Heterointerface Effect in Accelerating the Cathodic Oxygen Reduction for Intermediate-Temperature Solid Oxide Fuel Cells.

A solid-state mixing method was adopted to prepare a new Pr0.8Sr0.2Fe0.7Ni0.3O3-δ-Pr1.2Sr0.8Fe0.4Ni0.6O4+δ (PSFN113-214) composite cathode oxide for the solid oxide fuel cells (SOFCs). Herein, heterointerface engineering was investigated for the performance enhancement. It was found that the oxygen vacancy content could be increased by mixing the PSFN214 with PSFN113, which gave rise to the formation of a heterostructure, and resulted in the promotion of oxygen ion transport as well as the specific surface area. The optimum mixing ratio 5:5 resulted in the highest oxygen vacancy content and the largest specific surface area, indicating the strongest interface effect. Polarization resistance of PSFN113-214 (5:5) was 0.029 Ω cm2 at 800°C, which was merely 24% of PSFN113 and 39% of PSFN214. The corresponding maximum power density was 0.699 W cm-2, which was nearly 1.44 times of PSFN113 and 1.24 times of PSFN214. Furthermore, the voltage attenuation rate after 100 h was merely 0.0352% h-1. Therefore, the new PSFN113-214 composite could be a prospective cathode oxide for SOFCs.

Introduction

The development of intermediate-temperature solid oxide fuel cells (IT-SOFCs) is severely limited because of its restricted cathodic oxygen reduction reaction (ORR) rate (Huan et al., 2021; Ahmad et al., 2022). Therefore, high-performance cathode materials have become a research hotspot (Ajaa et al., 2020).

Perovskite oxides possess extraordinary conductivity but poor surface mobility. Ruddlesden-Popper (R-P) phase oxides possess a special structure of alternating layers of perovskite and rock salt, which provides more oxygen vacancies as well as oxygen gaps. Unfortunately, their conductivity is relatively lower (Liu et al., 2016; Lee and Lee, 2017; Yu et al., 2019). Hence, it is impossible for single-phase materials to fully satisfy the conditions of cathode materials. Heterostructure cathode materials possess sufficient contact sites between oxygen and heterointerfaces, which can not only enhance electronic or ionic conductivity but also have potential to enhance stability and catalytic performance. It has thus become a hot research area for IT-SOFCs cathode materials. Studies have shown significantly enhanced electrochemical performance of heterogeneous composite cathodes compared to single-phase materials, as seen in La0.5Sr0.5CoO3-δ-LaSrCoO4±δ (Li et al., 2020), La0.6Sr0.4Co0.2Fe0.8O3−δ-La0.6Sr1.4Co0.2Fe0.8O4−δ (Wang et al., 2020), La0.6Sr0.4Co0.2Fe0.8O3−δ-La2NiO4+δ (Dong et al., 2019; Ghamarinia et al., 2020), PrSrFe0.5Co0.5O4-Pr0.4Sr0.6Fe0.5Co0.5O3 (Yu et al., 2019), and Nd0.5Sr0.5CoO3−δ-Nd0.8Sr1.2CoO4±δ (Zheng et al., 2019; Zheng et al., 2020a; Zheng et al., 2020b). However, cobalt-based materials have some disadvantages, such as poor chemical stability, high price, and high coefficient of thermal expansion. Consequently, Cobalt-free ABO3-A2BO4 heterocomposite cathodes come into our sight. For example, the introduction of Sr ions into Pr2Ni0.5Mn0.5O4−δ could form a PrOx-(PrSr)Ni0.5Mn0.5O3−δ-(PrSr)2(MnNi)O4−δ heterocomposite cathode. Among them, the heterostructure formed at the interface of (PrSr)Ni0.5Mn0.5O3−δ and (PrSr)2(MnNi)O4−δ could improve the electrochemical performance, and the maximum power density (PPD) enhanced from 483 to 960 mW cm−2 at 800°C (Yang et al., 2019). Introducing La0.5Sr1.5MnO4+δ into La0.5Sr0.5MnO3−δ to form La0.5Sr1.5MnO4+δ-La0.5Sr0.5MnO3−δ, the ORR would be broadened to entire cathode region, and PPD was 936 mW cm−2 at 700°C (Hou et al., 2020). Porous (La0.6Sr0.4)0.98FeO3−δ electrodes impregnated with aqueous nitrate solutions of Pr2Ni0.6Cu0.4O4 and Pr2Ni0.7Cu0.3O4 greatly reduced the polarization resistance, from 0.98 Ω cm2 to 0.13 Ω cm2 and 0.16 Ω cm2 at 650°C (Khoshkalam et al., 2020).

