Active　and　stable　oxygen　electrode　is　probably　the　most　important　in　the　development　of　solid　oxide　electrolysis　cells　(SOECs)　technologies.　Herein,　we　report　the　successful　development　of　mixed　ionic　and　electronic　conducting　(MIEC)　La0.6Sr0.4Co0.2Fe0.8O3-δ　(LSCF)　perovskite　oxides　directly　assembled　on　barrier-layer-free　yttria-stabilized　zirconia　(YSZ)　electrolyte　as　highly　active　and　stable　oxygen　electrodes　of　SOECs.　Electrolysis　polarization　effectively　induces　the　formation　of　electrode/electrolyte　interface,　similar　to　that　observed　under　solid　oxide　fuel　cell　(SOFC)　operation　conditions.　However,　in　contrast　to　the　significant　performance　decay　under　SOFC　operation　conditions,　the　cell　with　directly　assembled　LSCF　oxygen　electrodes　shows　excellent　stability,　tested　for　300　h　at　0.5　A　cm−2　and　750　°C　under　SOEC　operation　conditions.　Detailed　microstructure　and　phase　analysis　reveal　that　Sr　segregation　is　inevitable　for　LSCF　electrode,　but　anodic　polarization　substantially　suppresses　Sr　segregation　and　migration　to　the　electrode/electrolyte　interface,　leading　to　the　formation　of　stable　and　efficient　electrode/electrolyte　interface　for　water　and　CO2　electrolysis　under　SOECs　operation　conditions.　The　present　study　demonstrates　the　feasibility　of　using　directly　assembled　MIEC　cobaltite　based　oxygen　electrodes　on　barrier-layer-free　YSZ　electrolyte　of　SOECs.　©　2018　Elsevier　B.V.