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• Protonic ceramic electrochemical cells (PCECs), utilizing proton-conducting electrolyte like BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb), efficiently produce hydrogen via water electrolysis at intermediate temperatures (350-700 ℃) [1]. The generated proton transport to the fuel electrode produces hydrogen gas, effectively separating dry hydrogen from water vapor and oxygen, reducing purification costs [2,3]. However, slow air electrode kinetics and degradation under high steam concentrations hinder their large-scale deployment [4]. Perovskite-based mixed ionic and electronic conducting (MIEC) materials like La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ have been explored as air electrodes for their high conductivity and catalytic activity [5,6]. While these materials show poor hydration and limited proton transport with the oxygen evolution reaction (OER) activity confined to the triple-phase boundaries. Additionally, high steam concentrations cause significant degradation, reducing the material’s performance and stability [7,8].

  Recently, triple-conducting oxides (TCOs) that concurrently transport electrons (e^-), oxide ions (O^2-), and protons (H⁺) have gained wide attentions [9]. The water electrolysis at TCOs involves adsorption of steam on the surface, oxygen formation and desorption, and proton transport to the electrolyte [10]. Among these, oxygen vacancies serves as adsorption sites for water, facilitating the proton defects (OHO·) formation via hydration reactions, as shown in Eq. 1.

  H2O+VO··+OO×↔2OHO·

  (1)

  The species OHO· is the main carrier of proton transport, enabling proton conduction by hopping between lattice oxygen atoms [11]. Adding proton conductivity into the MIEC electrodes can extend the reaction area from the electrode/electrolyte interface into the electrode bulk, thereby enhancing the electrode performance [12]. Several perovskite-structured TCOs electrodes, such as PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_6-δ [13], Ba_1-xGd_0.8La_0.2+xCo₂O_6-δ [14], and La₂NiO_4+δ [15] show potential in PCECs, but further improvement is needed in reaction kinetics and stability under harsh steam conditions.

  Recently, the double perovskite material Sr₂Fe_1.5Mo_0.5O_6-δ (SFM) has shown promise as an air electrode matrix due to its high OER activity and stability in water electrolysis [16]. However, the OER performance of pure SFM perovskite remains inadequate. Conventional strategies to enhance its properties involve introducing lower-valent cations (e.g., Zr⁴⁺, Y³⁺) into the B-site of SFM perovskite to increase oxygen vacancies and proton defects, facilitating the proton uptake and mobility [17,18]. Nevertheless, a challenge arises due to the large ionic radius mismatch between these dopants (e.g., Zr⁴⁺ = 0.72 Å, Y³⁺ = 0.90 Å) and the host Mo^5+/6+ cations (0.61 Å/0.59 Å). This mismatch induces local lattice distortion, primarily manifesting as tilting and deformation of BO₆ octahedra of SFM pervoskite. Such octahedral distortion disrupts the crystal symmetry, affecting the pathways for proton hopping and increasing the energy barrier for proton movement. While this distortion might modestly enhance OER activity to some extent, prolonged operation may induce further structural changes that compromise the material’s long-term stability [19]. Therefore, selecting dopant cations with closely matched ionic radii is critical to minimize lattice strain and ensure both high performance and long-term structural stability.

  Herein, tantalum (Ta⁵⁺) is introduced into SFM material to form a triple-conducting air electrode material Sr₂Fe_1.5Mo_0.4Ta_0.1O_6-δ (SFMTa) for PCECs. The heterovalent substitution of Mo^5+/6+ by Ta⁵⁺ creates more oxygen vacancies and the compatible ionic radius of Ta⁵⁺ (0.64 Å) and Mo^5+/6+ (0.61 Å/0.59 Å) minimize lattice distortion, facilitating the hydration process and improving proton mobility. Thanks to these advantages, the single cell with SFMTa air electrode exhibits excellent electrolysis performance, achieving a higher current density of 2.17 A/cm² at 1.3 V and 700 ℃, and maintaining stable operation for 120 h without significant degradation. This study presents a novel design strategy for developing PCEC air electrode, providing a practical approach for their application.