Among ABO3 perovskite materials, Pr1−xSrxFe1−yNiyO3−δ has broad application prospects due to its outstanding electrocatalytic activity and excellent electrical conductivity (Hashimoto et al., 2005; Larramendi et al., 2007; Pinedo et al., 2011; Giuliano et al., 2017; Liu et al., 2017). Especially, the conductivity of the Pr0.7Sr0.3Fe0.7Ni0.3O3−δ cathode at 600°C was up to 450 S cm−1, and the electrochemical performance was similar to La0.6Sr0.4Fe0.8Co0.2O3−δ (Hashimoto et al., 2005). Cathode materials containing Sr are prone to segregation of SrO during cell operation. Therefore, in the preparation process of ABO3 material, we could reduce the content of Sr as much as possible to obtain a more stable cathode material. Pr0.8Sr0.2Fe0.7Ni0.3O3−δ cathode reduced Rp by only 6% within 200 h, and was stable within 1000 h (Giuliano et al., 2017). In A2BO4 materials, Ln2NiO4+δ (Ln = La, Pr, Nd) material is the most widely studied cathode material. Pr2NiO4 (PNO) had the lowest polarization resistance and the highest oxygen surface exchange coefficient (k*) and diffusion coefficient (D*) therein (Bansod et al., 2018; Kim et al., 2019). However, its thermal stability was poor, and it was easy to decompose during operation. The structure stability is improved by doping Sr2+ at the Pr site (Bhoga et al., 2014; Kim and Lee, 2021). Among Pr2−xSrxNiO4 (x = 0.3, 0.5, and 0.8) cathode materials, Pr1.2Sr0.8NiO4 had the lowest area-specific resistance value when the Sr doping amount was 0.8, down to 0.112 Ω cm2 at 800°C (Yang et al., 2012). In addition, the substitution of Ni sites with Fe can increase the oxygen surface exchange and show excellent performance. For example, in the La1.5Sr0.5Ni1−yFeyO4+δ series, iron doping promoted the bulk diffusion of the sample, and the oxygen surface exchange was significantly increased when y = 0.4 (Gilev et al., 2018). La1.2Sr0.8Ni0.6Fe0.4O4+δ had the performance suitable for IT-SOFCs cathode. Its polarization resistance (Rp) was 0.078 Ω cm2, and its PPD was up to 781 mW cm−2 at 700°C (Miao et al., 2019). Therefore, this study chose Pr0.8Sr0.2Fe0.7Ni0.3O3−δ (PSFN113) and Pr1.2Sr0.8Fe0.4Ni0.6O4+δ (PSFN214) as the two components of the composite material. The structure, compatibility, microstructure, specific surface area, and electrochemical activity of single-phase materials and heterogeneous composite cathode materials were studied comparatively, and their potential as IT-SOFCs was evaluated.

Experimental

Chemicals

The chemicals utilized in this study, including Pr(NO3)3·6H2O (Aladdin, 99.9%), Sr(NO3)2 (Aladdin, 99%), Fe(NO3)3·9H2O (Aladdin, 98.5%) and Ni(CH3COO)2·6H2O (Aladdin, 99.9%), are used as received without further purification.

Preparation of Cathode Materials

Single-phase Pr0.8Sr0.2Fe0.7Ni0.3O3−δ (PSFN113) and Pr1.2Sr0.8Fe0.4Ni0.6O4+δ (PSFN214) cathode powders were synthesized via the sol-gel method. First, citric acid was completely dissolved in distilled water under stirring. Raw materials including Pr(NO3)3·6H2O (Aladdin, 99.9%), Sr(NO3)2 (Aladdin, 99%), Fe(NO3)3·9H2O (Aladdin, 98.5%), and Ni(CH3COO)2·6H2O (Aladdin, 99.9%) were joined in sequence in corresponding proportions. The proportion of citric acid was twice than of metal ions. Subsequently, 2 g of polyethylene glycol powder was added. After stirring for 2–3 h, the above solution was dehydrated in a water bath at 80°C for 12 h to gain a dry gel. This dry gel was heated on a hot plate to form a precursor powder. After sufficient grinding, the precursor powder was calcined in a muffle furnace at 600°C for 4 h. Ultimately, this pre-fired powder was ground and calcined in a muffle furnace at 900°C for 4 h to obtain the desired cathode material.

