Yb-Doped BaCeO3 and Its Composite Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells

BaCe0.9Yb0.1O3−α was prepared via the sol-gel method using zirconium nitrate, ytterbium trioxide, cerium nitrate and barium acetate as raw materials. Subsequently, it reacted with the binary NaCl~KCl salt to obtain BaCe0.9Yb0.1O3−α-NaCl~KCl composite electrolyte. The structure, morphology, conductivity and fuel cell performance of the obtained samples were investigated. Scanning electron microscope (SEM) images showed that BaCe0.9Yb0.1O3−α and NaCl~KCl combined with each other to form a homogeneous 3-D reticulated structure. The highest power density and conductivity of BaCe0.9Yb0.1O3−α-NaCl~KCl was 393 mW·cm−2 and 3.0 × 10−1 S·cm−1 at 700 °C, respectively.

In this study, BaCe 0.9 Yb 0.1 O 3−α was prepared via the sol-gel method and the composite electrolyte of BaCe 0.9 Yb 0.1 O 3−α -NaCl~KCl was also synthesized. The morphology, physical chemistry change, and the structure of BaCe 0.9 Yb 0.1 O 3−α were studied using SEM, Thermogravimetric Analysis and Differential Scanning Calorimetry (TGA-DSC) and X-ray diffractometer (XRD). The intermediate temperature electrochemical properties of BaCe 0.9 Yb 0.1 O 3−α and BaCe 0.9 Yb 0.1 O 3−α -NaCl~KCl were also investigated.

Materials and Methods
BaCe 0.9 Yb 0.1 O 3−α was prepared via the sol-gel method using zirconium nitrate, ytterbium trioxide, cerium nitrate and barium acetate as the raw materials. The stoichiometric metal ion salts (Ba 2+ :Ce 4+ :Yb 3+ = 10:9:1) were dissolved in deionized water. Citric acid was added (three times as much as the metal ion salts). The pH of the above solution was adjusted to 8.0 with ammonia and heated at 90 • C for 6 h until gelatinous. The xerogel was obtained at 130 • C and heated for the ashing treatment [35][36][37]. The calcination of the resultant ash was carried out at 1250 • C and 1550 • C for 5 h, respectively, to obtain BaCe 0.9 Yb 0.1 O 3−α .

Results and Discussion
TGA-DSC plots for the BaCe 0.9 Yb 0.1 O 3−α precursor were measured before and after the ashing treatment. In Figure 1a, the DSC curve has a sharp exothermic peak between 260 • C and 300 • C accompanied by 45% weight loss, mainly attributed to the decomposition of citric acid and ammonium salt. The weight loss is gentle, declining from 510 • C to 580 • C, which is attributed to the decomposition of the nitrate. As seen in Figure 1b, there was a decline in weight loss around 550 • C, which is ascribed to the incomplete decomposition of the nitrate [39,40]. There was almost no weight loss after 1070 • C indicating that the BaCeO 3 phase had begun to form.  Figure 1, the first sintering temperature of 1250 °C is suitable. There are some small additional peaks in the BaCe0.9Yb0.1O3−α-NaCl~KCl XRD spectrum, suggesting that NaCl~KCl inorganic salts exist as crystalline phases in the composite electrolyte [35].
The SEM external and cross-sectional surface images of BaCe0.9Yb0.1O3−α calcined at 1550 °C for 5 h (Figure 3a,b) and BaCe0.9Yb0.1O3−α-NaCl~KCl sintered at 750 °C for 2 h (Figure 3c,d) are displayed in Figure 3. As seen in Figure 3a,b, the degree of BaCe0.9Yb0.1O3−α particle agglomeration is good. However, the fractured surface image of BaCe0.9Yb0.1O3−α shows that there are still some holes after being calcined at 1550 °C for 5 h, as shown in Figure 3b. It has been proved by our experiments that they are closed holes. In Figure 3c,d, it is clearly visible that the particles of BaCe0.9Yb0.1O3−α are aggregated into clumps after the addition of NaCl~KCl inorganic salts sintered at 750 °C for 2 h. The regular polyhedron zones correspond to the BaCe0.9Yb0.1O3−α. Contrastingly, the amorphous areas point to the NaCl~KCl inorganic salt phase. Combined with the results of Figure 2, NaCl~KCl inorganic salts exist as both crystalline and amorphous phases [31,32].  Figure 1, the first sintering temperature of 1250 • C is suitable. There are some small additional peaks in the BaCe 0.9 Yb 0.1 O 3−α -NaCl~KCl XRD spectrum, suggesting that NaCl~KCl inorganic salts exist as crystalline phases in the composite electrolyte [35].
