stable sealing glass for planar solid oxide fuel cell

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Stable sealing glass for planar solid oxide fuel cell Sung-Bum Sohn a , Se-Young Choi a, * , Gyeung-Ho Kim b , Hue-Sup Song b , Goo-Dae Kim b a School of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, South Korea b Division of Materials, Korea Institute of Science and Technology, Seoul 136-791, South Korea Received 20 February 2001; received in revised form 25 October 2001 Abstract The thermal and chemical stability of glasses in the BaOAl 2 O 3 La 2 O 3 B 2 O 3 SiO 2 system were investigated as well as bonding characteristics and wetting behavior to yttria stabilized zirconia (YSZ) electrolyte, to develop a suitable sealing glass for planar solid oxide fuel cell operating at 800–850 °C. The thermal properties such as glass transition temperature, softening temperature and thermal expansion coefficient were found to depend on the B 2 O 3 :SiO 2 ratio in glass composition; thus the bonding characteristics of the glass to YSZ were also influenced by this ratio. The glass having a minimum thermal expansion mismatch with YSZ showed an excellent endurance during thermal cycling. No interface reaction was observed for all the glass/YSZ specimens heat-treated at 800–850 °C up to 100 h. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction Solid oxide fuel cells (SOFCs) provide advan- tages of high energy conversion efficiency, such as heat utilization and the ability to use a variety of fuels, due to their high operating temperature in the range from 800 to 1000 °C [1,2]. Such fuel cells can be classified into two large groups by unit cell stacking type, i.e. planar and tubular type SOFCs. Of these two types, the planar type SOFCs are superior to the tubular ones because of their sim- ple manufacturing process, relatively short current path and the resulting higher power density and efficiency [1,3]. In case of the planar type SOFCs, however, it is necessary to develop a suitable sealant for preventing the fuel gas and air from mixing during fuel cell operation and for insulat- ing the stack to the atmosphere. Within the fuel cell stack, an effective sealant must be compatible with the thermal expansion behavior of the fuel cell components, i.e. it has to have a suitable thermal expansion coefficient (TEC) that matches those of the other fuel cell components, and it must have no or little chemical reaction with the joining components and high chemical stability in both wet reducing (reducing fuel gas þ H 2 O) and oxidizing atmospheres [4–6]. Also, the sealant must be electrically insulating. Since it contacts both anode and cathode, electrical conductivity in the sealant causes an undesirable shunt cur- rent flow [4]. In addition, non-wetting (h > 90 °C) property of sealant to the fuel cell components should be considered to prevent the glass from Journal of Non-Crystalline Solids 297 (2002) 103–112 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +82-2 2123 2851; fax: +82-2 2365 0821. E-mail address: [email protected] (S.-Y. Choi). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(01)01042-0

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Stable sealing glass for planar solid oxide fuel cell

Sung-Bum Sohn a, Se-Young Choi a,*, Gyeung-Ho Kim b, Hue-Sup Song b,Goo-Dae Kim b

a School of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, South Koreab Division of Materials, Korea Institute of Science and Technology, Seoul 136-791, South Korea

Received 20 February 2001; received in revised form 25 October 2001

Abstract

The thermal and chemical stability of glasses in the BaO–Al2O3–La2O3–B2O3–SiO2 system were investigated as well

as bonding characteristics and wetting behavior to yttria stabilized zirconia (YSZ) electrolyte, to develop a suitable

sealing glass for planar solid oxide fuel cell operating at 800–850 �C. The thermal properties such as glass transitiontemperature, softening temperature and thermal expansion coefficient were found to depend on the B2O3:SiO2 ratio in

glass composition; thus the bonding characteristics of the glass to YSZ were also influenced by this ratio. The glass

having a minimum thermal expansion mismatch with YSZ showed an excellent endurance during thermal cycling. No

interface reaction was observed for all the glass/YSZ specimens heat-treated at 800–850 �C up to 100 h. � 2002

Elsevier Science B.V. All rights reserved.

