Enhanced electrocatalytic activity and stability of high performance symmetrical solid oxide fuel cells with praseodymium-doped SrCo_0.2Fe_0.8O_3−δ electrodes

Abstract

Symmetrical solid oxide fuel cells (SSOFCs) represent a promising path towards energy conversion and storage solutions characterized by reduced material costs, simplified manufacturing, and improved operational stability. The development of high-performance symmetrical electrodes with superior catalytic activity and durability remains a critical challenge. In this study, a series of praseodymium-doped SrCo_0.2Fe_0.8O_3−δ perovskites, Pr_xSr_1−xCo_0.2Fe_0.8O_3−δ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) are systematically explored, to evaluate their potential as efficient symmetrical electrodes for SSOFCs. Among the compositions studied, Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3−δ (P_0.2SCF) stands out, with an excellent crystal structure and microstructural stability demonstrated by XRD, XPS and SEM test results, and low polarization resistance values at 850 °C of 0.029 Ω cm² for the cathode and 0.054 Ω cm² for the anode. In particular, a single cell containing P_0.2SCF achieved an impressive maximum power density of 719.08 mW cm⁻² at 850 °C as well as good stability. The results of the DRT analysis show that the main rate-limiting steps of the cell are the gas diffusion and catalytic dissociation processes. These results underscore the potential of P_0.2SCF as a highly effective perovskite electrode material for SSOFCs and highlight the key role of Pr doping in enhancing perovskite electrode performance and advancing sustainable energy technologies.

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Introduction

Solid oxide fuel cells (SOFCs) can efficiently convert chemical energy into electrical energy. They have a wide range of fuel sources and offer advantages such as low pollution, all-solid-state design, and modular assembly.^1–5 As a result, SOFCs are widely recognized as one of the most promising energy conversion technologies of the 21st century, with significant development potential. In conventional SOFCs, the materials used for the cathode and anode are different. In particular, nickel-based anodes face challenges such as carbon deposition and sulfur poisoning when using hydrocarbons or sulfides are used as fuels.⁶ Using the same material for both the cathode and anode, and constructing a symmetric sandwich structure can simplify the fabrication process, reduce costs, and minimize compatibility issues. This configuration also allows the catalytic activity of the anode to be restored by switching the air flow to remove carbon and sulfur-containing impurities from the anode surface.⁷ Therefore, symmetric SOFCs (SSOFCs) have garnered increasing research interest and attention.8–11

As mentioned above, symmetrical cells use identical electrode materials, even though the anode and cathode play different roles in the electrochemical reactions. This places high demands on the electrode material. Currently, symmetrical electrodes for SSOFCs are mostly selected from SOFC cathode or anode materials,¹² which are then modified by elemental doping or surface treatment to improve the catalytic activity and stability of the electrode materials in both oxidizing and reducing atmospheres.¹³ Among these materials, SrFeO_3−δ-based perovskites have attracted the attention of researchers due to their excellent electrocatalytic activity and stability for the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR).¹⁴ They exhibit high oxygen vacancy concentration, surface oxygen exchange coefficient and excellent conductivity.¹⁵ The perovskite oxide SrFe_0.9Mo_0.1O_3−δ has been evaluated as an electrode for SSOFCs with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolytes, with a maximum power density of 253 mW cm⁻² at 700 °C.¹⁶ The SrFe_0.8W_0.2O_3−δ (SFW) perovskite oxide has been developed as an electrode material for SSOFCs, with polarization resistance (R_p) values as low as 0.084 and 0.20 Ω cm² in air and wet H₂ at 800 °C, respectively.¹⁷ SrFe_0.8Ti_0.2O_3−δ has been used in SSOFCs and shows the highest electrochemical performance with a maximum power density of 700 mW cm⁻² and a lower electrode R_p value (0.573 Ω cm²) at 800 °C.¹⁸ In addition, SrFe_0.9−xW_0.1Ti_xO_3−δ has been prepared as a promising electrode material for SSOFCs, with the R_p of a symmetric single cell recorded at 0.28 Ω cm².¹⁹ Other materials such as SrFe_0.9Si_0.1O_3−δ,²⁰ SrFe_0.75Mo_0.25O_3−δ,²¹ and SrFe_0.75Zr_0.25O_3−δ²² have also been investigated.

