The increasing use of renewables for energy production brings about challenges, for instance, regarding the storage of electricity. Highly efficient solid oxide electrolysis cells (SOECs) can become an attractive solution for converting excess electricity into hydrocarbons (Power-to-Gas). The use of electricity to produce synthetic methane, especially, is considered as a part of the energy transition due to available infrastructure for storage and distribution.

In this contribution, state-of-the-art (SoA) SOECs consisting of a Nickel-Yttria Stabilized Zirconia (Ni-YSZ) fuel electrode, YSZ electrolyte and a mixed ionic electronic conductor (MIEC) Lanthanum Strontium Cobalt Ferrite-Gadolinium doped Ceria (LSCF-GDC) oxygen electrode were tested under co-electrolysis (H₂O+CO₂) conditions where the ultimate goal is to produce methane. The SOEC durability tests are traditionally carried out in galvanostatic mode due to the straightforward test control and data interpretation. In this mode, the cell voltages change due to degradation and reach values that are either above or below the thermoneutral voltage which lead to either endothermic or exothermic processes lowering the cell efficiency. However, from a SOEC system point of view, operating the SOEC at thermoneutral voltage (i.e. potentiostatic) is attractive yielding a more efficient system with a comparatively easier heat management. The aim in this study was to compare the SOEC durability under co-electrolysis conditions between galvanostatic and potentiostatic mode. Specifically, the cell was operated at 0.75 A/cm² (galvanostatic) and at 1.2 V (potentiostatic) at 750 ^oC for 1000 hours and 500 hours respectively. A detailed electrochemical and microstructure analysis for the two testing regimes will be presented. The degradation mechanisms associated with individual electrode performance will be comprehensively discussed.