Electrochemistry Communications 12 (2010) 1326–1328

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Electrochemistry Communications

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Anode-supported planar SOFC with high performance and redox stability

Qianli Ma a,⁎, Frank Tietz a, André Leonide b, Ellen Ivers-Tiffée b

a Institute of Energy Research (IEF-1), Forschungszentrum Jülich GmbH, 52425 Jülich, Germanyb Institut für Werkstoffe der Elektrotechnik, Karlsruhe Institute of Technology, Adenauerring 20, 76131 Karlsruhe, Germany

⁎ Corresponding author. Tel.: +49 2461 614596; fax:E-mail address: q.ma@fz-juelich.de (Q. Ma).

1388-2481/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.elecom.2010.07.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 May 2010Received in revised form 5 July 2010Accepted 7 July 2010Available online 15 July 2010

Keywords:Donor-doped strontium titanateSolid oxide fuel cellsAnode materialRedox

Solid oxide fuel cells with full ceramic anodes have recently attracted increasing attention, because theconventional Ni/YSZ cermet anodes may fail during practical operation due to their weak mechanicalstability in the case of re-oxidation of the nickel. However, until now the reported fuel cells based on ceramicanodes have been fabricated only as small pellet-sized cells and electrochemical performance has beenbarely satisfactory, making it difficult to evaluate these attempts with respect to commercial feasibility.Herein, we report single cells based on Y-substituted SrTiO3 anode substrates. These planar cells have outerdimensions of 50×50 mm2, which has not been reached for a ceramic anode-supported cell before. Theyshow power densities of 0.7–1.0 W cm−2 at 0.7 V and 800 °C, which are sufficient for technical applications.The cells survived 200 anode-gas changes between fuel and air (redox cycles), providing a new direction forthe development and commercialisation of anode-supported solid oxide fuel cells.

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1. Introduction

The solid oxide fuel cell (SOFC) is a promising energy conversiondevice that directly produces electrical power from a wide variety offuels. With respect to commercialisation of SOFCs, the anode-supported cell (ASC) design has great prospects for success becauseof its remarkable electrical performance and reduced operatingtemperature [1]. The state-of-the-art anode material for SOFCs is aNi/YSZ cermet, which is both an excellent catalyst for fuel oxidationand an effective current collector. However, its poor mechanicalstability during reduction and oxidation cycles (redox stability) of theNi/YSZ cermet has significantly obstructed the development of ASCs[2–4]. During the practical application of SOFCs, situations such as sealleakage, fuel supply failure or system shutdown are almost inevitableand may re-oxidise the Ni component of the cermet. Even if SOFCs areoperated in safe conditions, re-oxidation of Nimay also happen duringhigh fuel utilisation, which corresponds to high water vapour content[2]. The re-oxidation of Ni is accompanied by large volume change,which is detrimental for the mechanical stability of the entire cellstructure, especially for ASCs. Hence, redox-stable anodes based onceramic materials have been under consideration for several years,e.g. Ti-substituted yttria-stabilised ZrO2 [5], Sr- and Mn-substitutedLaCrO3 [6], La or Y-substituted SrTiO3 [7,8].

However, the development of applicable SOFCs based on theseredox-stable anodes is still difficult. Although a few cells have alreadybeen reported [9–12], it is very hard to evaluate the novel anodes in

practical terms because most of the obtained performances are barelysatisfactory, and more importantly, the results are all based on smallpellet-sized cells or electrolyte supported cells. Hence mechanicalstresses or fracture formation of the anodes during re-oxidationcannot be observed because a pellet-sized cell surviving in certainredox cycle conditions does not imply that large planar cells with thesame materials and design will also survive this treatment [3,4], andthe redox stability of electrolyte supported cells comes from the lowchemical expansion of electrolyte materials. With respect to com-mercial purposes, at least 100 redox cycles should be tested todetermine the robustness of the cells. But until now no such results forASCs are reported, even for pellet cells.

In this study, single cells based on a Sr0.895Y0.07TiO3 (SYT) anodesupport and a (Sr0.89Y0.07)0.91TiO2.91/YSZ (2:1 in volume, SYT2–YSZ)anode layer were fabricated. The reason for using A-site-deficientSYT2 as anode material is the higher conductivity after mixing withYSZ [13]. The cells are of a planar design with dimensions of50×50 mm2, which is close to commercial cell dimensions for stackfabrication [4]. The electrochemical performances of the cells,especially the performances under redox conditions are investigatedin this study.

