Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/

Structural and electrical characterization of Y-doped SrZrO3
thin films for solid oxide fuel cells
Kiyoshi Uchiyama1, Yuri Isse1, Yuichi Hori1, Takashi Nishida1,
Yukiharu Uraoka1,2, Tetsuro Kariya3, and Katsu Yanagimoto3
1

Graduate School of Materials Science, Nara Institute of Science and Technology,
Takayama-cho 8916-5, Ikoma, Nara, 630-0192, Japan
2CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan3R&D Center, Sanyo Special Steel Co., Ltd., 3007, Nakashima, Shikama-ku,Himeji, Hyogo 672-8677 Japan

Keywords: Solid Oxide Fuel Cell (SOFC), Thin film, Electrolyte, Proton conductive oxide

Abstract. To achieve intermediate-temperature (400-600oC) operation of the solid oxide fuelcells (SOFCs), we proposed to use a porous stainless-steel plate with a Pd layer as a substrate.
Sol-gel deposition of proton conductive oxide thin film, i.e. Y-doped SrZrO3 (SZYO), wassuccessfully deposited on this substrate. The annealing temperature of 700oC was the best toobtain crystallized SZYO with lower leakage at a room temperature. The proton conductivity ofthis film was about 10-6 S/cm at 600oC in a dry condition and showed almost the same value inthe wet atmosphere. To achieve good SOFC operations, we are investigating the fabricating
method of the SOFC cells with a cathode electrode as well as the improvement of the proton
conductivity of the electrolyte. We believe we can solve these issues and obtain good SOFC
operations in a near future.
Introduction
Solid oxide fuel cells (SOFCs) are considered to be one of the candidates for future energy
sources. In these years, a new SOFC structure using a proton conductive oxide as an electrolyte
was proposed [1, 2] for intermediate-temperature (400-600oC) operations, which brings theSOFC cost reduction and ease of start-up. However, it was difficult to fabricate actual SOFCs
with proton conductive electrolytes because of the low conductivity (10-2-10-4S/cm) of theelectrolytes. To reduce resistivity, Ito et al. proposed to use a thin film of the proton conductiveoxide, doped-BaCeO3, as the electrolyte [3] and achieved good SOFC operations as low as
400oC. However, they need to use a thick Pd foil (>40Pm) as a substrate to support their SOFC
structure mechanically. As a consequence, their SOFC becomes too expensive and not realistic.
Recently, we have successfully deposited a solid proton conductive oxide directly on aporous stainless-steel (PSS) substrate by MOCVD [4]. Using this film, we can fabricate SOFCs
with a thin film electrolyte on an inexpensive substrate. We have been working to fabricate
SOFCs using this film and considering we would have a good result in a near future. However,using MOCVDs for fabricating SOFCs is not realistic neither because it is one of the expensive

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methods as an oxide deposition technique and its deposition conditions for fabricating solidoxide on the PSS is very narrow. Thus, other conventional and inexpensive oxide deposition
methods, i.e. sputtering, sol-gel, etc, is preferable for the SOFC fabrications. In order to use
these deposition techniques, the surface of the substrate needs to be smooth with no pores. To
achieve smooth surface on the PSS, we deposited a solid Pd layer on it. Though we use someamount of Pd in this structure, its thickness is several Pm and quite small compared to Ito etal’s[3]. Thus, we consider this substrate structure will bring realistic SOFC fabrications withgood SOFC operations.
In this paper, we chose a sol-gel spin-coating as an electrolyte deposition method because ofits ease of compositional control and lower deposition costs. In addition, we have alreadyachieved high quality epitaxial-growth of (Pb,La)(Zr,Ti)O3 thin films by sol-gel [5,6] andconsidered it would also effective for high quality deposition of electrolytes. Y-doped SrZrO3(SZYO) was chosen as an electrolyte because it has a good chemical stability and relatively
large proton conductivity with about 10-4S/cm at 300 oC [7].
Experimental
We use Pt deposited silicon wafers to reveal asinterability and a leakage current property of the SZYO
films and PSS substrates for SOFC cell fabrications. The PSS
substrate was fabricated by sintering of stainless steel
powders. After the sintering, the surface of the substrate waspolished and a solid Pd layer with several-mm in thick wasdeposited by plating on it.
SrZr0.8Y0.2O3-G (SZYO) thin films were deposited on Pt
deposited silicon wafers and Pd deposited PSS substrates bya sol-gel method. The sol-gel solution (Mitsubishi Material)with concentration of 5wt% was spin-coated on the substrates,
dried at 80oC, and pyrolyzed 300 and 400oC, subsequently.
This process was repeated several times until the film
thickness reaches at about 300nm. Finally, the deposited film
was annealed at 500 㹼 800oC in air for 60min. Typical
procedure of the sol-gel process was shown in Fig. 1.
The crystal structure was evaluated by the X-Ray
Diffraction (XRD). The cross- sectional structure observation
was performed by the Scanning Electron Microscopy (SEM).
A Pt top electrode was deposited by sputtering to measureelectrical leakage of the films at a room temperature.

