Solid oxide electrolysis cells (SOEC) represent a high-efficiency pathway for hydrogen production, leveraging elevated operating temperatures to achieve favorable thermodynamics and kinetics. Among the critical components of SOECs, perovskite oxide catalysts have emerged as promising electrode materials due to their mixed ionic-electronic conductivity (MIEC), structural flexibility, and tunable properties. Materials such as LaSrCoFeO3 exhibit exceptional catalytic activity for the oxygen evolution reaction (OER) at the anode, a key process in steam electrolysis. This article explores the role of perovskite oxides in SOECs, focusing on their MIEC behavior, degradation mechanisms, and strategies to enhance their durability through doping.

Perovskite oxides possess a general formula of ABO3, where A is typically a rare-earth or alkaline-earth metal and B is a transition metal. The MIEC properties of these materials arise from their ability to conduct both oxygen ions and electrons simultaneously. In LaSrCoFeO3, the partial substitution of La with Sr introduces oxygen vacancies, enhancing ionic conductivity, while the transition metals (Co, Fe) provide electronic conductivity through electron hopping between mixed valence states. This dual functionality is critical for SOEC anodes, where the OER occurs at the triple-phase boundaries involving the electrode, electrolyte, and gas phase. The high ionic conductivity facilitates oxygen ion transport from the electrolyte to the reaction sites, while electronic conductivity ensures efficient current collection.

The performance of perovskite oxides in SOECs is closely tied to their defect chemistry. Oxygen vacancies, formed due to Sr doping, act as charge carriers for ionic transport. The concentration of these vacancies can be described by the defect equilibrium equation, which depends on temperature and oxygen partial pressure. At high temperatures, the oxygen vacancy concentration increases, improving ionic conductivity. However, excessive vacancies can lead to structural instability, a key challenge in long-term operation. The electronic conductivity, on the other hand, is governed by the small polaron hopping mechanism, where electrons move between Co3+ and Co4+ or Fe3+ and Fe4+ sites. The balance between ionic and electronic conductivity is crucial for optimal electrode performance.

Despite their advantages, perovskite oxide catalysts face degradation issues during prolonged SOEC operation. One major degradation mechanism is cation segregation, where Sr or Co migrates to the surface, forming insulating phases such as SrO or Co3O4. This phenomenon is driven by electrostatic interactions and strain effects at the surface, leading to decreased catalytic activity and increased polarization resistance. Another challenge is chromium poisoning when SOECs are coupled with metallic interconnects. Volatile Cr species from interconnects deposit on the electrode surface, blocking active sites and accelerating degradation. Additionally, thermal cycling can induce mechanical stresses due to mismatched thermal expansion coefficients between the electrode and electrolyte, causing delamination or cracking.

To mitigate these degradation mechanisms, doping strategies have been extensively investigated. Doping at the A-site or B-site can tailor the defect chemistry and improve stability. For instance, substituting Sr with smaller cations like Ca or Ba can reduce Sr segregation by altering the electrostatic potential at the surface. Similarly, replacing Co with Fe or Mn can enhance redox stability while maintaining sufficient electronic conductivity. The introduction of dopants such as Nb or Ta at the B-site has been shown to suppress cation migration by stabilizing the perovskite lattice. These modifications not only improve durability but also optimize the ionic-electronic transport properties.

Another approach involves nanostructuring or composite formation to enhance performance. Infiltrating nanoparticles of catalytically active materials, such as CeO2 or Pd, into the perovskite scaffold can increase the density of active sites without compromising MIEC properties. Composite electrodes, combining perovskite phases with ionic conductors like gadolinium-doped ceria (GDC), have demonstrated improved electrochemical performance and resistance to degradation. The interfacial interactions between phases play a critical role in enhancing oxygen exchange kinetics and mechanical integrity.

The operating conditions of SOECs also significantly influence the stability of perovskite oxide catalysts. Lowering the operating temperature can reduce degradation rates but may compromise efficiency due to slower reaction kinetics. Optimizing the steam-to-hydrogen ratio is essential to prevent electrode oxidation or reduction, which can lead to phase decomposition. Advanced characterization techniques, such as in-situ X-ray diffraction and impedance spectroscopy, provide insights into the real-time structural and electrochemical changes, enabling better material design.

In summary, perovskite oxide catalysts like LaSrCoFeO3 are pivotal for advancing SOEC technology due to their MIEC properties and catalytic activity. However, challenges such as cation segregation, chromium poisoning, and thermal cycling-induced degradation must be addressed to ensure long-term durability. Strategic doping, nanostructuring, and composite formation offer viable pathways to enhance stability while maintaining high performance. Continued research into defect chemistry, interfacial engineering, and operational optimization will be critical for the widespread adoption of these materials in industrial-scale hydrogen production. The development of robust perovskite catalysts will play a central role in achieving efficient and sustainable hydrogen generation through solid oxide electrolysis.