Materials Letters 65 (2011) 2066–2069

Contents lists available at ScienceDirect

Materials Letters

j ourna l homepage: www.e lsev ie r.com/ locate /mat le t

Preparation of porous Ni–YSZ cermet anodes for solid oxide fuel cells by highfrequency induction heated sintering

Jong-Yeol Yoo a, Chun-Kang Cho a, In-Jin Shon a,b, Ki-Tae Lee a,b,⁎a Department of Hydrogen and Fuel Cells Engineering, Specialized Graduate School, Chonbuk National University, Jeonbuk, 560-756, Republic of Koreab Division of Advanced Materials Engineering, Chonbuk National University, Jeonbuk, 560-756, Republic of Korea

⁎ Corresponding author at: Division of Advanced MNational University, Jeonbuk, 560-756, Republic of Korea. T63 270 2386.

E-mail address: ktlee71@jbnu.ac.kr (K.-T. Lee).

0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.04.032

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 February 2011Accepted 7 April 2011Available online 14 April 2011

Keywords:Anode supportNi–YSZHigh frequency induction heated sinteringRapid sintering

Ni–YSZ cermet anodes for solid oxide fuel cells were successfully prepared by high frequency inductionheated sintering, producing a uniformly porous microstructure without abnormal grain growth found in theconventional sintering method. All sintering processes commenced below 1150 °C and finished within 2 min.The rupture strength and electrical conductivity of the sample sintered by high frequency inductionheated sintering without addition of a pore former were over 180 MPa and about 2000 S cm−1 at 800 °C,respectively.

aterials Engineering, Chonbukel.:+82632702290; fax:+82

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Solid oxide fuel cells (SOFCs) are attractive because of their manyadvantages when compared with other energy conversion systems[1], such as high energy conversion efficiency, low pollutionemissions, and various environmental advantages. The electrolyte inSOFCs should be as thin as possible to reduce ohmic loss, therebyresulting in improved performance. Therefore, anode-supportedSOFCs represent a promising approach for improving performanceand reducing operating temperature, because they reduce thethickness of the electrolyte below 20 μm [2,3]. Anode support requiresappropriate mechanical strength to support a whole single cell,porous microstructure to supply fuel gases and emit reactionproducts, high thermal shock resistance, and high electrical conduc-tivity. [4–6].

The state-of-the-art anode material for SOFCs is a composite of Niand yttria-stabilized zirconia (YSZ) called Ni–YSZ cermet. Ni acts as acatalyst and YSZ provides the porous structure and suppresses graingrowth of the Ni catalysts at the Ni–YSZ cermet anode. In addition, YSZalso acts as an ionic conductor, which transfers oxygen ions to thereaction site to finish the catalytic cycle. The triple-phase boundary(TPB) where electrolyte, gas, and electrode meet is an important partof the anode from a catalytic perspective, because the hydrogenoxidation reaction can only occur at TPB sites [7]. Therefore, extensionof the TPB provides excellent opportunities for performance enhance-

ment. Microstructure and connectivity must be controlled to extendthe TPB at the cermet anode.

High frequency induction heated sintering (HFIHS) is used toobtain fully dense sintered bodies and to control microstructureduring the sintering process [8–10]. While rapid grain growth occursduring conventional sintering processes, such as solid state reactions,the HFIHS method suppresses abnormal grain growth due to itsrelatively short sintering time (b5 min) and low sintering tempera-ture. Recently, we showed that rapid sintering methods, includingHFIHS, can be applied to synthesize dense electrolyte materials suchas YSZ, Ce0.8Sm0.2O1.9 (SDC), or Ce0.8Gd0.2O1.9 (GDC) in relative lowtemperature. [11–13]. In this study, we used the HFIHS method tocontrol the microstructure of porous anode supports withoutabnormal grain growth of NiO during sintering, thereby increasingthe length of the TPB and resulting in performance improvement. Theeffect of the pore former on the sintering process, microstructure, andmechanical and electrical properties has also been investigated.

2. Experimental

NiO–YSZ powders (Ni:YSZ=60:40 vol.%) were synthesized by theglycine nitrate process (GNP), calcined at 600 °C for 2 h, and then ballmilled for 24 h [14,15]. In order to control porosity, 10 vol.% of carbonblack (HI-BLACK 900 L) was added.

NiO–YSZ pellets were sintered by HFIHS for up to 2 min in agraphite die under a uniaxial pressure of 60 MPa. An induced current(frequency of about 50 kHz) was then activated and maintained untildensification was attained as indicated by a linear gauge measuringthe shrinkage of the sample. The process was carried out under a

W/ carbon black

0 20 40 60 80 100

500

600

700

800

900

1000

1100

1200

1300W/o carbon black

Time (s)

Tem

per

atu

re (

oC

)

4

3

2

1

0

W/o carbon blackW/ carbon black

Sh

rin

kag

e d

isp

lace

men

t (m

m)

Fig. 1. Temperature and shrinkage displacement of NiO–YSZ support with and withoutpore former with heating time during HFIHS.

