Concurrent surface and bulk modification of ceria-based cathodic catalyst for CO ₂ 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, yet currently face the critical challenge of phase instability and coke formation, especially under strong polarization conditions, which leads to device performance degradation or even failure. In this study, redox phase stable CeO₂ cathode materials were modified via co-doping with Mn and Ni, enabling the in situ formation of self-assembled and strongly coupled metal Ni/ceria heterostructures, and modification of bulk properties (increased redox Ce³⁺ active site numbers, Ni exsolution, oxygen defect concentration, and electrical properties), which consequently effectively enhances surface reaction kinetics and electrode series conductivity. The optimized Mn-Ni co-doped ceria (CMN) exhibited stable electrochemical performance for nearly 140 h at 750 ^oC with minimal degradation under mild electrolysis conditions. Under strong polarization (2.0 V), SOECs based on the CMN electrode delivered an impressive current density of 3.05 A cm^-2 at 800 ^oC, accompanied by reliable operational stability over the state-of-the-art Ni-based cermet electrode and widely investigated perovskite oxide cathode catalysts. These findings underscore the synergistic benefits of transition metal co-doping and in-situ heterostructure engineering in enabling high-performance, carbon-tolerant ceria-based electrodes for SOEC operation both under moderate and harsh conditions, approaching practical application.