When the operating temperature of a solid oxide electrolysis cell (SOEC) is lower than the outlet temperature of a nuclear reactor, the reactor can be directly coupled with the SOEC as a high-temperature heat source. However, the key to the efficiency and return on investment of this hybrid energy system lies in the expected lifetime of the SOEC. This study assessed Ni-YSZ|YSZ|GDC|LSC fuel electrode support cells’ long-term stability during electrolysis at \({650}\,^{\circ }\) C with a current density of \(-0.5\, \text {A}\,\text {cm}^{-2}\) over 1818 h. The average voltage degradation rate of \(2.63\%\, \text{k}\text{h}^{-1}\) unfolded in two phases: an initial rapid decay (90 to 1120 h at \(3.58\% \, \text {kh}^{-1}\) ) and a stable decay (1120 to 1818 h at \(2.14\% \, \text {kh}^{-1}\) ), emphasizing SOECs’ probability coupling with nuclear reactors at \({650}\,^{\circ }\) C. Post-1818-hour electrolysis revealed nickel particle formation associated with \(\text {Ni(OH)}_x\) diffusion and re-deposition, alongside a strontium-containing layer causing interface cracking. Despite minimal strontium segregation in the EDS, XPS data indicated surface segregation of Sr. This study provides crucial insights into prolonged SOEC operation, highlighting both its potential and challenges.
