As a highly efficient electrochemical energy conversion device with fuel flexibility and high energy efficiency, solid oxide cell (SOC) shows great potential in power generation, energy storage, and synthesis of high-value-added chemicals. As the core components of SOC, the fuel and air electrodes need to have excellent thermal stability, hybrid conductivity, catalytic activity, and carbon and sulfur poisoning resistance at high temperatures and in oxidizing/reducing environments. Sr₂Fe_2-xMo_xO_6-δ-based double perovskite (SFM) has become a research hotspot for SOC electrode materials in recent years due to its unique double perovskite structure, tunable chemical composition, high electron-ion mixed conductivity, and good redox stability. In this review, we systematically review the progress of SFM-based materials in applying SOC fuel electrodes and air electrodes, focusing on their structural properties and performance optimization strategies in different reaction environments.

The crystal structure of SFM double perovskite is characterized by alternately arranged FeO₆ and MoO₆ octahedra that can form two-dimensional oxygen ion transport channels and significantly reduce the oxygen ion migration barrier. Meanwhile, the degeneracy of Fe³⁺/Fe²⁺ and Mo⁶⁺/Mo⁵⁺ provides the electronic conductance and realizes the synergistic electron-ion transport. Since the A-, B-, and O-site elements of SFM have the potential to be replaced by other ions, the electronic structure and oxygen vacancy concentration of SFM can be precisely regulated when it is used as different reaction electrodes, thus optimizing its hybrid conductivity and electrocatalytic activity. As a fuel electrode, SFM-based bicarbonate shows excellent fuel adaptability, with excellent catalytic performance in a wide range of fuels such as hydrogen, alkanes, syngas, and ammonia, and its carbon resistance and long-term stability are better than those of traditional cermet electrodes. In addition, SFM-based electrodes also exhibit electrolytic performance and stability in the electrolysis of H₂O and CO₂ to produce fuels comparable to that of cermet oxides and other chalcogenide materials. As air electrodes, the high oxygen vacancy concentration of SFM-based bilayers also shows their potential application in oxygen reduction reaction, and the combination of proton conductive modifications, such as doping and in-situ dissolution, can also expand their application in the electrolysis of H₂O and CO₂ by proton-conducting SOC. The review also provides a comprehensive discussion on various strategies to improve the performance of the materials under different reaction conditions, including second-phase composite, impregnation, elemental doping, defect design, and in-situ dissolution, which can enhance the adsorption and activation capacity of the electrodes for reactants by increasing the reactive sites and constructing the heterogeneous interfaces, and improve the long-term stability of the cells.

Although SFM-based electrode materials show a broad application prospect in SOC, problems such as Sr elemental segregation at high temperatures, structural phase transition in reducing atmosphere, and performance degradation during long-term operation must be solved. In the future, we must combine in situ characterization and computational simulation to deeply analyze the charge transport mechanism, surface reaction kinetics, and decay mechanism of SFM-based double perovskite. Through electrode design and multifunctional synergistic modification strategies, it is expected to realize the precise control of nanoparticle components and crystal structure, and develop new electrode materials with both high activity and durability.

The metal-supported reversible proton ceramic cell (MS-rPCC) combines the dual advantages of metal support and proton conduction. It can simultaneously achieve efficient low-temperature operation, high mechanical strength, and excellent thermal cycling stability. However, a critical challenge in MS-rPCC fabrication lies in the element diffusion from the metal support and the mismatch between the metallic and ceramic layers. To address this, a rationally designed pure Ni metallic support combined with a transition layer (80 wt% NiO–20 wt% BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)) was introduced to engineer the interface, improving the strength and structural stability of MS-rPCC. The cell achieved a peak power density (P) of 0.8 W·cm⁻² in fuel cell (FC) mode at 650 °C and a current density (I) of −1.25 A·cm⁻² at 1.3 V in electrolysis cell (EC) mode. The cell exhibited no significant degradation in FC mode after 200 h of operation, with a degradation rate of 0.02 mV·h⁻¹. The cell demonstrated exceptional stability during 100 h of reversible fuel cell/electrolysis cycling, thermal cycling, and rapid startup tests. This work provides a new approach for the commercialization and widespread adoption of MS-rPCC for low-temperature, high-performance power generation and hydrogen production.

Electrochemical carbon dioxide reduction reaction (CO₂RR), powered by advanced technologies such as solid oxide electrolysis cells (SOEC), is a promising method to convert CO₂ into valuable carbon-based products using renewable electricity. The high chemical stability of CO₂ requires catalysts to exhibit both high activity and stable electrocatalytic performance. However, catalysts that deliver high performance in CO₂RR are rare and still require further improvement. Here, we report a strategy that can efficiently enhance catalyst activity through Zn doping, which introduces active frustrated Lewis pairs (FLP) to improve the catalyst's ability to activate small molecules. A high current density of −1.85 A cm⁻² at 800 ℃ under a bias voltage of 1.5 V was achieved using the Sr₂Fe_0.8Zn_0.2MoO_6-δ (SFZn_0.2M) cathode with pure CO₂ feeding gas, surpassing previously reported results for perovskite oxide cathodes. This SOEC device also demonstrates excellent stability, with negligible degradation over tests lasting up to 110 h.

