Reversible symmetrical solid oxide cells (RS-SOCs) exhibit significant potential in the fields of high-efficiency energy conversion and storage, due to their high energy conversion efficiency and simplified manufacturing processes. The key to their performance lies in the development of robust electrode materials that can function effectively as both the fuel electrode under reducing atmospheres and the air electrode under oxidizing atmospheres, during reversible operation between solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes. Sr₂Fe_1.5Mo_0.5O_6-δ (SFM)-based materials have attracted considerable attention in this field, owing to their excellent mixed ionic-electronic conductivity, high structural stability and remarkable tolerance to sulfur poisoning and carbon deposition. This study was to focus on Sr_1.85Fe_1.3Co_0.2Mo_0.5O_6-δ (SFCM), a modified composition, in which partial substitution with Co to further enhance electrochemical activity, while maintaining sufficiently high stability. The primary objectives are to synthesize high-purity SFCM powder via an optimized method, comprehensively characterize its phase evolution and structural stability under both oxidizing and reducing conditions relevant to SOC operation, and ultimately evaluate its practical electrochemical performance as a symmetrical electrode in a single cell.

The material was prepared using an EDTA-citrate dual-complexing sol-gel method. Crystal structure and phase evolution of the SFCM material were studied by using X-ray diffraction (XRD). XRD analysis was also performed on samples thermally reduced at 900 ℃ under a reducing atmosphere to simulate the fuel-electrode environment. Microstructure and electrode/electrolyte interface of the cells were examined by using scanning electron microscopy (SEM). The SFCM electrode was composited with gadolinium-doped ceria (GDC) to form an SFCM-GDC composite electrode, which increases the triple-phase boundary length and mitigates potential chemical reactions with the electrolyte. Electrochemical impedance spectroscopy (EIS) was employed to analyze polarization resistance of the electrodes. Finally, performance of the single cell was tested in SOFC mode using 3 vol.% H₂O-97 vol.% H₂ as fuel and air as oxidant and in SOEC mode under an applied voltage for electrolysis of a 50 vol.% H₂-50 vol.% CO₂ gas mixture.

XRD results confirmed that the powder calcined in air at 1200 ℃ is of a pure double perovskite structure, as the phase of Sr₂Fe_1.5Mo_0.5O₆, indicating success in synthesis of the target materials. After reduction at 900 ℃, significant phase evolution was revealed. While the primary double perovskite structure was largely retained, new phases emerged, including a distinct double perovskite phase identified as Sr₃FeMoO₇ and a metallic cubic phase corresponding to a CoFe alloy. This in-situ exsolution of alloy nanoparticles under reducing conditions is highly beneficial, as it can significantly enhance the electrocatalytic activity for fuel oxidation and reduction reactions. SEM results showed that the SFCM-GDC composite electrode adhered well to the SSZ electrolyte after sintering, forming a porous electrode layer with strong connectivity, which is crucial for gas diffusion and charge-transfer processes. Electrochemical tests demonstrated promising performance. In SOFC mode at 800 ℃, the symmetrical single cell with SFCM-GDC electrodes achieved a maximum power density of 0.52 W·cm^-2, when fueled with humidified hydrogen, indicating promising anodic and cathodic activity. More notably, in SOEC mode at 800 ℃ under an electrolysis atmosphere (50 vol.% H₂-50 vol.% CO₂), the single cell reached a current density of 1.106 A·cm^-2 at an applied voltage of 1.5 V.

SFCM material synthesized using the EDTA-citrate sol-gel method demonstrates excellent potential as a symmetrical electrode for RS-SOCs. The material exhibits remarkable structural adaptability, maintaining a stable double perovskite framework in air and undergoing favorable in-situ phase evolution under reducing conditions to form catalytically active CoFe alloy nanoparticles along with a stable Sr₃FeMoO₇ phase. This bifunctional character underpins its robust electrochemical performance. The fabricated symmetrical cell with an SFCM-GDC composite electrode on an SSZ electrolyte delivered competitive power output in fuel cell mode. More importantly, high current density was realized in electrolysis mode at 800 ℃. These results validate SFCM as a highly active and stable electrode material suitable for both power generation and fuel synthesis in reversible solid oxide cells. It is confirmed that strategic cationic substitution (Co for Fe) in the Sr-Fe-Mo-O system, combined with composite electrode engineering, is an effective approach for developing high-performance symmetrical electrodes.

Solid oxide fuel cell (SOFC) has garnered significant attention due to the clean and efficient energy conversion characteristics, with the performance of cathode materials playing a decisive role in the overall efficiency and lifetime of the device. However, the commercialization of SOFC in the intermediate- to low-temperature range (500–800 ℃) faces two major challenges: insufficient electrocatalytic activity for the oxygen reduction reaction (ORR) at the electrodes and poor long-term operational stability. Although traditional perovskite cathode materials (such as La_1–xSr_xCoO_3–δ, LSC, etc.) exhibit excellent catalytic activity for ORR, they still suffer from issues, such as element segregation, thermal expansion coefficient mismatch, inadequate high-temperature stability, and susceptibility to CO₂/Cr poisoning. In recent years, high-entropy perovskite oxide (HEPO) materials have emerged as a significant research direction for overcoming the limitations of traditional cathode materials by introducing multi-element lattice distortion effects, which demonstrate unique structural stability and performance regulation potential. The high configurational entropy of HEPO materials effectively inhibits cation segregation, enhances phase structural stability, and optimizes oxygen reduction reaction kinetics through the synergistic effect of multiple active sites. Furthermore, high-entropy design can improve the anti-poisoning capability and electronic/ionic conductivity of materials by adjusting the d–band center or oxygen vacancy concentration, providing new insights for the practical application of intermediate- and low-temperature SOFC.

