Understanding the Roles of Double- and Triple-Phase Boundaries in High-Temperature CO ₂ Electrolysis

High-temperature solid oxide electrolysis cells are promising for CO₂-to-CO conversion with high selectivity and energy efficiency. However, the correlation between the electrolysis performance and electrode interface structure remains poorly understood. Here, in a Ni/ceria system, we demonstrate that the segregation-free Ni-doped ceria forms double-phase boundaries (DPBs) with CO₂, offering a CO outlet concentration of 83.0 ± 0.2%. By contrast, carbon deposition was seen in controls with triple-phase boundaries (TPBs) formed by segregated Ni and ceria interfacing with CO₂. The electrochemical activity strongly correlates with oxygen vacancy (O_v) concentrations in Ni/ceria. The segregation-free Ni/ceria catalyst achieves 1.4 A cm^-2 at 1.65 ± 0.01 V and operates stably at 600 mA cm^-2 for 220 h without any decay in activity. The results indicate that enriching O_v at DPBs promotes high-temperature CO₂ electrolysis, with a more influential role than TPBs for these ceria-based catalysts.