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Solid State Ionics 177 (2

Ferrite-based perovskites as cathode materials for anode-supported

solid oxide fuel cells

Part II. Influence of the CGO interlayer

Andreas Mai *, Vincent A.C. Haanappel, Frank Tietz, Detlev Stover

Institute for Materials and Processes in Energy Systems, Forschungszentrum Julich, 52425 Julich, Germany

Received 31 August 2005; accepted 8 December 2005

Abstract

It was shown in part I (Variation of the cathode composition) [A. Mai, V.A.C. Haanappel, S. Uhlenbruck, F. Tietz, D. Stover, Solid State Ionics

176 (2005), 1341], that an interlayer is needed between yttria-stabilised zirconia (YSZ) electrolytes and (La,Sr)(Co,Fe)O3�d (LSCF) cathodes in

order to prevent undesired chemical reactions between these materials. This interlayer makes it possible to benefit from the superior

electrochemical properties of the LSCF perovskites in solid oxide fuel cells (SOFCs). In this study the influence of a Ce0.8Gd0.2O2�d (CGO)

interlayer on the electrochemical performance of LSCF-type SOFCs is investigated in more detail. For screen-printed and sintered interlayers the

grain size as well as the sintering temperature affected the electrochemical performance. The use of a fine powder with a mean particle size of

d50=0.2 Am and sintered at 1250 -C resulted in the best performance. Furthermore, reactive sputtering resulted in dense CGO interlayers at low

deposition temperatures, which led to improved properties regarding diffusion inhibition and electrochemical performance. SOFCs with sputtered

interlayers gave power densities of up to 0.9 W/cm2 at 700 -C and 0.7 V, with H2+3% H2O as fuel gas (approx. 10% fuel utilisation).

D 2006 Elsevier B.V. All rights reserved.

Keywords: Solid oxide fuel cells; LSCF; Cerium oxide; Interlayer

1. Introduction

La1�x�zSrxCo1�yFeyO3�d (LSCF) perovskites are currently

used as cathode materials for solid oxide fuel cells (SOFCs) due

to their high electrocatalytic activity. It was shown in the first

part of this series of publications [1] that SOFCs with

La0.58Sr0.4Co0.2Fe0.8O3�d cathodes delivered about twice the

power density than SOFCs with lanthanum manganite based

(LSM/YSZ-composite) cathodes. Since the LSCF perovskites

are chemically incompatible with the Zr0.85Y0.15O1.925 (8YSZ)

electrolyte forming insulating phases at high temperatures the

use of an interlayer between cathode and electrolyte is

necessary [1]. A widely used material for this interlayer is

substituted ceria [2], for example Ce0.8Gd0.2O2�d (CGO). This

interlayer should be as dense as possible for maximum

efficiency as a diffusion barrier layer and for maximum

conductivity. To reduce the porosity of sintered interlayers, a

0167-2738/$ - see front matter D 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2005.12.010

* Corresponding author. Tel.: +49 2461 61 3071; fax: +49 2461 61 2455.

E-mail address: mai_andreas@freenet.de (A. Mai).

higher sintering temperature is feasible, but this will increase

the formation of a solid solution between YSZ and CGO at the

interface, having a lower ionic conductivity [3,4] than both

YSZ and CGO. As a result of these two contradicting influences

on the performance, most interlayers presented in literature are

porous [5–7]. This paper concentrates on the influence of the

processing parameters and the resulting properties of the

interlayer on the electrochemical performance of the SOFCs.

2. Experimental

The ceria compounds used in this study were commercially

available (Ce,Gd)O2�d powders manufactured by Treibacher,

Austria and Rhodia Electronics and Catalysis, France. The

powders were ground by ball milling. NiO/YSZ anode

substrates (50�50 mm2) were used for fabrication of the

SOFCs. The substrates were co-fired with an anode functional

layer (Ni/8YSZ) and an electrolyte (8YSZ) at 1400 -C for 5 h.

The above-mentioned CGO powders were screen-printed and

sintered for 3 h on top of the electrolytes, resulting in layer

thicknesses of approximately 7 Am. The varied processing

006) 2103 – 2107

ww

a)

b)

c)

d) 10 m

5 m

5 m

5 m

Fig. 1. CGO interlayers sintered at 1300 -C for 3 h, made from different

powders: a) CGO from Treibacher Auermet ground to a d50 of 0.9 Am, b)

ground to a d50 of 0.22 Am, c) CGO from Rhodia ground to a d50 of 0.4 Am, d)

CGO mixed with cobalt oxide as sintering aid (note the different magnifica-

tion), with partial delamination of the interlayer.

