Reversible solid oxide cells (RSOCs) are electrochemical devices that can convert energy between chemical and electrical ones, which have attracted widespread attention due to their high energy conversion efficiency, low emissions, and lack of requirement for precious metals. They can be integrated with renewable energy systems to store excess electrical energy during periods of low demand (valleys) and efficiently release it during periods of high demand (peaks), demonstrating great potential for promoting the effective utilization of renewable energy and the achievement of the dual-carbon goals. The RSOCs typically operate at 700-800 ℃. The high operating temperature not only impose stringent requirements on the stability and durability of electrode and electrolyte materials, resulting in prolonged startup or shutdown times for RSOC stacks, but also poses significant challenges to the thermal stability, thermal expansion matching, and chemical stability of sealing materials and surrounding components. These collective requirements will lead to increased system costs and reduced operational lifespan, presenting critical barriers to practical application. Therefore, reducing operating temperature has become a hot topic in RSOC development. If the operating temperature of RSOCs can be reduced to 400-600 ℃, a better reliability of power generation and electrolysis in RSOC systems can be obtained, as well as a lower cost, a promoted industrialization, which aligns with the national strategy of achieving carbon peaking and carbon neutrality as soon as possible. However, as the operating temperature of RSOCs decreases, the oxygen reduction/evolution reactions (ORR/OER) kinetics become significantly sluggish due to the slow oxygen surface exchange rate, charge transfer, and limited bulk oxygen ion diffusion. Consequently, the polarization resistance (R_p) of the air electrode increases drastically, which severely constrains the practical application of RSOCs in low-temperature regime. Therefore, enhancing the ORR/OER catalytic activity of the air electrode at low temperatures is a key factor in reducing the R_p and achieving high performance RSOCs.

Generally, air electrode materials require high electrical conductivity (electronic, ionic, and/or protonic), high intrinsic electrocatalytic activity, structural and chemical stability, and durability in high oxygen partial pressure and steam content conditions. Over years of intensive research, substantial progress has been achieved through tailoring the elemental composition and stoichiometric ratios of air electrode materials, incorporating supplementary active species, and engineering microstructures that facilitate the ORR/OER processes of air electrode. Among these methods, nanotechnology plays a significant role in enhancing the ORR/OER catalytic activity of air electrodes and improving the electrochemical performance and stability of RSOCs. This review summarizes the application and research progress of nanotechnology in RSOCs air electrode materials from two perspectives: the development of novel air electrode materials and the surface modification of air electrode materials. The development of novel air electrode materials includes the preparation of nano-powders, in situ self-assembly methods, and in situ water-induced methods. The preparation of nanoscale powders is achieved through wet chemical methods, yielding high specific surface area and narrow particle size distribution, which is the first step toward obtaining highly active air electrode materials; In situ self-assembly methods involve rational design of air electrode materials composition to produce composite air electrodes with nano-particle modifications via a one-pot process; In situ water-induced methods utilize water molecule-induced cation segregation to form catalytically active phases in air electrodes. The surface modification of air electrode materials encompasses the nano-infiltration method, atomic layer deposition (ALD), pulsed laser deposition (PLD), and the construction of new air electrode structures. The nano-infiltration method can significantly improve the electrical conductivity, electrocatalytic activity, and expand the effective three-phase reaction sites by selecting an appropriate catalyst. ALD technology offers advantages such as precise regulation of membrane thickness and composition at the atomic level, low operating temperature (<300 ℃), and excellent reproducibility. PLD technology enables high-precision stoichiometric control of nanoscale films, compatibility with complex materials, and controllable microstructures. Novel air electrode structures are engineered to facilitate gas and ion transport, thereby increasing reactive sites and reducing activation losses. Finally, several suggestions for future research directions are proposed.