Preparation, Characterization and Intermediate-Temperature Electrochemical Properties of Er3+-Doped Barium Cerate–Sulphate Composite Electrolyte

In this study, BaCe0.9Er0.1O3−α was synthesized by a microemulsion method. Then, a BaCe0.9Er0.1O3−α–K2SO4–BaSO4 composite electrolyte was obtained by compounding it with a K2SO4–Li2SO4 solid solution. BaCe0.9Er0.1O3−α and BaCe0.9Er0.1O3−α–K2SO4–BaSO4 were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry. AC impedance spectroscopy was measured in a nitrogen atmosphere at 400–700 °C. The logσ~log (pO2) curves and fuel cell performances of BaCe0.9Er0.1O3−α and BaCe0.9Er0.1O3−α–K2SO4–BaSO4 were tested at 700 °C. The maximum output power density of BaCe0.9Er0.1O3−α–K2SO4–BaSO4 was 115.9 mW·cm−2 at 700 °C, which is ten times higher than that of BaCe0.9Er0.1O3−α.


Introduction
With the rapid development of the economy, energy problems are imminent. As one of the new energy sources, fuel cells are of great significance. Solid oxide fuel cells (SOFCs) have the advantages of high conversion efficiency, small size, no noise, reduced pollution, and so on [1][2][3][4][5][6][7][8]. However, higher operating temperatures often lead to serious performance degradation, longer start-up times and expensive interconnecting sealing materials, which are considered the main obstacles to SOFCs' commercialization. Therefore, it is urgent to explore SOFCs operating at intermediate temperatures (400-700 • C) and at high performance at the same time. Compared with oxygen ion-conductive SOFCs, proton-conducting SOFCs can operate at lower temperatures. An exploration of electrolyte materials with high protonic conductivities at 400-700 • C is of vital importance [9][10][11][12][13][14].

