Concurrent surface and bulk modification of ceria-based cathodic catalyst for CO₂ reduction in solid oxide electrolysis cell under strong polarization conditions

High-temperature solid oxide electrolysis cells (SOECs) offer high energy efficiency and environmental compatibility for CO₂ conversion but currently face the critical challenges of phase instability and coke formation, especially under the strong polarization conditions. In this study, redox phase-stable CeO₂ cathode materials were modified by co-doping with Mn and Ni, enabling the in situ formation of self-assembled and strongly coupled metal Ni/ceria heterostructures and the modification of bulk properties by increasing redox Ce³⁺ active site numbers, Ni exsolution, oxygen defect concentrations, and electrical properties, which consequently effectively enhanced surface reaction kinetics and electrode series conductivity. The optimized Mn–Ni codoped ceria (CMN) exhibited stable electrochemical performance for nearly 140 h at 750 °C with minimal degradation under mild electrolysis conditions. Under strong polarization (2.0 V) condition, SOECs based on the CMN electrode delivered an impressive current density of 3.05 A∙cm⁻² at 800 °C, accompanied by reliable operational stability over the state-of-the-art Ni-based cermet and widely investigated perovskite oxide cathode catalysts. These findings underscore the synergistic benefits of transition metal codoping and in situ heterostructure engineering in enabling high-performance, carbon-tolerant ceria-based electrodes for SOEC operation under both moderate and harsh operating conditions, advancing practical application.