In addition to doping the B-site with high-valence metals to improve stability and electrocatalytic activity, introducing high-valence rare earth elements at the A-site is also an effective strategy due to their unique 4f orbital electron properties.²³ This approach can significantly improve both electrocatalytic activity and stability. For example, the manipulation of the A-site deficiency in La_0.3Sr_0.7Ti_0.3Fe_0.7O_3−δ-based electrodes for SSOFCs was investigated, and the R_p values of (La_0.3Sr_0.7)_0.95Ti_0.3Fe_0.7O_3−δ were found to be 0.032 and 0.110 Ω cm² in air and wet H₂ at 800 °C, respectively.²⁴ In another study, SrFeO_3−δ perovskite (Ce_0.2Sr_0.8Fe_0.95Ni_0.05O₃) was modified with ceria and NiO doping on both the A- and B-sites to achieve a maximum power density of 580 mW cm⁻² and an R_p of 0.21 Ω cm² at 800 °C for SSOFCs.²⁵ Pr_0.6Sr_0.4Fe_0.8Cu_0.1W_0.1O_3−δ has also been investigated as a symmetrical electrode material, yielding R_p values of 0.069 Ω cm² under oxidizing conditions and 0.24 Ω cm² under reducing conditions at 800 °C.²⁶ Similarly, Pr_0.4Sr_0.6Co_0.3Fe_0.6Nb_0.1O_3−δ oxides have been evaluated as electrode materials for SSOFCs, with the lowest R_p values of 0.028 and 0.077 Ω cm² at 900 °C in air and hydrogen, respectively.²⁷

Although A-site doping with the rare earth element Pr can enhance the electrocatalytic activity, the specific mechanism of its effect remains unclear, and the optimal doping amount is also unknown. In this study, LSGM was chosen as the electrolyte support due to its higher ionic conductivity, better chemical compatibility with perovskite electrodes and chemical stability in both oxidizing and reducing environments. SrFeO_3−δ-based perovskite oxides doped with a small amount of the element Co were used as the substrate. Pr-doped Pr_xSr_1−xCo_0.2Fe_0.8O_3−δ (P_xSCF, x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) symmetric electrode materials were prepared using the sol–gel method. The impacts of different Pr doping level on the structure and electrochemical properties of the phases were investigated, and their AC impedance values were determined. The doping ratios with the best electrochemical performance were identified, and the stability of the crystal structure and microscopic morphology of these optimal ratios were studied in detail under oxidizing and reducing atmospheres.

Experimental

Powder synthesis

Pr_xSr_1−xCo_0.2Fe_0.8O_3−δ (P_xSCF, x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) powders with different Pr doping level were synthesized using the sol–gel method. For example, to prepare the Pr doping level of x = 0.2, stoichiometric amounts of Pr(NO₃)₃·6H₂O, Sr(NO₃)₂, Co(NO₃)₂·6H₂O, and Fe(NO₃)₂·9H₂O were weighed and dissolved in deionized water. Citric acid (CA) and ethylenediaminetetraacetic acid (EDTA) were added to the solution as complexing agents in a molar ratio of ions : CA : EDTA = 1 : 2 : 1. The pH of the solution was adjusted to 7–8 using ammonia, then stirred at 80 °C until a gel formed, which was then heated to 180 °C to produce the precursor. The precursors were then calcined in air at 900 °C for 6 h to finalize the electrode powders with the appropriate Pr doping level. To simulate the actual conditions of the electrode powder in the anode working atmosphere, the electrode powder was calcined at 800 °C for 2 h in a 5% H₂/95% Ar atmosphere to obtain the reduced powder, denoted as P_xSCF@2. To further investigate the stability of its crystal structure and microstructure in a long-term high temperature reducing atmosphere, the electrode powder was calcined at 800 °C for 100 h in a 5% H₂/95% Ar atmosphere, denoted as P_xSCF@100. The La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte was synthesized via a solid-state reaction. First, La₂O₃ was calcined at 1250 °C for 2 h, while MgO was calcined in air at 900 °C for 2 h. Then, stoichiometric amounts of La₂O₃, Ga₂O₃, MgO, and SrCO₃ powders were ball-milled in ethanol for 12 h. The resulting powder mixture was dried and calcined in air at 1100 °C for 5 h. The calcined powder underwent a second 12-hour ball-milling. To produce dense LSGM electrolyte discs with a thickness of 400 μm, the pre-calcined powder was dry-pressed with 1 wt% polyvinyl butyral (PVB) and held in air at 1450 °C for 10 h.