2. Experimental

2.1. Synthesis

SYT and La0.6Sr0.4CoO3− δ (LSC) powders were synthesised byspray pyrolysis as reported elsewhere [13]. SYT2 powder wasprepared by Pechini's method [14]. The powder was then mixedwith commercial 8YSZ powder (Zr0.852Y0.148O1.926, Tosoh, Japan) in a

1327Q. Ma et al. / Electrochemistry Communications 12 (2010) 1326–1328

volume ratio of 2:1 (1.7:1 in weight ratio) by ball milling for 72 h(SYT2–YSZ). The screen-printing pastes of LSC and SYT2–YSZ wereprepared by mixing the corresponding powders with a binderconsisting of ethyl cellulose in terpineol.

2.2. Cell fabrication

The fabrication of half cells, consisted of SYT (3 wt.% NiO) support,SYT2–YSZ (3 wt.% NiO) anode, YSZ electrolyte, was reported else-where [13]. A Ce0.8Gd0.2O1.9 (CGO) layer was applied as an interlayerbetween electrolyte and LSC cathode by magnetron sputtering with acommercial physical vapour deposition (PVD) cluster system (CS 400ES, von Ardenne Anlagentechnik, Germany). LSC cathodes werefabricated by screen printing. The effective cathode area was10×10 mm2. The cathodes were sintered directly at 900 °C duringperformance testing.

2.3. Characterisation

The microstructure of the cells after performance testing wasinvestigated by scanning electron microscopy (Zeiss Ultra55) and thecomposition in the YSZ layer was analysed by energy-dispersive X-rayspectroscopy (EDX).

The performance of the cells was tested with a single cell testbench [15]. The current–voltage curves of the cells were measured attemperatures ranging from 850 to 600 °C using dry H2 as fuel and airas oxidant. However, a steam content of 3 to 4% in the fuel gas wasproduced by undesired fuel utilisation from gas leakages. The redoxbehaviour of the cells was tested after performance testing. The anodegas was first changed from H2 to N2 and maintained for 1 min at a N2

flow rate of 0.3 l/min, then air (0.15 l/min) was fed to the anode for10 min. After 1 min of purging with 0.3 l/min N2 once again, 0.25 l/min H2 was delivered to the anode chamber for either 10 or 120 minto reduce the anode again.

3. Results and discussion

Fig. 1 presents the cross-sectional view of the cell after perfor-mance and redox testing. The cell architecture consists of a 1.2 mmthick SYT anode substrate and a 3 μm thin SYT2/YSZ anode bothinfiltrated with about 3 wt.% NiO, a YSZ electrolyte (~14 μm), a CGOdiffusion barrier layer (~1 μm) and a 50 μm thick La0.4Sr0.6CoO3 (LSC)cathode. A diffusion barrier layer of CGO was deposited on theelectrolyte by PVD. This 1 μm thick layer can effectively block the Sr2+

Fig. 1. Cross section of SYT-ASC composed of SYT/SYT2–YSZ/YSZ/PVD-CGO/LSC afterperformance testing.

diffusion from cathode to electrolyte, and remarkably improve theperformance of the cell [16]. A gradient of the Ti content from 4.5 to12.5 at.% in the electrolyte layer is observed by EDX, decreasing fromthe anode side to cathode side. An ionic transport number of 0.78 wasreported for 10 mol% TiO2-substituted YSZ at 1000 °C [17], indicatingpossible electronic conduction of the co-sintered electrolyte layer inthe present study. A very small amount of Ni was infiltrated in the SYTanode substrate and SYT2/YSZ anode. The Ni particles cannot act ascurrent collector because they do not have any connections to eachother, but they still function as fuel catalyst. Hence, depending ontheir amount and position, the dimensional change of the particlesduring the redox process will not affect the mechanical stability of thecell.

