ELECTRICAL PROPERTIES OF Sr0.86Y0.08TiO3 UNDER REDOX AND FULL CELL FABRICATION CONDITIONS

The effects of manufacturing and preparation conditions on the structural and electrical properties of Sr 0.86 Y 0.08 TiO 3 (SYT) reduced in 5% NH 3 (95% N 2 ) are discussed. The realization of an SYT-based SOFC anode is challenging because the conductivity of SYT is highly dependent upon the thermal history combined with heat treatment atmosphere used in manufacturing. To obtain highly conductive SYT as a candidate for an SOFC anode material, all samples in this study were pre-reduced to 1400 o C under reducing conditions (ammonia) for 8 hours. After pre-reduction, 3 samples were oxidized in air at 850 o C, 950 o C and 1050 o C, respectively, for 4 hours to evaluate the impact of oxidizing conditions in practical cell fabrication processes on the SYT conductivity. XRD analyses showed that the lattice parameter of SYT sintered in ammonia was slightly different than the sample sintered in air. Measured at 800 o C in reducing atmosphere (dry N 2 /4%H 2 ), the maximum electrical conductivity of 36.3 S/cm was observed in SYT reduced in ammonia at 1400 o C. However, the observed conductivities were not preserved after oxidation-reduction cycles. Various SYT samples pre-reduced in ammonia at 1400 o C and then oxidized in air at 850 o C, 950 o C and 1050 o C showed an irreversible drop on conductivity measured in a reducing atmosphere, and the higher the oxidation temperature, the lower the conductivity became. The conductivity results indicate a strong dependence upon the SYT manufacturing and processing conditions. Despite the irreversible drop due to the oxidation cycle, the conductivity of SYT sintered in ammonia


INTRODUCTION
Current state-of-art SOFC systems most frequently use a Ni/YSZ cermet anode materials, which possess excellent catalytic activity and conductivity. However, Ni/YSZ has many disadvantages, including nickel coarsening, sulfur poisoning and carbon deposition which can hinder the direct use of practical hydrocarbon fuels, and volume instability during redox cycles, which may cause fracture. What is more, nickel and nickel oxide may lead to allergies or cancer, adding difficulties in handling the material during manufacture [1].
Recently yttrium-doped strontium titanate (SYT) is considered to be a promising alternative SOFC anode material [1][2][3][4][5][6]. Several groups demonstrated that SrTiO 3 -based materials satisfied the anode requirements well, being thermodynamically stable in anodic conditions, electronically and ionically conductive, chemically compatible with the electrolyte and interconnect, and has similar thermal expansion coefficient as other cell components (The thermal expansion coefficient of yttrium doped SrTiO 3 and YSZ are (11-12) × 10 −6 K −1 and (10.6-10.9) × 10 −6 K −1 , respectively) [7][8][9][10]. One of the most attractive properties of SYT compared to the nickel-based anode is the intrinsic sulfur tolerance and coking resistance, which indicate that practical fuels such as natural gas could be directly applied in SOFC without addition of steam and extra balance of plant (BOP). These properties further enable SOFC use in stationary power generation to utilize fuels that are currently used such as natural gas and coal syngas. However, SYT still needs to be further improved to compete with Ni/YSZ in terms of conductivity and catalytic activity. Yttrium doped SrTiO 3 exhibits very poor electro catalytic activity for fuel oxidation, especially for the oxidation of methane, possibly due to the lack of significant ionic conductivity in the materials [7]. Catalytic infiltration or impregnation has been demonstrated to solve the problem. The catalytic activity issue could be addressed by Ni or Pd infiltration as demonstrated by Gorte's group [8,11,12].
The conductivity issue of donor doped SrTiO 3 has been reported extensively in the literature showing that to possess a high conductivity, the SYT must be pre-reduced in a reducing environment at elevated temperature and that the extent of reduction might not be preserved during other cell fabrication processes. In recent studies, the exact influence of the oxidizing heat treatment that might be inevitable in the cell fabrication process was not clearly reported. In the present work, A-site 2 Copyright © 2011 by ASME deficient yttrium-doped SrTiO 3 Sr 0.86 Y 0.08 TiO 3 (SYT) was prepared, characterized and the conductivities under various preparation conditions and redox cycles were tested.

