Pr- and Sm-Substituted Layered Perovskite Oxide Systems for IT-SOFC Cathodes

: In this study, the phase synthesis and electrochemical properties of A / A // A /// B 2 O 5+d (A / : Lanthanide, A // : Ba, and A // : Sr) layered perovskites in which Pr and Sm were substituted at the A/-site were investigated for cathode materials of Intermediate Temperature-Operating Solid Oxide Fuel cells (IT-SOFC). In the Pr x Sm 1 − x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1–0.9) systems, tetragonal (x < 0.4) and orthorhombic (x ≥ 0.5) crystalline structures were conﬁrmed according to the substitution amount of Pr, which has a relatively large ionic radius, and Sm, which has a small ionic radius. All of the layered perovskite oxide systems utilized in this study presented typical metallic conductivity behavior, with decreasing electrical conductivity as temperature increased. In addition, Pr 0.5 Sm 0.5 Ba 0.5 Sr 0.5 Co 2 O 5+d (PSBSCO55), showing a tetragonal crystalline structure, had the lowest conductivity values. However, the Area-Speciﬁc Resistance (ASR) of PSBSCO55 was found to be 0.10 Ω cm 2 at 700 ◦ C, which is lower than those of the other compositions.


Introduction
Solid Oxide Fuel Cells (SOFCs) are energy converters that directly convert the chemical energy of oxygen and hydrogen into electrical energy at high temperatures. Because they are made of ceramic and run at high temperatures (600-1000 • C), SOFCs have the advantage of not requiring any additional precious metal catalyst. There is a problem, however, in their long-term performance, such as in the chemical reaction of the electrode and the poisoning of Cr when devices are operating at high temperatures [1][2][3][4].
To solve these problems, many domestic and overseas research institutes are focusing on research and development toward Intermediate Temperature-Operating Solid Oxide Fuel Cells (IT-SOFCs). In the case of these IT-SOFCs, however, problems such as relatively lower ion conductivity and Oxygen Reduction Reaction (ORR) of the cathode when operating at a relatively low temperature have been reported, making it essential to develop advanced cathode materials.
For example, the properties of PrBaCo 2 O 5+d (PBCO), in which the lanthanide site of LnBaCo 2 O 5+d was replaced by Pr, were reported; the compound was found to show excellent Area-Specific Resistance (ASR). The layered perovskite oxide system formed a crystalline structure for the layered order of [CoO 2 6 ], in which it was shown that two types of cobalt coordination environments such as pyramidal CoO 5 and octahedral CoO 6 co-existed. Oxygen vacancy in the [LnO d ] layer was acknowledged to prevent spin glass behavior, resulting in excellent oxygen mobility and surface properties [5,6].

]-[BaO]-[CoO 2 ]-[LnO
Based on these studies, our group synthesized SmBa 0.5 Sr 0.5 Co 2 O 5+d (SBSCO) using a cathode mixed with Ce 0.9 Gd 0.1 O 2 (CGO91) and SBSCO at a mass ratio of 1:1; a cathode material with an ASR of 0.019 Ω cm 2 at a temperature of 700 • C was developed [7][8][9]. In addition, from the composition of Sm 1−x Nd x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) that our group recently reported, the crystalline structure was confirmed through the ionic radius substituted at the A / -site in the chemical composition of the layered perovskite oxide system A / A // A /// B 2 O 5+d (A / : Lanthanide, A // : Ba and A /// : Sr). In particular, Sm 0.2 Nd 0.8 Ba 0.5 Sr 0.5 Co 2 O 5+d (SNBSCO8), reported as having a tetragonal crystalline structure, was found to have an excellent ASR of 0.092 Ω cm 2 at 700 • C [10].
In this study, in order to develop a cathode that can be used in IT-SOFC, Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) layered perovskite oxide systems were synthesized by substituting Pr, with a larger ionic radius than that of Sm, in the A / -site based on SBSCO of the layered perovskite oxide system. Phase synthesis, electrical conductivity, electrochemical analysis, and X-ray Photoelectron Spectroscopy (XPS) analyses of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) were conducted for the cathode materials of IT-SOFC.

