3D microstructural characterization of Ni/yttria-stabilized zirconia electrodes during long-term CO₂ electrolysis

Solid oxide electrolysis cells (SOECs) are one of the most promising energy conversion devices due to their high efficiency and gas flexibility. However, the degradation of their performance during long-term operation limits their commercialization. Among the different phenomena, the degradation of Ni/yttria-stabilized zirconia (YSZ) fuel electrodes during long-term operation is recognized as one of the main causes for the loss in cell performance. Accordingly, numerous studies have focused on investigating the degradation of Ni/YSZ electrodes during steam electrolysis, whereas only limited studies have been performed on the stability of Ni/YSZ electrodes after long-term CO₂ electrolysis. In this work, the microstructure evolution of Ni/YSZ electrodes after long-term operation in a CO/CO₂ atmosphere was investigated. For this purpose, Ni/YSZ-supported planar-type SOEC cells were operated at 800 degrees C and -1 A cm^-2 or zero current with a mixture of CO₂/CO (90/10) supplied to the Ni/YSZ electrode. Subsequently, the 3D electrode microstructures of pristine cermet (reference) and after 1000 h operation were observed. A significant loss of Ni in the active electrode was observed in the cell operated at -1 A cm^-2 for 1000 h, along with an increased Ni fraction in the support layer. An increase in the Ni particle size (i.e., Ni coarsening) was also observed. In the cell operated with zero current for 1000 h, only Ni coarsening was observed, with no Ni migration. Our results show that Ni migration away from the electrode/electrolyte interface also occurs during long-term CO₂ electrolysis to a similar extent as in steam electrolysis.