Single-cell fabrication and fuel cell testing

The optimal Pr doping level was determined by fabricating P_xSCF|LSGM|P_xSCF symmetric cells and evaluating their electrochemical performance as both the cathode and anode. The P_xSCF was combined with a binder in a 1 : 1 weight ratio and manually ground for 40 min to produce the electrode slurry. The paste was then screen-printed onto both sides of the LSGM electrolyte and calcined in air at 950 °C for 2 h, forming electrodes with an area of 0.28 cm² on each side. A symmetric cell was assembled by applying DAD-87 silver paste and attaching silver wires to both sides. These P_xSCF|LSGM|P_xSCF symmetric cells were tested at open circuit voltage (OCV) with H₂ supplied to the fuel electrode at a flow rate of 40 ml min⁻¹ and the cathode exposed to ambient air. For the AC impedance test to evaluate anode performance, the cell was first reduced at 800 °C for 2 h in a 5% H₂/95% Ar atmosphere to simulate the anode environment, followed by a polarization impedance measurement. Current–voltage curves (I–V) and electrochemical impedance spectroscopy (EIS) measurements were carried out in the temperature range of 700–850 °C using a Zahner electrochemical workstation (Zennium). The EIS measurements covered a frequency range of 0.1–10⁶ Hz with an amplitude of 10 mV. The EIS data were analyzed using the Distribution of Relaxation Times (DRT) method.

Characterization

The crystal structure and structural stability of the powders were evaluated by X-ray diffractometry (XRD, Rigaku Ultima IV) using Cu Kα radiation with step widths of 0.02° and scan ranges from 20° to 90°. Detailed structural information was obtained by Rietveld refinement using the FullProf software. Scanning electron microscopy (SEM, ZEISS Sigma 300) was used to examine the microstructure and cross-sections of the samples before and after reduction. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) was used to analyze the chemical valence states of the P_xSCF samples. All XPS measurements were calibrated to a binding energy of 284.8 eV corresponding to the C 1s peak. Peak fitting was performed using XPS Avantage software to determine the valence states and chemical environments of the elemental ions. The conductivity of the electrode material in air was determined by the DC four-terminal method.

Results and discussion

Materials screening

The P_xSCF (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) electrode powders with different Pr doping levels were first analyzed for their physical structures as depicted in Fig. 1(a), for x ≤ 0.4, the P_xSCF samples exhibit a well-defined orthorhombic perovskite structure, with all diffraction peaks matching the standard reference (PDF#40-0906) and showing no impurity peaks. However, for x ≥ 0.6, a few impurity peaks were observed in P_0.6SCF, P_0.8SCF, and P₁SCF samples, identified as PrFeO₃ in comparison with the standard reference (PDF#19-1012). Consequently, only the electrode powders with Pr doping levels of 0, 0.1, 0.2, and 0.4 were selected for further electrochemical performance testing in half-cells. The R_p values for each doping ratio, derived from the tests are presented in Fig. 1(b)–(e). The R_p of P_xSCF decreases with increasing temperature both in air and in a 5% H₂/95% Ar atmosphere. This trend suggests that higher temperatures facilitate oxygen adsorption and desorption, as well as enhance the catalytic activity of the H₂. From Fig. 1(f) and (g), the R_p values of P_0.2SCF in air are lower than those of P_0.1SCF, indicating its superior oxygen catalytic activity. However, in a reducing atmosphere, P_0.1SCF has lower R_p values than P_0.2SCF at lower temperatures. As the temperature increases, the difference in R_p between the two materials decreases. After careful consideration of these factors, the P_0.2SCF electrode material with the highest catalytic activity was selected for further detailed study.