Solid State Reaction, X-ray Diffraction Measurement, and Microstructure Analysis
Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-1) layered perovskite oxide systems were synthesized by means of a traditional solid-state reaction (SSR).
To ensure an accurate experiment, reagent-grade oxides and carbides such as Praseodymium Oxide (Pr 6 O 11 , High Purity Chemicals, Kojundo, Japan), Samarium Oxide (Sm 2 O 3 , Alfa Aeser, Haverhill, MA, USA), Cobalt Oxide (Co 3 O 4 , Alfa Aeser), Barium Carbonate (BaCO 3 , Alfa Aeser), and Strontium Carbonate (SrCO 3 , Alfa Aeser) were dried in an electric furnace for 1 h under 150 • C conditions and then accurate weights were obtained. Table 1 summarizes the chemical compositions and abbreviations used in this study. A Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-1) layered perovskite composition was mixed through physical methods such as agate mortar and ball mill processes using an ethanol solvent, and then calcined at 1000 • C and 1100 • C for 6 h and 8 h, respectively, under air condition.
X-ray diffraction (XRD) patterns of the calcined samples were obtained by means of an X-ray diffraction analyzer (Model D/MAX 2500, Rigaku). An XRD measurement system was operated at 45 kW and 200 mA. The data were collected from 2θ = 10 to 90 • and the obtained data were matched with reference data for the identification of crystal structures [7][8][9][10]. The microstructure of a symmetrical half-cell was investigated using field-emission scanning electron microscopy (FE-SEM, JSM-7600 F, JEOL) combined with energy-dispersive spectroscopy.

Electrical Conductivity Analysis
To measure the electrical conductivity of the layered perovskite oxide systems, Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) oxide systems were put into a metal mold with a size of 6 mm × 25 mm and compressed to prepare rectangular-shaped bars. After sintering at 1100 • C for 3 h, bars showing sizes of 5 mm × 4 mm × 25 mm were prepared.
The bar-type samples were then connected using Pt-wire and Pt paste for a general 4-terminal DC method. These were measured at 50 • C intervals in a temperature range of 50-900 • C via a Keithley 2400 source meter.

Electrochemical Characterization
In order to fabricate a half cell, 2.5 g of Ce 0.9 Gd 0.1 O 2 (CGO91, Rhodia, Frankfurt, Germany) powder, an electrolyte material, was weighed and compression-molded at a pressure of 2 × 10 3 kg/m 2 ; it was then sintered at 1450 • C for 6 h. Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0-1) layered perovskite oxide systems, synthesized by SSR for electrochemical analysis, were fabricated as a cathode ink using Alpha-terpineol (KANTO CHEMICAL), Butvar (SIGMA Aldrich), and acetone. Afterwards, this ink was coated on the CGO91 electrolyte substrate by screen printing. After this material was sintered at 1000 • C for 1 h, a half-cell was finally prepared.
The electrochemical properties of the fabricated layered perovskite oxide half-cell were measured by means of an electrochemical analyzer (Model nStat, HS Technologies, Cavite, Philippine) under a frequency range of 0.05 Hz to 2.5 MHz and air conditions in a 500-900 • C temperature range with steps of 50 • C at a rate of 5 • C/min.

X-ray Photoelectron Spectroscopy (XPS) Analysis
X-ray photoelectron spectroscopy (XPS) measurements were carried out on Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d samples calcined at 1100 • C for 8 h.
For analysis of these samples, XPS was performed on a PHI VersaProbe XPS Microprobe with a monochromatized Al Ka source (hv = 1486.6 eV) under conditions of 25 W and 15 kV. The XPS data were analyzed using PeakFit version 4. Calibration of the binding energy (BE) was conducted with respect to the C 1s peak fixed at 284.4 eV.

XRD Analysis and Microstructure
Based on SmBa 0.5 Sr 0.5 Co 2 O 5+d (SBSCO), the XRD results of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0-0.9), which stepwise-substituted Praseodymium (Pr) with a relatively large ionic radius in the place of Samarium (Sm), are summarized in Figure 1.