  X-ray diffraction (XRD) patterns of the calcined SFM and SFMTa powders (Fig. 1a) reveal cubic phase structures without impurity phases, indicating successful Ta⁵⁺ doping in SFM. XRD Rietveld refinement further verifies that both samples crystallize in the double-perovskite structure with the Fm-3m space group. Notably, XRD refinement reveals a slight increase in the unit cell volume, from 482.49 ų for pristine SFM (Fig. S1 in Supporting information) to 483.04 ų for SFMTa (Fig. 1b). This minor lattice expansion is attributed to the partial substitution of Mo^5+/6+ with the larger Ta⁵⁺ ion, and the formation of extra oxygen vacancies [20]. The unit cell volume change is approximately 0.11%, compared to 4.69% of Sr₂Fe_1.5Mo_0.4Y_0.1O_6-δ (SFMY) [17] and 1.78% of Sr₂Fe_1.5Mo_0.4Zr_0.1O_6-δ (SFMZ) [18], indicating the favorable ionic radius compatibility between Ta⁵⁺ and the Mo-site cations. The X-ray photoelectron spectroscopy (XPS) spectra of Ta 4f further validate the successful incorporation of Ta⁵⁺ (4f_7/2, 25.14 eV) into the SFM structure (Fig. 1c) [21]. Scanning electron microscopy (SEM) images show smooth surfaces and uniform particle sizes of both samples, with an average particle diameter of approximately 900 nm for both materials (Fig. S2 in Supporting information). The High-resolution transmission electron microscopy (HR-TEM) diaplays the crystal structure of SFMTa (Fig. 1d). The two orthogonal facets are assigned to the (220) and (22¯0) planes, while the facet at a 45 ° angle corresponds to the (040) plane. The fast Fourier transform (FFT) of the electron diffraction patterns along the [001] zone axe aligns with the double perovskite structure. Additionally, energy-dispersive X-ray (EDX) mapping shows uniform distribution of elements within SFMTa, further supporting the successful integration of Ta⁵⁺ into the SFM phase (Fig. 1e)

  Figure 1

  

  Figure 1.  (a) XRD patterns of SFM and SFMTa samples. (b) Rietveld refinement plot of SFMTa sample. (c) XPS spectra of Ta 4f. (d) HR-TEM images of SFMTa along with corresponding FFT images. (e) EDX results of SFMTa sample.

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  The oxygen vacancy concentration and proton defects (OHO·) generation significantly impact the material’s OER performance [22]. During the hydration reaction, oxygen vacancies serve as active sites for water molecule adsorption. Thus, the increased number of oxygen vacancies in the air electrode facilitates the hydration process and proton defects formation [23]. Thermogravimetric analysis (TGA) reveals that SFMTa exhibits greater weight loss at 800 ℃ in argon atmosphere. The weight reduction typically suggests the formation of oxygen vacancies [21], indicating that SFMTa has a higher concentration of oxygen vacancies (Fig. 2a). Electron paramagnetic resonance (EPR) is used to quantify the oxygen vacancy concentration in the bulk (Fig. S3 in Supporting information). The EPR signals at g = 2.003 correspond to single-electron-trapped oxygen vacancies [24]. Notably, the SFMTa sample exhibits a more intense peak compared to SFM, indicating an increase in oxygen vacancy concentration. Temperature-programmed desorption of O₂ (O₂-TPD) is further investigated the oxygen desorption properties (Fig. S4 in Supporting information). SFMTa exhibits lower initial desorption temperature (400.5 ℃) and higher resolved oxygen content, suggesting its superior oxygen activation ability and greater oxygen vacancy content, which facilitates the hydration process [25,26]. The O 1s XPS spectra reveal the surface oxygen species, including adsorbed oxygen (O_ads, such as H₂O and O₂, ~530.9 eV), lattice oxygen (O_lat, ~528.8 eV) and physical adsorption of oxygen (carbonate or hydroxyl groups, ~532.9 eV) [27]. The higher binding energy is associated with the adsorbed oxygen in the oxygen vacancies [28]. As shown in Fig. 2b and Table S2 (Supporting information), the O_ads/O_lat ratios in SFM and SFMTa are 1.38 and 1.69, respectively, indicating an increase in the concentration of oxygen vacancies [12,17]. Additionally, thermogravimetric relaxation (TGR) shows SFMTa gains 0.81% weight under rapid humidity changes at 500 ℃, higher than SFM of 0.51%, further confirming the improved hydration capacity after Ta⁵⁺ doping (Fig. 2c).

  Figure 2

  

  Figure 2.  SFM and SFMTa samples: (a) TGA curves at 200-800 ℃ in an argon atmosphere. (b) O 1s XPS spectra. (c) Thermogravimetric relaxation curves. (d) ECR responses curves from pO₂ = 0.21 to 0.1 atm in dry air at 700 ℃. (e) ECR response curves when switching the atmospheres from dry to pH₂O = 0.05 atm at 700 ℃. (f) The fitted k_H,chem and D_H,chem value from 550-700 ℃.