Fig. 1 (a) XRD patterns of P_xSCF (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) calcined at 900 °C for 6 h in air; (b)–(e) impedance spectra of P_xSCF (x = 0, 0.1, 0.2, 0.4) under air and 5% H₂/95% Ar; (f) and (g) histograms of the R_p values of P_xSCF (x = 0, 0.1, 0.2, 0.4) under air and 5% H₂/95% Ar at different temperatures.

Materials characterization

To further investigate the crystal structure stability of the P_0.2SCF powder in reducing atmospheres, the powder was exposed to a 5% H₂/95% Ar mixture at 800 °C for 2 h and 100 h, respectively. The orthorhombic perovskite structure of P_0.2SCF maintained its structural stability after 2 h of reduction, as illustrated in Fig. 2(a). However, after 100 h of reduction, a localized magnification study of the diffraction angles revealed a slight shift to lower angles, indicating lattice expansion. This expansion is attributed to the reduction of Fe⁴⁺ to Fe³⁺ and the corresponding increase in lattice parameters in a reducing atmosphere, which is consistent with observations in similar electrode materials such as SrFe_0.7Ti_0.3O_3−δ.²⁸ Despite these changes, the orthorhombic perovskite structure remained intact, demonstrating that P_0.2SCF has excellent crystal structure stability in oxidizing and prolonged reducing atmospheres. The Rietveld refinement of the P_0.2SCF electrode powder in air and after reduction is shown in Fig. 2(b)–(d). The space group is P/mmm, and the refined cell parameters are provided in Table S1.† The results confirm that lattice expansion occurs after 100 h of reduction in a 5% H₂/95% Ar atmosphere, with slight increases in the cell parameters a, b, and c. This expansion is attributed to the reduction-induced decrease in the valence states of the Co and Fe elements over a prolonged period. Fig. 2(e) shows the SEM images of the P_0.2SCF electrode powder before reduction at different magnifications, revealing a loose, porous structure. Fig. 2(f) and (g) show the microscopic morphology of the P_0.2SCF electrode powders reduced in a 5% H₂/95% Ar atmosphere for 2 h and 100 h, respectively, at different magnifications. The images illustrate that the microscopic morphology remains unchanged regardless of the reduction time, maintaining a loose, porous structure. The micromorphology of P_0.2SCF after 100 h of reduction, along with the elemental distribution, was obtained through X-ray energy dispersive spectroscopy (EDS) analysis of P_0.2SCF@100. Fig. 2(h) shows that the elements Pr, Sr, Co, Fe, and O are uniformly distributed without any elemental enrichment. Combining the results of the XRD, SEM, and EDS tests, it can be concluded that P_0.2SCF exhibits excellent crystal structure and microstructure stability under redox conditions. This stability meets the basic requirements for a symmetric electrode.

Fig. 2 (a) XRD patterns of P_0.2SCF in oxidizing and reducing atmospheres; (b)–(d) XRD refinements of P_0.2SCF with different reduction times; (e)–(g) microscopic morphology of P_0.2SCF with different reduction times; (h) EDS patterns of P_0.2SCF reduced for 100 h in a 5% H₂/95% Ar atmosphere.