The SEM external and cross-sectional surface images of BaCe 0.9 Yb 0.1 O 3−α calcined at 1550 • C for 5 h (Figure 3a,b) and BaCe 0.9 Yb 0.1 O 3−α -NaCl~KCl sintered at 750 • C for 2 h (Figure 3c,d) are displayed in Figure 3. As seen in Figure 3a,b, the degree of BaCe 0.9 Yb 0.1 O 3−α particle agglomeration is good. However, the fractured surface image of BaCe 0.9 Yb 0.1 O 3−α shows that there are still some holes after being calcined at 1550 • C for 5 h, as shown in Figure 3b. It has been proved by our experiments that they are closed holes. In Figure 3c,d, it is clearly visible that the particles of BaCe 0.9 Yb 0.1 O 3−α are aggregated into clumps after the addition of NaCl~KCl inorganic salts sintered at 750 • C for 2 h. The regular polyhedron zones correspond to the BaCe 0.9 Yb 0.1 O 3−α . Contrastingly, the amorphous areas point to the NaCl~KCl inorganic salt phase. Combined with the results of Figure 2, NaCl~KCl inorganic salts exist as both crystalline and amorphous phases [31,32].    Figure 4 shows the log (σT)~1000 T −1 plots of BaCe0.9Yb0.1O3−α (1550 °C) and BaCe0.9Yb0.1O3−α-NaCl~KCl in the air from 400 °C to 700 °C. As seen in Figure 4, the conductivities of composite BaCe0.9Yb0.1O3−α-NaCl~KCl electrolytes are higher than that of the single BaCe0.9Yb0.1O3−α. The conductivities of BaCe0.9Yb0.1O3−α-NaCl~KCl vary from 2.0 × 10 −4 S·cm −1 to 3.0 × 10 −1 S·cm −1 in the range of 400-700 °C which is equivalent to BaZr0.85Y0.15O3−α-Li2CO3-K2CO3 in the air at 650 °C [31]. The single BaCe0.9Yb0.1O3−α electrolyte shows a linear Arrhenius curve in the air at 400-700 °C, whereas the     Figure 4 shows the log (σT)~1000 T −1 plots of BaCe0.9Yb0.1O3−α (1550 °C) and BaCe0.9Yb0.1O3−α-NaCl~KCl in the air from 400 °C to 700 °C. As seen in Figure 4, the conductivities of composite BaCe0.9Yb0.1O3−α-NaCl~KCl electrolytes are higher than that of the single BaCe0.9Yb0.1O3−α. The conductivities of BaCe0.9Yb0.1O3−α-NaCl~KCl vary from 2.0 × 10 −4 S·cm −1 to 3.0 × 10 −1 S·cm −1 in the range of 400-700 °C which is equivalent to BaZr0.85Y0.15O3−α-Li2CO3-K2CO3 in the air at 650 °C [31]. The single BaCe0.9Yb0.1O3−α electrolyte shows a linear Arrhenius curve in the air at 400-700 °C, whereas the  in the range of 400-700 • C which is equivalent to BaZr 0.85 Y 0.15 O 3−α -Li 2 CO 3 -K 2 CO 3 in the air at 650 • C [31]. The single BaCe 0.9 Yb 0.1 O 3−α electrolyte shows a linear Arrhenius curve in the air at 400-700 • C, whereas the conductivities of BaZr 0.85 Y 0.15 O 3−α -Li 2 CO 3 -K 2 CO 3 start to increase dramatically above 600 • C. The results indicate that the molten NaCl~KCl salt provides more ion transport channels at high temperatures [31,32,41]. Figure 5 shows the conductivities of BaCe 0.9 Yb 0.1 O 3−α (1550 • C) and BaCe 0.9 Yb 0.1 O 3−α -NaCl~KCl as a function of pO 2 from 1 × 10 −20 to 1 atm at 700 • C. The log σ~log pO 2 plot is usually used to estimate the ionic and electronic conduction of an electrolyte. Pikalova et al. reported that BaCe 0.89 Gd 0.1 Cu 0.01 O 3−α has a predominantly proton-conducting character at intermediate and low pO 2 values [9]. As shown in Figure 5, the conductivity is a horizontal line parallel to the X-axis, which indicates that the samples are almost pure ionic conductors. This may be ascribed to the molten salts acting as fast conduction paths for ionic charge carriers, which corresponds with related reports on composite electrolytes [25][26][27][28][29][30][31][32]. conductivities of BaZr0.85Y0.15O3−α-Li2CO3-K2CO3 start to increase dramatically above 600 °C. The results indicate that the molten NaCl~KCl salt provides more ion transport channels at high temperatures [31,32,41]. Figure 5 shows the conductivities of BaCe0.9Yb0.1O3−α (1550 °C) and BaCe0.9Yb0.1O3−α-NaCl~KCl as a function of pO2 from 1 × 10 −20 to 1 atm at 700 °C. The log σ ~ log pO2 plot is usually used to estimate the ionic and electronic conduction of an electrolyte. Pikalova et al. reported that BaCe0.89Gd0.1Cu0.01O3−α has a predominantly proton-conducting character at intermediate and low pO2 values [9]. As shown in Figure 5, the conductivity is a horizontal line parallel to the X-axis, which indicates that the samples are almost pure ionic conductors. This may be ascribed to the molten salts acting as fast conduction paths for ionic charge carriers, which corresponds with related reports on composite electrolytes [25][26][27][28][29][30][31][32].    