Modification of B-site substituted by high-valence Nb in PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ as a highly active air electrode for solid oxide fuel cell

Abstract

Solid oxide fuel cells (SOFCs) are a new type of power generation technology that directly converts chemical energy into electrical energy with advantages, such as low environmental pollution and high conversion efficiency. However, these fuel cells suffer challenges, including high operating temperatures and poor stability. In this paper, PrBa_0.5Sr_0.5Co_1.5−xNb_xFe_0.5O_5+δ series materials were prepared via the sol–gel method and applied as cathode material in SOFCs. X-ray diffraction patterns reveal that the addition of Nb to PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ did not change its crystal structure but increased the cell volume, leading to lattice expansion and improved thermal stability. Electron paramagnetic resonance quantitative analysis was conducted on PrBa_0.5Sr_0.5Co_1.5−xNb_xFe_0.5O_5+δ. At 800 °C, the lowest area specific resistance (ASR) at x = 0.04 was 0.0336 Ω cm², indicating that the addition of Nb significantly improved the oxygen reduction reaction catalytic activity. The power density of the electrolyte-supported single cell increased by 110.6 mW cm⁻². Thus, Nb addition enhances the electrochemical performance of PBSCF cathode materials, and PBSCFNb_0.04 is a promising cathode material for SOFCs.

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1. Introduction

Environmental problems, such as the energy crisis and global warming, have become increasingly prominent. Solid oxide fuel cells (SOFCs) play a crucial role in advancing future energy development and have garnered significant attention from researchers due to their cleanliness and efficiency advantages.¹ However, the high development costs and low durability of SOFCs hinder their commercialization.² As a vital component of SOFCs, the economic durability of the cathode is the focus of many scientific researchers. Cobalt-based materials offer advantages, such as high conductivity and low ASR, making them promising candidates for SOFC cathode materials. PrBaCo₂O_5+δ (PBC) demonstrates high conductivity and oxygen transport performance. This P-type conductor material exhibits a high spin state of Co ions and conductivity due to its thermal activation at low temperatures.³ However, in high-temperature environments, Co⁴⁺ is reduced to Co³⁺, and the lattice oxygen in PBC gradually detaches, resulting in the formation of oxygen vacancies and reduced conductivity. The reduction of lattice oxygen in PBC leads to a high thermal expansion coefficient (TEC) and poor thermal matching between the cathode and electrolyte, which can cause cracking or detachment.⁴ Therefore, this issue significantly limits the commercial development of PBC as a SOFC cathode material.⁵

To facilitate the commercialization of PBC cathode materials, many researchers have proposed various modification methods, with doping modifications becoming a research hotspot. Park et al.⁶ doped Sr at the Pr position of PrCoO_3−δ, achieving a considerable conductivity improvement of 2000 S cm⁻¹ at a doping concentration of 0.3. Subsequently, Lü et al.^4,7 and Liu et al.⁸ doped Sr into PBC to prepare PrBa_0.5Sr_0.5Co₂O_5+δ. This increased conductivity and improved ASR but the TEC mismatch between the cathode and electrolyte remained unchanged. The Fe-based double perovskite cathode material has low TEC and ASR values.⁹ Introducing appropriate amounts of Fe into the Co-based material not only avoids affecting the ASR but may also reduce the TEC. Zou et al.¹⁰ observed that Fe effectively reduces the average TEC. Bai et al.¹¹ revealed that the TEC between the cathode and electrolyte decreases with B-position Fe doping in PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) at 0.5, resulting in improved electrochemical performance; however, the chemical compatibility between the two materials was still not matched. Seonhye Park et al.¹² prepared a composite cathode material of PBSCF and the electrolyte Ce_0.9Gd_0.1O_1.95 (GDC), which can reduce the average TEC. Saccoccio et al.¹³ reported that doping Nb in PBC benefits its oxygen reduction reaction performance and can improve chemical compatibility. When the doping amount of Nb was 0.25, the ASR at 600 °C was reduced to 1.034 Ω cm², further enhancing the material's electrochemical performance.