From Figure 1, when the specific peaks of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d oxide systems measured at about 23, 33, 41, 47, 59, 69, and 78 o were compared with those of SBSCO, these XRD patterns corresponded to the same peaks of SBSCO, implying that these oxide systems were synthesized as a layered perovskite crystal structure [7,8].
In a similar research result, the composition of Nd x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) that our group recently reported represented a change in the crystalline structure due to the splitting and merging of XRD peaks. In other words, in the chemical composition of A / A // A /// B 2 O 5+d (A / : Lanthanide, A // : Ba and A /// : Sr), a layered perovskite oxide system, an orthorhombic crystalline structure was found when the A/-site was substituted by Sm, which had a relatively small ionic radius, while a tetragonal crystalline structure was formed when Nd, which had a relatively large ionic radius, was substituted.
Upon comparison of the previous results described above and the results summarized in Figure 1, it can be seen that all the XRD peaks were split in the case of Pr 0.3 Sm 0.7 Ba 0.5 Sr 0.5 Co 2 O 5+d (PSBSCO37) and Pr 0.1 Sm 0.9 Ba 0.5 Sr 0.5 Co 2 O 5+d (PSBSCO19). This showed the same phenomenon as the split XRD patterns of SBSCO, SmBaCo 2 O 5+d From Figure 1, when the specific peaks of PrxSm1-xBa0.5Sr0.5Co2O5+d oxide systems measured at about 23, 33, 41, 47, 59, 69, and 78 o were compared with those of SBSCO, these XRD patterns corresponded to the same peaks of SBSCO, implying that these oxide systems were synthesized as a layered perovskite crystal structure [7,8].
In a similar research result, the composition of NdxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) that our group recently reported represented a change in the crystalline structure due to the splitting and merging of XRD peaks. In other words, in the chemical composition of A / A // A /// B2O5+d (A / : Lanthanide, A // : Ba and A /// : Sr), a layered perovskite oxide system, an orthorhombic crystalline structure was found when the A/-site was substituted by Sm, which had a relatively small ionic radius, while a tetragonal crystalline structure was formed when Nd, which had a relatively large ionic radius, was substituted.
Therefore, the crystal structures of PSBSCO37 and PSBSCO19, with a lower molar ratio of Pr than Sm, existed in the orthorhombic form in PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0-0.9) oxide systems. However, the structures of PSBSCO91, PSBSCO73, and PSBSCO55, with the same or higher Pr substitution compared to that of Sm, existed in a tetragonal from.
Therefore, the crystal structures of PSBSCO37 and PSBSCO19, with a lower molar ratio of Pr than Sm, existed in the orthorhombic form in Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0-0.9) oxide systems. However, the structures of PSBSCO91, PSBSCO73, and PSBSCO55, with the same or higher Pr substitution compared to that of Sm, existed in a tetragonal from.
The field-emission scanning electron microscopy (FE-SEM) images in Figure 2 show the cross-section view of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) oxide systems of a symmetrical half-cell. It was confirmed that all of the cathode materials sintered a good balance of the conflicting electrode requirement of maintaining a porous and cathode thickness.

Electrical Conductivity of Pr
The electrical conductivity results of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.0-0.9) layered perovskite oxide systems are summarized in Figure 3.
As was confirmed in Figure 3a, all compositions of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.0-0.9) showed typical metallic conductivity behavior in which the conductivity decreased with an increasing temperature, similar to results reported for SBSCO and SNBSCO in the existing literature [7][8][9][10]20].
Perovskite or layered perovskite, which both have high electrical conductivity values in the relatively low-temperature region, were mainly associated with the change of the charge state of cobalt (Co) substituted at the B-site. For example, changes from a charge state of Co 3+ to Co 2+ and Co 4+ were observed; from this, it could be interpreted that an increase in the concentration of Co 4+ , which could be explained by small-polaron hopping, caused the higher electrical conductivity in the low-temperature region compared with the high-temperature region [10,11].
Perovskite or layered perovskite, which both have high electrical conductivity values in the relatively low-temperature region, were mainly associated with the change of the charge state of cobalt (Co) substituted at the B-site. For example, changes from a charge state of Co 3+ to Co 2+ and Co 4+ were observed; from this, it could be interpreted that an increase in the concentration of Co 4+ , which could be explained by small-polaron hopping, caused the higher electrical conductivity in the low-temperature region compared with the high-temperature region [10,11].
