Porous electrolyte-supported tubular micro-SOFC design

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Author's personal copy Solid State Ionics 192 (2011) 368–371

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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i

Porous electrolyte-supported tubular micro-SOFC design Amir Reza Hanifi a,⁎, Alireza Torabi a, Thomas H. Etsell a, Luis Yamarte b, Partha Sarkar b a b

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta Canada T6G 2V4 Carbon and Energy Management, Alberta Innovates — Technology Futures, Edmonton, Alberta Canada T6N 1E4

a r t i c l e

i n f o

Article history: Received 13 August 2009 Received in revised form 18 June 2010 Accepted 22 July 2010 Available online 16 August 2010 Keywords: μ-SOFC YSZ Tubular cell Porous support Slip casting

a b s t r a c t Due to the poor redox cycling resistance of the second generation of μ-SOFCs, a new generation of SOFC has been recently developed using a porous electrolyte-supported structure to overcome this problem. In this research, the porous structure was successfully fabricated with slip casting using calcined YSZ (ZrO2 + 8 mol% Y2O3) with or without graphite as a pore former. Calcination of YSZ powder at 1300–1500 °C prior to making the slip leads to growth of YSZ crystals and particle size which results in a decrease in surface area and powder sinterability. This was found to be an important criterion in developing the porous structure as, due to the high sinterability of non-calcined YSZ, even the addition of graphite is inadequate to generate sufficient open porosity. A dense YSZ electrolyte layer was immediately coated on the porous structure using YSZ calcined at 1300 °C with a sequential slip casting method. Sample thickness was found to be a function of both graphite content as well as YSZ calcination temperature. Physical properties of the porous YSZ supports and SEM analysis of the support and coated electrolyte are presented. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Development of the 1st generation novel tubular μ-SOFC design based on ~ 2 mm diameter thin (~ 200 μm) dense YSZ electrolytesupported cells was pioneered by Kevin Kendall in the 1990s [1,2]. Two major benefits of a small diameter thin walled μ-SOFC are high volumetric electrolyte surface area and extremely high thermal shock resistance. These benefits open up the possibility of using μ-SOFCs in portable applications with rapid on/off capabilities. However, due to the thickness (~ 200 μm) of the electrolyte, the cell has a relatively low power density and in order to achieve sufficient power, the cell should be operated at high temperatures leading to more rapid cell degradation [3]. Therefore, practical SOFC devices cannot be developed based on this tubular μ-SOFC single cell. Accordingly, an anode-supported thin (~10 μm) electrolyte layer 2nd generation tubular μ-SOFC was developed [4–9]. The thin electrolyte layer enhances the power density resulting in operating temperatures even below 800 °C. A major drawback, however, of the anode-supported cell is the anode oxidation–reduction related microcracking of the cell. Any accidental oxidation of the anode can terminally damage the anode-supported SOFC device [10,11]. To overcome the redox cycling related problems and in order to improve the thermal shock resistance of the tubular fuel cells, Sarkar et al. [12]

⁎ Corresponding author. Fuel Cell Laboratory, Chemical and Materials Engineering Building, University of Alberta, Edmonton, Canada. Tel.: + 1 780 710 7997; fax: + 1 780 492 2881. E-mail address: hanifi@ualberta.ca (A.R. Hanifi).

developed a 3rd generation ‘porous electrolyte-supported’ (PES) tubular μ-SOFC. The present paper describes the fabrication of such a ‘tubular micro-SOFC’ type single cell using the ceramic slip casting technique.

2. Experimental methods In order to make a YSZ suspension for slip casting of the porous substrate, YSZ (Tosoh, TZ-8Y, 8 mol% Y2O3) calcined at 1300–1500 °C was mixed with water in a ratio of 52:48 wt.% and milled using 5 mm YSZ balls in a jar mill rotating at 80 rpm for 72 h after which the pH of the slip was set at 4.0 using HCl. Graphite was added in different volume percentages and the suspension was mixed shortly before casting. Different samples were slip cast as tubes for 1 min and as pellets in plaster molds with gypsum/water of 1.5/1 (~ 40% porous mold). Samples were removed from the molds, dried for 2 h at 100 °C, heat treated at 700 °C for 1 h to burn off the graphite and sintered at 1350 °C for 3 h. Due to the difficulties associated with measuring the physical properties of tubes leading to large experimental errors, pellets of 20 mm (diameter) × 5 mm (height) were slip cast and used for this purpose. Bulk density and open porosity of the pellets were determined by Archimedes principle considering the dry weight, saturation weight after boiling in distilled water for 2 h and also immersion weight in water. Closed porosity content was found using the theoretical density of YSZ (6.1 g/cm3). Shrinkage and weight loss were calculated from the length and weight of the dry pellets before and after sintering. SEM was carried out on gold coated tubes using an FEI company XL30 SEM running at 20 kV.