Fig. 2. Current–voltage curves of SOFCs at 750 -C with La0.58Sr0.4Co0.2Fe0.8O3�d cathodes (cathode type I) and different interlayers: layers made from coarse

(d50=0.9 Am) and fine (d50=0.2Am) CGO and CGOwith cobalt oxide as sintering

aid, all sintered at 1300 -C, as well as a layer made by reactive sputtering.

A. Mai et al. / Solid State Ionics 177 (2006) 2103–21072104

parameters were the grain size of the powders and the sintering

temperature. Additionally, cobalt oxide was used as a sintering

aid for the interlayer. CGO interlayers were also deposited by

reactive sputtering similar to the deposition of 8YSZ described

by Wanzenberg et al. [8]. For this, metallic sputtering targets

were used, while the sample temperature and the oxygen partial

pressure in the reactor chamber were optimised, resulting in

dense layers with thicknesses of 3 to 5 Am. On top of the

interlayers, La0.58Sr0.4Co0.2Fe0.8O3�d cathodes were deposited,

as described in part I [1]. The interlayers were analysed by SEM

scanning electron microscopy (LEO 1530 (Gemini), Zeiss

Ultra55) equipped with an energy dispersive X-ray (EDX)

analysis system. Electrochemical measurements of the cell

performance and their analysis were conducted as described in

Ref. [1]. For some of the SOFCs the nickel oxide was reduced to

nickel with a different anode reduction procedure [9] (named

hereafter ‘‘reduction procedure II’’), while for the others, the

same procedure as in the first part [1] was used (‘‘reduction

procedure type I’’).

3. Results and discussion

3.1. Variation of the powder characteristics (particle size,

supplier)

Fig. 1a–c show CGO interlayers on YSZ electrolytes

sintered at 1300 -C, made with three different powders: CGO

from Treibacher, ground to an average particle size (d50) of 0.9

Am (Fig. 1a), to a d50 of 0.2 Am (Fig. 1b) and CGO from Rhodia

(Fig. 1c) ground to a d50 of 0.4 Am. According to the

expectation, the layer made from the coarser powder (Fig. 1a)

shows a coarser microstructure. The two layers made from the

finer powders (Fig. 1b and c), but received from different

suppliers, have a similar microstructure. All three pictures show

powder particles that are very well sintered together, providing

well-established conductivity paths. Fig. 2 shows I–V-curves

measured at 750 -C of SOFCs made with the coarse (d50=0.9

Am) and the fine (0.2 Am) powders (both originally from

Treibacher). Table 1 lists the corresponding current densities

and area specific resistances in the first two rows. The interlayer

made with the finer powder results in a slightly higher

performance of the SOFC in comparison to the SOFC with

Table 1

Current densities (A/cm2, at 700 mV) and area-specific resistances (mVIcm2) of SOFCs with La0.58Sr0.4Co0.2Fe0.8O3�d cathodes and various CGO interlayers

Temperature

(-C)

Variation of the powder properties

(sintering at 1300 -C)

Layer applied by

sputtering deposition

Variation of the sintering temperature (d50=0.2 Am)

d50=0.9 Am d50=0.2 Am With cobalt-oxide 1200 -C 1250 -C 1275 -C 1300 -C 1325 -C

Anode reduction type I Anode reduction type II

Current density (A/cm2, 700 mV)

800 1.38*+ 1.76*T0.08 1.40*T0.04 1.81*T0.08 2.10*T0.13 2.30*T0.07 2.20*T0.15 2.11*T0.05 1.95*+

750 1.13+ 1.43*T0.08 1.15T0.01 1.68*T0.03 1.59*T0.11 1.80*T0.06 1.73*T0.13 1.65*T0.05 1.56*+

700 0.78+ 0.99T0.06 0.80T0.05 1.30*T0.01 0.99T0.08 1.20T0.05 1.10T0.07 1.13T0.04 0.99+

650 0.48+ 0.58T0.03 0.46T0.03 0.86T0.01 0.47T0.07 0.65T0.01 0.60T0.03 0.58T0.02 0.60+

Area-specific resistance (mXIcm2)

800 195+ 179 222 165 150 136 147 148 163+

750 239+ 219 271 173 189 167 173 186 200+

700 326+ 297 380 196 267 215 232 244 277+

650 531+ 522 707 271 531 427 473 480 526+

+Results from only one cell measurement;

*Current densities above 1.25 A/cm2 were linearly extrapolated to 0.7 V from the last three measurement points (see Fig. 3).