1. Introduction

Solid oxide fuel cells (SOFCs) provide advan-tages of high energy conversion efficiency, such asheat utilization and the ability to use a variety offuels, due to their high operating temperature inthe range from 800 to 1000 �C [1,2]. Such fuel cellscan be classified into two large groups by unit cellstacking type, i.e. planar and tubular type SOFCs.Of these two types, the planar type SOFCs aresuperior to the tubular ones because of their sim-ple manufacturing process, relatively short currentpath and the resulting higher power density andefficiency [1,3]. In case of the planar type SOFCs,

however, it is necessary to develop a suitablesealant for preventing the fuel gas and air frommixing during fuel cell operation and for insulat-ing the stack to the atmosphere. Within the fuelcell stack, an effective sealant must be compatiblewith the thermal expansion behavior of the fuelcell components, i.e. it has to have a suitablethermal expansion coefficient (TEC) that matchesthose of the other fuel cell components, and itmust have no or little chemical reaction with thejoining components and high chemical stability inboth wet reducing (reducing fuel gasþH2O) andoxidizing atmospheres [4–6]. Also, the sealantmust be electrically insulating. Since it contactsboth anode and cathode, electrical conductivityin the sealant causes an undesirable shunt cur-rent flow [4]. In addition, non-wetting (h > 90 �C)property of sealant to the fuel cell componentsshould be considered to prevent the glass from

Journal of Non-Crystalline Solids 297 (2002) 103–112

www.elsevier.com/locate/jnoncrysol

* Corresponding author. Tel.: +82-2 2123 2851; fax: +82-2

2365 0821.

E-mail address: [email protected] (S.-Y. Choi).

0022-3093/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0022-3093 (01 )01042-0

infiltrating into the porous anode and cathode orthe electrolyte thin film and flowing out from thefuel cell stack during the sealing process or theoperation of the cell [7].In an effort to develop such a suitable sealant

for planar SOFCs, many studies have been carriedout over the application of the glass or glass–ceramics-based on alkali silicate, alkaline-earthsilicate, alkali borosilicate such as Pyrex, alumino-phosphate and so on [8–14]. Each material, how-ever, has a few drawbacks such as high thermalmismatch and poor long-term stability due tothe active interface reaction with other compo-nents of the fuel cell.In this work, therefore, thermal and chemical

stability of glasses in the BaO–Al2O3–La2O3–B2O3–SiO2 system were investigated as well as bondingcharacteristics and wetting behavior to yttria sta-bilized zirconia (YSZ), which is generally used as aelectrolyte for SOFC, to develop a suitable sealingglass for planar SOFC operating at 800–850 �C.The thermal stability was evaluated by investi-gating the thermal expansion mismatch betweenglass and YSZ and observing the interface-crack-ing after thermal cycling test. The long-termchemical stability was investigated from the pointof view of the interface reaction between glass andYSZ after 100 h duration at fuel cell operationtemperature.

2. Experimental

The compositions of the glass in this work aregiven in Table 1. To prepare a suitable sealingglass, B2O3 and SiO2 were chosen as a glassformer (B2O3=SiO2 were kept constant at 0.33–

0.71). Other components include BaO to increasethe TEC, Al2O3 to prevent the rapid crystalli-zation of glass during heat-treatment and also toincrease the surface tension of glass and La2O3to control the viscosity [15,16]. All the glass com-position in this work has no alkali oxide as aflux because alkali metal ions could be easilydiffused into the fuel cell components, to lead theinterface reaction or increase the electrical con-ductance.

2.1. Glass preparation

The glasses were made from reagent-gradeBaCO3, Al2O3, La2O3, H3BO3 and SiO2. Thethoroughly mixed batches were melted in an elec-tric furnace at 1550 �C for 3 h using a Pt–Rd 10%crucible, and poured onto the steel mold to pro-duce bulk glasses that were subsequently annealedat a temperature around glass transition temper-ature (Tg). Then, the bulk glasses were cut intothe bar type specimen (20 mm in length) for TECmeasurement or milled into the fine powders(38–45 lm) for the fabrication of the pressed bartype specimen or glass pellet.