EXPERIMENTAL
Sr 0.86 Y 0.08 TiO 3 powders were prepared by a modified Pechini method. A detailed description of the synthesis is published elsewhere [13]. The synthesized SYT powder was first calcined to 900 o C in air for 8 hours and selected powders were sintered to 1400 o C for 4 hours in 5% Ammonia/N 2 . After ball milling for 24 hours the sintered powders were pressed into pellets (13mm in diameter and 1mm in thickness) at 180 MPa using a manual uniaxial hydraulic press. The as-pressed pellets were sintered at 1400 °C for 4 hours in 5% Ammonia/N 2 . Selected pellets were then oxidized in air for 4 hours at 850 o C, 950 o C, or 1050 o C. The preparation of various samples investigated in the present work is listed in Table 1. The density of the as-sintered samples was measured using Archimedes method and the relative densities were calculated. The crystalline structures of series of SYT pellets with various thermal histories were examined using an X-ray diffractometer (XRD, Bruker D5000), and XRD patterns were taken using a Ni-filtered Cu-K α radiation source (operating at 40 V/30 mA), with a 2θ step of 0.02°.
For the conductivity measurements, platinum gauze (Alfa Aesar, 99.9%) and platinum paste (Engelhard, A3788A) were used as current collector. The SYT samples with platinum current collectors were fired at 950°C in N 2 for 1h to establish good bonding between the pellets and platinum paste. N 2 was also expected not to facilitate the oxidation of SYT and preserve extend of reduction. Four-probe electrical conductivity measurements were carried out in forming gas (4% H 2 /N 2 ) at temperatures ranging between 500 °C and 800°C with a temperature increment of 50°C. AC impedance spectra were collected to measure ohmic resistance using a Solartron 1260A/1480A system operating in the frequency range of 100kHz to 0.1Hz with an excitation voltage amplitude of 5mV.

RESULTS AND DISCUSSION
The relative density and the open porosity of samples are presented in Table 2. The relative densities of the pellets after heat treatment were all larger than 85% (density of SYT powder was taken as 5.02 g/cm 3 ). With the increasing relative density of the sample, the effective contact area between grains will be increased which is favorable to both the electron and ion mobility. As shown in the X-ray Diffraction patterns of Fig. 1, all assintered pellets showed a clear cubic perovskite structure regardless of their thermal history. Small traces of secondary phase Ti 2 Y 2 O 7 also presented in some of the samples (denoted with *). In a closer view the XRD peaks of A2-A4 samples were shifted to smaller angles compared to the peaks of the A1 sample. The shift indicates that the lattice constant varied slightly with various post-reduction oxidizing conditions. The change of lattice constant of SYT samples due to different heat treatment atmosphere is estimated to be less than 1% and the induced variation in residual stress should be negligible. The small change upon reduction/oxidation also suggests that the structural integrity of SYT is tolerant to oxidation/reduction cycling or to the variation of oxygen partial pressure between fuel inlet and outlet during cell operation [10].