0167-2738/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.07.010

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3. Results Fig. 1(a) and (b) shows the microstructure of the tubes made of non-calcined YSZ and non-calcined YSZ + 20 vol.% graphite, respectively. It can be seen that the YSZ powder becomes dense after sintering at 1350 °C and minor amounts of closed pores remain in the microstructure which is inevitable. The addition of graphite does not generate significant open porosity inside the material as evidenced by the microstructure. The elongated closed pores are formed by graphite flakes which are burnt leaving void space. This clearly shows that non-calcined YSZ is not suitable for fabrication of a porous structure. The microstructure of the tube made of YSZ calcined at 1300 °C is shown in Fig. 2(a). It appears that the microstructure becomes dense after sintering at 1350 °C but some spherical closed pores are left. The addition of 10 vol.% graphite to the YSZ calcined at 1300 °C enhances the formation of a porous structure as shown in Fig. 2(b). A point worth noting is that when YSZ calcined at 1400 °C and 1500 °C is used for fabricating the tubes, even without the addition of graphite, samples contain considerable amounts of open pores (Fig. 2(c) and (d)). However, it is obvious that when graphite is added to the composition, structures will become even more porous. Fig. 3 shows the porous support structures made of YSZ calcined at 1300 °C with 30 vol.% graphite, YSZ calcined at 1400 °C with 15 vol.% graphite and YSZ calcined at 1500 °C with 10 vol.% graphite coated with YSZ electrolyte still calcined at 1300 °C. The support and electrolyte were co-fired at 1350 °C. It can be seen that a thin layer of dense YSZ which can act as the electrolyte has been coated successfully on the porous supports. 4. Discussion Fig. 1. SEM micrographs of the microstructure of tubes made of (a) non-calcined YSZ and (b) non-calcined YSZ + 20 vol.% graphite, both sintered at 1350 °C.

Table 1 shows the physical properties of the tubes made using different calcining temperatures and graphite contents. It appears that

Fig. 2. SEM micrographs of the microstructure of tubes made of (a) calcined YSZ at 1300 °C, (b) calcined YSZ at 1300 °C + 10 vol.% graphite, (c) calcined YSZ at 1400 °C and (d) calcined YSZ at 1500 °C, sintered at 1350 °C.

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Fig. 3. SEM micrographs of the interface of the porous support made of (a) calcined YSZ at 1300 °C and 30 vol.% graphite, (b) calcined YSZ at 1400 °C + 15 vol.% graphite, (c) calcined YSZ at 1500 °C + 10 vol.% graphite with the dense electrolyte layer cofired at 1350 °C. (1): Porous support cross section, (2): Elecrtolyte cross section, (3): Electrolyte inner surface.

after sintering of non-calcined YSZ there is only a small number of closed pores left and the material approaches its theoretical density after considerable shrinkage of about 22%. The addition of 20 vol.% graphite mainly leads to an increase in closed porosity as verified in the microstructure (Fig. 1(b)). The fact that even at such high contents of graphite there are few connected pores can be explained by the high sinterability of non-calcined YSZ which dominates the shrinkage behavior of the tube even in the presence of graphite leading to high densification. The physical properties of the tube made of YSZ calcined at 1300 °C indicate that this dense sample has more closed porosity and lower density compared with the sample made of non-calcined YSZ. This can be attributed to the lower sinterability of YSZ after calcination as evidenced by lower shrinkage of about 15%. Increasing the content of graphite helps in the formation of highly porous samples and a decrease in bulk density. It is worth noting that