A. Mai et al. / Solid State Ionics 177 (2006) 2103–2107 2105

the coarser interlayer. This is probably the effect of a better

contact between the CGO particles due to the higher sintering

activity of the finer particles.

3.2. Cobalt oxide as sintering aid

To achieve a better densification of the CGO interlayer

during sintering, cobalt oxide as sintering aid for CGO, as

proposed by Kleinlogel et al. [10], was used. Therefore, fine

CGO powder (d50=0.2 Am) was impregnated with a cobalt-

nitrate (Co(NO3)2I6H2O, Sigma-Aldrich) solution and dried at

100 -C. The amount of cobalt-nitrate solution was chosen to

result in 2 wt% of cobalt oxide. The impregnated powder was

then processed to a paste and screen-printed on top of the

electrolyte. The nitrate decomposes during the sintering process,

which leads to a thin layer of cobalt oxide on the CGO grains.

As shown in Fig. 1d, the cobalt-containing interlayer has a

significantly denser structure than the corresponding interlayer

without cobalt (Fig. 1b). However, a large number of cracks have

evolved resulting in a partial delamination of the interlayer.

Lower sintering temperatures (1200 -C) resulted in a slightly

higher porosity, but cracks were still present. The cracking is

probably caused by stresses due to the increased shrinkage of the

interlayer on top of the non-shrinking, nearly dense electrolyte

(sintered at 1400 -C, before). This mismatch could explain, why

it is possible to get dense, crack-free pellets at sintering

temperatures as low as 1000 -C [10], while it is much more

complicated to get dense, crack-free CGO interlayers on pre-

sintered 8YSZ. Fig. 2 and Table 1 show the electrochemical data

frommeasurements of single cells with a cobalt-containing CGO

interlayer sintered at 1300 -C. The performance is lower than for

the corresponding cells without sintering aid, which is certainly

due to the presence of cracks in the interlayer.

3.3. Interlayers applied by reactive sputtering

As mentioned above, it would be desirable to deposit a

dense CGO diffusion barrier layer on the 8YSZ electrolyte and

to minimize the formation of a solid solution between the two

materials at the interface. The maximum temperature without

interaction at the interface is 1200 -C [11]. A possibility to

achieve dense layers with thicknesses of up to 5 Am at a sample

temperature of 700 -C within a few hours is the deposition of

CGO by reactive sputtering, as described for 8YSZ in Ref. [8].

Fig. 3a shows such a CGO layer with a columnar structure.

After sintering the cathode on top of this layer, small pores

have evolved in the layer due to sintering processes (Fig. 3b) as

it was also found in YSZ layers [8]. Despite these pores, no

strontium enrichment could be detected at the interface (Fig.

3c). This is in contrast to investigations of SOFCs with sintered

CGO layers, where the formation of SrZrO3 at the interface

CGO/8YSZ was clearly visible (see Fig. 4b of part I [1] and

Ref. [12]) after sintering of the cathode, being the result of Sr

diffusion through the interlayer. The results of the electro-

chemical characterisation (Fig. 2) clearly show an improvement

of the electrochemical performance for cells with a sputtered

interlayer in comparison to cells with sintered interlayers. To

estimate the effect of the decreased porosity on the area specific

resistance (ASR) of the cell, the area specific resistance of a

porous layer and of a dense layer was estimated. For a porous

layer as shown in Fig. 1b, this results in 41 mV cm2 at 700 -C(with a porosity of 33%, measured by image analysis, an

estimated tortuosity of 2, and a thickness of 6.5 Am), while a

dense layer as shown in Fig. 3a would have an ASR of 8.5 mV

cm2 at 700 -C (thickness 4 Am, bulk conductivity of

Ce0.8Gd0.2O2�d: 0.047 S/cm at 700 -C [13]). The difference

between these two calculated ASRs (32 mV cm2) is much

smaller than the difference (99 mV cm2) between the measured

ASRs of the corresponding cells (297 and 196 mV cm2,

respectively). The reason for the improved performance of the

cell with the sputtered CGO is therefore probably a decreased

resistance of the interface CGO-YSZ which is caused on the

one hand by less SrZrO3 formation due to an improved Sr-

blocking of the dense CGO layer and on the other hand by a

minimization of the formation of a solid solution between the

two materials at the interface.

