With the continuous utilization of fossil fuels, environmental pollution has become increasingly severe. Various renewable energy sources, such as solar and wind energies, have been widely deployed. However, their applications are limited by geographic constraints. Solid oxide fuel cells (SOFCs) are clean energy conversion devices capable of directly converting chemical energy into electrical energy, which have garnered significant attention. Traditional SOFCs require different electrode materials to function in distinct atmospheres, necessitating specific material properties. This not only requires thermal compatibility between the electrodes and electrolytes, but also increases manufacturing costs, due to the use of different materials. In symmetrical solid oxide fuel cells (SSOFCs), the anode and cathode are made of same materials, simplifying the fabrication process and hence cutting the down production costs. However, identifying materials that maintain structural stability and exhibit excellent catalytic activity in both oxidizing and reducing atmospheres remains a significant challenge. This paper was aimed to review recent research progress on iron-based perovskites as SSOFC electrode materials from the perspectives of single perovskites, double perovskites and Ruddlesden-Popper (RP) type perovskites. In addition, the catalytic activity and stability of different Fe-based perovskite structures and their applications in SSOFCs are also analyzed and summarized, along with an outlook on future developments of Fe-based perovskite electrodes in SSOFCs.
Most SSOFC electrode materials are modified from conventional anode and cathode materials. However, they often suffer from low catalytic activity, high thermal expansion coefficients (TECs) and instability in reducing atmospheres. Additionally, many electrode materials contain cobalt, which significantly increases the fabrication cost. Among all perovskite materials, Fe-based perovskites have similar TECs to those of electrolytes. Moreover, Fe exhibits variable oxidation and spin states, which endows them with high catalytic activity. However, most Fe-based perovskites are unstable in reducing atmospheres, necessitating modifications to enhance their stability and catalytic activity. Firstly, Fe-based single perovskites follow the general formula ABO3, where the A-site is typically occupied by large-radius alkaline earth metals or lanthanide elements (such as La, Pr, Ba), while the B-site is usually occupied by transition metals (such as Fe, Mn, Ni). By doping and modifying the A-site and B-site elements, as well as adjusting their types and ratios, electrochemical performance, structural stability and conductivity of the perovskite materials can be optimized. Secondly, Fe-based double perovskites can be categorized into A-site ordered double perovskites (AA'BBO6) and B-site ordered double perovskites (AABB'O6). In A-site ordered double perovskites, the A-site elements are lanthanides with smaller ionic radii (such as La, Pr, Nd), whereas the A' site is typically composed of larger cations (such as Ba, Sr, Ca). In B-site ordered double perovskites, the B-site elements are lower-valence cations (such as Fe, Co, Ni), whereas the B' site elements are higher-valence cations (such as Nb, Mo, W). Compared with Fe-based single perovskites, double perovskites offer greater flexibility in tuning the crystal structure and optimizing the catalytic performance. Then, for Fe-based Ruddlesden-Popper (RP) perovskites, the chemical formula is expressed as An+1BnO3n+1. In RP perovskites, the AO layers contain a large number of interstitial oxygen species, while the ABO3 layers have a small number of oxygen vacancies. The stability of RP perovskites is significantly influenced by the doping of transition metal elements at the B-site. Finally, regarding the stability of Fe-based perovskites under different fuel conditions, matching the TECs of the electrode materials with electrolytes can be used to enhance the long-term stability of the material. Additionally, the exsolution of metallic nanoparticles from the material in a reducing atmosphere may further improve the long-term stability of the SSOFCs.
Fe-based perovskites have attracted extensive attention in recent years as promising electrode materials for SOFCs. This paper was aimed to review the progress of Fe-based perovskite materials as symmetrical electrodes in SSOFCs. Since Fe-based perovskites are mainly derived from modifications of conventional cathode materials, they exhibit relatively high polarization resistance on the anode side. Overall, by adjusting the type and proportion of A-site and B-site elements, the electrochemical performance, structural stability, and conductivity of perovskite materials can be optimized. However, despite the development of many high-performance Fe-based SSOFC materials, the cell performance and durability still do not match those of traditional SOFCs. To address this issue, reducing the thickness of the electrolyte is necessary, such as fabricating electrode-supported cells via phase inversion techniques, which could better exploit the high catalytic activity of Fe-based perovskite materials. Additionally, in current SSOFCs, La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) has been widely used as electrolyte. From commercial application perspective, exploring more stable scandium-stabilized zirconia (ScSZ) electrolytes is important. Lastly, Fe-based perovskites still grape with issues associated with carbon deposition and sulfur poisoning, during the ultilization of hydrocarbon fuels. Further efforts are needed to solve these problems.
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