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  The generation and migration abilities of charge carriers (O^2- and H⁺) are crucial for optimal OER performance in air electrode materials. The electrical conductivity relaxation (ECR) measurements show that SFMTa has a shorter conductivity relaxation time (210 s) than SFM (490 s) at 700 ℃ when the oxygen partial pressure switches from 0.21 atm to 0.1 atm (Fig. 2d). The calculated oxygen bulk-diffusion coefficient (D_O,chem) are 2.43 × 10^-5 cm²/s for SFM and 3.40 × 10^-5 cm²/s for SFMTa, suggesting superior oxygen ion migration capability of SFMTa material (Fig. S5 in Supporting information). The ECR measurements after a sudden change in steam partial pressure reveal a nonmonotonic conductivity response as pH₂O increases from 0 to 0.05 atm, initially rising sharply, then decreasing and stabilizing (Fig. 2e). This phenomenon is attributed to the mobility of holes (h^·), oxygen vacancies (Vo..), and protons (H⁺). Upon a sudden increase in pH₂O, surface oxygen vacancies are occupied by oxygen from water, forming lattice oxygen and generating holes (O + Vo.. → OO× + 2h^·). The protons, produced by the combination of hydrogen in water and holes (H + h^· → H⁺), consume holes and combine with lattice oxygen to form OHO·. As the oxygen ions migrate faster than protons, the nonmonotonic response to increasing water pressure can be observed [29]. The ECR fitting results further confirm that Ta⁵⁺ enhances the surface exchange coefficient of water molecules (k_{H2O, chem}) and chemical diffusion coefficients of protons (D_H,chem) of the material. Specifically, the k_{H2O, chem} is 1.42 × 10^-5 cm/s for SFMTa and 6.48 × 10^-6 cm/s for SFM, while the D_H,chem for SFMTa is 1.06 × 10^-5 cm²/s², which is higher than SFM value of 0.58 × 10^-5 cm²/s² at 700 ℃ (Fig. 2f). The calculated D_O,chem (Fig. S5) exceeds D_H,chem, further supporting the observed trend in the ECR curve of water response. This enhancement is attributed to the use of radius-matched dopant Ta⁵⁺ ions, which contribute to improved proton mobility and benefit the OER reaction.

  The electrochemical impedance spectroscopy (EIS) spectra from symmetrical cells of SFM and SFMTa electrodes are tested in air atmosphere. The plots are fitted using an equivalent circuit model, L-R-QR_h-QR_i-QR_l. The polarization resistance (R_p) is composed by R_h, R_i, and R_l, representing the resistances associated with the primary processes, including gas adsorption/diffusion and charge transfer during the OER. Fig. 3a shows that R_p of the SFMTa cell is 0.129 Ω cm² at 700 ℃, lower than SFM electrode (0.166 Ω cm²). EIS plots of SFM and SFMTa cells from 550 ℃ to 700 ℃ are displayed in Fig. S6 (Supporting information). The reduced R_p of SFMTa indicates enhanced OER electrochemical activity. Arrhenius analysis reveals that SFMTa has a lower OER activation energy (0.948 eV) than SFM (0.993 eV), confirming the beneficial effect of Ta doping on the activity enhancement (Fig. 3b). Distribution of relaxation times (DRT) analysis of the EIS spectrum at 700 ℃ (Fig. 3c) reveals distinct peaks in three frequency ranges: Low frequency (LF, 10^-2-1 Hz), intermediate frequency (IF, 1-10⁴ Hz), and high frequency (HF, 10⁴-10⁵ Hz), corresponding to gas diffusion, surface exchange/ion transfer, and charge transfer, respectively [30]. Ta⁵⁺ doping significantly reduces resistances in the IF and HF regions, thus enhancing the surface gas adsorption and ion transfer process. Furthermore, the effect of different pO₂ on the SFMTa electrode is analyzed using EIS and DRT curves (Fig. S7 in Supporting information). The R_p value decreases from 0.129 Ω cm² to 0.070 Ω cm² as the pO₂ rises from 20% to 100%. Three peaks corresponding to distinct processes are observed, following the relationship of R_p = k(pO₂)^-n (Fig. S8 in Supporting information). The relatively high values of n in the LF (n = 0.43) and IF (n = 0.47) regions reflect the sensitivity to the changes in pO₂, indicating that O₂ content influences the gas diffusion process and surface reaction kinetics [31].

  Figure 3

  

  Figure 3.  (a) EIS spectra of BZCYYb electrolyte-supported symmetrical cells in dry air at 700 ℃. (b) Arrhenius plots for R_p from symmetrical cells in dry air at 700 ℃. (c) DRT analysis of SFM and SFMTa symmetrical cells at 700 ℃. I-V curves of (d) SFM and (e) SFMTa cell from 500 ℃ to 700 ℃ in 3 vol% H₂O/air. (f) Stability test of SFMTa cell at 1.3 V and 600 ℃ with 3 vol% H₂O/air and 30 vol% H₂O/air.