The electrical performance of the SYT-ASCs was characterisedelectrochemically by means of current–voltage (I/U) and impedancemeasurements. In Fig. 2, typical I/U characteristics recorded atdifferent temperatures are shown. At 800 °C, a typical operationtemperature for SOFC systems, an OCV of 1.09 V is obtained, very closeto the theoretical value and indicating that the Ti content in theelectrolyte layer, especially for the region close to cathode with only4–5 at.% of Ti, does not cause obvious electronic conduction. At thesame temperature a current density of 1.22 Acm−2 at 0.7 V isachieved, which corresponds to a power density of 0.85 W cm−2.The actual data for all the tested cells so far varied from1.0 to 1.5 Acm−2

at 0.7 V and 800 °C. This variation presumably comes from the differentquality of contact between cathode and current collecting layer duringtesting, because the produced cells still have some slight bendingproblems. Nevertheless, these performance values are remarkably highand are the best yet reported for ASCs with a ceramic anode. Therefore,the SYT-ASC has already proven a considerable potential for practicaluse and is a serious prospect for commercial application.

Redox tests were performed at 750 °C or 800 °C with the anode-gas input procedure described in Section 2.3. Fig. 3 shows the changesof OCV and performance at 0.7 V of the cell as a function of the numberof redox cycles. Impressively, after 200 redox cycles, the OCV onlydecreased by 1.3%, indicating the high robustness and stability of thecell. Fig. 1 also gives evidence for this stability, because afterperformance and redox testing no cracks were found in the anodesupport, anode and electrolyte layer or at the interfaces betweenthem. This is the first time that an ASC sustained 200 redox cycles andthe size of 50×50 mm2 should be emphasized here. In contrast, SOFCsbased on Ni cermets usually collapsed or had apparent losses of OCVafter just one redox cycle [2–4]. However, the performance of the celldecreased by 35% after 200 redox cycles. The reason is most likely the

Fig. 2. Current–voltage curves of the SYT-ASC for different temperatures ranging from600 to 850 °C.

Fig. 3. OCV (open circles) and current density at 0.7 V (closed circles) as a function ofthe number of redox cycles (10 min in H2 and 10 min in air) at 750 °C as well as currentdensity at 0.7 V (closed squares) as a function of the number of redox cycles at 800 °Capplying 2 h in H2 and 10 min in air.

1328 Q. Ma et al. / Electrochemistry Communications 12 (2010) 1326–1328

gradual loss of electrical conductivity after each redox cycle due to theslow kinetics between the reduced and oxidised state of SYTmaterials. It was reported that the conductivity of Y-substitutedSrTiO3 sintered in reducing atmosphere significantly depends on theoxygen partial pressure during testing [13,18]. The values are nearly100 S/cm after sintering in Ar/H2 and only about 0.01 S/cm in air, bothmeasured at 800 °C. If the atmosphere is changed from air back to Ar/H2, the conductivity can be restored, but this takes minutes to hoursdepending on the density of the anode substrate. In this study, it isvery likely that the 10 min interval for reduction of each redox cycle isnot long enough to restore the conductivity decrease of the anodeduring the oxidising period (also 10 min), which results in acontinuous performance decrease with the redox cycling. This canbe proved by redox tests with longer periods of reduction. Fig. 3 alsoshows the change of performance of a different cell at 0.7 V and 800 °Cas a function of the numbers of redox cycles. For this test the reducingtime during the cycles was prolonged to 2 h, and the oxidising periodremained 10 min. As anticipated, there is no apparent decrease inperformance after 50 redox cycles. During practical application ofSOFCs the event of re-oxidation is regarded as a kind of ‘accident’during operation and there is enough time to recover the anode statusafter such a failuremode. In this sense, the redox stability of the cells is

very adequate, indicating a good prospect as possible direction forfurther commercialisation.

4. Conclusions

Planar SOFCs with dimension of 50×50 mm2 based on SYTsupports and SYT2–YSZ anodes were prepared in the present study.Although Ti was found to diffuse from anode to YSZ electrolyte, itapparently did not deteriorate the OCV and performance of the cells.The performance of the cells was 1.0 to 1.5 Acm−2 at 0.7 V and 800 °C,which is enough for practical use. The cells can also sustain 200 redoxcycles between H2 and air without significant decrease in OCV.Although because of the slow redox kinetics of SYT, the performanceof the cell continuously decreased during the 200 redox cycles, it canbe solved by prolonging the reducing time during the redox cycles.

Acknowledgement

This work was financially supported by the European Commissionunder contract no. SES6-CT-2006-020089 as part of the IntegratedProject “SOFC600”.

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