On the other hand, electrical conductivity decreased in the high-temperature region. This could be interpreted as showing a decrease in the charge state from Co 4+ to Co 3+ and a sharp increase in the oxygen vacancy concentration, which increased as a function of the temperature inside the layered perovskite lattice, causing the decrease in electrical conductivity [10,11]. It can be seen that the electrical conductivity slope in the low-temperature region and the conductivity reduction tendency in the high-temperature region changed in the temperature range of 300-350 °C .
This temperature range was interpreted as the boundary between the low and hightemperature regions. This same phenomenon was found in the SBSCO composition in this study. Further, results of the Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) show that the temperature range of 300-350 °C was the same temperature range in which weight loss of the layered perovskite occurred. This phenomenon was related to the rapid occurrence of oxygen vacancies [11].
Therefore, the PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.0-0.9) oxide system applied to this study could also be judged to have the same characteristics as the existing results. In other words, the decrease in electrical conductivity at 300-350 °C found in the PrxSm1- On the other hand, electrical conductivity decreased in the high-temperature region. This could be interpreted as showing a decrease in the charge state from Co 4+ to Co 3+ and a sharp increase in the oxygen vacancy concentration, which increased as a function of the temperature inside the layered perovskite lattice, causing the decrease in electrical conductivity [10,11]. It can be seen that the electrical conductivity slope in the low-temperature region and the conductivity reduction tendency in the high-temperature region changed in the temperature range of 300-350 • C.
This temperature range was interpreted as the boundary between the low and hightemperature regions. This same phenomenon was found in the SBSCO composition in this study. Further, results of the Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) show that the temperature range of 300-350 • C was the same temperature range in which weight loss of the layered perovskite occurred. This phenomenon was related to the rapid occurrence of oxygen vacancies [11].
Therefore, the Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.0-0.9) oxide system applied to this study could also be judged to have the same characteristics as the existing results. In other words, the decrease in electrical conductivity at 300-350 • C found in the Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d composition was interpreted as being due to the rapid generation of oxygen vacancies in the crystal.
The electrical conductivity values of each composition at specific temperatures (500, 600, 700, and 900 • C) are summarized in Figure 3b. In the case of the Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) oxide system, differences in conductivity occurred depending on the composition. As the substitution amount of Pr existing in A / of A / A // A /// B 2 O 5+d in the Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d increased (as the substitution amount of Sm decreased), the conductivity decreased and the lowest conductivity properties were found in PSBSCO55, which had substitution ratios of 1 for Pr and Sm. Hence, it can be inferred that as the substitution amount of Pr decreased (as the substitution amount of Sm increased), the value of electrical conductivity increased.
In summary, it was confirmed that the electrical conductivity was decreased when Pr was substituted in the layered perovskite SBSCO. Therefore, the lowest conductivity values of the PSBSCO55 composition of the Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) oxide system have been found. However, these values were meant to be higher than the minimum electrical conductivity (100 S/cm) required for the IT-SOFCs cathode material [22]. Furthermore, all compositions were confirmed to have lower electrical conductivity values than those of SBSCO and PBSCO, but higher conductivity values than that of NBSCO [7,8,17,19].  . Impedance plots of PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) oxide systems measured at 700 °C in air on dense CGO91 electrolyte. The number in these plots corresponds to the logarithm of frequency. Figure 5 summarizes the ASRs according to the temperatures of all PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) layered perovskite oxides. In contrast to the low ASRs of cathodes with high electrical conductivity, the lowest ASR value can be found in PSB-SCO55, which has relatively low conductivity in the composition of PrxSm1-xBa0.5Sr0.5Co2O5+d. These properties will be further explained using the results of X-ray photoelectron spectroscopy surface analysis XPS.  