Electrolytes of 0.5M0.5Ce10ScSZ (M = Gd, Sm, Yb) where CeO₂ added to 1Ce10ScSZ (10 mol% Sc₂O₃-1 mol% CeO₂-89 mol% ZrO₂) was partially substituted with Gd₂O₃, Sm₂O₃, and Yb₂O₃ (CeO₂:MO₃ = 1 : 1 molar ratio) were synthesized by solid state reaction. Since all secondary stabilizing elements have the oxidation number of +3, and are cations having a relatively similar size (Gd⁺³: 1.053 Å, Sm⁺³: 1.079 Å, Yb⁺³: 0.985 Å) to the radius for cations of Sc⁺³ (0.87 Å) or Zr⁺⁴ (0.84 Å), a stable solid solution is formed with zirconia.¹⁴⁾ Also, electrolytes of xGd(1-x)Ce10ScSZ (x = 0.25, 0.5, 0.75, 1.0) where the added amount of Gd₂O₃ was increased to 0.25-1.0 mol% were synthesized. Compositions of the synthesized electrolytes are summarized in Table 1.

Stoichiometry was controlled by quantitative mixing of oxide raw material powders (ZrO₂, CeO₂, Gd₂O₃, Sm₂O₃, and Yb₂O₃). The mixed raw materials were wet-milled for 24 h in ethanol, and dried at 70°C for 24 h, followed by calcination at 1100°C for 5 h. The calcined powders were ball-milled and dried for 24 h again to be prepared as electrolyte powders for ceramic manufacturing. To produce ceramic specimens to be used for evaluation of electrical conductivities, the powder was uniaxially compressed (250 kgf/cm²) to produce plates of 40 × 40 × 4 mm³ and then sintered at 1450°C for 10 h.

Sintered densities of ceramic electrolytes were measured by Archimedes method, and all specimens showed more than 95% of relative sintered density as shown in Table 1. Crystal structures for a sintered body of electrolyte were analyzed by X-ray diffractometry (Rigaku, Model no. D/max-2500 VL/PC, Japan). The sintered ceramics were cut into bar specimens of 3 × 3 × 20 mm, and electrical conductivities were measured by 4-probe DC method. Specimens were painted with Pt electrode (Haeraus, Model 6026) and cured at 900°C for 1 h, all of which were loaded inside a tube-type chamber by connecting Pt wire. Electrical conductivity of each specimen in the atmosphere was calculated from the slope by measuring current-voltage curves in the temperature range of 500 ~ 850°C. By using Source-Measure- Unit (Keithley, Model 2400, USA), currents in the range of 1 × 10⁻⁵ - 1 × 10⁻⁴A were applied to the electrodes at both ends of each specimen, and voltage drops between 2 middle electrodes were sequentially measures by using a multimeter (Keithley, Model 2700, US) equipped with a switching module (Keithley, Model 7700, US). To compare degradation characteristics of the electrolyte materials, H₂ gas was supplied into the chamber, and electrical conductivities were measured at 850°C for about 50 h.

Through performance evaluation of single cells using the commercial electrolyte, 1Ce10ScSZ and 0.5Gd0.5Ce10ScSZ with 0.5 mol% Gd₂O₃ addition as an electrolyte, degradation characteristics of each electrolyte were observed. Disk-shaped sintered body of electrolyte formed by uniaxial compaction was polished and electrolyte-supported cells (ESC) of 20 mm in diameter, 0.3 mm in thickness were produced. To coat Ni-ScSZ cermet anode, a slurry was prepared by mixing commercial NiO (Kceracell, 99.8%, Korea) and 1Ce10ScSZ (Daiichi Kigenso Kagaku Kogyo, 99.9%, Japan) powders with a binder in the ratio of 57 : 43 wt. The prepared anode paste was screen-printed, and then sintered at 1300°C for 3 h by 2°C/min. For the cathode functional layer, a LSM-ScSZ composite was used. The LSM-ScSZ slurry was prepared by mixing commercial LSM (Kceracell, La_0.8Sr_0.2 MnO₃, Korea) and 1Ce10ScSZ powders in the ratio of 57 : 43 wt% with plastic additives. The printed cathode layer was cured at 1200°C for 3 h. LSM paste was screen-printed as a current collector layer onto the LSM-ScSZ composite layer, and cured at 1150°C for 3 h to produce ESC. Performance of single cells was evaluated at 850°C, with the both air and fuel fed by 200 ml/min. At operating temperatures, current-voltage characteristics were continuously measured for 250 h by using a Source-Measure-Unit (Keithley, 2400, USA) for the single cell.