increasing the content of graphite does not influence the shrinkage rate indicating that shrinkage is mainly controlled by YSZ. The main reasons which limit the addition of more graphite into the batch to generate further pores are firstly, difficulties associated with casting as the rheology of the slip changes and secondly, samples become very thick unless casting time is decreased significantly. In summary, the sample made of YSZ calcined at 1300 °C with 30 vol.% graphite which contains 36% open porosity can be a potential candidate for the porous support. Samples made of YSZ calcined at 1400 °C and 1500 °C contain considerable open pores after sintering at 1350 °C while this was not the case for YSZ calcined at 1300 °C possibly due to smaller particle size and higher surface area of this latter powder. This clearly shows that in order to create a YSZ porous structure, it is not always necessary to add pore former. Calcination at higher temperatures than the final sintering temperature leads to crystal and particle growth and a reduction in surface area and, consequently, in reactivity and sinterability of the YSZ powder. Therefore, during sintering at a lower temperature, the material is not able to completely eliminate the pores and the structure remains partially porous. However, addition of graphite to samples made of YSZ calcined at 1400 °C and 1500 °C still generates further porosity with a decrease in bulk density of the samples as shown in Table 1. The effect of increasing the calcination temperature on tube properties is reflected in a reduction of shrinkage rate and also the amount of closed porosity. Since the structures made of YSZ powder calcined at 1400 °C and 1500 °C are porous themselves, smaller graphite contents (10–15%) are required in order to generate 35–40% open pores in the final structure, as compared to the case where YSZ calcined at 1300 °C is used as the raw material. Similar difficulties in casting behavior and thickness of the samples appear when the calcination temperature increases. Therefore, in order to obtain a reasonably thin support with high porosity, both the calcination temperature and graphite content should be considered. In addition to the slip chemistry and slip solid loading, casting conditions such as casting time, porosity of the plaster mold and type of gypsum used play crucial roles on controlling tube thickness. When the porous electrolyte support is thinner, not only is it easier to infiltrate with anode material but also the final electrode resistance will be lower and gaseous reactant will be accessible resulting in lower concentration polarization [12]. The thickness of the porous tubes made of the potential compositions will decrease to about 500 μm when these parameters are optimized. As expected for graphite containing samples, the recorded weight loss of the samples was found to be proportional to the graphite content which burns during sintering. Tubes made of YSZ calcined at 1400 °C with 15 vol.% graphite and also YSZ calcined at 1500 °C with 10 vol.% graphite giving 39% and 36% porosity, respectively, can be two other potential candidates for the porous support. However, it should be noted that pores formed by using calcined powder (1400 °C and 1500 °C) are less than 2 μm, while elongated pores formed by graphite are much larger and can reach even 20 μm. Therefore, despite showing similar total porosity, the chosen porous structures may act differently when Ni is impregnated inside the structure due to different pore size as well as shape. This needs further analysis to find the best microstructure for the highest cell performance. Following fabrication of the porous supports, slips made of noncalcined YSZ and also YSZ calcined at 1300 °C which become dense after sintering at 1350 °C were considered as potential electrolyte material to be coated on the porous structure. The three mentioned potential porous supports were cast and after removal of the remaining slip from the plaster mold, the second slip for the electrolyte was cast for a brief time and the remaining slip was immediately poured out in order to form a thin layer. The results show that non-calcined YSZ is not a suitable candidate for the electrolyte

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Table 1 Physical properties of the tubes made of different types of YSZ sintered at 1350 °C. YSZ: ZrO2 + 8 mol% Y2O3, G: Graphite. Raw materials used

Sample thickness (μm)

Bulk density (g/cm3)

Open porosity (%)

Closed porosity (%)

Shrinkage (%)

Weight loss (%)

Non-calcined YSZ Non-calcined YSZ + 20 vol.% G Calcined YSZ at 1300 °C Calcined YSZ at 1300 °C + 10 vol.% G Calcined YSZ at 1300 °C + 20 vol.% G Calcined YSZ at 1300 °C + 30 vol.% G Calcined YSZ at 1300 °C + 40 vol.% G Calcined YSZ at 1400 °C Calcined YSZ at 1400 °C + 10 vol.% G Calcined YSZ at 1400 °C + 15 vol.% G Calcined YSZ at 1400 °C + 20 vol.% G Calcined YSZ at 1500 °C Calcined YSZ at 1500 °C + 10 vol.% G Calcined YSZ at 1500 °C + 15 vol.% G Calcined YSZ at 1500 °C + 20 vol.% G

