Modeling for optimizing performance has attracted substantial research efforts in the last twenty years with special focus on solid oxide fuel cells (SOFCs). However, limited amount of the modeling work has been focused on the solid oxide electrolysis cell (SOEC) operation mode and even less on degradation issues connected to SOEC operation. Despite the similarities between the two operation modes, the different operating voltage ranges and the gradients involved influence the long-term degradation and performance in different ways. When performing co-electrolysis of CO₂ and H₂O at high current densities, which provides the highest syngas production rate, degradation at the interface between the Ni-YSZ electrode and the YSZ electrolyte has been reported due to the formation of small amounts of carbon. The formation of carbon at the active sites of the Ni-YSZ electrode may reduce the amount of active sites and, in the worst case, cause a delamination of the electrode/electrolyte interface due to the pressure exerted by the growth of carbon fibers. The Boudouard reaction (2CO_(g) → C_(s) + CO_{2 (g)}) describes how carbon is formed. Carbon deposition could also occur by electrolysis of carbon dioxide to solid carbon (CO_2(g)+ 4e^-→ C_(s) + 2O^2-), which is thermodynamically equivalent to the two-step reaction of electrolysis of CO₂ to CO followed by the Boudouard reaction. Although carbon deposition is thermodynamically favored only at higher reactant conversion than typically used in the operation of an SOEC, it has been observed at “safe” conversion conditions. Thus, understanding why the thermodynamic threshold is violated and where this happens locally in the cell would be of great interest when choosing operating conditions and to avoid possible degradation of the cell. A three dimensional (3D) multiphysics model of an SOEC with cross-flows considering heat transfer is used to study and locate the degradation thresholds in these cells regarding carbon deposition at the Ni-YSZ electrode/electrolyte interface. The aim of this study is to show where and under which operating conditions the Boudouard reaction is locally induced. Only carbon dioxide electrolysis is considered as it is supposed to represent an extreme case compared with co-electrolysis, as the reverse water-gas shift reaction can mitigate carbon deposition. Moreover, possible solutions to the problem will be studied by varying structural parameters.