In this study, composite Nickel-yttria stabilized zirconia (Ni-YSZ) identically fabricated fuel electrodes were operated as solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) under the same gas environment, and their corresponding performances and microstructural evolutions were quantitatively evaluated. Fuel electrode performance degraded during the FC operation, whereas the performance was enhanced during the EC operation. Impurity segregation was clearly observed using scanning electron microscopy (SEM) observation with low acceleration voltage. Severe segregation phenomenon of impurity phase was observed after the EC mode operation, while less segregation of impurity segregation was found after the FC mode operation. From the three dimensional microstructure reconstructions based on focused ion beam-scanning electron microscopy technique, triple phase boundary (TPB) density showed drastic decrease after the EC mode operation due to the blocking of TPB by segregated impurity phase, which contradicts to the performance enhancement during the EC operation. Contrasting influences of FC and EC operation modes on electrochemical performance and microstructural evolutions were clearly observed. Performance enhancement after 100 hours of EC operation implies that impurity segregation cannot be simply regarded as one of the causes of electrode performance degradation. (C) 2017 The Electrochemical Society. All rights reserved.