Stable operation at elevated temperature has significant importance for the commercialization of solid oxide fuel cells. In particular, interdiffusion at the interface between the cathode and the electrolyte during long-term operation leads to the formation of insulating phases, significantly reducing the cell performance. In this study, a simple wet chemical coating method was employed to fabricate a thin, dense Gd_0.2Ce_0.8O_2−δ (GDC) diffusion barrier layer between the La_0.6Sr_0.4Co_0.2Fe_0.8-O_3−δ (LSCF) cathode and yttria-stabilized zirconia (YSZ) electrolyte with a gelatin hydrogel-based precursor solution. The fabrication of thin dense film GDC using a simple spin coating method was enabled by the interfacial self-assembly of organic/inorganic composites between the precursor and hydrogel. The anode-supported single cell with a hydrogel-assisted GDC chemical diffusion barrier layer showed improved electrochemical performance due to fast ionic conduction via short and highly percolated diffusion pathways, maintaining the initial performance after the accelerated heat treatment at 900 °C for 100 h by preventing the cation interdiffusion at the interface.