2067J.-Y. Yoo et al. / Materials Letters 65 (2011) 2066–2069

1 Torr vacuum. For comparison, another NiO–YSZ pellet was sinteredby the conventional solid state reaction method at 1400 °C for 4 h.

Phase analysis and crystal size determination were carried out byX-ray diffraction (XRD). Microstructures of the reduced and the Ni-removed samples were observed by scanning electron microscopy(SEM). Ni in the reduced samples was removed in HCl solution.Apparent porosity of the reduced samples was determined by theArchimedes method, and pore size distribution was measured with amercury porosimeter. Rupture strength tests were conducted on20×5×2 mm bar specimens using a 3-point support in a universal

(a)10µm

(d)10µm

10µm

10µm

Fig. 2. SEMmicrographs of the samples sintered by HFIHS without (a, d) and with (b, e) carbbody, and (d), (e), (f); Ni-removed body.

test machine at room temperature [16]. Electrical conductivity wasmeasured in a hydrogen atmosphere following a 4-probe dc methodin the temperature range of 400–800 °C.

3. Results and discussion

The HFIHS process consists of several steps. The first step is theintroduction of uniaxial pressure and evacuation, the second is theinduction of current, and the final step is densification. Current wasinduced in NiO–YSZ until therewas no change in the rate of shrinkage.Fig. 1 shows the sintering behaviors of all samples. The initialshrinkage displacement within 10 s that we observed might be due tothe abrupt temperature increase with current induction and appliedpressure of 60 MPa.With further current induction, the shrinkage rateincreases gradually with temperature, due to consolidation. Thesintering temperature was decreased by about 100 °C by adding apore former. The centerpiece of the HFIHS process is the generation ofeddy currents enabled by electrical conduction. Therefore, noncon-ductive powders obtain their heat sources from the graphite die,which is a conductor. Because carbon-based pore formers have goodelectrical conductivity, carbon black supplies an additional heatsource within the powder, resulting in uniform heating through thewhole sample and corresponding to a reduction of sinteringtemperature. More importantly, all sintering processes finish within2 min and below 1150 °C, which is a dramatic reduction compared tothe conventional sinteringmethod, which involves holding for severalhours at 1400 °C.

Fig. 2 shows images of the fractured surfaces of specimens sinteredby HFIHS and conventional sintering. In the samples sintered byHFIHS it is clear that fine pores are uniformly distributed, and YSZ hasa network structure with better connectivity. On the contrary, thereare large grains in the conventionally sintered samples due to the highsintering temperature of 1400 °C and the long (4 h) soaking time.Meanwhile, the samples with carbon black show relatively smallergrain sizes of both Ni and YSZ. Thismight be due to the lower sinteringtemperature of around 1050 °C for the samples with carbon black, asshown in Fig. 1.

(b) (c)

(e) (f)

10µm

10µm

on black, and by (c, f) conventional method without carbon black: (a), (b), (c); reduced

Table 1Porosity and average pore size of the sintered samples.

Sample Porosity (%) Average pore size (nm)

By Archimedesmethod

By porosimeter Reduced Ni-removed

W/o carbon black 24 33 93 100W/ carbon black 33 42 90 98W/o carbon black(1400 °C, 4 h)

24 30 178 489

400 500 600 700 8001000

2000

3000

4000

5000

6000 W/o carbon black (HFIHS) W/o carbon black (C.S.) W/ carbon black (HFIHS)

Temperature (oC)

Fig. 4. Electrical conductivity of the Ni–YSZ anode cermet measured in a hydrogenatmosphere.

2068 J.-Y. Yoo et al. / Materials Letters 65 (2011) 2066–2069

Porosity and average pore sizes of all samples are listed in Table 1.Porosity increases with the amount of pore former that is added.Because pores in the Ni–YSZ cermet anodes without pore formers aremainly derived from incomplete sintering processes and the reduc-tion of NiO, samples with the same amount of NiO show similarporosity regardless of the sintering processes used to fabricate them.However, despite having similar porosity, samples fabricated byHFIHS have much smaller pore sizes than those fabricated byconventional sintering. The large pore sizes in the samples sinteredby the conventional method are indicative of grain growth during theconventional sintering, as shown in Fig. 2.

The 3-point rupture strengths of the reduced samples arepresented in Fig. 3. The strength of the sample sintered by HFIHSdecreases after the addition of carbon black, due to an increase inporosity. Interestingly, the sample sintered by HFIHS shows higherstrength than the conventionally sintered sample (C.S.) even thoughthey have similar porosity. According to the Inglis theory [17], when apore acts as a failure origin, a spherical pore shape can relax the localstress at the tip. Therefore, the sample sintered by HFIHS withuniformly distributed fine pores may show higher strength than theconventionally sintered sample with irregularly large pores.

