Formation of secondary phases at the interface between cathode and electrolyte is one of the issues affecting the long-term stability of solid oxide fuel cells. La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) perovskite cathode is known to react with yttria-stabilized zirconia (YSZ) forming SrZrO₃ at the interface. To reduce secondary phase formation, a reaction barrier layer such as Gd-doped ceria (GDC) is inserted between LSCF and YSZ. However, the stability of the reaction barrier layer after long-term operation may be directly affected by the microstructure and cation diffusion through GDC. In this work, the microstructure of the reaction barrier layer is controlled by growing the GDC thin films on polycrystalline and single crystal YSZ (100) electrolytes using pulsed laser deposition technique. Electrochemical impedance spectroscopy (EIS) and microstructural analyses were performed on LSCF|GDC|YSZ|Pt cells with different GDC interlayers under an open-circuit condition at 800⁰C for 300 h in air. It is observed that SrZrO₃ formed primarily at the LSCF/GDC interface of cells with polycrystalline GDC interlayers resulting to a rapid increase in area-specific resistance (ASR) with time. In the case of single-crystalline GDC interlayer, no apparent formation of SrZrO₃ but a possible formation of GdFeO₃ or LaMO₃ (M= Co, Fe) at the LSCF/GDC interface was observed. EIS analysis of cells with single-crystalline GDC interlayer reveals a slow increase in the ASR with time. Microstructural evolution of the interfaces and type of secondary phase formation at the interface gives rise to a different rate of ASR and is shown to be directly related to the microstructure of GDC interlayers.