Pr0.8Sr0.2Fe0.7Ni0.3O3-δ-Pr1.2Sr0.8Fe0.4Ni0.6O4+δ (PSFN113-214) heterogeneous composite cathode was synthesized via solid-state mixing method, in which mass ratio of PSFN113 and PSFN214 was 4:6, 5:5 and 6:4 respectively. First, PSFN113 powder was poured into ethanol solvent according to the corresponding proportion, and zirconia ball was milled to make it evenly distributed. Subsequently, PSFN214 powder of the corresponding quality was poured into it and ball milled for 24 h. Above-mentioned mixture solution was put in an oven and dried continuously at 80°C for 12 h to gain dry powder. Finally, the gained powder was ground in a mortar for 48 h to gain a uniformly mixed composite cathode.

Cell Construction

The symmetrical cell was assembled as follows. First, Ce0.8Gd0.2O1.9 (GDC) powder was pressed at 10 MPa into a disc with a diameter of 13 mm and a thickness of 1 mm. A compact GDC electrolyte sheet was formed via sintering at 1500°C for 5 h in muffle furnace. Then, 6wt% terpineol/ethyl cellulose binder was mixed with PSFN113, PSFN214, PSFN113-214 heterogeneous composite cathodes, and the required five kinds of cathode slurries were obtained after the grinding for several hours. Cathode slurry was symmetrically coated on both sides of the Ce0.8Gd0.2O1.9 (GDC) electrolyte by screen printing, calcined in a muffle furnace at 900°C for 2 h. Ultimately, the collector silver slurry was symmetrically coated in the grid structure on both sides of the electrolyte, and dried in an oven at 200°C for 2 h.

Similarly, a single cell was assembled. The GDC electrolyte sheet was polished to ∼250 μm and ultrasonically cleaned to obtain the required electrolyte sheet. NiO-GDC anode was prepared by mixing NiO and GDC at a mass ratio of 6:4. The required PSFN113, PSFN214, PSFN113-214 (5:5) cathode slurry, and NiO-GDC anode slurry were constructed as described above. First, NiO-GDC anode paste was screen printed on one side of the GDC electrolyte. After calcination at 1250°C for 4 h, the cathode slurry was symmetrically coated on the other side of the electrolyte. After calcinating at 900°C for 2 h, current collector silver paste was coated on both sides of the cell to form a grid structure.

Characterization

An X-ray diffractometer with a Cu Kα X-ray source (λ = 0.15406 nm, 40 kV, 200 mA) was used to collect the X-ray diffraction (XRD) data from 20° to 80°, so as to determine the purity, thermal stability, crystal structure and chemical compatibility of the synthetic powder. Fourier infrared spectrometer was utilized to characterize the functional group structure of the synthesized powder in the range of 400–4000 cm−1. A field emission scanning electron microscope equipped with an X-ray spectrometer (EDS) was utilized to observe the cathode powder morphology and cathode/electrolyte interface adhesion. A high-resolution transmission electron microscope was utilized to further verify the existence of a heterointerface. Under the condition of degassing at 180°C for 12 h, a rapid specific surface area analyzer was utilized to measure the specific surface area of the material. X-ray photoelectron spectrometer was utilized to determine the oxygen vacancy content on the surface and valence state change.

Electrochemical Test

The symmetrical cell was tested by electrochemical impedance spectroscopy (EIS) at 500–800°C using an electrochemical workstation. The current-voltage (I-V) and current-power density (I-P) curves of the single-cell were measured at 500–800°C using the SI 1287 electrochemical interface. Ultimately, the long-term stability of the single-cell was evaluated under the condition of a constant current density of 0.3 A cm−2 at 700°C.

Results and Discussion

Phase Analysis

To study the structure and phase purity of obtained cathode powder, XRD tests are performed on PSFN113, PSFN214, and PSFN113-214, as shown in Figure 1A. PSFN113 powder showed a good orthorhombic perovskite structure with a space group of Pbnm, which matched well with the crystal structure of La0.7Sr0.3Co0.5Fe0.5O3 (PDF#89-1267) (Ajaa et al., 2020). PSFN214 had a typical K2NiF4 type tetragonal structure with a space group of I4/mmm, which matched well with the crystal structure of Sr2FeO4 (PDF#82-0414) (Giuliano et al., 2017; Miao et al., 2019). The diffraction peaks of PSFN113-214 heterogeneous composites were entirely in accordance with the above-mentioned two single-phase cathode, and no other impurity peaks were observed. In addition, corresponding characteristic peak intensity raised little by little with the increase of PSFN214 mass ratio, indicating that three different proportions of heterogeneous composite cathodes had been synthesized.