The specific valence states of each element on the surface of P_0.2SCF and P_0.2SCF@100 electrode powders were analyzed by XPS as shown in Fig. 3(a). It indicates that the elemental composition of the powder remains unchanged, confirming the stability of its physical phase structure. Fig. 3(b) shows the XPS pattern of Pr 3d. The binding energy peaks of Pr³⁺ are located at 929.9 eV, 949.4 eV, 934.1 eV, and 955.2 eV,²⁹ while the binding energy peaks of Pr⁴⁺ are at 932.6 eV and 952.6 eV. An increased content of Pr³⁺ in the P_0.2SCF@100 powders, with binding energies at 934.1 eV and 955.2 eV, suggests a decrease in the average valence state of Pr from 3.52 to 3.42 after 100 h of reduction. In Sr containing materials, the Sr element migrates from the bulk to the surface, forming compounds such as Sr(OH), SrCO₃, and SrO.^30–33 The XPS pattern of Sr 3d is shown in Fig. 3(c), exhibiting a bimodal pattern corresponding to the 3d_5/2 and 3d_3/2 orbitals of Sr, with a peak separation of 1.75 eV.³⁴ The binding energy peaks at 3d_5/2 and 3d_3/2 are attributed to the Sr²⁺ lattice (Sr_lattice) in both P_0.2SCF and P_0.2SCF@100 powders. The peaks at 133.1 eV and 134.9 eV are assigned to non-lattice Sr (Sr_non-lattice) compounds formed by interaction with carbon dioxide and oxygen in air.³¹ Sr_non-lattice forms an insulating layer on the surface, hindering electron transfer and degrading electrode performance. Semi-quantitative analysis of the Sr 3d spectra reveals that the Sr_lattice and Sr_non-lattice contents are 45.55% and 54.45% in P_0.2SCF, and 51.37% and 48.63% in P_0.2SCF@100, respectively. This indicates no significant Sr segregation after prolonged high temperature reduction, suggesting the stability of the P_0.2SCF electrode. The XPS pattern of Co 2p shown in Fig. 3(d) includes binding energy peaks at 780.6/795.6 eV, 779.9/794.6 eV, and 782.1/796.7 eV, corresponding to Co²⁺, Co³⁺, and Co⁴⁺, respectively.³⁵ This indicates that Co exists in mixed valence states (Co²⁺/Co³⁺/Co⁴⁺) in both P_0.2SCF and P_0.2SCF@100 powders. After prolonged reduction, there is a slight increase in Co²⁺ and Co³⁺ contents, while Co⁴⁺ decreases, reducing the average valence state of Co from 2.88 to 2.82. The XPS pattern of Fe 2p shown in Fig. 3(e) includes binding energy peaks at 709.8/722.6 eV, 711.8/724.6 eV, and 715.1/727.8 eV, corresponding to Fe²⁺, Fe³⁺, and Fe⁴⁺, respectively.³⁶ Fe exists in mixed valence states (Fe²⁺/Fe³⁺/Fe⁴⁺), predominantly as Fe²⁺ and Fe³⁺. The average valence state of Fe decreases from 2.74 to 2.59 after prolonged reduction. The XPS pattern of O 1s shown in Fig. 3(f) has two distinct peaks corresponding to lattice oxygen (O_lat) at 528.5 eV and surface adsorbed oxygen (O_ads) at 531.1 eV.³⁷ Semi-quantitative analysis shows that the O_ads and O_lat contents in P_0.2SCF are 59.72% and 40.28%, respectively, while in P_0.2SCF@100 they are 68.84% and 31.16%, respectively. Prolonged high temperature reduction increases the surface adsorbed oxygen content, indicating the formation of additional oxygen vacancies, which enhances the electrocatalytic activity. Overall, the average valence states of Pr, Co, and Fe decrease in the P_0.2SCF@100 powder, this reduction in valence states suggests improved catalytic performance due to increased oxygen vacancy concentration.

Fig. 3 XPS spectra of (b) Pr 3d, (c) Sr 3d, (d) Co 2p, (e) Fe 2p, and (f) O 1s in P_0.2SCF and P_0.2SCF@100 (a).