A. Mai et al. / Solid State Ionics 177 (2006) 2103–21072106

3.4. Optimization of the sintering temperature

Together with the aforementioned experiments with sintered

interlayers, three different sintering temperatures were tested:

1200, 1300 and 1350 -C (all for 3 h, with 1300 -C resulting in

the best performance). Due to the large influence found, the

sintering temperature of the interlayer was varied again with a

smaller step size. For these SOFCs, an optimised anode

reduction procedure [9] was followed (reduction procedure

a)

b)

c)

Sr Ce Zr

1 m

1 m

Fig. 3. CGO interlayers deposited by reactive sputtering deposition, a) layer

with columnar structure directly after deposition, b) similar layer after sintering

of the cathode (1080 -C, 3 h) on top of the interlayer with c) EDX line scans for

strontium, cerium and zirconium taken along the black line.

Fig. 4. a) Current densities (A/cm2) at 700 mV of SOFCs CGO (d50=0.2 Aminterlayers sintered at various temperatures (Current densities larger than

1.25 A/cm2 are calculated from a linear approximation of the linear part in the

I–V-curves to 0.7 V, as illustrated in the I–V-curves b) Current–voltage curves

of one of the SOFCs with a CGO interlayer sintered at 1250 -C.

)

type II). Therefore, a direct comparison with the electrochem-

ical results shown above is difficult. Fig. 4a shows the current

densities measured at 0.7 V with cells containing CGO

interlayers (d50=0.3 Am, Treibacher) that were sintered at

temperatures from 1200 to 1325 -C for 3 h. The highest power

densities were obtained for the cells with a sintering

temperature of the interlayer of 1250 -C. Fig. 4b shows

exemplarily the corresponding I–V-curves for one of these

cells. The decrease of the power density both with sintering

temperatures above 1250 -C is probably the result of the

increased formation of the solid solution between YSZ and

CGO, while the decrease of the power density at lower

sintering temperatures is due to the higher porosity and lower

adherence of the interlayer.

4. Conclusions

The investigations have shown that the structural properties

of the ceria interlayer strongly influence the electrochemical

performance of SOFCs with LSCF cathodes. Several

approaches were made to maximise the density of the interlayer

and at the same time minimize the formation of a solid solution

A. Mai et al. / Solid State Ionics 177 (2006) 2103–2107 2107

between CGO and 8YSZ. The experiments have shown that a

fine powder (d50=0.2 Am) leads to a better performance and

that for these layers 1250 -C is the optimum sintering

temperature. It was confirmed that cobalt oxide acts as a

sintering aid for CGO, but cracks evolved, probably due to the

high shrinkage of the layer relative to the underlying dense

electrolyte. It was further shown that reactive sputtering is a

method to deposit dense CGO layers on the electrolyte,

resulting in an improved performance of the SOFCs. An

estimation on the influence of the layers’ porosity on the ASR

leads to the conclusion, that the improved performance is not

only a result of an increase in conductivity of the layer due to a

lower porosity, but mainly a result of the improved buffering

properties of a dense CGO layer, inhibiting SrZrO3 formation,

and avoiding the formation of a solid solution between CGO

and 8YSZ due to the low deposition temperature. The tested

SOFCs reached power densities of up to 0.9 W/cm2 at 700 -C.

Acknowledgements

The authors thank the staff, namely G. Blah, M. Kampel, S.

Heinz, and G. Klein of the IWV department at Forschungszen-

trum Julich for processing the anode substrates and electrolyte

layers, for the SEM investigations, D. Sebold and for

performing the electrochemical measurements, C. Tropartz,

B. Rowekamp, and H. Wesemeyer. Thanks are also given to P.

Panjan (Institute Jo”ef Stefan, Ljubijana) for reactive sputter-

ing. The work was carried out in the networking project

‘‘Renewable Energies’’ under contract no. 01SF0039 and

financial support from the German Federal Ministry of Science

and Education (BMBF) is gratefully acknowledged.

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