FIGURE 1

(A) XRD of cathode powder; (B) infrared spectrum of cathode powder; (C) XRD of PSFN113-214 mixed in equal proportion at different calcination temperatures.

To further verify the (PSFN113:PSFN214) ratios of heterogeneous composite oxides, Fourier transforms infrared spectroscopy (FT-IR) tests are performed on all samples. Unlike simple ABO3 perovskites, A2BO4 consists of alternating perovskite (ABO3) and salt rock formations (AO) in the c-axis direction. Therefore, A2BO4 contains the characteristic peaks of ABO3 and AO formations. As shown in Figure 1B, the absorption peak intensity of the AO formation at 667 cm−1 gradually increased in proportion to the increase of PSFN214 content. It showed that the heterogeneous composite material had been mixed uniformly and satisfied the expected ratio change, which was consistent with the above-mentioned XRD peak intensity change.

To determine the stability of the PSFN113-214 heterogeneous composite cathode material, we mixed the quality mass of PSFN113 and PSFN214. After calcination at 900°C for 2 h and 1000°C for 10 h, XRD test is performed, as shown in Figure 1C. These results showed that the PSFN113-214 heterogeneous composite cathode still maintained its single-phase structure without any change in composition, which met the requirements of long-term operation.

Chemical Compatibility

In addition, the long-term stable operation of the cell is affected via chemical compatibility of the electrode and the electrolyte (Zhang et al., 2021). Therefore, PSFN113-214 (5:5) and GDC electrolyte oxides were mixed with equal mass in an ethanol medium, and the compatibility of the two was tested by calcination at 1000°C for 10 h, as shown in Figures 2A,B. XRD pattern only contained the characteristic peaks of the PSFN113-214 cathode/NiO anode component and the GDC electrolyte, and no other impurity peaks are detected. This showed that the electrode and the electrolyte existed stably with each other during the calcination process and could be used for long-term operation.

FIGURE 2

After calcination at 1000°C for 10 h, (A) XRD patterns of PSFN113-214 and GDC mixed in equal proportions and (B) XRD patterns of NiO and GDC mixed in equal proportions.

Microstructure Analysis

Electrode microstructure is one of the important factors affecting cell performance, which is closely related to material porosity and the three-phase interface (TPB) area (Han et al., 2021). The microstructure of the cathode material is shown in Figure 3A–E. All samples had a small and uniform particle size, good connectivity between the two-phase particles, and a porous structure. The porous structure facilitates gas diffusion and oxygen ion transport and also provides adequately active sites for ORR. In addition, heterocomposite cathode had no obvious bidirectional characteristics in appearance, indicating that the two single phases constituting the composite cathode were fully mixed and tightly wound together.

FIGURE 3

FE-SEM images of (A) PSFN113, (B) PSNF214, (C) PSFN113-214 (6:4), (D) PSFN113-214 (5:5), and (E) PSFN113-214 (4:6) cathode microstructure. (F) Specific surface area of all samples.

To further clarify the microstructure of cathode material, N2 adsorption-desorption test is carried out, as shown in Figure 3F. The results showed that the specific surface area of the five samples with mass ratios of PSFN113 and PSFN214 of 1:0, 0:1, 6:4, 5:5, 4:6 was 3.5399 m2 g−1 and 4.6552 m2 g−1 4.1249 m2 g−1, 5.1687 m2 g−1 and 4.5906 m2 g−1, respectively. It can be seen that when PSFN113 and PSFN214 particles were tightly wound to form a heterointerface, the specific surface area of the composite cathode oxides was greater than PSFN113, and the corresponding ORR reaction active sites were increased. When the mass ratio of PSFN113 to PSFN214 was 5:5, the specific surface area value was the largest, which was expected to have the best electrochemical performance.