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  The electrochemical performance for OER is assessed using single cells with SFM and SFMTa air electrodes with pure hydrogen in the fuel electrode and a 3 vol% H₂O/air mixture in the air electrode, tested at 500-700 ℃. The cross-sectional image of the tested cell shows a sandwich structure consisting of a porous SFMTa electrode (~22 μm), a dense BZCYYb electrolyte (~15 μm), and a NiO-BZCYYb support layer (Fig. S9 in Supporting information). After sintering, the cell structure remains intact with strong interfacial bonding, facilitating the water electrolysis reaction. Compared to the SFM electrode, Ta⁵⁺ doping notably enhances the current density. For instance, SFMTa based single cell exhibits the corresponding current densities of 2.17, 1.43, 0.92, 0.48 and 0.26 A/cm² at 700, 650, 600, 550 and 500 ℃ (Fig. 3e), respectively, which are higher than SFM cell (Fig. 3d). Furthermore, the SFMTa cell exhibits relatively higher current density than that of other recently reported air electrodes (Fig. S10 and Table S3 in Supporting information). Additionally, the durability of the SFMTa single cell is evaluated under a voltage of 1.3 V at 600 ℃ (Fig. 3f). The cell exhibits no significant performance degradation over approximately 120 h under 3 vol% H₂O/air, indicating the stability during steam electrolysis conditions. To evaluate its practical applicability, XRD analysis is conducted to assess the structural change after exposure to 30 vol% H₂O/air at 600 ℃ for 10 h. The material maintains a stable perovskite crystal structure (Fig. S11 in Supporting information). Moreover, the single cell is subjected to a stability test under 30 vol% H₂O/air atmosphere. The results indicate an improvement in current density as steam concentration increased, while electrolysis stability is still maintained throughout the 120 h test. The durability test confirms that radius-matched Ta⁵⁺ doping maintains perovskite structural stability under harsh steam electrolysis conditions and enhances the OER activity of the air electrode.

  In summary, a TCO air electrode of SFMTa has been successfully constructed and evaluated in PCECs. XRD Rietveld refinement confirms close ionic radius matching between Ta⁵⁺ dopants and host Mo-sites, ensuring the structural stability and reducing the distortion of the BO₆ octahedron in SFMTa. TGA, EPR and O₂-TPD results indicate that Ta⁵⁺ doping increases the oxygen vacancy concentration, providing multiple active sites for the proton defects formation. TGR and ECR results reveals the enhanced hydration capability and proton mobility of SFMTa. When tested in air with 3 vol% H₂O at 700 ℃, the SFMTa electrode-based single cell achieves a high current density of 2.17 A/cm² at 1.3 V. Besides, the cell operates stably for 120 h without noticeable degradation. This approach offers a practical solution using ionic radius matching strategy developing efficient and long-lasting air electrodes for PCECs.

  CRediT authorship contribution statement

  Zhen Liu: Writing – original draft, Methodology, Data curation, Conceptualization. Qi Zhang: Software, Methodology, Data curation. Yongqi Zhang: Methodology, Data curation. Rongzheng Ren: Writing – review & editing, Formal analysis, Conceptualization. Chunming Xu: Supervision, Resources, Methodology. Zhenhua Wang: Writing – review & editing, Methodology, Funding acquisition, Conceptualization. Jinshuo Qiao: Validation, Supervision, Project administration. Wang Sun: Validation, Methodology, Conceptualization. Kening Sun: Validation, Software, Methodology.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the National Key R&D Program of China (No. 2022YFB4002501).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111884.

  
  
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  Figure 1  (a) XRD patterns of SFM and SFMTa samples. (b) Rietveld refinement plot of SFMTa sample. (c) XPS spectra of Ta 4f. (d) HR-TEM images of SFMTa along with corresponding FFT images. (e) EDX results of SFMTa sample.

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  Figure 2  SFM and SFMTa samples: (a) TGA curves at 200-800 ℃ in an argon atmosphere. (b) O 1s XPS spectra. (c) Thermogravimetric relaxation curves. (d) ECR responses curves from pO₂ = 0.21 to 0.1 atm in dry air at 700 ℃. (e) ECR response curves when switching the atmospheres from dry to pH₂O = 0.05 atm at 700 ℃. (f) The fitted k_H,chem and D_H,chem value from 550-700 ℃.

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  Figure 3  (a) EIS spectra of BZCYYb electrolyte-supported symmetrical cells in dry air at 700 ℃. (b) Arrhenius plots for R_p from symmetrical cells in dry air at 700 ℃. (c) DRT analysis of SFM and SFMTa symmetrical cells at 700 ℃. I-V curves of (d) SFM and (e) SFMTa cell from 500 ℃ to 700 ℃ in 3 vol% H₂O/air. (f) Stability test of SFMTa cell at 1.3 V and 600 ℃ with 3 vol% H₂O/air and 30 vol% H₂O/air.

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