Unraveling reaction pathways in H2O–CO2 co-electrolysis for tunable syngas composition in solid oxide electrolysis cell

H₂O–CO₂ co-electrolysis in solid oxide electrolysis cells (SOECs) represents an efficient approach to syngas (H₂ + CO) production, enabling downstream synthesis of carbon-neutral fuels. However, complex competing pathways—particularly the poorly understood CO₂ reduction mechanism—hinder precise control of syngas composition. Here, we experimentally determine the onset voltage of CO₂ electrolysis under co-electrolysis conditions, addressing existing gaps in the early literature. By coupling electrochemical analysis with outlet gas analysis, two voltage-dependent regimes are identified: at low voltages, CO is primarily generated via the thermochemical reverse water-gas shift (RWGS) reaction, while at high voltages, both RWGS and CO₂ electrolysis occur, with RWGS remaining dominant. The onset voltages for CO₂ electrolysis are precisely determined as 0.9 V, 1.1 V, and 1.2 V for atmospheres of 20 % H₂O–80 % CO₂, 50 % H₂O–50 % CO₂, and 80 % H₂O–20 % CO₂, respectively, marking the transition from pure H₂O electrolysis to H₂O–CO₂ co-electrolysis. Feed composition is the dominant factor influencing the onset voltage and syngas selectivity, while operating parameters such as flow rate and temperature also have noticeable effects, enabling wide H₂/CO range ratio (1.09–5.40). These results provide new insights into the CO₂ reduction pathway and offer guidance for the rational design of SOEC operation for controllable syngas production.