This paper investigates the effect of Co replacement with Nb in PBSCF on its microscopic and electrochemical properties. The electrochemical performance of symmetric and single cells prepared with PBSCFNb_x as the cathode material was studied in detail.

2. Experimental

2.1 Preparation

The PrBa_0.5Sr_0.5Co_1.5−xFe_0.5Nb_x (x = 0, 0.02, 0.04, 0.06) materials were prepared via the sol–gel method. The raw materials included Pr(NO₃)₃·6H₂O(AR), Ba(NO₃)₂(AR), Fe(NO₃)₃·9H₂O(AR), Co(NO₃)₂·6H₂O(AR), Sr(NO₃)₂(AR), Ce(NO₃)₃·6H₂O(AR), Sm(NO₃)₃·6H₂O(AR), C₆H₈O₇·H₂O(AR), NiO(AR), and C₁₀H₁₆N₂O₈(AR). A solution was obtained through dissolution in deionized water at a ratio of metal ion: citric acid (CA): ethylenediaminetetraacetic acid (EDTA) = 1 : 1 : 5 : 2. CA was added, and the mixture was stirred until dissolved. Ammonia and deionized water were mixed in a 1 : 3 ratio and slowly added to a beaker containing EDTA. The EDTA mixed solution was then added to the mixed solution of metal ions and CA, and the pH was adjusted to 7–8 after thorough mixing. The final solution was placed in a water bath at a constant temperature of 80 °C and stirred until it formed a gel. The gel was heated in a box resistance furnace to obtain a fluffy precursor, which was placed in a muffle furnace and kept at 1200 °C for 5 h to acquire the target products, named PBSCFNb_x (x = 0, 0.02, 0.04, 0.06). The obtained cathode powder was pressed into cuboid strips in a mold with dimensions of 29.0 mm × 6.0 mm × 0.5 mm at 300 MPa, placed in a muffle furnace, held at 1200 °C for 5 h, and then sintered into dense strips. Conductivity was measured using the four-probe method.

The Sm_0.2Ce_0.8O_1.9 (SDC) precursor was prepared using the same method, and SDC powder was obtained after heating the precursor at 800 °C in a muffle furnace for 2 h. The SDC powder was poured into a circular mold (with a diameter of 12 mm), smoothed evenly, pressed at a pressure of 300 MPa to form a round sheet, and placed in the muffle furnace at a temperature rate of 2 °C min⁻¹ to 1450 °C for 5 h to obtain the SDC electrolyte sheet. The obtained PBSCFNb_x powder and mixed slurry (ethyl cellulose : terpinol = 6 : 94) were ground into mixed cathode slurry at a ratio of 7 : 3, coated on both sides of the SDC electrolyte sheet, and sintered at 1200 °C in the muffle furnace to obtain a symmetrical cell.

The electrolyte-supported single cell was prepared for testing. The electrolyte sheet, coated with NiO slurry (prepared using the same method as the cathode slurry) on one side, was placed in the muffle furnace at 1300 °C to form a half cell. Then, the other side was coated with cathode slurry, and the single cell was sintered at 1200 °C in the muffle furnace.