Experimental
BaCe 0.9 Er 0.1 O 3−α was prepared by a microemulsion method. Firstly, Er 2 O 3 was completely dissolved with concentrated nitric acid. Sixty milliliters of water was added to make Ba(CH 3 COO) 2 and (NH 4 ) 2 Ce(NO 3 ) 6 dissolve evenly. A mixture of cyclohexane, ethanol and polyvinyl alcohol (PVA) was added to the solution and stirred until it was completely emulsified to form Microemulsion A. Then, (NH 4 ) 2 CO 3 , NH 4 OH, cyclohexane, ethanol and PVA were mixed evenly to form Microemulsion B [38,39]. Microemulsion B was slowly added to Microemulsion A. In the process of dropping, the number of white precipitates increased, and a large number of bubbles emerged at the same time. The precipitation was filtered and dried under an infrared lamp to obtain the precursor powder. Finally, the precursor was calcined in a high-temperature furnace at 1250 and 1550 • C for 6 h to obtain BaCe 0.9 Er 0.1 O 3−α .
For intermediate-temperature electrochemical properties, BaCe 0.9 Er 0.1 O 3−α and BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 were polished to a thickness of 1.0 mm. Circles 8 mm in diameter were drawn in the center of both sides of the discs with a pencil, and a 20%Pd-80%Ag paste was coated on the circles (area: 0.5 cm 2 ). AC impedance spectroscopy was measured in a nitrogen atmosphere at 400-700 • C. The frequency ranged from 1 to 10 5 Hz, and the signal voltage was 0.05 V. The logσ~log (p O 2 ) curves of BaCe 0.9 Er 0.1 O 3−α and BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 were tested by adjusting different proportions of air, nitrogen, oxygen and hydrogen at room temperature (p H2O = 2.3 × 10 3 − 3.1 × 10 3 Pa). The two sides of BaCe 0.9 Er 0.1 O 3−α and BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 were in hydrogen and oxygen atmospheres, respectively, which constituted the following fuel cells: H 2 , Pd-Ag | sample | Pd-Ag, O 2 . We then measured their I-V-P curves.    It can be seen that BaCe0.9Er0.1O3−α (1550 °C) had a compact structure, complete grain growth, clear grain boundaries, and very few holes. The density of the BaCe0.9Er0.1O3−α (1550 °C) ceramic prepared by the microemulsion method was higher than that by the high-temperature solid-state method at the same sintering temperature. After adding sulphate, the boundaries between grains became not particularly distinct. There were different degrees of adhesion between grains [32,33]. This is due to the BaCe0.9Er0.1O3−α grains being wrapped in molten sulfate.   The density of the BaCe 0.9 Er 0.1 O 3−α (1550 • C) ceramic prepared by the microemulsion method was higher than that by the high-temperature solid-state method at the same sintering temperature. After adding sulphate, the boundaries between grains became not particularly distinct. There were different degrees of adhesion between grains [32,33]. This is due to the BaCe 0.9 Er 0.1 O 3−α grains being wrapped in molten sulfate. perovskite structure when sulphate and BaCe0.9Er0.1O3−α form a composite electrolyte. This is why CeO2 also appears in BaCe0.9Er0.1O3−α-K2SO4-BaSO4.  It can be seen that BaCe0.9Er0.1O3−α (1550 °C) had a compact structure, complete grain growth, clear grain boundaries, and very few holes. The density of the BaCe0.9Er0.1O3−α (1550 °C) ceramic prepared by the microemulsion method was higher than that by the high-temperature solid-state method at the same sintering temperature. After adding sulphate, the boundaries between grains became not particularly distinct. There were different degrees of adhesion between grains [32,33]. This is due to the BaCe0.9Er0.1O3−α grains being wrapped in molten sulfate.  The energy-dispersive X-ray spectroscopy result of BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 is shown in Figure 3. The spectrum had major peaks assigned to the Ba, Ce, Er, O, K and S elements. The atomic ratios of Ba/Ce, Ba/Er and S/K are 0.87, 9.21 and 1.54. The low content of the Ba element may be due to the formation of BaSO 4 by the reaction: BaO + Li 2 SO 4 = BaSO 4 + Li 2 O, resulting in segregation. The elements mapping images indicated that the spatial distribution of sulphate was uniform. The energy-dispersive X-ray spectroscopy result of BaCe0.9Er0.1O3−α-K2SO4-BaSO4 is shown in Figure 3. The spectrum had major peaks assigned to the Ba, Ce, Er, O, K and S elements. The atomic ratios of Ba/Ce, Ba/Er and S/K are 0.87, 9.21 and 1.54. The low content of the Ba element may be due to the formation of BaSO4 by the reaction: BaO + Li SO = BaSO + Li O, resulting in segregation. The elements mapping images indicated that the spatial distribution of sulphate was uniform.     Figure 5 shows the conductivities of BaCe0.9Er0.1O3−α (1550 °C) and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 in nitrogen measured from 400 to 700 °C. It can be seen from Figure 5 that the BaCe0.9Er0.1O3−α-K2SO4-BaSO4 had a beneficial effect on conductivity. With the addition of sulphate, the conductivity was significantly improved. This is because the sulphate distributed at the grain boundary and formed a continuous phase, so both the main phase and the grain boundary phase could conduct ions. The highest conductivities of BaCe0.9Er0.1O3−α (1550 °C) and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 achieved were 9.4 × 10 −3 and 1.8 × 10 −1 S·cm −1 at 700 °C. Under the same conditions, the conductivity of BaCe0.9Er0.1O3−α-K2SO4-BaSO4 was higher than that of BaCe0.7In0.3O3−δ-Gd0.1Ce0.9O2−δ [27] and comparable to values of BaCe0.83Y0.17O3−δ-Sm0.15Ce0.85O2−δ [29]. This indicated that the sulphate was conducive to the conduction of ion defects through the interface region in the BaCe0.9Er0.1O3−α-K2SO4-BaSO4 composite electrolyte. The conductivity of BaCe0.9Er0.1O3−α was equivalent to that of BaCe0.7In0.15Ta0.05Y0.1O3−δ [22] and BaCe0.5Zr0.3Y0.2−xYbxO3−δ in wet H2 (~3% H2O) [18]. This may be related to its high density, as shown in Figure 3.    [18]. This may be related to its high density, as shown in Figure 3.   Figure 5 shows the conductivities of BaCe0.9Er0.1O3−α (1550 °C) and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 in nitrogen measured from 400 to 700 °C. It can be seen from Figure 5 that the BaCe0.9Er0.1O3−α-K2SO4-BaSO4 had a beneficial effect on conductivity. With the addition of sulphate, the conductivity was significantly improved. This is because the sulphate distributed at the grain boundary and formed a continuous phase, so both the main phase and the grain boundary phase could conduct ions. The highest conductivities of BaCe0.9Er0.1O3−α (1550 °C) and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 achieved were 9.4 × 10 −3 and 1.8 × 10 −1 S·cm −1 at 700 °C. Under the same conditions, the conductivity of BaCe0.9Er0.1O3−α-K2SO4-BaSO4 was higher than that of BaCe0.7In0.3O3−δ-Gd0.1Ce0.9O2−δ [27] and comparable to values of BaCe0.83Y0.17O3−δ-Sm0.15Ce0.85O2−δ [29]. This indicated that the sulphate was conducive to the conduction of ion defects through the interface region in the BaCe0.9Er0.1O3−α-K2SO4-BaSO4 composite electrolyte. The conductivity of BaCe0.9Er0.1O3−α was equivalent to that of BaCe0.7In0.15Ta0.05Y0.1O3−δ [22] and BaCe0.5Zr0.3Y0.2−xYbxO3−δ in wet H2 (~3% H2O) [18]. This may be related to its high density, as shown in Figure 3.  The conduction characteristics of BaCe 0.9 Er 0.1 O 3−α and BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 were tested by adjusting different proportions of gases. As seen in Figure 6, the conductivities of the samples in a reductive atmosphere are very close to those in an oxidizing atmosphere. The logσ~log (p O 2 ) curves of BaCe 0.9 Er 0.1 O 3−α and BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 were almost horizontal straight lines. When the temperature exceeded the melting point of sulphate salts, the mobility of various ions (Ba 2+ , Li + , K + , H + ) was greatly enhanced, which led to a low activation energy for ion transport in the interface regions. The proton was the smallest cation, and the mobility of protons was greater than other ions (Li + , K + ), resulting in an increased conductivity. Therefore, ion conduction appeared to become dominant [34]. The conduction characteristics of BaCe0.9Er0.1O3−α and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 were tested by adjusting different proportions of gases. As seen in Figure 6, the conductivities of the samples in a reductive atmosphere are very close to those in an oxidizing atmosphere. The logσ~log (pO 2 ) curves of BaCe0.9Er0.1O3−α and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 were almost horizontal straight lines. When the temperature exceeded the melting point of sulphate salts, the mobility of various ions (Ba 2+ , Li + , K + , H + ) was greatly enhanced, which led to a low activation energy for ion transport in the interface regions. The proton was the smallest cation, and the mobility of protons was greater than other ions (Li + , K + ), resulting in an increased conductivity. Therefore, ion conduction appeared to become dominant [34]. Hydrogen/oxygen fuel cells were assembled with BaCe0.9Er0.1O3−α and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 as supporting electrolytes and Pd-Ag as electrodes. The current-voltage characteristic curves are shown in Figure 7. The resistance directed from current-voltage characteristic curve of BaCe0.9Er0.1O3−α-K2SO4-BaSO4 (2.76 Ω) was lower than that of the value (5.54 Ω) from AC impedance at 700 °C, implying that the protonic conduction was dominant under the fuel cell condition [45]. The maximum power density of BaCe0.9Er0.1O3−α was 10.9 mW·cm −2 at 700 °C. Because the fuel cell was supported by the electrolyte and the electrolyte was thicker (1.0 mm), the current and power density were relatively low. When the voltage was 0.6 V, the maximum output power density of BaCe0.9Er0.1O3−α-K2SO4-BaSO4 was 115.9 mW·cm −2 at 700 °C, which is ten times higher than that of BaCe0.9Er0.1O3−α. The results show that BaCe0.9Er0.1O3−α-K2SO4-BaSO4 is an excellent electrolyte material for medium-temperature fuel cells.   Figure 7.