2.2. Glass characterization

The as-prepared glass was tested by X-ray dif-fraction, using Ni-filtered CuKa radiation, for theamorphous nature of the glass. All the as-preparedglasses in this work have been found to be X-rayamorphous exhibiting a broad halo in the XRDpattern.The thermal properties of the glass such as

glass transition temperature (Tg), softening tem-

Table 1

Glass compositions in this work

Specimen Compositions (mol%)

BaO Al2O3 La2O3 B2O3 SiO2 B2O3=SiO2

S1 20 10 5 21.7 43.3 0.5

S2 25 10 5 20 40 0.5

S3 30 10 5 18.3 36.7 0.5

S4 35 10 5 16.7 33.3 0.5

S5 40 10 5 15 30 0.5

A 35 10 5 12.5 37.5 0.33

B 35 10 5 20.83 29.17 0.71

104 S.-B. Sohn et al. / Journal of Non-Crystalline Solids 297 (2002) 103–112

perature (Ts) and thermal expansion coefficient(TEC) were measured by a thermomechanicalanalyzer in air with a heating rate of 5 K/min,where TEC value was obtained from the tem-perature range of room temperature � below Tgfor each glass.

2.3. Wetting and chemical reactivity

In this work, YSZ electrolyte was chosen as asubstrate material for bonding test. YSZ plate wasprepared by uniaxial pressing of the powder of 8mol%-YSZ into the pellet and sequentially sinter-ing at 1400 �C in air to above 95% relative density.Then, the sintered YSZ plate was polished withdiamond paste (up to 0:04 lm) to obtain flat andsmooth surface.To examine the bonding characteristic and

wetting behavior of the glass to YSZ plate, theglass pellet was observed by a heating microscopewhere the glass pellet was put on YSZ plate andcontinuously heated up to 1100 �C at a heatingrate of 5 K/min.For the investigation of chemical reaction be-

tween glass and YSZ, the glass pellet placed onYSZ plate was heated up to 900–950 �C at 5 K/min and held for 20 min in order to bond the glassand YSZ plate together, and the resulting glass/YSZ specimen was cooled slowly down to 800–850�C at a rate of 3 K/min and maintained for 10–100h, then cooled down to room temperature. Afterheat-treatment, the cross-section of specimen waspolished and then the glass/YSZ interface wasexamined using electron probe micro analyzer andscanning electron microscopy to investigate theinterface reaction and microstructural change ofthe interface during the heat-treatment. All themicroscopic images were obtained as the typeof secondary electron image (SEI). It should benoted that boron (B) could not be detected withthe available EPMA system and boron is conse-quently not considered in detail in the followingresults.

2.4. Thermal cycling test

In the thermal cycling test, the glass/YSZspecimen bonded at 900 or 950 �C for 20 min

was cooled down to room temperature at a rateof 5 K/min after heat-treated at 800 or 850 �Cfor 1 h, and then heated from room temperatureto 800 or 850 �C at 5 K/min, maintained for 1 hand cooled down again to room temperature atthe same cooling rate as previous (5 K/min).

Fig. 1. Thermal properties of the glasses with various BaO

contents at constant B2O3=SiO2 ratio.

Fig. 2. Thermal properties of the glasses with various

B2O3=SiO2 ratios at constant BaO content.

S.-B. Sohn et al. / Journal of Non-Crystalline Solids 297 (2002) 103–112 105

3. Results and discussion

3.1. Thermal properties

The changes in thermal properties of glasses suchas Tg, Ts andTECare shown inFigs. 1 and 2 togetherwith TEC value of YSZ obtained for the tempera-ture range 25–700 �C fromTMAmeasurement. Fig.1 exhibits that the glasses S1–S5 prepared withvariousBaOcontents at constantB2O3:SiO2 ratio of1:2 have Tg and Ts values in the range 668–671 �Cand 735–742 �C, respectively. It indicates that Tgand Ts are almost independent of the change of BaOcontent under constant B2O3 : SiO2 ratio. It is alsoapparent from Fig. 2, which shows the changesin thermal properties of the glasses prepared with