Figure 5 summarizes the ASRs according to the temperatures of all Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) layered perovskite oxides. In contrast to the low ASRs of cathodes with high electrical conductivity, the lowest ASR value can be found in PSBSCO55, which has relatively low conductivity in the composition of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d . These properties will be further explained using the results of X-ray photoelectron spectroscopy surface analysis XPS.  Figure 5 summarizes the ASRs according to the temperatures of all PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) layered perovskite oxides. In contrast to the low ASRs of cathodes with high electrical conductivity, the lowest ASR value can be found in PSB-SCO55, which has relatively low conductivity in the composition of PrxSm1-xBa0.5Sr0.5Co2O5+d. These properties will be further explained using the results of X-ray photoelectron spectroscopy surface analysis XPS.  . From these experimental results, it can be confirmed that, among the oxides of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) layered perovskite, the activation energy of the oxide system with a tetragonal crystalline structure presented the same tendency as the existing results, showing a value relatively lower than the activation energy of the orthorhombic crystalline structure (PSBSCO19, PSB-SCO37) [7,8]. Figure 6 provides a summary of the relationships between the crystalline structure and electrical conductivity-ASR property of Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d . The activation energy values, calculated at 550-900 °C , of tetragonal crystalline structures PSBSCO55, PSBSCO73, and PSBSCO91 (1.23, 1.17, and 1.23 eV) were relatively lower than those of the orthorhombic crystalline structures SBSCO (1.21 eV), PSBSCO19 (Ea = 1.37 eV), and PSBSCO37 (Ea = 1.26 eV). From these experimental results, it can be confirmed that, among the oxides of PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) layered perovskite, the activation energy of the oxide system with a tetragonal crystalline structure presented the same tendency as the existing results, showing a value relatively lower than the activation energy of the orthorhombic crystalline structure (PSBSCO19, PSBSCO37) [7,8]. Figure 6 provides a summary of the relationships between the crystalline structure and electrical conductivity-ASR property of PrxSm1-xBa0.5Sr0.5Co2O5+d. Based on the layered perovskite SBSCO, it was confirmed that the electrical conductivity decreased when Pr was substituted for the A-site. In PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9), the tetragonal crystalline structure was identified when there was a relatively  Based on the layered perovskite SBSCO, it was confirmed that the electrical conductivity decreased when Pr was substituted for the A-site. In Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9), the tetragonal crystalline structure was identified when there was a relatively large ionic radius (Pr > Sm); lower electrical conductivity values were also obtained by replacing Pr by 50 mol % or more in Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d perovskite oxide systems. In addition, the PSBSCO55 composition showed the lowest electrical conductivity values at all temperature conditions measured. However, PSBSCO55 showed the lowest ASR values in these experiments.  In general, it is accepted that a Co 2p spectrum can be separated into a Co 2p3/2 spectrum with low binding energy (BE) and a Co 2p1/2 with high BE. In this study, it was likewise verified that all measured Co 2p3/2 spectra of the layered perovskite oxide systems were in the lower BE range of about 779.0-780.0 eV. In addition, it can be seen that Co 2p1/2 spectra existed in the higher BE range of about 794.0-795.0 eV [21,25,26].

XPS Spectra of Cobalt
Further, the range of 777.1-777.9 eV found in the PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) layered perovskite oxide systems shown in Figure 7 was similar to the BE range of the 2p3/2 found at 777.3 eV for Co metal [21]. Furthermore, the BE region of 792.0-792.5 eV was similar to the BE range of Co metal 2p1/2 found at 792.4 eV. Therefore, all the PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) layered perovskite oxide systems measured in this experiment showed that Co metal was present on the surface [21,27].
According to the XPS results corresponding to SBSCO which were studied by our group, relatively high electrical conductivity properties were present because a high concentration of Co 4+ was apparent at relatively low temperatures in the layered perovskite oxide system in which Co was substituted at the B-site [21]. In other words, it was clear that the presence of Co 4+ and high conductivity were directly proportional. In the case of  In general, it is accepted that a Co 2p spectrum can be separated into a Co 2p 3/2 spectrum with low binding energy (BE) and a Co 2p 1/2 with high BE. In this study, it was likewise verified that all measured Co 2p 3/2 spectra of the layered perovskite oxide systems were in the lower BE range of about 779.0-780.0 eV. In addition, it can be seen that Co 2p 1/2 spectra existed in the higher BE range of about 794.0-795.0 eV [21,25,26].