The electrical conductivities of Ni–YSZ measured in a hydrogenatmosphere are shown in Fig. 4. It is clear that conductivity decreaseswith increasing porosity. The electrical conductivity of the cermet alsodepends on its microstructure. Ni particle-to-particle contact is criticalto obtain higher conductivity. It also has been reported that electricalresistivity drops with a decrease in Ni particle size in a Ni–YSZ cermet[18–20]. This is a result of the larger probability of connection withsmaller Ni particles at the same volume. At the same Ni content, thesample sintered byHFIHS shows smaller grain size andmore uniformNi

100

120

140

160

180

200

220

W/o carbonblack (C.S.)

W/o carbonblack (HFIHS)

W/ carbonblack (HFIHS)

Ru

ptu

re S

tren

gth

(M

Pa)

Samples

Fig. 3. Rupture strengths of the reduced samples.

particle-to-particle contact than the conventionally sintered sample, asshown in Fig. 2, thus resulting in higher electrical conductivity.

4. Conclusions

Porous anodes for SOFCs were successfully fabricated by HFIHS.Compared with conventional sintering, the sintering time andtemperature can be dramatically reduced by HFIHS and themicrostructure can be maintained at the initial conditions of theraw powders without significant grain growth. When used as a poreformer, carbon black can reduce the sintering temperature further aswell as provide additional porosity. At similar porosities, the samplessintered by HFIHS show higher strength and electrical conductivitythan the conventionally sintered samples as a result of uniform poreand grain distributions, and better Ni particle-to-particle contact,respectively. Consequently, HFIHS enables control of the porousmicrostructure in Ni–YSZ cermet anode supports without graingrowth. Thus, tailor-made processes for increasing the TPB arepossible and can result in improved performance.

Acknowledgments

This work was supported by the New & Renewable EnergyProgram of the Korea Institute of Energy Technology Evaluation andPlanning (KETEP) grant funded by the Korean Ministry of KnowledgeEconomy (grant number 20093021030010-12-1-000).

References

[1] Minh NQ. J Am Ceram Soc 1993;76:563–88.[2] Giuseppe GD, Selman JR. J Mater Res 2001;16:2983–91.[3] Yoon SP, Han JH, Nam SW, Lim TH, Oh IH, Hong SA, Yoo YS, LimHC. J Power Sources

2002;106:160–6.[4] Aruna ST, Muthuraman M, Patil KC. Solid State Ionics 1998;111:45–51.[5] Anselmi-Tamburini U, Chiodelli G, Arimondi M, Maglia F, Spinolo G, Munir ZA.

Solid State Ionics 1998;110:35–43.[6] Skarmoutsos D, Tsoga A, Naoumidis A, Nikolopoulos P. Solid State Ionics 2000;135:

439–44.[7] O'Hayre R, Barnett DM, Prinz FB. J Electrochem Soc 2005;152:A439–44.[8] Oh DY, Kim HC, Yoon JK, Shon IJ. J Alloys Compd 2005;395:174–80.[9] Kim HC, Oh DY, Guojian J, Shon IJ. Mater Sci Eng A 2004;368:10–7.[10] Park HK, Park JH, Yoon JK, Lee KT, Shon IJ. J Electroceram 2009;23:542–7.[11] Shon IJ, Jeong IK, Park JH, Kim BR, Lee KT. Ceram Int 2009;35:363–8.[12] Shon IJ, Park HK, Nam KS, Lee KT. J Ceram Process Res 2008;9:325–9.[13] Shon IJ, Kim DK, Lee KT, Nam KS. Met Mater Int 2008;14:593–8.

2069J.-Y. Yoo et al. / Materials Letters 65 (2011) 2066–2069

[14] Chick LA, Pederson LR, Maupin GD, Bates JL, Thomas LE, Exarhos GJ. Mater Lett1990;10:6–12.

[15] Chen M, Kim BH, Xu Q, Ahn BK, Kang WJ, Huang DP. Ceram Int 2009;35:1335–43.[16] Lee KS, Lee SW, Yu JH, Seo DW, Woo SK. J Solid State Electrochem 2007;11:

1295–301.

[17] Inglis CE. Trans Inst Naval Architects 1913;55:219–41.[18] Tietz F, Dias FJ, Simwonis D, Stöver D. J Eur Ceram Soc 2000;20:1023–34.[19] Kim SJ, Kim KM, Cho PS, Cho YH, Lee CY, Park SY, Kang YC, Lee JH. Kor J Mater Res

2008;18:660–3.[20] Pratihar SK, Dassharma A, Maiti HS. Mater Res Bull 2005;40:1936–44.