Results and Discussion
The resistance directed from current-voltage characteristic curve of BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 (2.76 Ω) was lower than that of the value (5.54 Ω) from AC impedance at 700 • C, implying that the protonic conduction was dominant under the fuel cell condition [45]. The maximum power density of BaCe 0.9 Er 0.1 O 3−α was 10.9 mW·cm −2 at 700 • C. Because the fuel cell was supported by the electrolyte and the electrolyte was thicker (1.0 mm), the current and power density were relatively low. When the voltage was 0.6 V, the maximum output power density of BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 was 115.9 mW·cm −2 at 700 • C, which is ten times higher than that of BaCe 0.9 Er 0.1 O 3−α . The results show that BaCe 0.9 Er 0.1 O 3−α -K 2 SO 4 -BaSO 4 is an excellent electrolyte material for medium-temperature fuel cells. The conduction characteristics of BaCe0.9Er0.1O3−α and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 were tested by adjusting different proportions of gases. As seen in Figure 6, the conductivities of the samples in a reductive atmosphere are very close to those in an oxidizing atmosphere. The logσ~log (pO 2 ) curves of BaCe0.9Er0.1O3−α and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 were almost horizontal straight lines. When the temperature exceeded the melting point of sulphate salts, the mobility of various ions (Ba 2+ , Li + , K + , H + ) was greatly enhanced, which led to a low activation energy for ion transport in the interface regions. The proton was the smallest cation, and the mobility of protons was greater than other ions (Li + , K + ), resulting in an increased conductivity. Therefore, ion conduction appeared to become dominant [34]. Hydrogen/oxygen fuel cells were assembled with BaCe0.9Er0.1O3−α and BaCe0.9Er0.1O3−α-K2SO4-BaSO4 as supporting electrolytes and Pd-Ag as electrodes. The current-voltage characteristic curves are shown in Figure 7. The resistance directed from current-voltage characteristic curve of BaCe0.9Er0.1O3−α-K2SO4-BaSO4 (2.76 Ω) was lower than that of the value (5.54 Ω) from AC impedance at 700 °C, implying that the protonic conduction was dominant under the fuel cell condition [45]. The maximum power density of BaCe0.9Er0.1O3−α was 10.9 mW·cm −2 at 700 °C. Because the fuel cell was supported by the electrolyte and the electrolyte was thicker (1.0 mm), the current and power density were relatively low. When the voltage was 0.6 V, the maximum output power density of BaCe0.9Er0.1O3−α-K2SO4-BaSO4 was 115.9 mW·cm −2 at 700 °C, which is ten times higher than that of BaCe0.9Er0.1O3−α. The results show that BaCe0.9Er0.1O3−α-K2SO4-BaSO4 is an excellent electrolyte material for medium-temperature fuel cells.

Conflicts of Interest:
The authors declare no conflicts of interest.