Further, the range of 777.1-777.9 eV found in the Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) layered perovskite oxide systems shown in Figure 7 was similar to the BE range of the 2p3/2 found at 777.3 eV for Co metal [21]. Furthermore, the BE region of 792.0-792.5 eV was similar to the BE range of Co metal 2p 1/2 found at 792.4 eV. Therefore, all the Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) layered perovskite oxide systems measured in this experiment showed that Co metal was present on the surface [21,27].
According to the XPS results corresponding to SBSCO which were studied by our group, relatively high electrical conductivity properties were present because a high concentration of Co 4+ was apparent at relatively low temperatures in the layered perovskite oxide system in which Co was substituted at the B-site [21]. In other words, it was clear that the presence of Co 4+ and high conductivity were directly proportional. In the case of the spectra located at 786.0-790.0 eV of SBSCO, the crystalline structure was orthorhombic, and this property was caused by Co 2+ and Co 4+ ; at 804.0-805.0 eV, the phenomenon was caused by the coexistence of Co 3+ and Co 4+ [21,27,28].
The BEs of the Co 2+ and Co 4+ spectra of the PSBSCO19 and PSBSCO91 compositions, showing relatively high electrical conductivity, were located at 789.3 and 787.5 eV, respectively; the area % of the spectra was confirmed to be about 0.81 and 0.95, respectively, as shown in Table 2. The BEs of the Co 3+ and Co 4+ spectra were located at 804.8 and 804.3 eV, respectively, and the area % of the spectra was confirmed to be approximately 5.49 and 5.45%, respectively. On the other hand, in the case of PSBSCO55, which showed the lowest electrical conductivity, the BEs of Co 2+ and Co 4+ were located at 788.2 eV and the area % occupied by Co 2+ and Co 4+ in PSBSCO55 was 1.73. It was confirmed that the Co 3+ and Co 4+ spectra were located at 804.3 eV, occupying an area % of 4.38.
The relationships between the crystalline structure and the area percentage of Co, when Co has charge states of +2, +3, and +4, as calculated through the results of this experiment, with electrical conductivity of 50 • C, are summarized in Figure 8. The reason that the electrical conductivity was high in PSBSCO19 and PSBSCO91 and the electrical conductivity was low in PSBSCO55 was the correlation between Co 3+ + Co 4+ area % and Co 2+ + Co 4+ area %. In other words, the effects of the electrical conductivity and the Co 2+ + Co 4+ area % were inversely proportional. On the other hand, the electrical conductivity behavior was proportional to Co 3+ + Co 4+ area %. It is clear that the property of high electrical conductivity was caused by Co 4+ , which has a relatively high concentration at a high area % of Co 3+ + Co 4+ . the spectra located at 786.0-790.0 eV of SBSCO, the crystalline structure was orthorhombic, and this property was caused by Co 2+ and Co 4+ ; at 804.0-805.0 eV, the phenomenon was caused by the coexistence of Co 3+ and Co 4+ [21,27,28]. The BEs of the Co 2+ and Co 4+ spectra of the PSBSCO19 and PSBSCO91 compositions, showing relatively high electrical conductivity, were located at 789.3 and 787.5 eV, respectively; the area % of the spectra was confirmed to be about 0.81 and 0.95, respectively, as shown in Table 2. The BEs of the Co 3+ and Co 4+ spectra were located at 804.8 and 804.3 eV, respectively, and the area % of the spectra was confirmed to be approximately 5.49 and 5.45%, respectively. On the other hand, in the case of PSBSCO55, which showed the lowest electrical conductivity, the BEs of Co 2+ and Co 4+ were located at 788.2 eV and the area % occupied by Co 2+ and Co 4+ in PSBSCO55 was 1.73. It was confirmed that the Co 3+ and Co 4+ spectra were located at 804.3 eV, occupying an area % of 4.38.