Electrochemical performance

Fig. 4 presents the EIS and DRT analysis for P_0.2SCF used as both the cathode and anode, with the ohmic impedance of the electrolyte subtracted for a clearer comparison of the R_p. When P_0.2SCF is used as the cathode, the R_p values at 700 °C, 725 °C, 750 °C, 800 °C and 850 °C were 0.14 Ω cm², 0.098 Ω cm², 0.069 Ω cm², 0.041 Ω cm² and 0.029 Ω cm² as shown in Fig. 4(a), respectively. The results of the DRT analysis showed that the resistive response frequency of P_0.2SCF as a cathode is mainly in the mid-frequency range (10²–10⁴ Hz) as shown in Fig. 4(b), indicating that the process of absorption and desorption of oxygen is the rate-limiting step. As an anode, the R_p values at 700 °C, 725 °C, 750 °C, 800 °C and 850 °C were 0.34 Ω cm², 0.24 Ω cm², 0.17 Ω cm², 0.086 Ω cm² and 0.054 Ω cm² as shown in Fig. 4(c), respectively. The resistive response frequency of P_0.2SCF is mainly in the low to mid-frequency range (<10² Hz) as shown in Fig. 4(d), indicating that the rate-limiting step is mainly the diffusion and adsorption–desorption process of hydrogen on the electrode surface, with gas diffusion limitations being particularly prominent at low frequencies (1 Hz). The peak intensity decreases rapidly with increasing temperature, indicating that increasing temperature is favorable for increasing the electrocatalytic activity.

Fig. 4 (a) and (b) EIS and corresponding DRT plots of P_0.2SCF in an air atmosphere; (c) and (d) EIS and corresponding DRT plots of P_0.2SCF in 5% H₂/95% Ar atmosphere.

Both the ORR at the cathode and the HOR at the anode in SSOFCs involve complex reaction steps. To further clarify the kinetics of the ORR and the HOR catalyzed by the P_0.2SCF electrode, the R_p of P_0.2SCF as both the cathode and anode was examined under varying partial pressures of oxygen and hydrogen. This analysis aims to identify the rate-limiting steps of these reactions. Fig. 5(a) illustrates the R_p values at different partial pressure of oxygen (P_O2) at 800 °C. In addition, EIS data at other temperatures and P_O2 are shown in Fig. S1,† with the corresponding fitting results presented in Table S2.† The results show that a decrease in R_p with increasing P_O2, highlighting the significant influence of P_O2 on the ORR. In addition, temperature dependence tests show that R_p decreases with increasing temperature, suggesting that higher temperatures enhance electrode responses. The measured R_p values were fitted using Zsimpwin software and the equivalent circuit fitting model is shown in Fig. S2.† The contributions of the high frequency resistance (R_H) and low frequency resistance (R_L) to the overall R_p are critical. As shown in Fig. 5(c), for the P_0.2SCF cathode at P_O2 of 0.21 atm, the activation energies are 122.2 kJ mol⁻¹ for R_H and 134.68 kJ mol⁻¹ for R_L. This comparison reveals that the working temperature has a greater impact on the oxygen ion migration in the cathode, making R_H more sensitive to temperature changes. In contrast, the adsorption/desorption process on the electrode surface is less temperature-dependent, resulting in a higher activation energy for R_L. The relationships between P_O2 and R_p can be described using the following equation R_p = R₀P_O2⁻ⁿ, where different values of n indicate different rate-determining steps in the electrochemical reaction: (1) n = 1, O_2(g) → O_2,ads, indicating oxygen adsorption on the electrode surface and diffusion in the organic phase. (2) n = 0.5, O_2,ads → 2O_ads, indicating the dissociation of adsorbed oxygen into atomic oxygen. (3) n = 0.25, , indicating a charge transfer process. Fig. 5(e) shows the linear relationship between P_O2 and R_p for the P_0.2SCF cathode at different temperatures. In the 700–850 °C range, the n values are 0.2, 0.33, 0.34, and 0.35, which are close to 0.25. This suggests that charge transfer is the rate-limiting step for the ORR under these conditions.