While 1Ce10ScSZ electrolyte is known to have a stable cubic phase crystal structure which shows a high ionic conductivity, the cubic phase (c) and the rhombohedral phase (β) are known to be intermingled as a function of temperature for its crystal structures.¹³⁾ In the present study, the characteristics of 0.5M0.5Ce10ScSZ electrolyte were evaluated where CeO₂ was partially substituted with M₂O₃ (M = Yb, Sm, Gd) to improve the mixed phase crystal structures (c+β) in the 1Ce10ScSZ electrolyte and the resultant degradation problem in electrical conductivities.

Ambient-temperature XRD analysis result for the sintered specimen of 0.5M0.5Ce10ScSZ electrolyte with substitutional solid-solution of 0.5 mol% of secondary stabilizer (Gd, Sm, Yb) is shown in Fig. 1. As reported in the literature, 1Ce10ScSZ specimen was shown to have a mixture of cubic phase and some β phase. No β phase was detected in the electrolyte where 0.5 mol% Sm₂O₃ was substitutionally solid-solutioned among the sintered specimens of electrolyte with some CeO₂ substituted by secondary stabilizer. However, β phase was detected in the specimens with addition of Gd₂O₃ or Yb₂O₃.

Figure 2 shows the electrical conductivities in air for 0.5M0.5Ce10ScSZ ceramics with addition of 0.5 mol% of M₂O₃. In a high-temperature section above 700°C, 0.5Gd0.5 Ce10ScSZ with addition of Gd₂O₃ and 0.5Yb0.5Ce10ScSZ with addition of Yb₂O₃ showed slightly higher electrical conductivities compared with 1Ce10ScSZ. Electrical conductivities for the electrolytes with addition of 0.5 mol% of M₂O₃ were highest in the order of M = Gd > Yb > Sm. As shown in Table 1, such differences in electrical conductivities would not have been caused by the fraction of pores present inside the specimens, since all specimens fired at 1450°C for 10 h showed high sintered densities of more than 95%. In that case, the differences in electrical conductivities should be considered by division into grain conductivity and grain boundary conductivity. Grain boundary conductivity is determined by size of the grain, impurities in the grain boundary, space charge layer, etc. However, since the grain boundary resistance for zirconia is not large at a high temperature above 700°C,^9,15,16) its effect may be excluded.

First, grain conductivity is affected by the symmetricity caused by phase transition from the cubic fluorite structure to the tetragonal or rhombohedral phase. This is because the mobility of ions shows a higher value in a symmetrical structure. In Fig. 2, electrical conductivities of other specimens showed a gradual change as a function of temperature unlike 0.5Yb0.5Ce10ScSZ which showed a marked change in the slopes below 600°C. Also, activation energy values at high temperatures did not show a large difference, suggesting that there was no large difference in the crystal phases at a high temperature being attributable to additives. In Fig. 1, a rhombohedral phase (β phase) was observed when CeO₂ was partially substituted with Yb₂O₃, Sm₂O₃, or Gd₂O₃. However, since the specimen with addition of Yb₂O₃ where a relatively large amount of rhombohedral phase was observed showed a higher conductivity than the specimen with addition of Sm₂O₃, the effects of existence of β phase present at ambient temperature on electrical conductivity are presumably not large. However, in view that the specimen with addition of Yb₂O₃ exhibited a drastic reduction in conductivities below 600°C, the content of β phase with a relatively large resistance appears to have been increased. Based on the high-temperature XRD results, Yarmolenko et al.¹³⁾ reported that the commercial 1Ce10ScSZ ceramic electrolyte showing a metastable cubic phase at ambient temperature was transitioned back to a cubic phase at a higher temperature after transition to a rhombohedral phase around 300 - 500°C. These authors demonstrated it by obtaining 95% of rhombohedral phase after annealing the 1Ce10ScSZ electrolyte at 400°C for 12 h. Therefore, the ambient-temperature crystal phases of 0.5M0.5Ce10ScSZ shown in Fig. 1 appear to be metastable phases, most of which were transitioned to a cubic phase at temperatures above 500°C where electrical conductivities were measured. However, even if a small change in the amount of additives may not greatly affect the crystal structures at a high temperature in the present study, the fine difference in symmetricity of a cubic structure can bring about a change in the electrical conductivities. Since lanthanide ions substituted for Zr sites have respectively different ionic radii, the effects of the added stabilizers on lattice distortion and the corresponding reduction in oxygen ion mobility should be considered. However, additional studies such as high-temperature XRD analysis and impedance analysis are required for the clearer analysis.