393 348 575 597 660 1080 1248 525 750 1005 1985 589 952 1551 2350

5.977 5.215 5.629 4.857 4.326 3.675 3.130 4.713 4.131 3.578 3.173 4.424 3.737 2.935 2.664

0 2 0 15 23 36 46 21 30 39 46 25 36 50 54

2 12 7 5 6 4 3 2 2 2 1.5 2 2.5 1.5 2.5

22 22 16 15 15 15 15 8 10 11 13 6 7 9 7

– 9 – 4.5 9 14.5 20 – 4.5 7 9 – 4.5 7 9

since, after sintering of the tubes, the considerable difference in shrinkage rate between the electrolyte and porous support caused delamination of the thin coating and cracking of the electrolyte. However, YSZ calcined at 1300 °C was coated successfully on the porous support without any crack formation as observed in Fig. 3. The thickness of the coated electrolyte varied between 20 and 30 μm which can be decreased by lowering the solid loading of the slip. Impregnation of Ni into the porous supports that have been developed will be studied. A thin LSM cathode will be subsequently coated on the support and cell performance as well as thermal shock resistance and redox cycling will be evaluated in order to optimize the fabrication parameters. 5. Conclusions 1. Porous electrolyte supports were successfully fabricated using YSZ calcined at 1300–1500 °C with graphite as a pore former and a dense layer of YSZ calcined at 1300 °C was coated on the porous support to function as the cell electrolyte. 2. It was determined that calcination of the YSZ powder prior to casting leads to a decrease in shrinkage rate and sinterability of the powder so that with calcination at high temperatures, it is possible to generate significant porosity in the structure even without addition of any pore former. 3. Increasing the graphite content leads to an increase in the content of open pores, an increase in overall weight loss and a reduction in bulk density. Graphite does not appear to have a significant effect on the shrinkage rate of the tubes. 4. Thickness of the porous support is a function of both graphite content and calcination temperature and in order to achieve a thin layer, these parameters as well as casting conditions should be considered.

Acknowledgment This research was supported through funding to the NSERC Solid Oxide Fuel Cell Canada Strategic Research Network from the Natural Sciences and Engineering Research Council (NSERC) and other sponsors listed at www.sofccanada.com. References [1] K. Kendall, Proc. International Forum on Fine Ceramics, Nagoya, Japan, 1992, p. 143. [2] K. Kendall, in: S.C. Singhal, H. Iwahara (Eds.), Proc. Electrochemical Society, 1993, p. 813, NJ. PV93-4, Pennington. [3] J.W. Kim, A.V. Virkar, K.Z. Fung, K. Mehta, Singhal SC, J. Electrochem. Soc. 146 (1999) 69. [4] K. Kendall, G. Sales, Proc. 2nd International Conference on Ceramics in Energy Applications, London, UK, 1994, p. 55. [5] K. Kendall, M. Prica, Proc. 1st European SOFC Forum, Oberrohrdorf, Switzerland, 1994, p. 163. [6] Y. Du, N.M. Sammes, B. Eberly, in: M. Mogensen (Ed.), Proc. 6th European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, 2004, p. 125. [7] P. Sarkar, H. Rho, L. Yamarte, L. Johanson, in: M. Mogensen (Ed.), Proc. 6th European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, 2004, p. 278. [8] P. Sarkar, D. De, H. Rho, J. Mater. Sci. 39 (2004) 819. [9] P. Sarkar, L. Yamarte, H. Rho, L. Johanson, Int. J. Appl. Ceram. Technol. 4 (2) (2007) 103. [10] D. Waldbillig, A. Wood, D.G. Ivey, Solid State Ionics 176 (2005) 847. [11] D. Sarantaridis, A. Atkinson, Fuel Cells 7 (2007) 246. [12] P. Sarkar, L. Yamarte, G. Amow, Proc. 2009 CF/DRDC International Defence Materials Meeting, Victoria, B.C., Canada, 2009, p. 55.