High-temperature solid oxide cells (SOCs) offer one of the most efficient and versatile routes for producing electric power and H2. However, the practical use of nanomaterials in SOCs has been limited by their lack of thermal stability. In this study, we present an infiltration technique that enables the in situ synthesis of extremely small, thermally stable perovskite (Sm0.5Sr0.5)CoO3 nanocatalysts on the inner surface of porous SOC electrodes. We identified certain impurity phases, such as SrCO3, that cause fatal degradation and eliminated them using a rational complexation strategy optimized for individual constituent cations. Consequently, we fabricated ∼ 20 nm diameter, highly pure, single-phase nanocatalysts that achieved more than double the performance of a cell with standard (La,Sr)(Co,Fe)O3– and (La,Sr)CoO3-based air electrode. The cells stably operated during long-term tests in both the power generation and H2 production modes, with negligible degradation. Furthermore, we successfully scaled up this process to fabricate large-scale commercial cells using a fully automated process. The key findings of this study will resolve critical barriers with high-temperature nanomaterials and accelerate the commercialization of SOC technology.