Surface Chemical Environmental Analysis

To further investigate the influence of the presence of heterointerfaces on the oxygen reduction catalytic activity and structural stability of the cathode material, the orbitals of O 1s and Sr 3d were collected by XPS, as shown in Figure 4. Previous studies have shown that the O element can be separated into lattice oxygen (Olattice, ∼528.5 eV) and surface oxygen (Osurface) (Wang H. et al., 2019). Surface oxygen includes three types: O2− (∼529.6 eV), O− (∼531.1 eV), and (∼532.4 eV) (Bai et al., 2021). Among them, the content of Osurface ( O−) mainly affects the surface oxygen vacancies on the cathode and the ORR activity (Wang J. et al., 2019; Bai et al., 2021). So, the higher the content, the stronger the ORR catalytic activity. As shown in Table 1, the presence of heterointerface increased Osurface from 64.61% (PSFN113) and 83.96% (PSFN214) to 90.61% (PSFN113-214). It can be concluded that the existence of the heterointerface has a beneficial influence on the catalytic activity of oxygen reduction.

FIGURE 4

XPS spectra of PSFN113, PSFN214 and PSFN113-214 (5:5) cathode materials obtained for (A–C) O1s and (D–F) Sr 3d.

TABLE 1

The percentage of Osurface, Olattice, Srsurface, and Srlattice in PSFN113, PSFN214, and PSFN113-214 (5:5).

Cathode	Osurface (%)	Srsurface (%)	Srlattice (%)
PSFN113	64.61	56.42	43.59
PSFN214	83.96	47.84	52.16
PSFN113-214	90.61	49.88	50.12

Similarly, Sr elements can be separated into lattice strontium (Srlattice) and 1Q (Srsurface). Srlattice in perovskite lattices have less binding energies (∼131.7 eV, 3d5/2, ∼133.5 eV, 3d3/2) (Wang H. et al., 2019). Srsurface on perovskite surfaces has large binding energies (∼133.8 eV, 3d5/2; ∼135.2 eV, 3d3/2), such as Sr(OH)2, SrCO3, or SrO barrier layers (Bai et al., 2021). The higher the surface strontium content, the greater the degree of segregation of the Sr element, which will impair the electrical activity of the cell. Therefore, the less the content of Srsurface, the better the stability of the cathode (Wang J. et al., 2019; Bai et al., 2021). As shown in Table 1, the presence of heterointerfaces neutralized Srsurface from 56.42% (PSFN113) and 47.84% (PSFN214) to 49.88% (PSFN113-214). It can be concluded that the existence of the heterointerface has a neutralizing effect on the structural stability. Whether it can meet the requirements for long-term operation of cathode oxides will be confirmed in the follow-up long-term stability test.

High-Resolution Transmission Electron Microscope Analysis

To determine the micromorphology of the heterocomposite cathode material in one step, a high-resolution transmission electron microscope (HR-TEM) is utilized to observe the micromorphology of the PSFN113-214 (5:5) heterocomposite cathode, as shown in Figures 5A,B. It can be seen from Figure 5A that the PSFN113-214 heterocomposite cathode particles were tightly entangled and had a good contact area. Two distinct diffraction fringes can be observed in Figure 5B. The diffraction fringe with a pitch of 0.274 nm matched the orthogonal (112) plane of PSFN113, while the diffraction fringe with a pitch of 0.366 nm matched the (101) plane of the tetragonal phase of PSFN214 (Yu et al., 2019). There was an obvious interface between PSFN113 and PSFN214 nanoparticles. The electrons in PSFN113 could be transferred to PSFN214 through the heterointerface, and the oxygen ions in PSFN214 could also be transferred to PSFN113 through the heterogeneous interface. This synergistic effect of the two was expected to promote the catalytic activity of oxygen reduction. The above mechanism is shown in Figure 5C.

FIGURE 5

(A) TEM image, (B) HR-TEM image, and (C) performance improvement mechanism diagram of PSFN113-214 (5:5) heterocomposite cathode.

Electrochemical Impedance Spectroscopy Spectra

To research the effect of the presence of heterointerface on electrochemical activity, we constructed five symmetrical cells with mass ratios of PSFN113 and PSFN214 of 1:0, 0:1, 6:4, 5:5, and 4:6. At 500–800°C, the EIS test is performed on three symmetrical batteries at intervals of 50°C, as shown in Figures 6A–E. The cathodic polarization resistance (Rp) is the difference between the real axis intercepts of the impedance diagram (Li et al., 2017), and the corresponding values are shown in Table 2. Within the above temperature range, the RP of the three materials all followed the following sequence: PSFN113-214 (5:5)< PSFN113-214 (4:6)

FIGURE 6

Electrochemical impedance spectrum of a symmetrical cell: (A) PSFN113, (B) PSFN214, (C) PSFN113-214 (6:4), (D) PSFN113-214 (5:5), (E) PSFN113-214 (4:6), and (F) polarization resistance varies Arrhenius curve of temperature change.

