Proton ceramic cells (PCCs) are widely investigated as devices for power generation, energy storage, and sustainable chemical synthesis because of their moderate operating temperature, high efficiency and great application prospects. Nevertheless, a main challenge for oxygen electrode of PCCs remains the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at intermediate and lower temperatures as well as the insufficient stability of the oxygen electrode materials in the operating environment. Designing and developing high-performance and durable oxygen electrodes is thus crucial for promoting the industrial application of PCCs technology. This review discussed the relationships among composition, structure and electrochemical performance of oxygen electrode materials and summarized the factors that impact the stability of oxygen electrode. The stability of electrolyte and oxygen electrode interface under high steam condition, CO₂ atmosphere and Cr atmosphere was discussed. In addition, a future research in oxygen electrode stability was also proposed.

The elementary reactions at the oxygen electrode include oxygen adsorption and dissociation, charge/oxygen/proton transfer, as well as water formation and exhaust. It is considered that protons can play a critical role in the reaction process on the oxygen electrode in addition to electrons and oxygen ions based on the elementary reactions occurring on the oxygen electrode. Triple ion-electronic-protonic conducting (TIEC) materials can extend the triple-phase boundary (TPB) area to the entire surface of the oxygen electrode. Besides the basic requirements of high activity and electrochemical performance, the oxygen electrode also needs to have a sufficient porosity, a good stability, and a matched thermal expansion coefficient with the electrolyte.

Developing new materials or optimizing their compositions is a possible way to improve the intrinsic characteristics of materials. Effective design strategies include element doping (A/B/O-site doping) or the construction of A- and B-site defects. The A-site in the perovskite structure ensures a large lattice volume and prevents lattice distortion, and doping elements in the A-site with large radii is beneficial to ion transport. The acceptor doping on the B-site can increase the electron cloud density around the oxygen atoms, thereby facilitating proton absorption. Also, the oxygen-site doping is more complicated. The substitution of oxygen by elements with a lower valence state and a higher electronegativity can promote the migration rate of oxide ions and protons. However, electron conductivity and carrier concentration may decrease after anion doping. The realization of optimal TIEC in single-phase materials is a challenge. Composite electrodes with different functions exhibit synergistic effects and strong interactions at the nanoscale, thus garnering increasing attention. Composite oxygen electrode can improve the mismatched thermal mechanical issue between the oxygen electrode and the electrolyte, and enhance the chemical stability towards CO₂ and H₂O, thereby increasing the long-term stability of PCCs. Methods for preparing composite oxygen electrodes include mechanical mixing, impregnation, self-assembly, and dissolution. Among these methods, the last three can be used to fabricate nano-sized composite oxygen electrodes with a superior performance.

The stability of the oxygen electrode and the interface between the oxygen electrode and the electrolyte has a significant impact on the lifespan of the PCCs. The adhesion between the electrode and the electrolyte is strengthened via depositing an intermediate layer to enhance PCCs stability. Active material with a high coefficient of thermal expansion can be combined with a negative thermal expansion material to form a composite oxygen electrode, exhibiting a well-matched thermal expansion characteristic with the electrolyte, which is beneficial to the thermal cycling stability of the PCCs. In addition to interface stability between the electrode and electrolyte, the stability of the oxygen electrode material is also crucial. Most oxygen electrode materials contain Ba and Sr elements, which tend to enrich and segregate on the electrode surface, especially in atmospheres containing H₂O and CO₂. Water can promote element segregation, leading to the formation of secondary phases through reactions with impurities in air, which affects the catalytic activity of the electrode material. The stability of oxygen electrode materials can be significantly improved via doping, developing materials without alkali elements, and using composite oxygen electrodes. In addition, other pollutants (such as Cr, Si, B) may be generated in the stack, which also affects the stability. Furthermore, the oxygen electrode of PCCs is exposed to humidified air when operated in the electrolysis cell mode. However, there is a limited research on the poisoning of PCCs oxygen electrodes towards high steam and other elements (Cr, Si).