2.2 Characterization

The phase composition and crystal structure of the samples were analyzed using an X-ray diffractometer (XRD, Malvern Panalytical, Empyrean) at a scanning rate of 5° min⁻¹, a scanning range of 20°–80°, and Cu Kα radiation (40 kV, 40 mA, λ = 1.5418 Å). The results were refined using Rietveld methods with GSAS/EXPGUI software. The lattice spacing of the materials was characterized via transmission electron microscopy (TEM, FEI Tecnai F20). Energy-dispersive X-ray and high-resolution TEM analyses were performed. The surface element valence was analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific, EscaLab250Xi). A thermal dilatometer (NETZSCH, DIL 420C) was used to test the thermal expansion coefficient (TEC) of the cathode material from 30 °C to 750 °C. The oxygen vacancy of PBSCFNb_x was quantitatively analyzed through electron paramagnetic resonance (EPR, Bruker, EMX PLUS). Scanning electron microscopy (SEM, TESCAN, GAIA3) was used to test the cross-section of the symmetric cell. Electrical conductivity (300–800 °C), electrochemical impedance spectra (EIS, 600–800 °C, 100 kHz–0.1 Hz), and power density (650–800 °C) were tested at 50 °C intervals using an electrochemical workstation (Metrohm, PGSTAT302).

3. Results and discussion

Fig. 1(a) presents the XRD patterns of PBSCFNb_x at room temperature after calcination at 1200 °C for 5 h. According to a comparison of standard XRD patterns, no other diffraction peaks appeared, and the peak positions matched well with those of undoped materials. This finding proves that no other impurities were produced in the prepared cathode materials, and all of them exhibited a single tetragonal structure.¹⁴ Fig. 1(b) shows a partial amplification of the XRD pattern from 32° to 34°. The ionic radius of Nb is 0.64 Å, larger than those of Co⁴⁺ (0.53 Å) and Co³⁺ (0.61 Å), which increased the cell volume of PBSCF. With the increase in the Nb content, the diffraction peak gradually shifted to a lower angle. Rietveld refinement was used to further study the effect of Nb doping on the crystal structure. Fig. S1 and Table S1† show the Rietveld-refined XRD patterns of PBSCFNb_x. The cell volume increased with an increase in Nb content. TEM and EDS analysis further corroborate these findings on PBSCFNb_0.04, and the analysis results are shown in Fig. 1(c) and (d), respectively. Each element displayed a uniform distribution with no aggregation. Fig. 1(d) and Table S2† present the elemental analysis results for region 1. Nb was successfully added to PBSCF. The lattice spacing of PBSCF and PBSCFNb_0.04 was calculated (Fig. 1(c) and (d), respectively). The lattice spacing of 100 crystal planes increased from 0.384 nm to 0.387 nm; this slight increase confirms the successful doping of Nb. The calculated results of lattice spacing were consistent with those obtained by refinement. The Goldschmidt tolerance factor t is used to predict the stability of PBSCFNb_x materials.¹⁵ The calculation results for t are summarized in Table S3.† The results show that the addition of Nb reduces the t of PBSCF, bringing it closer to 1. Therefore, the introduction of Nb to PBSCF improves the thermal stability of the material.where r_A, r_B, and r_O refer to the average ionic radii of positions A, B, and O, respectively. The ionic radii of each element in PBSCFNb_x are 1.305 Å (Pr³⁺),1.61 Å (Ba²⁺), 1.44 Å (Sr²⁺), 0.61 Å (Co³⁺), 0.53 Å (Co⁴⁺), 0.645 Å (Fe³⁺), and 0.585 Å (Fe⁴⁺).

Fig. 1 (a) XRD patterns of PBSCFNb_x cathode materials. (b) Magnified XRD patterns of PBSCFNb_x. (c) High-resolution TEM of PBSCF. (d) High-resolution TEM of PBSCFNb_0.04, and (e) EDS of PBSCFNb_0.04.

The TEC of SOFC components should be as close as possible to effectively prevent cell cracking and degradation under high-temperature conditions. Fig. 2 presents the average TEC curve of PBSCFNb_x in an air atmosphere over a temperature range of 30–750 °C. This finding demonstrates that the thermal expansion degree (dL/L₀) in this temperature range decreased with an increasing Nb doping amount, as the bond energy of the Nb–O bond is higher than that of the Co–O bond. When the amount of Nb doping reached 0.06, the average TEC decreased from 21.1 × 10⁻⁶ K⁻¹ to 18.0 × 10⁻⁶ K⁻¹. Table S4† shows that the addition of Nb greatly improved the average TEC between PBSCFNb_x and the SDC electrolyte, potentially extending the service life of the cell.