various B2O3:SiO2 ratios at same composition forother components (S4, A andB), that the changes inTg and Ts are strongly dependent on B2O3 : SiO2ratio. These results are quite correspondent withthe previous report studied on the thermal proper-ties of borosilicate glasses [4]. TEC of the glass,however, increases with increasingBaO content andB2O3 : SiO2 ratio. Thus, it can be suggested thatB2O3:SiO2 ratio in the BaO–Al2O3–La2O3– B2O3–SiO2 system is an important factor in the prepa-ration of the sealing glass suitable for SOFC inthermal properties. Among the glasses prepared inthis work, the glass S4 shows a minimum thermalexpansion mismatch with YSZ ð3:8� 10�7=KÞ attemperatures below about Tg ð� 670 �CÞ and it hasa B2O3:SiO2 ratio of 1:2 ðB2O3=SiO2 ¼ 0:5Þ.

Fig. 3. Shape change of the pressed glass pellet of the S4 composition as a function of temperature (heating rate of 5 K/min).

106 S.-B. Sohn et al. / Journal of Non-Crystalline Solids 297 (2002) 103–112

3.2. Bonding characteristics and wetting behavior ofthe glass to YSZ

The bonding characteristics and wetting be-havior of the glass to YSZ were investigated byobserving the shape change of the glass pellet onYSZ plate with increasing temperature. Since the

shape change of the glass occurs due to the vis-cosity change, it is consequently influenced by thethermal properties of the glass such as Tg and Ts.As a result, the glasses S1 to S5 showed quitea similar tendency in shape change due to theirsimilar values of Tg and Ts. Fig. 3 depicts the de-formation of S4 glass pellet on YSZ plate with

Fig. 4. SEM cross-sectional views of the glass S4/YSZ interface heat-treated at 900 �C for 20 min followed at 800 �C for (a) 20,(b) 50, and (c) 100 h and at 950 �C for 20 min followed at 850 �C for (d) 20, (e) 50, and (f) 100 h.

S.-B. Sohn et al. / Journal of Non-Crystalline Solids 297 (2002) 103–112 107

increasing temperature. The glass S4, which hasTs of 742 �C, shows a shrinking behavior withincreasing temperature up to 775 �C due to theliquid phase sintering and then it begins to bondto YSZ with the completion of shrinkage (aboveabout 775 �C). It is also found from Fig. 3 thatthe glass S4 shows a non-wetting behavior up to

the temperature below 1130 �C due to its highsurface tension property. The glasses A and B,however, showed an unsuitable property as a seal-ing glass for SOFC having the operating temper-ature in the range from 800 to 850 �C. The glass Ashowed a relatively high bonding temperature dueto its Ts of 776 �C; thus its bonding strength was

Fig. 5. EPMA line analyses for the glass S4/YSZ interface heat-treated at 900 �C for 20 min followed at 800 �C for (a) 10, (b) 20,(c) 50 and (d) 100 h.

108 S.-B. Sohn et al. / Journal of Non-Crystalline Solids 297 (2002) 103–112

to be weak when bonded to YSZ at 900–950 �C.To the contrary, the glass B exhibited a goodbonding characteristic to YSZ when bonded at900–950 �C, but it began to reveal a wetting be-havior at the temperature above 1050 �C; thus it isapt to flow out from the fuel cell stack during longtime operation.

3.3. Interface reaction between glass and YSZ

Fig. 4 shows microstructures of the glass S4/YSZ interface heat-treated at 800 or 850 �C forvarious times after being bonded at 900 or 950 �Cfor 20 min, respectively. It is apparent not onlythat the glass S4 is completely bonded with YSZ

Fig. 6. EPMA line analyses for the glass S4/YSZ interface heat-treated at 950 �C for 20 min followed at 850 �C for (a) 10, (b) 20,(c) 50 and (d) 100 h.

S.-B. Sohn et al. / Journal of Non-Crystalline Solids 297 (2002) 103–112 109

Fig. 7. EPMA area mapping analyses for the glass S4/YSZ interface heat-treated (a) at 900 �C for 20 min followed at 800 �C for 100 h and (b) at 950 �C for

20 min followed at 850 �C for 100 h.