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Copyright © 2011 by ASME The results of conductivity are shown in Fig. 2. For all samples investigated, at a temperature range of 500 o C-800 o C the conductivity increases with increasing of temperature after being reduced in dry N 2 /4%H 2 at 800 o C for 36 hours, indicating a negative temperature coefficient of resistance and a nonmetallic type conductive behavior. The results suggest that the electronic conduction is not the pre-dominant conduction mechanism in the material. Consequently, SYT samples exhibit mixed electron-ion conductivity under the current preparation conditions. The Arrhenius plots were not perfectly linear for some of the samples, and the relation between conductivity and temperature suggests a fairly clear Arrhenius dependence. Averaged activation energy is derived from the slope of the Arrhenius plot in the range 500-800 o C, and lies in the range of 0.31eV to 0.95eV. The activation energy of conductivity is an important parameter to evaluate the electrical conductivity of materials. In our study, it is clear that conductivity and activation energy decrease with increasing oxidization temperature exposure during processing. The electrical conductivity at 800 o C in forming gas was found to be 12.1, 9.9, and 6.1 S/cm for the samples with oxidation at 850 o C, 950 o C, and 1050 o C in air, respectively. The A1 sample, which had not been sintered in air, has the highest conductivity of 36.3 S/cm at 800 o C. The conductivity of SYT (Sr 0.86 Y 0.08 TiO 3-δ ) with similar heat treatment process reported was 22 S/cm [13] and 82 S/cm [10] at temperature of 800°C.  The defect chemistry and its effect on conductivity in yttrium-doped SrTiO 3 has been widely reported in the literature; the extra positive charge of the donor (Y 3+ ) can be compensated either by strontium vacancies (ionically) in an oxidizing atmosphere or conduction electrons (electronically) provided by the Ti 4+ reduction to Ti 3+ in reducing atmospheres [14]. The more dominant the electronic compensation mechanism, the higher the conductivity of SYT has been observed. The temperature and atmosphere of the final heat treatment steps determine the compensation mechanism, which therefore affect the conductivity. The results show that the A1 sample possesses the highest degree of reduction, which suggests that the electronic compensation mechanism accounts for a larger portion of the conductivity than for the A2-A4 samples. While the degree of reduction of A2-A4 are lowered with increasing oxidation temperature, therefore showed a decreasing trend in conductivity, which indicates that part of the electronic compensation mechanism is converted to ionic compensation, and the conversion tends to be proportional to the increasing oxidation temperature. The rate of reduction of SYT with various thermal histories had been studied by measuring the change in the electrical conductivity as a function of time, as all samples were tested in 4% H 2 for 36 hours at 800 o C (shown in Fig. 3). The conductivity increased rapidly with the exposure to the reducing atmosphere and tended to reach a plateau after some time. The changes of conductivities show noticeable parabolic time dependence features, which was also reported in Petric et al.'s study [9]. Two processes in series have been proposed for the reduction reactions, the gas solid interface reaction and bulk diffusion [9,15]. The behavior showed in Fig. 3. indicates a diffusion controlled process, as the rate of the reduction decreases with time. After the samples are exposed to the reducing atmosphere, fast surface reactions occurred and triggered the composition change, followed by mass transport via diffusion from the gas/solid interface into the bulk and formed a reduced/oxidized interface. With the extent of 4 Copyright © 2011 by ASME reduction increased, the reduced/oxidized interface moved further from the gas/solid interface into the bulk (increased diffusion length), therefore the rate of reduction decreases with time. It has been reported that the rate of oxidation is much faster than that of reduction. The reason for a much slower rate of reduction is due to that fact that the incorporation of oxygen into lattice is much easier than the release of oxygen from lattice. The results also suggest that to fabricate a SYT-based single cell, any co-sintering processes in air will cause a relatively long reduction period to restore the extent of reduction and fulfill the conductivity requirement. However, if the degree of reduction of SYT was partly preserved during cell fabrication processes, one can expect increasing performance of an SYT anode in time, since the diffusion controlled reduction processes will continuously occur under an anode atmosphere at the SOFC operating temperature.
The electrical conductivity of an SYT-YSZ composite anode (SYT: YSZ: Pores=30:30:40 in vol. %) at various temperatures were calculated and the ASR of 1mm thick anode (assuming an anode-supported cell) are plotted in Fig. 4 (A). The model is based on the percolation theory developed by McLachlan et al. [16], which is in general employed to calculate the electrical conductivity of a composite. The equation is presented in equation (1): where t is a system specific parameter, and typical values of the parameter t fall in the range of 1.2-1.6 [17]. In this study t=1.6. V c is the critical (percolation) volume fraction of SYT, and V SYT is the volume fraction of SYT phase in the composite. σ comp and σ SYT are the electrical conductivities of the composite and SYT phase, respectively. In our model, it is assumed that SYT is a discontinuous conductive particle embedded in a YSZ (and pore) insulating matrix, and the SYT loading is above the percolation threshold. The critical volume fraction of SYT (V c ) is estimated by the microstructure of STY-YSZ composite showed in Fig. 4 (B). The microstructure of the composite suggested that the particle size of SYT (dark color) and the YSZ (light color) were approximately identical, as a result V c tend to be in the range of 0.16 according to McLachlan et al. [16]. As the particle size of SYT decrease below the particle size of YSZ, the value of critical volume fraction will drop, and as a result the conductivity of the composite will be increased with a more effective conducting network. Based on McLachlan's percolation model, the critical volume fraction will drop and reach about 0.05 when the SYT particle is 30 times smaller than that of the YSZ particle [16], and the predicted ASR will be halved for all cases discussed.
The ASR of a 1mm thick composite anode has been calculated and with results presented in Fig. 4 (A) using the predicted composite conductivities of the previous model. The results show that an SYT-YSZ composite anode that has been pre-reduced in ammonia at 1400 o C (same thermal history as A1) has the lowest ASR (~0.05 Ωcm 2 ) compared to the other composite anodes oxidized in air at various temperatures. It is argued that the ASR of a 1mm thick anode lower than 0.1 Ωcm 2 is required in order to achieve high performance with alternative ceramic anode materials [18,19]. The results, therefore, show that the porous SYT-YSZ composite anode that is reduced at 1400 o C without any oxidative manufacturing processes in air meets the minimum conductivity requirements. Whereas any additional heat treatments processed in air at elevated temperature will result in the increase of ASR and reduce the current composite anode from being a viable candidate anode materials set, in proportion to the oxidation Copyright © 2011 by ASME temperature. The results also suggest that to fabricate an SYTbased single cell, any co-sintering processes in air should be avoided or should be accomplished in Ar or N 2 atmosphere (as an alternative) to fulfill the anode conductivity requirements. Under some circumstances, co-sintering processes in air at the temperature range of 850 o C-1100 o C is inevitable, an alternative way to lower the anode ASR is to optimize the percolation of SYT particles in YSZ and optimize the microstructure of the composite.

CONCLUSIONS
The XRD results indicate that all as-sintered in ammonia SYT pellets showed perovskite structure regardless of the thermal history, and the lattice constant varied slightly due to the various preparation conditions.
The conductivity results suggest that the SYT reduced at high temperature (1400 o C) in ammonia has sufficient conductivity for use as an SOFC anode material. In addition, to fabricate an SYT-based single cell, any co-sintering processes in air should be avoided or one should use Ar or N 2 atmosphere as an alternative to preserve the degree of SYT reduction. Although the conductivity value of SYT is still orders of magnitude lower than the Ni-based anode materials, it was demonstrated that SYT or SYT composite materials can be used as an anode or anode current collector through careful morphological design and controlled fabrication processes.