Fig. 2 The thermal expansion of PBSCFNb_x and SDC.

The valence states of surface ions in PBSCFNb_x were analyzed via XPS. The peak-fitting results for the O 1s orbit are shown in Fig. 3(a). The activation energy was divided into three oxygen ion states, namely. O_adsorbed, O_lattice, and O_vacancy, from 525 eV to 536 eV.¹⁶ The lattice oxygen at 531.7 eV corresponds to the oxygen state of the perovskite structure, the characteristic peak at 528.3 eV corresponds to chemisorbed oxygen, and the peak at 529.3 eV represents the oxygen vacancy.^17,18 Doping with high-valence elements will reduce the oxygen vacancy concentration. According to the semi-quantitative analysis of XPS, the oxygen vacancy concentration decreases from 20.35% to 13.27% (Table S5†), which coincides with the EPR analysis of oxygen vacancy. The reduction in oxygen vacancy concentration will directly affect cell performance tests. Fig. 3(b) shows the XPS of Fe ions in PBSCFNb_x, which lacks symmetry. The peaks at binding energy values of 723.7 and 714.6 eV correspond to Fe³⁺ 2p_3/2 and Fe⁴⁺ 2p_3/2, respectively.^19–21 Meanwhile, the peaks at 710.8 eV correspond to the spectral peaks of Fe³⁺ 2p_1/2. The absence of Fe²⁺ in the XPS of PBSCFNb_x indicates a high oxidation state of Fe ions. Fe⁴⁺ was reduced from 15.15% to 3.03% (Table S5†). Fig. 3(c) shows the XPS of Co ion in PBSCFNb_x. The binding energy values of 795.3 and 780.4 eV correspond to Co³⁺ 2p_1/2 and Co³⁺ 2p_3/2, respectively. The Co²⁺ 2p_1/2 and Co²⁺ 2p_3/2 spectral peaks in PBSCF had binding energy values of 793.1 and 777.4 eV, respectively. A Co⁴⁺ spectral peak of 789.8 eV was also observed. The Co in PBSCFNb_x presented three valence distributions.^15,22 According to the principle of electric neutrality, the addition of Nb⁵⁺ reduces the conversion of Co⁴⁺ to Co³⁺. The semi-quantitative analysis of the Co content showed that the doping of Nb reduced the Co⁴⁺ content from 4.77% to 3.4% (Table S5†), thus influencing the properties of conductivity and thermal expansion. Fig. 3(d) shows the XPS of Pr ions. The spectral peaks of Pr⁴⁺ 3d_3/2 and Pr⁴⁺ 3d_5/2 were located at 953.3 and 932.8 eV, respectively. The energy spectra of 948.9 eV and 928.4 eV correspond to Pr³⁺ 3d_2/3 and Pr³⁺ 3d_5/2, respectively, indicating that Pr³⁺ and Pr⁴⁺ coexist in PBSCF.^23,24 Thus, Fe³⁺/Fe⁴⁺, Co²⁺/Co³⁺/Co⁴⁺, and Pr³⁺/Pr⁴⁺ exist in PBSC and PBSCFNb_0.04, and the presence of mixed oxidation states enhances the oxygen reduction reaction in the cathode materials.

Fig. 3 XPS curves of PBSCFNb_x: (a) O 1s, (b) Fe 2p, (c) Co 2p, (d) Pr 2p.