The relationships between the crystalline structure and the area percentage of Co, when Co has charge states of +2, +3, and +4, as calculated through the results of this experiment, with electrical conductivity of 50 °C , are summarized in Figure 8. The reason that the electrical conductivity was high in PSBSCO19 and PSBSCO91 and the electrical conductivity was low in PSBSCO55 was the correlation between Co 3+ + Co 4+ area % and Co 2+ + Co 4+ area %. In other words, the effects of the electrical conductivity and the Co 2+ + Co 4+ area % were inversely proportional. On the other hand, the electrical conductivity behavior was proportional to Co 3+ + Co 4+ area %. It is clear that the property of high electrical conductivity was caused by Co 4+ , which has a relatively high concentration at a high area % of Co 3+ + Co 4+ .

XPS Spectra of Oxygen
The XPS spectra of O1s for PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) are shown in Figure  9. It can be seen that all layered perovskite oxide systems were split into three spectra according to the binding energy (BE) differences.

XPS Spectra of Oxygen
The XPS spectra of O1s for Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) are shown in Figure 9. It can be seen that all layered perovskite oxide systems were split into three spectra according to the binding energy (BE) differences.
In the low BE (LBE) region, the oxygen was inside the lattice. Oxygen ions inside the lattice became oxygen vacancies at high temperatures, which affected the adsorption and activation energy of oxygen molecules. In the high (HBE) BE region, this phenomenon was associated with the adsorption of hydroxyl species (OH-) and surfaces. Finally, in the intermediate BE (IBE) region, this phenomenon was associated with CO 3 2− [21,29,30]. Table 3 summarizes the peak shifts and areas for the corresponding deconvolution results shown in Figure 8. In the Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) layered perovskite oxide system, the LBE was in the range of 527.6-527.9 eV. The area % of orthorhombic structures of PSBSCO19 and PSBSCO37 was analyzed and found to be 17.07 and 18.51, with tetragonal structures PSBSCO55, PSBSCO73, and PSBSCO91 occupying approximately 20.29, 17.36, and 16.65 of area %.
For HBEs found between 530.7 and 530.8 eV, PSBSCO55 occupied the largest area % of 74.18. On the other hand, PSBSCO37 and PSBSCO19, whose substitution amounts of Sm were higher than that of PSBSCO55, showed a lower HBE area % value than that of PSBSCO55. This tendency was also found in the oxide systems, such as those of SBSCO and NdBaCo 2 O 5+d (NBCO), in which the A-site was partially removed [21,31]. In the low BE (LBE) region, the oxygen was inside the lattice. Oxygen ions inside the lattice became oxygen vacancies at high temperatures, which affected the adsorption and activation energy of oxygen molecules. In the high (HBE) BE region, this phenomenon was associated with the adsorption of hydroxyl species (OH-) and surfaces. Finally, in the intermediate BE (IBE) region, this phenomenon was associated with CO3 2- [21,29,30]. Table 3 summarizes the peak shifts and areas for the corresponding deconvolution results shown in Figure 8. In the PrxSm1−xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9) layered perovskite oxide system, the LBE was in the range of 527.6-527.9 eV. The area % of orthorhombic structures of PSBSCO19 and PSBSCO37 was analyzed and found to be 17.07 and 18.51, with tetragonal structures PSBSCO55, PSBSCO73, and PSBSCO91 occupying approximately 20.29, 17.36, and 16.65 of area %. For HBEs found between 530.7 and 530.8 eV, PSBSCO55 occupied the largest area % of 74.18. On the other hand, PSBSCO37 and PSBSCO19, whose substitution amounts of Sm were higher than that of PSBSCO55, showed a lower HBE area % value than that of PSBSCO55. This tendency was also found in the oxide systems, such as those of SBSCO and NdBaCo2O5+d (NBCO), in which the A-site was partially removed [21,31].
The results of XPS analysis and ASR performed at 700 °C are shown in Figure 10. In Figure 10, it can be seen that PSBSCO91 with a tetragonal structure and PSBSCO19 with  The results of XPS analysis and ASR performed at 700 • C are shown in Figure 10. In Figure 10, it can be seen that PSBSCO91 with a tetragonal structure and PSBSCO19 with an orthorhombic crystalline structure showed the highest ASR values. It can also be seen that PSBSCO55 with a tetragonal structure shows the lowest ASR value. The properties were inversely proportionate, as follows: When the area % in LBE was large, ASR had its lowest value; on the contrary, ASR had its highest value when the area % was small. In addition, PSBSCO55, which had the largest area % (74.18%) in the range of HBE, had the lowest ASR of all samples. Comparing these results, LBE and HBE with a high area % had effectively lower ASR values.