Fig. 5 (a) EIS for different P_O2 at 800 °C; (b) EIS for different P_H2 at 800 °C; (c) Arrhenius plots of R_H and R_L changes with temperature at P_O2 = 0.21 atm; (d) Arrhenius plots of R_H and R_L changes with temperature at P_H2 = 0.05 atm; (e) correlation between P_O2 and R_p at different temperatures; (f) dependence of R_H and R_L on the P_H2.

As an anode, R_H represents the charge transfer process at the electrode–electrolyte interface, while R_L represents the hydrogen adsorption and dissociation process at the anode surface. Fig. 5(b) shows the R_p values at 800 °C for different hydrogen partial pressures (P_H2). In addition, EIS data at other temperatures and P_H2 are shown in Fig. S3,† with the corresponding fit results presented in Table S3.† The R_p decreases with increasing P_H2, indicating the influence of the P_H2 on the HOR. Temperature-dependent tests of the P_H2 reveal that R_H is consistently greater than R_L, indicating that the P_0.2SCF anode has a high dependence on the working temperature. According to the fitting results in Fig. 5(d), the activation energies of R_H and R_L are 80.21 kJ mol⁻¹ and 88.1 kJ mol⁻¹ at a P_H2 of 0.05 atm, respectively. Fig. 5(f) shows the relationship between R_H, R_L, R_p, and P_H2. The slope of the R_L fitted line (−0.71) is smaller than the slope of the R_H fitted line (−0.25), indicating that hydrogen adsorption and dissociation on the anode surface are highly sensitive to the P_H2 during the reaction process at the P_0.2SCF anode.

For an in-depth evaluation of the electrochemical stability of P_0.2SCF as the anode, the long-term impedance stability test in a 5% H₂/95% Ar atmosphere at 800 °C was performed, as shown in Fig. 6(a), the R_p of the P_0.2SCF anode decreases from 0.088 to 0.077 Ω cm² over 120 h and remains stable. This decrease in R_p suggests that P_0.2SCF exhibits excellent catalytic activity for the HOR process when used as an anode, which is in good agreement with the XPS test results very well. Fig. 6(b) displays the SEM cross-sectional morphology of the P_0.2SCF anode after the long-term stability test. The P_0.2SCF anode retains a loosely packed structure and adheres tightly to the LSGM electrolyte without significant delamination or cracking, indicating good interfacial stability between the electrode and the electrolyte. These results demonstrate that P_0.2SCF possesses excellent and stable electrocatalytic activity for the HOR in a reducing atmosphere, making it a promising candidate for use as an anode in SSOFCs.

Fig. 6 (a) 120 h stability test of the P_0.2SCF anode in 5% H₂ at 800 °C; (b) microscopic cross-section of a half-cell after the stability test; (c) I–V–P curves of the single cell with P_0.2SCF in wet H₂ at 700–850 °C; (d) polarization impedance values for a single cell; (e) DRT analysis of the P_0.2SCF single point cell at 700–850 °C, (f) stability test for a single cell.

The I–V–P curves of the symmetric cell P_0.2SCF|LSGM|P_0.2SCF are shown in Fig. 6(c). The maximum power density (MPD) of the cell increases with temperature, and the current and voltage exhibit a clear linear relationship, indicating minimal concentration and activation polarization. The MPD of the single cell reached 719.08 mW cm⁻² at 850 °C. Fig. 6(d) presents the EIS data of the P_0.2SCF single cell at different temperatures. The intercept of the high-frequency arc with the real axis represents the ohmic resistance (R_o), while the intercept of the low-frequency arc with the real axis indicates the total resistance (R_t). The R_p of the single cell is calculated as the difference between R_t and R_o. At 850 °C, the R_o and R_p of the single cell are 0.3 Ω cm² and 0.42 Ω cm², respectively. Due to the high conductivity of the electrode material (Fig. S4†), the primary source of the R_o of the cell is the electrolyte. These results highlight the potential of P_0.2SCF as a high-performance electrode material in SSOFCs, demonstrating both excellent electrochemical stability and catalytic activity.