To consider the degradation characteristics of 0.5M0.5 Ce10ScSZ where a part of Ce was substituted from 1Ce10 ScSZ electrolyte, changes in electrical conductivity at 850°C under hydrogen atmosphere were observed. Fig. 3 represents normalized (σ/σ₀) electrical conductivities for electrolyte specimens of each formulation under H₂ atmosphere. (σ₀ being the initial conductivity value.) Whereas 1Ce10ScSZ electrolyte exposed to H₂ atmosphere at 850°C for about 50 h showed the highest degradation rate (~ 28%), 0.5Gd0.5 Ce10ScSZ with substitution of Gd showed the lowest degradation rate.

A study on the degradation phenomenon of 1Ce10ScSZ has been conducted by Omar et al.¹⁷⁾ These authors reported that continued reduction in conductivities was observed in air and the degradation progressed faster particularly under the reducing atmosphere although addition of 1mol% CeO₂ improved the phase stability of scandia stabilized zirconia. The cause for such degradation in a long term test at 600°C for 3,000 h was considered to be caused by an increase in grain boundary resistance, which was attributed to formation of blocking layers impeding diffusion of oxygen vacancies as Ce⁺³ was gradually diffused to the grain boundary from inside the grain. Also, Ni solid solution from an anode material such as Ni-based cermet was reported to affect the phase transition of ScSZ and the corresponding degradation characteristics. ^15,18) Therefore, since the concentration of Ce⁺³ was reduced when a part of CeO₂ was substituted with lanthanum oxide (M₂O₃, M = Yb, Gd, Sm), more excellent durability is expected to be observed in comparison with 1Ce10ScSZ. However, additional studies are required to find the causes for delaying of the degradation rates by addition of Gd₂O₃ in comparison with that of Yb₂O₃ or Sm₂O₃.

Meanwhile, concerning the degradation phenomenon under an oxidizing atmosphere, Du et al. reported that almost no effects of aging were observed in 1Ce10ScSZ while tetragonal → monoclinic phase transition and serious degradation phenomenon occurred only in ScSZ with additions of 4 mol% and 6 mol% Sc₂O₃.³⁾ Such results are in agreement with the results of Haering and Yamamoto on the effects of aging at 1000°C in air.^11,19) Haering et al. attributed such degradation in the electrical conductivities to the lowered mobility of oxygen vacancies as dipoles [ScZr′-VO··]· formed a strong defect association with tripoles [ScZr′-VO··-ScZr′]X. According to their analysis, no severe degradation occurred due to the already high ratio of tripole/dipole when Sc₂O₃ dopant of a high concentration of more than 10 mol% was added. In the present study, the degradation progressed more slowly, although the electrolytes doped with Sm₂O₃ and Yb₂O₃ showed a lower ionic conductivity compared with 1Ce10ScSZ. In terms of the cause, there is a possibility that acceptor concentrations, [Sm′_Zr] and [Sb′_Zr] were increased as Ce⁺⁴ was substituted with Yb⁺³ or Sm⁺³, resulting in a slight increase in oxygen vacancy concentrations, [ VO··]. Since, however, the effect of co-doping with 0.5 mol% on the oxygen vacancy concentration of zirconia stabilized with 10 mol% Sc₂O₃ will be around 5%, the effect is considered to be not large. Meanwhile, the degradation occurring in YSZ or ScSZ may be attributed to formation of the tetragonal or the ordered rhombohedral phase.¹⁹⁾ As with Ce, Yb or Gd apparently suppresses transition from a cubic phase to a rhombohedral phase, and maintenance of a cubic structure is known to occur at higher concentrations compared with Sc.⁹⁾ For a clear explanation, however, studies of high-temperature structure analysis need to be implemented in the future.