Obtaining high-performance cathodes is critical for protonic ceramic fuel cells (PCFCs), as cathode performance significantly impacts fuel cell performance. A full understanding of the interactions among the diverse properties of cathode materials would benefit cathode design. In this study, PrBaFe₂O_6−δ (PBF) was doped with various dopants, including cobalt (Co), Ni, Cu, Zn, and Mn. Experiments and first-principles calculations are used to study the key properties of dopant-modified PrBaFe₂O_6−δ, including oxygen vacancy (V_O) creation, hydration ability, proton mobility, and oxygen reduction reaction (ORR) activity. There is no perfect dopant that can improve every property to its full potential. Instead, different dopants can impact different properties of the material. Co-dopant has the best cathode performance since it balances the material’s instinctive properties, even though it does not provide a significant advantage in the formation of V_O. PCFC utilizing Co-doped PrBaFe₂O_6−δ cathode has a high performance of 1680 mW·cm⁻² at 700 °C, which is greater than that of the other dopant-tailored PrBaFe₂O_6−δ cathodes reported in this study and is one of the largest ever recorded for PrBaFe₂O_6−δ-based cathodes for PCFCs. Co-doped PrBaFe₂O_6−δ cathode is further demonstrated to be robust, with excellent operational stability. This study not only provides a potential cathode candidate for PCFCs but also suggests an intriguing approach to cathode design by carefully examining and balancing different vital properties of the material.

BaCe_0.8Fe_0.1Ni_0.1O_3−δ (BCFN) in a perovskite structure is impregnated consecutively by BCFN solution and BCFN suspension into a phase-inversion prepared NiO–Gd_0.1Ce_0.9O_2−δ (GDC) scaffold as an anode for solid oxide fuel cells (SOFCs) with on-cell dry reforming of methane (DRM). The whole pore surface of the scaffold is covered by small BCFN particles formed by BCFN solution impregnation; the large pores near the scaffold surface are filled by BCFN aerogels with a high specific surface area produced by BCFN suspension impregnation, which act as a catalytic layer for on-cell DRM. After reduction, the anode consists of a Ni–GDC scaffold and BCFN particles with exsolved FeNi₃ nanoparticles. This BCFN-impregnated Ni–GDC anode has higher electrical conductivity, electrochemical activity, and resistance to carbon deposition, with which the cell shows maximum power densities between 1.44 and 0.92 W·cm⁻² when using H₂ and between 1.09 and 0.50 W·cm⁻² when using CO₂–CH₄ at temperatures ranging from 750 to 600 °C. A stable performance at 400 mA·cm⁻² and 700 °C is achieved using 45% CO₂–45% CH₄–10% N₂ for more than 400 h without carbon deposition, benefiting from the impregnated BCFN aerogel with a high specific surface area and water adsorbability.

Reliable and economical energy storage technologies are urgently required to ensure sustainable energy supply. Hydrogen (H₂) is an energy carrier that can be produced environment-friendly by renewable power to split water (H₂O) via electrochemical cells. By this way, electric energy is stored as chemical energy of H₂, and the storage can be large-scale and economical. Among the electrochemical technologies for H₂O electrolysis, solid oxide electrolysis cells (SOECs) operated at temperatures above 500 ℃ have the benefits of high energy conversion efficiency and economic feasibility. In addition to the H₂O electrolysis, SOECs can also be employed for CO₂ electrolysis and H₂O–CO₂ co-electrolysis to produce value-added chemicals of great economic and environmental significance. However, the SOEC technology is not yet fully ready for commercial deployment because of material limitations of the key components, such as electrolytes, air electrodes, and fuel electrodes. As is well known, the reactions in SOEC are, in principle, inverse to the reactions in solid oxide fuel cells (SOFCs). Component materials of SOECs are currently adopted from SOFC materials. However, their performance stability issues are evident, and need to be overcome by materials development in line with the unique requirements of the SOEC materials. Key topics discussed in this review include SOEC critical materials and their optimization, material degradation and its safeguards, future research directions, and commercialization challenges, from both traditional oxygen ion (O²⁻)-conducting SOEC (O-SOEC) and proton (H⁺)-conducting SOEC (H-SOEC) perspectives. It is worth to believe that H₂O or/and CO₂ electrolysis by SOECs provides a viable solution for future energy storage and conversion.

Slow oxygen reduction reaction (ORR) involving proton transport remains the limiting factor for electrochemical performance of proton-conducting cathodes. To further reduce the operating temperature of protonic ceramic fuel cells (PCFCs), developing triple-conducting cathodes with excellent electrochemical performance is required. In this study, K-doped BaCo_0.4Fe_0.4Zr_0.2O_3−δ (BCFZ442) series were developed and used as the cathodes of the PCFCs, and their crystal structure, conductivity, hydration capability, and electrochemical performance were characterized in detail. Among them, Ba_0.9K_0.1Co_0.4Fe_0.4Zr_0.2O_3−δ (K10) cathode has the best electrochemical performance, which can be attributed to its high electron (e⁻)/oxygen ion (O²⁻)/H⁺ conductivity and proton uptake capacity. At 750 ℃, the polarization resistance of the K10 cathode is only 0.009 Ω·cm², the peak power density (PPD) of the single cell with the K10 cathode is close to 1 W·cm⁻², and there is no significant degradation within 150 h. Excellent electrochemical performance and durability make K10 a promising cathode material for the PCFCs. This work can provide a guidance for further improving the proton transport capability of the triple-conducting oxides, which is of great significance for developing the PCFC cathodes with excellent electrochemical performance.