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without cracks but also that no reaction productlayer due to interface reaction between the glass S4and YSZ is observed during the heat-treatment fora duration up to 100 h. The absence of the inter-face reaction between glass and YSZ can be alsoconfirmed from Figs. 5 and 6, which exhibitEPMA results for a cross-section of the glass S4/YSZ interface for the specimen heat-treated at 800or 850 �C for various times. It is apparent fromthose figures that no diffusion of elements in glasstoward YSZ is found and in YSZ toward glassirrespective of the heat-treatment temperature andtime. It is also noted that the thickness of the in-terface increases continuously according to heat-treatment time up to 50 h and then becomes settledto be about 8 lm after 50 h, equally for the heat-treated specimens at 800 and 850 �C. Fig. 7 ex-hibits EPMA area mapping results for the glass S4/YSZ interface heat-treated at 800 or 850 �C for100 h. From the degree of brightness shown ineach image, the distribution of concentration ofthe element could be easily observed. As shown inthis figure it could be found that no diffusion ofLa, Si and Y occurs from glass into YSZ and viceversa. From the other mapping results, it was alsoconfirmed that other elements in glass and YSZdid not diffuse for a duration up to 100 h at 800–850 �C.The experimental results for the interface reac-

tion between YSZ and other glasses examined in

this work were almost similar with the result forthe glass S4 as mentioned above. Accordingly nointerface reaction due to the diffusion of elementsbetween the glasses and YSZ was observed duringheat-treatment for a duration up to 100 h at 800–850 �C.

3.4. Thermal cycling test

In planar SOFCs, the component such as theelectrolyte is apt to be broken from the area incontact with the sealing glass during thermal cy-cling. This is caused by the stress due to thedifference in thermal expansion coefficient be-tween electrolyte and glass, and thus the thermalexpansion mismatch must be minimized to im-prove the endurance against thermal cycling.As mentioned previously (Section 3.1), the glass

S4 among the glasses in this work showed a min-imum thermal expansion mismatch with YSZð3:8� 10�7=KÞ at temperatures below Tgð� 670�CÞ, so it could be expected that no cracking wouldhave occurred at the glass S4/YSZ interface afterthermal cycling. Fig. 8 exhibits the microstructuresof the glass S4/YSZ interface thermally cycledtwice between room temperature and 800 or 850�C. It is confirmed that no crack or delaminationis observed at the interface; therefore the glassS4 shows an excellent endurance against thermalcycling.

Fig. 8. SEM cross-sectional views of the glass S4/YSZ interface thermally cycled twice between room temperature and (a) 800 �Cor (b) 850 �C.

S.-B. Sohn et al. / Journal of Non-Crystalline Solids 297 (2002) 103–112 111

4. Conclusions

The thermal properties and chemical stability ofglasses in the BaO–Al2O3–La2O3–B2O3–SiO2 sys-tem as well as bonding characteristics to YSZelectrolyte have been investigated to develop asuitable sealing glass for planar SOFC having anoperating temperature of 800–850 �C. From theobservation over the changes in the thermalproperties such as Tg, Ts and TEC, it was revealedthat B2O3:SiO2 ratio in the BaO–Al2O3–La2O3–B2O3–SiO2 system is an important factor for pre-paring the sealing glass suitable for SOFC, so thatthe bonding characteristics of the glass to YSZ arealso influenced by this ratio. Among the glassesexamined in this work, the glass S4 ð35BaO–10Al2O3–5La2O3–16:7B2O3–33:3SiO2 in mol%)showed not only the most appropriate bondingcharacteristics and wetting behavior as a sealingglass but also a minimum thermal expansion mis-match with YSZ ð3:8� 10�7=KÞ, which conse-quently resulted in an excellent endurance againstthermal cycling.Interface reaction between the glass and YSZ

was not observed for all the glass/YSZ specimensheat-treated at 800–850 �C for a duration up to100 h.Based on these experimental results it can be

concluded that the glass S4 is the most suitablesealing glass for planar SOFC operating at 800–850 �C.

Acknowledgements

This work was supported by the Korean Min-istry of Science and Technology through Korea

Institute of Science and Technology (Project No.2E16460).

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