The oxygen vacancy in Nb-doped PBSCF was measured via EPR (Fig. 4). The results showed symmetrical oxygen vacancy signal peaks, with symmetrical signals appearing at g = 2.003. The peak of oxygen vacancy varied significantly among each sample, mainly because the addition of the high-valency Nb ion reduces the average valency of the other elements, affecting the content of oxygen vacancy. To further study the effect of Nb addition on oxygen vacancy concentration in PBSCF, we conducted quantitative tests on the oxygen vacancy concentration in PBSCFNb_x; the results are shown in Table S6.† The oxygen vacancy concentration decreased gradually with the addition of Nb, from 6.936 × 10¹⁶ to 3.539 × 10¹⁶. The oxygen vacancy concentration in SOFC affects conductivity, ASR, and power density. Thus, oxygen vacancies serve as channels for O²⁻ transmission in the cathode material and are important indicators of catalytic activity on the cathode surface.²⁵ The higher the oxygen vacancy concentration, the stronger the oxygen adsorption capability, which aids the oxidation reaction of SOFC. Therefore, the addition of Nb will reduce the surface exchange of oxygen molecules and limit oxygen reduction.

Fig. 4 The oxygen vacancy test curve in PBSCFNb_x.

Fig. 5 shows the cross-section microstructure of the symmetrical cell. The symmetric cell had a cathode coating with a porous structure and a dense SDC electrolyte layer. The porous cathode material facilitated oxygen transport and provided a site for the oxygen redox reaction. The compact SDC electrolyte layer was tightly bound to the cathode coating, and the microstructure of the cathode coating did not change noticeably, ensuring the transfer of O²⁻ to the SDC electrolyte at the three-phase interface (TPB). The thickness of the coating also had a certain effect on the performance of the symmetrical cell. The symmetric batteries obtained using each component displayed similar thicknesses, reducing the influence of the cathode thickness on the ASR.

Fig. 5 Cross-section of SEM images of (a) PBSCF, (b) PBSCFNb_0.02, (c) PBSCFNb_0.04, and (d) PBSCFNb_0.06.

Fig. 6(a) presents the conductivity (σ) test curve of PBSCFNb_x at various temperatures in an air atmosphere. The conductivity increased first and then decreased with temperature. Therefore, PBSCFNb_x is a P-type conductor, and when the temperature was below 400 °C, its conductivity followed a small polarized molecular transition and increased with the temperature. The small polaron transition is represented by the following conductivity model:¹¹

σ = (C/T) exp(−E_a/kT), (2)

where σ represents the electrical conductivity; C denotes the pre-exponential factor unique to the material; T is the temperature under test; E_a indicates the activation energy required for the reaction, and k is the reaction rate constant. When the temperature is below 400 °C, kT < E_a, the conductivity depends on the exponential term and increases with temperature. When the temperature exceeds 400 °C, kT > E_a, the conductivity depends on the pre-exponential factor term and decreases with increasing temperature. When kT = E_a, the conductivity of the material is at its highest. Meanwhile, the electrons in PBSCF are mainly transported by Co⁴⁺–O²⁻–Co³⁺ and Fe⁴⁺–O²⁻–Fe³⁺, and the conductivity conforms to the small polaron transition at temperatures below 400 °C. At temperatures greater than 400 °C, Fe⁴⁺ and Co⁴⁺ reduce to Fe³⁺ and Co³⁺, respectively, resulting in an increase in oxygen vacancies and hindering the transport of Co⁴⁺–O²–Co³⁺ and Fe⁴⁺–O²⁻–Fe³⁺. The PBSCF reaction can be expressed using eqn (3) and (4). With the progress of the reaction, the oxygen vacancy concentration gradually increased, and the conductivity decreased.²⁶where and are the lattice oxygen and oxygen vacancy, respectively. Fig. 6(b) shows the relationship between Nb content and conductivity at 400 °C. All materials reached their maximum value at 400 °C, with conductivities at x = 0, 0.02, 0.04, and 0.06 being 1111.72, 1004.54, 986.81, and 808.92 S cm⁻¹ (errors due to conductivity fitting are shown in Table S7†), respectively. It can be seen from TEM that the addition of Nb causes lattice distortion in PBSCF, evidenced by the lattice constants. The XRD refinement results indicate that the lattice constant gradually increases, the lattice barrier decreases, and the oxygen vacancy concentration decreases, suggesting that conductivity should increase. However, in perovskite materials, carriers predominantly transmit through the B–O–B structure. Some Co–O bonds are replaced by longer Nb–O bonds, lengthening the carrier transmission path, and decreasing the carrier migration rate. According to the principle of charge conservation and XPS analysis, it can be concluded that B-position Nb doping leads to an increase in non-conductive Nb–O bonds, reducing the spin of Co⁴⁺ to Co³⁺, which is the main reason for the higher conductivity of cobalt-based materials, and thus, the electron conductivity decreases. When Co and Fe coexisted, the ion compensation mechanism showed that Fe⁴⁺ preferentially obtained electrons compared to Co⁴⁺.²⁷ However, the addition of Nb in this experiment inhibited the increase in the content of Fe⁴⁺. In contrast to the system where Co and Fe coexisted, Co⁴⁺ obtained electrons more easily, promoting the stabilization of high spin states of Co and Fe, which led to a decrease in conductivity. Based on the Arrhenius diagram in Fig. 6(a) and the small polaron transition model, the activation energy and pre-exponential factor of the material at the maximum conductivity were obtained (Fig. 6(c) and (d)). The pre-exponential factor of the material itself gradually decreased with the doping of Nb. The activation energy increased, and conductivity decreased with the rising Nb content at 400 °C.

