A Review on In-situ Methanation Based on CO2/H2O Co-electrolysis in Solid Oxide Electrolytic Cells Addressing the Thermo-Kinetic Mismatch

Solid oxide electrolysis cells (SOECs) offer a compelling platform for converting renewable electricity and captured CO 2 into methane by integrating CO 2 /H 2 O co-electrolysis with in-situ methanation. This single-reactor concept promises process intensification, superior heat integration, and compatibility with existing natural gas infrastructure. However, its development is fundamentally constrained by the thermo-kinetic mismatch: efficient co-electrolysis typically requires temperatures above 600°C to minimize ohmic and polarization losses, whereas methanation is thermodynamically favored below 400°C. This review examines recent progress in direct methane synthesis within SOECs from the perspective of mitigating this mismatch systematically. We first clarify the reaction pathways, which including CO 2 /H 2 O electroreduction, reverse water-gas shift, methanation, carbon formation, and electrochemical promotion, and discuss the reaction mechanism of the direct methane synthesis within SOECs. Subsequently, we analyze material design strategies with emphasis on multifunctional cathodes, integration of methanation-active catalysts, and the enabling roles of oxygen electrodes and electrolytes in achieving medium-temperature operation. Reactor engineering approaches, ranging from single-temperature-zone to two-temperature-zone and thermally intensified configurations, are then evaluated to illustrate how spatial separation and flow-field regulation can enhance methane selectivity and overall efficiency. Finally, we identify key challenges and future directions in mechanistic understanding, integrated materials design, reactor optimization, and long-term durability. This review highlights that coordinated advances across reaction mechanism, thermo-electrocatalytic materials, and reactor engineering are essential to translate SOEC in-situ methanation from laboratory demonstration into practical power-to-methane applications.