Compared with conventional doping methods, the high-entropy strategy exhibits greater flexibility in component regulation, enabling precise adjustments of the chemical composition, defect characteristics and the disordered/ordered structures of materials, thereby achieving targeted optimization of material properties. In contrast to traditional perovskite materials, the introduction of multiple ions within specific lattices through the high-entropy strategy significantly expands the design space and customization potential of cathode materials, owing to the highly uniform distribution of various elements in the lattice. The preparation methods for high-entropy perovskite materials are relatively diverse, with liquid-phase methods currently being predominantly used to synthesize the high-performance high-entropy perovskite cathode materials. The A-site in the lattice is typically occupied by rare earth or alkaline earth metal cations with larger ionic radii, while the B-site is filled by transition metal cations with relatively smaller ionic radius. HEPO materials exhibit the novel functional characteristics and infinite possibilities, due to the extensive selectivity in elemental composition and content. Depending on the doping elements, they are primarily categorized into ABO₃-type A-site, ABO₃-type B-site, AA'BB'O₆-type and other types of high-entropy perovskite cathode materials. This approach significantly improves the catalytic activity of cathodes, enhances oxygen ion transport capabilities and increases stability, providing new perspectives and solutions to the numerous challenges faced by the traditional SOFC cathode materials, thereby demonstrating cutting-edge and innovative features.

Starting from the design and preparation of high-entropy perovskite materials, the application of high-entropy perovskite cathode materials was focused on, as an emerging and highly potential material system in SOFC. The performance breakthroughs and the mechanism of action are discussed in detail. The high-entropy perovskite material system demonstrates significant enhancement in oxygen reduction activity under medium- and low- temperature operating conditions, effectively suppresses element segregation and simultaneously exhibits excellent CO₂ tolerance, resistance to Cr poisoning and long-term operational stability. The high-entropy strategy has opened new avenues for innovation of SOFC cathode materials. However, current research primarily focuses on the performance enhancement of HEPO, while the elucidation of their mechanisms and theoretical studies still face challenges, mainly manifested in the three aspects. (1) It is necessary to systematically analyze the synergistic mechanisms of multiple elements and quantify the correlation between entropy value and performance through a combination of experimental and theoretical computational methods. (2) It is essential to establish a multi-dimensional evaluation system encompassing electrochemical catalytic activity, oxygen ion transport, chemical compatibility and durability to systematically verify the universality of the four core effects of high entropy under actual SOFC operating conditions. (3) It is crucial to systematically reveal the dynamic regulatory mechanisms of configurational entropy on crystal structure, oxygen vacancy distribution and oxygen reduction kinetics, thereby elucidating the multi-scale correlation laws of entropy-structure-performance.

Solid oxide electrolysis cells (SOECs) are among the most promising technologies for hydrogen production, particularly when coupled with renewable energy sources, such as solar and wind power. Operating at high temperatures, SOECs achieve high electrochemical efficiency, effectively converting electrical energy into hydrogen with lower operational costs. However, their long-term stability and performance degradation remain significant challenges, impeding large-scale commercialization. A fundamental factor influencing SOEC performance is the impedance contributions of its components. Therefore, a comprehensive understanding of the electrochemical impedance characteristics in SOECs is crucial for performance optimization and durability improvement.

In this study, electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) were employed to systematically analyze the impedance contributions of various SOEC components. Experimental measurements were conducted under different operating conditions, including temperature, fuel utilization, steam content and oxygen partial pressure at the oxygen electrode. The impedance response of the fuel and oxygen electrodes was analyzed to identify the dominant electrochemical processes and their respective frequency-dependent characteristics.

DRT analysis revealed multiple characteristic peaks, each corresponding to a distinct kinetic process: P1 (1×10³–1×10⁴ Hz) was associated with oxygen ion transport in the Ni-YSZ fuel electrode, P2 (1×10²–1×10³ Hz) corresponded to charge transfer reactions within the fuel electrode, P3 (50–100 Hz) reflected the charge transfer and ionic transport in the LSC oxygen electrode, P4 (1–10 Hz) represented gas diffusion within the fuel electrode, and P5 (0.1–1.0 Hz) was attributed to gas-phase diffusion in the oxygen electrode and gas conversion reactions in the fuel electrode. Based on these findings, an equivalent circuit model (ECM) was developed to accurately describe the electrochemical behavior of the SOEC. The proposed ECM incorporated a combination of resistive and capacitive elements to represent various charge transfer, mass transport and gas diffusion processes. The model successfully quantified the contributions of different polarization losses, offering valuable insights into the dominant limitations affecting the performance of SOEC. Moreover, it is demonstrated that increasing the operating temperature led to a significant reduction in polarization impedance, due primarily to enhanced oxygen ion conductivity and faster charge transfer kinetics. Similarly, increasing the steam content reduced gas-phase diffusion resistance, thereby improving overall SOEC efficiency. Conversely, higher fuel utilization ratios resulted in increased concentration polarization, emphasizing the importance of optimizing gas composition and flow rates for stable long-term operation.