However, ASR is a resistance that occurs when oxygen molecules are electrochemically reduced from the three-phase boundary (TPB) of an electrolyte/cathode/oxygen to oxygen ions. In order to achieve low ASR properties, therefore, the interface properties where the area of the three-phase interface was relatively extended were very important. In other words, since the value of HBE is directly related to this property, it can be asserted that PSBSCO55, with its outstanding surface oxygen activity, had the lowest ASR properties. Therefore, compared to the properties of oxygen ions in the lattice, low ASR was directly related to the surface or interface HBE properties [21,[29][30][31]. that PSBSCO55 with a tetragonal structure shows the lowest ASR value. The properties were inversely proportionate, as follows: When the area % in LBE was large, ASR had its lowest value; on the contrary, ASR had its highest value when the area % was small. In addition, PSBSCO55, which had the largest area % (74.18%) in the range of HBE, had the lowest ASR of all samples. Comparing these results, LBE and HBE with a high area % had effectively lower ASR values. However, ASR is a resistance that occurs when oxygen molecules are electrochemically reduced from the three-phase boundary (TPB) of an electrolyte/cathode/oxygen to oxygen ions. In order to achieve low ASR properties, therefore, the interface properties where the area of the three-phase interface was relatively extended were very important. In other words, since the value of HBE is directly related to this property, it can be asserted that PSBSCO55, with its outstanding surface oxygen activity, had the lowest ASR properties. Therefore, compared to the properties of oxygen ions in the lattice, low ASR was directly related to the surface or interface HBE properties [21,[29][30][31].
When Pr (x = 0.1 to 0.3) was substituted with Sm, an orthorhombic crystalline structure was formed. However, when the replacement amount of Pr (x = 0.5-0.9) was the same or higher than that of Sm, a tetragonal crystal structure was formed.
From electrical conductivity values of all chemical compositions, electrochemical analyses, and XPS analysis results, tetragonal Pr0.5Sm0.5Ba0.5Sr0.5Co2O5+d (PSBSCO55), possessing a low Co 4+ concentration in the Co 2p range, presented the lowest electrical conductivity values of 1091, 374, and 236 S/cm at 50, 600, and 700 °C . For PSBSCO55, which had the largest area % of LBE and HBE spectra in PrxSm1-xBa0.5Sr0.5Co2O5+d (x = 0.1-0.9), the ASR value was 0.10 Ωcm 2 at 700 °C , the best ASR among all chemical compositions, confirming that PSBSCO55 is suitable as a cathode material for IT-SOFCs.

Conclusions
In this study, Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9) was synthesized by substituting Pr, possessing a large ionic radius, and Sm, possessing a small ionic radius, at the A / -site in the chemical composition of A / A // A /// B 2 O 5+d (A / : Lanthanide, A // : Ba and A /// : Sr).
When Pr (x = 0.1 to 0.3) was substituted with Sm, an orthorhombic crystalline structure was formed. However, when the replacement amount of Pr (x = 0.5-0.9) was the same or higher than that of Sm, a tetragonal crystal structure was formed.
From electrical conductivity values of all chemical compositions, electrochemical analyses, and XPS analysis results, tetragonal Pr 0.5 Sm 0.5 Ba 0.5 Sr 0.5 Co 2 O 5+d (PSBSCO55), possessing a low Co 4+ concentration in the Co 2p range, presented the lowest electrical conductivity values of 1091, 374, and 236 S/cm at 50, 600, and 700 • C. For PSBSCO55, which had the largest area % of LBE and HBE spectra in Pr x Sm 1−x Ba 0.5 Sr 0.5 Co 2 O 5+d (x = 0.1-0.9), the ASR value was 0.10 Ωcm 2 at 700 • C, the best ASR among all chemical compositions, confirming that PSBSCO55 is suitable as a cathode material for IT-SOFCs. Data Availability Statement: The study did not report any data.