To elucidate the reaction kinetics of the P_0.2SCF symmetric single cell, the EIS results of a symmetric single cell using the DRT method were analyzed. This analysis directly calculates the distribution function of the relaxation times and the relaxation amplitude of the impedance-dependent processes in Fig. 6(e). The DRT peaks at different response frequencies correspond to different electrochemical processes. The DRT results for the three groups of peaks were categorized, labeled P1 to P3, based on their response frequencies. The relative heights of these peaks indicate the extent to which each reaction process contributes to the overall reaction in the cell. Specifically, the high-frequency peak P3 (10³ to 10⁴ Hz) represents the ion transport processes within the P_0.2SCF electrode, the LSGM electrolyte, and their interface. The mid-frequency peaks P2 (10¹ to 10³ Hz) represent the HOR and ORR on the electrode surface, and the gas adsorption/desorption on the electrode material surface, respectively. The low-frequency peak P1 (<10¹ Hz) corresponds to that of hydrogen and oxygen on the porous electrode surface. Our observations indicate that the processes represented by P3 (ion transfer) are relatively insensitive to temperature. In contrast, P1 and P2 (gas diffusion and catalytic dissociation) decrease fast with increasing temperature in the range of 700–850 °C, indicating a strong temperature dependence. This suggests that the primary rate-limiting step for the single cell is the diffusion, adsorption and desorption of gas on the P_0.2SCF electrode. These results highlight the critical role of gas diffusion, adsorption and desorption in determining the overall reaction rate, providing valuable insights for optimizing the performance of P_0.2SCF-based SSOFCs. The cell showed no significant degradation during the 120 hours of operation, as shown in Fig. 6(f), providing an initial indication of its stability. However, slight voltage fluctuations were observed. Therefore, further testing over a longer period is necessary to confirm its stability.

Conclusions

This study investigated the effects of various Pr doping levels on the physical phase structure and electrochemical properties of Pr-doped SrCoFeO_3−δ (P_xSCF) materials. AC impedance measurements identified Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3−δ (P_0.2SCF) as having the optimal electrochemical performance, maintaining a stable orthorhombic perovskite structure at doping ratios ≤0.4 and exhibiting the highest HOR and ORR activities. Reduction tests at 800 °C in a 5% H₂/95% Ar atmosphere for 2 and 120 h demonstrated that P_0.2SCF retains its crystal structure and micro-morphology, indicating excellent structural stability. The polarization impedance values at 850 °C were 0.029 Ω cm² for the cathode and 0.054 Ω cm² for the anode. Long-term stability tests showed a decrease in polarization impedance from 0.088 Ω cm² to 0.077 Ω cm² at 800 °C. The maximum power density of a single cell using LSGM as the electrolyte was 719.08 mW cm⁻² at 850 °C. Overall, P_0.2SCF exhibits outstanding high electrochemical catalytic activity, making it a promising candidate electrode for SSOFCs.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Peng Li: investigation, methodology, writing – original draft. Bing Yang: data curation, formal analysis, investigation, methodology. Jing Chen: methodology, data curation, supervision, writing – review & editing. Bo Li: methodology, funding acquisition. Lushan Ma: methodology, project administration, funding acquisition. Mengjia Wang: investigation. Xuzhuo Sun: methodology, project administration, funding acquisition, supervision. Yunfeng Tian: methodology, supervision, conceptualization writing – review & editing. Bo Chi: methodology, funding acquisition, project administration.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22279029 and 52302334), Key Scientific and Technological Research Project in Henan Province (No. 242102241022), Jiangsu Province (BZ2022027), the Innovative Funds Plan of Henan University of Technology (2022ZKCJ01) and Changzhou City (CZ20230010).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06084a

‡ These authors contributed equally to this work.

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