Fig.1 Schematic process of thesol-gel method

Results and Discussion
Figure 2 shows the XRD patterns of the SZYO thin films deposited on the Pt/SiOx/Si
substrate annealed at various temperatures. Below 600oC annealings, the films did not show anydiffraction peaks, which means these films were not crystallized. In turn, the films annealed
above 600oC showed only diffractions which are belong to those of SZYO. In addition, theintensities of the SZYO peaks do not change seriously when it was annealed above 600oC. Thus,
the annealing as low 600oC as is sufficient to obtain crystallized SZYO films.
The leakage current of the SZYO films were shown in Fig. 3. The films annealed at 500 and100228102-2

550oC showed lower leakage compared to the others. As shown in Fig. 2, these films isamorphous and this low leakage profile might be originated from their amorphouscharacteristics. In turn, the film annealed above 600oC shows larger leakage, which might befrom the leak passes at the grain boundaries. The reason why the film annealed at 600oC shows
the largest leakage is not sure; however, its leakage is larger than almost 2 decades and too large
for the electrolyte use. In addition, the annealings above 700oC bring the PSS deformation andoxidation, which is not desirable for the SOFC fabrications. So we consider annealings at 700oCis the best annealing temperature to obtain good electrolyte in our experiment. The conductivityof this film was about 10-6 S/cm at 600oC in a wet condition and is a little higher than that of thevalue measured in the dry atmosphere. We consider this difference in the conductivities is fromproton conduction in the SZYO as described in ref. 7. However, the value obtained here is not
sufficient for the good SOFC operations. Thus, we need further improvement in the proton
conductivity (at least 10-4S/cm) and the optimization of the SZYO fabrication including dopantconcentration and the sintering atmosphere is in progress.

Fig.2 XRD patterns of the SZYO
films annealed at various temperatures

Fig.3 Electrical leakage at a room
temperature of the SZYO films annealed
at various temperatures

Finally, we deposited SZYO thin films on thePSS substrate with the Pd layer as shown in Fig.4.
Solid SZYO thin film was successfully obtainedon the substrate by sol-gel method. We arefabricating SOFC cells based on this structure but
still suffering the cathode (top electrode)fabrication. Because of the sinterability ofcathode materials, we need sintering temperaturesof >1000 oC, which causes the deformation of thesubstrate. We are also investigating sinterable
materials as low as 700oC for SOFC cellfabrications and have been trying to use a new Fig.4 Cross-sectional SEM image ofmaterial as the cathode. We believe we can sol-gel derived SZYO deposited onsolve this issue in a near future.
Pd/porous stainless-steel sub.

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Summary
The good SZYO thin film fabrications were demonstrated by sol-gel method on thePt/SiOx/Si and PSS substrates. Sol-gel-derived films were crystallized above 600oC and theannealing temperature of 700oC was the best to obtain crystallized SZYO with lower leakage.
The proton conductivity of this film was about 10-6 S/cm at 600oC in a dry condition andshowed almost the same value in the wet atmosphere.
To achieve good SOFC operations, we are studying for fabricating SOFC cells with cathodeelectrode as well as the improvement of the proton conductivity of the electrolyte. We are
studying for fabricating SOFC cells with cathode electrode as well as the improvement of theproton conductivity of the electrolyte. However, we have been investigating the optimum
fabrication conditions of SZYO and cathode, we consider they would be solved in a near future
and will bring good SOFC operations at the intermediate-temperatures.
Acknowledgments
The authors would like thank to the Foundation for Nara Institute of Science and Technology
for the support of this study.
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