Stabilizing lifetime syngas production of thermoneutral H2O-CO2 co-electrolysis

High-temperature co-electrolysis of H 2 O and CO 2 in solid oxide electrolysis cells (co-SOEC) is a promising pathway for renewable syngas production in future Power-to-Liquid systems. However, stable operation under thermoneutral conditions remains challenging due to the need to simultaneously control thermal balance, syngas composition, and carbon stability, particularly with gas recirculation. In this work, we investigate thermoneutral co-SOEC operation through a combined experimental and numerical approach, coupling single-cell electrochemical characterization and gas analysis with thermodynamic modelling and multiphysics simulations. The results show that carbon deposition is governed by local thermodynamic conditions within the Ni-YSZ fuel electrode, which can significantly deviate from bulk gas equilibrium. Increasing the inlet flow rate at constant reactant utilization promotes earlier carbon formation, highlighting the limitations of conventional bulk thermodynamic approaches. These findings provide guidelines for defining stable operating strategies for co-SOEC systems with controlled syngas composition and reduced carbon risk.