In the 3.1 above, 0.5Gd0.5Ce10ScSZ with addition of Gd₂O₃ showed the lowest degradation rate. To investigate the effects of Gd content on electrical conductivity, xGd(1-x) Ce10ScSZ specimens were prepared with the added amounts of Gd₂O₃ being varied within 0.25 - 1 mol%. In Fig. 4, XRD analysis results for the ceramic specimens are shown as a function of the amounts of Gd-substituted solid-solution. In the specimens with formulations of 0.25 - 0.5 mol% for the substituted solid-solution amount of Gd₂O₃, the metastable β phase was observed, while a single cubic phase was confirmed with a formulation of more than 0.75 mol% for the substituted solid-solution amount. Namely, the amounts of metastable phase (rhombohedral, β) at ambient temperature were reduced, as the more Gd₂O₃ was substituted in lieu of CeO₂ as a structure stabilizer.

Figure 5 shows changes in the electrical conductivity and the activation energy at 600, 700, and 800°C as a function of an increase in added amounts of Gd₂O₃. When the Gd₂O₃ addition were 0.25 - 0.5 mol%, the highest electrical conductivity occurred, and a tendency for reduction was observed with an increase in the concentrations. Such feature is a typical phenomenon occurring when the acceptor concentrations are increased in YSZ or ScSZ, and caused by reduction in the concentration of mobile oxygen vacancies due to association of oxygen vacancies [MZr′-VO··]· or short-range-ordering at high defect concentrations.^11,16,20) When the added amounts of Gd₂O₃ were increased to more than 0.5 mol%, the gradually increased activation energies showed an evidence for supporting such explanation.

Meanwhile, addition of 0.25 - 0.5 mol% Gd₂O₃ not only enhanced the conductivity of 1Ce10ScSZ, but also improved the durability under hydrogen atmosphere. Also, the durability was increased under hydrogen atmosphere, the more increased the added amount of Gd₂O₃. This is probably associated with stabilization of the cubic structure resulting from an increase in Gd₂O₃ contents as shown in Fig. 4.

In Fig. 6, a change in output performance as a function of time for ESC where 1Ce10ScSZ electrolyte and 0.5Gd0.5 Ce10Sc SZ electrolyte were used is shown along with a current- voltage curve. Considering the thickness of electrolyte (300 μm), a relatively high output density of more than 0.8W/cm² was observed at 850°C. Open-circuit voltages (OCV) of two cells were different, being 1.08 V and about 1.31 V, respectively. The low OCV observed with 1Ce10ScSZ was caused by an increase in the electronic concentrations due to a reduction of Ce+4 (CeZr×→CeZr•+e′) under the reducing atmosphere and the resulting decrease in Nernst potential. Despite the difference in OCV, however, the two cells showed almost similar maximum outputs. Actually, the difference in conductivities between the two electrolytes was not large, and the out performance may be considered almost the same when other variables in the single cell preparation processes are considered. However, it is noteworthy that the single cell of 0.5Gd0.5Ce10ScSZ with substitution by Gd₂O₃ showed a lower degradation rate compared with the single cell of 1Ce10ScSZ. After measurement for about 250 h, the single cell with application of 10Sc0.5Gd0.5CeSZ electrolyte showed a performance reduction by 2.3%, while the single cell with application of 1Ce10ScSZ electrolyte showed a degradation rate (4.2%) which was about twice as fast. Such result is in agreement with the earlier result where addition of Gd₂O₃ suppressed the degradation in electrical conductivities for 1Ce10ScSZ. Degradation of a single cell is caused primarily by an increase in ohmic resistance due to electrolyte and by an increase in polarization resistance resulting from degradation in catalyst characteristics of the electrode. Generally, the increase in polarization resistance arises from coarsening of electrode particles due to high-temperature operation or reduction in active areas due to deposition of impurities from outside and reaction, etc. Since the same electrodes were used on both faces of each electrolyte support in the present study, the difference observed between the two types or cells was presumably caused by an increase in ohmic resistance of the electrolyte.