Fig. 6 (a) The change of conductivity with temperature and the relation between Arrhenius and conductivity. (b) The relation between conductivity and Nb content at 400 °C. (c) The relation between pre-exponential C and Nb content at 400 °C. (d) The relation between E_a and Nb content at 400 °C.

Based on the Alder theory, the equivalent circuit diagrams in Fig. 7(a)–(c) were established. In the circuit, L represents the inductance generated by the test device and the electrode lead, and R_s is the ohmic resistance from the cathode, the Pt line, the contact between the electrolyte and the cathode, and the electrolyte. The main role is the contact resistance between the cathode layer and the electrolyte. R_HF and R_LF refer to mid-frequency and low-frequency polarization resistors, while CPE1 and CPE2 are constant-phase elements corresponding to each frequency range.²⁸ The ASR values of PBSCF and PBSCFNb_0.04 decreased with increasing temperature because Co⁴⁺ and Fe⁴⁺ in PBSCF underwent reduction reactions in a high-temperature environment, leading to lower valence states, increased oxygen vacancy concentration, and decreased ASR. Fig. 7(c) and (d) show the ASR diagrams of PBSCFN_x at 800 °C and PBSCFN_x with different Nb content, respectively. At the same temperature, the ASR decreased with increasing Nb content, although it increased when the doping level was raised to 0.06 Nb. At 800 °C with a doping amount of 0.04, the ASR reached its minimum at 0.0336 Ω cm². The ASR is closely related to the oxygen vacancy concentration, but it can be seen from Fig. 7(c) and (d) that as the oxygen vacancy concentration decreases, the ASR value decreases, instead of increasing. The phenomenon can be attributed to the fact that R_HF is related to the migration of oxygen ions, while R_LF is related to the adsorption, dissociation, and diffusion of oxygen in the oxygen reduction reaction (ORR). The calculations for R_HF and R_LF are as follows (R_HF: PBSCF, PBSCFNb_0.02, PBSCFNb_0.04, PBSCFNb_0.06 are 0.0247, 0.0123, 0.0109 and 0.0172 Ω, respectively, R_LF: PBSCF, PBSCFNb_0.02, PBSCFNb_0.04, PBSCFNb_0.06 are 0.0246, 0.0232, 0.0227, 0.0238 Ω, respectively). Errors due to ASR fitting are shown in Table S8.† R_HF and R_LF demonstrated a trend of decreasing and then increasing. However, the decrease in R_LF indicated that the oxygen surface exchange performance improved with the doping of Nb. The increase in oxygen vacancy concentration led to defect association of the material, which caused oxygen vacancy localization.²⁹ The distribution of relaxation time (DRT) analysis results of the PBSCFNbx series materials is shown in Fig. S2.† The results indicate that, in the high-frequency region, the impedance (R_HF) shows a pattern of first increasing and then decreasing, consistent with the pattern in Fig. 8(c). From x = 0 to 0.04, the peak width of the high-frequency peak gradually increases, indicating that the time for oxygen ions to transition from the reaction transient state to the steady state becomes shorter, improving the chemical reaction kinetics and accelerating the reception and escape speed of oxygen ions,^30,31 thereby increasing the chemical reaction rate of the material. When x = 0.06, the peak width decreases, the oxygen ion transference number reduces, and the impedance increases. Table 1 shows that the ASR value of PBSCFNb_0.04 is considerably lower than that reported in the literature.

