Operando observations of active three phase boundary of patterned nickel - Yttria stabilized zirconia electrode in solid oxide cell

In solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), the substantial performance degradation of nickel (Ni) - yttria-stabilized zirconia (YSZ) fuel electrode has been a growing challenge for commercialization. The microstructure of Ni-YSZ electrode is not stable during long-term operation due to Ni phase redistribution. In the present study, operando observations of patterned Ni film electrodes under SOFC, SOEC and reversible solid oxide cell (RSOC) operation modes are conducted. Confocal laser scanning microscopy is applied to acquire real-time microscopy images of the three phase boundary (TPB) under three operation modes to study the Ni mobility on the YSZ surface. The operando observation showed that the Ni phase dynamically spreads and splits up only under the SOFC mode, while this phenomenon is not observed under the SOEC mode. Ni coarsening occurs regardless of the operation mode. The dynamic motion of active TPB qualitatively correlates with the variation in the current density during the operation. The contact angles of Ni on flat YSZ surface after operation are the smallest for SOFC, followed by RSOC and the largest for SOEC. Competition between the altered Ni wettability and the Ni agglomeration finally determines the complicated Ni morphological changes.