**1. Introduction**

In view of the currently increasing demand for energy resources, gradual depletion of fossil fuels, and growing environmental problems, the development of alternative hydrogen energy is an essential and highly promising R&D direction. For further progress in this area, the development of advanced materials for electrochemical power systems is required. Particularly, promising solid oxide fuel cells (SOFCs) provide the efficiency of up to 85% in couple with almost double cost savings and a 100 times reduction of harmful emissions compared with conventional power sources due to the absence of direct chemical contact between the fuel and the oxidizer [1]. The application of SOFC-based power sources a significant power and fuel savings. The operation temperatures as high as 700–950°C afford increased rates of electrode reactions without using expensive catalysts. Another advantage of SOFCs is the absence of strict requirements for fuel purity. Besides hydrogen, any hydrocarbons converted into synthesis gas (H2-CO) can be used. High thermodynamic efficiency, continuous operation, and environment-friendly performances make solid oxide fuel cells more promising compared with such conventional systems as internal combustion engines, solar panels, and wind turbines. As a promising power storage system, reversible SOFCs can provide an economically effective approach to the power management using discontinuous energy sources [2, 3].

Thus, the development and implementation of SOFC-based fuel cells for commercial production are becoming a priority to address the problems of distributed power supply, energy saving, cogeneration, and saving fuel resources.

The decrease of working temperature and development of medium-temperature SOFCs is an important goal in materials science since high-operating temperatures cause problems with the compatibility of electrode materials and electrolytes. The implementation of medium-temperature SOFCs can provide an extension of the range of applied materials, reduce degradation of the devices, and increase their operational lifetime. The main components of SOFC include cathode, anode, and electrolyte. The applied electrolytes differ in their anionic, protonic, or ion-mixed ion-transport mechanism. The basic principle of fuel cell operation is that the transport of oxygen ions (O<sup>2</sup>) from the cathode to the anode can only proceed in the presence of oxygen vacancies. In this regard, the optimal electrolyte materials must contain anion vacancies in the crystal lattice. Currently, cerium dioxide (CeO2)-based nanomaterials with oxygen-ionic conductivity are considered promising as medium-temperature electrolytes affording the reduction of the fuel cell operating temperature by 300–400°C. In respect of electric performances, these electrolytes are not inferior to conventional zirconia-based YSZ materials (particularly, (ZrO2)0.92(Y2O3)0.08 ceramics) [4, 5]. Furthermore, they are thermodynamically stable at relatively low-operating temperatures of 600–800°C that provide a long lifetime. Moreover, relatively low-operating temperatures prevent from the interlayer diffusion and interfacial layers between SOFC components exclude solid-phase interaction. All the existing SOFC cathodes have certain disadvantages that determine the growing interest in R&D in the development of new cathode materials, particularly for medium-temperature (500–700°C) SOFCs.

During SOFC operation in the middle-temperature range, the power characteristics of the cell are limited by the cathode operation [6]. Since the SOFC cathode reduction of molecular oxygen and transport of oxygen ions to the electrolyte surface take place, the developed medium-temperature cathode materials should meet several requirements. Particularly, they should be ultrafine and have high electronic or mixed electron-ion conductivity. To reduce the diffusion drag at the cathode, a welldeveloped porous structure is required, since the rate of molecular oxygen reduction depends on the specific surface area. Furthermore, the mechanical compatibility of the cathode and electrolyte is important. These requirements are fulfilled by complex metal oxides and their nanocomposites, particularly complex oxides based on rare earth elements (REE) and 3D transition metals combining high electric and catalytic properties. Currently, single-phase complex perovskite materials such as LnMO3-<sup>δ</sup> (M-Cr, Mn, Fe, Ni, and Co) are proposed as new materials for medium-temperature SOFC cathodes, featuring stability in an oxidizing atmosphere in a wide temperature
