Bismuth　oxide　is　as　an　active　promoter　in　enhancing　the　ionic　conductivity　and　electrocatalytic　activity　of　manganite　oxygen　electrodes　of　solid　oxide　cells,　but　there　are　very　limited　reports　on　the　formation　and　evolution　of　electrode/electrolyte　interface　of　bismuth　oxide-manganite　composite　electrode　under　the　influence　of　electrochemical　polarization.　Herein,　we　report　the　effect　of　electrochemical　polarization　and　direction　of　polarization　current　on　the　electrocatalytic　performance　and　electrode/electrolyte　interface　of　a　(La0.8Sr0.2)(0.95)Mn0.95Pt0.05O3+delta-Er0.4Bi1.6O3　(LSMPt-ESB)　composite　oxygen　electrode　assembled　on　zirconia　electrolyte.　The　cell　with　the　LSMPt-ESB　electrode　produces　outstanding　performance　for　power　generation　and　steam　splitting,　and　it　is　stable　without　noticeable　degradation　during　operation　at　600　degrees　C　for　350　h　in　the　fuel　cell　mode.　The　cathodic　polarization　induces　in　operando　formation　of　electrode/electrolyte　interface　with　observation　of　an　Er-deficient　LSMPt-ESB　dense　layer　and　Er-rich　(Er,Bi,Mn)O-x　particles　on　the　zirconia　electrolyte　surface.　This　is　different　to　the　case　of　dwell　under　open　circuit　and　in　particular　under　anodic　polarization　conditions.　The　present　study　gains　insights　into　the　development　of　high　performance,　reliable　bismuth　oxide-manganite　composite　oxygen　electrode　for　reduced　temperature　solid　oxide　cells.