conductivities of BaZr0.85Y0.15O3−α-Li2CO3-K2CO3 start to increase dramatically above 600 °C. The results indicate that the molten NaCl~KCl salt provides more ion transport channels at high temperatures [31,32,41]. Figure 5 shows the conductivities of BaCe0.9Yb0.1O3−α (1550 °C) and BaCe0.9Yb0.1O3−α-NaCl~KCl as a function of pO2 from 1 × 10 −20 to 1 atm at 700 °C. The log σ ~ log pO2 plot is usually used to estimate the ionic and electronic conduction of an electrolyte. Pikalova et al. reported that BaCe0.89Gd0.1Cu0.01O3−α has a predominantly proton-conducting character at intermediate and low pO2 values [9]. As shown in Figure 5, the conductivity is a horizontal line parallel to the X-axis, which indicates that the samples are almost pure ionic conductors. This may be ascribed to the molten salts acting as fast conduction paths for ionic charge carriers, which corresponds with related reports on composite electrolytes [25][26][27][28][29][30][31][32].     which correspond to the ohmic and total resistances, respectively. Additionally, the difference between them from the intercept with the real axis at high frequencies to the juncture point of the semicircle and radial, represents polarization resistance (R p ) [18]. The semicircle gradually disappears as the temperature increases [42,43]. In Figure 6, the polarization resistance (R p ) for BaCe 0.9 Yb 0.1 O 3−α (1550 • C) and BaCe 0.9 Yb 0.1 O 3−α -NaCl~KCl are 1.72 Ω·cm 2 and 0.31 Ω·cm 2 , respectively. This result indicates that the molten salt cannot only generate fast transport ways but also enhance its long-range mobility, which leads to lower resistance and higher performance.
curve includes a semicircle and a radial at high (1 KHz-100 KHz) and low (1 Hz-1 KHz) frequencies which correspond to the ohmic and total resistances, respectively. Additionally, the difference between them from the intercept with the real axis at high frequencies to the juncture point of the semicircle and radial, represents polarization resistance (Rp) [18]. The semicircle gradually disappears as the temperature increases [42,43]. In Figure 6, the polarization resistance (Rp) for BaCe0.9Yb0.1O3−α (1550 °C) and BaCe0.9Yb0.1O3−α-NaCl~KCl are 1.72 Ω·cm 2 and 0.31 Ω·cm 2 , respectively. This result indicates that the molten salt cannot only generate fast transport ways but also enhance its long-range mobility, which leads to lower resistance and higher performance.
The H 2 /O 2 fuel cell using BaCe 0.9 Yb 0.1 O 3−α -NaCl~KCl (thickness = 1.0 mm) as electrolyte achieves the highest power density (P h ) of 393 mW·cm −2 when the voltage is 0.64 V at 700 • C. The SrCe 0.6 Zr 0.3 Lu 0.1 O 3−α only has 34.8 mW·cm −2 under the same conditions. The P h value of our result is higher than the fuel cell performance of 60 wt% Ce 0.8 Sm 0.2 O 1.9 -40 wt% (Li/Na) 2 CO 3 (575 • C) and BaCe 0.7 In 0.15 Ta 0.05 Y 0.1 O 3−δ (thickness = 25 µm, 700 • C), however, lower than 80 wt% BaCe 0.7 Zr 0.1 Y 0.2 O 3−δ -20 wt% (Li/Na) 2 CO 3 (thickness = 0.4 mm, 600 • C) as shown in Table 1 [18,32,44]. This may be due to the different electrolyte and inorganic salt types and fuel cell construction. curve includes a semicircle and a radial at high (1 KHz-100 KHz) and low (1 Hz-1 KHz) frequencies which correspond to the ohmic and total resistances, respectively. Additionally, the difference between them from the intercept with the real axis at high frequencies to the juncture point of the semicircle and radial, represents polarization resistance (Rp) [18]. The semicircle gradually disappears as the temperature increases [42,43]. In Figure 6, the polarization resistance (Rp) for BaCe0.9Yb0.1O3−α (1550 °C) and BaCe0.9Yb0.1O3−α-NaCl~KCl are 1.72 Ω·cm 2 and 0.31 Ω·cm 2 , respectively. This result indicates that the molten salt cannot only generate fast transport ways but also enhance its long-range mobility, which leads to lower resistance and higher performance.