Fig. 7 ASR shows the temperature dependence of (a)PBSCF, (b) PBSCFNb_0.04, (d)PBSCFNb_x, and an (c) ASR diagram of PBSCFNb_x at 800 °C.

Fig. 8 (a) The power density of a single cell PBSCF|SDC|Ni + SDC at 650–800 °C and (b) the power density of single cell PBSCF_0.04|SDC|Ni + SDC at 650–800 °C.

Table 1 The ASR of other Co-based materials R_p (Ω cm²)

Sample	Temperature (°C)	Rp (Ω cm^2)	Ref.
LnBaCo	800	0.046	18
PBCF	800	0.049	32
PBSCF	800	0.067	15
PBSCFNb0.04	800	0.034	This work
PBSCFNb0.04	700	0.057	This work
PBCM	700	0.067	33

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To investigate the effect of PBSCFNb_x as a SOFC cathode on power density, we prepared an electrolyte-supported single cell. Fig. 8(a) and (b) display the power density of PBSCF and PBSCFNb_0.04 in a hydrogen atmosphere from 650 °C to 800 °C. The power density increased with temperature due to the acceleration of oxygen ion transport and gas diffusion as the temperature rose. At 650 °C, the open-circuit voltage values were 0.91 and 0.98 V, all lower than the theoretical voltage of 1.1 V because SDC has a certain N-type electronic conductivity.³⁴ Furthermore, a portion of Ce⁴⁺ was reduced to Ce³⁺ under a high-temperature reduction atmosphere, resulting in a lower voltage. The power density of the single cell prepared using PBSCF at 800 °C was 217.7 mW cm⁻², while that prepared with PBSCFNb_0.04 was 328.3 mW cm⁻², indicating an increase of 110.6 mW cm⁻². This finding was observed because the power density of a single cell increases as the ASR value of a symmetrical cell decreases.

4. Conclusion

A series of materials PBSCFNb_x (x = 0, 0.02, 0.04, 0.06) was prepared as cathode materials for SOFC via the sol–gel method, and all of them exhibited a single tetragonal structure. With the increase of Nb content, the oxygen vacancy concentration decreased, but the exchange performance of the oxygen surface was effectively improved and the ASR was reduced. The TEC of PBSCFNb_0.04 decreased from 21.1 × 10⁻⁶ K⁻¹ to 18.0 × 10⁻⁶ K⁻¹, which effectively reduced the thermal expansion of the material. At 800 °C, the power density of PBSCFNb_0.04 increased by 110.6 mW cm⁻². The ASR reached a minimum of 0.0336 Ω cm² at x = 0.04, implying its potential as a SOFC cathode material.

Data availability

The data are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the Inner Mongolia Autonomous Region universities' basic scientific research project (2023CXPT002), the Baotou Science and Technology Bureau project (YF2022014), the Open Research Project for Innovation Center of New Rare Earth Materials Technology (CXZX-D-202409-0020), and the integrated research platform for novel and important energy comprehensive utilization technology in Inner Mongolia Autonomous Region.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra09097g

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