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OPTIMIZED MIXTURE RATIO IN YSZ-SUPPORTED NI-YSZ ANODE MATERIAL FOR SOFC

Ni-YSZ cermets are generally used as anodes in SOFCs. The authors have proposed a new concept for the microstructure of anodes, referred to as “YSZ-supported anodes.” A feature of this material is that, unlike other materials, the YSZ powders are divided into coarse and fíne particles. Comparative analysis of the properties of the material has revealed improved long-term stability. Samples were prepared with varying levels of coarse and fine YSZ and NiO powders, with average particle sizes of 27.0, 0.4 and 1.0/zm, respectively. To optimize the mixture ratio of the three powders, volume shrinkage, porosity ratios and electrical conductivity were investigated under both SOFC fabricatrion and operation atmospheres. Electrode performances were also measured and compared. The results provide evidence for an optimal mixture ratio for YSZ-supported anodes.


INTRODUCTION
To use solid oxide fuel cells (SOFCs) for practical applications, it is necessary to both reduce fabrication costs (1) and to achieve higher and more stable levels of performance. Electrodes for fuel cells provide sites for electrochemical reactions and electrical paths under operation conditions. In SOFCs, nickel (Ni) and yttria stabilized zirconia (YSZ) cermets, which are generally used as the anode, are made by heat-treating in air after powder mixtures of nickel oxide (NiO) and YSZ, and then NiO is reduced to Ni in initial cell operation. YSZ particles are added to the anode material to make its thermal expansion behavior compatible with other components, particularly the YSZ electrolyte, and to inhibit the agglomeration of Ni particles (2). An optimal distribution of Ni particles would increase the amount of the reaction sites, that is, the length of the three-phase boundary (3) and would form a current path (4,5). A network of micro-pores in the anode is also required to provide a diffusion path for gaseous reactants and products. These functions are greatly dependent on the morphology of the anode, consisting of the Ni particles, the added YSZ and the micro-pores.
In the research and development of SOFCs at CRIEPI, serious reductions in cell performance have been observed due to deterioration of the anode, which, in turn, is due to the agglomeration of the Ni particles and the resulting shrinkage of the anode (6)(7)(8).
A new concept for the microstructure of anode has been proposed to solve this problem. A feature of this material, referred to as "YSZ-supported anodes," is that the YSZ powders are divided into coarse and fine particles. Measurements of cell performance 750 Electrochemical Society Proceedings Volume 2001-16 have indicated improved long-term stability compared to the previous material. Analyses of the configuration and the electrical behavior of the anode, tested with the total Ni volume ratio set at 40 %, have shown that the YSZ structure made from the two different particle sizes leads to more stable properties (9).
In this paper, experimental results are discussed concerning the changes and differences in the dimensional stability, electrical conductivity and electrode characteristics of different YSZ-supported anode samples under both cell fabrication and cell operation atmospheres. Furthermore, based on these results, the optimal mixture ratios for the three powders are determined.

Sample Preparation
The preparation procedure for the YSZ-supported anode consists of two main steps. The first step is the control of the particle size distribution and the second is the mixing of the powders. Details of this process have been described elsewhere (6)(7)(8)(9). The mixture ratios for the samples are shown in Fig. 1. The weight ratio values are expressed in the form, x-y-z, (where x + y + z = 100, apart from the series 4-6-z) for the coarse YSZ, the NiO and the fine YSZ, respec tively.

Measurement of Anode Characteristics
For reduction tests and electrical conductivity measurements, the anode powders were molded and then sintered at 1673 K for 10 h. The samples were placed in an alumina tube and were held at 1273 K for 12 to 300 h in a H2 flow, humidified by water at around 293 K at about 100 cm3/min. The sizes and weights of the samples were then measured to derive their volumes and porosities. Electrical conductivity measurements were conducted after the reduction tests with either an ac impedance spectroscopy or the dc four-probe method in a H2 atmosphere at 1273 K. The samples were set in a special rig, similar to that used by MacDonald (10).
Two anodes, one with and the other without fine YSZ (z=0 and z=2 in the series of 4-6-z samples, respectively), were used to examine anodic polarization. YSZ disks with relative densities of > 95.0% and about 1.0 mm in thickness were used as electrolytes. Anodes were slurry-coated with an electrode area of 0.8 cm2 and fired at 1673 K onto one side of the disks.
Cathodes, (La0.8Sro.z)o.97Mn03, were also slurry-coated and baked at 1423 K on the opposite sides. A reference electrode was made with platinum (Pt) and Pt wire at the edge of the cathode side of the disks. In the electrochemical measurements, gaseous Hz humidified by water at 293 K (50 cm3/min) as a fuel and air (200 cm3/min) as an oxidant were supplied to the anode and the cathode, respectively. Anode characteristics were measured using an impedance technique. Data concerning current flow through the cell and steady-state impedance spectroscopy were measured simultaneously within conditions of almost open circuit voltage and anodic polarization. At least 20 minutes was allowed for stabilization before each impedance run. After measurements, the polished cross sections of the anodes were observed with an electron-probe micro-analyzer (EPMA, JEOL Ltd., JXA-8900R).

