli M. Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition. Appl Phys A 2008;93:639–43. 8. Ignatiev A, Chen X, Wu N, Lu Z, Smith L. Nanostructured thin solid oxide fuel cells with high power density. Dalton Trans 2008:5501–6. 9. Gannon P, Sofie S, Deibert M, Smith R, Gorokhovsky V. Thin film YSZ coatings on functionally graded freeze cast NiO/YSZ SOFC anode supports. J Appl Electrochem 2009;39:497–502. 10. Park Y-i, Chen Su P, Cha SW, Saito Y, Prinz FB. Thin-film SOFCs using gastight YSZ thin films on nanoporous substrates. J Electrochem Soc 2006;153:A431–6. 11. Su P-C, Chao C-C, Shim JH, Fasching R, Prinz FB. Solid oxide fuel cell with corrugated thin film electrolyte. Nano Lett 2008;8(8):2289–92. 12. Chun S-Y, Mizutani N. The transport mechanism of YSZ thin films prepared by MOCVD. Appl Surface Sci 2001;171:82–8. 13. Hoon Joo J, Man Choi G. Simple fabrication of micro-solid oxide fuel cell supported on metal substrate. J Power Sources 2008;182: 589–93. 14. Oh E-O, Whang C-M, Lee Y-R, Park S-Y, Hari Prasad D, Joong Yoon K, Son J-W, Lee J-H, Lee H-W. Extremely thin bilayer electrolyte for solid

15.

16.

17.

18.

19.

20.

21.

22.

23.

24. 25.

1515

oxide fuel cells (SOFCs) fabricated by chemical solution deposition (CSD). Adv Mater 2012;24:3373–7. Oh E-O, Whang C-M, Lee Y-R, Lee J-H, Joong Yoon K, Kim B-K, Son J-W, Lee J-H, Lee H-W. Thin film yttria-stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition. J Eur Ceram Soc 2012;32:1733–41. Van Gestel T, Sebold D, Buchkremer HP, Stöver D. Assembly of 8YSZ nanoparticles into gas-tight 1–2 ␮m thick 8YSZ electrolyte layers using wet coating methods. J Eur Ceram Soc 2012;32:9–26. Van Gestel T, Feng H, Sebold D, Mücke R, Menzler N, Buchkremer HP, Stöver D. Nano-structured solid oxide fuel cell design with superior power output at high and intermediate operation temperatures. Microsyst Technol 2011;17(2):233–42. Han F, Mücke R, Van Gestel T, Leonide A, Menzler NH, Buchkremer HP, Stöver D. Novel high-performance solid oxide fuel cells with bulk ionic conductance dominated thin-film electrolytes. J Power Sources 2012;218:157. Cassir M, Gourba E. Reduction in the operating temperature of solid oxide fuel cells – potential use in transport applications. Ann Chim Sci Mat 2001;26(4):49–58. Irvine JTS, Dobsona JWL, Politovaa T, García Martín S, Shenouda A. Co-doping of scandia–zirconia electrolytes for SOFCs. Faraday Discuss 2007;134:41–9. Park J-Y, Wachsman ED. Stable and high conductivity ceria/bismuth oxide bilayer electrolytes for lower temperature solid oxide fuel cells. Ionics 2006;12:15–20. Butz B, Störmer H, Gerthsen D, Bockmeyer M, Krü ger R, Ivers-Tiffée E, Luysberg M. Microstructure of nanocrystalline yttria-doped zirconia thin films obtained by sol–gel processing. J Am Ceram Soc 2008;91(7):2281–9. Scherrer B, Martynczuk J, Galinski H, Grolig JG, Binder S, Bieberle-Hütter A, Rupp JLM, Prestat M, Gauckler LJ. Microstructures of YSZ and CGO thin films deposited by spray pyrolysis: influence of processing parameters on the porosity. Adv Funct Mater 2012;22:3509–18. Mücke R, Menzler N, Buchkremer HP, Stöver D. Sintering of thin zirconia SOFC electrolyte films. ECS Trans 2007;7(1):2175–85. Tikkanen H, Suciu C, Waernhus I, Hoffmann AC. Examination of the cosintering process of thin 8YSZ Films obtained by dip-coating on in-house produced NiO-8YSZ. J Eur Ceram Soc 2011;31:1733–9.