TABLE 2

Rp values of PSFN113 and PSFN214 cathode oxides with different mass ratios.

Temperature/°C	1:0 RP/Ω·cm2	0:1 RP/Ω·cm2	6:4 RP/Ω·cm2	5:5 RP/Ω·cm2	4:6 RP/Ω·cm2
800	0.12	0.075	0.047	0.029	0.042
750	0.17	0.10	0.067	0.040	0.060
700	0.24	0.15	0.098	0.059	0.089
650	0.36	0.23	0.15	0.098	0.14
600	0.61	0.40	0.26	0.18	0.26
550	1.16	0.72	0.61	0.42	0.59
500	2.77	1.61	1.58	1.14	1.70

Interface Microstructures

To research the thermal compatibility of cathode material and electrolyte material in symmetrical cells, the cross-section FE-SEM of cathode and electrolyte is observed, as shown in Figure 7. It can be seen from the figure that both the electrolyte GDC and the cathode material had a clear interface. Particles were evenly distributed and very tightly attached to the GDC electrolyte, without obvious delamination and cracks, which was conducive to gas transmission and oxygen diffusion.

FIGURE 7

After the EIS test, the cross-sectional SEM of the symmetrical cell with (A) PSFN113, (B) PSFN214, (C) PSFN113-214 (6:4), (D) PSFN113-214 (5:5), and (E) PSFN113-214 (4:6) as the cathode.

Single Cell Performance

To further evaluate the effect of the heterointerface on the cathodic oxygen reduction reaction, an electrolyte-supported NiO-GDC/GDC/[PSFN113/PSFN214/PSFN113-214 (5:5)] single cell was constructed, denoted as Cell-I, Cell-II, and Cell-III. Figure 8A represents the cross-sectional morphology of a single cell. As shown in the figure, the thickness of the GDC electrolyte was about 250 μm, and the thickness of the cathode and anode was between 20 and 30 μm. The current-voltage (I-V) and current-power density (I-P) curves between 650 and 800°C are shown in Figures 8B–D. These corresponding values are displayed in Table 3. The maximum open-circuit voltage (OCV) of Cell-I, Cell-II, and Cell-III were 0.807, 0.898, and 0.836 V, respectively, which were all lower than the theoretical value of 1.04–1.1 V (Wang H. et al., 2019; Chen et al., 2020). In the high-temperature range, the OCV was further reduced. This was due to the partial reduction of Ce4+to Ce3+in the high-temperature reduction atmosphere, and the GDC electrolyte had a certain n-type conductivity, resulting in the internal short circuit of the cell and the decrease of OCV value (Bai et al., 2021; Han et al., 2021). At 650–800°C, the power densities of the three materials all followed the following order: PSFN113 < PSFN214 < PSFN113-214 (5:5), which corresponded to the above-mentioned RP results. At 800°C, the maximum power density (PPD) of Cell-III (0.699 W cm−2) was 1.44 times that of Cell-I (0.485 W cm−2) and 1.24 times that of Cell-II (0.562 W cm−2). Cell performance was significantly enhanced. The PPD value of the PSFN113-214 (5:5) cathode was compared with the cathode performance reported in the literature, which further illustrated the superiority of its performance, as shown in Table 4.

FIGURE 8

(A) Cross-sectional morphology of a single cell and I-V-P curves: (B) PSFN113, (C) PSFN214, and (D) PSFN113-214 (5:5).

TABLE 3

OCV (V) and PPD (W·cm−2) of Cell-I, Cell-II, and Cell-III in the range of 650–800°C.

Temperature (°C)	Cell-I		Cell-II		Cell-III
	OCV	PPD	OCV	PPD	OCV	PPD
800	0.748	0.485	0.779	0.562	0.733	0.699
750	0.777	0.235	0.801	0.481	0.755	0.586
700	0.778	0.146	0.856	0.409	0.801	0.528
650	0.807	0.103	0.898	0.347	0.836	0.333

TABLE 4

Comparison table of PPD value of PSFN cathode and other cathode reported in literature.