Configurational Stability
Configuration stability and compatibility with other components in terms of thermal expansion behavior are important factors for anodes. The thermal expansion behaviors of both NiO-YSZ composites and Ni-YSZ cermets have been already clarified in air and in Hz atmospheres (11,12). As Dees et al. reported (13), the agglomeration of Ni particles under SOFC operating conditions leads to a reduction in reaction sites and the cutting-off of current paths. Furthermore, shrinkage of the electrode upon firing also leads to decreases in the gas permeability of the electrode layer, the occurrence of shear stresses at the electrolyte/electrode interfaces, and increases in contact resistance between the electrode and the current collector when assembled as a stack. Figure 2 shows the changes in linear shrinkage for the various YSZ-supported samples and "YSZ composites" which consisted of only coarse and fine YSZ (y=0 series) after sintering in air. The shrinkage in the samples decreases gradually with increases in the content of coarse YSZ. From the values of the raw materials it is clear that the NiO densified more than the coarse YSZ, but less than the fine one.  When the NiO only sample (0-100-0), indicated by a gray square, was reduced and held for 300 h, the porous specimen became agglomer ated and shrank by more than 50%. However, the shrink age for the z=0, coarse YSZ added NiO (apart from the 0-100-0 sample), decreased with increases in the content of coarse YSZ. This indi x/(x+ y) (%) cates that added coarse YSZ inhibits the coarsening of Ni particles and the resultant di mensional changes. From a comparison o f the data for the 0-100-0 sample and the 0-91-9 sample, it is also clear that fine YSZ had a similar effect. All samples for which the ratio (x/(x+y)) was below 0.4 showed remarkable volume decreases during the initial 50 h, and then became constant over time. With increases in the content o f coarse YSZ, volume contractions became milder and only small differences were seen at x/(x+y)>0.5.
The linear shrinkage for our previous anode after sintering in air was 15.6 %. When sintered with either a 3 m/o YSZ plate (4 cm diameter and ca. 300 /Zm) or an 8 m/o YSZ plate (10 cm diameter and ca. 300/Zm) using the slurry-coating and firing process, the anode caused warping o f the plates due to large shrinkage. During long-term SOFC operation, this anode deteriorates remarkably after about 20 h because the anodic over-potential curve at 0.2 A/cm2 rises abruptly. From the re sults o f reduction test, this anode contracted at a rate o f about 15.0 % after 24 h, and continued to shrink even after 300 h. These phenomena would be accom panied by strong agglom eration, and w ould cause shearing stress at the interface with the electrolyte under SOFC fabrica tion conditions. In general, for m aterials w ith iso tro p ic volum e change behavior, the linear expansion is one third of the volume. There fore, the permissible linear shrinkage can be calculated to be 5.0 %. Fig  ure 4 shows the total linear shrinkage for samples, excluding y=0 sample, which were added to the rate after sintering in air to the rate after 300 h reduction. This clearly shows the suitable range for permissible shrinkage.