This work provides a detailed electrochemical analysis of SOEC impedance contributions using advanced characterization techniques. The combination of EIS, DRT and ECM modeling offers a comprehensive framework for understanding the key kinetic processes governing the performance of SOEC. The findings contributed to the ongoing efforts to enhance SOEC stability, reduce polarization losses and improve overall efficiency, paving a way for more robust and commercially viable electrolysis systems.

The concentration of carbon dioxide (CO₂) as the main greenhouse gas continues to rise in the Earth's atmosphere. Reducing CO₂ emissions and converting them into high value-added fuels are necessary strategies to achieve the carbon peaking and carbon neutrality goals. Solid oxide electrolysis cell (SOEC) is an energy conversion device that can efficiently convert CO₂ into high value-added fuels, which has great application prospect in the efficient utilization of renewable energy. The cathode materials used to catalyze the CO₂ reduction reaction is one of the key components of SOEC. In addition to traditional cermet materials, various types of perovskite materials (single perovskite, double perovskite and Ruddlesden-Popper phase) are employed as new cathode materials for SOEC, due to their excellent carbon resistance, improved impurity tolerance, high redox stability and adequate ionic and electronic conductivity. However, the relatively low catalytic activity is the main factor restricting the development of perovskite cathode materials. In order to solve this problem, various methods have been adopted to improve the electrolytic catalytic activity of perovskite cathode materials, such as impregnation, A/B site doping and in-situ exsolution. Among them, the B-site doping with transition metal to promote the metal dissolution of perovskite in reducing atmosphere is considered to be an effective way to improve the electrocatalytic activity of perovskite materials.

Compared with traditional deposition technology, exsolution is an "inside-out" technology, in which the metal catalyst migrates from the solid lattice and segregates or aggregates on the substrate surface to produce precipitation, namely, the migration of metal ions to the surface of perovskite and the phase transformation into a metal phase. The exsolution technology of perovskite-type materials (ABO₃) is considered to be an effective approach to improve the catalytic activity of perovskite-type materials through nanoparticle modification. Usually, transition metal ions (e.g., Fe, Co, Ni, Cu, Ru, etc.) are incorporated into the B-site of the perovskite lattice during oxidation and then migrate from the main lattice to the surface during reduction, thus forming metal nanoparticles distributed on surface of the parent perovskite and hence metal/perovskite carrier heterogeneous interface. For exsolved perovskite cathode, after reduction, more CO₂RR active sites are generated, which enhances the catalytic activity of CO₂RR. The exsolved perovskite-based catalytic materials exhibit various advantages. (1) Highly active metal nanoparticles and oxygen-rich perovskite support are generated at the same time. (2) The metal/oxide interface promotes the adsorption and dissociation of CO₂, reduces the charge transfer barrier and has a significant catalytic CO₂RR synergistic effect. (3) The anchoring structure effectively inhibits carbon deposition and metal nanoparticle aggregation. (4) The exsolution/dissolution process is reversible in the redox cycle of practical application. In recent years, the introduction of A-site non-stoichiometry to promote exsolution of the first transition metals (e.g., Fe, Co, Ni, Cu) has attracted extensive attention. Exsolution bimetallic nanoparticles also show excellent synergistic effects in various applications, and catalysts modified with bimetallic nanoparticles (e.g., CoFe and NiFe) show significant performance improvements. The ternary/quaternary alloys proved to be more active than mono - or bimetallic catalysts.

The research progress of metal/alloy exsolved perovskite SOEC cathodes for CO₂ electrolysis have been discussed in detail, while the exsolution mechanism of cathodes in the CO₂RR electrocatalysis was summarized from the perspectives of active site, oxygen site content, CO₂ adsorption/dissociation, metal-oxide interface co-catalysis and so on. The efficient dissolution methods include B-site doped transition metal elements, A-site non-stoichiometric regulation, reduction condition regulation, phase transition, voltage drive and induced lattice strain. Although anchoring nanoparticles with high catalytic activity to perovskite cathodes through in-situ exsolution has various advantages, there are challenges for large-scale long-term applications, focusing on (1) developing and designing a controllable and efficient exsolution method for stable and durable nanoparticles, (2) identifying the influence of material composition on the synergistic catalytic effect between perovskite carrier and exsolution particles, (3) tracing reversible exsolution/dissolution and analyzing the influence of the structural change of perovskite on the performance of CO₂ electrolysis according to the in-situ characterization method and (4) designing a multi-functional, efficient and stable exsolved perovskite cathode material.