Cathode	Cell configuration	PPD/(W·cm−2)	Temperature/°C	Reference
Ba0.5Sr0.5Fe0.8Cu0.2O3-δ	NiO-GDC|GDC|BSFC	0.51	700	Bai et al. (2021)
Pr0.5Ba0.5Fe0.8Ni0.2O3-δ	NiO-GDC|GDC|PBFN	0.52	700	Meng et al. (2021)
Pr1.91Ni0.71Cu0.24Ga0.05O4-Ba0.5La0.5CoO3	Ni-Fe|LSGM| PNCG-BLC	0.117	400	Xie et al. (2013)
La0.6Ca0.4Fe0.8Ni0.2O3-δ-Sm0.2Ce0.8O1.9	LCFN-30SDC/SDC/LCFN-30SDC	0.303	800	Ding et al. (2017)
PrLaNiO4-(La0.75Sr0.2Ba0.05)0.175Ce0.825O1.891	NiO-SDC|SDC| PLNO-LSBC	0.606	800	Chiu et al. (2017)
Pr2NiO4-Pr0.2Ce0.8O1.9	NiO-GDC|GDC|PNO-PCO	0.57	800	Chen et al. (2020)
Ce0.8Sm0.2O2-δ-La0.25Sr0.75Ti1O3-δ-Ni0.8Co0.15Al0.05LiO2-δ	Ni-LST-SDC-NCAL|LST-SDC-NCAL|LST-SDC-NCAL-Ni	0.222	550	Gao et al. (2020)
Pr0.8Sr0.2Fe0.7Ni0.3O3-δ-Pr1.2Sr0.8Fe0.4Ni0.6O4+δ (5:5)	NiO-GDC|GDC|PSFN113-214 (5:5)	0.699	800	This work

Furthermore, to study the stability of Cell-III, a 100-h long-term stability test was conducted under the conditions of 0.3 A cm−2 and 700°C, as shown in Figure 9A. These results showed that the initial voltage of Cell-III decreased from 0.738 to 0.712 V after 100 h of polarization, and the degradation rate of OCV was about 0.0352% h−1. Figure 9B shows the change in electrochemical impedance spectrum of cell-III before and after the stability test. Simultaneously, the XRD and SEM of PSFN113-214 (5:5) cathode after the long-term stability test were tested, as shown in Figure 10. The results showed that the XRD pattern only contained the characteristic peaks of cathode and electrolyte, and there was no impurity peak. And the cathode morphology had no obvious change. These results further proved that PSFN113-214 (5:5) could be used for long-term operation. PSFN113-214 (5:5) had good single-cell activity and stability, so it had broad application prospects in the intermediate temperature range. Afterward, cell activity can be improved via applying anode-supported single cells or diminishing the thickness of the electrolyte.

FIGURE 9

(A) The long-term stability test of a single cell constructed via PSFN113-214 (5:5) cathode (B) cross-sectional view of it.

FIGURE 10

(A) XRD comparison pattern of PSFN113-214 (5:5) cathode before and after long-term stability test and SEM images of PSFN113-214 (5:5) cathode: (B) before long-term stability test; (C) after long-term stability test.

Conclusion

Herein, new Pr0.8Sr0.2Fe0.7Ni0.3O3−δ-Pr1.2Sr0.8Fe0.4Ni0.6O4+δ (PSFN113-214) cathode materials were prepared. The effect of the existence of heterointerfaces on the structures and properties of Pr0.8Sr0.2Fe0.7Ni0.3O3-δ (PSFN113) and Pr1.2Sr0.8Fe0.4Ni0.6O4+δ (PSFN214) was systematically studied. PSFN113 showed a good orthorhombic perovskite-type structure. PSFN214 showed a typical K2NiF4-type tetragonal structure. They were stable to each other and compatible with GDC electrolytes. Two single phases that make up the composite cathode were well mixed and tightly intertwined. When the heterocomposite cathode obtained by mixing at a mass ratio of 5:5 had the largest oxygen vacancy content (0.9836%) and specific surface area (5.1687 m2 g−1), the heterointerface was maximized. At 800°C, the RP value of the heterocomposite cathode PSFN113-214 (5:5) was down to 0.029 Ω cm2 and the PPD for the corresponding single cell was as high as 0.699 W cm−2. The voltage decay rate was merely 0.0352% h−1 after 100 h. Therefore, PBFN113-214 (5:5) has broad application prospects.