Changes in Porosity
The porosity o f electrode is also an important factor, because of its effect on gas permeation, where low permeability can give rise to increases in over-potential related to gaseous diffusion. For our previous anode, over-potential occurred at about 10 h after commencing operation. The porosities for the z=0, 9, 23 samples after sintering in air increase with decreases in the content of fíne YSZ, and a similar behavior was observed for other samples. However, the curve for changes in porosity for the z=23 sample shows a gradual increase from 3.2 % up to 17.8 % with increases in the x/(x+y) value, while the changes for the samples z=0 and z=9 show only increases in the minimum region. The respective porosities o f the coarse YSZ only, the fine YSZ only and the NiO only were 43.5 %, 1.5 % and 32.0 %. The differences in the porosity o f the samples, made from three different powders, are due to different sintering properties of each of the powders.
During reduction, the large initial increase in porosity, which occurs for all samples apart from the x-0-z sam ple, is due to the reduction in NiO to Ni with a simultaneous decrease in the volume and an agglomeration o f Ni particles.
Samples with high fine YSZ contents and samples for which x/(x+y)>0.5 did not show any changes after the initial increment. Figure 5 shows the final porosities of the z=0, 9, 23 samples after sintering in air and reducing at 1273 K for 300 h, and the applicable range for SOFC anodes. Porosity was below about 30 % and the decrease to 20 % was continuous, as reported in ref.(l). Therefore, sam ples with a porosity o f less than 30 % are unsuitable for SOFC anodes. The electrical conductivities o f all the Ni-YSZ cermets were measured in a Hz atmosphere at 1273 K. As reported and discussed with respect to the change in conductivity for the y=56 sample (9), samples that have low NiO contents compared to either coarse or fine YSZ have low conductivities because the connections between the Ni particles are lost. Dees et al. (13) state that normally more than 1 h was required to attain steady states for their samples, although more than 90% of the change occurred within the first 10 min. In contrast, some o f the YSZ-supported anodes required more than 50 h to stabilize. Figure 6 shows the electrical conductivities for the z=0, 9, 23 sam ples maintained in a reducing condi tion for 300 h, as a function o f x/(x+ y). C o n d u ctiv ity w as im proved by three orders of magnitude by increases in the content of coarse YSZ. The conductivity o f the z=23 sample either becam e constant or slightly decreased with increases in the coarse YSZ content up to 30%, and then decreased gradually thereaf ter, although the conductivity o f the z=0 and z= 9 sam ples e x h ib ite d abrupt decreases at the 50-60 % level. This difference in the conductivity curves can be attributed to the fact that there is a dependency on both the size of the YSZ particles and the con tent o f both kinds o f YSZ, which is related to the level o f Ni. Such size effects have been observed in other cermet anodes, for example, by Dees et al. (13). Malliaris et al. (14) and Aharoni (15) have also investigated the effect of particle size on the conductivity of composites made with conductive and insulative powders. The suitable range for SOFC anodes is also shown in Fig. 6, which is more than 100 S/cm. This value was determined from the initial conductivity o f our previous anode when treated under the same conditions, where cell performance deteriorated immediately. 80 100

Characteristics of Anodes with and without the Fine YSZ
The relation betw een polarization behavior and m icrostructure has been investigated by electrochemical techniques (15,16). In this study, we focus on the influence of content levels o f fine YSZ, which was examined by means of ac impedance spectroscopy under non-zero, stepwise direct current densities and EPMA analysis.  distribution map for the 4-6-2 sample, the fine YSZ is seen as dark or white gray parts, which are extended around the coarse YSZ to cover the Ni particles and to form bridges connecting the coarse YSZ particles. Comparison with the 4-6-0 sample indicates that the coarse YSZ contact becomes more solid when fíne YSZ is used, and the dispersion and connection o f the Ni is also subtly changed in the presence of the fíne YSZ.
With respect to the current den sity-cell voltage characteristics of the respective single cells with anodes, the measurements indicate that cell per formance was better for the 4-6-2 sam ple than that for the 4-6-0 sample, with the current density o f the 4-6-2 sample cell reaching approximately 0.40 A/cm2 when the potential between the refer ence and anode was 0.4 V, although this was limited to about 0.28 A/cm2 for the 4-6-0 sample. The results o f the spec troscopy were applied to compensate for IR and the analysis o f electrode reaction (16,17). Figure 9 shows the polarization curves after excluding the IR for each polarized value. The over- potential of the 4-6-0 sample becomes higher and shifts to a lower current density than the 4-6-2 sample. It is thought to depend on the distribution o f the electrochemically active sites, electrodes with firm YSZ networks have been found to exhibit smaller over-potentials.

Optimal Mixture Ratios
An optimal mixture o f the three powders was determined from the present results, as shown in Fig. 10. Although some o f the x-y-0 samples exhibited excellent characteristics and are applicable for SOFC anodes in terms o f dimensional stability and electrical conductivity, these were not included because the performance o f the anode without fíne YSZ was low. Fig. 10 Area of applicable mixture ratios for the three powders for YSZ-supported anodes.

CONCLUSIONS
The present investigation o f YSZ-supported anodes suggests that coarse and fine YSZ grains form a solid structure and prevent damage due to reduction of the NiO or Ni agglomeration. However, coarse YSZ alone is not a suitable material for long-term stability, since slight changes in volume and porosity were observed. The addition of fine YSZ is effective in achieving a more stable anode, without any substantial decreases in electrical conductivity and alterations to the microstructure. The present results indicate that conductivity depends not only on the Ni content but also on the particle sizes of the YSZ grains and suggest an optimal mixture ratio for the coarse YSZ and fine YSZ. The present investigation has shown that the microstructure of YSZ-supported anodes is highly correlated with the characteristics o f the electrode.
Electrode polarization phenomena were strongly affected by the fíne YSZ content o f the powder. These results indicate a suitable range for the mixture ratios of the three powders for SOFCs.